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“Helmets for Preventing Head and Facial Injuries in Bicyclists (Cochrane
Review)” D C Thompson et al, Issue 1, 2003

This is a review of the validity and findings of five, previously published, casecontrolled studies from different countries into the effectiveness of cycle
helmets. The review found that all the studies provided consistent evidence that
wearing an approved cycle helmet significantly reduces the risk of head or brain
injuries in a crash or collision. It concluded that overall, cycle helmets decrease the risk of head and brain injury by 65% to 88%, and decrease the risk of facial injury by 65% (but do not protect the lower face or jaw). The review also stated that helmets are effective for cyclists of all ages and in accidents involving collisions with motor vehicle as well as those which do not.

It is a feature of Cochrane Reviews that responses to the review from other
researchers are published along with the replies by the original authors. This
review generated several responses that were critical of the review’s findings,
mainly on the following grounds:

-- helmets are not designed to protect the brain from rotational injuries, which
are the most serious type

-- helmet laws discourage cycling which is one reason for any apparent
reduction in head injuries and also means the health and environmental
benefits of cycling are lost

-- cyclists who wear helmets feel safe and so cycle in a less cautious manner,
hence increasing their accident risk (risk compensation).


The authors disagreed with these arguments on the grounds that:

-- helmets do protect against the most common types of head and brain
injuries, and the research studies prove this

-- there is no scientific evidence that mandatory cycle helmet laws discourage
cycling

-- there is no scientific evidence that cyclist who wear helmets take more risks.

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The Cochrane database has established a reputation as a key resource for medical research. This particular review, however, has attracted considerable criticism. Some of this criticism is included with the reviewers' paper on the Cochrane database and thus forms an integral part of the review. However, submissions were sometimes shortened and summarised in a way that omits key concerns. See here for the full version of one detailed critique (BHRF, 1243).
Further peer-reviewed criticism has appeared in the medical press, in which the studies selected for the review were accused of not possessing the necessary scientific rigour to be a reliable guide to helmet effectiveness, particularly with regard to efficacy against fatal and disabling injury to the brain (Curnow, 2005).

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Criticisms
Principal criticisms of the review are:

The review is not independent. Four of the seven papers selected for inclusion were the work of the reviewers themselves and their data dominate the analysis, comprising 77% of the cyclists on whom the review is based. Furthermore, these four papers are based on only two data sets and have themselves been much criticised for fundamental methodological shortcomings (BHRF, 1068).
Only case-control studies were considered for inclusion, although non-randomised studies of this type are acknowledged to be prone to bias because of the difficulty in controlling for the many independent variables (BHRF, 1052).
The paradox presented by the failure of other types of studies (e.g. whole population and time-series data) to show any benefit from large increases in helmet use - let alone the substantial benefits predicted by the studies reviewed - is left unstated and unaccounted for (BHRF, 1096).
The authors are dismissive of the possibility of risk compensation. However, it has subsequently been demonstrated that child cyclists often ride more riskily and suffer more crashes when wearing a cycle helmet (Mok et al, 2004) and that adults are more likely to ride on busier roads if helmeted (Gregory, Inwood and Sexton, 2003).
Similarly no consideration is given to rotational injuries, which dominate the most serious injuries. Helmets have not been shown to mitigate rotational brain injury and there is evidence they may increase the risk and/or severity of rotational injury.(BHRF, 1039).
Claims are accepted of efficacy for which no plausible mechanism exists (e.g. the prevention of mid-face injuries), where the reviewed data set is too small to make reliable inference (e.g. the result of collisions with motor vehicles), and which would not be possible even if helmets prevented all head injuries (e.g. an increase of 35% in cyclists wearing helmets leading to 66% fewer head injuries).
There is misleading interpretation of 'odds ratio', which is used interchangeably in the review with 'percentage reduction in head injuries'. This exaggerates the predicted benefit of helmets and masks the fact that studies of this type are not truly predictive, being essentially the authors' estimate of what proportion of the observed differences between two groups can be assigned to a single factor. Furthermore, the reviewed paper showing the least benefit from helmets is omitted from computation of odds ratio, thus again exaggerating benefit.