The state of head injury biomechanics: past, present, and future: part 1.

This article is the first of two parts of a comprehensive survey of the biomechanics of head injury since its inception in 1939 in the United States, the separation being made for temporal and spatial reasons. The second portion of this material will be published at a later time in this journal. The discussion will be almost exclusively limited to nonpenetrating events. The topics presented in the following sections include an introduction that discusses the magnitude of the problem, the basic tools of biomechanics, and significant major reference sources covering this subject. This is succeeded by a brief description of the components of the head, classification of head injuries, early experimental investigations and human tolerance considerations, measurement techniques of kinetic parameters, and head motion and head injury investigations prior to 1966. A Head Injury Conference sponsored by the National Institutes of Neurological Diseases and Stroke in 1966 changed the landscape of investigations in this area. While informal collaboration between neurosurgeons and engineers had existed prior to this time, the conference established a permanent mechanism of synergism between these disciplines, produced the first zero-order realistic model of biomechanical head injury investigation, and established a 4-year program of federally funded research into the mechanical properties of the tissues of the cranium. While a recession precluded a continuation of the national sponsorship of such work, this 4-year period of intensive research resulted in a nationwide individual effort to develop further knowledge in this area. The current presentation, then, covers the mechanical and structural properties of solid and fluid tissues of the head, emphasizing progress during the past 3 decades; fetal cranial properties; analytical and numerical head injury models; experimental cranial loads applied to human volunteers and cadaver heads, dynamic loading of surrogate heads; and, finally, head injury mechanisms. The future publication will encompass experimental, analytical, and some numerical and regulatory information and that will be divided into the following sections: 1. head injury experimentation involving translatory and rotational motion: equipment, subjects and mechanical and physiological consequences 2. diffuse axonal injury: production and traumatic effects; mechanical properties at the axonal and neuronal level 3. vehicular crash investigation and simulation: reconstruction methodologies, staging, surrogate validation, and occupant protection, including vehicular design 4. injury thresholds and tolerances, including skull and vessel failure and brain and brainstem damage, including consideration of loading directions 5. criteria for head injury: governmental and industry regulations, including effects of combined motion- and tissue-level loading 6. further discussions of cranial component properties and injury mechanisms 7. sports head injury considerations: boxing, baseball, softball, football, ice hockey, and skiing activities; protective head devices for these activities 8. vehicular protective devices: design, efficacy, standards, and limitations; models for helmets and experimental validation. This presentation is based on my nearly 4 decades of head injury research, continuous collaboration and discussions with prominent members of the neurosurgical and orthopedic community, and an exhaustive, 2-year search of the literature. While every effort has been made to include all relevant information, it is inevitable that some important research has not come to my attention, and I apologize for any such omissions. It is hoped that this survey will serve as a resource for researchers and practitioners in the area of traumatic head injury and provide a roadmap for further investigations that are urgently needed. For example, this could include a determination of the rate of absorption of blood emitted from broken vessels, and, hopefully, some correlation between mechanical failure and physiological dysfunction of the various relevant tissues of the head. Although a good beginning has been initiated, additional information at the neuronal and axonal level concerning the effect of loading on function as well as age-related changes in geometry and tissue properties is also needed.