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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 Alabama Mobile, AL 36688 USA June 29 , 2007.

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  1. FORENSIC BIOMECHANICS IN THE COURT OF LAWAli Erkan Engin, Ph.D., FASME, FSDPS(Professor Emeritus, The Ohio State Univ.)Professor of Biomechanics & Mechanics Mechanical Engineering DepartmentUniversity of South AlabamaMobile, AL 36688 USAJune 29, 2007

  2. TOPICS & RESEARCH AREAS IN 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

  3. BASIC MECHANICAL PROPERTIES OF BIOLOGICAL MATERIALS • Individual Cells • Various Tissues • Organs and Complex Body Systems

  4. ANALYSES OF RESPONSE TO INTERNAL, BIOLOGICAL FORCES • Circulation and Microcirculation • Respiration • Locomotion Kinetics in Normal, Abnormal, and Amputee Gait

  5. 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.

  6. ANALYSES OF RESPONSE TO REPLACED PARTS AND ASSISTIVE DEVICES • External Orthoses/Prostheses • Internal Orthoses/Prostheses • Biomechanical Compatibility of previous two items.

  7. Forensic Biomechanics can be defined as the application of biomechanics in the court of law.

  8. Forensic biomechanics cases may be put into the following categories: • Motor vehicle accidents and related injury cases (single and multiple vehicles involving single and multiple vehicle occupants and/or pedestrians), • 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.


  10. 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.

  11. 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.

  12. COURT TESTIMONY • Why the back support failed 2. What the manufacturer could (or should) have done 3. Biomechanics of the injury

  13. 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.

  14. 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.

  15. 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. or 2. 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.

  16. Warning Sticker Plastic Plug Back support stem WARNING Remove 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!

  17. Biomechanics of Injury • Angular velocity of her torso when she hit the floor • Her impact velocity with the floor • Magnitude of the force at the lower back just prior to floor impact • Magnitude of the maximum shear force at the lower back during floor impact

  18. 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: • Her lumbar region was subjected to hyperextension • Under this hyperextension intervertebral discs opened up anteriorly and closed down posteriorly, meanwhile, the facets were highly loaded in a compressive manner, • 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.


  20. 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.

  21. 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.

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

  23. 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.

  24. OBSERVATIONS • In order for this machine to perform its intended function, rotating brushes and the resulting danger points appear to be unavoidable design feature. • Safe engineering design practice requires appropriate shield and guarding when the hazard cannot be eliminated thorough design.

  25. 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.”

  26. 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.

  27. OPINIONS • 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. • 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.

  28. 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.


  30. DESCRIPTION OF THE ACCIDENTThis 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.

  31. 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.

  32. 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.

  33. 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.

  34. 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.

  35. 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.

  36. 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.

  37. 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.

  38. 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.

  39. 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.

  40. OBSERVATIONS ON THE HELMETThe 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.

  41. 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.

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