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Motorcycle Helmets and Spine Injuries, Revisited

By Jonathan P. Goldstein  
   February 16, 2011

A recently published Johns Hopkins study concluding that motorcycle helmets reduce spine injuries elicited this response from a Maine-based economics professor whose own 1986 study suggested that the weight of a helmet could cause significant torque on the neck that would be devastating to the spine:

The recent publication of "Motorcycle Helmets Associated with Lower Risk of Cervical Spine Injury: Debunking the Myth" by Compton et al. (Journal of the American College of Surgeons, 2011) affords me the opportunity to reflect on the current state of the motorcycle helmet effectiveness debate on the twenty-fifth anniversary of my contribution to that literature. In 1986, I published a study, "The Effect of Motorcycle Helmet Use on the Probability of Fatality and the Severity of Head and Neck Injuries: A Latent Variable Framework" (Evaluation Review, 10:3, June).

The statistical analysis in that paper established the existence of a head-neck injury tradeoff in accident situations faced by the users of motorcycle helmets. In particular, helmets were found to exacerbate the severity of neck injuries past realistic critical impact forces to the helmet at the same time that helmets were found to reduce the severity of head injuries. As the title of the recent paper by Compton et al. implies, their study claims to have debunked the myth that helmets increase the risk of cervical spine (neck) injuries. They cite the source of the myth as my 1986 study. I respectfully disagree with their conclusion.

Far more important than the statistical results in my 1986 paper were the establishment of two methodological conditions that must be met to prevent regression estimates from systematically overstating (understating) the role of helmets in the reduction (exacerbation) of head (neck) injuries.

Unfortunately, in the intervening twenty-five years, the helmet effectiveness literature has been more concerned with overturning my results, by any means necessary, rather than taking the necessary methodological steps to obtain a valid assessment of helmet effectiveness. The current study, while more sophisticated than previous studies, is no exception.

The first methodological condition requires controls for the more (less) risk-averse behavior of helmeted (unhelmeted) riders such as lower (higher) crash speeds and blood alcohol levels. Failure to include crash speed and blood alcohol levels in the analysis will incorrectly assign the injury reducing (increasing) effects of lower (higher) values of these variables to the use (non-use) of a helmet. As a result, the effect of helmets on head injury reduction is overstated. The Compton et al. paper confirms the risk-averse hypothesis by showing that helmeted riders are more likely to be insured, yet the study does not control for crash speed and blood alcohol levels.

The second methodological condition recognized that the engineering limits of helmet effectiveness --  helmets are only capable of absorbing a 13 mph impact before transferring impact forces to the contents of the helmet – must be incorporated in a regression analysis.  This requires that the resulting reduction in helmet effectiveness be captured by an additional helmet variable -- a multiplicative interaction between the helmet use variable and the impact speed to the helmet.

Any regression analysis that excludes this interaction variable will overstate the effectiveness of helmets in reducing head injuries by ignoring the critical mechanism via which helmet effectiveness is reduced. This omission will also systematically understate the potential impact that helmets have in causing neck injuries -- past impact speeds of 13 mph , the helmet no longer absorbs/dissipates the impact forces and sets in motion a now heavier helmeted human head transferring those forces to the cervical spine.

The analysis performed in the Compton et al. study omits the crucial helmet/impact speed interaction variable that captures the neck injury enhancing effects of helmet use. As a result, the author's produce estimates of the neck injury inducing (reducing) effects of helmets that are understated (overstated).

The resulting bias in their evaluation of the effect of helmets on cervical spine injuries is obvious from the illogical conclusion reached in their paper. In particular, the authors find the statistically significant result that the use of a helmet reduces the odds of cervical injury by 22% in motorcycle accidents.  The authors do not delineate any mechanism that substantiates their main finding.  Yet, they accept these incredulous results at face value.

To the best of my knowledge, no proponent of the effectiveness of motorcycle helmets has advanced an argument for how helmets protect the cervical spine. Even if a mechanism is advanced, no matter how farfetched, the omission of the critical helmet-impact speed interaction variable still produces results that systematically understate the potential for helmets to cause neck injuries. Thus, the authors' main conclusion expressed in the subtitle of their paper, "Debunking the Myth", is at best presumptuous.

Jonathan P. Goldstein is a professor in the Department of Economics at Bowdoin College in Brunswick, ME.

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