HEAD INJURY : WOULD A HELMET HAVE HELPED?
Those of us who work in the legal and insurance industries are aware of the benefits gained through use of protective equipment such as seatbelts and helmets. However, when an accident occurs and liability is in issue, the plaintiff who failed to make use of protective equipment will typically argue that the equipment, if used, would not have reduced the injury suffered. These liability determinations are made by the judge or jury, and establishing the necessary causal connection between the injury suffered and equipment used (or not used) is a matter of expert scientific evidence.
Walters Forensic Engineering recently evaluated the severe closed head injury of a cyclist who had collided with a car. He was not wearing a helmet. Defense counsel required more than general evidence as to the effectiveness of helmets. We were asked to determine specifically if the brain injury suffered by the cyclist would likely have been prevented had he been helmeted at impact.
In general, brain injuries caused by impacts are produced by one of two mechanisms. Sudden deceleration is one of these, and in such a case the brain is injured due to its motion inside the head, including collisions with the inside of the skull. The notable feature of this kind of injury is that the brain may suffer serious trauma even where the skull remains undamaged. The other mechanism of brain injury occurs when the skull is cracked or penetrated, in which case brain injury can occur regardless of the level of head acceleration during impact.
Our assignment involved a cyclist who suffered an "epidural hematoma associated with a cracked skull". This injury occurs when a crack in the skull (caused during impact) passes through the location of a 'meningeal artery', a small artery of which there are many within the skull bone. The artery may be suddenly severed in the creation of a fracture, and bleeding will occur under arterial blood pressure levels. This hemorrhage will force its way between the skull and the 'dura mater', a skin-like liner of the inner skull. The dura mater then separates from the skull and an expanding pocket of blood exerts pressure against the brain itself. The pressure restricts normal vascular supply to the underlying area and results in death of brain tissue. The inset figure shows the mechanism of this injury.
The cyclist collided with a slow-moving car, striking his head against the windshield and windshield frame. The minimal damage to the automobile suggested a low severity impact as did the minor general injuries suffered by the cyclist. We concluded that this was not a collision for which brain injury due to rapid deceleration would be expected. Thus, the remaining possibility for brain injury was penetration or cracking of the skull, which did occur, manifesting itself in the form of an epidural hematoma.
The cyclist's skull fracture resulted from high localized forces generated during the impact between his head and the windshield and possibly the windshield frame. Helmets significantly reduce the possibility of a skull fracture. The impact force of a striking object against the helmet is applied over a small area at the outer edge of the helmet. The purpose of the helmet is to distribute this force over a larger area at the interface of the helmet with the head. This results in a decreased maximum force at the skull relative to an equivalent non-helmeted impact and decreases the probability of a skull fracture.
Had the cyclist been helmeted at impact, it is likely that his skull would have not been fractured. The area of the cyclist's head that was struck during impact was an area that is protected by a typical bicycle helmet. We were thus able to conclude that had the cyclist been helmeted, the brain injury suffered by him would probably not have occurred.
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