Helmets and Head Injury Protection
A forensic examination of a collision-involved helmet is an important part of many biomechanical head injury investigations. The most common helmets encountered in forensic investigations are motorcycle, bicycle and sport helmets. The following is a brief primer on some of the relevant issues for head injury and helmet investigations.
Mechanical devices have been used since the time of antiquity to protect the head from direct impact. Most early helmets were crude shells worn over the head with little or no padding that were effective against projectiles and penetrating objects. A gladiator’s battle helmet is a good example. Early in the 1900s, when automobiles, motorcycles and aviation were in their infancy, padded leather helmets became fashionable, although their protective ability was limited. In the 1950s and 1960s, biomechanical scientists started to understand the impact loads that could cause skull fractures and brain injuries. With this understanding, instrumentation and tests were devised to quantify head impact exposure and helmet performance. This testing spurred the development of the modern helmet. A modern helmet has energy absorbing and impact distributing properties to mitigate lacerations, contusions, skull fractures and brain injuries.
The modern helmet is constructed of a hard outer shell to resist penetration and an inner liner to absorb energy and spread impact forces over a larger area. The combined effect of the functional layers reduces the injurious forces applied to the head by lengthening the total time of impact. A strap or restraint system keeps the helmet on the head where it is needed. Most helmets also have a comfort liner immediately adjacent to the head.
For helmets that are designed to protect the user from severe impacts, such as motorcycle, bicycle and equestrian helmets, the most common material used to construct the liner is expanded polystyrene (EPS). Energy absorbing liners for these helmets are good for one serious impact only. Once struck, the energy absorbing material will have been crushed, cracked and deformed; the material will no longer adequately absorb energy. Some sport helmets, such as those for hockey and football, are designed for multiple impacts (albeit they are designed to manage lower energy impacts). The energy absorbing material of these helmets recovers from impact and does not need to be replaced.
Technical performance standards have been established through the consensus of biomechanical and helmet experts. Specific standards have been developed for each type of helmet. Motorcycle helmets, for example, must adhere to different performance criteria than bicycle helmets or hockey helmets. The most common motorcycle helmet certification standards are the United States Department of Transportation (DOT) and the Snell Memorial Foundation standards. For bicycle helmets, the most common standards are administered by the U.S. Consumer Product Safety Commission (CPSC) and the Snell Memorial Foundation.
Novelty motorcycle helmets come up frequently in forensic helmet investigations. A novelty helmet looks very slim fitting because it does not have an energy absorbing liner. Do not mistake comfort foam for an energy absorbing liner. Novelty helmets will not meet an impact protection standard and are illegal in jurisdictions where helmets are required by law. These helmets are typically used for their visual appeal rather than their functional impact protection. Users may purchase a sticker closely resembling a certification label and place it on the helmet to mislead law enforcement that their helmet meets the required standards. Recent changes to the U.S. Federal Motor Vehicle Safety Standard, FMVSS 218, require improvements to the Department of Transport certification label that the DOT feel will deter attaching misleading helmet labels.
The development of the modern helmet is a biomechanical engineering success story. Modern helmets with energy absorbing liners are very effective at reducing the incidence of head injuries. Dubious claims that helmets increase other injuries are not supported by reliable scientific investigations and literature.
Different helmet makes, models and styles provide different head coverage and, as a result, the injury mitigation varies depending on the point of impact and the coverage area of the helmet. A full-face helmet can also protect the user from facial impacts. Helmets are least effective when the impact is to the edge of the helmet.
Mild traumatic brain injury (MTBI) or concussion is a topic that receives considerable attention in both litigation and in the press. Helmets reduce the transmission of forces applied to the head that cause MTBI; however, even a properly fitted and properly worn helmet can allow the transmittal of forces that will result in MTBI symptoms in a portion of the population. Helmets are most effective at preventing serious brain injuries and skull fractures. The most recent research indicates they may reduce but are not capable of preventing MTBI.
Even a certified helmet can only perform well when worn properly. When a helmet fits poorly or is not fastened securely with the chin strap, it can shift during usage resulting in reduced visibility or sub-optimum protection. Worse, the helmet can be ejected from the head during impact resulting in a serious injury.
An expert helmet inspection can identify whether a helmet has been subjected to an impact and can determine the direction and magnitude of the impact. A visual helmet inspection may be sufficient to identify and document the physical evidence. In some cases, the energy absorbing liner will need to be removed from the helmet for examination; this is a more involved procedure. Tests can be conducted to evaluate the effect of appropriate helmet fit, chin strap usage and helmet retention. An associated biomechanical investigation can identify the range of injuries expected given the physical damage to the helmet. An expert inspection can also identify marks that are fraudulently placed on a helmet to appear like impact evidence.
The scientific evaluation of injury outcome considering contributory negligence is often evaluated in a forensic helmet investigation. In many cases, an investigation reveals that the use of a helmet would have reduced the magnitude of the head injury. When a helmet is not worn, impact loads to the head are much higher and, as a result, injuries are much more severe than they would have been had a helmet been worn; however, a high severity impact can overwhelm the protective capabilities of a helmet and still result in an injury.
The U.S. Insurance Institute for Highway Safety (IIHS) reports that unhelmeted motorcycle riders are three times more likely to suffer a traumatic brain injury in a crash than helmeted riders and that helmets reduce motorcycle crash fatalities by 37 to 42 percent. The IIHS also reports that bicycle helmets reduce head injury risk by an estimated 85 percent.
Despite these convincing figures, many people prefer not to wear helmets, and helmet laws vary greatly from province to province and state to state. Only four Canadian provinces require all bicyclists to wear helmets and no U.S. states require adult helmet usage. Others have helmet laws applying to minors only, or none at all, although there are many local helmet by-laws in U.S. cities. Motorcycle helmets are mandatory for all riders in Canada, but in the U.S., many motorcycle helmet laws have been repealed, leaving only 19 states and the District of Columbia with helmet laws covering all riders and three states with no motorcycle helmet laws.
By Dr. Craig A. Good, Ph.D., P.Eng. - Collision Analysis, Inc.ABOUT THE AUTHOR: Craig A. Good, Ph.D., P.Eng.
Expert Website: https://www.collisionanalysis.com/
Call (888) 427-2747
Expert Website: https://www.collisionanalysis.com/
Call (888) 427-2747
In 2007, Dr. Good received a Ph.D. in Mechanical Engineering specializing in Injury Biomechanics from the University of Calgary. He previously earned his B.A.Sc. and M.A.Sc. in Mechanical Engineering at the University of Waterloo.
Collision Analysis is involved in ongoing research programs designed to learn more about helmeted head impact protection. The most recent program is attempting to understand how helmet age affects the impact performance of energy absorbing liners. A sample of approximately 800 used bicycle helmets up to 20 years old were collected and impact tested. The preliminary results were presented at the 2014 World Congress of Biomechanics in Boston, Massachusetts. The results show that the impact performance degrades very little as a helmet ages. The impact performance of aged helmets is relevant to forensic investigations as many accidents involve older helmets.
Copyright Dr. Craig A. Good, Ph.D., P.Eng. - Collision Analysis, Inc.
Disclaimer: While every effort has been made to ensure the accuracy of this publication, it is not intended to provide legal advice as individual situations will differ and should be discussed with an expert and/or lawyer.For specific technical or legal advice on the information provided and related topics, please contact the author.