Spaite DW, Murphy M, Criss EA, Valenzuela TD, Meislin HW.
Journal of Trauma, 1991;31(11):1510-6.
Medical data for 198 bicyclists involved in collisions with motor vehicles and seen at the emergency department of University Medical Center, Tucson, Arizona from January 1986 to January 1989 were evaluated. Of 284 patients whose helmet status at the time of injury was known, 116 (40.9%) wore helmets. 85 patients were severely injured (ISS>15), being only 5.2% of helmet users but 47.0% of non-users. Mortality was also higher for non-users (6.0%) than users (0.9%). However, a striking finding was noted when the group of patients without major head injuries was analysed separately. Helmet users in this group had much less serious injuries than non-users. The researchers concluded that helmet non-use is strongly associated with serious injuries even if those without serious head injuries are excluded. This implies tht non-users of helmets tend to be in higher impact crashes than helmet users, since the injuries suffered in body areas other than the head also tend to be much more severe. It is possible that at least some of the 'protection' afforded helmet wearers in previous studies may be explained by safer riding habits rather than solely a direct effect of the helmets themselves.
This was the first study to question whether perceived helmet benefits are in fact due to other mechanisms such as the difficulty of matching cases and controls.
Spaite looked only at bicyclists hit by motorists. These injuries differ greatly from those caused by simple falls and are much more likely to result in serious injury. Within that set of injured bicyclists, Spaite found that unhelmeted bicyclists had more severe injuries to non-head areas. This implies that the presence or absence of the helmet isn't the controlling factor in the outcome. It's the 'choice vs. chooser' problem, or in epidemiological terms, confounding.
Towner E, Dowswell T, Burkes M, Dickinson H, Towner J, Hayes M, 2002. Bicycle helmets - a review of their effectiveness: a critical review of the literature. Department for Transport Road Safety Research Report 30.