a case-control study of biomechanical and psychosocial risk factors for occupational low-back pain

Institute for Work & Health, 20018

A Case-Control Study of Biomechanical and Psychosocial

Risk Factors for Occupational Low-Back Pain

Michael S. Kerr, PhDInstitute for Work & HealthToronto, Ontario, CANADA

PREMUS 2001, Amsterdam

Kerr et al. (2001) Am J Pub Health 91:1069-1075


Study Collaborators

Institute for Work & Health:Kerr MS, Frank JW, Shannon HS, Bombardier C

University of Waterloo:Norman RW, Wells RW, Neumann PGeneral Motors: Mr. Elmer BeddomeCAW: Mr. John Graham

The Ontario Universities Back Pain Study (OUBPS) Group

Andrews D, Beaton DE, Dobbyn M, Edmonstone E, Ferrier S, Hogg-Johnson S, Ingelman P, Mondlock M, Peloso P, Smith J, Stanfield SA, Tarasuk V, Woo H


Why a case-control study?

• Uncertainty about role of physical demands necessitated concentration of effort on developing valid workplace measures

• Cohort model required multiple measures at multiple times to assess valid exposure

Does a prospective cohort study with limited (e.g. once only) exposure assessment really provide more rigorous evidence than a case-control study with more detailed assessments?


Research Question

After controlling for individual characteristics, what are the main work-related biomechanical and psychosocial risk factors for reported low-back pain?


STUDY SETTING

GM-Oshawa autoplex65 km east of TorontoTotal hourly-paid work force – approx. 12,0003 Divisions: Car Assembly (7,000)

Truck Assembly (3,000)Fabrications N/A

Nursing stations – handle ALL WCB reports as well as most other health problems occurring at work-site Truck = 1 station; Car = 4 stations


The GM Study of Low-back Pain

baseline questionnaire and

physical exam

10,000 car and truckplant workers (hourly-paid)

random

selection(job-matched)

accrued viaworkplace health

stations

conducted

at home

physical demands assessment

CASES

workers without

LBP

STUDY POPULATION

workers with LBP

CONTROLS

Incidence density sampling

at work doingusual job

video, EMG, checklists, posture

(n=137)

(n=179)(n2=65)


SUBJECT ENROLMENT

(Voluntary)

179

137

65

Random controls

Cases

A total of 381 subjects enrolled

in the study

Job-matched controls(used for proxy data)


CASE Definition

• Full-time, hourly-paid worker with “sprain-strain” LBP (reported to nursing stations)

• No lost-time or WCB claim requirement

• No previous worksite LBP report (90 days)


SF-36, Health-related quality of life

PhysicalFunction

Role –Physical

BodilyPain

SocialFunction

Vitality Role –Emotional

MentalHealth

GeneralHealth

Were cases and controls comparable except for LBP?

sd f

rom

mal

e re

fere

nce

popu

latio

n

Ref: Garrat et al. (1993) BMJ 306:1440-4


INDIVIDUAL Characteristics

Age, height, weight, body mass index, sex education, marital status, preschool children, main wage earner, non-occupational physical activity, smoking, alcohol consumption

STUDY VARIABLES - 1


BIOMECHANICAL Factors

• Peak Forces:compression; shear; hand force

• Cumulative Forces:average and integrated compression;

• Low-level (static) Forces:compression

• Posture and Movement:peak flexion; time non-neutral; trunk kinematics

(reduced a priori to about 20 key variables, NO EMG)

STUDY VARIABLES - 2


STUDY VARIABLES - 3

PSYCHOPHYSICAL Factors

self-rated physical exertion measures

distinct from "objective" measurements of physical demands analyses, but may also have additional role to play over and above the measured demands

e.g. tolerance levels, "job stress", etc.


• Karasek-Theorell Job Content Instrument

• "psychological demand", decision latitude, supervisor support, coworker support, workplace social environment, job self-identity

• - job dissatisfaction, mastery, empowerment, status inconsistency ("over-education")

STUDY VARIABLES - 4

PSYCHOSOCIAL Factors


Summary of Key Risk Factors

0

1

2

3

Peak Shear

Peak hand force

Disc Compression

Series 4

Self-rated demands

Series 6

Poor Social Environment

Over-education

High Co-worker support

High Job Satisfaction

Low Job Control

PsychosocialBiomechanical

Odds Ratio

(Cumulative)

(Exertion)

(* p=0.07)

(p<0.05)

(after adjusting for individual characteristics)


Study Strengths

• Directly measured physical demands data combined with (basic) physical exam and interview-assisted psychosocial data for individual subjects

• Comprehensive workplace job demands assessments (generalizable – i.e. not specific to automobile manufacturing)

• Well defined study base for subject selection


Study Weaknesses

• Modest participation rate (approx 60%)• Relied on workplace reporting

mechanism to identify cases• Self-report only for psychosocial factors• Case-control design

But … does a prospective cohort study with limited once only exposure assessment really provide more rigorous evidence than a case-control study with more detailed assessments?


How did we control for Bias?

• Directly measured physical demands data rather than self-report

• Job-matched controls used to examine potential recall bias (none observed)

• Used newly incident cases rather than prevalent cases

• Compared cases with non-participating compensation claimants (no differences)

• A priori reduction in variables eligible for regression modeling (multi-methods)


Possible Biological Mechanisms

HORMONESe.g. cortisol

MUSCLETENSION

PSYCHOSOCIAL LOAD

INDIRECT EFFECTS

PSYCHOSOCIAL LOADe.g. job control

BIOMECHANICAL LOAD

e.g. cumulativecompression

DIRECT EFFECTS

BIOMECHANICAL LOAD

TOLERANCEREDUCED

TOLERANCEEXCEEDED

TISSUE RESPONSE


Main Conclusions

• Consistency and strength indicates biomechanical load increases LBP risk

• Psychosocial factors also shown to be associated with reporting LBP

• Job dissatisfaction not a risk factor for reporting LBP in this setting

• Psychosocial instruments (especially Karasek’s demands scale) may require further refinement for MSK studies


For additional information please contact:

Mickey KerrThe Institute for Work & Health

481 University Ave., Suite 800Toronto, ON M5G 2E9Phone: (416) 927-2027

Fax: (416)-927-4167Website: http://www.iwh.on.ca

E-mail: [email protected]

The Institute for Work & Health operates with the support of the Ontario Workplace Safety & Insurance Board

Kerr et al. (2001) Am J Pub Health 91:1069-1075

a case-control study of biomechanical and psychosocial risk factors for occupational low-back pain

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