and protecting the head from penetration. Early helmets were designed to prevent such injuries as skull fractures as well as moderate to severe brain injuries such as focal contusions and hemorrhages. The typical helmet has a comfort liner, an impact energy attenuating liner, a restraint system, and a shell. Some helmets, such as those used in motor sport, bicycling, and alpine skiing, are designed to attenuate a single impact. Once one of these single-impact helmets has sustained an impact, it must be replaced. Other helmets, such as those used in ice hockey, football, and lacrosse, are designed to withstand multiple impacts over a season of games and practices (Hoshizake and Brien, 2004). Part of the difference between single-impact helmets and multiple-impact helmets lies in the materials used. For example, multiple-impact helmets, such as those for hockey and football, use materials that do not permanently deform but rather compress and return to their original dimensions. Inner shells can be made of vinyl nitrile or expanded polypropylene, and outer shells use lightweight plastics and composites for durability and protection. Single-impact helmets contain materials that are frangible and deform or fracture permanently upon impact as part of their energy management strategy.
Helmet design involves a series of trade-offs between optimal safety and parameters such as the thickness and other characteristics of the attenuation material, the size and mass of the helmet, comfort, and acceptability. A primary goal of the attenuation layer is to decrease the peak deceleration and to increase the time duration over which the deceleration occurs; this can be achieved by a thicker or more compliant layer of material which improves the energy management by reducing the peak linear deceleration upon impact. However, better energy management via an increased thickness results in a large helmet that may be unacceptable from a style, agility, or visibility standpoint. A helmet with increased mass would have reduced linear head acceleration for a given force; however, it may actually increase the rotational acceleration generated from an impact because there would be an increased radius over which the forces are acting.
Review of the Biomechanics of Concussion
In order to determine if helmet design can indeed be protective against concussion, one must first understand what mechanical events lead to concussions and then determine whether the helmet can mitigate those mechanical forces. The key biomechanical principles that define the mechanics of concussion were discussed in detail in Chapter 2 and are only briefly reviewed here. Local brain tissue deformation (i.e., strain) has been shown to cause brain injury as defined by loss of consciousness, white matter injury, hemorrhage, cell death, or some combination of these (Cater et al., 2006; Elkin and Morrison, 2007; LaPlaca et al., 2005; Margulies and Coats,