A study of lower back strain injuriesresulting from road accidents

Prepared for Vehicle Standards and Engineering and RoadSafety Divisions, Department of the Environment, Transportand the Regions (now the Department for Transport,Local Government and the Regions)

R Minton (TRL Limited), P Murray, W Stephenson andC S B Galasko (University of Manchester)

TRL Report TRL532

First Published 2002ISSN 0968-4107Copyright TRL Limited 2002.

This report has been produced by TRL Limited, under/as partof a contract placed by the Department of the Environment,Transport and the Regions (now the Department forTransport, Local Government and the Regions). Any viewsexpressed in it are not necessarily those of the Department.

TRL is committed to optimising energy efficiency, reducingwaste and promoting recycling and re-use. In support of theseenvironmental goals, this report has been printed on recycledpaper, comprising 100% post-consumer waste, manufacturedusing a TCF (totally chlorine free) process.

CONTENTS

Page

Executive Summary 1

1 Introduction 3

1.1 Background to present project 3

1.2 Lower back pain in the general population 3

1.3 Anatomy of the human spine 4

1.4 Injury mechanisms 4

2 The lumbar injury/vehicle study 5

2.1. Background 5

2.2 Recruitment phase 5

2.2.1 Objectives 5

2.2.2 Methodology 5

2.2.3 Actual numbers of patients recruited 5

2.3 Follow-up phase 6

2.4 Analysis phase 6

3 Accident analysis 6

3.1 Introduction 6

3.2 AIS 1 spine injuries 7

3.2.1 Influence of gender 7

3.2.2 Influence of impact direction 7

3.2.3 Influence of impact speed 7

3.2.4 Occupant contacts 7

4 Contents of the databases 8

4.1 Vehicle-based factors 8

4.1.1 Vehicle ages 8

4.1.2 Vehicle make/model 8

4.1.3 Occupant seating positions 9

4.1.4 Head restraints 9

4.1.5 Vehicle type, objects hit and number of impacts 10

4.1.6 Travel and impact speeds 10

4.1.7 Airbags and pretensioners 11

4.2 Occupant-based factors 11

4.2.1 Gender distribution 11

4.2.2 Age distribution 11

4.2.3 Occupant awareness 11

4.2.4 Neck strains (whiplash) 11

4.2.5 Prior back problems 11

4.2.6 Disability scores 12

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5 Analysis of data 13

5.1 Introduction 13

5.2 Analysis of medical assessment variables 13

5.2.1 Activities and movements 13

5.2.2 Symptoms 15

5.3 Psychological disability 17

5.4 Effect of occupant characteristics on disability scores 18

5.5 Effect of vehicle parameters on disability scores 19

5.6 Stacked regressions 22

5.6.1 Time-interactive factors 23

5.6.2 Non-time-interactive factors 25

5.6.3 Other factors 26

5.7 Survival 27

5.7.1 Lumbar ‘Survival’ 27

5.7.2 Neck ‘Survival’ 28

5.8 Separation of vehicle models 28

5.8.1. Ford Escorts 28

5.8.2 Ford Fiestas 30

5.8.3 Discussion of separate make/model results 32

6 Conclusions 32

6.1 Conclusions from accident analysis 32

6.2 Conclusions from lumbar injury/vehicle study 32

7 Acknowledgements 34

8 References 34

Abstract 36

Related publications 36

1

Executive Summary

the impending impact was detrimental for those with priorback problems, while there were indications that beinginvolved in a right side impact was beneficial. No benefitsfrom the use of head restraints were demonstrated fromthis analysis. Indeed, the analysis suggested there weresome detrimental effects, although the number of subjectswithout a restraint (the comparison group) was rathersmall. There are also other factors (such as the rear impactcharacteristics of the vehicle and the performance of theseat under impact) that could not be evaluated with thismethodology. Where a head restraint was present, resultsindicated that it should either be hard or, if soft, be thicklypadded. There was some indication that greater horizontaldistance between head and restraint may be detrimental.Results for vertical head restraint adjustment wereinconclusive. There were indications that hinged seatbackrests may be beneficial, but only for those withoutprior back problems. Hard structure at the base of thebackrest which was close enough to the surface to bedetected by the vehicle examiners was detrimental forwomen and for those with prior back problems. Restrictionof this sample to particular makes and models of vehicleswas not found to clarify which factors were significantlyrelated to injury risk, as the sample size became too small.

This project was jointly funded by Road Safety Divisionand Vehicle Standards and Engineering Division of theDepartment of the Environment, Transport and theRegions (DETR) (now Department for Transport, LocalGovernment and the Regions (DTLR)), and was carriedout by three contractors: the Department of OrthopaedicSurgery, University of Manchester, the DETR VehicleInspectorate and TRL Limited.

The aim of the project was to study the long-termdisabling effects of AIS1 injuries to the lumbar region of theback which have been sustained in road traffic accidents,and to attempt to relate the severity and duration of theseinjuries to factors such as vehicle damage, estimatedcollision speed, impact type, head restraint fitting/adjustment and seat type and adjustment. Information onsuch causative factors could possibly enable effectivecountermeasures against these injuries to be developed.

Although the main thrust of the study was towards lowerback injuries, people who had suffered whiplash injuries,with or without concomitant lower back injury, were alsoincluded in order to follow up on a previous project whichwas primarily intended to examine whiplash injury. It wasduring the course of this previous project that thesignificance and extent of the problem of lumbar injurieswas first recognised.

A brief analysis of the Co-operative Crash InjuryStudy’s databases was also carried out, to obtain a broaderview of lumbar strain injuries and to place them in thecontext of other spinal injuries resulting from road trafficaccidents. Results from these databases indicated thatwomen were more at risk than men of incurring straininjury in all regions of the spine. In addition, rear impactcarried a higher risk of spinal strain injury (again, in allregions), but frontal impacts produced higher actualnumbers of injuries. Finally, many of the spinal straininjuries were described as non-contact injuries, or as beingdue to indirect loading, indicating the difficulty the crashinvestigators had in finding occupant contacts within thevehicle to correlate with these injuries.

Approximately one third of subjects in the present studystill had some degree of disability two years after theiraccidents. A previous history of back problems was verysignificantly correlated with long-term disability, althoughin a number of cases, these previous back problems hadthemselves been caused by road accidents. Womensuffered higher disability than men. Older peoplerecovered more slowly, although their initial disabilitytended to be lower than that of younger people. Afterallowing for the difference in height between men andwomen, height itself was not associated with disability,although back length was, with longer back length beingbeneficial. However, trends for back length in relation tothe height of the seat backrest were contradictory andinconclusive. Neither weight, neck length, neckcircumference nor body mass index (obesity) showed anyconclusive correlations with disability. Being braced for

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1 Introduction

1.1 Background to present project

A previous research project studied the long-term disablingeffects of whiplash injuries sustained in road trafficaccidents, attempting to relate the severity and duration ofthe injuries to a number of vehicle-based factors(Minton et al., 1998). During the course of this study, itbecame apparent that a significant number of these AIS1neck injury patients also suffered from pain and disabilityassociated with strains in the lumbar region of the spine.Since patients tend to present at hospital Accident andEmergency departments complaining of neck paininitially, the fact that low back pain is present cansometimes be missed, as pain in this area is not so acute atthis early stage. Over the subsequent 24-72 hours this lowback pain can increase, often overshadowing the neckpain, causing serious movement problems and having longterm effects. The association between this type of injuryand road traffic accidents appeared to be somewhat under-researched, and the present project was therefore initiatedto follow on from the whiplash project, using a similarmethodology, but concentrating on lumbar strain injuries.

1.2 Lower back pain in the general population

The problem of back pain in the general UK population hasbeen reviewed by the Clinical Standards Advisory Group(1994), and the following is based on this report. Back pain istaken to mean low back pain with or without sciatica, whichis far more common than pain in the upper back.

Humans have suffered back pain throughout recordedhistory, and probably longer. Egyptian texts from 1500BCrefer to back pain but, until the 19th century, people appear tohave simply accepted the condition as a fact of life, and goton with living and working as best they could. During thebuilding of the railways in the UK, an injury known as‘railway spine’ came to prominence, and this seems to be thefirst time that simple back ache became linked in the minds ofthe medical profession and the public with some kind ofinjury to the spine. Coincidentally, it also became linked withideas of compensation for injury, with a flurry ofcompensation claims and the start of the modern socialsecurity system. The diagnosis of railway spine quickly fellinto disrepute, with no physiological basis ever being found,but the idea of back pain as an injury and the principle that itshould attract compensation remained. However, only withthe discovery of ‘the ruptured disc’ in 1934 was it generallyaccepted that back pain was coming from the spine itself, thatit was commonly due to injury and that it should be treated byrest. It is this idea of treatment with rest, coupled with theintroduction of widespread access to health care and socialsecurity after World War II that could be said to have usheredin the present epidemic of low back disability. It is but a shortstep in the minds of many people from the idea that a shortperiod of rest is ‘good’ to the idea that activity, andparticularly work, is ‘bad’. Currently, low back pain is one ofthe commonest and most rapidly increasing causes of workloss, demand for health care and need for state benefit, andthis pattern is repeated in all industrialised societies.

It is important to differentiate between back pain anddisability due to back pain. There is evidence that theactual prevalence of back pain in the general populationhas not increased, certainly not over the last 40 years.What have changed are people’s attitudes andexpectations, medical ideas and management and socialprovisions. In former times, people may have continued towork despite the pain, either out of necessity or a sense ofduty or loyalty (although there is also evidence that earlierestimates of the rate of recovery from back pain were over-optimistic and over-emphasised return to work). It couldbe said that today’s society is fairer, in that people who arein pain are no longer required to work to supportthemselves and their families, but such a situation is boundto lead to a certain amount of abuse from people who areaware that spinal medicine is not so well developed that aspurious report of back pain or an exaggeration of theeffects of real back pain can be reliably refuted by doctors.In addition, there is a growing awareness that gentleactivity can have therapeutic effects, and that ‘bed rest’can contribute to transforming a pain into a disability.

In the UK, back pain is reported by about 60% of peopleat some time in their life. Social Security statistics for1991-92 indicated 81 million days Sickness and InvalidityBenefit paid for back incapacities, and this was estimated tohave risen to 106 million days by 1993-94. This does notinclude short periods off work, which are self-certified orcovered by employers’ Statutory Sick Pay; these areestimated to total about 50 million days, so the grand totalamounts to about 150 million days work lost - approximatelyfour days for every person of employable age in the country.Low back pain accounts for approximately ten times morework incapacity, state health care use and state benefits thandoes neck pain.

Although there is conflicting evidence as to whetherback pain is related to social class, work loss due to lowback pain is greater in lower social classes. However, sincedivision into social classes tends to be determined byoccupation, this is probably explained by the unsurprisingfact that people in heavy manual jobs lose significantlymore time and longer periods off work when they haveback pain than do those in non-manual jobs. Althoughonly 5% of work loss due to back pain is due to reportedaccidents at work, surveys have indicated that as many ashalf of male back pain sufferers (and a quarter of females)believe that their condition was either caused or madeworse by their employment. There is some evidence thatpeople in driving jobs are particularly prone to back pain,and this is supported by a press article (Rowlinson, 1997),which claims that two thirds of drivers who drive morethan 10,000 miles per year suffer from back pain causedby badly adjusted seats. Even well-designed seats canresult in back problems if drivers fail to adjust them toachieve a ‘spine-friendly’ posture while driving.

Against this background of the huge and growingsocietal cost of back pain, the discovery that low backstrains occur in road traffic accidents with a frequency andin conditions comparable to whiplash led directly to thepresent project to investigate the problem. As indicated inthe foregoing paragraphs, back pain is a difficult quantity

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to measure. The perception of pain is subjective, there isfrequently no clinically detectable injury, and time takenoff work is certainly not an objective measure of disability,being heavily influenced by the nature of the work and bypersonal needs and attitudes. However, it was felt that thedisability scoring system developed at Hope Hospital,which had been used in the whiplash study, wouldintroduce a degree of objectivity into the measurement ofthe severity of these sub-clinical back injuries. Road trafficaccidents may not be the most important cause of low backpain, but if they are contributing to the problem, then itwould be worthwhile attempting to identify any vehicle-related factors which might increase the risk. The resultsmight lead to recommendations on improvements in seator vehicle design, and so help to alleviate the problem.

1.3 Anatomy of the human spine

The human spine is a strong, flexible column which extendsfrom neck to coccyx and consists of a series of bones knownas vertebrae. In an average adult male it measures about71cm in length, and about 61cm in the average adult female.The spine encloses and protects the spinal cord, supports thehead and provides points of attachment for the ribs and themuscles of the back. It transmits body weight in walkingand standing, is capable of movement in the anterior(forward), posterior (backward) and lateral directions, andcan also rotate. Between the vertebrae are openings calledintervertebral foramina, through which pass nervesconnecting the spinal cord to various parts of the body. Thespaces between the vertebral bodies are filled byintervertebral discs. Each disc is composed of a soft centre,the nucleus pulposus, contained within a tough, flexiblefibrous ring, the annulus fibrosus.

The normal adult spine contains 33 vertebrae, groupedas follows: seven cervical vertebrae (C1 to C7) in the neckregion, twelve thoracic vertebrae (T1 to T12) at the rear ofthe thoracic cavity, five lumbar vertebrae (L1 to L5)supporting the lower back, five sacral vertebrae which arefused together in the adult to form one bone called thesacrum and usually four coccygeal vertebrae, fusedtogether to form the coccyx. Thus, after fusion of thesacral and coccygeal vertebrae, which occurs betweenbirth and adulthood, there are 26 separate bones. From aninjury point of view, it is the cervical, thoracic and lumbarregions of the spine which are most important, becausedamage to the spinal cord in these areas may result inparalysis, or even death. Less severe damage, to thevertebrae themselves, the intervertebral discs or theassociated joints, ligaments and muscles can result inprolonged pain and disability.

As a whole, the vertebral column, between sacrum andskull, is equivalent to a joint with three degrees offreedom; it allows flexion and extension in the sagittalplane, lateral flexion to right and left and axial rotation.The range of these basic movements at each vertebra isquite small, but the cumulative effect over the largenumber of vertebrae present is significant. However, theactual overall range of movement available to any givenperson is highly dependent on the individual, and varies

enormously with age. Table 1, taken from Kapandji(1974), shows the natural ranges of angular movement forthe three regions of the adult spine, in a particularly suppleperson (note that the figures for the cervical spine includeC1 and the skull).

Table 1 Natural ranges of angular spinal movement

Lumbar Thoracic Cervical

Flexion 60ο 45ο 40ο

Extension 35ο 25ο 75ο

Lateral flexion ±20ο ±20ο ±(35ο - 45ο)Rotation ±10ο ±35ο ±(80ο - 90ο)

Notwithstanding the above, within each spinal region,the range of movement is not evenly distributed betweenthe vertebrae. For example, in the lumbar spine, the rangeof flexion and extension is greatest between L4 and L5,and decreases progressively at higher levels.

As regards flexion and extension, the lumbar spinedisplays a much greater range of movement than thethoracic spine, which is hampered by the attached rib cage.With only five vertebrae, the range of lumbar flexion isquite remarkable - the thoracic spine only manages 75% ofthe movement with more than twice as many articulatingvertebrae. Even in cervical extension, the averagemovement of each vertebra is less than in lumbar flexion.Flexion in the lumbar spine is limited by tension in theligaments of the posterior part of the lumbar spinalcolumn, while extension is mainly limited by contactbetween the bony vertebral arch structures of adjacentvertebrae, with some assistance from the anteriorlongitudinal ligament. In principle, the complete cervicalspine is much more flexible than either the thoracic orlumbar regions, although in practice cervical flexion islimited by contact between chin and chest.

Lateral flexion in all parts of the spine is limited bycontact between the articular processes of adjacentvertebrae and by ligaments on the convex side. The ribcage again hampers the thoracic spine.

The lumbar spine is least flexible in rotation. Movementhere is limited by the orientation of the articular facets ofthe vertebrae and by shearing forces in the intervertebraldiscs. The thoracic spine is capable of much greaterrotation, and even the average rotation per vertebra isgreater in the thoracic than the lumbar spine. This is partlybecause the geometry of the articular facets is different,and also because the thoracic intervertebral discs rotateand twist, rather than undergoing shearing movements, asin the lumbar spine. The very large degree of rotationexhibited by the cervical spine is predominantly due torotation of C1 (the Atlas) about C2 (the Axis).

1.4 Injury mechanisms

Much of the research that has been done on the injurymechanisms of the lumbar spine has related to theproblems experienced by military pilots ejecting fromaeroplanes and concerns serious injuries such as bonyfractures. Even in these cases, definition of biomechanicaltolerance limits for the spine as a whole, as opposed to

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individual vertebrae, is problematic, because the behaviourof the bony parts of the spine is so heavily influenced bythe surrounding soft tissues, such as ligaments and discs,which display non-linear, visco-elastic behaviour underload. Injury outcome is also heavily dependent on theinitial configuration of the spine, with small changes inconfiguration producing subtle changes in the load paththrough the spine, which can then result in significantlydifferent injury mechanisms.

The relatively minor strain-type injuries which result inlower back pain, and which are the subject of the presentreport, are even less well understood or researched. King(1993) has proposed a secondary load path for compressiveloads in the lumbar spine through the articular facets, withconcomitant capsule and ligament damage being responsiblefor lower back pain. However, many people who sufferfrom lower back pain (including a substantial minority ofthe subjects in our study) report associated radiated pain intothe buttocks and thighs, and this implies that some nerveirritation is also involved. As with neck strain injuries(whiplash), there is usually no observable tissue damage onX-ray or MRI images; the injury is self-reported and definedentirely by its symptoms. However, a proportion of thosewho suffer long-term low back pain after a road accident dogo on to develop identifiable and serious disc problems,perhaps after a delay of a year or more. The problem is, it isvery difficult to prove a causative link between this injuryand the original traumatic incident after such a long period,and this is exacerbated by the high prevalence of low backpain in the population at large, much of which is attributableto other causes. In the face of such uncertainties, lower backstrains are probably even more difficult to pin down to aspecific cause than is whiplash.

2 The lumbar injury/vehicle study

2.1. Background

The lumbar injury/vehicle study was set up by Road SafetyDivision and Vehicle Standards and Engineering Divisionof the Department of the Environment, Transport and theRegions (DETR), and involved three separate contractors:the Department of Orthopaedic Surgery, University ofManchester, based at Hope Hospital, Manchester, theVehicle Inspectorate (VI) for the Manchester area and theTransport Research Laboratory (TRL). The project wasdivided into three phases - a ‘recruitment’ phase, a‘follow-up’ phase and an ‘analysis’ phase.

2.2 Recruitment phase

2.2.1 ObjectivesDuring the recruitment phase, the aim was to select, frompatients presenting at Hope Hospital Accident andEmergency Department as a result of a road trafficaccident, a random sample diagnosed as having lumbarsprain either on its own or in combination with soft tissuecervical spine injury. Further entry criteria, at this stage,were that the vehicles should be cars or light vans only andno older than a ‘G’ registration (ie seven years old at that

time). It was anticipated that patients associated with 350vehicles could be recruited over a period of 18 months. Itquickly became obvious that the recruitment rate amongstvehicles younger than ‘G’ registration would not yield therequired numbers in any reasonable timescale, so thevehicle entry criteria were amended to accept vehicles ofall ages. Partly because of the slow start caused by this, therecruitment timescale was extended by three months, andthe target number of vehicles reduced to 200.

2.2.2 MethodologyEach patient recruited to the study was interviewed athome by the Research Sister, usually within 24-48 hoursafter the accident. A comprehensive questionnaire,covering details of the accident, the vehicle involved, theinjuries and what effect they had, was completed at thistime. An example of the questionnaire used appears inMinton et al. (1997). Details of where and when thevehicle could be examined were also noted, and this basicinformation was faxed to the vehicle examiners as soon aspossible to allow examination of the vehicle prior togarage repairs commencing. Patients were advised thatcontact would be made again in 6 months, and were askedto keep details of GP visits, any further hospital orspecialist visits, as well as time off from normal duties inthe intervening time.

The vehicle examinations were carried out by membersof the Vehicle Inspectorate. On receipt of notification fromthe medical team, a VI engineer made an appointment toexamine the vehicle. Damage to the vehicle was recordedin sufficient detail to allow an estimate of collision severityto be made in terms of an Equivalent Test Speed, related tobarrier impacts, using the ‘Crash 3’ computer programme(NHTSA, 1982). Patients were encouraged to be present atthe examination so that details of seat and head restraintadjustment at the time of impact could be discussed.Photographs of the vehicle and, where possible, of theoccupant in the vehicle were taken. An example of theforms used for recording the vehicle data is given inMinton et al. (1997).

2.2.3 Actual numbers of patients recruitedInitially, it was planned to recruit occupants of 350vehicles, implying a need to recruit some 450 patients, ofwhom about 400 were expected to become ‘completecases’ (ie with both medical and vehicle details recorded).However, as mentioned above, the recruitment rate neverreached the level required to achieve this total in thedesignated timescale, so the targets were revised. The finalfigures for the numbers of subjects were:

Subjects recruited: 235Complete cases: 219Vehicles: 200

During the course of the study, contact was lost with anumber of people, mostly due to their moving away fromthe study area. Thus, the 219 subjects available at the firstassessment fell at subsequent assessments (6 months, 12months, 18 months and 24 months) to 216, 209, 207 and204 respectively.

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2.3 Follow-up phase

Follow-up medical interviews with patients were at six-monthly intervals to assess their recovery (or lack thereof)from their injuries. All patients in the study had at leastfour follow-up interviews, over a period of two years.Some of the earlier recruits have had as many as threeyears follow-up, but the data reported here only relate tothe two-year follow-up applicable to the entire sample.

2.4 Analysis phase

In the final phase of the project, the medical- and vehicle-based information was combined and analysed with a viewto gleaning new insights into the problem of lower backinjuries in car accidents, and to developing effectivecountermeasures against such injuries. However, it wasalways recognised that, as with the previous whiplashstudy, definitive causal relationships were unlikely toemerge from this study. The mechanism of spinal straininjury is very complex and poorly understood, and the lackof clinically observable injuries makes it very difficult toidentify specific features of the vehicle interior which aredirectly linked to the resulting neck/back injury. All thatwas hoped for were statistically significant associationswhich may indicate important areas for further research.Another possible problem is that all investigation in thisstudy is carried out post-accident, relying on the memoriesof occupants as to how they were sitting and how theirseats were adjusted before the accident. However, all themeasurements were taken within a few days of theaccident, so patients’ memories should not havedeteriorated, and all patients were assured that the researchdid not form part of any police or insurance companyinvestigation, so as to encourage them to be as open andtruthful as possible. Inevitably, the data obtained will berather imprecise compared to that obtained from laboratorycrash tests. However, obtaining this data immediately priorto a real accident is completely impractical, and since realhumans cannot be subjected to tests likely to result inactual injury, and there are grave biofidelity problemsassociated with dummies and cadavers as regards theserelatively minor but debilitating injuries, it was felt thatthis study could still produce worthwhile contributions tothe debate, and would form a valuable basis for anysubsequent research programme based on dummy testing.

3 Accident analysis

3.1 Introduction

An in-depth accident investigation study, the Co-operativeCrash Injury Study (CCIS), is co-ordinated by TRL and hasbeen running for a number of years. Detailed investigationsof a sample of road traffic accidents in the UK are carriedout, including a comprehensive examination of the vehicleinvolved, collection of medical data on any injuriessustained by the occupants, and an attempt to correlate theseinjuries with observed occupant contact points in thevehicle. A brief analysis of these databases has beenundertaken, to obtain a preliminary understanding of the

factors influencing spinal injury in general, and lower backinjury in particular. The databases relating to Phases 4 and 5of the CCIS project were examined. These contain details of5,700 accidents, involving 11,837 occupants. The samplingmethodology is biased towards more severe crashes, so anyinjury rates quoted in the following analysis are likely to behigher than those applicable to the general motoringpopulation in the UK.

The AIS system of injury classification which is used inthe following analysis is in fairly widespread use. Nearlyevery conceivable traumatic injury is given a six-digitcode, the first few digits of which indicate the body regionand/or organs involved. A seventh digit indicates how life-threatening the injury is, on a scale of 0 (uninjured) to 6(unsurvivable).

A subset of occupants who received an injury (of anyseverity) to the spine (AIS Body Region 6) was identified,and these 3,238 occupants formed the basis of thisanalysis. These occupants received a total of 3,726 spinalinjuries, of which 419 were at AIS 2 or above, affecting314 occupants. One hundred and six of the occupants died(though not necessarily from the spinal injury). The rate ofAIS 1 spinal injury, at 25%, is quite high, outnumberedonly by minor cuts and bruises to the arms, legs and facein this relatively severe sample. In contrast to otherinjuries, however, the rate of AIS 1 cervical spine injury,in particular, has been shown not to fall off to any greatextent in lower severity accidents, which do not form partof this sample. The rate of AIS 2+ spinal injury, on theother hand, is fairly low (2.7%), considering the highproportion of relatively severe crashes in the sample.Table 2 shows the gender distribution in the whole datasetand in the spinal injury subset.

Table 2 Occupant gender distribution and injury rates

With spine With AIS 1 With AIS 2+All injury spine inj. spine inj.

Sex occupants (rate %) (rate %) (rate %)

Male 6,787 1,661 (24.5) 1,495 (22.0) 182 (2.7)Female 4,458 1,568 (35.2) 1,450 (32.5) 132 (3.0)N/K 592 9 9 0

Total 11,837 3,238 (27.4) 2,954 (25.0) 314 (2.7)

Note that some occupants received spinal injuries atAIS 1 and AIS 2+ (in different spine regions), so the totalnumbers will be less that the sum of those in the severitybands. Rates have not been calculated for the ‘gender notknown’ cases, since they would not be very meaningful. Inaddition, the relatively high incidence of this group in theoverall sample means that the injury rates calculated in theknown male and female groups are slightly higher thanwould otherwise be the case. This should be borne in mindin subsequent sections.

Although the rate of AIS 2+ spinal injury is slightlyhigher in females compared to males (in line with thefindings of Hassan et al., 1996), the difference in theinjury rates for AIS 1 is quite marked. As will becomeclear later, the vast majority of these AIS 1 injuries were to

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the neck, so this result also fits in with findings elsewherethat women are more susceptible to whiplash injury thanmen. AIS 2+ injuries will not be considered further, sincethey are outside the scope of this report.

3.2 AIS 1 spine injuries

3.2.1 Influence of genderFor each region of the spine (Cervical, Thoracic andLumbar) the AIS 90 manual lists only one injury at aseverity of AIS 1 - i.e. ‘strain, acute with no fracture ordislocation’ (injury codes 640278, 640478 and 640678 forthe cervical, thoracic and lumbar regions respectively).Table 3 shows the distribution of occupants with AIS 1injuries to the three spine regions by gender, with ratesbased on the total numbers of occupants, as in Table 2.

impacts, although these are not calculated in every case.They are available for 1,572 of the MAIS 1 spine injurycases, and for 6,201 occupants in the overall dataset. Table 5shows average ETS values (in km/h) for males and femalesin the various spine region categories, with figures for theoverall dataset again included for comparison.

Table 3 Occupants with AIS1 spine injuries and injuryrates by gender and spine region

Cervical Thoracic Lumbar

Sex Number Rate % Number Rate % Number Rate %

Male 1358 20.0 84 1.2 219 3.2Female 1340 30.1 95 2.1 196 4.4N/K 9 – 0 – 0 –

Total 2707 22.9 179 1.5 415 3.5

Table 4 Impact direction by spine region injured (AIS 1)

All occupants Cervical Thoracic LumbarImpactdirection Num. % Num. Risk % Num.Risk % Num. Risk %

Right 1109 10.4 218 19.7 11 1.0 39 3.5Back 924 8.7 362 39.2 25 2.7 64 6.9Left 1009 9.5 186 18.4 14 1.4 32 3.2Front 6770 63.8 1504 22.2 88 1.3 209 3.1Non-horizontal 802 7.6 160 20.0 13 1.6 27 3.4

Total 10614 100 2430 22.9 151 1.4 371 3.5

Table 5 Impact severity by gender and region of spineinjured (AIS 1)

All occupants Cervical Thoracic Lumbar

Ave Ave Ave AveSex Freq. ETS Freq. ETS Freq. ETS Freq. ETS

Male 3541 29.8 720 27.2 37 30.0 112 29.0Female 2378 27.6 732 25.3 52 24.8 103 25.1N/K 282 23.3 6 19.0 0 – 0 –

Total 6201 28.7 1458 26.2 89 26.9 215 27.1

As mentioned above, the vast majority of AIS 1 spineinjuries, in both males and females, are to the neck(whiplash injury), with incidence being some 50% higherin women than in men. By comparison, AIS 1 thoracic andlumbar spine injuries are rare, though lumbar injury isslightly more than twice as common as thoracic spineinjury. Again, women are more susceptible than men toinjury in these other spine regions.

3.2.2 Influence of impact directionImpact direction information was only available for 2,619of the occupants under consideration. Table 4 shows howthese impact directions were distributed, with thedistribution of impact directions in the overall datasetincluded for comparison (10,614 occupants with knownimpact direction). Risk in this table is calculated as thenumber of occupants in a given direction band receivinginjury to a particular spine region as a proportion of alloccupants in that direction band. For each of the threespine regions, the greatest number of injuries is in thefrontal impact group. However, this is simply due to thepredominance of that particular impact direction inaccidents generally. The Risk figures, on the other hand,indicate that rear impacts actually carry a greater risk ofAIS1 spine injury in all regions. As far as neck injury isconcerned, this would be in line with findings elsewhere(Morris & Thomas, 1996).

3.2.3 Influence of impact speedEstimates of impact severity are available in the database,in the form of an Equivalent Test Speed, related to barrier

In the overall dataset, there is a trend for males to havereceived their injuries in higher severity accidents thanfemales. This is repeated among those with AIS 1 spineinjuries, and is even more pronounced in the thoracic andlumbar injury groups. Comparing figures along the ‘Total’row, there is a slight tendency for AIS 1 spine injuries to havebeen incurred in lower severity impacts than those for theoverall dataset. This is to be expected, since the overalldataset covers all injury severities. There is little differencebetween the individual spine regions, although in the thoracicand lumbar injury columns, this trend towards lower impactseverity is mainly due to the influence of the female group.

3.2.4 Occupant contactsDetails of the likely cause of each injury suffered by anoccupant are recorded in the database, although the levelof detail recorded changed substantially half-way throughPhase 5. Phase 5b data must therefore be presentedseparately from that relating to Phases 4 and 5a. Tables 6and 7 show the numbers of injuries subdivided by likelyoccupant contacts for the three spine regions for the Phase 4and 5a data and the Phase 5b data respectively.

More than half the neck injuries in Table 6 are ‘non-contact’ injuries - i.e. caused purely by deceleration, orinertial forces, a mechanism which is widely recognised inassociation with whiplash injuries, even though it isunclear exactly how, or at what stage in the impact, theseforces produce the injury. Almost one quarter of the neck

8

injuries are associated with a head impact, although it isalways possible that many of these were really decelerationinjuries, with the head contact being coincidental, so thatprevention of the head contact would not have preventedthe neck injury.

As one progresses down the spine, it clearly becomesmore difficult to assign causes to the injuries, with wellover half the lumbar injuries in the ‘Contact not known’category. Only about 8% of the thoracic and lumbar spineinjuries are clearly correlated with a spine impact, whileindirect loading (where the investigator was unable toidentify a specific impact to the spine to account for theinjury, so that the loading was assumed to be via anotherpart of the body) accounts for between 30 and 40% of theinjuries in these areas.

In Table 7, non-contact injuries again form a very largeproportion of the total neck injuries, but here they alsoconstitute half of the thoracic and over 60% of the lumbarspine injuries. This contact category was not available forthoracic and lumbar injuries in Phases 4 and 5a, so it isdifficult to relate these high figures to the distribution inTable 6, unless ‘indirect loading’ can be taken to beequivalent to ‘non-contact’. Although Side Structuresstand out in the neck injury contacts, this is partly becausethe steering wheel has been separated from the otherfrontal structures (Fascia/A-pillar/windscreen). Althoughthe head restraint and seat have been lumped together, itwas the seat which was the important contact for thethoracic and lumbar spine injuries.

4 Contents of the databases

This chapter presents some summary statistics taken fromthe databases. The tables which follow consist largely offrequency counts, and these are compared with thedistributions found in the previous Whiplash study.

4.1 Vehicle-based factors

4.1.1 Vehicle agesTable 8 shows the age distribution of the examined vehicles,as represented by registration letters. The distribution found inthe Whiplash study is included for comparison.

Table 6 Occupant contacts by region of spine injured(Phases 4 and 5a)

Type of contact Cervical Thoracic Lumbar

No Impact 1174 – –Neck Impact 32 – –Head Impact 456 – –Spine Impact – 10 26Indirect Loading – 51 102Other 9 4 5Not Known 371 63 175

Total 2042 128 308

Table 7 Occupant contacts by region of spine injured(Phase 5b)

Type of contact Cervical Thoracic Lumbar

Non-contact 441 22 66Airbag 2 – –Head restraint/seat 10 10 13Seat belt 11 – 1Fascia/A-pillar/windscreen 17 – 2Steering wheel 12 1 –Side structures 26 – 1Roof/sunroof 9 1 –Knee impact – – 1Side loading of pelvis – – 1External object 1 – –Other 54 – 4Not known 69 10 14

Total 652 44 103

Table 8 Distribution of vehicle ages

W/lash Lumbstudy W/l % study Lumb%

Registration Letter (Suffix) T 0 – 1 0.5" W 0 – 1 0.5" X 1 0.5 3 1.5" Y 3 2 1 0.5

Registration Letter (Prefix) A 14 9 4 2" B 11 7 4 2" C 11 7 8 4" D 15 10 17 8.5" E 14 9 18 9" F 22 14 14 7" G 7 5 22 11" H 16 11 15 7.5" J 12 8 15 7.5" K 10 7 17 8.5" L 9 6 19 9.5" M 6 4 14 7" N 0 – 13 6.5" P 0 – 9 4.5" R 0 – 4 2

Cherished/other 1 0.5 1 0.5

Total 152 200

The recruitment period for the Whiplash study covered1994 to mid-1995, whereas data collection for the currentstudy began in mid-1996. The peak in the Lumbar studydistribution would therefore be expected to be displaced bya year or two compared to the Whiplash distribution and,indeed, it is.

4.1.2 Vehicle make/modelAnalysis of the makes and models of the vehicles in thestudy indicates that, as with the Whiplash sample, FordEscorts and Fiestas were the most common vehiclesalthough, unlike the Whiplash study, Escorts outnumberedFiestas in the current sample. Vauxhall Astras and FordSierras trailed some way behind. Conversely, RoverMetros, which were about equal with Escorts in theWhiplash sample, did not feature very strongly in thecurrent sample. The Whiplash sample contained 53different models; the Lumbar sample contained 58, evenafter aggregation of Rover, Austin and MG Metros,Maestros and Minis.

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4.1.3 Occupant seating positionsBy far the greater proportion of the study group weredrivers, 190 (86.8%) as opposed to 29 (13.2%) passengers.Of the passengers, six were in the rear of the vehicle; fivebehind the driver and one behind the front seat passenger.The preponderance of drivers is not an indication that theyare more prone to suffer this injury, but rather that the vastmajority of vehicles on the road only have one occupant -the driver. In addition, if the only casualty was a passenger,he or she was often not able to give permission for thevehicle to be examined (e.g. a passenger in a friend’s car),so this accident could not be included in the study. Table 9shows the distribution of occupants in the vehicle, and theirbelt use status, including corresponding figures from theWhiplash study for comparison. Clearly, the proportion offront seat passengers was lower than that in the Whiplashstudy. Belt use was very high in both samples - over 93%for male drivers, and higher for other Lumbar studyoccupants. The numbers in the rear offside female group inthe Whiplash study are too small to be meaningful.

4.1.4 Head restraintsTable 10 gives an indication of the range of head restraintadjustments adopted by the subjects in the two studies.

Only 51% of occupants had their heads closer than 10cmfrom the restraint, and a very small proportion (7%) hadtheir restraints ‘correctly’ adjusted vertically (ie with centreof restraint at or above ear level). These proportions wereslightly lower than those encountered in the Whiplash study.The proportion of people in the supposedly very riskyposition of having the top of the head restraint at or belowear level was also slightly lower, at 14%.

Table 11 shows the average head restraintmeasurements, by seat position and gender, for theWhiplash study and the present study, and compares thesewith population averages derived from large-scaleobservational studies (Parkin et al., 1994, Cullen et al.,1996). The population averages for drivers have beenadjusted to allow for a different measurement base-line.

No population figures are available for rear seatpassengers. The horizontal measurements indicate thatmale drivers in the current sample were sitting closer to therestraint than either the whiplash sample or the generalpopulation, whereas for female drivers the reverse is true.Front passengers had even larger horizontal distancemeasurements than those in the Whiplash sample, whowere themselves considerably further from the restraintthan the general population. In the vertical dimension,drivers in the Lumbar sample appear to be similar to theWhiplash sample, and much better positioned than thegeneral population. Male passengers were not toodissimilar from either the Whiplash sample or the general

Table 9 Occupant positions and seat belt use (Lumbar dataset, w/lash in italics)

Belt used (lum/w/lsh) Not used (lum/w/lsh) Total (lum/w/lsh)

Sex Numbers % Numbers % Numbers %

DriverF 107/(112) 50.1/(50.8) 1/(1) 16.7/(14.3) 108/(113) 49.3/(49.8)M 77/(71) 36.2/(32.3) 5/(5) 83.3/(71.4) 82/(76) 37.4/(33.5)

Front passengerF 17/(23) 8.0/(10.5) 0/(0) –/(–) 17/(23) 7.8/(10.1)M 6/(10) 2.8/(4.5) 0/(0) –/(–) 6/(10) 2.7/(4.4)

Rear o/sF 4/(3) 1.9/(1.4) 0/(1) –/(14.3) 4/(4) 1.8/(1.8)M 1/(0) 0.5/(–) 0/(0) –/(–) 1/(0) 0.5/(–)

Rear n/sF 1/(0) 0.5/(–) 0/(0) –/(–) 1/(0) 0.5/(–)M 0/(1) –/(0.5) 0/(0) –/(–) 0/(1) –/(0.4)

Total 213/(220) 100/(100) 6/(7) 100/(100) 219/(227) 100/(100)

Table 10 Head restraint adjustment

W/lash Lumbstudy W/l % study Lumb%

Number of occupants with head ....<10cm from restraint (h/r) 53 51.5 62 50.8

≥10cm from restraint (h/r) 50 48.5 60 49.2centre of h/r at or above ears 10 10 9 7.4top of h/r at or below ears 16 15.5 17 13.9

Table 11 Occupant distribution and head restraintmeasurements (Lumbar dataset - w/lashdataset in italics. Distance measurements are incentimetres)

Seat position Ave Popu- Ave Popu-Number hor dist lation vert dist lation

(lumb/ (lumb/ ave (lumb/ aveSex w/lsh) w/lsh) (hor) w/lsh) (vert)

DriverM 46/(42) 9.3/(10.6) 10.1 6.4/(6.8) 10.0F 61/(46) 10.4/(9.8) 10.1 3.3/(3.5) 8.0

Front passengerM 5/(4) 11.6/(10.5) 6.6 8.8/(7.0) 8.0F 10/(10) 11.5/(9.3) 6.6 0.5/(3.0) 4.0

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population as regards vertical positioning, and femalepassengers were better positioned than either the Whiplashsample or the general population.

In the Medical interview, patients were asked whethertheir head restraints were correctly adjusted. Theirsubjective responses can be compared with the objectivemeasurements of head restraint positioning carried out atthe vehicle examination (for those patients who attendedthe examination). However, the question then arises as towhat exactly constitutes ‘correct’ head restraintadjustment. The advice usually given is that the centre ofthe restraint should be level with the back of the headvertically and ‘as close as possible’ horizontally. Ahorizontal distance of 10cm is frequently quoted in theliterature as representing a threshold for increased risk ofsustaining whiplash injury. For the purposes of thefollowing table (Table 12), ‘correct’ head restraintadjustment is assumed to be closer than 10cm horizontallyand within ±2cm of ear level vertically. However, it shouldbe borne in mind that the previous Whiplash studyindicated that people who conformed to these criteria were,on average, no better off than those who did not, indicatingeither that the criteria are inadequate, or that other factorswere at least as important as restraint adjustment, thusconfounding the results. trend towards people who had been stationary having lower

disability than those who had been moving before impact, butthis was not statistically significant. However, the importantparameter, in terms of occupant kinematics in the impact, isthe velocity change (∆v) of the vehicle, rather than the travelspeed before impact, and this can only be estimated from theobserved damage to the vehicle. In most cases, the vehicles inthis study struck a deformable second vehicle, and calculationof the actual ∆v experienced would require a knowledge ofthe mass and stiffness of this second vehicle, along with thepost-impact speeds of the vehicles if they were not brought torest in the collision. Generally, this information was notavailable, so the impact damage was converted to an‘Equivalent Test Speed’ (ETS), i.e. the speed at which thevehicle would have to collide with a rigid, immovable barrierin order to suffer an equivalent amount of damage. Table 14shows the distribution of ETS for those cases where it can becalculated. ETS cannot be calculated in cases where there isno measurable damage to the vehicle, and the combination ofcharacteristically low-speed impacts in the current study,

Table 12 Subjective vs Objective assessments of headrestraint adjustment

Objective assessment

Subjective response Correct Incorrect

Correct 18 116Incorrect 1 9

Table 13 Various vehicle statistics

W/lash LumbarVehicle type study W/l% study Lum%

Number of occupants in ....Saloon cars 39 22 42 19Hatchback cars 119 69 156 71Estate cars 9 5 14 6.5Multi-purpose/off-road 0 – 3 1.5Sports cars 2 1 2 1Vans/light commercials 5 3 2 1

First object hitNo. of occs. in collision with a ....Car 145 83 191 87Two-wheeler 0 – 0 –Light commercial etc 8 5 11 5HGV 17 10 13 6Pole/narrow object 0 – 1 0.5Wide object 4 2 3 1.5

Number of impactsNumber of occupants suffering ....1 impact 142 81 178 812 impacts 27 16 34 15.53 or more impacts 5 3 7 3.5

Table 14 Impact severity distribution (lumbar/whiplash)

ETS (km/h) Number Percentage

0 - 4.9 10/(6) 4.5/(2.6)5.0 - 9.9 27/(17) 12.5/(7.4)10.0 - 14.9 53/(28) 24/(12.3)15.0 - 19.9 55/(39) 25/(17.1)20.0 - 24.9 32/(49) 14.5/(21.5)25.0 - 29.9 15/(29) 7/1(2.8)30.0 - 34.9 1/1(1) 0.5/(4.8)35.0 - 39.9 1/(8) 0.5/(3.9)40.0 - 44.9 0/(2) –/(0.9)45.0 - 49.9 0/(1) –/(0.4)75.0 - 79.9 1/(0) 0.5/(–)Not known 24/(37) 11/(16.3)

Total 219/(227) 100/(100)

Table 12 shows that a considerable number of subjectswere either unwilling to admit failing to adjust theirrestraints, or they had a genuinely false idea of ‘correct’restraint adjustment, according to the objective criteriaused in this table. It is also interesting to note that 24subjects admitted to not knowing whether their headrestraint was adjustable or not, while 12 subjects thoughttheir restraints were fixed when they were actuallyadjustable or vice versa.

4.1.5 Vehicle type, objects hit and number of impactsTable 13 shows the distribution of these parameters, takenfrom the Vehicle database.

4.1.6 Travel and impact speedsPatients were asked about their travel speed prior to impact.Of the 96 people who said they were stationary at the time ofimpact, 61 were women and 35 men. These figures represent47% of all women, but only 39% of all men. Women thusseem to be at slightly higher risk of having other people runinto them in this sample of accidents. Of those who weremoving prior to impact, the average estimated cruising speedwas 21.5mile/h (34.4km/h), and there was virtually nodifference between males and females. There was a slight

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together with the introduction on some vehicles of energy-absorbing bumpers which regain their shape after such animpact has resulted in a significant number of such cases.Multiple impact cases have also been excluded where thesecond impact was of a severity comparable to the first, sinceit is impossible to say which impact caused the injury in suchcases. The mean ETS in the current study is 16.1km/h, whilethat for the Whiplash study was 19.7km/h. These aresignificantly different (p<0.0001). Figure 1 shows thedifference between the two distributions graphically.

this, with percentages calculated relative to the totals ineach bracing category. Apart from the ‘aware, not braced’category (where numbers are very small), the proportionsare roughly what would be expected from the overallmale:female ratio in the study.

Table 15 Age distribution

Cumulative Males FemalesAge band Number Percent % (%) (%)

<10 0 – 0 (-) 0 (–)10-14 1 0.5 0.5 0 (-) 1 (0.8)15-19 11 5.0 5.5 7 (7.9) 4 (3.1)20-24 51 23.3 28.8 17 (19.2) 34 (26.1)25-29 39 17.8 46.6 16 (18.0) 23 (17.7)30-34 37 16.9 63.5 12 (13.5) 25 (19.2)35-39 16 7.3 70.8 10 (11.2) 6 (4.6)40-44 19 8.7 79.5 9 (10.1) 10 (7.7)45-49 18 8.2 87.7 9 (10.1) 9 (6.9)50-54 7 3.2 90.9 1 (1.1) 6 (4.6)55-59 13 5.9 96.8 5 (5.6) 8 (6.2)60-64 4 1.8 98.6 1 (1.1) 3 (2.3)65-69 2 0.9 99.5 1 (1.1) 1 (0.8)70-74 1 0.5 100 1 (1.1) 0 (–)>74 0 – 0 (–) 0 (–)

Total 219 100 89 (100) 130 (100)

Table 16 Awareness of impending impact and bracing

Awareness

Bracing Male (%) Female (%) Total (%)

AwareBraced 23 (44) 29 (56) 52 (100)Not braced 8 (73) 3 (27) 11 (100)Bracing n/k 13 (35) 24 (65) 37 (100)

UnawareNot braced 45 (38) 74 (62) 119 (100)

4.1.7 Airbags and pretensionersTwenty-eight vehicles had drivers’ airbags fitted, and sevenof these were involved in frontal impacts. Only one airbagdeployed, at an ETS of 27km/h, compared to an average of19km/h for the five airbags which did not deploy in frontalimpacts and for which impact speed could be estimated.

4.2 Occupant-based factors

4.2.1 Gender distributionThe male:female ratio in the sample was 89:130, i.e. 59.4%of the total were female. This is not dissimilar from the62.6% females found in the Whiplash study sample. Thepolice were informed immediately after the accident by 136people, and a further 28 people informed the police later.There was no indication that women were more likely to callthe police than men. As might be expected, men were tallerthan women on average (176cm compared to 162cm), andthey had longer backs (86cm compared to 80cm)

4.2.2 Age distributionTable 15 shows that well over 50% of the subjects wereaged between 20 and 34 years, but this is largely a functionof the female distribution - the distribution for males ismuch flatter. The average ages were almost identical (33.4for females, 33.9 for males).

4.2.3 Occupant awarenessSome people reported being aware of the impendingimpact, and some also braced for impact. Table 16 shows

4.2.4 Neck strains (whiplash)In the previous (whiplash) study, recruitment was based onthe presence of a neck strain injury, but it was found that45% of the subjects recruited also had a lower back straininjury, and this had to be taken into account in the analysisof the results. In the present study, recruitment was basedon the presence of a lower back strain injury, but it hasbeen found that the vast majority of subjects (95%) alsohad a neck strain injury. Only 11 subjects suffered purelumbar strain injury. Thus it would appear that neck straincan quite commonly occur without accompanying lumbarstrain, but lumbar strain very rarely occurs withoutconcomitant neck strain. This has serious implications forthe analysis, since separation of the ‘pure lumbar’ subsetwould produce a very small sample. It has been judgedthat any conclusions based on such a small subset wouldnot be reliable, and the analysis has therefore been basedon the entire sample.

4.2.5 Prior back problemsThis is a potential source of error in the results. A priorback problem could predispose a person to injury and

Figure 1 Cumulative speed distribution (Lumbar andWhiplash)

0

20

40

60

80

100

120

0 5 10 15 20 25 30 35 40 45

Speed (km/hr)

Cum

ulat

ive

%

Lumbar

Whiplash

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confound any attempt to link injury severity to impactseverity, seat adjustment parameters or vehicle/seat designfeatures. In this study, 72 people (34.5% of all females,30.5% of all males) reported having had a previous backproblem. Of these 72 people, 44 (61%) stated that theywould definitely be pursuing a claim for compensation fortheir current injury while, of the 147 people with noprevious back problem, 97 (66%) intended to claimcompensation. This difference was not statisticallysignificant. The effect of prior back problems on overalldisability is considered in Chapter 5. Table 17 shows thedistribution of previous back problems by gender and areaof back affected.

carrying out a range of activities and movementsassociated with everyday life. Copies of the fullquestionnaire and the method of calculating disabilityscores can be found in Minton et al. (1997). It should benoted that this assessment of ‘macroscopic’ disability cantell us nothing about the source of that disability - thismust be deduced from the initial selection of the patients inthe sample. As mentioned above, most of the people in oursample suffered from both lumbar and neck pain. Analysisof the disability scores, therefore, can only allowconclusions to be drawn about the combined effects ofneck and lumbar pain. However, one form of analysis thathas been carried out does allow neck pain to bedifferentiated from lumbar pain (see Section 5.7.)

Table 18 shows the average disability scores at the fiveassessments (i.e., immediately after the accident, and atsix-monthly intervals to two years post-accident)

Patients’ recovery from their injuries over time is clearlyshown in this table by the decrease in average disabilityscores over the five assessments. As can be seen, the totalnumber of patients in the study fell from 219 to 204 by thetime of the 24 month assessment. This was due to peoplemoving out of the study area, or becoming unavailable forsome other reason. The average disabilities for males andfemales are significantly different at the second and allsubsequent assessments. See Table 23 for details.

Tables 19a and 19b show the full distribution ofdisability scores in the sample.

One male scored zero at the first assessment (Table 19b).A zero score at this stage does not necessarily mean he hasno problem, but rather that he may not yet have attemptedany of the prescribed activities or movements. Scores of

Table 19a Distribution of disability scores (Female)

Assessment 1 Assessment 2 Assessment 3 Assessment 4 Assessment 5Disabilityscore Number Cum. % Number Cum. % Number Cum. % Number Cum. % Number Cum. %

0 0 – 26 20.5 54 43.2 64 52.0 75 61.51 5 3.8 3 22.8 11 52.0 16 65.0 15 73.82 18 17.7 16 35.4 25 72.0 17 78.9 15 86.13 33 43.1 40 66.9 14 83.2 14 90.2 8 92.64 36 70.8 24 85.8 14 94.4 7 95.9 3 95.15 20 86.2 10 93.7 3 96.8 2 97.6 2 96.76 13 96.2 3 96.1 0 96.8 2 99.2 3 99.27 4 99.2 4 99.2 2 98.4 0 99.2 1 1008 1 100 0 99.2 1 99.2 1 100 0 1009 0 100 1 100 1 100 0 100 0 100

Totals 130 127 125 123 122

Table 17 Distribution of prior back problem location

Area of back affected

Cervical Thoracic Lumbar

Male 6 3 18Female 22 0 23

Totals 28 3 41

Table 18 Mean disability scores

Assessment 1 Assessment 2 Assessment 3 Assessment 4 Assessment 5

Mean No. Mean No. Mean No. Mean No. Mean No.

Male 3.51 89 2.04 89 1.02 84 0.74 84 0.54 82Female 3.83 130 2.80 127 1.64 125 1.22 123 0.95 122

Overall 3.70 219 2.49 216 1.39 209 1.02 207 0.79 204

Clearly, prior lumbar problems predominated overall,though this was almost entirely due to their relatively highincidence among men. Women were almost equally likelyto report prior neck problems as prior lumbar pain.

4.2.6 Disability scoresA patient’s overall disability score is calculated byreference to the degree of difficulty experienced in

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zero at the second (6 month) and subsequent assessments,on the other hand, indicate complete recovery from injury.The general movement of the distribution down thedisability scale as people get better can be seen in thesetables, but there is also some movement in the oppositedirection. Between the first and second assessments, forexample, 43 subjects (26 female, 17 male) actually gotworse, some by as much as three points. Between the secondand third assessments, 5 subjects (4 female, 1 male) gotworse, though none by more than one point.

5 Analysis of data

5.1 Introduction

In the early part of this chapter, each of the fiveassessments is treated separately, and all statistical analysisis based either on least squares regressions of disabilityscore onto the variable of interest (for continuousvariables), or on straightforward Analysis of Variance(ANOVA) for categorical variables. Significance levels aredetermined by Student’s t-test. The disability scores areassumed to be normally distributed and to represent alinearly increasing scale of disability - i.e. someone with ascore of four is taken to be twice as disabled as someonewith a score of two. Both these assumptions are rathersuspect, since the scores are restricted to a range of 0 - 9,and also disability is not a quantity which can berealistically measured on a linear scale. To overcome theseproblems, all the regressions detailed in the tables havebeen repeated using Ordered Logit analysis, whichassumes neither linearity nor that the scores themselves arenormally distributed. Score 2 is simply assumed to beworse than score 1, score 3 worse than score 2 and so on.In most cases, these alternative analyses confirmed theconclusions drawn from the regressions; where there werediscrepancies, these are discussed.

Later parts of the chapter contain some rather morecomplex analyses - stacked regressions and survivalanalysis. As the name indicates, stacked regressions stackthe data from all five assessments, and analyse them alltogether in a three-dimensional space, whose axes areDisability, Time and the variable of interest (eg impactspeed - ETS). There are several different approaches to

this type of analysis, and these are described in more detailat the beginning of the relevant section. A completelydifferent way of looking at the data is to use SurvivalAnalysis. This takes its name from one of its main uses,which is to compare groups of terminally ill patients, whenone group is given an experimental form of treatment andthe other is not. The technique looks at differences in thesurvival times of patients in the two groups to give anestimate of how effective the new treatment is inpreventing death. It can be adapted to look at whether anyof the variables measured in this study had an effect onhow long the subjects took to recover from their injuries.

Finally, all the analyses are repeated for two specificvehicle models - Ford Escorts and Ford Fiestas (chosenbecause they were the most numerous vehicle models inthe sample), to try to determine whether differences invehicle mass and structure could be masking importantrelationships in the data. The previous whiplash studyconcluded that this was a serious problem in the analysis,because so many different makes and models of vehicleswere represented in the sample. The disadvantage ofseparating specific vehicles out in this way is that samplesize is dramatically reduced, so that the results are likely tobe less reliable.

5.2 Analysis of medical assessment variables

The medical assessment of patients concentrated on twomain areas:

i The degree of difficulty experienced in carrying out arange of activities and movements associated witheveryday life.

ii The presence or absence of certain specified symptomsin a number of body regions (eg headache, numbness inthe legs, pins and needles in the hands).

5.2.1 Activities and movementsTable 20 is based on the ‘raw’ assessment scores for each ofthe activities and movements assessed in the questionnaire.These scores are not comparable with the disability scores inTables 18 and 19 (which are derived from the individualscores by a fairly complex and non-linear formula), and theyhave not been subjected to statistical testing. However, theycan give an indication of the areas of activity which people

Table 19b Distribution of disability scores (Male)

Assessment 1 Assessment 2 Assessment 3 Assessment 4 Assessment 5Disabilityscore Number Cum. % Number Cum. % Number Cum. % Number Cum. % Number Cum. %

0 1 1.1 32 36.0 52 61.9 56 66.7 63 76.81 5 6.7 6 42.7 3 65.5 11 79.8 8 86.62 16 24.7 14 58.4 15 83.3 7 88.1 3 90.23 27 55.1 16 76.4 7 91.7 6 95.2 5 96.34 20 77.5 11 88.8 5 97.6 2 97.6 1 97.65 10 88.8 4 93.3 0 97.6 1 98.8 1 98.86 8 97.8 6 100 2 100 1 100 1 1007 1 98.9 0 100 0 100 0 100 0 1008 0 98.9 0 100 0 100 0 100 0 1009 1 100 0 100 0 100 0 100 0 100

Totals 89 89 84 84 82

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Table 20 Summary of individual activity scores

Assessment 1 Assessment 2 Assessment 3 Assessment 4 Assessment 5

Mean Min/ No No Mean Min/ No No Mean Min/ No No Mean Min/ No No Mean Min/ No NoAbility to .... (var) max >2 =0 (var) max >2 =0 (var) max >2 =0 (var) max >2 =0 (var) max >2 =0

Cut food 5.5(5.31) 3/8 34 102 3.2(0.16) 3/4 22 193 3.2(0.19) 3/4 13 197 3.1(0.14) 3/4 7 201 3.0(0.00) 3/3 3 202Carry a medium weight 5.2(2.93) 3/9 105 13 4.1(0.99) 3/9 151 65 3.9(0.99) 3/6 96 114 3.8(0.67) 3/6 79 129 3.8(0.61) 3/6 57 148Open jars & bottles 5.0(4.42) 3/9 32 79 3.3(0.45) 3/6 29 187 3.4(0.24) 3/4 17 193 3.1(0.13) 3/4 14 194 3.0(0.00) 3/3 6 199Run a short distance 5.0(-) 5/5 1 4 4.7(3.18) 3/9 142 66 4.7(4.11) 3/9 85 121 4.8(4.41) 3/9 63 141 5.1(4.95) 3/9 49 152Bend to pick up an object 5.0(1.95) 3/9 173 10 4.1(0.93) 3/6 152 64 3.9(0.92) 3/6 93 117 3.8(0.90) 3/6 74 134 3.9(0.83) 3/6 58 147Shampoo hair 4.8(2.21) 3/9 109 41 3.9(1.04) 3/8 120 96 3.7(1.00) 3/8 71 139 3.6(0.76) 3/5 52 156 3.6(0.81) 3/5 36 169Reach up 4.7(1.64) 3/9 147 41 3.9(0.99) 3/6 114 102 3.8(0.90) 3/6 67 143 3.7(0.94) 3/6 54 154 3.5(0.80) 3/6 34 171Dress (inc buttons & laces) 4.7(1.62) 3/8 190 26 3.3(0.52) 3/7 117 99 3.3(0.66) 3/7 56 154 3.4(0.53) 3/5 30 178 3.5(0.55) 3/5 22 183Get in & out of bed 4.6(1.18) 3/8 201 17 3.7(0.86) 3/7 135 81 3.6(0.86) 3/7 75 135 3.4(0.51) 3/5 55 153 3.4(0.39) 3/5 40 165Wash & dry whole body 4.2(1.17) 3/9 134 63 3.4(0.68) 3/7 82 134 3.5(1.02) 3/7 30 180 3.4(0.81) 3/6 19 189 3.4(0.57) 3/5 14 191Get up from a chair 4.2(1.04) 3/8 200 19 3.3(0.43) 3/5 98 118 3.3(0.48) 3/5 39 170 3.4(0.65) 3/5 26 182 3.3(0.35) 3/5 17 187Open doors 4.0(1.50) 3/9 63 136 3.2(0.18) 3/4 10 206 3.3(0.25) 3/4 4 206 3.3(0.25) 3/4 4 204 3.3(0.25) 3/4 4 201Turn taps on & off 3.9(2.03) 3/9 39 169 3.3(0.23) 3/4 13 203 3.3(0.27) 3/4 6 204 3.0(0.00) 3/3 5 203 3.0(0.00) 3/3 4 201Climb stairs 3.8(1.04) 3/7 151 59 3.4(0.45) 3/5 66 147 3.4(0.40) 3/5 27 180 3.4(0.51) 3/5 24 182 3.7(0.80) 3/6 18 186Get on & off toilet 3.8(1.06) 3/7 92 118 3.3(0.36) 3/5 36 180 3.2(0.36) 3/5 11 199 3.2(0.44) 3/5 9 199 3.3(0.67) 3/5 6 199Walk outdoors 3.8(0.97) 3/7 175 42 3.5(0.62) 3/5 77 139 3.7(0.83) 3/6 37 173 3.8(0.83) 3/6 29 179 3.8(0.96) 3/6 21 184Lift a drink 3.8(0.89) 3/6 62 156 3.1(0.14) 3/4 7 208 3.3(0.33) 3/4 3 207 3.0(-) 3/3 1 207 – – 0 205Cut toe nails 3.0(0.00) 3/3 2 5 4.0(1.92) 3/8 139 77 3.7(1.64) 3/8 79 131 3.7(1.84) 3/8 53 155 3.8(2.09) 3/8 36 169

Total number in sample 219 216 210 208 205

15

find most difficult after suffering the kind of back and neckstrain injuries under investigation. The means are based onscores of 3 or more, since Score 1 means ‘not applicable’and Score 2 means ‘have not tried this yet’. The table showsthe mean score in a category (based on those scoring morethan 2), the variance on the mean, the minimum andmaximum scores, the number who claimed some degree ofdifficulty (i.e. the number with score greater than two) andthe number of people who reported a score of zero(unaffected) at each assessment. The scores are ranked inorder of the mean value at the first assessment. Note that,because people who recover are excluded from the means,so that the means indicate the degree of difficulty only forthose who remain affected, time-wise trends should beviewed with caution.

Among the categories where more than 100 peoplereported experiencing difficulty at the first assessment,bending down to pick up an object and, to an even greaterextent, carrying that object afterwards, presented thegreatest difficulty. Scores between four and five indicate afair degree of difficulty and/or pain, and such basiceveryday functions as getting in and out of bed, washingand dressing show average scores in this range for largenumbers of patients. Getting up from a chair, reaching foran object off a high shelf and shampooing hair also fall inthis range. Scores between three and four indicate somedifficulty, and a necessity to adapt one’s approach to anactivity so as to minimise pain or discomfort. Climbingstairs and walking outdoors show average scores in thisrange for large numbers of patients.

Cutting toe nails and running a short distance had beentried by very few people at the first assessment but, by thesecond assessment, well over half the sample had triedthese activities, and found them to be difficult. Runningranks as the most difficult activity at this stage, and cuttingtoe nails, picking up and carrying objects also give averagescores of four or more here. Cutting food and opening jarsand bottles, which ranked in the top three at the firstassessment, albeit among a minority of the sample, drop toamong the least difficult tasks by the second assessment,and the number of people affected at all remains small.Shampooing hair, reaching up to a shelf and getting in andout of bed remain difficult for large numbers of people, butthe rank position of dressing has dropped to among theleast difficult activities, though about half the sample stillhave some degree of difficulty in this area.

At the third assessment, as indicated by the reducednumbers in the ‘number greater than score 2’ column,many people have recovered from their injuries, leaving acore of long-term sufferers. Among these, running remainsthe most difficult activity, for a relatively large number ofpeople. This is probably to be expected, since runninginvolves movement of the whole body, along withsignificant jarring. Pain anywhere is likely to be a barrierto this kind of activity. It could be argued that some peoplefind running very difficult at the best of times, but patientsare asked in the interview to compare their situation nowto that before they were injured, as opposed to comparingthemselves to a young Olympic athlete. A patient whocould not run before injury could reply that the question is

not relevant. Picking up and carrying objects remain in thetop three most difficult activities, again with large numbersof people (just under half the sample) affected. Theseactivities rely mainly on the muscles of the back, shouldersand neck. Reaching up, cutting toe nails and shampooinghair all involve reaching and stretching the muscles of theback and shoulders, while getting in and out of bedinvolves the shoulder muscles and a twisting of the spineas the torso is eased from a sitting to a prone position. Allthese latter activities still cause difficulty to significantnumbers of people at the third assessment stage.

This pattern continues at Assessments 4 and 5; runningremains the most difficult activity, for 30% and 24% ofpatients respectively at these assessments. Lifting andcarrying objects are also difficult, and affect more people(38% and 28% respectively). Walking outdoors andcutting toe nails show mean scores comparable withlifting/carrying, but far fewer people are affected. Otheractivities found to be difficult by at least 10% of thesample at the fifth assessment include shampooing hair,reaching up, dressing and getting in and out of bed.

5.2.2 SymptomsTable 21 is based on the particular symptoms noted by thepatients. These are recorded on a simple present/absentbasis, at the first assessment only, and are not used in thecalculation of the overall disability score. Two furthervariables - Prior Back Problems and PsychologicalProblems - are also included in this table, because they canalso be thought of as being ‘internal’ to the subject.Psychological problems are considered in more detail inSection 5.3. but, for now, their relationship to overalldisability is analysed. The purpose of the analysispresented in Table 21 is to see whether there is anysignificant difference in overall disability between thegroup reporting presence of a particular symptom,compared with the group who did not. This may indicatethat certain symptoms are predictors of high disabilityscores and/or of long-lived disability. The table shows thenumber in the group with the symptom and, for each of thefive assessments, the average disability score for the groupwithout the symptom (the Base Score), the differencebetween the Base Score and the average disability (at thatassessment) of the group with the symptom (the ScoreIncrement) and the associated p-value (ie the probabilitythat this difference could have occurred by chance). Asingle asterisk in the ‘Sig’ column flags cases for whichthe difference is significant at the 10% level, while twoasterisks indicate better than 5% significance. The resultsare ranked in order of decreasing score increment at thefirst assessment.

Since the presence of lumbar pain and/or neck painconstituted part of the selection criteria for this study, andsince most subjects suffered both, the lack of significantcorrelations with these symptoms reflects the fact thatthere were very few people in the ‘no symptom’ groups,and that those who were in the ‘no symptom’ group forone necessarily had the other. At the first assessment,difficulty in micturition, weakness and pins & needles inthe legs and radiated pain in the thighs and buttocks seem

16

Table 21 Influence of symptoms on disability scores

Assessment 1 Assessment 2 Assessment 3 Assessment 4 Assessment 5

No. with Base Score P- Base Score P- Base Score P- Base Score P- Base Score P-Symptom symptom score incr. value Sig score incr. value Sig score incr. value Sig score incr. value Sig score incr. value Sig

Pins & needles (legs) 26 3.61 0.778 0.012 ** 2.38 0.983 0.016 ** 1.34 0.453 0.238 1.01 0.161 0.626 0.77 0.149 0.634Difficulty in micturition 15 3.65 0.753 0.058 * 2.38 1.55 0.003 ** 1.29 1.374 0.003 ** 0.93 1.35 0.001 ** 0.69 1.380 0.000 **Weak legs 55 3.52 0.718 0.002 ** 2.31 0.750 0.014 ** 1.26 0.546 0.055 * 0.95 0.306 0.212 0.74 0.181 0.435Pain in thighs 63 3.50 0.710 0.001 ** 2.39 0.381 0.194 1.28 0.415 0.127 0.95 0.268 0.254 0.71 0.270 0.226Pain in buttocks 87 3.42 0.691 0.001 ** 2.19 0.750 0.005 ** 1.08 0.789 0.001 ** 0.79 0.584 0.006 ** 0.62 0.414 0.043 **Lumbar pain 205 3.07 0.670 0.103 2.00 0.525 0.327 1.07 0.342 0.485 0.86 0.179 0.671 0.79 0.000 0.999Weak arms 93 3.41 0.673 0.001 ** 2.22 0.641 0.016 ** 1.24 0.339 0.169 0.88 0.331 0.120 0.68 0.237 0.243Interscapular pain 176 3.16 0.667 0.008 ** 1.84 0.816 0.013 ** 0.69 0.875 0.004 ** 0.48 0.686 0.009 ** 0.28 0.634 0.012 **Shoulder pain 182 3.16 0.646 0.016 ** 2.16 0.396 0.257 1.00 0.471 0.145 0.61 0.499 0.073 * 0.43 0.430 0.106Psychological problems 170 3.20 0.637 0.008 ** 1.98 0.661 0.035 ** 0.94 0.585 0.045 ** 0.83 0.251 0.320 0.63 0.200 0.406Pain in arms 102 3.41 0.619 0.002 ** 2.14 0.762 0.004 ** 1.12 0.588 0.016 ** 0.84 0.400 0.058 * 0.64 0.322 0.109Headache 177 3.24 0.570 0.025 ** 2.27 0.275 0.414 1.00 0.480 0.126 0.67 0.440 0.103 0.46 0.400 0.117Numbness in arms 57 3.56 0.526 0.021 ** 2.38 0.406 0.175 1.29 0.393 0.155 0.94 0.313 0.190 0.70 0.342 0.133Numbness in legs 22 3.65 0.487 0.146 2.43 0.670 0.132 1.37 0.232 0.578 1.00 0.305 0.394 0.76 0.238 0.482Neck pain 208 3.27 0.448 0.331 2.18 0.325 0.588 0.727 0.700 0.202 0.55 0.505 0.284 0.40 0.405 0.384Pins & needles (hands) 62 3.61 0.330 0.139 2.51 0.055 0.851 1.38 0.037 0.892 1.01 0.036 0.876 0.78 0.021 0.925Lower leg pain 41 3.68 0.101 0.697 2.43 0.340 0.322 1.34 0.294 0.354 0.98 0.260 0.340 0.74 0.229 0.380Prior back problems 72 3.69 0.035 0.870 2.21 0.843 0.002 ** 1.07 0.957 0.000 ** 0.75 0.825 0.000 ** 0.53 0.760 0.000 **

* Significant at <10%** Significant at <5%Bold O-logit gives a change in significance level

17

to have particularly large effects on disability in thissample, although difficulty in micturition was onlysignificant at the 10% level, probably due to the relativelysmall numbers involved. All these symptoms are indicativeof nerve problems in the lower spine. It is perhapsunsurprising that psychological problems are a highlysignificant indicator of increased disability at this stage.

At the second assessment, the rank order of thesymptoms changes, with difficulty in micturition beingassociated with the largest increase in disability(accompanied by an increase in significance level),followed by pins & needles in the legs, interscapular painand pain in the arms. The latter two were also highlysignificant at the first assessment, but were lower down thescale in terms of the increase in disability associated withthem. Weak legs and radiated pain in the buttocks alsofeature strongly at the second assessment, but pain in thethighs has dropped out of the picture at this stage.

The main correlates of increased disability at the thirdassessment can be seen to be arm, buttock andinterscapular pain and, particularly, difficulty inmicturition. Pain in the arms becomes less significant atthe fourth and fifth assessments, but the other threecontinue to be the initial symptoms which are most highlypredictive of increased long-term disability. Difficulty inmicturition, in particular, shows a score increment whichremains virtually constant, despite a declining base score,and, at Assessments 4 and 5, the smallest p-values of allthe physical symptoms. Radiated pain into the buttocksand difficulty in micturition are both indicative of damageto the lower spinal area, while interscapular pain isprobably indicative of neck damage (most subjectssuffered both lower back and neck strain injuries).

Turning to the two ‘non-symptom’ variables,psychological problems reported at the time of the firstassessment are still significant at the third assessment,although the p-value has risen to 0.045, and they cease tobe significant at the fourth and fifth assessments. Theinfluence of prior back problems is interesting. This factorlies at the bottom of the list, because it is a very poorpredictor of increased disability at the first assessment.However, at Assessment 2, it constitutes the third largest,and the most statistically significant influence on disabilityscore. At the third assessment, it moves up to second placein the rank order of score increments, and again is the mosthighly significant factor, and it maintains this ranking andsignificance level at the fourth and fifth assessments.Previous experience of back problems is thus seen to bevery significantly correlated with long-term disability dueto back pain from a road accident, regardless of the

circumstances of that accident. This factor has to be takeninto account in subsequent analyses.

Ordered logit results

In general, the O-logit results confirmed those obtainedfrom the simple regressions, the only major disagreementbeing that ‘Weak Arms’ continued to be highly significantat Assessments 3, 4 and 5 (p=0.038, 0.028 and 0.036respectively). Other slight discrepancies were: atAssessment 1, ‘Difficulty in Micturition’ was not significant(p=0.126), while ‘Lumbar Pain’ was slightly significant(p=0.080); at Assessment 2, ‘Numbness in the Legs’ wasslightly significant (p=0.081); at Assessment 4, ‘ShoulderPain’ was not significant (p=0.133), while ‘Pain in Arms’showed a slight increase in significance (p=0.044).

5.3 Psychological disability

In addition to physical problems, some patients had adegree of nervous reactions which affected their lives insome way. Nervousness reported at the first assessmentwas significantly associated with increased disability at thefirst three assessments (Table 21), although its significancedropped as time went on, and it was not significant atAssessments 4 and 5. Nearly 78% of patients (170)initially said that they felt more nervous than previously,although many people got over this problem as timepassed. At the second assessment, the number had fallen to103 (about 48%); by the third assessment, only 65 (about31%) were affected while, at Assessments 4 and 5, thetotals were 54 (26%) and 40 (20%). However, continuednervousness reported at the second assessment was verysignificantly correlated with increased disability right up toAssessment 5 (p-value<0.0005).

When asked in which of four main areas of activity (dailyliving, work, leisure and on the road) this nervousness mostmanifested itself, the majority of people, not surprisingly,said it was associated with use of the road, and this wasrepeated at all five assessments (Table 22). Note that all thepeople in this table suffered some degree of psychologicalproblem - those in the ‘unaffected’ group for one of thefour categories would have been in the ‘affected’ group forat least one of the other categories.

In the daily activities (‘Living’) category, at the firstassessment, the spread was fairly even between thoseaffected and those not, but at subsequent assessments,most people said that they were unaffected in this area. Inthe work and leisure categories the majority said they wereunaffected at all five assessments. The highest categorywas the reaction to being either a driver or a passenger in a

Table 22 Breakdown of areas affected, for subjects claiming to have suffered nervous problems

Assessment 1 Assessment 2 Assessment 3 Assessment 4 Assessment 5Type ofactivity Affected Unaffected Affected Unaffected Affected Unaffected Affected Unaffected Affected Unaffected

Living 89 81 38 65 23 42 13 41 9 31Work 70 100 31 72 15 50 10 44 7 33Leisure 69 101 31 72 16 49 11 43 8 32Road 167 3 102 1 65 0 53 1 40 0

18

car, with 167 patients having a problem at Assessment 1,and eight of these (all drivers) being unable to bringthemselves to get in a car again at all at this early stage. Ofthese 167 patients, 145 (87%) had been drivers. Driversalso constituted 87% of the overall sample, so there was noindication that drivers were more likely to bepsychologically affected than passengers. A similar patternmanifests itself at all the subsequent assessments. Fivesubjects at the second assessment were still completelyrefusing to get into a car, but this number dropped to four,then three then two at the third, fourth and fifthassessments respectively.

5.4 Effect of occupant characteristics on disability scores

Regression analyses were carried out to seek correlationsbetween a number of personal characteristics and overalldisability score. The results appear in Table 23. The scoreincrement in this table is the increase in disability score perunit increase in the characteristic, in the units given in thefirst column (ie, it is the gradient of the line). Thus, forexample, disability score at the first assessment decreasesby 0.016 for each year increase in age. The scoreincrement for ‘Maleness’, on the other hand, is simply thedifference in average score for the male group compared tothe female group. The Base Score (as given in Table 21),applied to continuous variables such as age, wouldrepresent the disability score at age 0. This would not bevery meaningful, so it has been omitted. The Base Scorefor ‘maleness’ can be found in Table 18. Again, the lastcolumn flags the significance level, one asterisk for 10%significance, two for 5%. Neck length is the distance fromAtlas to Spur (ie the bony prominence on the top thoracicvertebra) with the head upright. NL (flexed) is theequivalent measurement with the neck flexed. BMI (BodyMass Index) is a crude measure of obesity.

Rather surprisingly, bearing in mind that most subjectssuffered whiplash as well as lumbar strain, neither necklength (straight or flexed) nor neck circumference haveany correlation with disability score. Apart from a slightlysignificant correlation with the second assessment scores,weight also appears to be irrelevant to disability outcome.

The correlation with age at the first assessment issignificant at between 5 and 10%, but the trend indicatesthat older people are less severely injured. However, at thesecond and all subsequent assessments, increased age isvery significantly associated with increased disability(although the p-values do rise as time goes on), and this ismore in line with expectations.

Back length (ie seated height) is highly significant atmost of the assessments, and the trend indicates that longerback length is associated with lower disability.

Height and gender are not correlated with the firstassessment scores, but become significant in relation to thesecond assessment scores, with the trends indicating thattaller people and males tend to suffer lower disability. Thesetwo findings could be thought to be linked - males aregenerally taller than females (see discussion of Table 30).Although gender remains highly significant at Assessments3, 4 and 5, height becomes rather less significant.

Tab

le 2

3 In

flue

nce

of s

ome

pers

onal

cha

ract

eris

tics

on

disa

bilit

y sc

ore

Ass

essm

ent

1A

sses

smen

t 2

Ass

essm

ent

3A

sses

smen

t 4

Ass

essm

ent

5

Uni

tsSc

ore

incr

.P

-val

ueSi

gSc

ore

incr

.P

-val

ueSi

gSc

ore

incr

.P

-val

ueSi

gSc

ore

incr

.P

-val

ueSi

gSc

ore

incr

.P

-val

ueSi

g

Mal

enes

s–

-0.3

250.

112

-0.7

580.

004

**-0

.616

0.01

3**

-0.4

810.

025

**-0

.414

0.04

3**

Age

year

s-0

.016

0.05

7*

0.0

220.

041

** 0

.027

0.00

7**

0.02

40.

004

**0.

021

0.01

0**

Nec

k le

ngth

cm 0

.012

0.73

3 0

.019

0.67

7-0

.019

0.68

7-0

.032

0.38

0-0

.013

0.70

9N

L (

flex

ed)

cm 0

.004

0.93

1 0

.004

0.93

8-0

.021

0.67

3-0

.033

0.44

7-0

.010

0.80

3N

eck

circ

umf.

cm-0

.016

0.50

3-0

.026

0.40

6-0

.007

0.81

60.

002

0.94

00.

003

0.88

8B

MI

kg/m

2 0

.000

0.99

1-0

.006

0.78

7-0

.010

0.61

6-0

.002

0.92

30.

005

0.73

5B

ack

leng

thcm

-0.0

390.

017

**-0

.041

0.06

0*

-0.0

450.

024

**-0

.044

0.01

3**

-0.0

380.

022

**H

eigh

tcm

-0.0

100.

292

-0.0

310.

011

**-0

.020

0.07

4*

-0.0

180.

075

*-0

.016

0.08

7*

Wei

ght

kg-0

.003

0.58

1-0

.012

0.08

2*

-0.0

080.

212

-0.0

050.

332

-0.0

030.

573

* Si

gnif

ican

t at

<10

%**

Sig

nifi

cant

at

<5%

.B

old

O-l

ogit

giv

es a

cha

nge

in s

igni

fica

nce

leve

l

19

Ordered logit resultsIn most cases, the O-logit calculations confirmed the resultsobtained from regression. Three slight discrepancies werenoted: Gender became slightly significant at the firstassessment (p=0.083), and Age became slightly moresignificant at Assessment 1, and slightly less atAssessment 2 (p=0.047 and 0.061 respectively).

5.5 Effect of vehicle parameters on disability scores

Tables 24 to 29 show the results of the regressions carriedout on vehicle-based parameters. Table 24 showscontinuous variables, while Table 25 containsdichotomous variables (ie where a feature such as lumbarsupport is either present or absent). Tables 26 to 29 containcategorical variables, i.e. where a parameter such asvehicle type can take more than two values.

ETS is the calculated Equivalent Test Speed (see Section4.1.6). Normalised seat back height is the ratio of the seatback height to the occupant’s seated height, whereas Seatback height difference is the difference between the seatback height and the occupant’s seated height (i.e. s/backheight - subject back length). Seat back padding is thethickness of padding over any hard structure that wasdetected by the vehicle examiners at the bottom of the seatbackrest. It is only recorded in cases where underlying hardstructure was detected. Head restraint padding is thehorizontal depth of padding between the front of the headrestraint and the underlying support structure, in those caseswhere this padding was soft. Head restraint horizontal andvertical distances refer to the distance of the subject’s headfrom the restraint, as demonstrated at the vehicleexamination by those subjects who attended.

Few of these correlations are anywhere near significant,and several of the trend directions (for car age, engine size,seat back and seat base angles, seat back padding thicknessand head restraint vertical distance) reverse at the differentassessments.

In Table 24, higher ETS is consistently associated withincreased disability at all five assessments, becomingslightly significant at Assessments 4 and 5, indicating thathigher impact speed tends to lead to greater long-termdisability, in line with expectations. Seat back heightdifference is slightly significant at Assessment 1 only.Because of the way this factor is calculated (see above), allthe values are negative, and the graph of seat back heightdifference against disability lies in the second quadrant, witha positive slope. Greater back length, which tends toproduce a greater negative value of seat back heightdifference, is thus beneficial, and this is in line with thefindings in Table 23. However, the corollary of this is thatgreater seat back height will tend to have a detrimentaleffect, so that recent trends towards the introduction ofhigher seat backs to provide greater protection in highseverity rear impacts may be having an undesirable effect inlow severity crashes. Thicker head restraint padding tends tobe beneficial (i.e. leads to lower disability), and this ishighly significant at the second assessment, though less so atthe third and fourth, and it fails to achieve significance at thefifth. For the subgroup of subjects who attended the vehicle

examination, and for whom we have head restraintmeasurements, greater horizontal distance between head andrestraint is detrimental at all five assessments. At the thirdassessment, the correlation is significant at between 5 and10%, and the significance level continues to rise through thefourth and fifth assessments, leaving this as the most highlysignificant of the continuous vehicle-based factors inpredicting long-term injury outcome. This is in line with theconventional wisdom as regards protection from soft tissueneck injury, and it may be the neck-induced aspect ofdisability that is being picked up here. The previousWhiplash study failed to find any consistent correlationbetween disability and horizontal distance between head andrestraint, but that study only followed subjects for twelvemonths post-accident - i.e. as far as Assessment 3. Thepresent correlation only becomes strong at Assessments 4and 5 (but see below).

Ordered logit results (Table 24)Again, in most cases, the O-logit results confirmed thoseobtained by regression. The only major discrepancy was inthe results for head restraint horizontal distance, whereAssessments 3 and 4 were both non-significant (p=0.171and 0.281 respectively), and Assessment 5 was onlyslightly significant (p=0.082). This rather negates thefindings from the regression analysis discussed above and,since ordered logit is probably the better model, in that itdoes not assume linearity in the disability scale, this resultis likely to be the more reliable. If the study had beencontinued, then it is possible that O-logit would also havebecome highly significant at subsequent assessments, inline with, but later than, the regression results. But, withthe available results, this factor is insignificant up toAssessment 5, and then only slightly significant. Otherminor discrepancies are in the results for ETS, which isnon-significant at Assessment 4 (p=0.151), and headrestraint padding thickness, which becomes slightlysignificant at Assessment 1 (p=0.065).

In Table 25, Energy-absorbing Bumper refers to thebumpers fitted on some modern vehicles which are capableof absorbing minor impacts without distortion of otherbody panels, and which can regain their shape afterimpact. The effect of their presence on occupantkinematics, if any, is expected to be small - their majoreffect is to make it almost impossible to estimate theimpact speed, because there is frequently no visibledamage when the vehicle is inspected. They are includedin the analysis for completeness, and the trend directionassociated with their presence changes from beingbeneficial at the first assessment to being detrimental at allother assessments, even achieving slight significance atAssessments 3 and 4.

Only two of the parameters considered - ‘Hinged SeatBack’ and ‘Hard Structure (in seat back) Felt (at time ofimpact)’ - do not show a change in trend direction over thecourse of the five assessments, from beneficial todetrimental or vice versa. The presence of either a lumbarsupport or of some other underlying hard structure in theseat back was (non-significantly) beneficial at the firstassessment, but detrimental at Assessments 3, 4 and 5.

20 Table 24 Influence of continuous vehicle-based parameters on disability scores

Assessment 1 Assessment 2 Assessment 3 Assessment 4 Assessment 5No. in

Units group Score Incr. P-value Sig Score Incr. P-value Sig Score Incr. P-value Sig Score Incr. P-value Sig Score Incr. P-value Sig

Car age years 218 -0.014 0.588 0.001 0.980 -0.025 0.435 -0.009 0.740 -0.003 0.912Engine size litres 219 0.021 0.948 -0.365 0.393 0.099 0.802 0.062 0.856 0.009 0.982ETS km/h 195 0.005 0.731 0.025 0.167 0.022 0.276 0.030 0.087 * 0.031 0.056 *S/back angle deg 217 0.013 0.476 -0.012 0.598 -0.004 0.853 -0.002 0.935 0.010 0.578S/base angle deg 206 0.002 0.921 0.017 0.555 -0.005 0.844 -0.015 0.522 -0.022 0.335Norm. s/back ht. – 201 1.962 0.204 0.358 0.865 1.819 0.344 1.624 0.330 1.415 0.368S/back ht diff. cm 201 0.087 0.085 * 0.013 0.537 0.023 0.228 0.020 0.224 0.017 0.285S/back padding cm 79 0.018 0.846 -0.109 0.348 -0.056 0.595 -0.037 0.694 -0.043 0.614H/rest padding cm 196 -0.082 0.102 -0.146 0.030 ** -0.122 0.075 * -0.100 0.066 * -0.067 0.194H/rest hor dist. cm 122 0.029 0.179 0.040 0.165 0.049 0.057 * 0.048 0.040 ** 0.052 0.022 **H/rest vert dist. cm 122 -0.025 0.450 0.012 0.789 0.010 0.803 0.022 0.551 0.021 0.555

* Significant at <10%** Significant at <5%Bold O-logit gives a change in significance level

Table 25 Influence of dichotomous vehicle-based parameters on disability scores

Assessment 1 Assessment 2 Assessment 3 Assessment 4 Assessment 5No. with

parameter Base Score P- Base Score P- Base Score P- Base Score P- Base Score P-Parameter present score incr. value Sig score incr. value Sig score incr. value Sig score incr. value Sig score incr. value Sig

Energy-absorbing bumper 46 3.71 -0.086 0.727 2.44 0.240 0.464 1.28 0.582 0.057 * 0.93 0.448 0.089 * 0.73 0.272 0.276Pretensioner fitted 25 3.71 -0.066 0.835 2.50 -0.057 0.889 1.39 0.009 0.982 1.03 -0.073 0.823 0.78 0.018 0.954Pretensioner activated 5 3.70 -0.101 0.881 2.48 0.521 0.553 1.39 0.213 0.791 1.02 0.385 0.577 0.78 0.016 0.980Hinged seat back 77 3.75 -0.156 0.459 2.61 -0.339 0.218 1.60 -0.591 0.020 ** 1.20 -0.500 0.023 ** 0.92 -0.396 0.059 *Hinged seat base 16 3.69 0.190 0.623 2.50 -0.058 0.909 1.41 -0.222 0.631 1.06 -0.433 0.276 0.82 -0.487 0.207Seat broken in impact 12 3.69 0.143 0.748 2.51 -0.255 0.658 1.38 0.162 0.769 1.03 -0.122 0.797 0.79 -0.156 0.726H/rest present 203 3.94 -0.258 0.506 2.33 0.169 0.745 0.39 1.075 0.034 ** 0.15 0.929 0.033 ** 0.08 0.745 0.081 *Hard h/rest 10 3.65 0.654 0.177 2.47 0.195 0.773 1.46 -0.574 0.351 1.10 -0.769 0.147 0.85 -0.737 0.141Adjustable h/rest 181 4.09 -0.426 0.362 2.00 0.518 0.400 1.10 0.351 0.549 0.80 0.274 0.578 0.67 0.141 0.777Lumbar support 15 3.72 -0.391 0.328 2.47 0.099 0.854 1.35 0.512 0.296 0.99 0.510 0.228 0.76 0.381 0.340Hard struct. in st bk 82 3.77 -0.179 0.391 2.47 -0.056 0.839 1.37 0.016 0.951 1.00 0.067 0.764 0.77 0.057 0.787Hard structure felt 3 3.69 2.646 0.002 ** 2.41 1.589 0.162 1.33 1.005 0.332 0.99 0.344 0.702 0.78 0.225 0.829Aware 94 3.76 -0.143 0.483 2.57 -0.182 0.495 1.38 0.041 0.870 0.99 0.077 0.721 0.70 0.196 0.337Braced 52 3.67 -0.018 0.942 2.62 -0.326 0.306 1.40 0.017 0.954 1.00 0.143 0.588 0.70 0.318 0.203Hit head 67 3.71 -0.094 0.745 2.54 -0.282 0.468 1.45 -0.070 0.848 1.08 -0.077 0.806 0.79 0.103 0.727

* Significant at <10%** Significant at <5%Bold O-logit gives a change in significance level

21

However, feeling the hard structure in the seat back as thetorso sank into the padding during the impact wasconsistently detrimental, by a very large amount at the firstthree assessments. Despite this, however, it was onlysignificant at the first assessment, possibly due to the verysmall numbers involved, and by the fifth assessment,several other factors showed much larger values of scoreincrement. Impact speed for these three ‘felt structure’cases was lower than average, though not significantly so.The padding over the hard structure was, not surprisingly,thinner for those who felt it during impact.

Hitting one’s head on the head restraint was non-significantly beneficial at the first four assessments, anddetrimental at the fifth. However, this parameter does relyon the subject’s recollection of a very transient eventduring the impact, so the responses may not be as reliableas is the case for more objectively-measurable parameters.

The presence of a hinged seat back is consistentlyassociated with a reduction in disability score at allassessments, and this is highly significant at the third andfourth, though less so at the fifth. It is not clear why such aseat should be better than a non-hinged seat in this respect.Breakage of the seat back had no significant effect at anyof the five assessments.

The presence of a head restraint, although (non-significantly) beneficial as regards first assessment scores,becomes detrimental at all subsequent assessments, andvery significantly so (with large score increment values) atthe third and fourth. This is difficult to explain, and alsodifficult to reconcile with the previous finding that largehorizontal distance from the restraint (where one exists) isdetrimental. One would have expected that not having ahead restraint would be equivalent to being a long wayfrom one, and would be similarly detrimental. Onepossible explanation for this may be that there were veryfew people without a head restraint (only 16), and nearlyhalf of these were rear seat occupants who, in our study,had higher average disability scores than the front seatoccupants at the first assessment, but lower scores atsubsequent assessments (see Table 27). This reversalmatches the reversal in trend direction for the ‘headrestraint present’ results. Other studies have found a lowerrisk of neck strain injury for rear seat occupants (egCarlsson et al., 1985). However, another complicatingfactor is that there was a very high proportion of femalesamong the ‘no head restraint’ group, and this should havegiven this group a higher average score, even if headrestraints had no effect one way or the other.

Neither being aware of the impending impact nor bracedfor impact had any significant effect on disability scores,and the trend directions at the five assessments wereinconsistent.

Ordered logit results (Table 25)Once again, the O-logit analysis produced a broadlysimilar picture. Energy absorbing bumper dropped below10% significance at Assessments 3 and 4 (p=0.129 and0.180 respectively). Hinged seat back was only slightlysignificant at Assessment 4 (p=0.067), and non-significantat Assessment 5 (p=0.109). ‘Head restraint present’

showed a lower significance level at Assessment 3(p=0.060), and ‘Hard structure felt’ became slightlysignificant at Assessment 2 (p=0.082).

Table 26 Disability vs Car body type

Ave disability at assessment

Number 1st 2nd 3rd 4th 5th

Saloon 42 3.62 2.79 1.66 1.34 1.03Hatchback 156 3.72 2.46 1.40 1.00 0.77Estate 14 3.71 2.15 0.55 1.55 0.27MPV/off road 3 4.67 2.33 1.67 1.00 1.00Sports 2 3.00 1.50 0.00 0.00 0.00Other 2 2.50 2.00 1.00 1.50 0.00

Table 27 Disability vs Seating position

Ave disability at assessment

Number 1st 2nd 3rd 4th 5th

Driver 190 3.72 2.50 1.37 1.01 0.75Front passenger 23 3.39 2.61 1.81 1.38 1.24O/S rear 5 4.00 1.40 0.40 0.20 0.00N/S rear 1 5.00 4.00 2.00 1.00 1.00

Table 28 Disability vs Impact direction

Ave disability at assessment

Number 1st 2nd 3rd 4th 5th

Rear 105 3.58 2.68 1.42 1.04 0.75Frontal 56 3.77 2.80 1.54 1.15 1.00Left hand side 27 4.19* 2.23 1.42 0.96 0.73Right hand side 31 3.55 1.47** 1.00 0.79 0.59

* Significant at <10%** Significant at <5%

Tables 26 to 29 show the average disabilities associatedwith the various categories of Vehicle Type, SeatingPosition in the car, Impact Direction and Seating Posture.

No body type was significantly different to any other atany of the five assessments, and this was confirmed byOrdered Logit analysis.

Table 29 Disability vs Seating posture

Ave disability at assessment

Number 1st 2nd 3rd 4th 5th

Normal 138 3.61 2.57 1.34 1.05 0.78Leaning forward 9 4.67** 2.56 2.00 1.50 1.50Looking left 7 3.00 1.86 1.00 0.43 0.43Looking right 12 3.67 1.58 1.17 0.83 0.67Looking back 1 3.00 3.00 0.00 0.00 0.00Looking down 1 4.00 4.00 0.00 0.00 0.00Looking up 0 – – – – –Other 1 3.00 3.00 0.00 0.00 0.00Not known 50 3.90 2.52 1.58 1.04 0.82

** Significant at <5%

22

No seat position (Table 27) was significantly differentto any other at any of the five assessments, and this wasconfirmed by Ordered Logit analysis. There is a tendency forfront passengers to have higher disability than drivers, fromAssessment 2 on, but this may be explained by the muchhigher proportion of women in this group (see Table 9).

Looking at Table 28, at the first assessment, averagedisability after left hand side impacts was significantlyhigher than the average disability over all other directions(p=0.069), but this trend direction was not maintained atthe other assessments, and there were no other significantdifferences. At the second assessment, average disabilityafter right hand side impacts was very significantly lowerthan that associated with other impact directions,(p=0.002). Although average disability after right handimpacts was consistently lower than the others at allassessments, none of the other differences were significant.Ordered Logit confirmed all these results except for RearImpact at Assessment 2, where there was a slightlysignificant difference between this impact direction and theothers (p=0.095).

At the first assessment, people who had been leaningforwards at the moment of impact (Table 29) sufferedsignificantly higher disability than the average over allother people (p=0.024). This is in contrast to the Whiplashstudy, which indicated that people who had been leaningforwards suffered significantly lower disability than others.If these effects are real, then they may indicate that leaningforwards is beneficial in reducing the severity of whiplashinjury, but detrimental in terms of lower back strain injury.All results were confirmed by O-Logit.

5.6 Stacked regressions

In addition to the separate regressions for eachassessment carried out up to now, it is possible to carryout ‘stacked regressions’, using the data from all fiveassessments in a single regression. For a dichotomousvariable, such as sex, this method can be thought of asgenerating two lines on a graph of disability against time- one for males and one for females. We then test whetherthese lines are significantly different. A number ofdifferent statistical models can be used, and four havebeen tried in the present analysis. These are:

Model 1: This performs a least-squares regression with allthe data stacked, but the errors (termed ‘Robust Errors’) arecalculated in a way which does not assume that all theobservations are independent - i.e. clusters of observations(the five assessments for each subject) are allowed to becorrelated. The results of such a calculation are not biassed,but in theory the method is not as efficient as a GeneralisedLeast Squares calculation which uses a reasonable estimateof the variances and covariances at each assessment.

Model 2: Generalised Least Squares. Here, it is usuallynecessary to know in advance the correlations within theclusters, but the calculation can be done without thisinformation, and it will give an estimate of what thiscorrelation is.

Model 3: Random Intercepts. Rather than all the Male datapoints being fitted to one line, as in Model 1, each

individual has a separate line, all parallel within eachgender group. The significance of any difference betweenthe intercepts for Males and Females is then assessed.

Model 4: This is the Ordered logit version of Model 1 - i.e.using Robust Errors.

There are advantages and disadvantages associated withall four of these models. None of them is ideal but, byusing a number of different models in this way, it isintended that a more balanced picture will emerge.

The decay of the average disability in the sample overtime is closely matched by an equation of the form

Dn = D

1 - λlog(n)

where D1 is the average disability at Assessment 1, D

n is

the average disability at Assessment n and λ is a constant.Note that this is not a normal exponential decay curve forD (which would be represented by D

n = D

1e-λn), and, in

time, the equation used would allow D to become negative,which is not possible. However, over the time periodconsidered, it does give the best fit to the available data, soit has been used as the basic model. In addition, the O-logitmodel handles the data in such a way that this problem ofD becoming negative does not arise - another advantage ofthe O-logit model.

A graph of D vs log(n) is therefore a straight line withnegative slope. In the case of a dichotomous variable suchas sex, which can take only two values (male and female),the data are fitted to two separate straight lines on thisgraph. For continuous variables, such as age, the data arefitted to a surface on a 3-dimensional graph, whose thirdaxis is Age (or whatever variable is under investigation).Within each model, it is possible to make differentassumptions about the behaviour of the data. For example,for sex, the data can be fitted to two lines with parallelslopes - i.e. it is assumed that people get better at the samerate, no matter what their initial disability score. Thestatistical tests then determine whether these two lines are,in fact, significantly different. Alternatively, it can beassumed that the data fit two non-parallel lines, allowingthe possibility that people in one group might recover at adifferent rate from those in the other - i.e. there is aninteraction with time (or Assessment number). Thestatistical tests then determine firstly whether the twoslopes are significantly different, and secondly whether theintercepts on the Disability axis are significantly different.Each of the models has been run with each of these twosets of assumptions. Note that, in the time interactionversion of Model 3 (Random Intercepts), the slopes are notrandom - all the individual lines are parallel within eachgender group.

Only the results for personal and vehicle-based factors arepresented, as these are most relevant to the purpose of theproject. For those factors where a significant time interactionwas found, the results are displayed in Table 30. If nosignificant time interaction was found, then the ‘parallelslopes’ assumption was used, and the results are displayed inTable 31. Factors which did not produce any significantresults under either assumption have been omitted.

Because gender and prior back problems have been sosignificant in previous tables, Model 4 has also been run

23

for females only and for males only, and then again forthose with prior problems and for those without. Theresults of these exercises are commented on in the text.

5.6.1 Time-interactive factorsTable 30 presents the Slope Increments (the amount bywhich the slope, or rate of recovery, changes per unitincrease in the factor considered), and the correspondingp-values found in each of the regressions, with bold typeemphasising those which are significant at better than 5%,and italics for those between 5 and 10%. A positive valueof the Slope Increment means that an increase in the‘factor’ variable produces an increase in the slope of theDisability/Time graph - i.e. the slope becomes lessnegative, and the line takes longer to reach the Time axis,indicating an increase in the time taken to recover frominjury. For example, increasing age produces a consistentincrease in recovery time across all the models.

Considering first those factors which are significant atless than 5% in three or more models, and which alsodisplay a consistent sign to the Slope Increments (allpositive or all negative):

i Age is highly significant in all models, and the resultsindicate that older people recovered more slowly. Thiswas also true within all the gender and prior problemsubgroups.

ii Prior back problems are highly significant in all modelsbut one, and people with this condition recovered moreslowly than those without. This effect was also significantwithin the separate male and female subgroups.

iii People who remembered feeling the hard structure at thebase of their seat backrests during the impact, althoughinitially suffering significantly higher disability (seeTable 25), recovered more quickly. However, it shouldbe born in mind that there were only three of thesepeople, so the result cannot be considered conclusive,and also, it was not worthwhile breaking this factordown into gender and prior problem subgroups.

iv The presence of a head restraint is associated withlonger recovery times, but if the head restraint was hard,recovery tended to be quicker than if the restraint wassoft. The detrimental effect of a head restraint has beendiscussed previously, in terms of the small numbersinvolved (only 16 people without a restraint) and thesignificant proportions of rear occupants and females inthe group. In fact, the trend was predominantlyassociated with the female group (SlopeIncrement=1.579, p=0.001), and was maintained whenthe sample was restricted to female front seat occupants(Slope Increment=1.935, p<0.0005). The male groupshowed no significant effect (Slope Incr=0.613,p=0.537), although the trend was in the same direction.It may be that the lack of significance in the male groupis simply due to low numbers (only 3 males withouthead restraints, compared to 13 females, of whom 8were front occupants). The male and female SlopeIncrement values were not significantly different fromeach other, so the effect of the head restraint upon mencan not be said to be different from its effect uponwomen. Separating out the Prior Problem subgroups hadno effect - both groups showed almost identical andsignificant (p=0.032 and 0.005) responses to thepresence of a head restraint. For the hard head restraintcases, the numbers involved were even smaller (only 10),and average ETS in this group was some 15% lower thanthe overall average, so again, this result may be ratherunreliable. This factor has not been significant previously,and splitting the sample by gender did not produce aclearer picture, although the beneficial effect wasconfirmed. These trends all agree with the trends in thevalues of the score differences across the five assessmentsin Tables 21 to 25. It was not worthwhile splitting thesample by prior back problems, since only one subjecthad a hard head restraint and prior back problems.

Considering next those factors which are significant atless than 5% in two models (and which also display a

Table 30 Results of ‘Stacked Regression’ analyses (Time-interactive factors)

Model 1 Model 2 Model 3 Model 4

Factor Slope incr P-value Slope incr P-value Slope incr P-value Slope incr P-value

Age (yrs) 0.023 0.001 0.014 0.011 0.022 0.000 0.029 0.000Nervous -0.285 0.115 -0.358 0.029 -0.239 0.074 -0.150 0.570Prior probs 0.463 0.006 0.242 0.097 0.447 0.000 0.682 0.001Aware 0.215 0.174 0.316 0.022 0.283 0.012 0.197 0.328Braced 0.226 0.240 0.427 0.010 0.322 0.017 0.130 0.580Saloon car 0.258 0.211 0.192 0.267 0.278 0.041 0.336 0.149Sports car -0.132 0.491 0.186 0.790 -0.144 0.799 -1.835 0.012En. Abs. bumper 0.297 0.106 0.054 0.755 0.295 0.032 0.341 0.096Left side impact -0.324 0.227 -0.300 0.145 -0.339 0.042 -0.469 0.151Hard struct. in s/back 0.156 0.354 0.212 0.135 0.253 0.029 0.291 0.173Hard structure felt -1.595 0.000 -1.257 0.073 -1.481 0.009 -1.149 0.000H/rest present 0.789 0.000 0.451 0.124 0.847 0.000 1.336 0.002Adj. h/rest 0.353 0.282 0.369 0.274 0.658 0.015 0.338 0.468Hard h/rest -0.968 0.003 -0.823 0.013 -1.005 0.000 -1.353 0.014H/rest vert. dist. (cm) 0.028 0.204 0.039 0.101 0.039 0.045 0.029 0.311Lumbar support 0.555 0.078 0.366 0.176 0.614 0.005 0.354 0.323Hinged seat back -0.200 0.181 -0.081 0.577 -0.255 0.029 -0.285 0.167Hinged seat base -0.435 0.088 -0.489 0.062 -0.493 0.021 -0.329 0.344

24

consistent sign to the Slope Differences):

i Both awareness of the impending impact and beingbraced for the impact are associated with slowerrecovery in Models 2 and 3 only. The trends in the scoredifferences in the previous tables are consistent with thepresent result, and the trend directions were confirmedin both male and female subgroups, but none of theresults was significant. Among those who were bracedfor impact, there was a significant difference betweenthe reaction of those with prior back problems comparedto those without. In the former, being braced wasdetrimental to speed of recovery (Slope Incr=0.769,p=0.007), while among the latter it was beneficial, butnot significantly so (Slope Incr=-0.177, p=0.573). Thesetwo slope increments were significantly different(p=0.026). Splitting the ‘Aware’ group into PriorProblem subgroups produced no significant results.

Moving on to factors which are significant at less than5% in only one model, but which have consistent SlopeIncrement signs:

i Nervousness at Assessment 1 is significantly associatedwith shorter recovery times in Model 2, and it alsoachieved slight significance in Model 3. These people hadsignificantly higher scores than others at the first threeassessments (see Table 21), but they were not significantlydifferent from the rest at Assessment 5. Splitting thesample by gender and by prior back problems (usingModel 4) did not clarify the picture, but see Section 5.6.2.

ii Occupants of saloon cars tended to recover moreslowly than occupants of other cars. This is the firsttime this factor has shown significance, but only inModel 3 - it is not even slightly significant in any othermodel, nor in the male and female subgroups.However, separation of the sample by prior backproblems confirmed this result for those with no priorproblems (Slope Incr=0.779, p=0.008) while, for thosewith prior problems, saloon occupants tended to recovermore quickly (non-significant, Slope Incr=-0.428,p=0.282). These opposing trends were significantlydifferent (p=0.015). Interestingly, the opposite situationis found among occupants of hatchback cars: for thosewithout prior problems, being a hatchback occupantwas beneficial, while for those with prior problems, itwas detrimental. Although these two trends were bothnon-significant, they were significantly different fromeach other (p=0.028).

iii The presence of an energy absorbing bumper isassociated with slower recovery. This is significant inModel 3, and just manages slight significance inModel 4. It was significant in the male subset (SlopeIncr=0.711, p=0.031), but not in the female subset,although the trend direction was similar. The male andfemale Slope Increments were not significantlydifferent from each other. Similarly, those with priorback problems were not significantly different fromthose without in their response to the presence of anenergy absorbing bumper, although the overall trenddirection was confirmed among those with no priorproblems (Slope Incr=0.642, p=0.012). This factor

achieved slight significance at Assessments 3 and 4 inTable 25, but the equivalent O-logit calculations failedto confirm those results. The present result agrees withthe trend directions in Table 25.

iv People involved in left side impacts tended to recovermore quickly (Model 3 only), although Table 28 showsthat this was primarily due to their high average scoreat Assessment 1, from which they rapidly merged tobecome indistinguishable from the other impactdirection groups. This factor was significant among thefemale subgroup (Slope Incr=-0.669, p=0.050) andamong those with no prior back problems (SlopeIncr=-1.039, p=0.018), but not among the males(Slope Incr=-0.113, p=0.886), nor among those withprior problems (Slope Incr= -0.417, p=0.415). Neitherthe male and female slope increments nor the with/without prior problem slope increments weresignificantly different from each other.

v Hard structure in the seat backrest, adjustable headrestraints and greater vertical distance between thecentre of the head restraint and ear level on theoccupant are all associated with longer recovery times(Model 3). Again, these are not significant in any othermodel, nor have they been in any previous table. Whensplit into gender groups, hard structure was significantamong females (Slope Incr=0.661, p=0.012), but notamong males (Slope Incr=-0.539, p=0.168). Note,however, that the trend directions are opposite here,and indeed, they are significantly different from eachother (p=0.011). The most highly significant effect ofhard structure, however, is among those with priorback problems (Slope Incr=0.968, p=0.001). Hardstructure had no significant effect among those with noprior problems, and the two groups were significantlydifferent from each other in this respect (p=0.024). ForAdjustable Head Restraint, splitting Model 4 intogender and prior problem subgroups did not clarify thepicture - this factor was still not significant. For HeadRestraint Vertical Distance, splitting Model 4 didproduce a significant result for females, in agreementwith the detrimental effect of greater vertical distance(Slope Incr=0.067, p=0.015), whereas males showed anopposite but non-significant trend (Slope Incr=-0.013,p=0.841). The male and female trends were not,however, significantly different from each other.Splitting the sample by prior problems produced nosignificant results for the vertical distance factor.

vi The presence of a lumbar support is also associatedwith longer recovery times (Model 3 and slightly inModel 1). Again, these are the first significant resultsfor this factor, but they are not borne out in Models 2and 4. In terms of Slope Increment, splitting the sampleby gender had no effect, although it did producesignificant results in terms of the intercept on theDisability axis (female Score Incr=0.847, p=0.019,male Score Incr=-0.654, p=0.142, male and femaleScore Incs significantly different, p=0.010). Thus, interms of initial disability scores, a lumbar support issignificantly detrimental for women. For men, the

25

presence of a lumbar support is non-significantlybeneficial, but this beneficial effect is significantlydifferent from the effect on women. Splitting thesample by prior back problems did not improve on thebasic Model 4 result.

vii People whose seats had hinged backrests tended torecover more quickly, as well as tending to start offwith a lower disability score, to the extent that they hadsignificantly lower scores from Assessment 3 (seeTable 25), although the equivalent O-logit calculationswere rather less significant. Here, this factor was notsignificant in either of Models 2 or 4, nor in the Model 4gender subgroups, although it was slightly significantin the No Prior Problems only subgroup (Slope Incr=-0.542, p=0.052). Those with prior problems showedan opposite (detrimental), but non-significant response(Slope Incr=0.147, p=0.608). The difference betweenthese two responses was slightly significant (p=0.085).

viii The presence of a hinged seat base was beneficial(Model 3, and slightly in Models 1 and 2), but it wasnon-significant in Model 4. This factor has not beensignificant previously, although the trend directions inTable 25 are consistent with the present results. Interms of Slope Increment, splitting the sample bygender produced no significant results. However, interms of intercepts on the Disability axis, men andwomen showed opposite responses to having a hingedseat base - detrimental for women and beneficial formen. The actual score increments for each group werenon-significant, but the difference between them wasslightly significant (p=0.059). When Model 4 was splitby Prior Problem status, those with prior problems hada significant response to this factor (Slope Incr=-1.069,p=0.044). Those with no prior back problems showedan opposite but non-significant trend. The difference inthe responses of the two groups was not particularlysignificant (p=0.092).

Finally, Sports Car was significant in only one model(Model 4), but the signs of the Rate Differences wereinconsistent in the various models, so this factor has beencarried forward to Table 31, along with Nervousnesswhich, alone of the rather inconclusive factors in Table 30,‘performed’ more consistently in the non-time-dependentanalysis.

5.6.2 Non-time-interactive factorsTable 31 includes the two factors mentioned above, alongwith factors which were not significant in any of the timeinteraction models.

Note that this analysis concerns Score Increments, i.e. theseparation between two parallel lines, rather than SlopeIncrements. Significant Score Increments indicate that thefactor concerned is significantly associated with higher (orlower) disability scores, but that the rate at which people gotbetter was independent of the factor. However, note alsothat a group which gets better at the same rate as anothergroup, but which starts off at a higher average disability thanthe other group, will still take longer to fully recover. ‘Non-time-interactive’ thus relates to the rate of reduction ofdisability score, not to the total time to recover.

The two factors carried forward from Table 30 bothshow consistent Score Increment signs, and are significantunder more models. Looking first at factors which arebetter than 5% significant in at least three models:

i The result for sex indicates that being male is stronglyassociated with lower disability, in agreement withprevious results. When Model 4 was applied to the PriorProblems subgroups, sex became less significant(probably due to the reduction in sample size), althoughthe trend direction was maintained.

ii Nervousness at the time of the first assessment isassociated with higher disability scores, also inagreement with previous results (Table 21).Confirmation of this was found within the separategender and prior back problem subgroups.

iii Thicker head restraint padding is associated with lowerdisability scores, while greater horizontal distancebetween head and restraint resulted in higher scores.Both these were significant in Table 24, although therewas then some doubt over the horizontal distance result,because it was not fully confirmed by the O-logitcalculations, and indeed, it is not significant here underModel 4. The result for padding thickness can be tracedto the male subgroup (Score Incr=-0.146, p=0.047). Itwas non-significant for females, but the trend directionwas similar, and the male and female groups did notrespond significantly differently to this factor, so it is thecombined result which should be used. In the PriorProblem subgroups, padding thickness was significant

Table 31 Results of ‘Stacked Regression’ analyses (Non-time-interactive factors)

Model 1 Model 2 Model 3 Model 4

Factor Slope incr P-value Slope incr P-value Slope incr P-value Slope incr P-value

Maleness -0.519 0.005 -0.329 0.041 -0.547 0.005 -0.632 0.003Height (cms) -0.019 0.051 -0.012 0.105 -0.021 0.019 -0.023 0.056Back length -0.028 0.071 -0.027 0.015 -0.029 0.034 -0.035 0.072Nervous 0.471 0.019 0.248 0.191 0.470 0.041 0.541 0.028Sports car -0.995 0.000 -0.633 0.430 -0.981 0.318 -1.441 0.000Rt side impact -0.465 0.039 -0.124 0.587 -0.484 0.079 -0.544 0.071ETS (km/hr) 0.021 0.164 0.024 0.061 0.028 0.070 0.017 0.292H/r pad. (cm) -0.101 0.038 -0.077 0.054 -0.107 0.030 -0.118 0.035H/r hor dist (cm) 0.043 0.047 0.044 0.009 0.044 0.030 0.040 0.106Leaning forward 0.625 0.333 0.853 0.033 0.655 0.182 0.564 0.424

26

for those with prior problems (Score Incr=-0.271,p=0.003), but not for those without (Score Incr=-0.031,p=0.564). The responses of the two groups weresignificantly different (p=0.025). Horizontal Distancewas no more significant when Model 4 was applied tothe gender subgroups than it was in the overall sample.It was significant in the Prior Problems only subgroup(Score Incr=0.046, p=0.035), but the response of thosewith no prior back problems was non-significant,although the trend direction was the same (ie greaterdistance is detrimental).

Moving on to factors which have two highly significantresults:

i Back length was significant under Models 2 and 3, withModels 1 and 4 being slightly significant. The indicationis that people with longer backs tend to have lowerdisability scores. It has been postulated previously thatthis may be linked to the fact that men are generally tallerthan women. However, when the sample was split bygender, back length was only important for males (ScoreIncr=-0.052, p=0.045). It was not significant for females(Score Incr=-0.002, p=0.957), but Slope Increment forfemales was significant (Slope Incr=-0.055, p=0.045).Slope increment for males showed the opposite trenddirection, but was non-significant (Slope Incr=0.051,p=0.110), but the male and female slope increments weresignificantly different from each other (p=0.011). Tosummarise: longer back length among men tends to resultin lower initial disability (sig.), but slower recovery (non-sig.); among women, it results in very slightly lowerinitial disability (non-sig), but quicker recovery (sig.), andthe effect of back length on rate of recovery issignificantly different for men and women. In the PriorProblem subgroups, longer back length was beneficial forboth groups, and this was significant for those with priorback problems (Score Incr=-0.073, p=0.021).

ii The sports car category also shows two highlysignificant results (Models 1 and 4), although the othertwo models were not even slightly significant.Occupants of these vehicles had lower disability scoresthan occupants of other vehicle types but, with only twopeople in this group, the result is likely to be ratherunreliable. It was not worthwhile splitting this groupinto gender and prior problem subgroups.

Now considering factors with a highly significant resultin only one model:

i Height and right side impact yielded one significantand two slightly significant results each, with tallerpeople and those involved in right side impacts tendingto suffer lower disability. In the gender-segregatedsample, height was not significant in either subgroup,and the trend directions were opposite for males andfemales, indicating that the overall result for this factormay be an artifact, due to men being generally tallerthan women. In the Prior Problem subgroups, heightjust failed to achieve slight significance for those withprior back problems (Score Incr=-0.033, p=0.110), butthe trend was the same for both groups. Right side

impact was significant at Assessment 2 in Table 28.The present results can be traced to women (ScoreIncr=-0.586, p=0.073). For men, the Score Incrementwas non-significant, but they got better much morerapidly than men involved in other impact directions(Slope Incr =-4.279, p=0.005), and they weresignificantly different from women in this respect.There were, though, only 8 men involved in right sideimpacts, so the result may not be reliable. ApplyingModel 4 to the Prior Problem subgroups did not revealany more significant results. The beneficial effects of aright side impact were not dependent on seat positionin the vehicle.

ii Leaning forward is significant in Model 2 only, and thisagrees with the previous result in Table 29. However,there were only nine people in this category, and only oneof them was male, and only two had prior back problems.When the gender and prior problem subgroups wereanalysed, the only significant results were for the singlemale and the two prior problem cases, so these may berather unreliable. The female and non-prior-problemgroups did not produce significant results.

Finally, ETS has been included in Table 31 because,intuitively, it would be expected to be important indetermining disability outcome. In Table 24, it was onlyslightly significant at Assessments 4 and 5, and only theAssessment 5 result was confirmed by O-logit. Here, it isslightly significant under Models 2 and 3, but not underModels 1 and 4; the indication is that higher impact speedresults in higher disability, as would be expected. In thegender-segregated subsets, the trend direction for ETS wasconfirmed in the female group (Score Incr=0.036, p=0.085),but reversed among the males (Score Incr=-0.012 - i.e.higher speed is beneficial). Although this latter result wasnon-significant, (p=0.228), the difference in responsebetween men and women was significant (p=0.037). In thePrior Problem subgroups, ETS was no more significantthan in the full sample. Overall, these results are too weakto be conclusive.

5.6.3 Other factorsThere were three factors which had not been significantin the overall stacked regressions, but which producedsignificant results in the separate gender and priorproblem analyses:

i Both measures of relative seat back height producedsignificant results in the gender segregated analysis, butnot in the prior problem subgroups. Seat back heightdifference was significant in terms of both scoreincrement and slope increment for men only (ScoreIncr= 0.044, p=0.049, Slope Incr=-0.062, p=0.030), i.e.increasing this factor is detrimental as regards initialdisability, but beneficial as regards rate of recovery. Interms of more fundamental parameters, increased backlength tends to result in reduced initial disability, butslower recovery, and increased seat back height tends toproduce higher initial disability, but faster recovery.Women demonstrated trends opposite to those of menfor both score and slope increments, although neither

27

was significant. The trends for men were, however,significantly different from those for women (p=0.033and 0.010 respectively). Normalised seat back heightproduced a similar picture, with significant results forboth score increment and slope increment in the malesubgroup, but not in the female subgroup. Again, thetwo groups showed opposing trends, which weresignificantly different from each other. Overall, theseresults involving seat back height are contradictory,confusing, and probably best ignored.

ii Hitting one’s head on the head restraint during impactproduced opposing trends in the with/without prior backproblem subgroups. For those without prior problems itwas beneficial in terms of score increment (Score Incr=-0.734, p=0.013), while for those with prior problems itwas detrimental (Score Incr=0.881, p=0.093). These twoscore increment values were significantly different fromeach other (p=0.008). The slope increments also showedopposing trends between the two groups, but neither wassignificant. This factor was not significant in either ofthe gender-segregated subgroups.

5.7 Survival

As mentioned previously, this technique is usually used tolook at differences in the survival times of patients in twogroups, one of which is given an experimental form oftreatment, while the other is not. The technique gives anestimate of how effective the new treatment is inpreventing death. It can also cope with continuousvariables - i.e. where each patient is given a different doseof the drug or treatment. For the present study, dosage ofdrug can be replaced by the presence or absence of asymptom or a head restraint or awareness of the impendingimpact (ie the ‘two group’ approach) or by a continuousvariable such as age, ETS etc. Rather strangely, theanalogue of death is recovery in our study, since thisresults in the subject dropping out of the Lumbar study inthe same way that death results in a patient dropping out ofa clinical trial study. At each assessment, subjects areasked whether they have recovered from their neck injuryand/or their lumbar injury and, if so, when the recoverytook place (subjects keep a diary of their progress from thetime of the first assessment). The time variable is thuscontinuous, rather than the discrete, 6-monthly timeintervals used in the disability analysis, and it is possible todifferentiate between recovery from lumbar injury andrecovery from neck injury (this is the only type of analysispresented in this report which is capable of distinguishingbetween these two injuries). Disability scores are not used.

Two methods of carrying out this analysis were tried - aLog-normal (‘Accelerated Failure’) model and the ‘CoxProportional Hazard’ model. Again, only personal andvehicle-based factors were included.

5.7.1 Lumbar ‘Survival’Table 32 presents the ‘lumbar survival’ results, only forthose factors which were found to be statistically significantin one or the other of the two methods (bold indicates betterthan 5% significance, italics indicates 5-10%).

Where the Accelerated Failure ratios are greater thanone, the corresponding Cox ratios are less than one (andvice versa). This is because the former is looking at thetime to recover, so that, for example, suffering pain in thebuttocks is associated with recovery times 1.606 timeslonger than the recovery time of people with no buttockpain. The Cox ratio, on the other hand, indicates that the‘risk of recovery’ by a given time for people with buttockpain is less than the ‘risk’ with no buttock pain by a factorof 0.617 - i.e. people with buttock pain are less likely torecover in a given time. The two methods are thus givingthe same basic message.

For continuous variables, such as age, the relationship is(for Accelerated Failure):

Recovery time at age n = (1.015)nRecovery time at age 0

For Cox Proportional Hazard:

Risk of recovery at age n = (0.987)nRisk at age 0

Although not shown in the table, both the AcceleratedFailure and Cox models show a very strong correlationwith the first assessment score. This may, perhaps bethought of as being unsurprising but, on the other hand, itwould have been very worrying if this had not been thecase. In fact, this strong correlation provides confirmationthat the system of disability scoring is valid, to the extentthat people with higher initial disability scores really dotake longer to recover, as one would expect them to.

Age, sex, back length, prior back problems and rightside impact are all highly significant in both models, andthe indication is that older people and those with priorback problems recover more slowly, while men, those withlonger backs and those who have been in right sideimpacts recover more quickly. All these findings are inline with previous results.

Height is slightly significant under the AcceleratedFailure model only, with taller people tending to recovermore quickly. The result for height ties in with the stackedregressions, where this factor was significant in one (andonly one) of the time-interactive models.

Neither of the two measures of neck length has beensignificant previously. They are not very significant here,under either model. Greater neck length is beneficial, andcareful perusal of Table 23 indicates that this is inagreement with the trends in the magnitudes of the scoredifferences at the five assessments, although none of these

Table 32 Lumbar ‘Survival’ analysis

CoxAccelerated proportional

failure hazard

Parameter Units Ratio P-value Ratio P-value

Age years 1.018 0.010 0.985 0.031Maleness – 0.696 0.037 1.412 0.042Height cm 0.986 0.081 1.014 0.100Back length cm 0.964 0.011 1.038 0.006Neck length cm 0.950 0.094 1.049 0.076N L (flexed) cm 0.944 0.117 1.055 0.085Prior probs. – 1.804 0.001 0.550 0.002Right side impact – 0.557 0.016 1.740 0.018

28

was significant. However, it is very strange that theseparameters should be even slightly significant in relation tolumbar injury, rather than neck injury.

5.7.2 Neck ‘survival’In Table 33, sex, back length, right side impact, prior backproblems and hard structure in the lower seat back are allhighly significant in both models. The latter two aredetrimental to a speedy recovery, while being male, havinga longer back and being in a right side impact arebeneficial. All featured prominently in Table 32, apartfrom Hard Structure, which, surprisingly, has a highlysignificant effect on neck recovery, but was not asignificant factor in lumbar recovery.

non-significant in the other. Although weight has shownone or two significant results previously, its influence hasbeen rather vague, in that trends have not been consistent.The present result is again too weak to draw firmconclusions. The presence of a head restraint is again anappropriate neck-related parameter, but it is detrimental.However, this rather surprising result is in agreementwith previous findings in both the time-interactivestacked regressions (Table 30) and in the individualassessment regressions (Table 25).

5.8 Separation of vehicle models

One of the main conclusions of the Whiplash study wasthat vehicle structural design is probably very important indetermining spinal strain injury outcome, and that the largevariety of vehicle makes and models in that study mayhave masked trends in the data. In an attempt to examinethis possibility in the present data, the two commonestvehicle models in the sample (Ford Escorts and Fiestas)have been separated out and analysed individually. Thefollowing two sections give a summary of the significantresults, for personal and vehicle-based factors only (i.e.Symptoms have been omitted). The analysis has beenrestricted to simple regressions and Ordered Logits, basedon the individual assessment scores.

5.8.1. Ford EscortsThere were 30 occupants of Ford Escorts in the sample(see Table 34), of which 21 (70%) were female, comparedto 59.4% in the full sample. This overrepresentation offemales in this subset most likely arises from anoverrepresentation of women among the generalpopulation of Escort drivers, rather than a tendency forwomen to be preferentially injured when driving Escorts.Sex and age are included in this table, even though theyare not significant, because such a situation is itself quiteinteresting, given that these two factors have been found tobe highly significant in the main analyses. Otherpreviously significant factors which have not shownsignificance in this subset include:

Back Length, Height, H/rest Padding, H/rest Hor. Dist:all significant in full sample at several assessments, andin the non-time-interactive stacked regressions.

Hinged Seat Back, H/rest Present: both significant in fullsample at several assessments, and in the time-interactivestacked regressions.

Hard H/rest, Aware, Braced: all significant in fullsample in the time-interactive stacked regressions only.

Hard Structure Felt, Lumbar Support: there was nobodyin these groups in the Escort subset.

In the cases of ‘Head restraint present’ and ‘Hard headrestraint’, their failure to show significance may be relatedto the fact that this is a single-vehicle subset, so if headrestraints are fitted, they will tend to be fitted to all thevehicles, and to be all the same type. There will be few, ifany people in the ‘other’ category for comparison. In fact,there were only two Escort occupants without headrestraints - one was a rear occupant and the other hadremoved the restraint from the (front) seat.

Table 33 Neck ‘Survival’ analysis

CoxAccelerated proportional

failure hazard

Parameter Units Ratio P-value Ratio P-value

Age years 1.013 0.044 0.988 0.072Maleness - 0.680 0.014 1.563 0.007Back length cm 0.970 0.021 1.039 0.006Weight kg 0.993 0.075 1.006 0.205Prior probs. - 1.490 0.016 0.640 0.010Sports car - 0.532 0.049 1.779 0.094Rt side impact - 0.599 0.018 1.683 0.018Hinged seat back - 0.760 0.095 1.475 0.020Hard struct present - 1.491 0.014 0.666 0.016S/back padding cm 0.869 0.064 1.197 0.022H/rest present - 1.348 0.340 0.555 0.079Hit head - 0.675 0.086 1.508 0.072H/rest padding cm 0.934 0.097 1.073 0.082

Age, sports car, hinged seat back and seat back paddingthickness are all highly significant under one model, but lessso under the other. Age was also significant for lumbarrecovery, and the trend direction is the same here, withyounger people recovering sooner. The apparent beneficialeffects of being a sports car occupant are probably spurious- there were only two such people in the entire sample so,although the result is statistically significant, it cannot berelied upon. Having had a hinged seat back is beneficial forrecovery from neck injury, in agreement with previousgeneral findings. Thicker seat back padding is beneficial,but this factor has not been significant previously.Intuitively, one would have expected this factor to be moreimportant for lumbar than for neck injury.

Thicker head restraint padding and having hit one’shead on the restraint during impact are both beneficial andslightly significant under both models. Thicker paddinghas been found to be beneficial previously, in the non-timeinteractive stacked regressions. Hitting one’s head has onlybeen significant previously in the prior problem-segregatedsubgroups, where it was beneficial for those without priorback problems, but detrimental for those with. It isappropriate that both these factors should occur in theneck, rather than the lumbar survival analysis.

Finally, occupant weight and presence of a headrestraint are both slightly significant in one model, but

29

Table 34 Influence of personal and vehicle-based parameters on disability scores (Ford Escorts only)

Assessment 1 Assessment 2 Assessment 3 Assessment 4 Assessment 5No. with

parameter Base Score P- Base Score P- Base Score P- Base Score P- Base Score P-Parameter present score incr. value Sig score incr. value Sig score incr. value Sig score incr. value Sig score incr. value Sig

Maleness 9 3.905 0.540 0.496 3.143 -0.921 0.297 2.286 -1.036 0.243 1.619 -0.619 0.345 1.381 -0.381 0.559Age (yrs) 30 – -0.055 0.114 – -0.014 0.717 – 0.000 0.993 – 0.012 0.665 – 0.016 0.564Neck length (cm) 29 – 0.242 0.052 * – 0.169 0.243 – 0.017 0.909 – -0.036 0.742 – -0.032 0.768NL (flexed) (cm) 28 – 0.472 0.006 ** – 0.327 0.110 – 0.179 0.393 – 0.080 0.605 – 0.084 0.579BMI (kg/m2) 28 – 0.126 0.087 * – 0.015 0.860 – -0.039 0.646 – -0.024 0.693 – -0.016 0.799Weight (kg) 28 – 0.039 0.067 * – -0.002 0.948 – -0.015 0.537 – -0.011 0.518 – -0.008 0.644Prior back problems 10 3.850 0.650 0.398 2.400 1.400 0.097 * 1.368 1.832 0.023 ** 1.000 1.300 0.029 ** 0.789 1.411 0.016 **ETS (km/hr) 26 – 0.129 0.058 * – 0.175 0.015 ** – 0.166 0.021 ** – 0.100 0.063 * – 0.124 0.018 **Energy-absorbing bumper 6 4.125 -0.292 0.749 2.625 1.208 0.230 1.652 1.681 0.081 * 1.261 0.906 0.208 1.087 0.913 0.199Frontal impact 5 3.760 1.840 0.052 * 2.520 2.080 0.049 ** 1.750 1.450 0.165 1.375 0.425 0.586 1.167 0.663 0.411Hard struct. in st bk 14 4.750 -1.464 0.037 ** 3.062 -0.420 0.607 1.875 0.279 0.730 1.188 0.582 0.323 1.125 0.337 0.566S/back padding (cm) 14 – 0.177 0.735 – -0.569 0.279 – -0.833 0.105 – -1.000 0.016 ** – -0.917 0.027 **Leaning forward 2 3.632 2.868 0.054 * 2.864 1.316 0.368 1.611 1.889 0.141 1.167 1.833 0.093 * 0.944 2.056 0.056 *H/rest vert dist. (cm) 14 – -0.337 0.176 – -0.478 0.017 ** – -0.389 0.054 * – -0.318 0.070 * – -0.290 0.129

* Significant at <10%** Significant at <5%Bold O-logit gives a change in significance level

30

Turning to the factors which were significant in theEscort subset, neither body mass index, nor either of thetwo measures of neck length have been significantpreviously. They are significant here only at the firstassessment. O-logit analysis indicated that flexed necklength was also significant at Assessment 2 (p=0.047), butthat BMI was non-significant at Assessment 1 (p=0.112).Straight neck length and BMI both show inconsistent trenddirections over the five assessments, as does weight, whichis also only (slightly) significant at the first assessment. Inthe full sample, weight was (slightly) significant atAssessment 2 only.

Prior back problems are consistently detrimental in thissubset, and significantly so from Assessment 2 on. TheO-logit analysis indicated that this factor was non-significantat Assessment 2 (p=0.206), but it was in agreement onAssessments 3, 4 and 5. This closely matches the situation inthe full sample.

Higher Equivalent Test Speed is significantly associatedwith increased disability at all assessments in this subset.O-logit analysis only disagrees on the degree ofsignificance (p=0.038 at Assessment 1, p=0.060 atAssessment 3), This is in contrast to the full sample, whereETS was only slightly significant at Assessments 4 and 5(only Assessment 5 confirmed by O-logit), with only twoslightly significant non-time-interactive stacked regressionresults. ETS was more significant in the overall femalesubgroup, which may partly explain its strong showinghere, but it is more likely that it is the restriction of thesample to one vehicle model which has brought out theeffect of impact speed.

Energy absorbing bumper was slightly significant atAssessment 3 in this subset, although this was notconfirmed by the O-logit analysis (p=0.209). Previously, itwas slightly significant at Assessments 4 and 5 (althoughagain, this was not backed up by the O-logit results), andin two of the time-interactive models.

Those who had been involved in a frontal impact hadsignificantly higher disability scores at Assessments 2and 3. Although this trend was maintained at the otherassessments, it was non-significant. O-logit analysisimproved the significance level at Assessment 1 (p=0.026),but failed to confirm the Assessment 2 result (p=0.113).This is the first time that frontal impacts have shownsignificance anywhere.

Seat back padding thickness has also not beensignificant previously; it is significant here only at thefourth and fifth assessments, with inconsistent trenddirections over the other assessments possibly indicatingsome time dependence. The presence of 14 seats with hardstructure present in this single-vehicle subset must be dueto different amounts of wear in the overlying padding,allowing the structure to be felt at the vehicle examinationon some vehicles but not others. The presence of hardstructure is significant at Assessment 1 only, butinconsistent trend directions again indicate the possibilityof some time dependence. Previously, this factor achievedone significant result in one of the time-interactive stackedregression models (not the best one), and it was alsosignificant among females (rather than males) and among

those with prior back problems, (rather than thosewithout). The high proportion of females in the Escortsubset may account for its appearance here; the proportionof prior back problem cases in the Escort subset wassimilar to that in the overall sample.

Leaning forward at the time of impact was consistentlydetrimental in this subset, significantly so at Assessments1, 4 and 5. O-logit improved the significance level atAssessment 1 (p=0.026), but failed to confirm theAssessment 4 result (p=0.121). There are only twoprevious significant results for this factor (Tables 29 and 31).There were only two people in this group in the Escortsubset, so the results are likely to be unreliable.

Greater vertical distance of the head restraint below theears was consistently beneficial, very significantly so atAssessment 2, rather less significantly at Assessments 3and 4. There is some indication that this beneficial effectbecomes less marked as time goes on (after Assessment 2),so that large vertical distance is associated with lowerdisability, but slower recovery. This would tie in with theonly significant result for this factor in the full sample,which was in one of the time-interactive stackedregressions, and also with the related finding that thisslower recovery is primarily associated with females.

Ordered logit results

Generally, the O-logit calculations confirmed the findingsof the normal regressions. Exceptions have already beendetailed in the text above.

5.8.2 Ford FiestasThere were 23 occupants of Ford Fiestas in the sample (seeTable 35). The proportion of females here was evenhigher, at 74%, but again, it is probable that drivers ofFiestas generally are more likely to be women than men,so that this finding says nothing about the relativesusceptibility to injury of female Fiesta drivers. Previouslysignificant factors which have not shown significance inthis subset include:

Height, H/rest Padding: both significant in full sampleat several assessments, and in the non-time-interactivestacked regressions.

Energy-absorbing Bumper, H/rest Present, HardStructure Felt: all significant in full sample at severalassessments, and in the time-interactive stackedregressions.

Hard H/rest, H/rest Vert. Dist, Aware, Braced, HardStruct. Present: all significant in full sample in thetime-interactive stacked regressions only.

Lumbar Support: nobody in this subset had a lumbarsupport.

Only one person felt hard structure in the seat backduring impact and, although this resulted in much higherdisability than the average, the difference was notsignificant.

Although sex is significant in the Fiesta subset, this onlyoccurs at two assessments, and one of those is onlysignificant at between 5 and 10%. Males have lower

31

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disability at all five assessments, and the lack ofsignificance may be due to the relatively small number ofmales in this subset. In addition, all the males hadcompletely recovered by Assessment 3 (as indicated by theScore Increment being equal and opposite to the BaseScore). As the average female score continues to fall, thedifference between the two groups necessarily dwindles.

After Assessment 2, there are consistent and significanttrends towards higher disability scores in older subjects,although at Assessments 1 and 2, age appears to have verylittle effect. O-logit analysis reduced the significance level atAssessment 3 (p=0.057), but confirmed the results atAssessments 4 and 5. There is no indication of any timedependency in the significant results, in contrast to the fullsample, where the stacked regressions indicated a significanttendency for older people to get better more slowly.

Greater back length is consistently beneficial (inagreement with the full sample results), though again, thisis only significant after the second assessment, and again,O-logit analysis reduced the significance at Assessment 3(p=0.058), but confirmed the fourth and fifth assessmentresults. There is some indication of time dependency, withthe score increments tending to get bigger, on the whole,as time passes. Back length was beneficial in the non-time-interactive stacked regressions, but there was a definitetime interactive effect in the female subgroup. Sincefemales are overrepresented in this Fiesta subset, this mayaccount for the trends observed here.

The result for Prior Back Problems is interesting - theirpresence at the first assessment is actually beneficial, andsignificantly so. They only become detrimental afterAssessment 2, but none of these later results is significant.However, this time dependency (prior back problemsbeing associated with slower recovery) is in agreementwith the stacked regression results for the full sample.

There is one slightly significant result for engine size, atAssessment 2, indicating that larger engines are beneficial.This factor has not shown significance in any of theprevious analyses, and it is unlikely that any generalconclusions can be drawn from its appearance here.

Apart from Assessment 2, higher ETS is consistentlybeneficial, significantly so at Assessment 3, rather lesssignificantly at Assessments 4 and 5. The results wereconfirmed in the O-logit analyses, but they are unexpected,and fly in the face of all previous results. They are mostlikely to be an artefact, introduced by the drasticallyreduced sample size in this subset.

People involved in rear impacts had lower averagedisabilities than others at all assessments except thesecond. This was slightly significant at Assessments 1 and4, although the latter was not confirmed by the O-logitanalysis (p=0.182). Rear impacts have not been significantpreviously, and their appearance here is too weak to justifyany general conclusions. Left side impacts, on the otherhand, were detrimental, and very significantly so atAssessments 1, 3 and 4. O-logit confirmed these and alsofound Assessment 5 to be significant (p=0.021). The scoreincrements reduce in size as time goes on, indicating apossible time-interactive effect. In Table 28, left sideimpacts produced much higher average disabilities than

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the other impact directions, but this was only slightlysignificant. This factor was also significant in one of thetime-interactive stacked regression models, but it wasbeneficial, in that these people recovered more rapidlyfrom their high initial disability scores. This beneficialeffect was traced to the female subgroup, and the presenttrend may be linked to the high proportion of women inthe Fiesta subset. However, there were only three people inthis group, so the result may not be very reliable.

Seat height difference (ie the difference between theheight of the seat back and the occupant’s seated height)was significant at Assessments 3, 4 and 5. Since, in asingle-vehicle subset, all the seats are likely to be ofsimilar height, this result is essentially a reflection of thetrend already seen for back length.

Horizontal distance between head and restraint yieldsone significant result, at Assessment 2, indicating thatgreater distance is detrimental. However, this trenddirection reverses at Assessments 4 and 5. This is incontrast to the full sample, where this factor was not foundto be time-interactive, but was detrimental at allassessments. Furthermore, this factor did not previouslyperform well under the (preferable) O-logit models, andalthough the present result was confirmed by O-logit, theoverall picture can not be said to be conclusive.

Finally, a hinged seat back was beneficial at allassessments, significantly so at Assessment 4, lesssignificantly at Assessment 5. However, O-logit failed toconfirm either of these results (p=0.138 at Assessment 4,p=0.189 at Assessment 5). The trend in the magnitude ofthe Score Differences indicates a time interactivebehaviour which is in agreement with that found in the fullsample stacked regressions, although this factor was onlysignificant under Model 3, with no improvement when thesample was split into gender and prior problem subgroups.

5.8.3 Discussion of separate make/model resultsThis exercise was carried out in an attempt to eliminatesome of the variability in the sample, due to variations inmass, stiffness and seat design between different makesand models. It was hoped that this might allow trends toshow through which had been masked in the full sample.The significant correlations with ETS in the Escort subset,indicating that increased ETS is detrimental, thereforeappear at first sight to vindicate this approach. ETS wouldbe expected to be correlated with injury severity in thisway, and its insignificance in the full sample wasdisappointing. However, in the Fiesta subset, severalsignificant results indicated that increased impact speedwas beneficial. It is unlikely that changes in speed wouldhave opposite effects on different vehicles, so theconclusion must be that selecting out these single modelsreduced the sample size to such an extent that quirks in thedata had an undue effect. This was always a possibility,and it casts doubt on all the other results obtained forEscorts and Fiestas separately. Something useful mightstill have been gained if a particular personal characteristicor aspect of seat design or adjustment had shown strongtrends in both the subsets considered, but this was not thecase - the only parameters which are common to both

Tables 34 and 35 are prior back problems (which havealready appeared in the full sample) and ETS. In summary,this attempt to clarify the data by separating out particularmakes and models of vehicles has proved unsuccessful,due to the reduction in sample size involved.

6 Conclusions

6.1 Conclusions from accident analysis

1 Women were confirmed to be more at risk of neckinjury than men; similar trends were observed for thethoracic and lumbar regions.

2 Rear impact appeared to carry a higher risk of AIS 1spine injury in all three spine regions, although frontalimpacts produced greater absolute numbers of injuries.

3 There was a slight trend towards females being morelikely to receive spine injuries in lower severity impacts.

4 Many of these low-severity spine injuries were found tobe non-contact injuries, or to involve indirect loading ofthe spine. Head impacts were quite common in neckinjury cases.

6.2 Conclusions from lumbar injury/vehicle study

1 It was not possible to include uninjured people in thisstudy. This may have an effect on the results obtained.

2 Two years after their accidents, more than a quarter ofsubjects were still having difficulties lifting and carryingobjects. Running a short distance was even moredifficult, but for a slightly smaller number of people.More than 15% of subjects were still experiencingdifficulty with such everyday activities as shampooinghair, cutting toe-nails and getting in and out of bed.

3 Difficulties in micturition, weakness and pins andneedles in the legs, and radiated pain in the thighs andbuttocks were significantly associated with high initialdisability in this sample. Buttock and interscapular painand, especially, difficulty in micturition, were verysignificantly correlated with long-term disability.

4 More than three quarters of the subjects reportedpsychological problems in relation to confidence on theroad. There was no indication that drivers were affectedmore than passengers in this respect, and the numbersaffected decreased to 20% of the sample after 24months. As might be expected, there was a very highcorrelation between continuing nervousness andcontinuing disability

5 Having had a previous back problem was not aparticularly good predictor of initial disability score, butit was very significantly correlated with long-termdisability and with increased recovery times. This seemsto indicate that, even after apparent recovery from oneinjury, the ability of the back to recover from asubsequent injury can be compromised.

6 Most analyses showed that women sufferedsignificantly higher disability scores than men. Theyalso took longer to recover, but this was probably due

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to starting from a higher level, rather than anydifference in recuperative powers.

7 There was a significant trend towards reduced initialdisability in older subjects. However, all methods ofanalysis indicated strongly that older people recoveredmore slowly, so that, from six months on, they hadsignificantly higher disability scores.

8 Longer subject back length was beneficial, regardless ofgender or prior back problem status - i.e. it was notlinked to the fact that men, on average, had longerbacks than women. However, this association all butdisappeared when seat back height was taken intoaccount, with confusing and opposing trends amongmales and females. Thus, it was absolute back lengththat was important, not back length in relation to seatback height. An apparent association between disabilityand overall height was found to be probably due to thedifference in height between the sexes.

9 Body mass index failed to show any association withdisability, apart from an inconclusive result in one ofthe single vehicle subsets, which was not confirmed bythe (preferable) Ordered Logit technique. Two isolatedsignificant results for weight were insufficient to allowconclusions to be drawn. The trends were, in any case,contradicted in one of the single vehicle subsets. Thetwo measures of neck length yielded one or two slightlysignificant but contradictory and inconclusive results,while neck circumference was not significant at all.

10 Being braced for the impending impact was detrimentalin the full sample, and especially so for those with priorback problems.

11 Being involved in a right side impact was beneficial in anumber of models. If this had been dependent on seatposition within the vehicle, it might have beenexplicable, but the trend was the same for both driversand passengers, so it is rather difficult to account for.

12 Very few vehicle-based parameters had any significantcorrelation with disability scores. There were a fewsignificant results for factors with very low numbers ofcases (ie hard structure in seat back felt at impact, sportscar, leaning forward at time of impact), and these can bediscounted as being unreliable. In addition, there weresome factors with larger numbers of cases, but whichonly produced one or two isolated significant results,which can also be discounted as being inconclusive.These were: engine capacity, seat back paddingthickness, energy-absorbing bumper, hinged seat base,adjustable head restraint, lumbar support, aware ofimpending impact, saloon car, rear impact, frontalimpact, left side impact. Impact speed, as measured bythe Equivalent Test Speed, produced a few significantresults in the overall sample, but the trends for malesand females were in opposite directions.

13 Separation of particular makes and models of vehiclesin an attempt to reduce the variability in the data wasunsuccessful, due to the drastic reduction in sample sizeinvolved.

14 The presence of a head restraint was consistentlydetrimental (with regard to the AIS1 strain-type injuriesunder consideration), especially in the female subgroup.

There were only 16 people without a restraint, and mostof these were women. This may explain why a similartrend among men was not significant - there were veryfew men without a restraint for comparison. The trendamong women could not be explained by the fact thatfive of the 13 women without head restraints were rearoccupants - among female front seat occupants, theeight without head restraints had significantly lowerdisability scores than the 117 who had head restraints.This comparison group of eight is rather small, butstatistical testing is designed to take account of this. Inaddition, we were unable to take account of differencesin seat design and vehicle structural characteristics,which may also have influenced the results, and theresults themselves are based on a combination of neckand lumbar strain injuries, of which the latter areintuitively more likely to be influenced by seat backrestcharacteristics than by the presence or absence of a headrestraint. Since there is evidence, and an understandablemechanism, for head restraints mitigating serious neckinjuries, it would probably be unwise to abandon theuse of these items even if they were proved to bedetrimental in terms of AIS1 neck strains.

15 Hard head restraints were beneficial, although with onlyten in this category, the result may not be very reliable.Where the head restraint had soft padding, thickerpadding (in a horizontal direction) was consistentlybeneficial in a number of different models. It was alsoassociated with shorter recovery times. This seems tocontradict the result for hard head restraints, but sincethe presence of a soft or hard restraint will tend to bedetermined by the particular model of vehicle involved,it is possible that it is some other correlated aspect ofseat or vehicle design which is influencing this result.Greater horizontal distance between head and restraintwas detrimental in a number of models, but these tendedto be the simple regression models, rather than the morereliable Ordered Logit models. Vertical distancebetween head and restraint was significant in only onemodel for the overall sample; a rather inconclusiveresult, indicating that greater distance is detrimental.Finally, among those who remembered hitting theirhead on the head restraint during impact, this contactwas beneficial for those who did not have prior backproblems, but detrimental for those who did. Overall,despite nearly all the sample having neck strain injuries,no particular aspect of head restraint construction oradjustment had any conclusive effect on injury severity.This may be because virtually all these people also hadlumbar strain injuries which may have masked anycorrelation between overall disability and head restraintparameters.

16 A hinged seat back was beneficial, but mainly in thesimple regression models, rather than the OrderedLogits. This effect was confirmed among those with noprior back problems, but those with prior problemstended to find a hinged seat back detrimental. Thisfactor was also significant for recovery from neck injury(as opposed to lumbar injury). It is possible that theseseat backs are weaker, and do not produce as much

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post-impact rebound as rigid seats (rebound is generallyaccepted to be undesirable as regards neck straininjuries). Hard structure at the base of the seat back wasvery detrimental among those with prior back problemsand among women, regardless of whether theyremembered contacting this structure during the impact.Strangely, hard structure was detrimental for the fullsample in terms of recovery from neck injury, but wasnot significant in terms of lumbar injury.

Generally, it should be borne in mind that statisticaltesting only gives the probability that a result could beobtained by chance. Five percent significance simplymeans that the result could occur by chance in only fivepercent of cases. It follows that, if 20 statistical tests arecarried out on a random sample, one of them is very likelyto indicate 5% significance. Many of the significant resultsin the present analysis have been inconsistent with resultsfor other, related factors, or with the same factor at otherassessments, or under other models, and it is likely thatthese are simply ‘random significances’. Only factorswhich showed consistent effects across a number ofdifferent models should be accepted as having any realinfluence. These included sex (being male is beneficial),age (being younger is beneficial), prior back problems(lacking prior problems is beneficial), back length (longeris beneficial), head restraint padding thickness (thicker isbeneficial) and head restraint present (restraint absent isbeneficial). This latter result should perhaps be viewedwith caution, because of the relatively small number ofpeople without restraints, although it was significant in anumber of different models.

It is regrettable that this project has not yielded anymore conclusive results, particularly as regards the effectof vehicle-related factors on disability outcome. It is likelythat there were simply too many uncontrolled variables.The large number of different vehicle makes and models inthe sample was a major factor here, and this was why theoriginal intention was to recruit occupants of 350 vehicles,these vehicles all less than seven years old. Any futurestudy which attempted to repeat this would either have toallow a much longer recruitment period or would have torecruit from several different hospitals. Restrictingrecruitment to one particular make/model would alsorequire much longer recruitment periods or multi-hospitalsampling, and any results could only be confidentlyapplied to the model chosen. Ideally, one would like to beable to look at the effects of, for example, a range ofdifferent seats in one vehicle model, but manufacturerstend to limit the options on seats merely to the upholstery.

Another imponderable is the susceptibility to injury ofindividuals in the population. Macroscopic propertiessuch as age, sex, height and weight can be measured, andwere allowed for in the calculations, but even outwardlysimilar individuals can vary considerably in theirsusceptibility, especially to the soft tissue injuries underconsideration. Ideally, one would like to be able to carryout repeated tests on the same individual but, especiallybearing in mind the likely long-term consequences of theinjuries under investigation, such tests would be veryhard to justify ethically.

Finally, a better appreciation of the impact experience ofthe victim would be beneficial. ETS, as calculated by theCRASH3 program, is a fairly crude measure of impactseverity. Routine fitment of simple crash recorders to newvehicles is being undertaken in Sweden, and mayeventually happen in the UK. Access to accurateinformation on impact acceleration and ∆v would be veryuseful in attempting to correlate impact severity withinjuries received.

7 Acknowledgements

The authors wish to thank Messrs H Gregory and J Moss,of the Manchester area Vehicle Inspectorate, who carriedout the vehicle examinations for this study.

8 References

Carlsson G, Nilsson S, Nilsson-Ehle A, Norin H,Ysander L and Örtengren R (1985). Biomechanicalconsiderations to improve head restraints. Proc. 1985IRCOBI Conf.

Clinical Standards Advisory Group (1994). Back pain.London: The Stationery Office.

Cullen E, Stabler K, Mackay G M and Parkin S (1996).Head restraint positioning and occupant safety in rearimpacts: the case for smart restraints. Dublin: Proc. 1996IRCOBI Conf.

Hassan A M, Parkin S and Mackay M (1996). Spinalinjuries in car collisions. Vancouver: Proc. 40th AAAMConf.

Kapandji I A (1974). The physiology of the joints (Vol 3:The Trunk and Vertebral Column). Edinburgh: ChurchillLivingstone.

King A I (1993). Injury to the thoraco-lumbar spine andpelvis. In, Nahum & Melvin: Accidental Injury:Biomechanics and Prevention. London: Springer-Verlag.

Minton R, Murray P A and Stephenson W (1997).Lower back injuries resulting from road accidents: InterimProgress Report. Unpublished Report PR/SE/278/97.Crowthorne: TRL Limited. (Unpublished report availableon direct personal application only).

Minton R, Galasko C S B, Murray P A and Pitcher M(1998). Whiplash injuries, causative studies: final report.TRL Report TRL257. Crowthorne: TRL Limited.

Morris A P and Thomas P (1996). A study of soft tissueneck injuries in the UK. Melbourne: Proc. 15th ESV Conf.

National Highway Traffic Safety Administration(1982). CRASH3 User’s Guide and Technical Manual.Washington DC: US Dept of Transportation.

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Parkin S, Mackay G M and Cooper A (1994). Howdrivers sit in cars. Munich: Proc. 14th ESV Conf.

Rowlinson A (1997). Inside story - sitting comfortably?Auto Express, 4th June, 1997.

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Abstract

A sample of 219 road accident victims suffering from lower back strain injuries were studied over a three yearperiod. The vehicles they had been travelling in were examined to assess the impact severity and, where possible,measurements were made of seat and head restraint adjustment with the subject sitting in the vehicle. Each subjectwas interviewed to assess the disability resulting from their injuries, and their progress was followed for two yearspost-accident. It proved impossible to recruit a sample consisting purely of lower back injury cases - 95% of thesample also had neck strain injuries. Several personal factors (age, sex, back length and experience of previous backproblems) were found to influence injury severity and/or rate of recovery from injury. Very few vehicle-relatedfactors were found to have any consistent effect; those that were are discussed. Separate analysis of particularmodels of vehicles failed to clarify the picture, probably because of the drastic reduction in sample size involved.Recommendations are given for the conduct of any future, similar studies.

Related publications

TRL272 National hospital study of road accident casualties by H F Simpson. 1998 (price £35, code H)

TRL257 Whiplash injuries, causative studies: final report by R Minton, C S B Galasko, P A Murray andM Pitcher. 1998 (price £35, code H)

RR136 An in-depth study of road accident casualties and their injury patterns by R J Tunbridge, J Everest,B R Wild and R A Johnstone. 1988 (price £20, code B)

CT97.1 Injuries from traffic accidents update (1998-2000) Current Topics in Transport: selected abstractsfrom TRL Library’s database (price £20)

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