“Ski helmets reduce head injuries by 35% in adults and 59% in children under 13,” BBC News reported. The news is based on Canadian research which looked at whether helmets prevent head and neck injuries for skiers and snowboarders.
The research combined a number of studies that compared injured and uninjured skiers and snowboarders to determine the effects of wearing a helmet. It found that helmet use reduced the risk of head injury in both adults and children, but did not increase the risk of neck injury, as some people have suspected.
However, some shortcomings of this research, including the quality and methods of the original studies pooled in the analysis, mean that we can be less confident in its estimated risk-reduction figures. Importantly, while the risk reductions reported may sound large (35% for adults and 59% for children), skiing-related head injuries are rare: based on the study’s data we estimate that one head injury would be expected among 11,111 skiing outings. It is important to bear this low risk in mind when interpreting the results of this review.
Where did the story come from?
The British Medical Journal (BMJ) recently published an editorial drawing together the findings from several different studies on the use of helmets in skiing and snowboarding. When reporting on this discursive piece, BBC News included some risk-reduction figures, which were sourced from a recent review of studies conducted by researchers from the University of Calgary. It is this review that has been appraised in this Behind the Headlines article.
The study was published in the peer-reviewed Canadian Medical Association Journal. The authors report that their study received no external funding.
BBC News reported the issue well, giving the context provided in the BMJ’s editorial. However, it attributed the risk-reduction figures of 35% in adults and 59% in children under 13 to a study of ski helmets by the University of Innsbruck, whereas these figures are derived from a review conducted by researchers at the University of Calgary.
What kind of research was this?
This was a systematic review of studies that assessed head injuries in skiers and snowboarders. To gather studies, the reviewers searched through sources of research literature, including electronic databases of studies published up to 2008, conference proceedings and reference lists of other research. They only included studies that had a control group, i.e. a comparison group of uninjured people. This allowed them to assess the effect of wearing a helmet on the risk of head and neck injuries.
What did the research involve?
The studies included were cohort, case-control or case-crossover studies. Three researchers separately extracted data from each of the included studies to ensure that data were extracted appropriately. The data extracted included the study design, the characteristics of the participants and the study results. A statistical technique called meta-analysis was then used to combine the results of the studies. This gave a pooled estimate of the risk of head injury associated with not wearing a helmet while skiing or snowboarding.
When performing a meta-analysis, it is important to assess whether it is appropriate to pool the results of the included studies. One way to measure the appropriateness of pooling a set of results is to determine how different the studies are from one another through measuring a statistical property called “heterogeneity”. The researchers measured the heterogeneity, which allows us to have a sense of the robustness of the pooled estimate. They also performed analyses using only the high-quality studies and only the low-quality studies to determine what range of results might be plausible around their estimate.
While they initially identified 36 studies, the researchers included 12 studies in their analysis after applying their inclusion criteria. Ten were case-control studies, one was a case-control/case-crossover and one was a cohort study. In total, they pooled data from 9,829 participants who were wearing helmets and 36,735 who were not.
The researchers presented two different analyses of the studies, one assessing the effect of helmet use on the risk of head injury and a second reporting the effects on the risk of neck injury.
What were the basic results?
The review found that wearing a helmet reduced the risk of head injury by about 35-40%. This variation arose due to studies comparing skiers with head injury against different control groups, including uninjured people, or a mix of uninjured people and those who were injured but not with head or neck injuries. For example, skiers and snowboarders with a helmet were significantly less likely to have a head injury than injured and uninjured people without a helmet (odds ratio [OR] 0.65, 95% confidence interval [CI] 0.55 to 0.79).
Helmet use was associated with a significant reduction of about 55% in potentially severe head trauma in the studies that assessed this outcome. In subgroup analyses, the researchers investigated the effect of helmet use in children under 13 and found that use reduced the risk of head injury by 59% (OR 0.41, 95% CI 0.28 to 0.62).
Six studies specifically looked at neck injury. Analysis of these studies showed no evidence of a reduction in the risk of neck injury using helmets. The researchers report that this was true in both adults and children.
How did the researchers interpret the results?
The researchers concluded that helmets reduce the risk of head injury among skiers and snowboarders but that there is no evidence of an increased risk of neck injury.
This systematic review and meta-analysis addressed the issue of helmet use for snow sports. It has some shortcomings, some of which the researchers discussed:
- The studies were generally of only moderate quality, and many failed to adequately adjust for potential confounding factors, such as activity while being injured, jumping and participants’ age.
- The control groups in the studies varied, for example some studies compared uninjured people and others looked at people who were injured but not with a head or neck injury.
- The definition of head injury varied across studies.
- There was a lack of information about the quality or fit of the helmet. The researchers say that if poor-quality helmets or helmets that didn’t fit well were worn, the helmets’ potential to reduce head injury may have been underestimated.
Importantly, there were a number of differences between the various studies being pooled (high heterogeneity). This appears to be due to the different methods of the studies and the different way that samples were selected. This high heterogeneity means that we can be less confident in the results of this study. For these studies, the I² statistic (a measure of heterogeneity) was about 75%, which can be interpreted as the percentage of variation across study results that is due to heterogeneity rather than chance.
As they are not based on a population sample, it is impossible for case-control studies to calculate the absolute risk of a head or neck injury. While it appears that helmets reduce the risk of injury, the large relative risk reductions (35% and 60%) mask the fact that a head or neck injury is a rare event. The authors of this study report that the rate of head injury is about 0.09 per 1,000 outings. This means that if you went skiing or snowboarding 11,111 times, you would be likely to injure your head just once. For neck injuries, the rate is reported to be 0.46 per 1,000 outings, equating to going skiing or snowboarding 2,174 times and receiving one neck injury