Bhh

PCa

b

c

d

e

a

ARRA

KBCHBI

B+

h0

Accident Analysis and Prevention 70 (2014) 1–7

Contents lists available at ScienceDirect

Accident Analysis and Prevention

jo u r n al homepage: www.elsev ier .com/ locate /aap

icycle helmets are highly effective at preventing head injury duringead impact: Head-form accelerations and injury criteria forelmeted and unhelmeted impacts

eter A. Criptona,b,c,d,∗, Daniel M. Dresslera,b,d, Cameron A. Stuarte,hristopher R. Dennisona,b, Darrin Richardse

Department of Mechanical Engineering, University of British Columbia, Vancouver, CanadaInternational Collaboration on Repair Discoveries, University of British Columbia, CanadaCentre for Hip Health and Mobility, University of British Columbia, CanadaOrthopaedic and Injury Biomechanics Group, University of British Columbia, CanadaSynaptic Analysis Consulting Group, Vancouver, British Columbia, Canada

r t i c l e i n f o

rticle history:eceived 18 February 2013eceived in revised form 8 January 2014ccepted 19 February 2014

eywords:rain injuryoncussionelmeticycle

njury prevention

a b s t r a c t

Cycling is a popular form of recreation and method of commuting with clear health benefits. However,cycling is not without risk. In Canada, cycling injuries are more common than in any other summer sport;and according to the US National Highway and Traffic Safety Administration, 52,000 cyclists were injuredin the US in 2010. Head injuries account for approximately two-thirds of hospital admissions and three-quarters of fatal injuries among injured cyclists. In many jurisdictions and across all age levels, helmetshave been adopted to mitigate risk of serious head injuries among cyclists and the majority of epidemio-logical literature suggests that helmets effectively reduce risk of injury. Critics have raised questions overthe actual efficacy of helmets by pointing to weaknesses in existing helmet epidemiology including selec-tion bias and lack of appropriate control for the type of impact sustained by the cyclist and the severityof the head impact. These criticisms demonstrate the difficulty in conducting epidemiology studies thatwill be regarded as definitive and the need for complementary biomechanical studies where confoundingfactors can be adequately controlled. In the bicycle helmet context, there is a paucity of biomechanicaldata comparing helmeted to unhelmeted head impacts and, to our knowledge, there is no data of thistype available with contemporary helmets. In this research, our objective was to perform biomechanicaltesting of paired helmeted and unhelmeted head impacts using a validated anthropomorphic test head-form and a range of drop heights between 0.5 m and 3.0 m, while measuring headform acceleration andHead Injury Criterion (HIC). In the 2 m (6.3 m/s) drops, the middle of our drop height range, the helmetreduced peak accelerations from 824 g (unhelmeted) to 181 g (helmeted) and HIC was reduced from 9667(unhelmeted) to 1250 (helmeted). At realistic impact speeds of 5.4 m/s (1.5 m drop) and 6.3 m/s (2.0 m

drop), bicycle helmets changed the probability of severe brain injury from extremely likely (99.9% riskat both 5.4 and 6.3 m/s) to unlikely (9.3% and 30.6% risk at 1.5 m and 2.0 m drops respectively). Thesebiomechanical results for acceleration and HIC, and the corresponding results for reduced risk of severebrain injury show that contemporary bicycle helmets are highly effective at reducing head injury metricsand the risk for severe brain injury in head impacts characteristic of bicycle crashes.

∗ Corresponding author at: Department of Mechanical Engineering, University ofritish Columbia, 6250 Applied Science Lane, Vancouver, BC V6T 1Z4, Canada. Tel.:1 604 675 8835.

E-mail address: cripton@mech.ubc.ca (P.A. Cripton).

ttp://dx.doi.org/10.1016/j.aap.2014.02.016001-4575/© 2014 Elsevier Ltd. All rights reserved.

© 2014 Elsevier Ltd. All rights reserved.

1. Introduction

Cycling is a popular form of recreation and it is used for com-muting and other forms of transportation. It is generally safe andthe health benefits of it are clear (Hamer and Chida, 2008; Wen and

Rissel, 2008), which is in sharp contrast to motorized transportationof any type. However, cycling is also not without risk. In Canada,cycling injuries are the most common injury occurring from sum-mer sports; over 4300 people were hospitalized due to a cycling

2 alysis

i2ietcpe

actrsebfcwaaomtspamHrS

bhphaaoigbtmoapia2tdtoHe

TC

P.A. Cripton et al. / Accident An

njury in 2009–2010 (Canadian Institute for Health Information,010). According to the National Highway and Traffic Safety Admin-

stration (NHTSA), between 600 and 800 cyclists are fatally injuredach year in the United States and 52,000 cyclists were injured inhe US in 2010 (NHTSA Traffic Safety Facts 2010 Data, 2010). Amongyclists, head injuries account for approximately two-thirds of hos-ital admissions and three-quarters of fatal injuries (Thompsont al., 1999).

Epidemiological studies show that helmets are highly effectivet preventing head and brain injury amongst riders who crash. Aase–control study conducted by Thompson et al. (1989) in Seat-le over a period of 1 year found that bicycle helmets reduced theisk of head and brain injury by 85% and 88%, respectively. In aecond larger case–control study by the same group (Thompsont al., 1996), helmets decreased the risk of head injury by 69%,rain injury by 65%, and severe brain injury by 74%. Helmets wereound to be equally effective in accidents involving motor vehi-les and those not involving motor vehicles. Furthermore, helmetsere found to provide substantial protection from head injuries

cross all age groups. Amoros et al. (2012) recently conducted case–control study in France and studied helmet effectivenessver more than 13,500 cyclist injuries. They concluded that hel-ets were associated with a decreased risk of head injury in cyclist

rauma and this decrease seemed to be more pronounced forevere head injuries. Maimaris et al. (1994) studied over a thousandatients that sustained cycling-related injuries who were treatedt an emergency department in England. They concluded that hel-ets reduced the risk of head injury by a factor of more than three.eng et al. (2006) found that helmet use significantly reduced the

isk of head and facial injury in a 2006 study of cycling trauma iningapore.

Despite the protection provided by helmets, as demonstratedy the epidemiological studies above, the safety benefits offered byelmets are not universally accepted. Many cities, towns, states androvinces do not have helmet laws and many cyclists do not wearelmets (Page et al., 2012). Anti-helmet groups state that helmetsre not effective and that, in some cases, due to the increased size of

helmeted head compared to a bare head or due to the compliancef the shell or presence of vent holes, helmets can cause “rotational”njuries such as diffuse axonal injury (DAI). In the lay press, someroups claim that helmets cause injuries by obstructing vision orlocking sound. Researchers have also published articles, critical ofhe many epidemiological studies (cited above) that show that hel-

ets are highly effective at preventing head injuries, accusing themf bias and conflicts of interest (Curnow, 2006; Elvik, 2011). Curnowrgued that bicycle helmets are not as effective as claimed becauserevious epidemiological studies have not considered rotational

njury (Curnow, 2003). There is considerable debate on the meritnd limitations of the epidemiological evidence (Curnow, 2006,003; Elvik, 2011; Hagel and Barry Pless, 2006). One limitation ofhe epidemiological approaches is that it infers helmet performanceuring the impact from evidence collected after the impact and thushe severity of the head impact under study is never known. It is not

ur purpose to debate the merit of the epidemiological literature.ere we aim to explore the extent to which the epidemiologicalvidence of helmet efficacy can be supported or contradicted by

able 1omparison of several bicycle helmet standards.

Standard Reference

Consumer Product Safety Commission (CPSC) 16 CFR Part 1203

Snell Memorial Foundation (Snell) BF95 (1998 Revision)

American Society for Testing and Materials (ASTM) ASTM F1447F12

Canadian Standards Association (CSA) CSA D113 2FM89 (ReaffirmEuropean Standards (CEN) EN 1078

and Prevention 70 (2014) 1–7

a biomechanical study that allows study of helmet performanceduring the impact.

Biomechanical investigations of helmet efficacy, and indeed hel-met certification standards, simulate helmeted head impact bydropping helmeted headforms onto prescribed impact surfaces.In helmet certification standards, the primary metric to assessimpact management efficacy is linear headform acceleration mea-sured during a drop test; helmets are considered to have metthe certification criteria if the helmeted headform acceleration isbelow a prescribed threshold. The threshold varies from standard tostandard (Table 1), and is not directly correlated to established riskcurves. The standards generally require that helmets be certifiedusing a magnesium headform. The range of drop heights associ-ated with these standards is from 1.5 m (EN1078) to 2.2 m (SnellB95A) (Table 1).

In biomechanical investigations, linear and rotational headaccelerations are measured during the impact and helmet effi-cacy is determined by comparing these accelerations, and otherderived metrics such as the Head Injury Criterion (HIC), to injuryrisk functions. For example, Mertz et al. have established headinjury probability curves, in terms of HIC and linear acceleration, forthe Hybrid III headform which was originally developed for auto-motive crash testing (Mertz et al., 2003). Because injury tolerancesexist for this headform, the Hybrid III is increasingly applied inbiomechanical helmet and head impact studies (Beckwith et al.,2012; Kendall et al., 2012; Pang et al., 2011; Pellman et al., 2003;Scher, 2006; Scher et al., 2009; Viano and Halstead, 2012; Viano andPellman, 2005). Overall, the biomechanical studies indicate thathelmets significantly reduce head accelerations relative to unhel-meted impacts (Benz et al., 1993; Hodgson, 1990; Mattei et al.,2012; Scher, 2006) or to impacts with thin uncertified noveltyhelmets (DeMarco et al., 2010; Scher et al., 2009). Furthermore,because linear head acceleration is known to be monotonically cor-related to concussion and skull fracture risk (Greenwald et al., 2008;Mertz et al., 2003; Pellman et al., 2003) they are therefore knownto reduce the risk of sustaining head injury.

The biomechanical comparison that best matches the epidemi-ological studies, and thus that would be best able to augment thedebate in that field, is a comparison of helmeted and unhelmetedhead impact under identical impact conditions. Unfortunately,these tests are difficult to perform because of limitations of themagnesium head forms that are mandated in bicycle helmet stan-dards and that have thus most often been used in bicycle helmetimpact tests. The magnesium head forms are at high risk of damageif they are tested with no helmet and they have not been validatedto match the expected human response for bare head impacts. Thusthere is no test series available to our knowledge that contrastshelmeted and unhelmeted impacts for contemporary bicycle hel-mets under direct matched impact. Hodgson contrasted early 1990sera helmets with an unhelmeted impact using a “small humanoidheadform”, Benz et al. dropped an unhelmeted Hybrid II headformfrom a lower height than their helmeted impacts and Mattei et al.dropped human cadaver skulls with and without helmets from

six and nine inch drop heights (Benz et al., 1993; Hodgson, 1990;Mattei et al., 2012). All of these studies demonstrated a dramaticdecrease in head accelerations for the helmeted compared to the

Drop height (m) Drop height (feet) Criteria (g’s)

2 6.6 3002.2 7.2 3002 6.6 300

ed 2004) 1.6 5.2 2501.5 4.9 250

P.A. Cripton et al. / Accident Analysis and Prevention 70 (2014) 1–7 3

F Hybrida op-tow

uhaahcdh

chsfHht(Ha

2

tahtmaiabw

has2wwmhhm

nr

ig. 1. Photograph showing helmeted Hybrid III headform (left) and unhelmeted

ttached to the ball arm which was mounted to a linear bearing on the monorail dr

nhelmeted impacts. However, to the best of our knowledge, thereave been no studies that compare the crucial situation of helmetednd unhelmeted impacts using contemporary bicycle helmets, with

head form that is validated for bare head impacts and from dropeights that compare to bicycle helmet standards and real worldycling falls. The data that would result from such a test series isirectly relevant and indeed central to the ongoing debate of bicycleelmet efficacy.

The objective of this study was to assess the biomechanical effi-acy of bicycle helmets to reduce risk of head injury in simulatedead impacts from drop heights consistent with bicycle helmettandards and real world cycling head impacts. We designed andabricated a custom-made test fixture that allowed us to attach aybrid III headform to a monorail drop tower and performed bothelmeted and unhelmeted drops. The Hybrid III head form can beested without a helmet and it is validated in bare head impactsFoster et al., 1977). Linear head acceleration was measured, andIC and injury risk were determined from these accelerations toscertain the efficacy of helmets to reduce risk of head injury.

. Methods and materials

We simulated head impacts using a monorail drop tower similaro those specified in helmet certification standards. We fabricated

custom-made test fixture that allowed us to attach a Hybrid IIIeadform (Humanetics Inc., Plymouth, MI, USA), that correspondedo a 50th percentile male head, to the drop tower. A ball-arm was

ounted to a monorail drop tower that was purpose-built for thispplication. Paired tests were performed in order to study the risk ofnjury in helmeted and unhelmeted impacts. A paired test is defineds two drops onto an identical anvil and from identical drop heightsoth with and without a helmet. The impact surface for all dropsas a flat, fixed steel anvil.

Fig. 1 shows the anvil, helmeted and unhelmeted Hybrid IIIeadform and features of the bearing and guide rail. Translationalcceleration along the direction of impact was measured using aingle axis accelerometer (±2000 g range, Endevco model 7264C-000, Meggitt Sensing Systems, San Juan Capistrano, CA, USA),hich was mounted to the center of the ball-arm which in turnas placed within the Hybrid III head close to the head center ofass. The mass of the entire drop assembly, including the Hybrid III

eadform, ball-arm, and linear bearing was 5.05 kg. The mass of theelmet was approximately 0.25 kg and was considered additional

ass for the helmeted drops.The helmeted and unhelmeted drops were conducted from

ominal heights starting at 0.5 m to 3 m in 0.5 m increments. Thisange brackets heights used in certification standards but exceeds

III headform (right) in contact with the steel anvil. The Hybrid III headform waser.

the maximum height of typical standards (CSA D113.2-M89 (1.7 m),CPSC (2.0 m), ASTM F1447 (2.0 m), EN1078 (1.5 m) Snell B95A(2.2 m)) to allow study of higher energy impacts that can occurin real-world cycling where falls can happen while traveling atconsiderable speed. Our testing range also brackets the range ofperpendicular impacts documented for reconstructed bicycle falls(Fahlstedt et al., 2012). Two drops were performed at 0.5 m, 1 m,1.5 m, 2.5 m and 3 m; one drop for a helmeted Hybrid III headformand one unhelmeted. Six drops, three helmeted and three unhel-meted, were performed from 2 m. More drops were performed at2 m than other heights to obtain drop speed and acceleration datathat would allow a limited investigation of the repeatability of theexperiment and to do so at common drop height used in bicycle hel-met standards. To assess repeatability, we calculated the maximumdifference in both peak acceleration and HIC and expressed thesedifferences as a percentage of the mean peak acceleration and HIC.In total sixteen drops were conducted (8 helmeted, 8 unhelmeted).The headform was adjusted so that impacts took place to the fore-head of the headform as seen in Fig. 1. Actual drop heights wereincreased by approximately 5 cm above the nominal drop heightto account for friction in the drop rail. Speed at impact was calcu-lated using high-speed video and was found to be within 5% of theexpected velocity for each respective drop height.

All helmets used in this work were CCM V15 Backtrail bicyclehelmets (Reebok-CCM Hockey, Montreal, QC, Canada). The helmetswere constructed with a micro-shell and an expanded polystyreneliner. The helmets conformed to the standards set out by the Con-sumer Product Safety Commission (CPSC) (CPSC, 1998). In impactswhere helmets were used, the helmet was placed on the HybridIII headform in a standardized fashion. The orientation of the head-form was held constant for all drop heights using angle and positionlandmarks drawn on the Hybrid III headform (Fig. 1). The chinretention strap was tightened to secure the helmet to the Hybrid IIIheadform (Fig. 1). The helmets were also equipped with a ratchet-ing tension system that is designed to pass inferior to the occipitalprotuberance. This was tightened prior to all drop tests. A checkfor helmet fit and secure attachment to the Hybrid III headforminvolved manipulation of the helmet on the head to ensure no vis-ible relative motion. Each helmet was used for a single drop andthen replaced with a new helmet.

An Analog Devices (Analog Devices Inc., Norwood, MA) dataacquisition system was used to collect the data, with the acceler-ation signal sampled at 39 kHz and hardware anti-alias filtered to

comply with SAE J211-1 (“SAE J211 Instrumentation for Impact Test– Part 1: Electronic Instrumentation”). In addition, the accelerom-eter data were low-pass filtered at 1650 Hz (CFC1000) duringpost-processing as per SAE J211-1.

4 P.A. Cripton et al. / Accident Analysis and Prevention 70 (2014) 1–7

Fmi

huwIpda

pim

H

bw(tttMi

3

(douim

oIttaarto

Fig. 3. Peak accelerations for both helmeted and unhelmeted drops. Numbers over

Fig. 5 shows the calculated risk of a severe brain injury (AIS 4+)for all helmeted and unhelmeted drops. Overall, the helmeted dropsdramatically reduced the risk across all drop heights. For drops of 1meter and greater, the unhelmeted condition resulted in essentially

ig. 2. Typical acceleration data plotted versus time for both a helmeted and unhel-eted Hybrid III. Data shown is for the 2 m drop height. Acceleration is expressed

n g, where one g corresponds to 9.81 m/s2.

Peak head accelerations were used as a biomechanical metric ofelmet efficacy and were compared for the paired helmeted andnhelmeted tests. In order to assess the risk of injury associatedith these impacts, head accelerations were also compared to the

njury Assessment Reference Value (IARV), and probability curvesublished by Mertz et al. (2003). For presenting peak accelerationata, we use a 5% risk threshold for skull fracture based on peakcceleration (180 g).

The Head Injury Criterion (hereafter HIC) was calculated duringost-processing using Eq. (1). The HIC quantifies head impact sever-

ty by incorporating time of acceleration exposure and accelerationagnitude.

IC15 ={[

1t2 − t1

∫ t2

t1

a(t)dt

]2.5

(t2 − t1)

}max

(1)

For this analysis, a(t) is the head acceleration, in g, as measuredy the single axis accelerometer, and the time interval (t2 − t1)as chosen to maximize HIC over a maximum duration of 15 ms

Eppinger et al., 1999). In the subject testing the sensing axis ofhe accelerometer was aligned with the direction of impact andhus captured the resultant acceleration. Similar to the accelera-ion analysis, HIC15 values were compared to the IARV of 700 which

ertz et al. have reported corresponds to a 5% risk of AIS ≥ 4 brainnjury for the adult population (Mertz et al., 2003).

. Results

Repeatability was evaluated by analyzing multiple drops at 2 m3 helmeted drops and 3 unhelmeted drops). The maximum inter-rop difference in peak acceleration was 1.5% and 3.3% (percentagef mean peak accelerations for 2 m drop) for the helmeted andnhelmeted Hybrid III headform, respectively. Similarly, maximum

nter-drop differences in HIC were 6.0% and 5.0% (percentage ofean HIC), respectively.Fig. 2 shows typical acceleration curves plotted over the time

f the impact event for both the unhelmeted and helmeted HybridII headform. In general, the acceleration magnitudes plotted overime exhibited a single abrupt increase in acceleration, which con-inues to the peak acceleration, followed by an abrupt decrease incceleration. Following this, the accelerations fluctuate (e.g. Fig. 2,

fter 5 ms for the unhelmeted data) and these fluctuations cor-espond to head/helmet–anvil interactions that are secondary tohe initial head/helmet-to-anvil impact (i.e. the head “bounces”ff of the anvil). In general, peak accelerations, HIC and injury

bars indicate peak acceleration. For 2 m drop height, results stated are the meanvalue calculated from three drops. Horizontal dashed line indicates the IARVof 180 g(5% chance of skull fracture) for a midsize male.

probability were all of smaller magnitude in drops where theHybrid III was helmeted. The duration of the impact pulse waslarger in helmeted drops relative to unhelmeted. As the head decel-erated, a small amount (approximately less than 5 mm) of slidingoutward occurred between the helmet shell and the impact surfaceas the helmet shell and liner deformed during impact.

Peak accelerations (Fig. 3) were smaller in helmeted drops rel-ative to unhelmeted drops, for all drop heights. On average andconsidering all drops, the peak accelerations for helmeted dropswere smaller by a factor of 4.2 relative to unhelmeted. For theseverity of impacts tested, peak acceleration exhibited a linear rela-tionship with drop height. In the unhelmeted situation, the headaccelerations were above the IARV of 180 g for every drop from0.5 m to 3 m. For drop heights of 0.5–1.5 m, helmets decreased thepeak accelerations to a value below the IARV (Fig. 3).

Fig. 4 shows maximum HIC for helmeted and unhelmeted dropsfrom all heights. For each drop height, helmets reduced HIC relativeto unhelmeted drops. The mean interval required to maximize HICfor unhelmeted and helmeted drops was 1.0 ms and 5.0 ms, respec-tively. The increased HIC interval for helmeted drops is consistentwith the considerably wider (in the time domain) acceleration peakfor helmeted drops relative to unhelmeted shown in Fig. 2.

Fig. 4. Head Injury Criterion (HIC) calculated using HIC15 convention for both hel-meted and unhelmeted drops. Numeric values over bars indicate HIC values andlong dashed line indicates IARV based on HIC.

P.A. Cripton et al. / Accident Analysis

Foi

1ta9

metfst

4

camapalds(Hb

bttsttt(h1tItbt(p

ig. 5. Calculated probability of sustaining a severe brain injury (i.e. AIS 4+) basedn the HIC values for the helmeted and unhelmeted drops. Numeric values over barsndicate risk magnitude.

00% chance of an AIS 4+ brain injury. For all drops of 2 m or less,he helmeted condition resulted in a risk of below 35%. However,t the 3 m drop height the risk of an AIS 4+ brain injury exceeded0% for the helmeted condition.

For each of the helmeted drops, various levels of helmet defor-ation and helmet shell and foam damage were evident. The

xpanded polystyrene (EPS) foam cracked near the impact loca-ion in all but the lowest drop height (0.5 m) and the micro-shellractured in the 3 m drop. Upon further inspection, all of the testcenarios resulted in plastic deformation of the EPS foam liner nearhe impact location.

. Discussion

The overarching objective of this study was to assess the effi-acy of certified contemporary bicycle helmets to mitigate skullnd brain injury risk in head impacts with characteristic velocitiesatching impact velocities from helmet certification standards and

lso matching those from the biomechanical literature. We used aurpose-built drop tower with a Hybrid III test headform and a uni-xial accelerometer aligned to the direction of impact to measureinear head accelerations during both helmeted and unhelmetedrops. Linear head acceleration is one accepted mechanical mea-ure that can be related to both skull injury risk and brain injury riskMertz et al., 2003; Pellman et al., 2003) both directly and throughIC (Mertz et al., 2003), and therefore acceleration was our primaryiomechanical measure to assess efficacy.

We have characterized the ability of one typical contemporaryicycle helmet to reduce the severity of a head impact and reducehe risk of severe life-threatening skull and brain injury, comparedo not wearing a helmet, in matched impact tests where impacteverities (i.e. drop height and pre-impact head velocity) were iden-ical for the case of the helmeted and unhelmeted headform. Theested helmet dramatically decreased peak linear head accelera-ion (Fig. 3), HIC15 (Fig. 4) and the potential for severe brain injuryFig. 5) in all impacts. Considering peak acceleration (Fig. 3), theelmeted headform experienced accelerations below the IARV of80 g (Mertz et al., 2003) in drops from 0.5 m, 1.0 m, and 1.5 m whilehe unhelmeted headform experienced acceleration well above theARV in drops from all heights. In drops from 2.0 m up to 3.0 m,he helmeted headform experienced accelerations above the IARV,

ut helmeted headform accelerations were at least 4 times smallerhan those of the unhelmeted headform (Fig. 3). Evaluation of HIC15Fig. 4) was consistent with the acceleration results when com-ared with the IARV of 700. Probability of skull fracture (not shown)

and Prevention 70 (2014) 1–7 5

and severe brain injury (Fig. 5) is reduced, for all drop heights, forthe helmeted headform relative to the unhelmeted headform. Thisbiomechanical evidence clearly indicates that contemporary bikehelmets are highly effective at reducing injury risk through pairedhelmeted and unhelmeted impacts with realistic drop heights andimpact speeds. For example, the helmets reduced the head peakacceleration from 824 g to 181 g for drops of 2.0 m reducing therisk of skull fracture from 99.9%+ to 5%.

The reductions in head acceleration and HIC15 described abovedemonstrate that certified helmets significantly reduce the risk ofsustaining severe and even fatal injuries. It is worth noting thatcommon helmet standards do not seem to be designed to pre-vent head accelerations from exceeding the IARV values for severeskull and brain injuries published by Mertz et al. (2003), althoughit is noted that the helmets reduced the accelerations to a valuewell below that specified in the standard. The helmeted drop from2.0 m resulted in a HIC of 1250 that corresponds to a 34% chance ofsevere brain injury. It may be necessary to re-evaluate the helmetstandards and contemplate lowering the allowable accelerations insome standards and to require testing from multiple drop heightsin order to decrease the potential for serious and severe skull andbrain injuries from falls of various drop heights.

Common injuries coded AIS 4 (severe) and above include pen-etrating skull injuries leading to brain injury, large contusions(e.g., coupe-contrecoup, intermediate, and gliding) and hematomas(e.g., subdural, subarachnoid, and intracerebral), as well as diffuseaxonal injury with associated loss of consciousness for a periodexceeding six hours. The severity of these injuries when codedas 4+ range from severe to maximum (usually fatal) (Gennarelliand Wodzin, 2005). Considering a realistic bicycle accident sce-nario documented in the literature (Fahlstedt et al., 2012) wherea cyclist was thrown at 20 km/h (i.e. 5.6 m/s which corresponds toa drop height of approximately 1.5 m), our analysis indicates thata helmeted cyclist in this situation would have a 9% chance of sus-taining the severe brain and skull injuries noted above whereas anunhelmeted cyclist would have sustained these injuries with 99.9%certainty. In other words, a helmet would have reduced the prob-ability of skull fracture or life threatening brain injury from verylikely to highly unlikely.

Evaluation of the 3 m drops demonstrate that helmets only offera finite amount of protection. However, the 7.7 m/s impact speedis not representative of most real-world bicycle impacts (Fahlstedtet al., 2012). At impact speeds of this velocity the energy manage-ment capability of the helmet is saturated (Newman, 2002) andthe EPS liner bottoms out. The range of drop heights that we testedwas consistent with the range of impact speeds that has been doc-umented as plausible for cyclist impact scenarios. Our 3.0 m, 2.5 m,and 2.0 m drops were consistent with the resultant cyclist headvelocity in the Fahlstedt study that was oblique to the ground (i.e.glancing off it). However, if the cyclist hit a curb or the wheel ofa car at the point of glancing off the ground then there would behead impact of similar severity and impact direction to the higherimpact severity tests that we carried out.

The biomechanical results for acceleration and HIC15, and thecorresponding results for reduced risk of severe skull and braininjury, complement epidemiological studies that have sought toassess the protective efficacy of bicycle helmets. The majority ofthe epidemiological studies suggest that helmets are effective atreducing injury risk in a range of sports and across both adult andyouth segments of the population. However, concerns have beenraised in recent years over issues of selection-bias, failure to com-pensate for time-trending and public policy, and lack of control for

confounding aspects of the input data for statistical studies includ-ing type and mechanism of head/brain injury and severity of thehead impact (Curnow, 2006; Elvik, 2011). A fundamental reasonfor this study is that the lack of appropriate knowledge of and

6 alysis

srwtthiuttfiooa

bma“aahachwph1uoar(mapltaiouotWaih

dwhiiw8tocfitet

P.A. Cripton et al. / Accident An

tatistical control for the severity of the impact confounds theesults and decreases the significance of results below what theyould be if appropriate controls were applied. As a consequence,

he results in some studies suggest helmets are only slightly effec-ive at preventing head injuries or, in extreme cases, actually causeead injuries (Elvik, 2011). However, the results of our biomechan-

cal study, where impact severity was controlled in helmeted andnhelmeted impacts, strongly refute the epidemiological studieshat suggest helmets are marginally effective. Indeed, in all impactshe risk of sustaining injury was reduced when the Hybrid III head-orm was helmeted, and in the case of a realistic bicycle headmpact (5.5 m/s impact speed which corresponds to a drop heightf approximately 1.5 m), a helmeted cyclist would have a 9% chancef sustaining AIS 4+ injuries whereas an unhelmeted cyclist wouldlmost certainly sustain these injuries.

The results of this study are in good agreement with previousiomechanical research on the protective efficacy of bicycle hel-ets. For example, Benz et al. (1993) conducted a study of child and

dolescent helmets in drops from 1 m to 1.5 m using an unspecifiedchild headform” (Benz et al., 1993) and a Hybrid II headform fordolescent helmets, both dropped onto a flat anvil, and reportedn overall threefold decrease in HIC as a result of protecting theeadform with a helmet. Hodgson (1990) conducted a study using

Hodgson-WSU headform dropped from 1 m and 2 m onto flat andonvex surfaces (Hodgson, 1990). For 1 m drops, the unhelmetedeadform was at increased risk of injury (70–99% of the populationould be injured) relative to the helmeted headform (<1% of theopulation would be injured) as indicated by the severity index foread impact and results were similar for drops of 2 m (Hodgson,990). Mattei et al. (2012) used pediatric skulls, both helmeted andnhelmeted, in drops from 6 inches (0.15 m) to 9 inches (0.23 m)nto a flat surface and showed a maximum 87% reduction in meancceleration as a result of skull protection through helmet use. Theesults of our study were overall reductions in peak accelerationhelmeted accelerations on average 4 times smaller), HIC15 (hel-

eted HIC15 were on average 6 times smaller) and risk of skullnd AIS 4+ brain injury, which are in overall agreement with therevious literature. Direct comparison of our work to the previous

iterature is difficult because our study has several key strengthshat are lacking in previous work. Unlike previous studies, we used

Hybrid III headform that is validated for bare-head impacts ands capable of surviving bare-head impacts from drops in excessf 3 m. Therefore, we were able to perform paired helmeted andnhelmeted drops over a greater range of drop heights than thatf previous work, up to 3 m, and were further able to documenthe protective efficacy of bicycle helmets over this entire range.

e also used contemporary off-the-shelf helmets and tested over much broader range of impact severities than the previous stud-es and our drop experiments bracketed and incorporated the dropeights prescribed in contemporary bicycle helmet standards.

Like all biomechanical studies relying on anthropomorphic testevices (i.e. the Hybrid III headform), there are limitations of thisork. In our study drop height, impact speed, impact location on theead/helmet, and helmet fit on the head were all controlled, which

s in contrast to the real world where these parameters vary frommpact to impact and from individual to individual in the helmet

earing population. Depreitere et al. (2004) showed in a sample of6 bike accidents that impacts to the front of the head were 30% ofhe total and the front of the head was the second most likely regionf impact. Therefore, our experimental protocol is representative ofommon realistic impact scenarios that occur in cycling, despite theact that it does not simulate all possible impacts. We conducted

solated headform impacts in this study to simulate short dura-ion head impacts. It is possible that, in some cycling impacts, theffective mass of the head would be larger than the head mass weested here because some of the neck mass would couple to the

and Prevention 70 (2014) 1–7

head. However, torso mass generally has little effect on the headacceleration because of lag in time between head acceleration andneck loading (Nightingale et al., 1997). A limitation of this work isthat we used only one model of helmet and thus were not able toevaluate variation in the helmet mechanics as a function of designfeatures. However, this allowed us to minimize the variation in ourresults and to study the repeatability of our tests. Also, the helmetsthat we tested were certified to the CPSC standard and represent acommon design widely used in North American cities.

Bicycle helmets are effective at reducing peak translationalacceleration and HIC values; parameters that have been correlatedwith risk of skull fracture and severe brain injury. For a 1.5 m hel-meted drop, the risk of severe brain injury was reduced from 99.9%+to 9.3%. Thus, for realistic impact speeds (Fahlstedt et al., 2012) bicy-cle helmets changed the probability of severe brain injury from verylikely to highly unlikely. A contemporary helmet can transform ahead impact that would result in severe brain injury (which in somecases could result in lasting disability) into an impact with littlepotential for skull fracture or severe brain injury.

Acknowledgments

We gratefully acknowledge the Natural Sciences and Engi-neering Research Council of Canada (NSERC - Research Tools andInstruments Program) and the Canada Foundation for Innovation(New Opportunities Fund) for providing funding to purchase someof the measurement equipment used in this research. We also thankNSERC for supporting author CD with a postodoctoral fellowship.

References

Amoros, E., Chiron, M., Martin, J.-L., Thélot, B., Laumon, B., 2012. Bicycle helmetwearing and the risk of head, face, and neck injury: a French case–control studybased on a road trauma registry. Inj. Prev. 18, 27–32.

Beckwith, J.G., Greenwald, R.M., Chu, J.J., 2012. Measuring head kinematics in foot-ball: correlation between the head impact telemetry system and Hybrid IIIheadform. Ann. Biomed. Eng. 40, 237–248.

Benz, G., McIntosh, A., Kallieris, D., Daum, R., 1993. A biomechanical study of bicyclehelmets’ effectiveness in childhood. Eur. J. Pediatr. Surg. 3, 259–263.

Canadian Institute for Health Information, 2010. Cycling Injury Hospitalizations inCanada, 2009–2010.

Consumer Product Safety Commission, Safety Standard for Bicycle Helmets; FinalRule, 16 CFR Part 1203, Federal Register, Vol.63 No.46, Tuesday March 10, 1998.

Curnow, W.J., 2003. The efficacy of bicycle helmets against brain injury. Accid. Anal.Prev. 35, 287–292.

Curnow, W.J., 2006. Bicycle helmets: lack of efficacy against brain injury. Accid. Anal.Prev. 38, 833–834.

DeMarco, A.L., Chimich, D.D., Gardiner, J.C., Nightingale, R.W., Siegmund, G.P., 2010.The impact response of motorcycle helmets at different impact severities. Accid.Anal. Prev. 42, 1778–1784.

Depreitere, B., Van Lierde, C., Maene, S., Plets, C., Vander Sloten, J., Van Audekercke,R., Van der Perre, G., Goffin, J., 2004. Bicycle-related head injury: a study of 86cases. Accid. Anal. Prev. 36, 561–567.

Elvik, R., 2011. Publication bias and time-trend bias in meta-analysis of bicycle hel-met efficacy: a re-analysis of Attewell, Glase and McFadden, 2001. Accid. Anal.Prev. 43, 1245–1251.

Eppinger, R., Sun, E., Bandak, F., Haffner, M., Khaewpong, N., Maltese, M., Kuppa, S.,Nguyen, T., Takhounts, E., Tannous, R., Zhang, A., Saul, R., 1999. Development ofImproved Injury Criteria for the Assessment of Advanced Automotive RestraintSystems – II. NHTSA Report.

Fahlstedt, M., Baeck, K., Halldin, P., Vander Sloten, J., Goffin, J., Depreitere, B., Kleiven,S., 2012. Influence of impact velocity and angle in a detailed reconstructionof a bicycle accident. In: Proc. of the 2012 International IRCOBI Conf. on theBiomechanics of Impacts, pp. 787–799.

Foster, K.J., Kortge, J.O., Wolanin, M.J., 1977. Hybrid III-a biomechanically-basedcrash test dummy. In: SAE International 770938.

Gennarelli, T.A., Wodzin, E., 2005. Abbreviated Injury Scale 2005.Greenwald, R.M., Gwin, J.T., Chu, J.J., Crisco, J.J., 2008. Head impact severity measures

for evaluating mild traumatic brain injury risk exposure. Neurosurgery 62, 789.Hagel, B.E., Barry Pless, I., 2006. A critical examination of arguments against bicycle

helmet use and legislation. Accid. Anal. Prev. 38, 277–278.

Hamer, M., Chida, Y., 2008. Active commuting and cardiovascular risk: a meta-

analytic review. Prev. Med. 46, 9–13.Heng, K.W.J., Lee, A.H.P., Zhu, S., Tham, K.Y., Seow, E., 2006. Helmet use and

bicycle-related trauma in patients presenting to an acute hospital in Singapore.Singapore Med. J. 47, 367–372.

alysis

H

K

M

M

M

N

NN

P

P

P.A. Cripton et al. / Accident An

odgson, V.R., 1990. Impact, Skid and Retention Tests on a Representative Groupof Bicycle Helmets to Determine Their Head-neck Protective Characteristics.Wayne State University, Department of Neurosurgery.

endall, M., Walsh, E.S., Hoshizaki, T.B., 2012. Comparison between Hybrid III andHodgson–WSU headforms by linear and angular dynamic impact response. Proc.Inst. Mech. Eng. P: J. Sports Eng. Technol. 226, 260–265.

aimaris, C., Summers, C.L., Browning, C., Palmer, C.R., 1994. Injury patterns incyclists attending an accident and emergency department: a comparison ofhelmet wearers and non-wearers. BMJ 308, 1537–1540.

attei, T.A., Bond, B.J., Goulart, C.R., Sloffer, C.A., Morris, M.J., Lin, J.J., 2012. Perfor-mance analysis of the protective effects of bicycle helmets during impact andcrush tests in pediatric skull models. J. Neurosurg.: Pediatr., 1–8.

ertz, H.J., Irwin, A.L., Prasad, P., 2003. Biomechanical and scaling bases for frontaland side impact injury assessment reference values. Stapp Car Crash J. 47, 155.

ewman, J.A., 2002. Biomechanics of head trauma: head protection. In: Acciden-tal Injury: Biomechanics and Prevention. Springer Verlag, New York, USA, pp.303–323.

HTSA Traffic Safety Facts 2010 Data, 2010. Bicyclists and Other Cyclists.ightingale, R.W., Richardson, W.J., Myers, B.S., 1997. The effects of padded surfaces

on the risk for cervical spine injury. Spine 22, 2380–2387.age, J.L., Macpherson, A.K., Middaugh-Bonney, T., Tator, C.H., 2012. Prevalence of

helmet use by users of bicycles, push scooters, inline skates and skateboards inToronto and the surrounding area in the absence of comprehensive legislation:an observational study. Inj. Prev. 18, 94–97.

ang, T.Y., Thai, K.T., McIntosh, A.S., Grzebieta, R., Schilter, E., Dal Nevo,R., Rechnitzer, G., 2011. Head and neck responses in oblique motorcycle

and Prevention 70 (2014) 1–7 7

helmet impacts: a novel laboratory test method. Int. J. Crashworthiness 16,297–307.

Pellman, E.J., Viano, D.C., Tucker, A.M., Casson, I.R., Waeckerle, J.F., 2003. Concussionin professional football: reconstruction of game impacts and injuries. Neuro-surgery 53, 799–812 (discussion 812–814).

SAE J211 Instrumentation for Impact Test – Part 1: Electronic Instrumentation.Scher, I., 2006. Head injury in snowboarding: evaluating the protective role of hel-

mets. J. ASTM Int. 3.Scher, I.S., Harley, E.M., Richards, D., Thomas, R., 2009. Likelihood of brain injury in

motorcycle accidents: a comparison of novelty and DOT-approved helmets. In:SAE Technical Paper 01-0248.

Thompson, D.C., Rivara, F., Thompson, R., 1999. Helmets for preventing head andfacial injuries in bicyclists. In: The Cochrane Collaboration, Rivara, F. (Eds.),Cochrane Database of Systematic Reviews. John Wiley & Sons, Ltd., Chichester,UK.

Thompson, D.C., Rivara, F.P., Thompson, R.S., 1996. Effectiveness of bicycle safety hel-mets in preventing head injuries. A case–control study. JAMA 276, 1968–1973.

Thompson, R.S., Rivara, F.P., Thompson, D.C., 1989. A case–control study of the effec-tiveness of bicycle safety helmets. N. Engl. J. Med. 320, 1361–1367.

Viano, D.C., Halstead, D., 2012. Change in size and impact performance of footballhelmets from the 1970s to 2010. Ann. Biomed. Eng. 40, 175–184.

Viano, D.C., Pellman, E.J., 2005. Concussion in professional football: biomechanics ofthe striking player—Part 8. Neurosurgery 56, 266–280 (discussion 266–280).

Wen, L.M., Rissel, C., 2008. Inverse associations between cycling to work, publictransport, and overweight and obesity: findings from a population based studyin Australia. Prev. Med. 46, 29–32.