Injury risk assessment of wheelchair occupant restraint

\.V4.,\ Department ofVeterans Affairs

Journal of Rehabilitation Research andDevelopment Vol . 37 No . 5 September/October 2000Pages 573-589

Injury risk assessment of wheelchair occupant restraint systemsin a frontal crash : A case for integrated restraints

Gina E. Bertocci, PhD, PE and Jonathan Evans, BSDepartment of Rehabilitation Science and Technology, University of Pittsburgh, Pittsburgh, PA 15260

Abstract—Obtaining proper occupant restraint fit when usinga wheelchair as a motor vehicle seat is often difficult to attainwith vehicle-mounted restraint systems . The comprehensiveevaluation conducted in this study illustrates the occupant crashprotection benefits of wheelchair-integrated restraint systems,as compared to vehicle-mounted restraint systems . Using com-puter crash simulation, occupant kinematic and biomechanicalmeasures associated with a 20g/30mph frontal impact wereevaluated and compared to injury criteria and SAE J2249WTORS kinematic limits . These measures were also used tocompile a Motion Criteria (MC) index and Combined InjuryCriteria (CIC) index for each evaluated restraint scenario.These indices provide a composite method for comparing var-ious crash scenarios . With the exception of an unsafe 36-inchheight off-shoulder shoulder belt anchor scenario, the MCindex was minimized for the integrated restraint scenario.Similarly, the CIC index was also minimized for the wheel-chair-integrated restraint scenario . This preliminary studyemphasizes the need for transfer of integrated restraint technol-ogy to the wheelchair transportation industry .

Key words : injury prevention, injury risk assessment, inte-grated restraints, occupant restraints, wheelchair occupantprotection, wheelchair safety, wheelchair transportation.

INTRODUCTION

The enactment of the 1990 Americans withDisabilities Act has led to an increased need for trans-portation services for persons with disabilities to allowthem to commute safely to work, participate in recre-ational activities, and carry out activities of daily living.Many of these persons are wheelchair users who areunable to transfer to a vehicle seat, thus necessitating thatthey travel seated in their wheelchairs . Vehicle seatingsystems provide inherent crash protection to their occu-pants through properly positioned occupant restraint sys-tems and crashworthy seat design.

In contrast, wheelchair-seated travelers are unable tobenefit from vehicle seat safety features because wheel-chairs are not typically designed with crash safety as aprimary function. Furthermore, occupant restraint sys-This material is based on work supported in part by the STTR-NIH grant

"Evaluation of a Wheelchair-Integrated Restraint System" awarded to theUniversity of Pittsburgh and the University of Pittsburgh NIDRR RERC

tems used by vehicle-seated occupants have beenon Wheeled Mobility.

designed to accommodate and provide effective protec-Address all correspondence and requests for reprints to : Gina E . Bertocci, PhD,

tion to a diverse population of able-bodied occupants.PE, University of Pittsburgh, Department of Rehabilitation Science andTechnology, 5044 Forbes Tower, Pittsburgh, PA 15260 ; email : ginaber@pitt .edu .

Conversely, wheelchair occupants of differing size typi-573

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Journal of Rehabilitation Research and Development Vol . 37 No . 5 2000

tally use wheelchairs of varying size and configuration,rendering them less likely to accommodate vehicle-mounted restraint systems . In fact, field investigationhas shown that occupant size differential often rendersvehicle-mounted occupant restraint systems unusable,requiring that wheelchair users travel without protectionafforded by occupant restraints . On those occasionswhere the wheelchair user is able to use vehicle-mount-ed occupant restraints, proper belt angles and position-ing required for effective restraint are often inhibited bywheelchair structures such as armrests . The additionalconstraints often posed by these wheelchair structuresinvariably lead to poorly positioned lap and shoulderbelts, which has been documented as a source of belt-related injury (1,2).

In an attempt to provide the wheelchair travelerwith crash protection similar to the able-bodied traveler,this study evaluated the crash protection advantages thatmay be realized by integrating occupant restraints onwheelchairs . By physically anchoring the occupantrestraint to the wheelchair, a custom fit is achieved foreach wheelchair user. Also, this approach removes thecomplexity associated with attempting to fit a vehicle-mounted occupant restraint to the wide range of occu-pant-wheelchair size combinations . It is proposed thatthis integrated restraint concept, which has been provenin the automobile industry, will inherently increase thefrequency of use, as well as the comfort and effective-ness of the wheelchair occupant restraints.

The Need for Wheelchair Integrated RestraintSystems

For wheelchair users unable to transfer from theirwheelchair to a vehicle seat, the wheelchair must serveas their seat during transportation . Because most wheel-chairs were not designed for this purpose, the wheel-chair-seated occupant is not offered the same level ofsafety as those occupants using automotive originalequipment manufacturers (OEM) vehicle seats . Unlikewheelchairs, OEM vehicle seats have been designed toanticipate and accommodate the loads and occupantresponse associated with crash conditions . Furthermore,vehicles have been designed so that the combined seatand occupant restraint protection system is optimized toprovide effective crash protection for a cross-sectionalpopulation . Conversely, occupant protection for thewheelchair-seated occupant requires that after-marketequipment, designed independent of the wheelchair andoccupant, be installed to secure both the wheelchair and

the occupant. In many cases, the result of these circum-stances is poor fit or unusable occupant restraints, lead-ing to ineffective crash protection or belt-inducedinjuries (1,2).

Voluntary standards have been developed to estab-lish general design and performance requirements forwheelchairs intended to be used as a vehicle seat (i .e .,transport-safe wheelchairs), and for wheelchairtiedowns and occupant restraint systems (WTORS) . AnANSI/RESNA standard addressing wheelchairs used asseats in motor vehicles (WC-19) was adopted this year(3) . Similarly, a Society of Automotive Engineers (SAE)standards group has completed a WTORS standard(SAE J2249), which was adopted as recommended prac-tice in 1996 (4) . Based on current practices for the useof occupant restraints, these voluntary standards incor-porate guidance for installation and usage of occupantrestraints . The current practice for wheelchair occupantpelvic restraints is to anchor the belts to the vehiclefloor or to the rear wheelchair tiedowns . Current prac-tice for the shoulder restraint consists of a fixed upperanchor point on the vehicle wall or ceiling, and a loweranchor located on the pelvic belt . ANSI/RESNA WC-19proposes the addition of a pelvic restraint on thosewheelchairs that will be used in motor vehicles . A two-year integrated pelvic restraint phase-in period will begranted to wheelchair manufacturers.

Several problems are encountered with currentpractices employing vehicle-mounted occupantrestraints in wheelchair transportation . The location offixed vehicle anchorages (particularly the upper shoul-der restraint anchorage) is often limited to sites that arestructurally suitable . Location of windows, positions ofseating, and the vehicle's structural integrity often resultin less than optimal placement of these anchor points inpublic transit vehicles . Additional problems with cur-rent restraint practices occur due to different wheelchairseat heights and various occupant populations requiringuse of the same fixed shoulder belt anchorage, leadingto compromised belt fit, comfort, and occupant protec-tion (5).

Figure 1 illustrates the range of shoulder beltanchorage locations required to accommodate adult andchild wheelchair users when following Federal MotorVehicle Safety Standards (FMVSS) shoulder belt comfortzone recommendations (6) . This figure highlights the sig-nificant effect of varying wheelchair seat heights . A fixedshoulder belt anchorage configured for the 50th-per-centile male population would clearly lead to the shoul-

575

BERTOCCI and EVANS : Wheelchair Occupant Restraint Systems

der belt passing over the face or upper neck of smallerwheelchair occupants, rendering the restraints useless, oreven dangerous . Such belt configurations could lead tocompromised protection of a child in a crash or duringnormal driving maneuvers . It is likely that in these casesthe occupant restraint is simply not worn (7).

55 deg

""" 55 deg

Adjusted 6 yr old

Seat Plane 15"

(381 mm)

Figure 1.Variation in vehicle-mounted upper restraint anchor point needed toaccommodate 50th-percentile male and 6-year-old wheelchair userswith varying seat height.

Vehicle-mounted restraints used to accommodatewheelchair occupants also lead to poor belt fit due toobstructions or constraints presented by the wheelchairstructure . Armrests, clothing shields, and other wheel-chair components often obstruct clear paths required toobtain the belt position and angles that provide the mosteffective occupant protection . Poorly positioned lap beltscan lead to "submarining," which can induce abdominaland lumbar vertebra injury in frontal crashes (1,2).Similarly, improperly positioned shoulder belts have beenfound to lead to excessive head excursions (8), increasingthe risk of secondary impact with vehicle surfaces, and tocause internal injuries to vital thoracic cavity organs (9).In addition to safety and comfort concerns, the currentmethods of restraint engagement typically require theassistance of an attendant or operator, thereby leading to

undesirable contact between the disabled passenger andthe vehicle operator.

A recent study conducted by the authors on shoulderbelt anchor influence on wheelchair-seated occupants in afrontal crash shows that varying shoulder belt anchoragelocation impacts occupant protection (8) . In this study,computer crash simulation was used in the evaluation ofvarious Hybrid III ATD (anthropomorphic test device)biomechanical measures and injury criteria while varyingthe position of the shoulder belt anchorage . Anchorsresulting in belt geometries consistent with the NHTSA-proposed shoulder belt comfort zone were found toproduce the most effective occupant protection.Unfortunately, achieving this belt fit in wheelchair trans-portation scenarios using a fixed anchor, vehicle-mount-ed restraint system is difficult to attain across a mixedoccupant population having varying wheelchair seatheights . Recently, WTORS manufacturers have begunoffering"track-type" restraint anchoring systems thathave been mounted longitudinally along van or bus walls,providing horizontal shoulder belt anchorage adjustment.Such installations allow for moving the shoulder beltanchorage fore or aft within the wheelchair securementstation, providing improved belt fit.

A potentially simple, yet effective, solution to theproblem of inadequate wheelchair occupant protection isoffered through equipping the wheelchair with anchorpoints and belts for a 3-point occupant restraint . This inte-grated restraint approach has been successfully implement-ed and has been shown to provide superior occupantprotection in the automotive industry (10,11,12) and inschool buses used to transport infants (13) . Integratedrestraints have also been employed in infant strollers usedfor the transport of disabled children . This approach hasnot, however, been investigated for application to wheel-chairs used as seats during vehicle transportation.

Integrating restraints on wheelchairs used for trans-portation will potentially resolve several operationalproblems associated with present wheelchair restraintpractices.

• First, vehicle-mounted restraint anchor locations currentlyused in public wheelchair transportation are typicallyselected based upon guidelines established to promoteeffective occupant protection for an average or 50th-per-centile male . Application of this anchoring configurationby wheelchair users of a size other than the 50th-percentilemale can be ineffective or even hazardous to the occupantif the belt transmits forces to body areas other than boneystructure . In many cases the restraint is deemed unusable

50th%tile Male

Sternum

Reference 16"

(406 mm)

Adult Seat

me 20" (508 mm)

576


due to poor or uncomfortable fit with the mismatchbetween occupant and restraint further compounded inwheelchair transportation by the variations in wheelchairsize and the size of the user. Integrating the occupantrestraint into the wheelchair serves to resolve these prob-lems of restraint use across a mixed population becausethe restraint will inherently be customized to each wheel-chair and its user, thereby providing optimal occupant pro-tection . It can further be asserted that the frequency ofrestraint use will increase because comfort and fit of therestraint system will be enhanced.

• Second, when attempting to install an occupant restraintanchor on a public transportation vehicle, it is common tofind that the vehicle's structure dictates the use of ananchorage location that is less than optimal . As mentionedabove, despite an effort to optimize vehicle-mountedrestraint anchor location for various occupant sizes, loca-tions of windows, seats, or structurally unsuitable vehiclecomponents commonly preclude favorable installationintentions . Once again, by integrating restraint anchoragesonto the wheelchair, users are afforded a customization inbelt fit that serves to optimize crash protection.

• Third, lap belt angle and positioning play a crucial role inoccupant protection. Numerous factors such as belt slack,lap belt angle, seat design, and position of the body rela-tive to the lap belt have been found to influence sub-marining and belt-related injuries in frontal crashes.When lap belt forces are >680 lb and the belt is posi-tioned over soft tissue, internal injury to abdominalorgans can result (14) . Because wheelchairs introduce anumber of components that inhibit belt path, by limitingwheelchair users to vehicle-mounted restraint systems itis difficult, if not impossible, to obtain a reasonable lapbelt fit. A typical wheelchair equipped with armrests andclothing shields makes it difficult to identify a clear paththrough which the lap belt can pass to a vehicle flooranchorage. Often the resulting lap belt configuration con-sists of passing the belt over the armrest, introducingextensive belt slack and resulting in poor belt positioningon the occupant . Such a situation will greatly increase therisk of injury to the occupant because armrests will like-ly impinge upon the occupant in a crash, rendering the lapbelt ineffective and possibly dangerous to the occupant.Through the use of wheelchair integrated restraints, thefit of the lap belt will inherently be customized to eachindividual wheelchair user, because wheelchair prescrip-tion is dependent upon occupant size .

Fourth, currently most wheelchair users rely on an atten-dant or vehicle operator to engage vehicle-mounted occu-pant restraints . In addition to causing a delay in thevehicle route schedule, this restraint engagement processusurps the independence of the wheelchair user . Throughrestraint integration, wheelchair users with the neededupper extremity function would be able to engage theiroccupant restraints independently, eliminating the needfor vehicle operator assistance. For those wheelchairusers who are unable to independently engage their occu-pant restraints, parents or caregivers would have theopportunity to properly secure the wheelchair user'soccupant restraints prior to their boarding a vehicle.

Injury Risk Assessment Method to EvaluateWheelchair Transportation Scenarios

To compare various wheelchair crash scenarios,such as securement methods, occupant restraint config-urations, seating positions, or variations in seating mate-rials, an injury risk assessment (IRA) methodappropriate to the evaluation of wheelchair crash safetywas developed (15) . This method evaluates both thekinematic occupant response and biomechanical loadsplaced on the wheelchair occupant in a frontal crash(Table 1) . An index that compares occupant motion/kinematic response to SAE J2249 Standard limits,(Motion Criteria [MC] index) and an index that com-pares biomechanical loading to injury tolerances,(Combined Injury Criteria [CIC] index) result throughapplication of the IRA method (4) . In a previous study,this IRA method was applied through crash simulationto evaluate the influences of wheelchair securementpoint location on occupant injury risk in a 20g/30mphfrontal impact (15).

The MC index is based upon evaluation of peakforward excursions of the head, wheelchair, and knee,and comparison to limits established by the SAE J2249Standard to prevent against secondary impact with thevehicle interior. The expression for the MC index isshown in the equation below.

[I]

EXCHead + 0.25 EXCKneeEXCHead - Limit

EXCKnee -Limit

+ 0.25

EXCwc ___EXCwc - Limia

MC=0.25

0 .

577


Table 1.Injury and Motion Criteria Used in the Injury Risk Assessment Method.

Injury or Motion Equation Tolerance Value/Criteria Abbraviations Source

Head Injury Criteria HIC 1000FMVSS

Neck Flexion Mflex 1681 in-lbGM-IARV

Neck Axial Tension Ftens 247 lb for 45 msecGM-IARV

Neck Compression Fcomp 247 lb for 30 msecGM-IARV

Neck Fore-Aft Shear Fshear 247 lb for 45 msec337 lb for 25 msec697 lb (0 msec)GM-IARV

Chest Acceleration a 60 g FMVSS

Forward Head Exchead 25 6"Excursion SAE J2249

Forward Excwe 7 .9"Wheelchair Excursion SAE J2249

Forward Knee Excknee 14 .8"Excursion SAE J2249

WC to Knee Excursion Excwc/ <=1 .1Ratio Excknee SAE J2249

In this equation EXC represents the forward excursion ofthe designated body segment or wheelchair and the sub-script limit represents the SAE J2249 WTORS sled testperformance limitation.

The CIC index is based upon biomechanical mea-sures of the head, neck, and thorax regions, and compar-ison to injury tolerance levels . Injury tolerance levelsused in calculating CIC include the FMVSS and GMInjury Assessment Reference Values (IARV; 16,17).Injury criteria tolerance levels are typically based upon alevel at which 25 percent of the test population experi-ence serious injury (18) . Individual body regions consid-ered in the CIC index are also weighted based upon theirinjury significance derived from accident statistical stud-ies (19) . The CIC index results from the weighing of bio-mechanical measures based upon their injury significance

in a crash, with normalization to injury tolerances.Weighing of injury significance to distinct body regionswas based upon accident studies from the US and Swedencompiled by Viano and Arepally for restrained occupants(19) . Significance values when considering only thehead, neck, and chest are 57 percent, 14 percent, and 29percent, respectively. Where multiple tolerance criteriaare available for a given body region, these values areweighed equally. The derived expression for the CICindex is shown in the equation below.where Faeial, F eomr, Fshear represent axial, compressive, andshear forces acting on the neck, and Mflex represents theflexion moment of the neck . Here, a designates chestacceleration, and HIC is the abbreviation for Head InjuryCriteria . The subscript t affixed to variables is used todesignate injury tolerance levels.

CIC = 0.57[HIC

+ 0.14HICt ]head

4

F axial + F comp + F shear + M flexF axial-t F comp-t F shear-t M flex-t + .29 C a ]neck

a t chest

[2]

578


Methods similar to the wheelchair occupant-basedIRA method have been developed and applied by Vianoand NHTSA in an effort to provide a comprehensive eval-uation of various crash safety features or various motorvehicles (19) . The NHTSA-developed assessmentmethod is known as the New Car Assessment Program(NCAP; 20). The NCAP is a simplistic rating systemassigning a 1 to 5 star rating (5 stars offering the bestcrash protection) to compare vehicle frontal crashworthi-ness . This assessment method evaluates only head accel-eration, as described by the HIC, and chest acceleration.Using HIC and chest acceleration, probabilities of AISlevel-4 chest and head injuries are then predicted basedupon injury risk function curves . A rating of 5 stars isequivalent to combined injury probability of 10 percentor less . Although the NHTSA NCAP system provides acomparative tool for consumers, it is not complete in itsability to predict life-threatening injuries . That is, addi-tional biomechanical measures associated with the head-neck complex, sternum compression and abdominalcompression are not included as part of the evaluation.Bending moments and axial forces applied to the neck,and compression of the thoracic cavity or abdominalregion, not included in the NCAP assessment method,can, however, result in disabling conditions or death.

A rating system that encompasses critical injurymodes and is based upon a simplistic rating system can bea useful comparative tool that allows wheelchair users toevaluate the crashworthiness of various transit wheel-chairs.

METHODS

To evaluate the effects of occupant restraint configura-tion and anchorage location on the crash response of awheelchair user, a lumped-mass model of the SAE surro-gate wheelchair (21) with a 50th-percentile male Hybrid IIIanthropomorphic test dummy was used . The SAE surrogatewheelchair (Figure 2a), a structurally enhanced wheelchair,was constructed for the purposes of repeated sled testing toevaluate the performance of wheelchair tiedowns and occu-pant restraints (WTORS ; 4) . Wheelchair securement in themodel is accomplished using a four-point tiedown system.The occupant restraint system used in the model was eithera vehicle-mounted lap and shoulder belt, or a fully integrat-ed lap and shoulder belt . In both the integrated and vehicle-mounted scenarios, the lap belt angle was positioned 50degrees from horizontal .

Figure 2a.SAE J2249 surrogate wheelchair and Hybrid 11 ATD.

The described model, developed for research associ-ated with the SAE J2249 WTORS, uses the ArticulatedTotal Body/Crash Victim Simulator code (Wright-Patterson Air Force Base) . Validation of the SAE surro-gate wheelchair model has been conducted throughinterlaboratory sled impact testing (21) . The model sub-jects the vehicle, transporting a forward oriented wheel-chair and occupant, to a 20g, 30mph frontal impact, incompliance with SAE J2249 deceleration pulse corridor.

Using the wheelchair/occupant model describedabove, simulations were run with varying shoulder beltconfigurations while maintaining all other conditions con-stant, or an integrated restraint system . Upper anchorpoints investigated were based upon either the SAE J2249-recommended anchorage zone, or actual anchorage loca-tions found in public transit vehicles . Table 2 describes theevaluated anchorage locations and their origins.

For each case, wheelchair and occupant kinematicswere measured, along with occupant biomechanics nec-essary to describe the MC and CIC indices . Individualmeasures include wheelchair, head and knee forwardexcursion, HIC value, head acceleration, neck force andmoment, and chest acceleration. Values were determinedfor each simulation and were used to calculate the MCand CIC indices associated with each restraint scenario.Individual measures were also compared to their specific

579


Table 2.Upper Shoulder Belt Anchorage Configurations Evaluated and Their Origins.

Upper Shoulder Belt AnchorageHeight Derived From

36" vehicle mounted

47" vehicle mounted

63" vehicle mounted

67" vehicle mounted

Wheelchair integrated

Actual vehicle installation - below window

SAE J2249 50th %tile MaleRecommendedZone - lowest pain

SAE J2249 50th %tile MaleRecommendedZone - highest point

Actual vehicle installation - above window

Proposed

injury criteria or kinematic limit as appropriate . Time his-tories through 100 msec were recorded for each measureand are presented graphically for each scenario . (Wherenecessary to ascertain compliance with injury criteria orkinematic limits, time histories were extended .) Overallcrash response of the wheelchair and occupant was alsoevaluated and visually compared at 80 msec.

RESULTS

Comparison of gross occupant motion across thevarious scenarios at 80 msec shows that occupant excur-sion appears minimized and better controlled with anintegrated occupant restraint (Figure 2b) . The integratedrestraint configuration also appeared to reduce overallforward excursion. The off-shoulder configurationappears to offer the least control in terms of occupantresponse, since the shoulder slips free of the shoulderbelt, allowing for increased torso excursion and rotation.

MC IndexWheelchair Excursion . Peak forward wheelchair excur-sions and time history profiles for each scenario did notvary significantly. Peak values for all scenarios were near2.5 inches (Figure 3) . No scenarios produced wheelchairexcursion values in excess of the 7 .9-inch limit estab-lished by SAE J2249.Head Excursion. Peak forward head excursion occurringbetween 0—120 msec was minimized at 12 .0 inches with

the use of an integrated restraint (Figure 4). The 36-inchhigh anchor scenario with the belt positioned on theshoulder also reduced forward head excursion, with apeak of 12.4 inches . The 36-inch anchor height with thebelt positioned off the shoulder resulted in the largesthead excursion (27.3 inches), and was continuing toincrease at 120 msec. This peak head excursion associat-ed with the 36-inch off-shoulder scenario exceeded theSAE J2249 limit of 25 .6 inches.Knee Excursion . None of the evaluated restraint scenar-ios produced knee excursions in excess of the 15-inchSAE J2249 limit between 0—100 msec (Figure 5) . Thelargest knee excursion (2 .7 inches) resulted from the 67-inch high anchor scenario, and was continuing to increaseat 100 msec . The integrated restraint scenario resulted ina peak knee excursion of 2.5 inches . The smallest kneeexcursion (2 .2 inches) was associated with the 36-inchheight off-shoulder scenario.Wheelchair/Knee Excursion Ratio . The wheelchair-to-knee excursion ratios were calculated in accordance withSAE J2249, using the peak excursion values of each vari-able, regardless of the time at which they occurred . Noneof the scenarios exceeded the SAE J2249, which limitsthe ratio to 1 .1.MC Index . Using the above wheelchair and occupantmotion measures, the MC index was calculated for eachof the restraint scenarios . As shown in Figure 6, the MCindex is highest (0 .60) in the 36-inch height, off-shoulderscenario . The MC index is minimized (0 .47) in the inte-grated restraint scenario .

580

Journal of Rehabilitation Research and Development Vol . 37 No. 5 2000

ail

it*

.D

,

-S

U

L,

d ,'r Res ty

.i

floor, off shoulder

Wheel char '

. .R.estramf

01 .

rail It ;'

'

6 " above floor

Figure 2b.Wheelchair and occupant crash responses at 80 msec .

581


Figure 3.Wheelchair forward excursion time histories for varying restraint scenarios.

Figure 4.Head forward excursion time histories for varying restraint scenarios .

582


Figure 5.Knee forward excursion time histories for varying restraint scenarios .

I

Figure 6.CIC and MC indices for varying restraint scenarios .

583


CIC IndexHIC . The HIC values were calculated for each restraintscenario (Figure 7), and showed that the integratedrestraint (261) and 36-inch off-shoulder (215) scenariosminimized HIC values . The largest HIC value (629)occurred in association with the 36-inch on-shoulder sce-nario . None of the scenarios exceeded the FMVSS limitof 1,000.Chest Acceleration . Peak chest acceleration is minimized(33.9 g) in the 36-inch height, on-shoulder anchor sce-nario (Figure 8) . The largest chest acceleration (50 .5 g)occurred in association with the 36-inch height off-shoul-der anchor scenario . None of the scenarios exceeded theFMVSS 60-g limit.Neck Axial Force . The GM IARV for axial neck force is atime-weighted criterion that limits neck tension to 247 lb forno more than 45 msec, and neck compression to 247 lb forno more than 30 msec . Although all scenarios except the inte-grated restraint exceed the 247-lb tension limit, none of thescenarios maintains that level of force for more than 45 msec(Figure 9) . The largest peak neck tension (400 lb) occurred

Figure 7.HIC values for varying restraint scenarios .

in the 36-inch height, off-shoulder scenario . In compression,all scenarios exceed the 247-lb level, but again none of thescenarios maintain this force for the 30-msec tolerance dura-tion . The largest neck compressive force (393 lb) occurred inassociation with the 47-inch height anchor scenario.Neck Flexion Moment . Three scenarios, the 36-inch on-shoulder, the 47-inch, and the integrated restraint,exceeded the GM IARV of 1,681 in-lb (Figure 10) . Thelargest moment (2,870 in-lb) occurred in the 36-inch on-shoulder scenario . The 36-inch off-shoulder scenario pro-duced the smallest neck flexion moment (111 in-lb).Neck Shear Force . The GM IARV for neck shear is atime-weighted criterion that limits shear to no more than247 lb for 45 msec, or no more than 337 lb for 25 msec,or 697 lb for any duration . The 36-inch on-shoulder and47-inch anchor scenarios nearly exceed the 2nd tier of theIARV; both exceed 337 lb for 24 msec (Figure 11) . Thisis only 1 msec less than the allowed duration at 337 lb.The largest neck shear forces occurred in association withthe 36-inch on-shoulder (633 lb) and the 47-inch anchor(665 lb) scenarios. The 36-inch off-shoulder scenarioresulted in the smallest (92 lb) peak neck shear, with theintegrated restraint scenario producing the lowest on-shoulder peak neck shear value (357 lb).CIC Index . Using the biomechanical measures describedabove, the CIC index value was calculated for each sce-nario (Figure 6) . The 36-inch height, off-shoulder sce-nario resulted in the lowest CIC index value (0 .39), withthe integrated restraint scenario producing the lowest on-shoulder scenario CIC value at 0 .51 . The largest CICvalue (0.84) occurred in association with the 36-inch on-shoulder restraint scenario.

DISCUSSION

The comprehensive evaluation conducted in thisstudy illustrates the crash-protection benefits of an inte-grated restraint system for wheelchair occupants . As longas individual injury criteria are not exceeded, lower CICand MC index values are more desirable, as they repre-sent reduced injury risk . Trends in these indices can beused when attempting to compare various occupant pro-tection features . The MC index is minimized in the inte-grated restraint scenario, and with the exception of theunsafe 36-inch off-shoulder scenario, the CIC index isalso minimized in the integrated restraint scenario.

This study highlights the need to examine both kine-matic and biomechanical responses to crash conditions

584


Figure 8.Chest acceleration time histories for varying restraint scenarios.

Figure 9.Neck axial force time histories for varying restraint scenarios .

585


Figure 10.Neck moment time histories for varying restraint scenarios .

r POD

Figure 11.Neck shear force time histories for varying restraint scenarios .

586


when comparing various scenarios . For example, 36-inch height, off-shoulder scenario results in the lowestCIC value because occupant loading is reduced throughineffective restraint ; but this scenario produces the highestMC value and exceeds the SAE forward head excursionlimit (Table 3) . The absence of belt loading on the occu-pant results in a low CIC value representative of reducedinternalized crash forces . Despite lower internalized crashforces, the excessive forward head excursion resultingfrom this scenario could lead to severe injury in the caseof secondary impact with the vehicle interior . Clearly, the36-inch off-shoulder scenario is an unsafe and ineffectiverestraint scenario as evidenced by exceeding the SAEhead excursion limit . Table 3 highlights other anchor sce-narios that exceed, or approach, injury criteria or kinemat-ic limits. Those scenarios that produce an occupantresponse for which any of the injury criteria or kinematiclimits are exceeded should be viewed as unsafe.

Table 3.Anchor Scenarios Exceeding Individual Injury Criteria. _Anchor Scenario

Criteria or Limitation Exceeded

36" height off-shoulder

Head Excursion

36" height on-shoulder

Neck Shear Force (I msec short ofexceeding limit)

47" height

Neck Shear Force (1 msec short ofexceeding limit)

With the exception of the unsafe 36-inch off-shoul-der scenario, the CIC value decreases with increasingvehicle-mounted shoulder belt anchor height . This trendin the CIC value is largely dominated by HIC values,neck moments, and neck shear forces . In general, config-urations that effectively restrain the occupant are associ-ated with higher forces applied to the body. However,occupant excursions are reduced in scenarios with effec-tive restraint. For example, peak head excursions arelower in the integrated, 36-inch height on-shoulder, and47-inch height scenarios . This reduction in head excur -sion is likely due to the shorter lengths of webbing, whichlimit belt stretching.

As indicated in Table 3, two of the restraint anchor-age scenarios approached the GM IARV limit for neckshear force . Of particular concern is the 47-inch heightconfiguration, which complies with SAE J2249 designguidelines for shoulder belt anchorage installation.(Anchorage configurations located above 47 inches pro-

duced neck loads with GM IARVs .) It isimportant to note that neck forces and moments were notexperimentally measured in the validation efforts of thesimulation model used in this study . (The Dynaman neckmodel, as well as the entire body representation, used inthis study has been validated to the Hybrid III 50th-por-centile AT[) [22,23,24]) . Unfortunately, sled tests con-ducted to evaluate the Wheelchair Tiedowns andOccupant Restraint Systems (WTORS) or wheelchaircompliance with standards do not typically mea-surement of neck forces and moments . However, one pre-vious series of 20g/30mph frontal impact sled tests,conducted using the SAE surrogate wheelchair andHybrid III 50th-percentile male ATD measured neckshear forces that exceeded the second tier of the GMIAR'V (337 lb for 24 msec ; reference 25) . Although it isnot possible to directly compare these sled test results tosimulations conducted in this study, these experimentalresults suggest that neck shear produced during20g/3omph frontal impacts may expose wheelchair-seat-ed occupants to increased risk of neck injury.

Injuries that may result from excessive shear forceapplied to the neck include anterior and posterior atlantoax-ial subluxation, transverse ligament ruptures, or odontoidprocess fractures (26,27,28) . Future efforts should includesled testing with ATD neck instrumentation to measureneck forces and moments for use in injury risk assessmentand model validation . It should be noted, however, that pre-vious studies have questioned the biofidelity of the HybridIII neck, documenting uloading response that differs fromthat of human cadavers and volunteers (29,30) . Additionalstudies have also challenged the usefulness of the neckshear injury criterion, suggesting that neck torque about theoccipital oondylea ` rather than neck shear or axial force, isa better predictor of neck trauma severity (31,32).

Through the use of integrated restraints, the occu-pant is better "coupled" to the wheelchair, and these inte-gratedrcmtnaiutn allow the occupant to "ride down thecrash" at the same rate as the wheelchair. Occupant cou-pling with the wheelchair, which is secured to the vehicle,effectively reduces the occupant crash pulse . This effectis reflected in the lower CIC and MC values . Previousstudies, such as by Wainright and others, of integratedrestraint systems used with OEM vehicle seats haveshown reduced HIC values and lower chest accelerationsas compared to vehicle-mounted restraint systems (11).Similarly, Johnson Controls has indicated that theirIntegrated Structural Seat (ISS) produced HIC values 40percent lower than conventional seats and vehicle mount-

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ed restraint systems (10) . Our findings replicate those ofWainwright and Johnson Controls, with the wheelchairintegrated restraint scenario producing the lowest on-shoulder HIC value and chest acceleration.

Alternate methods to those proposed could investigateweighing head and knee excursion measures (used to cal-culate MC) based upon their probability of inducing severeinjury. That is, head excursion could be given greaterweighting than knee excursion because secondary headimpact with the vehicle interior would present a greater riskof severe injury than impact of the knee with the vehicleinterior. Using the weighting method established for theCIC index based upon accident statistics and severity data,a modified MC index would weight head excursion by 0 .89and knee excursion by 0 .11 . Wheelchair excursion could beeliminated from the modified MC equation because injuryassociated with forward wheelchair excursion would bedetected through head and knee excursions . Wheelchairloading of the occupant as described through the ratio of

wheelchair excursion to knee excursion could also be elim-inated from the MC index, as occupant loading would beobvious through neck moments and loads evaluated in theCIC index . Such modifications to the MC index wouldresult in modified MC values shown in Figure 12 . Asshown, the modified MC index would place greater empha-sis on the increased injury risk associated with the excessivehead excursion of the 36-inch off-shoulder scenario . If theCIC index and the MC index were to be combined into oneindex then it is necessary that the appropriate weighting beapplied (i .e., the modified MC index) so as to not dilutethose scenarios that produce occupant responses thatexceed injury criteria or limits.

A limitation of this study is that injury risks associ-ated with chest compression and abdominal loading/com-pression are not included in the CIC index. Future effortswill include the development of an occupant model capa-ble of predicting submarining and evaluating chest com-pression . To develop a validated model, it will be

MC Index - original

Modified MC Index

0 .9

0.8

0 .7

0 .6

0.5

0.4 -

0 .3 -

0 .2

0 .1

0 -

00CQ

Figure 12.Comparison of modified MC versus MC index for varying restraint scenarios .

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necessary to conduct a series of sled impact tests using anappropriately instrumented Hybrid III ATD . This wouldinclude the use of a frangible abdomen, as well as poten-tiometers positioned to evaluate chest compression.

This study supports the ANSI/RESNA WC-19requirement of wheelchair integrated lap belts to bephased in over a 2-year period . The ANSI/RESNA WC-19 standard, along with the SAE 12249 WTORS stan-dard, have served to greatly improve wheelchairoccupant protection during transportation . A major fac-tor leading to this improved protection is that thesestandards require dynamic testing to 20g/30mph frontalimpact conditions . Accordingly, wheelchairs with theirintegrated lap belts (ANSI/RESNA WC-19), as well asvehicle-mounted occupant restraints provided withWTORS (SAE J2249), are subjected to 20g/30mphimpact testing for compliance with these standards.Complete occupant restraint integration, as described inthis study, represents the next step in providingenhanced wheelchair occupant crash protection.

CONCLUSIONS

Previous automotive safety studies have shownthat integrated restraints provide superior occupantcrash protection. This preliminary study reviewed theoperational benefits and evaluated the crash effective-ness of the integrated restraint concept applied to usinga wheelchair as a motor vehicle seat . Through a com-prehensive comparative analysis of injury risk associat-ed with various restraint configurations, it was shownthat a wheelchair integrated restraint system providesthe most effective crash protection . The benefits asso-ciated with the customized fit of integrated restraintsare even more profound in wheelchair transportationbecause variations in wheelchair seat height across amixed occupant population lead to poor belt fit whenattempting to use vehicle-mounted restraint systems.This preliminary study emphasizes the need for transferof integrated restraint technology to the wheelchairtransportation industry.

ACKNOWLEDGMENTS

This work was supported by the STTR-NIH grant"Evaluation of a Wheelchair-Integrated RestraintSystem" awarded to the University of Pittsburgh .

Additional support was provided by the University ofPittsburgh NIDRR RERC on Wheeled Mobility . Theopinions expressed in this manuscript are those of theauthors and do not necessarily reflect the views of thefunding agencies.

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Submitted for publiccation May 13, 1999 . Acceptedin revised form February 24, 2000 .

Injury risk assessment of wheelchair occupant restraint

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