Helmets (and motor vehicles)

The Risk of Spinal Cord Injury?  Is there a Solution?

helmet It was Sunday, October 22, 1991. Twenty-two year old Brian Yarrusso decided to take his off-road motorcycle over to the neighborhood dirt track and have some fun. As a serious rider, Brian dressed-out in boots, riding pants, gloves, chest and back protectors, and his Bell Moto-5 helmet. He had bought his helmet a few months earlier, and intentionally chose the most expensive helmet available because he believed "you get what you pay for." In fact, the Owner's Manual for the helmet affirmed that this safety equipment would protect against helmet impacts by causing the inside liner to crush and absorb the forces of impact.

After riding around the track for about an hour, Brian approached four small hills (separated by about 20 feet each) and launched himself from the first hill—at a speed of about 20 mph. Unfortunately, his approach to the second hill was a little long and the cycle "cased-out" on the second hill. The cycle then moved off the second hill and came down at the base of the third hill with the front tire angled down. This impact caused Brian and the bike to separate. Brian moved through the air about 20 feet and landed on the dirt surface with his helmeted head. Brian continued to tumble forward about 20 feet. Brian was unable to move; he was paralyzed from the shoulders down. Brian had no external injuries and his helmet was only slightly damaged; the inside styrofoam liner was only crushed about 3/16th's of an inch.

Brian initiated a helmet safety lawsuit against Bell Sports, Inc. on the basis that the Moto 5 was negligently designed and that unsafe characteristics in this safety equipment constituted a breach of both express and implied warranties. The plaintiff asserted that this helmet was too stiff and, therefore, in this low speed impact into a relatively resilient surface, it did not crush and manage the energy of the head impact. In fact, in the area of the helmet (the crown) where the blow had to have occurred to cause this injury, there was virtually no deformation of this stiff liner. Bell Sports' retort was that the helmet was built to meet the most stringent helmet standard in the world (Snell 85), that the helmet had done its job since Brian did not suffer a head injury, that the blow was forward of the crown of the helmet in the area of crush, and that helmets cannot prevent spinal cord injury.

Thus, the issues were joined: can helmets make a difference and prevent some forms of spinal cord injury? The jury said yes and found that Bell Sports had breached its warranties and that those breaches were a proximate cause of Brian's injuries. What was the evidence that led to this result, and what can safety advocates learn from this trial?

Spinal cord trauma is a devastating and life-altering injury. The National Head and Spinal Cord Injury Survey has estimated that every year in this country over 10,000 Americans suffer spinal cord injury resulting in quadriplegia. The real cost to these individuals, their families, and to society of the loss of these otherwise productive individuals is difficult to grasp, but it is enormous.

The primary activities associated with spinal cord injuries are highway accidents and sporting events. For years, scientists and product designers have researched how product design can be improved to minimize the risk of cervical spine injury associated with these activities. Despite some rather apparent risk prevention solutions, very little by way of product design has been effectuated. While both motor vehicle manufacturers and helmet companies are in a position to make a difference, in most instances they have chosen not to make meaningful change in the design of their products to address spinal cord injury. This article reviews some of the available science and some of the product deficits associated with cervical spine injury.

Mechanism of Injury

As early as 1971, researchers documented that most spinal cord injuries occurring in automobile and motorcycle accidents involve compression fractures with dislocation. The exact same mechanism has been identified as the primary causative event in football, hockey, and equestrian sports. The classic mechanics of this injury are depicted below:

In virtually all of these accident events, the injured victim is decelerated forward, his or her head flexes forward and impact occurs to the crown or vertex of the head--when the victim either strikes a stationary surface or one moving toward the victim. With the head rotated slightly forward, the natural lordodic curve in the cervical spine is eliminated and the head and spine are axially aligned. Initially, the imparted force is transmitted to the neck through the head or it may --under abnormal circumstances-- result from torso loading after the head is arrested. The applied force to the neck in axially alignment causes compression, which then produces a wedging of a vertebral body as it is squeezed between the segments above and below. With increasing compression, and when the tolerance level of the spine is exceeded, the vertebral body bulges and typically cracks, resulting in disc material herniating into the vertebral body and the body breaks up. This event results in either anterior or posterior fragments displacing into the spinal cord, which causes irreversible cord injury.

The cervical spine, like most other vital organs, is viscoelastic. That is, injury to the spine is dependent upon both force and the length of time of the application of force. Researchers have, over the years conducted extensive laboratory testing of both cadavers and cadaveric spines to provide estimates of the human tolerance of the cervical spine. It has been deduced that people can endure compressive forces for short durations of time without injury. Typically, spinal fractures are produced in the laboratory when the applied force ranges between seven hundred and fifty and one thousand pounds. However, researchers believe that the injury tolerance for most healthy adults may be as much as twice the force to injury produced in cadavers.

Design Issues

It is obvious that the risk of axial/compression spinal injury can be minimized or eliminated by taking out of the injury equation any one of the following elements: (1) forward head flexion putting the cervical spine in an axial posture; (2) avoiding substantial head contact; or (3) minimizing the transmitted forces to the cervical spine.

Eliminating element number one is not feasible in the motor vehicle environment, but can be accomplished in some helmet circumstances. In the early 1980's, nationally recognized experts, Dr. Voigt Hodgson (NOCSAE director) and neurosurgeon Dr. L. Murray Thomas published their research regarding football helmet design to reduce the risk of cervical spine injury, and found that adding a 4 point chin strap with an attached cushion (measuring 2" x 4") below the chin cup would substantially reduce the risk of inadvertent head flexion. Element number two can be effectively addressed in motor vehicle design only. Appropriately designed seat belt systems can address both forward travel (in frontal crashes) and vertical travel in rollover accidents. Most head strikes in frontal crashes involve contact with the vehicle's A pillar (the pillar supporting the windshield). A fairly well tuned seat belt system can reduce the occupant's forward excursion so that head contact with the A pillar is kept to a minimum--assuming the A pillar does not intrude into the occupant space in a frontal crash. In a rollover accident, the roof structure needs to be designed to prevent substantial intrusion which can load the head and spine. And, finally, element number three can only be addressed through the characteristics of the design of either the impacting surface--in the motor vehicle--or the helmet's designed crush characteristics. In other words, by designing the vehicle's roof or A pillar, or the helmet so that these components attenuate the energy of the head impact, some of the forces of impact are managed/absorbed, and cause a reduction in the forces imparted to the neck. As an example, in 1978 researchers tested seven different helmets and measured the transmitted forces in axial compression to the test dummies neck, and showed that some but not all of the helmets were able to attenuate the forces below the injury threshold.

In connection with helmet design, it is important to appreciate that most modern-day sports helmets include a shock absorbing liner. The shock absorbing liner in a helmet is positioned on the inside of the shell and it is designed to "manage" whatever force is transmitted through the shell. The liner provides protection by compression under load. Its function is to absorb the force so that little if any load is transmitted to the motorcyclist's head or spine. The energy of the impact is absorbed as the material compresses. How well a liner is designed to absorb energy is dependent upon its physical dimensions and characteristics. If, for instance, the liner is very dense/stiff and, therefore, does not crush readily under foreseeable impacts, then the primary force of the impact is passed along to the helmet wearer. Consequently, the correct choice of shock absorbing liner is one which manages predictable levels of force foreseeable in accidents by crushing and absorbing force at such a rate of time (in milliseconds) that either all the force is "spent" in the liner, or so much of it is managed that the wearer's anatomy can safely tolerate the balance.

Once again, the material of choice for liners varies with the style and use of the helmet. Helmets used in motorcycling and bicycling generally use expanded polystyrene bead (EPB) foams. Football helmets use a liner system which combines air bladders and foam materials. The crushing properties of all of these materials is determined by its density and thickness. The design of a shell and liner system requires a balance between shell flexibility and stiffness, and liner density and thickness. The choice of a very flexible shell (due to shell thickness and material) will result in more direct loading of the liner. In turn, the liner needs to be relatively thick and dense to accommodate these localized forces. A more rigid shell, which spreads the load, allows for larger surface load to the liner, thereby necessitating a relatively thick and less dense liner to allow for absorption by crush.

Conclusion

The object of a helmet (or interior surfaces of a motor vehicle) is to provide as much protection from injury due to head impact as possible, consistent with the environment of usage. A helmet, for instance, must reduce the injurious effect of blows of various shapes, sizes and speeds delivered to the head. In the design process, the manufacturer must appreciated the risks of cervical cord injury, the human tolerance to such injury and the envelop of circumstances within which the manufacturer has an opportunity to make a difference. Because no helmet can protect against injury at head impact speeds in excess of 20 mph, and because head injury is rarely seen from blows to the top of a helmeted head, and because spinal cord injury forces can be somewhat reduced in impact speeds under 10 mph, it seems reasonable to at least design helmets to provide absorption of impact up to a 10 mph crown impact. Providing protection to the limits of technology and human tolerance is both the least and most that careful manufacturers should do.

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