The protective effect of helmet use in motorcycle and bicycle accidents: a propensity score–matched study based on a trauma registry system

Traveling by motorcycle has been a common part of daily living for years and remains a crucial mode of transportation in Taiwan, due to the country’s suitable climate and high population density [1, 2]. According to a published study in Taiwan, 3323 persons were killed by traffic accidents in 2011, and more than 60 % of whom were motorcycle riders or passengers [2]. Bicycling has also become popular in Taiwan, not only as a means of transportation, but also as a symbol of personal fitness. In the United States, national statistics report that 500,000 people sustain bicycle-related injuries per year, resulting in approximately 800 deaths [3]. It is estimated that for every 2 million trips, 600 injuries will occur and one bicyclist will die in a collision [4]. In Taiwan, a retrospective cohort study revealed that bicyclists had a 1.2-fold higher adjusted odds ratio (AOR) of in-hospital mortality than motorcyclists [5]. Among patients with injury severity score (ISS) ≥25, bicyclists had a 4.4 times increased odds of mortality compared to motorcyclists (95% confidence interval [CI]: 1.95–9.82) [5].

Although the protective effect of motorcycle helmets is already well-established in the literature [6‐14], head injury is still regarded as a critical cause of mortality among victims of motorcycle collisions. Head injury is also the main cause of hospitalization of bicycle-related injuries [15]. Data from an estimate in 2000 noted that approximately $8 billion is spent annually in the United States in the care of bicycle crash victims, which is a significant cost from a public health perspective [16]. Head injuries often occur in motorcycle and bicycle traffic accidents and have severe consequences [17, 18], however, the current laws in Taiwan only enforce the use of helmets on motorcyclists, while helmet use remains optional for bicyclists. As a result, helmet use is more common among motorcycle riders than bicycle riders. Furthermore, most bicycle helmets are smaller, thinner, and lighter, than motorcycle helmets, thus they may provide less protection to riders during collisions. Moreover, because of different riding speeds, the impact energy may differ between collisions involving motorcyclists and those involving bicyclists. There is scarce information on the protective effect of helmet use in Taiwan, where motorcycle and bicycle accidents occur on relatively crowded streets [19]. Therefore, in this study we aimed to investigate the protective effect of helmet use during motorcycle and bicycle accidents by using a propensity score–matched study based on trauma registry system data over a seven-year period.

This study was approved by the institutional review board (IRB) of the Kaohsiung Chang Gung Memorial Hospital (reference number 201600005B0), a Level I regional trauma center providing care to trauma patients, primarily from southern Taiwan. We designed a retrospective study to review the data of all adult motorcyclists and bicyclists (n = 9321) entered into the Trauma Registry System between January 1, 2009 and December 31, 2015 (Fig. 1). Patients who had sustained an injury other than motorcycle or bicycle accident or whose registered data were incomplete were excluded. Detailed patient information was retrieved from the Trauma Registry System of our institution, including the following variables: age; sex; body mass index (BMI); co-morbidities, such as diabetes mellitus (DM), hypertension (HTN), coronary artery disease (CAD), congestive heart failure (CHF), cerebral vascular accident (CVA), and end-stage renal disease (ESRD); vital signs on arrival; blood alcohol concentration (BAC) on arrival; Glasgow Coma Scale (GCS) score; Abbreviated Injury Scale (AIS); and ISS on arrival; length of stay (LOS) in the hospital and intensive care unit (ICU); in-hospital mortality; procedures performed in the emergency department (ED); and associated head and maxillofacial trauma.

In order to evaluate the relationship between helmet use and traffic accident injury severity and outcome, the data of patients presenting to the ED following motorcycle and bicycle traffic accidents were analyzed. Motorcyclists with helmet use (n = 7735) were compared with motorcyclists without helmet use (n = 863), and bicyclists with helmet use (n = 76) were compared with bicyclists without helmet use (n = 647) using the SPSS v.20 statistical software (IBM, Armonk, NY). The primary outcome of the study was in-hospital mortality, and the secondary outcomes were hospital LOS, ICU admission rate, and ICU LOS. For categorical variables, Chi-square tests were used to determine the significance of the associations between the predictor and outcome variables. For continuous variables, Student t-tests were applied to analyze normally distributed continuous data, while Mann-Whitney U tests were used to compare non-normally distributed data. Univariate logistic regression analyses were performed to identify the significant predictor variables of the mortality risk. The corresponding crude odds ratio (OR) with 95% CI for each variable was calculated. Pearson’s correlation coefficient (r) was used for bivariate correlation to analyze the relationships between items of co-morbidity, including DM, HTN, CAD, CHF, CVA, and ESRD. Thereafter, to minimize the confounding effects due to a nonrandomized assignment in the evaluation of the effect of helmet use on mortality, propensity scores were calculated using SPSS v.20 statistical software with helmet use as dependent variable and the following covariates as independent variables: sex; age; and comorbidities (Additional file 1: Table S1). Using NCSS software (NCSS 10; NCSS Statistical software, Kaysville, Utah), the optimal method was used to create 1:1 matched study groups with a 0.2 caliper width. After adjusting for these confounding factors, binary logistic regression was used to evaluate the interventional factor of helmet use by motorcyclists and bicyclists on mortality. All results are presented as the mean ± standard deviation. A p-value <0.05 was considered statistically significant.

The results of this study highlight the protective effect of motorcycle helmets, with a significantly lower mortality rate among motorcyclists with helmet use than among motorcyclists without helmet use. The well-balanced propensity score-matched model, which eliminated the confounding effects of sex, age, and comorbidities, further strengthened our conclusion on the protective effect of motorcycle helmets. Furthermore, motorcycle helmet users also had higher GCS scores and a lower ISS scores on presentation, shorter hospital LOS, and lower ICU admission rates. The protective effect of bicycle helmets was not demonstrated in the bicyclists group, despite the use of propensity score-matched populations of bicyclists.

The protective effect of motorcycle helmets is well established in the literature [6‐14]. Helmet use among motorcyclists was found to benefit both riders and society, with improved discharge outcomes, as well as a reduction in mortality rates, traumatic brain injuries, and costs of hospitalization [6‐14]. Hooten et al. concluded that motorcycle helmets significantly decrease overall mortality, improve outcome at discharge, and are cost-effective, resulting in healthcare savings [6]. A systemic review published in 2008 by Liu et al. also concluded that motorcycle helmets reduce the risk of death and head injury among motorcycle riders involved in collisions [7]. Another retrospective study conducted by Sosin et al., found that motorcycle helmets not only reduce the severity of nonfatal head injuries, but also lower the rate of fatal injuries [8]. The legal enforcement of helmet use for motorcycle riders is therefore of crucial importance. Kraus et al. found that the enactment of motorcycle helmet law in California of USA significantly reduces the fatality rate in motorcycle accidents, as well as the number and severity of head injuries [9]. Moreover, Hotz et al. reported a significantly increased number and severity of brain injuries following the repeal of a motorcycle helmet law [10]. The data on cervical spine protection is controversial, with some studies reporting that the use of motorcycle helmets may increase the rate of cervical spine injury [20, 21], and others reporting statistically similar, or even lower risks of cervical spine injury [22‐26]. In this study, although the neurological damage from cervical spine injury was not evaluated, motorcycle helmet use was associated with a significantly lower rate of cervical vertebra fractures.

The protective effect of bicycle helmets has been reported in the literature. Heng et al. found that helmet use was associated with fewer injuries to the head and face, as well as a lower ISS, in bicycle crashes [15]. In two case-control studies published by Thompson et al., bicycle helmet use was associated with a decreased risk of head injuries among injured bicyclists [27, 28]. A case-control study also reported a 74% reduced risk of head injury among bicyclists with helmet use involved in accidents with motor vehicles [29]. Another recent study of 13,500 bicyclist injuries in France, conducted by Amoros et al., found a lower risk of all head injuries among bicyclists with helmet use, in both urban and rural environments [30]. However, Sethi et al. studied injured bicyclists in a level I regional trauma center and found no significant difference in mortality rates between patients with and those without helmet use [3]. In this study, we only observed a reduced risk of developing SDH among bicyclists with helmet use compared to those without after propensity score matching, while the mortality rate and the risks of all the other associated head injuries, other than SDH, were not significantly reduced. Therefore, unlike helmet use among motorcyclists, protective effect of helmet use in lowering mortality rates could not be identified among bicyclists.

A number of reasons for the lack of a protective effect of bicycle helmets in lowering mortality rates and rates of a range of head injuries in this study were identified. Firstly, the design and material of bicycle helmets are significantly different to those of motorcycle helmets, thus less protection could be expected. Secondly, it is reasonable to postulate that the distinct contusion force and trauma mechanism of bicycle traffic accidents can result in differences in the severity and presentation of intracranial hemorrhage between traffic accidents involving bicyclists and those involving motorcyclists. Therefore, the protective effect of bicycle helmets against death and different types of head injuries could not be demonstrated in relatively minor crashes. Thirdly, the sample size of bicyclists in our study may not be sufficiently large to generate a significant difference in mortality rates and rates of head injuries between bicyclists with and without helmet use. Further studies involving a larger series or involving a controlled crash scenario may provide more robust evidence of a protective effect of bicycle helmets.

A specific strength of our study was the propensity score-matching model, which markedly reduced bias on mortality. However, the results after matching depended on the specification of the logistic regression model and the potential confounders measured in this study. Therefore, balance between the comparison groups regarding unmeasured confounders could not be fully guaranteed. This study also presented a number of limitations. First, the inherent bias of retrospective studies must be considered. Second, the lack of available data regarding the circumstances of the crash, the mechanism of injury, the number of riders and their status of helmet-wearing on the same motorcycle or bike, the speed of the motorcycle or bike, and the type of helmet used, could also lead to a bias. Third, injured patients who were discharged against advice from the ED, those who were not brought to a level I trauma center after the crash, on-site deaths, those who were discharged but got re-admitted into hospital not via the ED, and those who were discharged but died from a complication directly related to the trauma, were not included in this study. This may represent a sample bias in the assessment of mortality. Notably, because there were more on-site deaths in motorcycle crashes than in bicycle crashes in our hospital, the extent of difference in terms of mortality in this study would be expected to be greater in the motorcyclists compared with that in the bicyclists. In addition, other important data, such as surgical outcome and complications, were not evaluated in this study. Finally, the study population, limited to a single urban trauma center in southern Taiwan, may not be representative of other populations.

This study, conducted at a Level I trauma center, revealed that motorcycle helmets provide protection to adult motorcyclists involved in traffic accidents and their use is associated with a decrease in mortality rates and the risk of different head injuries. Motorcycle helmet users also had a higher GCS score and a lower ISS score, a shorter hospital LOS, and a lower ICU admission rate. In contrast, for bicyclists, helmet use was not associated with a decrease in mortality rates and the risk of different head injuries other than SDH. This study therefore did not identify any protective effect of bicycle helmets for adult bicyclists involved in traffic accidents in Taiwan to keep them away from the risk of different head injuries and mortality.