PROTECTION AGA I NST REAR-END ACC I DENTS

by

C . Thomas* , G . Faverj on* , F . Hartemann* , C . Tarri ere* , A . Pate l ** , C . Got**

* Laborato i re de Phys i o l og i e et ·de B i omecan i que de l ' Assoc i at i on Peugeot S . A . / ReAau l t , Rue i l -Ma l ma i son ( 92 ) , France .

**· I nst itut de Recherches ürthoped iques et Acc idento l og i ques ( I . R . B . A . ) , H6p ital Raymond Po i nc are , Garches ( 92 ) , France .

After a presentat ion of a l itterature rev iew, a techn i c a l and med i c a l survey an car acc idents conducted by the Assoc i at ion Peugeot S .A . / Renau lt i s presented .

L I TERATURE REV I EW

Literature rev iews dea l i ng w ith rear-end impact s , cerv ica l i njur ies and the effect i veness of head restra i nts have a l ready been made by severa l authors ( 1 ) ( 2 ) . Consequent l y , we sha l l on ly deal with the fol l owing subjects :

- frequency and sever ity of rear-end impact

cerv i c a l hyperextens i on , nature of i n j ur ies and symptoms ,

- effect i veness of head restra i nt ,

- r i sk factors of cerv i c a l i n j ury by hyperextens ion .

The percentage of fata l it ies due to rear-end i mpact i s very l ow ( around 3 % ) i n Europe ( 3 ) and i n the Un ited States ( 2 ) . Fata l acc i dents descr i bed by C . J . Kahane ( 2 ) are essent i a l ly re l ated to severe compartment col l apse ( tr ucks or f i xed obstac l es ) , to eject ion or f i re .

I n f act , few peopl e are very ser ious ly i n jured i n a rear-end impact . Accord ing to the American N .A . S . S . 1 979 f i les , out of 1 000 frontseat occupants i n vol ved in rear-end acc i dents , on ly one d ied and t h i rty are admitted to hospita l . Among v i ctims , 78 % susta i ned 1 1wh i p l ash 1 1 i n j ur ies .

Sudden cerv i c a l hyperextens ion pathol ogy , without any head or neck contact i s rather unknown for living persons . Results of researches made on an i ma l revea l muscu l ar and l i g amentous tears , muscul ar hemorrhages and troubles coming from the encepha l on or from the nervous system because of forces transmi tted by the superior part of the sp i na l cord ( 4 ) .

On the bas i s of 1 1 c i nerad i ograp h i c 11 analys i s of cadavers , Ne l son ( $ ) po i nted out abnormal mod i f i c at ions of the cerv ica l s p i ne due to soft cerv i c a l t i ssue i nj ur ies after hyperextens i on ( o r hyperf lexion ) .

( * ) Numbers i n parentheses des ignate references at the end of paper .

l 7

There i s a l ack of object i v ity and def i n i t ion about 1 1wh i pl a sh 1 1 i n jur ies . Th i s i s the reason why med i cal techno logy uses words to qual ify such i njur ies , taken from ·k i nemat i c descr i pt ions used by b i omechan i c i an s .

If the Eng l i s h term 1 1wh i p l ash1 1 i n vented in 1 928 by Crowe has often been d i scus sed ( 6 ) , it i s the s ame for the French term 1 1coup du l ap i n 1 1 wh ich i s even less pert i nent .

There i s a l so a problem with symptoms of i n jury ( headaches , neck pa i n s , percept ion d i sturbances , t i ng l ing sensat ions i n h and s ) wh ich are often del ayed hours or days so that the i n j ury i s not ev ident at the scene of the acc i dent ( 7 ) . Kahane ( 2 ) po i nts 'out that 55 % of wh i p l ash i nj ur ies are not taken i nto account i n pol i ce reports .

Bes i des , the author reports that , accord i ng to N . C . S . S . f i les , 1 1 wh i p l ash 11 v i ct ims have four days of d i sab i l i ty on an average .

Th i s wou l d be un i mportant i f v i ct i ms d id not sti l l suffer from l ong term consequences several years after the i r acc i dents . Tous sa i nt et Fateck ( 8 ) descr i be a who le group of cerv i c a l -cephal i c troubles i nc l ud i ng psyc h i c symptoms wh ich are s i mi l ar to symptoms due to cerebral concu s s i ons .

Frequency of psyc h i c problems prior to acc i dent ( 30 % i n Tous s a i ntFabeck report ) and the c l a i m i ng tendency of these v i ct ims are frequent ly emphas i zed .

. Brauste i n and Moore ( 6 ) concl ude that the consc ious or unconsc i ous des i re for g a i n on the part of patients may mot i vate them to exaggerate the i r i njur ies and d i sab i l it ie s . Gotten ( 9 ) reported a rev iew of one hundred pat ients with neck i njur ies from rear-end i mpact car acc i dents i n wh ich 88 % recovered after l i t i gat ion was sett led . He stated that 85 % of them had psychosomat i c comp l a i nts . Accord i ng to Fourn ier ( 1 0 ) , 9 0 % of cases of d i sab i l ity due to cerv i c a l s p i ne i n j ury d i s appear 1 8 months or 2 years l ater . The 1 0 % l eft are e ither cerv i ca l vertebra fractures consequences or psychosomat i c cases .

Head restrai nts effect i veness for reduc i ng wh i p l ash cerv ica l i n j ur ies was def i ned in 8 studies (Table 1) . Effect i veness var ies from - 22 to + 55 % . Such a d i spar ity revea l s uncontro l led b i a s wh ich we sha l l study i n

Tab l e - Head Restra i nts Effect i veness and Cerv ica l Injuries Reduction Data Sources

Sweden , 1 973

Los Ange les , 1 970 I nsurance C l a i ms . . . . . . . . • . . . • • . . . . .

Rochester , 1 972 • . . • • • . • • .

Au s t r a 1 i a , 1 9 7 9 North Caro l i n a , 1 972/73 . •

A . C . I . R . . . • . . . . . • . . . . . . . .

M . D . A . I . . . . . . . . . . . . . . . . . .

N . C . S . S . . . . . . . . . . . . . . . . . . o (* ) s ig n i f i c ant stat i st i ca l tests

Ref erences

1 2

1 3

1 1

1 4

1 6

1 5

3

Effect i veness ( % )

55*

1 8*

1 5 1 0

6 0

- 5

- 22

the next paragraph . Majority of stud ies points out that the benef i c i a l effect

1 8

was more not i ceab le for fema le occupants than for male ones ( 1 ) ( 2 ) ( 1 1 ) ( 1 3 ) . Same i n vest i g ators report the less effect i veness of head restrai nts i n the down pos i t i on ( respect i ve ly 1 7 % and 1 0 % for i ntegral and adju stab le restra i nts ) ( 2 ) . One of them poi nts out that head restra ints do not reduce death r i s k .

F i n a l l y , a l l the authors agree to s ay that wh i p l ash i n j ur ies can occur with head restra int , even if it i s proper ly adjusted . Head restra int can very rarely be the c ause of cerv ica l i njur ies ( 1 7 ) ( 1 8 ) .

Major ity of peo�l e i�vol ved i n rear-end impacts does not suffer from cerv ical i n jury ( 1 )(2)( 1 1)(14) . States ( 1 ) gave a l i st of factors wh ich cou ld change the r i sks . They are e ither human or techn i ca l .

·

Human factors : sex ; age , he ight , cerv i ca l antecedents , seat pos i -t ion and att itude at the moment of the impact .

Techn i c a l factors : seat damage , reduct ion of the passenger compartment and head restra int .

Sex has a major i nf l uence over cerv i c a l i n j ury frequency . The fema l e i nj ury rate i s 1 , 5 ( 2 ) to twice ( 1 9 ) h i g her than the mal e i n j ury one . On the bas i s of anthropometr ica l dat a , States ( 1 ) observed fema les have sma l l er necks re l at i ve to head s i ze .

Other parameters have been stud i ed with the fol l owing resu lts : - con s idering adu lt occupants : age , height and we ight do not exert

a detectable i nf l uence on cerv ica l i n j ury frequency ( 1 ) ( 1 9 ) - front seat damages reduce cerv i ca l i njury frequency by 2 7 % for

dr i vers and by 8 % for front pas sengers ( 1 9 ) . Methodol ogy and resu lts hereafter try to point out the i nf l uence

of techn i ca l factors ( rear-end impacts desc r i pt ion , crash v i o lence , head restra i nt and seat damage i nf l uence ) upon cerv i c a l i n j ur ies frequency .

METHODOLOGY

Cases or 1 g 1 n - The Peugeot S . A . /Renau lt acc ident f i l e conta i n s 481 rear-end impact cases i n vo l v i ng 759 f.ront seats and 287 rear seats occupants , wh i ch occurred between 1 970 and 1 98 1 . Car crash c i rcumstances are des c r i bed i n po l i ce reports . Med i ca l data are commun i cated by hospital s . A rev i s i on of the d i agnost ic s i s made two months after the acc i dent .

Each car i s examined by a spec i a l i st . Invol ved cars exterior deformat ions are photographied and analysed in order to eva l u ate the i mpact veloc ity . Passenger compartment examinat i on reve a l s whether the occupants were restrained or not , if there were head restra int and · seat damgge after the i mpact .

Rear-end i mpact ve loc ity - Est i mat ion of impact ve loc ity i s ca l cul ated from occupant speed var i at ion ( AV ) and from mean acce lerat i on ( mean r ) accord i ng to the method descr i bed by Tarr i ere ( 20 ) for fronta l impact . Est i mat i on of AV has been done for 1 72 car-to-car rear-end impacts and for 9 car-tof i xed-obstac les rear-end i mpacts . These cases are g i ven in f i g . 1 hereafter . The compar i son with exper imental i mpacts ECE 34 and FMVSS 30 1 h i gh l ights that average ve l oc ity in exper imenta l tests i s genera l ly h igher . The d i fference i s due to the fact that exper imental tests are f u l l y d i stri buted on the rear end of the car whereas the majority of real crashe� are with offset .

1 9

Figure 1 : Violence of Rear Col l i s i ons

/\.V 60 -- - -'-- "- -'-- -- - - - · -· � . -._____

50 ,___ -- ---.__ _ ---- -- · - - .__ .___ __ ·-�---- - - t- ------- ----- -- '------t--- -

�--··-- - ---- . tJ-- • „ . „ : t" ::- eo--- -·-----• • e 40 --,__ � - � -�- - z;,-.____ >----" --�- L-----,__ _ ... A � � :- . . „ • • „ • • • - � --- � I+--.__ _ !ü - -----1------• • • „ „ 30

-,_ __ �- · II\ � • Real accidents � � 2 0 - . E. ·• „ :,; J �· -l·· 0 - ----- . „ ,). • o? • 0 experi nental tests � .I- ' 1·1 · n • •ii• - -� ,_____ t-- ,,.., ... _ --1-- -- - ,_ car-to-car 1---• i.- , t-· - ->---' /). moving barrier or wal 1 1 � 1------ - --� - -- -- ECE 34 or FMVSS 301 1 0 1 0

8 9 10 1 1 1 2 1 3 1 4 1 5 1 6 1 7 1 8 1 9 2 0 2 3 4 5 6

C A R .. 1fa ta l i t ir TO . „ i 1 1 vo l�ed � �� ylre ly fatal i t ie

..-.----i -: tr. l l!U Hf 1

„ . CA R 8 4 1 1 62 1 2 0 1 --;

. „TRU C K 1 3 'V;l ! 65 ____ ...... _ o __ J ____ � ·- - '--J--� 1 1 4 1 5 „ . F I X E D j O BSTACLE j

20

d:i c--1 _ _ _ Ll_

m e a n �

F i gure 2 : Distri bution of inj urPd occupants by type of obstac l e .

RESULTS

1 . Rear-end impacts character i st i c s

. Re l at i ve Frequency - In the APR samp l e , rear-end impact represents 7 % of i n volved, 3,8 % of ser ious ly injured and 1 , 6 % of k i l led .

. Obstac les and Inj ury Severity ( f i g . 2 ) - For 84 % , the str i k i ng veh i c l e i s another car but this type of collis ion represents only 20 % of fatal case s . But for two-t h i rds of the fata l rear-end impact s , the car i s struck by a truck .

. Occupant i n j ury sever ity and occupied seat - They are g i ven i n the fol l ow ing table :

Severe ly Sl ightly injured Ki l led Tota l

Un i njured i n j ured ( 1 ) ( 2 ) ( 3 ) ( 1 ) +( 2 ) / ( 3 )

. Front seats 290 447 1 8 1 3 768 4 , 0

. Rear seats 92 1 5 1 7 7 257 5 , 4

Tota l : 382 598 25 20 1 025

The gravity rate i s h igher for rear seats than for front one s , ma i n ly because of i ntrusions of trucks in the rear-end compartment .

• Number of i mpacts - In the present s amp l e , the rear-end impact was u n i que in 39 % of the whole c ases . The over-representat ion of motorway acc i dents i n our f i l e can exp l a i n t h i s percentage . Out of 297 cases among wh i ch the pr i nc i pa l rear-end impact was assoc i ated w ith another impact , t he f i rst was the rear-end one for 248 cases ( for 13 cases , t he order is unknown ) . Rearend i mpacts assoc i ated with secondary ones are thus i nc l uded i n t h i s study because the pos i t i on of a l most a l l the front seat occupants i s not infl uenced by the secondary impacts .

. Impact D i rect i on - lt i s an ax ia l one ( 6 o ' c l ock for 83 % of the cases ) .

. Struck Area - The two t h i rds r i ght and l eft rear-end areas represent 55 % of the cases ( f igure 3 ) .

Figure 3 - Frequencies of deformat ion area of the car

� � lY � � � 3 1 % 2 8 % 24 °/o 9 % 6 % 2 %

Speed var i at i on for the occupant (/,. V ) - The d i st r i bution ( i n % )of 6. V i s g i ven hereafter ( un km/h) : 6V< 1 6 t::,.V 1 6-25 t;.V 26-35 tlV 36-45 /JV 46-55 CY.V 56-65 [1V > 65 km/h

26 , 6 % 45 % 1 9 , 4 % 7 , 8 % 0 , 9 % 0 , 3 % 0

2 1

lt can be observed that more than 70 % of /J.V are i nfer ior to 25 km/h .

2 . Severity of Head and Neck Injuries - Head and neck are the most frequent body areas injured.

Head : The fol l owing table g i ves head i n jury sever i ty in the A IS scale ( 2 1 ) : --

( * ) N · A I S head

Front occupants 759

Rear occupants ( � 1 2 year s ) 9 1

Rear occupants (.:::::: 1 2 years ) 70

0 2 3 4

69 22 8 , 4 0 , 4

40 48 1 1

30 57 8 , 6 1 , 5 2 , 9

5

0 , 4 1 00%

1 00%

1 00%

- Front seat occupant : A IS � 2 head i njuries are observed in the most severe rear-end i mpact s , where seat damage ( open ing of seat back ang le or seat tra�k fa i l ure ) occurs more frequent ly as we see in the fol lowing tabl e :

( * )

. A I S Head : ;;;;:::- 3

= 2

Who le s amp l e :

N

7

64

759

tJ.V known

7 cases

38 c ases

320 cases

% {j V >35 km/h

1 00 %

47 %

9 %

Seat Damage f requency

70 %

60 %

45 %

Head A I S � 3 are due to d i rect i mpact aga i n st the structure of the rear passenger compartment area wh ich moved forward due to the i ntrus i on effect .

Head trauma with lass of consc i ousness < 1 5 1 ( AI S 2 ) are observed i n on ly 1 5 % of damaged seat cases . The majority of A IS 2 to head refers to these cases . Usu a l l y , they are due to head i mpacts aga i n st the bench seats or aga i nst the rear e l ements of the compartment .

Among the 64 i njured who sustai ned A IS 2 to the head , 9 ( 1 4 % ) had head restra i nts . These i n j uries are not only due to seat damages because seats remai ned i ntact 4 t i mes out of 9 .

- Rear seat occupants : rear seat occupants suffer more often head i n j uries than front seat 1 s ones because they d i rect ly susta in the effects of i ntrus ion .

( * ) Ki l led persons non autops ied and rear seat occupants of unknown age are not taken into account .

2 2

Neck : the fo l low ing table g i ves cerv i ca l i n j ury by degree of A I S :

N

Front occupants : 759

Rear occupants (� 1 2 years ) 9 1

Rear occu pants (< 1 2 years ) 70

Frequency of cerv i c a l i n j ur ies for 1 , 3 % ( 1 0 cases ) . Fracture i n 4 cases and burn i n 1 case . w i ng factors :

A I S Neck 0 2 3 4 5

73 25 , 7 0 , 4 0 , 8 0 , 1 1 00%

68 3 1 1 00%

94 6 1 00%

of A IS � 2 for front seats occupants accounts or l uxat ion occur i n 5 cases , contus ion or sprain A I S > 2 cerv ica l i n jur ies are due to the fo l l o-

- cerv i c a l s p i ne hyperextens ion : with worsen i ng f actor ( prev ious cerv ica l fracture , arthro s i s ) : 4 cases

. W ithout worsen i ng factor : 2 cases - Head i mpact : 3 cases - Burn : 1 case

· One of these ten v ict ims uses a head restra i nt . Head restra i nt as a countermeasure wou l d have been effect i ve i n the case where the rear seat of a station-wagon d r i ver cou l d not open because there was a transversal bar s i tuated between the B-pi l l ars . The majority of occupants d id not s uffer from cerv i ca l i nj ury , but neck pa i n s frequency ( neck A I S = 1 ) accounts for 25, 7 % . Th i s percentage i s very h igh i n compari son with the one observed i n other types of acc i dents ( 6 to 8 % ) .

3 . Neck A I S = 1 and preva i l i ng factors .

Factors i n f l uenc i ng neck i n j ury frequency are e ither human or techn ica l .

Human f actors : the d i fference between cerv i ca l pa i n s frequency for ma les ( 23 %) and for fema les ( 43 % ) i s h i ghly s i g n i f i cant . lt i s bec ause fema l e neck muscu l ature i s weaker than ma les ' one . For adu l t s , age , he ight and we ight do not exert a detectable i nf l uence on neck A I S = 1 frequency .

Techn i ca l factors : - Aead restraint ( f i g . 4 ) - The repart i t ion of the d ifferent head

restr a i nt types is : adju stable ( 56 % ) , i ntegrated ( 25 % ) , f itted headrest with the seat-back ( 1 9 % ) . Among adj u stable headrestra i nts , one fourth was i n h igh pos i t i on .

For ma l e s , neck A I S = 1 frequency without head restra i nt i s equa l to 22 % whereas it i s equa l to 27 % with head restra int . The d i fference i s not stat i st i c a l l y s i g n i f i c ant . For fema l es , neck A I S = 1 frequency without head restra int i s equal to 44 % and to 39 % with head restra i nt . The d ifference i s not stat i st i c a l ly s ig n i f i cant .

- Seat damage ( f i g . 5 ) - Seat damage ( seat back ang le � 1 40° or seat track fa i l ure s ) s i gn if icantly reduces of 28 % AIS neck = 1 frequency for fema l e s . Cerv i ca l pa i n s frequency for i n vol ved who d i d not susta i n head impact accounts for 30 % ( 1 23/399 ) . It accounts for only 20 % ( 48/233 ) for those who susta i n a head i mpact . Desp ite the fact that seat damage provokes m i nor head contact , it has a pos i t i ve effect i n reduc i ng the r i sk of cerv ica l pai n .

23

Fig . 4 : frequency of AIS neck = 1 accord i ng to head restraints and sex .

AIS neck: 1 HEAD mm w i t h frequency R ESTRAINT� · h 50 "1,__JWlt out

10%

0 M A L E

•••• •••• •••• •••• •••• •••• •••• •••• •••• •••• •••• •••• •••• •••• •••• •••• ••••• •••• •••• •••• •••• •••• •••• •••• •••• •••• •••• •••• •••• •••• •••• •••• ••••

F E MA LE

Fig . 5 : Frequency of AIS neck = 1 Fig . 6: Frequency of Ais neck = 1 accord ing to seat d amage and sex. accord ing to V and sex .

A IS neck: 1 frequency

50%

0

S E A T D A MAG E

MA L E

24

� w i t h c:::Jwithout

F EMALE

A IS neck: 1 f requency

50°

10%

0

A V

M A L E

c:::J E 21 k.p.h. � > 2 1 k.p)I.

F EMALE

- Rear-end impact ve l oc i ty ( A V ) ( see f ig . 6 ) - The re l at i ve frequency of A I S 1 for females i s lower at iS V> 21 than at (l V � 2 1 . Th i s tendency i s s ign if i cant .

As a conc l us i on , i t cou l d be s a i d that techn ica l f actors da not exert a detectable i nf l uence an cerv ica l i n j ury r i s k for ma les . For fema l e s , the r i sk i s ma i n ly i nf l uenced by 6, V , seat damage and head restra int .

The two f i rst techn ica l f actors 1 1 seat damage and '' A. V '' cannot be d i ssoc i ated . They are corre l ated with the d i minution of cerv i ca l i n j ury r i s k .

4 . Cerv i e a l - i njuries - frequency and -· seat -damage acco!'lEl i ng to sex and /::, V .

F igure 7 g i ves A I S neck = 1 frequency for 348 front seat occupants for wh ich seat damage after impact , sex and a V are known .

Seat damage i s effect i ve for females on ly and reduce the i r neck i n j ury r i sk by 45 % .

FR'.EcpENCY OF A IS NECK=1 FOR .348 FRONT OCaJPAN TS INVOLVED I N RE.AR IMPACT INFLUENCE OF SFAT DAMAGE , SEX AND t,.V ( Fi5 · 1)

SEX ' SFAT f � 21 km/h AV > 21 km/h J 1 I'

1 0 % 20 % 4-0 % 60 % 0 % 20 % 40 % 60 %

MEN

WOMEN

1 1 1 1

INTACT

DAMAGED

IN TA CT

DAMAGED

1 1 1 1 1 1

1 20 , 3 % 1 22, 2 % - 22, 6 % - 17 , 6 %

] 5 2 , 7 % 1 4o % J 4 , 8 % - 1 6 , 6 %

5 . Comparat i ve i nf l uences of neck restra int and of seat damage u pon cerv ica l pains frequency (fig . 8).

For males , seat damage reduces AIS n eck = 1 of 4 % whereas head restra int i s not effect i ve . For fema les , reduct ion of cerv i c a l p a i n frequency i s , an an average , equal to 1 1 % due to seat damage , and 3 % due to head restraint . lt can be not i ced that head restra i nt exerts a detectable i nf l uence most l y when cervi c a l i n j ury r i sk i s great, that i s to s ay when there i s no seat damage , it pr inc i pa l ly occurs i n rear-end impact with AV < 2 1 km/h .

6 . Lang term d i sabi l ity .

Al l v i ct ims suffering from cerv i ca l pa ins recei ved a mai l i ng q uest ionna ire . Out of 52 answers , 33 s tty that they da not suffer any more from any pa i n due to the i r acc idents • . Si x years after the acc ident , 1 9 s ay that they have d i sabi l i t ies wh ich are e i ther st iffness of the back of the neck or mood troubles . The proport ion of v i ct ims with previous cerebral or cerv ica l troub les ( past acc idents , headaches , arthros i s ) or who susta i ned a head impact

25

NECK

% 6 0

50

40

30

2 0

1 0

Figure 8 NECK INJURIES FREc;uENCY FDR FRON T OCQjPAKTS IN REAR IMPA CTS

IN FLUEN CE OF HEADREST, SEX AND SEAT DAMAGE

MEN �MEN IN JURY NECK IN JURY

% 6 0 decrease due t o headrest

-3%

5 0 SEAT DA�GE

WITHOUT

SEA T DA.'-1AGE 4 0 \\"ITH increase due to headrest +7%

\o."I lliOUT decrease due 3 0 t o seat damage

- 1 1%

2 0 pecrease due

· · to seat damage -'-'='*' 1 0

WITHOUT 'WITI-IOUT WITH

HEADREST HEADREST

during the acc i dent i s equa l to 64 % ( 1 2/ 1 9 ) among people who have d i s abi l ity whereas the rat io i s on ly equ a l to 2 1 % ( 7/33 ) for people who do not have any . lt c an be conc l uded that l ang term consequences are not only re l ated to neck hyperextens i on . They c an be obser ved with i n jured peop le who , before the acc i den t , a l ready had a frag i l e cerv i ca l sp ine , or who sustained a head i mpact . Med ica l l i terature on head trauma often desc r i bes sub ject i ve syndromes whose l i keness with neck d i sabi l ity i s obvious ( 22 ) .

26

D I SCUSSION

The poor head restra i nt effect i veness is probably due to the excess i ve vert ica l and most ly hor i zontal d i stance between the head and the restrain� The m i sposit i on i ng of adj usta ble restraint i s not a suff ic ient exp l anat i on because injured whose c ars aY'e equ i pped with " i ntegra l '' seats st i l l suffered from cerv ica l pa i n s . Beyond head restra i nt he ight , the hori zonta l d i stance between head and the restraint seems to be a pert i nent factor . Th i s d i stance i s probably too i mportant in most cases . B i omechan i c a l tests cou ld a l l ow to eva l uate the opt i mum d i stance for head restra i nt effect i veness .

F i n a l l y , the excess i ve st i ffness of the seat , wh ich prevents a more i mportant bend i ng of the seat-bac k , i ncreases A I S neck = 1 r i sk for females . But , on the one hand , i f seat damages reduce neck i n j ury frequency , on the other hand , it cou l d a l so increase mi nor head i mpact frequency , and , by the way , lessen the overal l benef i t s . But such hypothes i s wou l d only occur i n severe rear-end acc idents .

The i nf l uence of the seat st iffness upon the neck i n j ury r i sk must be taken into account in head restra i nt survey . In a recent NHTSA research ( 2 ) , the head restra i nt effect i veness accounts for 1 0 to 1 7 % accord ing to the various types . lt may ex i st a b i as due to the h i gher proport i on of bench seats and sp l it benchesin the sample without head rests . Cons ider ing that f a i l u res occur less frequently for benches than for bucket seat s , the benef it attri buted to head restra int i n stead of rearward bend i ng of the seat back i s quest ionab l e .

CONCLUSlONS

1 . Rear-end i mpact represents 7 % of i n vo l ved , 4 % of ser iously i n j ured and 1 , 6 % of k i l l ed peopl e . Consequent ly , its importance is m i nor compared with fronta l or s ide impacts .

2 . Car-to-car crashes are the source of 84 % of rear-end i mpacts with i n j ured occupants and of on ly 20 % of fatal rear-end acc idents . Car-totrucks account for 1 3 % of the overa l l cases , but for two-th irds of fatal rearend impacts .

3 . Peop l e i n vol ved in rear i mpacts whose /1 V i s inferior or equa l to 25 km/h account f or 70 % .

4 . Severe head and neck i njur ies ( A I S � 3 ) occur for l ess than 1 % of front seat occupants .

5 . Cerv ica l pa i n s ( A IS 1 ) i n fronta l occupants are four t i mes more frequent ( 27 % ) i n rear-end i mpacts than i n other types ( 7 % ) .

6 . Warnen susta i n A I S neck = 1 twice more frequent ly than men ( respect i ve ly 43 % and 23 % ) .

7 . Head restra int effect i veness i s rather sma l l . lt reduces by 1 1 % ( stat i st i c a l l y not s i g n i f i cant ) the A I S neck = 1 for fema les . Whereas there i s no reduc i ng effect for mal e s , the most obvi ous pos i t i ve effect of head restra int i s observed when , for fema les , cerv i c a l pa in r i sk are h igh ( seat i s i ntact at l ow-speed i mpact ) .

,

8 . Damaged seat-back or seat-track f a i l ure have a greater effect i ve ness than head restra i nt , cons ider ing cerv i c a l pa ins reduct ion . The h i ghest · seat damage frequency i ncreases with ß.V and exp l a i n s that , paradox ica l ly , A I S neck = 1 are l ess often observed in rear i mpacts super ior to 20 km/h of 6,V .

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ACKNOWLEDGEMENTS

Our thanks to the Gendarmerie Nat ionale of Mantes- l a-Jol i e and Sai nt-Germa i n -en-Laye , the C . R . S . n° 2 and a l so the Po l i ce of Hauts de Se i ne and Yve l ines for the i r cont i nued a s s i stance .

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