Although the development of head injury tolerances began over 40 years ago and made use of a broad range of innovative experimental techniques, including human volunteer and cadaver tests and animal experiments, conducted by John Stapp and other pioneers in the field (Stapp, 1955), continued progress has been slow. The tests and experiments were extremely useful in developing an understanding of biodynamic responses under well-controlled crash situations. Denny-Brown and Russell (1941) conducted a series of animal experiments to study concussion. As a result of their work, estimates were developed regarding the amount of pressure in head impact that causes concussion. Researchers replicated those pressure levels in head impact tests on human cadavers, and determined average head accelerations required to cause injury across different impact durations (Viano et al., 1989; King et al., 1995). Subsequent analyses showed that mathematical relationships, known as the Severity Index and the Head Injury Criterion (HIM), could be developed from head acceleration data to establish a threshold for life-threatening injury. This elegant evolutionary process in the development of head impact protection stopped, however, when the HIC was adopted as one of the standards for evaluating motor vehicle safety performance. However, research on brain injury mechanisms have shown that HIC may not be appropriate for predicting specific forms of injury, notably diffuse brain injuries (Margulies and Thibault, 1992). Unfortunately, this work has been inadequately funded and restrictions on the use of animal and cadaver surrogates have impeded further progress.

Although there are other areas of biomechanics research (e.g., severe maxillofacial, thoracic, abdominal, and internal organ injuries), neurotrauma to the head resulting in traumatic brain injury continues to be a critical area. In the United States, someone suffers a head injury every 15 seconds. Every 5 minutes, one of those individuals dies and another becomes permanently disabled. There are 75,000 deaths from head injuries every year, and another 70,000 to 90,000 suffer permanent disability. Furthermore, 5,000 of those individuals develop epilepsy and another 2,000 remain in a chronic vegetative state. Traumatic brain injury is primarily a disability of the young, and the economic costs alone approach $25 billion annually (U.S. DHHS, 1992). Thus, research on neurotrauma to the head remains a priority area for biomechanics researchers.

To date, limited funding (Chapter 8) and difficulties in conducting animal and human cadaver research remain significant impediments to biomechanics research. Animal and cadaver use has greatly enhanced our understanding of injuries to specific areas of the body. Unfortunately, societal stigma and institutional policies limit the availability of animals and cadavers for research, particularly on pediatric injury. As a result, most knowledge of injury biomechanics is based on average-sized adults, and child injury (as well as injury to small women) assessment reference values are actually scaled-down estimates based on tests on adults, despite uncertainty regarding the accuracy of the scaling assumptions. There is also limited knowledge of the biomechanics of whiplash-associated disorders, even though neck strains and sprains result in significant



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