Damage to bicycle helmets involved with crashes

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Abstract

The objective was to evaluate the relationship between helmet damage and head injuries in helmeted bicyclists in a sub-study of a large case-control study of bicycle injuries and helmet effectiveness. The setting consisted of seven hospital emergency departments in Seattle, WA. Hospitalized patients and medical examiners cases were included. The participants in the study were helmeted bicyclists who suffered a head injury or who damaged or hit their helmet in a crash. The Snell Memorial Foundation laboratory evaluated the helmets, blinded to crash circumstance and injury diagnosis. Damage was scored on a five-point scale (0=none to 4=destroyed). The damage location for each helmet was coded into regions (six longitudinal and three latitudinal) and mapped onto a three-dimensional CAD (computer-aided design) model of a helmet. The same procedure was also followed for injury location, which was mapped onto a three-dimensional ISO (International Organization for Standardization) headform for visualization of head-injury distribution. 785 helmeted subjects met the criteria for inclusion in the sub-study, and 527 helmets were purchased and evaluated (67%). 316 (60%) of the helmets had no or minimal damage, and 209 (39.7%) had significant damage (score 2, 3 or 4). Helmet types were 49.7% hard shell, 34.2% thin shell and 16.1% no shell. The risk of head and brain injury increased if the helmet was destroyed: OR=5.3 (95% CI 2.9, 9.9) and OR=11.2 (95% CI 3.5, 37.9), respectively. A high proportion of helmet impacts were along the front edge of the helmet, with a preponderance of head injuries in the same region. The large number of impacts to the front rim of the helmet, combined with the substantial number of riders with injuries to the forehead, indicate that some helmets, because of poor fit or wearing style, expose the forehead to injury. In addition, the data indicate that for a small proportion of injuries, the energy to the helmet may exceed design limits.

Introduction

While bicycle helmet use has increased dramatically in the last decade, few data are available on the actual damage to helmets, the location of this damage and the correlation of damage to injuries which have occurred. Four prior studies have examined damaged helmets. Fisher and Stern (1994)examined 1 100 Bell helmets, using self-reported injury data. Williams (1991)evaluated 64 damaged bicycle helmets obtained from an emergency-department based study of 1 892 injured bicyclists. He found that serious injuries occurred if the helmet came off, or if the impact was below the level of the rim of the helmet. Smith et al. (1994)examined 72 helmets sent to manufacturers for replacement; they found that the most common site of impact was the left frontal region. Cameron et al. (1994)found that one third of impacts were below the helmet test line, and that the 1.5 m Australian drop test may be too low.

Data from epidemiological studies indicate that helmets are very protective against head and brain injury, although this protective effect is not 100% (Thompson et al., 1989; McDermott et al., 1993; Maimaris et al., 1994). The potential reasons for this lack of complete protection are many, and include poor fit, improper wearing because of style or fit, helmet failure, energy exceeding the helmet protective capabilities, and impact occurring outside the covered area of the head. Data available to date do not allow us to determine which of these mechanisms is most important.

The purpose of this study was to examine a large series of helmets involved in crashes in order to determine the location and type of damage to the helmet to gain a better understanding of why some helmeted cyclists sustain head injuries.

Section snippets

Design

This study was part of a larger case-control study of bicycle injuries and helmet effectiveness, based on the case-control design used in our previous investigation of bicycle helmet effectiveness (Thompson et al., 1989). Subjects were recruited from seven Seattle area hospitals: Central and Eastside Hospitals of Group Health Cooperative (GHC) of Puget Sound, a large staff-model HMO; Harborview Medical Center (HMC), a Level I Regional Trauma Center; University of Washington Medical Center, a

Results

During the study period, there were 3 849 eligible subjects treated in the emergency departments of the seven study hospitals. In addition, five subjects died from bicycle-related injuries prior to ER arrival, and were identified from medical examiner's records. We obtained completed questionnaires and injury data on 3 390 subjects for an overall response rate of 88.0%. There were 1 718 people (50.6% of the study subjects) who were helmeted at the time of the crash. Of these, 785 met the study

Discussion

This study provided important information on the events during a bicycle crash and the impacts to the head, face and helmet. The information should be of use to manufacturers and standards organizations, as well to groups conducting interventions to increase helmet use and decrease injuries.

The relatively large proportion (39%) of helmets which had damage to the liner indicates that the present policy of many manufacturers to offer free replacement for their helmets involved in crashes is very

Acknowledgements

This work was supported by a grant from the Snell Memorial Foundation, 6731 A 32nd St., North Highlands, CA 95660. The authors are indebted to Edward Becker and Frank Lin of the Snell Memorial Foundation for assisting with damaged helmet evaluation, to Alexandra Kaufman and Viviana Rebolledo (project coordinators), to Bob Soderberg for programming assistance, and to the emergency department and medical records personnel at our seven participating hospitals; Harborview Medical Center, Central

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