topics & research areas in biomechanics

FORENSIC BIOMECHANICS IN THE COURT OF LAW

Ali Erkan Engin, Ph.D., FASME, FSDPS(Professor Emeritus, The Ohio State Univ.)

Professor of Biomechanics & Mechanics Mechanical Engineering Department

University of South AlabamaMobile, AL 36688 USA

June 29, 2007

TOPICS & RESEARCH AREAS TOPICS & RESEARCH AREAS IN BIOMECHANICSIN BIOMECHANICS

• Basic Mechanical Properties of Biological Materials

• Analyses of Response to Internal Biological Forces

• Analyses of Response to External Forces

• Analyses of Response to Replaced Parts and Assistive Devices

BASIC MECHANICAL PROPERTIES OF BIOLOGICAL

MATERIALS

• Individual Cells

• Various Tissues

• Organs and Complex Body Systems

ANALYSES OF RESPONSE TO INTERNAL, BIOLOGICAL FORCES

• Circulation and Microcirculation

• Respiration

• Locomotion Kinetics in Normal, Abnormal, and Amputee Gait

ANALYSES OF RESPONSE TO EXTERNAL FORCES

• Steady-State and Transient Pressure and Sound Applications

• Various Acceleration Environments a) Body Vibration b) Impact and Crash Protection (head,

neck, chest and abdominal injury) c) Hypo and Hypergravity Conditions• The Diagnostic and Therapeutic Sound and

Force Applications.

ANALYSES OF RESPONSE TO REPLACED PARTS AND

ASSISTIVE DEVICES

• External Orthoses/Prostheses

• Internal Orthoses/Prostheses

• Biomechanical Compatibility of previous two items.

Forensic Biomechanics can be defined as the

application of biomechanics in the

court of law.

Forensic biomechanics cases may be put into the following categories:1. Motor vehicle accidents and related injury

cases (single and multiple vehicles involving single and multiple vehicle occupants and/or pedestrians),

2. Occupation related accidents and injury cases,

3. Product failure and related injury cases,

4. Sports and recreation related accident and injury cases,

5. Slip and fall accidents and related cases.

OFFICE CHAIR CASE

This case is about a 41 year old female office worker who was injured when she fell backward from the chair to the floor after the back support (backrest) broke. At the time she was moving the chair away from the desk to open her desk drawer.

MEDICAL SUMMARY After the fall she complained of severe back pain. Initial diagnosis was a sprain and irritation of one of the lumbar discs causing mild sciatica on the left, and physiotherapy was recommended. Pain continued; several months later, her myelogram showed a severe spinal stenosis L3-4 and L4-5. Subsequently, she had to have decompressive laminectomy L2-L5 lumbar surgery. Eventually, she was found to be totally disabled.

COURT TESTIMONY

1. Why the back support failed

2. What the manufacturer could (or should) have done

3. Biomechanics of the injury

For the safe operation of the chair the back support stem, i.e., the strut which holds the backrest, must be held 100% by the support bracket. To ensure this, the manufacturer provided a plastic plug to be inserted at the end of the strut.

In the event that the insertion length is less than 50% of the normal insertion length, a catastrophic failure of the support bracket will occur which is what happened in this case.

What the Manufacturer Could Have Done To ensure the safe operation of the backrest, the manufacturer had at least two options: 

1. Ship the chairs in a fully-assembled state with a final check in the assembly line, making sure that the plastic plug is inserted.

or2. If the backrest is shipped unassembled, make

sure that the plastic plugs are inserted in the assembly line (i.e. separate packaging of the plastic plugs is not acceptable). Under this scheme a warning sticker should be placed next to the plug.

Warning Sticker

Plastic Plug Back support stem

WARNINGRemove the plastic plug in order to insert the stem into the support bracket. Insert the plastic plug back into its hole. FAILURE OF NOT INSERTING THE PLASTIC PLUG CAN RESULT IN THE BREAK-DOWN OF THE SUPPORT BRACKET AND SERIOUS INJURY!

Biomechanics of Injury

1. Angular velocity of her torso when she hit the floor

2. Her impact velocity with the floor3. Magnitude of the force at the lower

back just prior to floor impact4. Magnitude of the maximum shear

force at the lower back during floor impact

I explained to the jury that during the fall and impact with the floor the lower back was subjected to a very complex and severe dynamic loads. In particular:

a) Her lumbar region was subjected to hyperextension

b) Under this hyperextension intervertebral discs opened up anteriorly and closed down posteriorly, meanwhile, the facets were highly loaded in a compressive manner,

c) At the same time the lumbar spine, the functional spinal units were subjected to a dynamic shear loading which was much higher than safe load levels that can be sustained without injury.

APPLE BRUSHING MACHINE CASE

APPLE BRUSHING MACHINE CASE The machine is electrically powered and is approximately 9 ½ feet long, 3 feet wide and 5 ½ feet high. Apples are introduced into the machine through an opening in the top of one end.

The apples are tumbled and brushed as they move along toward the outlet opening at the opposite end.

The apples are brushed by six rollers, each about 5” in diameter and made with very stiff close to 1” long bristles.

When viewed from the outlet end, five of the rollers rotate in a counterclockwise direction and the top left roller rotates clockwise. This top left roller and the one adjacent to it (rollers 1 & 2) form an in-running nip point on the machine which is the mechanical equivalent of a very fast acting quicksand.

Examples of in-running nip points – protection against such hazards is of prime

importance

The center of the outlet opening is only 1.5 feet away from the side of the machine and it’s about 4 feet above the floor where the machine is installed. Due to the machine’s dimensions, it is very easy to extend a hand and come into contact with the dangerous continuous in-running nip point along the longitudinal axes of rollers 1 and 2.

OBSERVATIONS 1. In order for this machine to

perform its intended function, rotating brushes and the resulting danger points appear to be unavoidable design feature.

2. Safe engineering design practice requires appropriate shield and guarding when the hazard cannot be eliminated thorough design.

3. The National Safety Council, in their publication on machine safeguarding, states (in part):

“The purpose of machine safeguarding is to minimize the risk of accidents of machine-operator contact. The contact can be either: 1. An individual making the contact with the machine – usually the moving part – because of inattention caused by fatigue, distraction, curiosity, or deliberate chance taking.”

4. OSHA regulations require machine safeguarding to protect operators and others in the area from hazards such as those created by point of operation hazards such as in-running nip points, rotating parts, etc.

5. Safeguarding provided by the equipment manufacturer is usually better suited to the design and operation of the machine than those purchased and installed by the user.

OPINIONS 1. Manufacturer should have known of the

dangers to operators and other users of the in-running nip point created by the counter rotating brushes in the subject apple brushing machine.

2. The design of this machine is defective in that appropriate safeguarding was not provided to keep operators and other users from entering the machine while in operation and thereby being exposed to the hazards of the in-running nip points.

3. The design of the machine is defective in that the machine lacked clear and legible warnings concerning the dangers of the in-running nip points of the brush rollers. Manufacturer’s failure to provide these warnings deprived the operator of necessary information concerning his personal safety.

4. Further, the design of the machine is also defective since the defendant manufacturer did not provide an emergency shutoff switch at the outlet end of the machine or instructions directing that an emergency shutoff switch be installed in the proximity of the outlet end.

ATV - HELMET CASE

DESCRIPTION OF THE ACCIDENT

This accident involved two teenagers (driver and passenger) who were riding on an all terrain vehicle (henceforth ATV). According to testimonial evidence both boys were wearing helmets when they were riding the ATV. As the ATV proceeded forward on a dirt and gravel road which had a slight downgrade and entered a moderate left hand curve, the driver was unable to negotiate the curve and was unable to

bring the ATV back onto the path.

The ATV's solid rear axle design combined with narrow track width, short wheel base, and high center of gravity made the machine inherently unstable and difficult to steer. This inherent instability and difficulty in steering was a substantial factor causing problem for the driver of the ATV to properly steer and negotiate the curve.

According to the testimony of the driver, the ATV was traveling at 15 to 20 mph (24.1 - 32.2 km/hr) when the vehicle veered out of control off the pathway. The ATV traveled approximately 200 feet (61 m) down a grassy, unimproved terrain, and eventually stopped by striking a tree. The investigating officer identified three segments of the path the ATV took; the first one was 75 feet (22.9m) from the road to the grassy area.

The next segment which was also about 75 feet was more rough and included broken branches and one fallen tree branch, the last segment was 50 feet (15.2 m) from the fallen tree to the tree that the ATV eventually impacted.

The driver of the ATV was separated from the vehicle when the ATV hit a fallen tree branch on the ground. He sustained a brief loss of consciousness, multiple abrasions and contusions in the right upper and lower extremities, moderate swelling and tenderness of the right knee, and wrist with fractured right distal radius. These injuries were consistent with the driver leaving the ATV from the right side of the vehicle, and breaking his fall with his right arm. His helmet provided sufficient protection for this fall and he had no cerebral concussion.

The passenger who was sitting behind the driver on a long banana shape seat was most likely holding the grab bar on the rack, rather than the shoulder or waist of the driver, since the vehicle was on a rough and downward slope after leaving the pathway. The testimonial evidence suggested, at the instant of driver's separation from the ATV after hitting a tree branch or log, on the ground, the ATV's speed was about 20 mph (32.2 km/hr). At least with this speed the ATV struck the tree about 1.7 seconds after hitting the log on the ground.

According to the final rest position, and the damage done on the vehicle, the ATV struck the tree slightly eccentric manner (right of the centerline) with its front bumper bar and frame, causing the ATV to pitch as well as yaw clockwise. The pitching motion accelerated the passenger's body in the vertical direction, and upper portion of his body rotated while the whole body moved toward the tree.

Note that, at the vicinity of the tree, the terrain had two slopes, one forward down slope, the other left to right slope with respect to the longitudinal axis of the ATV.

Thus, at the moment of impact the passenger separated from the ATV with a momentum vector which had a vertical component (due to pitching motion of the ATV), a lateral component (due to clockwise yaw motion of the ATV), and a predominant longitudinal component (due to linear velocity of the ATV). With this kind of motion he had a direct head impact with a horizontally positioned large tree branch.

Since this was a litigation case in the USA, the defendant's expert witness presented arguments suggesting that the rider of the ATV was non-helmeted, and the helmet, which was found at the accident site, was not subjected to any impact. I will next present sections dealing with observations on the subject helmet as well as biomechanical foundations dealing with how one can get a head injury even wearing a DOT (Department of Transportation) certified helmet.

OBSERVATIONS ON THE HELMET

The helmet has a movable (rotatable about the mediolateral axis) plastic face shield which is connected to a support base which is press-buttoned to the helmet shell . Initial (partial) impact occurred when the casing of the plastic shield made a contact with the tree branch.

Under compressive dynamic load, the top plastic casing which has a ring shape went to a large radial deformation causing large tensile stresses in the interior surface of the casing which removed a chunk of the material.

Meanwhile, due to excessive radial deformation, at the symmetrically opposite side (5 cm left of the centerline), high tensile stresses on the outer surface of the casing cracked not only the casing but also the plastic shield. Scuff marks on the helmet were consistent with the color of the tree bark according to the observations made by the investigating officer.

BIOMECHANICS OF INJURY

The CT scan of the head revealed closed head injury lesions and hematoma which are consistent with deceleration and contre-coup injury. What we have here is a three-collisions situation. The first collision is the impact of the helmet with the tree branch, the second collision is the collision between skull and the liner of the helmet, the third one is the (pile-up) motion of the brain toward the skull interior. Note that there is always a certain amount of mismatch between the local geometries (curvatures) of the liner and the skull.

Partially because of this mismatch, and partially because of the magnitude of deceleration, at the impact site scalp is loaded locally between the skull and the liner. It is this reason the ATV rider also had a frontal scalp hematoma with “no associated depressed calvarial fractures” were seen. If the ATV rider were non-helmeted, as suggested by the defendant’s expert witness, he would have received not only massive depressed calvarial fractures and external bleeding but also distal linear skull fractures.

Although helmets provide some energy absorbing and the peak-impact-force reduction capabilities, they do not completely eliminate energy transfer to the head. In fact, a good helmet designed according to ANSI Z90.4-1984 standards can only absorb about 100 ft-lb (135.6 J) of energy. Anything above that amount has to be absorbed by the head and neck.

If we accept a 22 ft/sec (6.7 m/s) impact for the head of the ATV rider with the tree branch, and also consider only the body masses above L5/S1 level are associated with this impact, the total available energy will be 466 ft-lb (631.9 J) which is much above the helmet’s protective range. This implies that more than 496 J of energy must be absorbed by the head and neck region. This level of energy is certainly much above the tolerable range. A more precise analysis should be based on continuum mechanics approach.

Theoretical foundations of “coup” and “contre-coup” injuries in the brain for locally impacted head and impulsively loaded head were initially introduced by this author (Engin, 1969, 1971). For the helmeted head, deceleration of the brain matter toward the skull interior is essentially controlled by the thickness and crushing strength of the liner. Thus, collision of the helmeted head with an immovable solid object such as a tree can be modeled by a fluid-filled spherical shell traveling with a speed of V and brought to a stop with a constant deceleration within time to.

During deceleration phase the liquid occupying the interior space of the shell is subjected to a global axisymmetric pulse on its boundary and the resulting excess pressure distribution is given by    

where

, are the unit step functions, m are fluid-shell

natural frequencies, cs = [E/s (l - 2)]1/2 wave speed on the shell

in terms of E (Young’s modulus) , s (mass density of shell),

(Poisson’s ratio); s = c/cs and c is the compressional wave speed

in the fluid.

p r g r u u oms

mm

1 10

12

2, , , sin

u o

msm s

o

sin sin

2 2

g rV

cxm

s o

, cos

4

3 1 1 1

12 2 1 3 2

sin cos sin sin cos

sin sin cos

m m m m mr mr mr

r m m m m m m m

u o u

The figure above presents a three dimensional plot of the excess pressure distribution for various values of nondimensional radius r1 (r/a) and

nondimensional time (ct/a). For a 22 ft/sec (6.7 m/s) impact velocity and 0.01 sec deceleration time, this analysis gives a maximum negative contre-coup pressure of 24 psi (165.5 k Pa) which is more than sufficient to cause contre-coup injuries in a helmeted head.

HARD HAT CASE

A construction worker was struck on the head by a piece of falling concrete masonry block weighting approximately 20 pounds. The piece is about one third of a whole masonry block weighing 60 pounds. The initial impact point of the block was about 2” right of the center line of the hard hat and about 1.5” anterior to the frontal plane. The worker was unconscious for about 15 minutes. After regaining consciousness he was able to stand and walk.

MEDICAL SUMMARY Although initial neurological examination suggested a closed head injury with its related symptoms along with cervical herniated disk, a subsequent CT scan of the cervical spine indicated essentially no abnormal intraspinal or paraspinal soft tissue density to suggest disk herniation. However, patient’s headaches, frequent upper and lower extremity pain and related lower back pain continued. As a result of his accident the worker (the plaintiff) was totally disabled from employment since the date of the accident.

ANALYSIS When the concrete block struck him, the worker was sitting on a cinder block and grinding a cement wall. Thus, the reported height of the fall of 14’ gives delta-h of 10’, which results in 200 lb-ft of impact energy. Assuming a trapezoidal impact force profile, and total impact time duration of 15 milliseconds we obtain a maximum impact force of 1576 lbs.

Further biomechanical analysis of the head impact yields maximum head acceleration of 157g’s and the SI (Severity Index) value of over 4466 units. These values are much beyond the tolerable limits.

According to the ANSI test procedures (ANSI Z89.1-1997) the helmets are subjected to impact kinetic energies at the following levels: a) 40.24 lb-ft for force transmission tests, b) 22.6 lb-ft for impact attenuation tests, c) 18.07 lb-ft for apex penetration tests. In this case, the impact energy of a piece of concrete block is an order of magnitude higher, thus, caused puncture and partial penetration of the hard hat.

OPINIONS

1. In general, the helmets provide limited head injury protection by resisting penetration via outside shell, absorbing impact energy via inside liner, and extending impact duration to reduce the maximum value of the impact force.

2. In this particular case, the hard hat that meets ANSI Z89.1-1959 standards has no inside liner; however, it has a suspension which is a portion of the harness with crown straps designed to act as an energy-absorbing mechanism.

3. Finally, severity of impact was such that it was transmitted with some attenuation through vertebral column. This was appropriately stated by the plaintiff “It felt like it went right through me, not just my head” (p.21 of EBT).

SCOOP TRACTOR CASE

A coal miner was injured while he was operating his low profile scoop tractor (“scoop”) when it ran over a 6” x 6” wooden beam (“crib block”).

The miner sustained a neck injury when he was propelled vertically into the overhead canopy made of thick, non-yielding, steel plate after the scoop dropped 6” from the crib block it ran over in the under- ground mine. The helmet he was wearing cracked with the impact.

MEDICAL SUMMARY

The coal miner had a significant injury to his cervical spine, failed conservative treatment, had a C6-7 fusion and cervical diskectomy which provided minimal relief of his symptoms. More than two years after the accident his chronic pain continued and he had difficulty turning his head repeatedly.

ISSUES

• Plaintiff claimed cervical spine injury was caused by impacting the overhead canopy as a result of scoop tractor’s lack of suspension system or seatbelt, i.e. the scoop tractor’s inability to effectively reduce shock to the operator.

• Defendant claimed that the miner was not injured when the scoop ran over the crib block. His pre-existing condition was aggravated during the mishap.

ANALYSIS  In a systematic way I determined:

a) scoop’s wheel drop velocity as a function of vehicle velocity, wheel radius, and obstacle height,

b) corresponding velocity for the operator’s location and related average acceleration,

c) maximum value of head-canopy impact force,

d) normal and shear forces at C6 disk level,

e) stress on the disk surface, nucleus pulposus pressure, and the maximum circumferential tensile stress on the inner edge of the annulus fibrosus. I showed that this crucial stress is much higher than the allowable annulus fibrosis rupture stress which resulted in C6 disk herniation.

SUMMARY OPINION Subject product/machine is unreasonably dangerous because the hazard it provided was not open and obvious to the operator. Furthermore, the subject hazard of the machine can be very economically and feasibly corrected. A simple lap-type seatbelt, requiring no sensing devices, is all that is needed to remove the defective nature of this machine.

TAILGATE EJECTION CASE

At an intersection of two roads a SUV (Sports Utility Vehicle) was side impacted at its right quarter panel by a pickup truck.

During the SUV’s clockwise yaw motion, two children (8 year old boy and 7 year old girl), who were sleeping in the cargo space (extended by folded-down bench seat) were ejected from the SUV as a direct consequence of the failure of the tailgate. After the accident, the boy was found crawling and crying; the girl was lying on the ground in comatose state.

MEDICAL SUMMARY Although the boy came out of the accident with only a few bruises and scratches the girl’s situation was critical. According to medical records the girl had the following injuries:

a) Large open scalp wound with a large scalp flap b) Underlying skull fracture with missing segments

of bone c) Protruding CNS tissue d) Fracture of the left distal radius and ulna e) Multiple contusions and abrasions f) Comatose state

Injuries (a) – (c) occurred in the left fronto-parietal area of the head.

ANALYSISAfter reviewing all relevant documents and examining the SUV, I performed the following analysis:

1. Determined kinematics of the SUV and children during accident,

2. Calculated the ejection speed of the children from the tailgate,

3. Carried out a computer simulation of the ejection process,

4. Calculated the girl’s maximum head impact force, the maximum head acceleration and the SI, severity index.

OPINIONS

1. This was a relatively low-level side impact with delta-V less than 15 mph. The tailgate, because of its lack of structural integrity coupled with poor choice of material, was defective in design and failed in three areas under a relatively low-level side impact.

2. As a direct consequence of the failure of the tailgate, two children who were sleeping in the extended cargo space were ejected from the vehicle. In a rotational motion, the interior surfaces, in particular the tailgate, can serve as a restraining device for the occupant just like the seat belts do in the linear deceleration situation. Thus, failure of the tailgate destroys this safety aspect of the vehicle.

3. Based on her initial position in the cargo space along with the computer simulation of the ejection process, I determined that the girl was ejected in a head first mode. In fact, her initial impact with the ground was with her left arm and left fronto-parietal area of her head. Her head was subjected to approximately 2176 lb. of maximum normal force and a similar magnitude of tangential force which caused a large open scalp wound with a large scalp flap. The combination of high-level normal and tangential forces fragmented the skull and exposed the brain matter.

4. As expected, the severity index, SI, that I computed was 7120 which is much higher than the tolerable level, i.e. 1500. Her head was subjected to a maximum deceleration of 362g which is also much higher than suggested limits of 140g for the threshold of concussion, and 200g for the skull fracture.

5. In contrast to the girl, her brother was ejected feet first, and did not impact the ground head first. When he was found, he was crawling in the grass and crying. Whereas, his sister was lying on the grass without any motion and sound.

6. In conclusion, if the child was not ejected from the vehicle as a consequence of the failure of the tailgate, she would not have sustained the severe injuries which resulted from her head impact with the ground.

CONCLUDING REMARKSIn personal injury and product liability litigation cases forensic biomechanics is now a recognized science by the officers of the state and federal courts of the United States.

Forensic biomechanics expert witness must perform essentially three tasks:

1. Thorough investigation of the case, which may involve an accident reconstruction, inspection of broken part or failed device or product, reading depositions and examining the medical records, site visits and gathering relevant data,

2. Analysis phase which should include determination of the mechanism of injuries and factors responsible for the causation of the accident. Analysis should be based on sound scientific principles accepted by the expert’s peers and should be void of any theories which include speculations and conjectures,

3. Formal written expert report which, at a minimum, should include sections dealing with materials reviewed, description of the accident or event, summary of the expert’s opinions, and a brief resume of the author at the end of the report.

QUESTIONS?