ABSTRACT

THORAC IC INJURY POTENTIAL

Dav i d C . V i a no , Ph . D . B i omed i c a l Sci ence Dep artment

General Motors Res earc h Laboratori es Warren , M ich i gan 48090 U . S . A .

Nonskel eta l i nJ u r i es , such as l es i ons to organs and ruptures to major vessel s , pose the most s i g n i fi cant "t hreat -to- l i fe 1 1 res u l t i ng from thora c i c i mpact . Pre v i ous human cadaver impact stud i es have concent rated on assess i na the severi ty of human i nj u ry accord i nq to a cumu l at i ve A I S , wh i c h c l ass i f ies ·

ske l et a l and nons kel etal trauma on a common sca l e . I n many si tuati ons ske l etal i nj u r i es domi nate t he response p i cture and , as suc h , may have i n fl uenced u l t i ­mate i nterpretat i ons of i nj u ry potent i al away from the l ess frequent nonskel e ­ta l trauma . I n -depth stat i s t i c a l anal yses of 52 cadaver tests , i nvo l v i nq b l unt t horac i c i mpact , were conducted to del i neate paramet r i c dependenc i e s for both nonske l et a l ( i . e . , organ and vascu l ar trauma ) and ske l etal i nj uri es w ith b i o ­mechan i c a l responses and spe c i men characteri st i cs . Des p i te a demonstrated l i near dependence between overa l l and ske l etal i nj u ry severi t i es on the l evel of norma l i z ed thoraci c def l ec t i on , " l i fe- threateni ng" i nj u ry seems more reason ­a b l y associ ated wi t h a norma l i zed defl ect i on l i m i t ( P/ D r , 0 . 32 ) . Deforma t i on beyond thi s l i m i t appea rs to tri gger a struc tu ra l i nstab i l i ty and cause a col l apse of the r i b cage , wh i ch resu l t s i n mu l t i p l e ri b fractures and a real i s ­t i c hazard for i nt rathora c i c organ and vascu l ar i nj u ry . The l atter encompass a s pectrum of serious i nj u ry ( A I S 3 -6 ) . Non ske l eta l i nj u ry occu rrences are a l s o shown to be a stat i st i ca l l y si g n i f i cant funct i on o f the spec i men ' s age , l eve l of norma l i zed thorac i c defl ect i on and exten s i veness of s ke l etal damage .

OVERV I EW

Restra i nt and protect i on of oc cupants i n an automot i ve col l i s i on i s cu rrent l y prov i ded , i n part , by d i stri buted thorac i c l oads app l i ed by torso be l t s , i nf l atabl e a i r cush i on s and i n some i nstances by a steeri ng col umn . In a l l cases the thorax encou nters restra i nt l oads . Load i n g t he thorax can produce var i ou s bi omec han i ca l re sponses i n the h i g h l y comp l i ant and anatomi ca l l y comp l ex reg i on of the body . Cons i d era b l e l i terature has devel oped duri ng recent years coveri ng experimental and ana l yt i ca l effort s to i mprove our unders tand inq of b i omechan i ca l responses and i nj ury mechan i sms . Thi s work has l ed to the deve l ­opment of i nj ury avoi dance g u i de l i nes for the huma n thorax , so t hat mean i ngfu l measures of i nj u ry potent i a l are s l ow l y becom i ng avai l ab l e . Suc h too l s are v i t a l to des i gn and devel opment efforts a i med at i mprov i ng automot i ve i nteri or and re stra i nt system performa nce dur ing a car cras h . Cu rrent forms of act i ve and pass i ve occupant restra i nt genera l l y ut i l i ze t horac i c l oads to m i t i gate t he effects of t he secondary i mpact of the occupant w ith t he automot i ve i nteri or . I nc reased usage of restra i nts shou l d resu l t i n a decrease i n the frequency and severi ty of head i mpact . The overa l l potent i a l for thorac i c i nj u ry shou l d a l so decl i ne but may i ncrease i n promi nence as the re l at i ve s i te of seri ous i nj u ry . De s i g n re l ated questi ons a l ready strai n our understand i ng of the b i omechan i ca l response and i nj u ry phenomena associ ated wi t h t horac i c i mpacts . Mul t i d i s c i -p l i na ry researc h u s i ng many forms of su rrogates i s the o n l y means of cont i nu i ng to i mprove our understand i nq of i mportant fa ctors i nf l uenc i n g human i nj u ry .

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INTRODUCT ION

A comprehens i ve ser ies of cadaver experiments i nvo l v i ng t horac i c impact ha s been sponsored by the General Motors Research Laborator i e s at a wi l l ed- body research program at the Un i vers i ty of Cal i fo rn i a a t San Di ego . Near ly ten years of research have been i nvested i nto s tu d i e s of the t horac i c response of human cadavers exposed to bl unt fronta l impact . The i nvest i ga t i ve efforts have been ana l yzed and reg u l ar ly reported to the automot i ve safety­occupant protect i on commu n i ty by the pri mary researchers : Mes s rs . Kroe l l , Schneider or Nahum [ l - 3] . I n add i t i on , a deta i l ed summary of t horac i c re s ponses to b l unt fronta l l oad i ng was recent l y presented by Kroel l [ 4 ] . Th i s b i omechan­ics data has a l so formed the bas i s for subsequent dummy deve l opment efforts [ 5 , 6 ] , i mproved thorac i c response gui del i nes and i nj u ry cri teri a [ 7 , 8 ] and a n i ma l model experi mentat i on [9 - 1 2] .

Kroe l l ' s tests [ l -4 , 1 3 ] i ncorporated a ri g i d stri ker ma ss acce l e rated to i mpact v e l oci ty by e l as t i c s hoc k cords . The free s l i d i ng i mpactor was a l l owed to axi a l l y impi nge aga i ns t the thorax of an upri ght ori e nted specimen . The contact i ng i nterface was an unpadded 1 5 . 2 cm d i ameter wooden b l oc k w i t h rounded edge t o prevent l oca l i zed l oads a t the pe ri meter . The axi s of contact wa s a l i gned perpend i c u l ar and centered midsag i ta l l y at the l eve l of the 4th costal i nters pace on the sternum ( see F i g . 3 , Ref . [ 4] ) . I nerti a l l y compensated l oad and t hora c i c penetrat ion measurements were obtai ned i n mos t cases wi th frequent sterna l and/or sp i na l accel erat i on and i ntrathora c i c fl u i d pressures i n more recent experi ments . The chronol ogy of thi s experimental effort evi nces an i mproved unders tand i ng of the fundamenta l bi omechan i cs and trauma mechani sms associ ated w ith t horac i c impact s .

Gadd and Kroe l l [ l ] perce i ved a tendency i n the sternal i mpact data t hat the l eve l of resu l tant i nj u ry was dependent upon the deqree of thorac i c penetrat i on . Kroe l l reasoned that such a dependence was qui te p l a u s i b l e and demonstrated that a rea l i st i c cause -effect re l at i onsh i p was actual l y more va l i d t han a re l at i onsh i p between i nj u ry and force [3] or thora c i c acce l e rati on [ 1 4 , 8 ] . Kroel l ' s [3 , 1 4] and Neathery ' s [ 7 , 8 ] ana l yses of the UCSD cadaver data supp l i ed substant i ve evi dence for the uti l i zat i on of a norma l i zed thorac i c def l ect i on measure of i nj u ry potenti a l wh i ch wou l d supe rs cede a cu rrent s p i na l acce l e rat i on i nj u ry cri teri o n .

I n add i t i on , Neathery [ 5 , 7 ] ut i l i zed adj u sted force-def l ect i o n corri dors establ i s hed by Kroel l [6 ] a s re s ponse gu i de l i nes for t he devel opment of a b i omechani cal l y s im i l ar dummy ehest . Further stat i s t i ca l ana l yses of t he UCSD data by Neathery [ 7 ] a l so expanded the Kroe l l force-def l ect i on corri dors accord i ng to occupant anthropometry . He a l s o cata l oged the degree of thorac i c defl ect i on whi c h wou l d correspond to a seri ous thora c i c i nj u ry A I S 3 .

Neat hery obtai ned i nj ury potent i a l -normal i zed def l ect i on re l at i on s h i p s by mu l t i l i near reg ress i on anal yses o f the cadaver data . Cadaver c haracteri s t i c s were i nvesti gated to determi ne potent i a l effects of b i omecha n i c a l re spon ses and i nj u ri es . Neathery ' s work [ 7 , 8 ] consti tutes a c l a s s i c a l uti l i zati on of stat i st i ca l tec hni ques to extract i nterde penden c i e s between comp l ex mu l t i ­d imens i ona l data . A noteworthy consequence of h i s ana l ys i s was the i dent i fi ­cat i on of the i nfl uence of s pec imen age at death on the degree of expected overa l l i nj u ry outcome . In part i c u l ar , an age d i fferent i a l of 25 years ( 40 versus 65 ) cou l d be shown to aggravate the l eve l of expected i nj u ry severi ty by

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one u n i t of A I S ( A I S 3 vers u s A I S 2 ) . S i nce the average cadaver specimen ' s age for the UCSD s e r i e s was approx i mate ly 65 year s , Kroe l l ' s and Neathery ' s average 65 year o l d subject ' s l i near regres s i on l i nes were shown to be i n essent i a l agreement .

V i ano [ 1 0 ] emphasi z ed that a s many a s e l even di f ferent i mpact con ­d i t i ons were ut i l i zed i n the UCSD expe r i ments and t hat the numbe r of cadavers su bj ected to a part i cu l a r impact cond i t i on vari ed from a l ow of 1 to a h i gh of 1 1 . He reasoned t hat the stat i s t i ca l col l at i on of the i nd i v i dua l data cou l d b i as vari ous re l at i onsh i ps toward the impacts w i t h more specimen exper iments . U s i ng average cadaver responses for each set of d i s t i nct impact cond i t i ons ( t hereby equ a l l y we i gh i ng each type of i mpact event ) , a seri es of new featu res i n the l i near regres s i on analyses were exposed ( see Fi g . 1 4 , Ref . [ 1 0 ] ) . Al t hough V i a no ' s analyses undoubted l y b i ased the res u l t s toward t he s i ng l e or few s pec imen impact test s , the dependence of re s u l tant i nj ury on the degree of norma l i zed defl e ct i on was s hown to l ose spec i f i c i ty in pr�d i c i ng i n j u ry when P/D l eve l s approac hed 0 . 40 , i . e . , res u l tant i nj u ri es were shown to vary from A I S 3 -6 wi thout spec i f i c dependence of t he l eve l of norma l i zed def l ect i o n .

The l as s o f i nj u ry-def l ec t i o n spec i fi c i ty for moderate to l arge l evel s of thora c i c compress i on l ed V i ano to ques t i on the val i d i ty of a general l i near i nj u ry-defl ecti on re l at i onsh i p . I n add i t i on , the mode of spe c i f i c i nj u ry , whet her i t be pr imar i l y ske l etal o r nons kel etal i n nature , and i t s rel at i onsh i p t o b i omechan i ca l response parameters had not been i nves t i gated . Certa i n ly the re l at i ve l y frequent occu rrence of s ke l etal i nj u ri es , wh i ch typi ca l l y domi nate the exper imental response p i cture , may actua l l y have over­s hadowed potent i a l l y more s i g n i fi cant nonskel eta l i nj u r ies wh i ch i n prac t i ce can pose a seri ous threat-to- l i fe .

Epp i nger [ 1 5 ] recent l y anal yzed t he UCSD data for i nterdependenc i es between t he extent of s kel etal i nj ur i es and bi omechani cal responses and cada ver characteri s t i c s . In essence he extended Neat hery ' s mu l t i l i near regress i on ana lys i s by determi n i ng t hat t he degree of norma l i zed thorac i c defl ect i on and specimen age are reasonab l y predi ct i ve of the t horac i c ri b fracture outcome . Epp i nge r ' s rel at i ons h i p i s s l i g h t l y i nfe ri or to Neathery ' s due to a much wider d i spers i on i n t he res u l tant cadaver ri b fract u re data . Epp i nger a l s o anal yzed the r i b fractu re occurrences i n the experi menta l res u l ts of 6 i ndependent s l ed test i nves t i gat i ons i nvol v i ng be l ted cadavers by Cromack , Fayon , Mel v i n , Patri c k , Schmi dt and Wa l s h . He found that t he number of observed thorac i c ri b fractures was a stat i st i ca l l y s i gn i f i cant funct i on of the ma xi mum upper torso be l t force and the cadaver ' s age and we i gh t .

lt may seem surpri s i ng that Ep p i nger i dent i fi ed a funct i ona l depen­dence between torso be l t force and res u l tant ske l etal damage but it j u s t e l l u c i dates a factor that needs further emphas i s . Kroe l l ' s earl i e r ana lys i s work ( s ee F i g s . 1 6 and 1 7 , Ref . [ 3 ] ) demonstrated that peak app l i ed force was a rather poor corre l ator wi th re s u l tant i nj u ry a s compared to norma l i zed defl ec­t i on . Thi s was spec i fi cal l y true for the wi de range of b l u nt i mpact cond i t i ons u t i l i zed i n the UCSD test seri e s . When the res u l ts of a s i mi l ar i mpact con ­d i t i on are v i ewed i ndependent l y , appl i ed force i s genera l l y a moderate p re ­d i ctor of the res u l tant i nj u ry outcome . Th i s featu re has been shown [ 1 0 ] to become more pronounced as the ma s s of the impactor i ncreases a bove 20 kg . B l unt thorac i c impacts to an anima l mode l ( see mu l t i p l e correl at i on coeffi ci ent Tab les 9- 1 1 , Ref [ 1 1 ] ) demonst rated that bot h the overa l l i nj u ry sever ity and

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number of r i b fra ctures were we l l corre l ated w i t h t he peak app l i ed force for reason a b l y s imi l a r impact sever i t i es ( i . e . , 2 1 kg i mpactor wi th a stri ki n� vel oci ty of 8. 1 to 1 0 . 6 m/s ) . Large vari ati ons i n t he i mpact cond i t i o n s re sul ted i n dras t i c reducti ons i n the correspondence o f the l evel of peak force and i nj ury out comes . The authors noted that extrapo l at i on of the observed forc e- i nj ury c orre l at i on to a wi der range of i mp act exposu res wou l d be fa l a ­ci ous .

To fol l ow thi s poi nt a l i ttl e further , V i a no observed that res u l tant i nJ u ry and t hora c i c deformati ons i n the UCSD data were we l l correl ated wi t h t he i n i t i a l ki net i c energy of t he impact ( see F i g s . 1 2 , 1 3 and 1 5 , Ref . [ 1 0] ) . The study showed t hat very d i fferent impact cond i t i on s , i . e . , pai rs of i mpactor mass and ve l oc ity wi t h s i mi l ar i n i t i al energy produced extreme l y s i mi l ar defl ec­t i on and i nj ury responses . Ana l ys i s of response parameters for a ranqe of i mpact cond i t i ons fu rther demonstrated t hat the peak app l i ed force ( see F i g . 1 6a , Ref . [ 1 0 ] ) i s essent i a l l y impact vel oc i ty dependent a nd only for re l a ­t i ve l y l arge impactor mas s do the trends i n peak force moderately correspond to defl ect i on and i nj ury vari at i ons . When human cadavers l oad aga i nst an upper torso bel t , the exposure consti tutes an impact envi ronment somewhat equ i va l ent to a hi g h ma s s fronta l i mpact expos ure so t hat resul tant i nj u ry may re asonably be rel ated to the upper be l t force or i mpact vel oci ty . Eppi nger d i scusses other more subt l e dependen c i e s by a nond i mens i onal ana lys i s wh i ch para l l el s Neathery ' s tec hni ques [ 7 ] and res u l t s i n a ri b fract i on pred i c t i on equat i on rel ated to the peak be l t l oad norma l i zed by the subj ect ' s mas s .

Pre v i ous analyses of the UCSD data concent rated on the devel opment of rel a t i o n s h i p s between overa l l or cumu l at i ve i nj u ry observ at i ons and meas ures of bi omechan i ca l response . The cu rrent ana lyses recognizes a potent i al i nherent l i mi tat i on of assess i ng s ke l eta l and nonske l et a l i nj u ri es on a common sca l e . Eppi nger ' s anal yses attempted to remedy thi s s hortcomi ng by i nvest i gat i ng ri b fractures on l y , but we shou l d recog n i z e the i mportance of i mprov i ng our under­stand i ng of t he causat i ve factors associ ated wi t h the occu rrence of potent i a l 1 1 l i fe- threaten i n g 11 trauma , whi ch i s general l y nons kel etal i n nat u re . I n fact , s i nce the skel eta l i nj u ries typi ca l l y domi nate t he experimental response p i cture they may have a l ready bi ased our i nj u ry cri teri a i nterpretat i on away from the more s eri ous nonske l eta l trauma to the more frequent s k e l etal i n ­j u ri es . An i n-depth stat i st i c a l ana lys i s of the UCSD data was undertaken where resu l t ant i nj u ry data was cata l oged accord i ng to three i nj u ry severi ty measures : 1 ) cumu l at i ve overa l l i nj ury ( A I S ) , 2 ) max i mum skel etal i nj ury ( A I Ss K ) and 3 ) maximum nonske l etal i nj ury ( A I SNs ) . Paramet ri c interdepende nc i e s are i nves­t i gated between the vari ous measures of i nj u ry in t he cadave r , i nc l udi ng the number of ri b fractu res , and the avai l a b l e measures of b i omechan i c a l response .

EXPER IMENTAL DATA ANALYS I S

The bl unt thorac i c i mpact data from c adaver tests conducted by Kroel l , Sc hnei der and Nahum [ l -4] a l so form the ba s i s for thi s mode of i nj ury potent i a l ana lys i s . I nj ury resu l t s from thi s mu l t i year experimental effort were rev i ewed and corrected for consi stency w i t h the cu rrent 1 976 Abbrev i ated Inj ury Code [ 1 6 ] . Thus the mod i fi ed cumu l at i ve A I S ( Tabl e l ) shou l d be con ­si stent i n i nj ury severi ty score for a l l of the experi mental observat ion of resu l t ant trauma . The actual reported A I S i s tabul ated for re fe rence purposes on ly a nd wi l l not be used i n thi s stat i s t i c a l ana lys i s . In addi t i on to a general revi s i on of the o bs erved i nj u ry score s , two modes of res u l tant trauma

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were catal oged separate ly : 1 ) s kel etal i nj u ry and 2 ) nonskel etal i nj u ry . Thu s , for each impact event at l east three mod i f i ed i nj ury s cores are defi ned :

i njury ( A IS ) , 2 ) s kel etal i nj ury (A I SsK ) and 3 ) non-1 ) overa l l or cumu l at i v e s kel etal i njury ( A I SNs ) .

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The experi mental data are further grouped accord i ng to the s i mi l ari ty of i mpact cond i t ions so that average bi omechanical and i nj ury response data can be obtai ned for the el even reasonably di fferent impact sever i t i es . The reg i on 1 1 response data i nvol ves ri g i d back restra i nt , whereas the test data from reg i ons 1 - 1 0 i nvol ve free torso impacts .

Underlyi ng features of i njury response characteri st ics a re best descri bed by the average grou ped data responses ( Tab l e 2 , see a l so Tabl e 1 and I I , Ref . [ 1 0] ) . These averages wi l l a l so consti tute the bas i s for the i nj ury ana lyses i n the ma i n body of thi s paper. The importance of anal yses based on the i nd i v i dua l data i s acknowl edged by para l l el i ng the types of stat i s t i cal and parametri c presentat i on i n t he body of thi s paper wi t h s i mi l ar anal yses based on i nd i v i dual dat a . Al though i nd i v i dual data stat i st i c s are not presented i n th i s paper , resu l ts and i nterpretations of grouped responses have been checked and veri f i ed for cons i stency with i nd i v i dua l response stati st ics .

1 4 6

,--

TABlE 2

Gtntr•I Dltl Tflbu1n1on (Gro111)td Olta l

·-· ,,, SubjtcU fyrl

1 1

· -·

" ., 60 " ,, „ " „ II 1) "

- . -

we19ht

(�9)

66.0 b1.l 7 1 , , 8 1 . 9 6S.6 81 .4 S9.0 6S.O 61.6 )4.l 01.�

ChHt Oepth (C11) Ae91on ll.S " ll.• " 24.6 " zs.s „ lJ.1 RI 2it.l „ 20.) " 22.I •• 24.8 " 20.9 '10 n . 1 11:1 \ . llB

tQllPuttCI on <111u •n11ol�1nq noMktleul tr&�

Ktnet1c „ ... Vtlocjty Ener9y lkg l (111/s) (J)

-----

---· n.o . .. 18( n.J i . ' 190 23.0 10.0 l l SO 19. 6 S .! 260 19. 3 ' .I l'O 19.6 10.0 980 10.0 '>.9 llC ... lJ. S 420 1 . 9 1 1 l 110 : . b l].l 110

10.S ·.o 1•0

CU11Ulativt Non s k•ltUI lnJuri

Pe.k irtonaaliztd lnjury ft0. -�'lM:L��v

ry

Hol"N l ln<1• Ho. 1uo• Force Oeflect ton l�dlf1td) XISs1: Oefltction rnctvrti freqweflCy „„! ,kH) IP/O) rn (II (P/D) (f) (\I

· - -----

:.ac '" 1 . 6 6.2 2 .4 ' ' " 400 J.I , . � i. . i 401 9.f 60 • . l 6.19 „, u 1 1 . S ). � "' l i . s 100 1.1 l.OS .J01 J.S 14,0 l.I .i.41 "' H 1.1 ' ·' "' · .c "' 1.0 l.O< . 401 6 . 0 b . O 401 ß.( 100 �.eo .220 1 .1 • 0 a.oe )11 l .I 8 . 8 .19S 1 1 . G II > .14 181 t.8Z .zsc 1 . 1 1 . 1 . . ?.99 .413 1.1 '·' 1.0 )) . ..

A cons i derabl e range i n the severi ty of frontal impacts has been used i n the cadaver experi mentat i on at UCSD . Var i at i on s i n impactor masses of 1 . 6-23 . 0 kg and vel oci t ies of 4 . 9 - 1 3 . 8 m/s cover a n order of magni tude i n the amount of ava i l ab l e ki net i c energy for t he event . These impacts produced a spectrum of i njuri es i n the cadavers rangi ng from mi n i ma l abrasi ons and con­tusi ons to severe vascu l a r , myocard i al and l i ver l acerat i ons wi t h exten s i ve skel etal damage.

Grouped Data Analyses : U s i ng the mod i f i ed i nj ury rat i ngs , the over­a l l i nj u ry severity i s h i g h l y correl ated wi th the de�ree of max i mum norma l i zed t horac i c defl ect i on ( F i g . 1 , see al so F ig . 1 4 , Ref . L l O] ) . A thresho l d l evel of nonnal i zed defl ect i on at P/D � 0 . 20 seems to be consi stent wi th revers i b l e mechan i ca l l oads wh i c h do not produce i nj ury i n the cadavers ( see a l so Fi g . 37 , Ref [4] ) . Lobdel l a nd Kroel l [6] noted that rel axed and tensed human vol unteers cou l d a l so be subjected to abrupt thorac i c defl ect i ons produci ng thorac i c def l ec t i ons u p to P/D � 0 . 1 6 wi thout i nj ur i ou s effect s . For thi s ana l ys i s the l i near regress ion equat i on ha s pract i ca l s i g n i fi ca nce i n defi n i n g t he expected i nj u ry response for mi drange thorac ic defl ect i ons . A not i cea b l e l as s of s peci f i c i ty i n t he i nj ury deformat i on dependence i s encountered as the thora c i c compre s s i on approaches P / D � 0 . 40 , s i nce the overa l l i nj ury severity vari e s nonspec i fi cal l y from A I S 3-6 .

' Blunt Thorac1c Impact Data •Average o-;erattin1ury

per Test Region

Overall lnjuries for Free Torso Impacts

AIS • ·3.44 • 20.05 (P/01 r • 0.92. SAIS • 0.74

R6 •

. R3

R8 • .•R2

R5

O '--�--'-�-'<,;.;.;....��""-�-'-�---.J 0 LI l2 0 3 l4 0 5

Normalized Thorac1c Oeflecllon Pr D

F1g. l ; Ovor1l l tnjury dependence on the degree of normalized thor1c1c deflect ton for the grouped response data.

Average Nonskelelal 1n1ury R6 ·

AISNS ' 5. 16 1• 0.631 P/D • 0.400 <• 0 016! \ 1\3

R8-: R5

.,, Blunt Thorac1c lmpacl Data R2 :;;: 4 • Average Skeletal " 5! ln1ury per Tes< Region

0R4 R3o � Average Nonskeletal 1n1ury tor Occurrence R8 ; 3 • • R6

'-' • • R2 ... . R5 ;; Skelelal lnjuries lor g Free Torso lmpac<s Rt

AISSK • -2.07 • 1322 IPIDI r • 0.86. SAISsK ' 0 67 •

RIO

Normallled Thorac1c Deflection P/D

Ftg. 2; flonskeletal and skelet&l injury dependence on the degree of norma11zed thoractc deflectton for the grouped response data.

1 4 7

The amb i gu i ty i n t he sever i ty of t he i nj ury severi t i e s i s a resu l t of nonskel eta l i nj ury occurrences ( Fi g . 2 ) . An overa l l . i nj u ry rat i ng i s actual ly a compos i te i nd ex of t he d egree of s ke l eta l a nd nonskel eta l gross i nj ury . Ske1 et a l trauma typ i ca l l y dom i nates t he response p i cture i n the l ow- to mid ­range of thorac i c defl ect i o n . As the 1 eve l of def1 ecti on i ncreases s ke l etal i nj uri es may be of secondary s i g n i fi cance a s seri ous organ and vascu l a r i nj ury occurrences become more frequent . I n part i cu l ar , the average nonske l etal i nj ury severi ty i s a pproximately two u n i t s of AIS greater than that a s soci ated wi th the s ke1 eta1 i nj uri es . The l ac k of s pec i fi c i ty of the l i near regre ss i on equa t i on i n th i s reg ime i s a resu l t of the l ea s t squares approxi ma t i on of response data near P/D � 0 . 40 that comprom i s e s t he sever i ty d i fferences between s ke l eta l ( A I S � 2-4 ) and nonskel etal ( A I S � 3-6 ) i nj ur ies a s the most ser i o u s resul tant i nj ury .

The so l i d dots ( Fi g . 2 ) depict the a ve rage severity of skel etal dama g e . As the norma l i zed defl ect i on i ncreases the extent of skeleta l damage severi ty beg i n s to c l u s ter around A I S 3 . I n part t h i s i s a d i rect res u l t of the method of rati ng ri b fractures by the Abbre v i a ted Injury Sca l e [ 1 6 ] , wh i c h c l a s s i fi es s ke l eta l trauma accord i ng to a code t hat g i ve s an A I S 3 to 2 o r more ri b fracture s . By th i s general i nj u ry seve r i ty grou p i ng* of r i b fracture s , s ome l ac k of practi ca l i nj ury sever i ty must be l o s t , i . e . , ten r i b fracture s may we l l be more severe than four r i b fractures but both wou l d be coded A I S 3 . The sens i t i v i ty or s l ope of the l i near regre s s i on for the skel etal trauma i s much l ower than observed for the overa l l i nj ury regre s s i o n but both re l at i on­s h i p s demonstrate an i nj ury thre s ho l d at P/D � 0 . 20 .

At h i g h penetra t i on s a threat of nonskel etal i nj ury exi s ts . I n these tests t he nonske l etal i nj ur i e s were genera l l y l i mi ted to a narrow range of t horac i c defl ect i o n s . wh i ch con s i ti t utes a severe i nj ury res ponse a ve ra ge at P/ D = 0 . 40 . lt i s these serious i nj u r i e s that a re the pri mary causati on of the i ncrea sed i nj u ry sens i t i v i ty of the overa l l i nj ury regre ss i on l i ne . S i nce the overa l l A I S i s a compo s i te of two d i st i nct mode s of i nj u ry , the h i g h correl a t i on coeffi c i ent for the overa l l i nj ury regres s i on may actua l l y g i ve a fa l se im­pres s i o n of pred i ct i ve confi dence i n the l i neari ty of a potent i al i nj ury outcome . I n fact , overa l l i nj u ry severi ty pred i cti ons beyond P/D � 0 . 32 may not be practi ca l . due to the va r i ou s modes and sever i t i e s of resu l tant trauma . Cert a i n l y a seri ous i nj u ry hazard exi sts for norma l i zed defl ect i ons beyond P/D � 0 . 32 , but the actual extent or seve r i ty dev i ates greatl y and i s not l i near l y correl ated wi th c hanges i n t h e degree o f thora c i c compre s s i o n .

I n summary , the overa l l i nj ury sever i ty dependence on nonna l i zed thora c i c defl ect i on i s actua l l y a compo s i te of two d i s t i nct modes of t horac i c i nj u ry : 1 ) a m i d-ra nge s kel etal i nj ury res ponse , wh i c h i s wel l correl ated w ith the degree of defl ect i o n , and 2 ) a h i gh penetrati on range whi ch approac hes a n 1 1ava l anche l im i t 11 where extens i ve skel etal damage may be concomi tant w ith a w i de range of ser ious nonske l etal trauma . l t seems c l ear that the l atter l i mi t i s of most concern due to a potent i a l for these i nj ur ies to be 1 1 l i fe-threaten­i ng . 1 1 An aval anche defl ec t i on l i m i t seems apparent ( Fi g . 3) when t he frequency

* The 1 9 76 Abbrev i ated I nj ury Scal e c l a s s i fi es ri b fractures accord i ng to : A I S 2 : 1 -2 ; A I S 3 : 2 or more ; A I S 4 : fl a i l ehest w ith +l i n each category for ope n a nd/or d i s p l aced fract ures .

1 4 8

of nonskel etal i nj ur i e s i s compared aga i nst the extent of norma l i zed defl ect i on . Th i s representat i on sol i d i fi es the prev i ous d i scuss i on i n that the occurrence of nonskel etal i nj ury i s i so l ated to h igh 1 eve1 defl ect i ons . An aval anche l i mi t i s a l so s upported by the l a rge dev i a t i on i n occurrence frequency cl ustered around P/D = 0 . 40 . Th i s wide dev i a t i on i ndi cates that actua l A IS l evel pre­d i cti ons a re not poss i b l e ; i . e . , a gamut of i nj ur i e s may occur wi thout control .

100 A6:• AJ

%

80 Average Frequency ot "'

j Nonskeletal Trauma

lor A2. A3. AS. A6 & R8 � � 60 % = 67.0 (t32.7)

• R2 P/O = 0.400 it-0.016) � Blunl Thoracic Impact • 0 --- - -�-----

R5 z •Average Response per ö .1() Tes1 Region

g - - · - - _ _ ,

:il' .:: 21)

• RB

0 A9 A7 RIO Al 0 0. 1 0.2 0.3 0.4 0.5

Normalized Thoracoc Oeflectoon P/O

F1g . 3: Frequency of nonskeletal 1njuries as dependent on the degree of norma l i zed thorac1c deflection for the grouped response data.

u.. a:

� "' "

12

10 Average Skeletal Damage with Nonskeletal lnjury

RF • 9.34 (tl.92) P/O = 0.396 (t-0.018)

� 6 1 Blunt Thoracoc Impact

„ • Average Fracture

;;: per Test Region o Average Frac1ure

lor Nonskeletal : lnjury

0.1 0.2 0.3

A8 "°R3 o �

Aib Fractures tor Free Torso lmpac1s

AF • ·8.66 • .:3.80 (P/Ol r = 0.94. SAF • 1.37

0.4 0.5 Normalized Thoracoc �llection P/0

Fig. 4: Hurrt>er of rib fractures resulting from a pa rticu l ar level of nonnalized thorac ic deflection for the grouped response data.

The extens i veness of rib fractures seems reasonably correl ated w i th the degree of thorac i c compress i on ( Fi g . 4 ) . Al though a l i near regress i on l i ne apparent l y fits the data , there are i nsuff i c i ent mi d-defl ect ion reg i on responses to substanti ate a pract i ca l trend . In fact , another r ib fracture response mechan i sm may be s howi ng up that i nvol ves two modes of skel etal react i on : 1 ) reasonab ly reversi b l e l ow l evel thorac i c compre s s i ons wh i c h may produce l i ttl e or no s kel etal damage and l i mit the extent of thorac ic compress i on , and 2 ) reasonab ly i rrevers i b l e h igh l evel thorac i c compre s s i ons wh i c h seems to exceed the structural stabi l i ty of the thorax and may produce extens i ve skel etal damage by a dynami c co l l apse of the r ib cage . The l ack of response data i n the defl ecti on mid reg i on l ends some support to a thorac i c stabi l i ty l i mit* some­where between P/ D = 0 . 28 -0 . 36 . Be l ow that l evel the thorax i s structura l l y stabl e and protects the i ntrathoraci c contents . Beyond that l evel the thorax may become structura l l y unstabl e . The r i b cage may become extens i vely broken thus a l l ow i ng the i nt rathorac i c contents to be l oaded and become more vu l nerab l e

* The concept of a thoracic 11 i n stab i 1 i t,y 11 i s somewhat s upported by hi gh- speed X-ray eva l uation of s kel etal and i ntrathorac i c ki nemat i c s dur i ng b l unt thorac i c impacts [ 1 7-2 1 ] . The r i b ca ge has been observed to compre ss by i n­creased l ateral bending of the rel at ive ly l oosely coupl ed r ibs . I f one or two r i bs fa i l at moderate l evel s of compress ion and force , the rema i n i ng r ibs wi l l be rap i d l y s ubj ected to a h i gher ben d i n g stress . I f i n di v i dua l r i b s are at the el a st i c l i mi t when a few r i b s brea k , a cascade effect i s q u i te l i ke ly to res u l t i n extens i ve s kel etal dama ge . As the structura l i ntegr i ty of the r ib cage i s destroyed , the compre s s i ve l oa ds on i ntrathorac i c organs and vesse l s wi l l dramat ica l l y i ncrease and serious i nj uries may occur .

1 4 9

to i nj ury . When the ri b cage i s exten s i ve ly damaged , the occurrence of non­s kel etal i nj ury may or may not occur i n the cadaver s ubj ects . The c l ose prox­i m i ty of s kel etal responses when nons ke l etal i nj ur i e s were or were not observed seems to i ndi cate that extens i ve s kel etal damage i s necessary but not suffi c ient for nonskel etal i nj ury occurrences for th i s type of thorac i c impac t . By thi s observat ion I don ' t mean to prec l ude the pos s i bl i l ty of very se rious nonskel eta l t rauma i n the absence of exten s i ve s kel eta l damage for other types of impact exposure s . I n fact , response data i s now ava i l ab l e to support se rious i nj ury occurrence when the bl unt impact exposures are midsagi tal l y centered h i gh on the sternum [ 2 2 ] and res u l t i n on ly mi nor s kel etal damage . Certa i nl y , other forms of ser i ou s non s kel etal i nj ury ( e . g . myocard i a l conducti ng system dys­funct i ons ) may occur in vivo [ 1 2 , 1 1 , 1 9 and 23] , wh i ch are beyond i nves t i gat ion i n cadaver based programs .

Si nce the overal l i nj ury sever it ies and ri b fractures produced i n the UCSD b l unt impact exposures were reasonab ly correl ated w ith the degree of norma l i zed thorac i c compress ion ( a l though some pract i ca l l i mi tations have been i denti f i ed ) , a correspondence i s pos s i b l e between the two i nj ury cl ass i fi cati ons ( Fi g . 5 ) . As observed earl i er , skel etal i nj ur ies determi ne the overa l l i nj u ry sever i ty up to A I S 3 where thi s mode of i njury approaches a severi ty asymptote . Exten s i ve s ke l etal damage has a h i gh probabi l i ty of concomi tant nons kel etal trauma ( see a l so Fi g . 6 ) , but the correspondence exh i b i ts much greater d i s per­s i o n . Aga i n , the aval anche effect i s a pparent wi th the l i mi t somewhat above s i x to seven fractured r ibs . Skel etal damage that extens i ve may substant i a l l y i nc rease the vul nerabi l i ty of i ntrathoracic organ and vessel i nj ury . l t shoul d be cauti oned that the standard error of the estimate ( S = l . 1 4 , Fi g . 5 ) assoc i ­ated w i t h the overa l l i nj ury dependence on r i b fractures i s q u i te l arge . Th i s i nd i cates that cons i derabl e vari at i on i n probab l e overal l i nj ury outcome ( i . e . actual A I S ) may be ant i c i pated w ith the degree of skel etal dama ge .

.,

0 "9

: O!unt fhOtK•C lmo1C1 0.1111

1 - - -• Av11101 Ovfl'lll ln1ury t>lf Tu1 Rt9•on

o Av1uoe Nonsu11111 �!> • 1n1u1y lot Occurrenu / per hsl Rto.on �ve119e Sh1e111

- - - J1m1oe w11� · �Ot'ISktlelilll ll\tUl'f'

--�RI AISfllS ' !> 16 : 0 6J•

••o

R1 1·A8 .

Rf • 93'1 • • ' 92· i:n • • • R!> AS

" .

Q � e111l 11 1 1111 + tS In• F ret IOISO llTIOatlS AIS • 1 08 • 0 )• JH • 1 0 11 s . .•

! •O R1b F11c11.11u Af

'80

Fig. 5 : Overol 1 ond nonskeletal injury dependencies on the number of fractured rtbs for the grouped response data.

•• .

•00 . � . 116 . 113

90 .\ve119e Frequencv 01 „OftSUltlll l11um1

� • 0 1 A1 A l R� A6 4 Ra "'-._ 610 • r 321•

; " a 60 .• ' 13 � 60 oRI < � ••1 ö , "' Slunl lhO•IC•t lflDICI

i • .\ve1.a9e Resoonst oe• � ·u1 Ae910�

• Ra

.".l

, , 6 •C •;

R1D Fuc1u1rs Rf

F 1 g . 6: Frequency of nonskeletal injury as dependent on the number of fractured ribs for the grouped response data.

A s ummary representat i on of the current observat i ons ( Fi gs . 7 and 8 ) was made s o that they cou l d be compared with the a va i l ab l e stati st i cal eval u­ati ons of prev i ous i nvesti gators . lt i s hoped that thi s compa ri son wi l l establ i sh a rea l i st i c range of appl i cabi l i ty for the a va i l ab l e anal ytical resu l t s , espec i a l l y i n the context of actual i nj u ry -type outcomes . A subjec­t i ve i nterface between not 1 1 1 i fe-threaten i ng 1 1 s kel etal trauma and potent i a l l y

1 5 0

6

(/) 4 < c: .2 1ü � ·;;; 3 "' „ ü >-:; E' 2

0 0

Average Nonskeletal lnjury (AISNsl ....

lnjury Potential From Blunt Thoracic Impact

/ .. ..... /

1 , ...... . 65' . /

/

. · / /

Not "Life Threatening" �/O • . . / ; ; \ (' ·V / Linear Reoress1on

Skeletal Trauma i . ... ; / / / Neathery's Muluple

Kroell's linear .· ·

/ RO<Jrtss� _,;; .. ;)

/ //

Cumul111v1 lnjury / /.; //

S

k

e

l

::�:�:,:gres�sron IAISI

' ..-o //" / / / / /� \

P/D '!'> "Life Threatening" Linear Regrtssron / Nonskeletal Trauma

CAIS5Kl , .·· //

0.20

"· . / �.· · / /

0.30 0.40 Normatized Thoracic Deflection P/D

F1g. 7: SulT'IT'lary representation of injury dependenc1es an the degree of normal hed thoraclc deflect ion ( pub 1 1 shed data taken from Kroell (3] end Neathery [8]).

0.50

6

5

(/) 4 < c: .2 1ü u � 3 „ ü

>-:; E' 2

lnjury Potential From Blunt Thoracic Impact

Not "Life Threatening" Sketetal Trauma

/L RF---.. "'------i '

1

Cumulatlve lnjury " Linear Reoress1onv<AIS)

.,,.,, "' ,. /· .A / /

AVlf19" Nonskoletl

l

lnjUry \Al�/

4:::.:./ " RF i) ,,,..,,

,;

Neathery"s & Epp1nger s "Ute Threatentng" "' Mulliple Linear Regression Nonskeletal Trauma \

Sk1l11al ln1ury Linear ReQressron 1AIS5KI

O l--�--l.��-L-��..L..��..__�_....��-':-�---:-'. 0 6 10 12 14 Rib Fracture RF

Fig. 8: Sumary representatton of tnjury dependencies on the number of fracture rtbs (pub l i s hed data derived from Neathery (8) and Epptnger ( 15) ) .

1 1 l i fe-threaten i ng 11 nonskel eta l tra uma i s superi mposed on the graphs as a pract i ca l gu idel i ne whi ch l im its the real i st i c appl i cabi l i ty of l i near regres ­s i on rel at i onsh ips i nvol v i ng potent i a l overal l i nj ury outcomes . The l i near regress i on equat i on of Kroel l ( s ee Fi g . 17, Ref . [3] ) and the mul t i l i near regre s s i on equa t i on of Neathery ( see Eq . 2 , Ref . [8] , a ssumi ng a mean cadaver age of 65 years ) are i n mutual ag reement ( F i g . 7 ) even though a d i fferent s ubset of the UCSD data were u sed i n each ana l ys i s . Neathery ' s dependence for a mean subj ect age of 40 years i nd i cates that a drop of near l y one un it of AIS can be expected for a part i cu l ar l evel of thorac i c compre s s i o n . The cumu l ati ve 1 nJ ury regress i on of thi s study shows the same i njury sens i t i v i ty as the pre ­v i ous re l at i onshi p s but the i ntercept i s offset , probably due to the uti l i zation of average impact responses based on a l l UCSD data and mod i f i ed i nj ury c l ass i fi ­cat i ons . Al so s hown on thi s graph are the s kel etal i njury reg ress i on l i ne wi th i t s l ower i njury sen s i t i v i ty and the average nonske l etal i nj u ry res ponse . l t seems real i st ic t o su pport the ut i l i z at i on o f l i near regress i on re l at i onsh ips of th i s k i nd for i nj ury pred i ct i ons at l east up to the aval anche l i mi t of P/D � 0 . 28-0 . 36 , but beyond that l evel of compres s i on a wide range of outcomes shou l d be a nt i c i pated . l t i s c l ear that spec i men 1 s age drama t i ca l l y affects the extent of s ke l etal damage ( a l so see Fi g . 2 , Ref . [ 1 5 ] ) and a s such may wel l affect t he l oad or response l evel at wh i ch a thorac i c structura l i nstabi l i ty may be expected . At thi s t ime such a dependency has not been i nvesti gated i n deta i l ( see the sect i on on i nfl uence of spec i men characteri st ics for add i t i onal age re l ated effects ) but it s hou l d be borne i n m i nd when i nterpret i ng the data . lt i s probabl e that the cons i derabl e range i n expected overa l l i njury outcome between Neathery 1 s 40 year and 65 year average s pecimen i s domi nated by skeletal response effects . Thi s i s substanti ated i n part by t he s i m i l ari ty of the same

1 5 1

two ( 40 and 65 yea r ) age responses when r i b fractu res are assumed the i ndepen­dent var i a b l e ( Fi g . 8* ) . Spec imen age effects seem to be impl i c i t l y i ncorpor­ated i n the ri b fracture pa rameter whereas age effects a re more extr ins ic to the resu l t ant degree of nonna l i zed thorac i c compress ion ( see a l so Fi g . 9 ) . The cumu l at i ve a nd skel etal i nj u ry regress i ons exhi b i t very s i mi l ar response sens i t i vi t i es wi t h i nd i v i dual vari abi l i ty prima r i l y i n the i nj u ry i ntercept . In thi s i nj ury potent i al summary , the aval anche l imi t i s l ocated at about s i x t o seven r i b fractures , beyond whi c h the l i near regress i ons l ose pred ict ive spec i f i c i ty and shou l d not be uti l i zed . Ser ious 1 1 l i fe-threateni ng 11 nonskel etal trauma may wel l occur with exten s i ve skel etal damage ( see Fi g . 6 ) for thi s type of impact exposure .

Infl uence of Seec imen Character i sti cs : Neathery uti l i zed mul t i l i near regres s i on tec hn i ques i n an attempt to extract the dependenc ies of thorac ic i nj ury on bi omec han ica l response parameters and specimen characteri sti cs . H i s work was based o n a sel ect subset o f the UCSD data whi ch i nc l uded experi ments from most impact severi t i es . Because of some spec i fi c l i mitati ons on thi s stat i st i c a l techn i que an a lternate i nvesti gat i on o f the i nfl uences of spec imen age was undertaken to ref ine our understandi ng of i t s s i gn i f i cance i n the i nj ury proces s . By u s i ng the i nd i v i dual mal e res ponse data for i denti ca l impact severi ty tests ( Fi g . 9 ) , i t was hoped that the i nfl uence of age on i nj u ry and b i omechani cal response cou l d be more cl early i dent i fi ed , i . e . with as many of t he other confoundi ng factors hel d constant . lt i s c l ear that the number of fractu red ri bs and extent of overal l i nj ury are a ffected by the s pec imen ' s age. The potent i al for exten s i ve skel etal damage , probab l y re­s u l t i ng from a t hora c i c ri b cage col l a pse dur i ng l oadi ng , a nd concomi tant nons kel etal i nj ur i es i s potent i ated i n the o l der spec i men .

Even though the ol der spec i men ' s thoraces are on ly s l i ghtly more compl i ant , res u l tant i nj uri es are si gn i f i cantly greater . In fact , a 25 year age d i fferent i al has an effect ( Tabl e 3 ) on t he proba b i l i ty of more seri ous i nj ury. Th i s age d i fference i s prec i se l y that encountered i n comparing average l a boratory a nd f i e l d experi ence subj ect s . The l aboratory spec i men a veraged 65 years of a ge whereas the average ma l e dr iver i n the U . S . i s l es s than 40 years of age. ** Thi s age d i fference i nc reases the probab i l i ty of serious i nj ury and exten s i ve ri b fractures by 40% to 80% . The potentia l for greater thora c i c defl ect i ons and more ri b fractures i s of more p ract i ca l s i gn i f i cance s i nce they i ncrease the potent i a l for surpa ss i ng t he stabl e defonnat i on l i mi t of t he ri b cage , wh ich i t sel f may be age affected .

* Actua l l y , the mul t i l i near regress i on equat i ons of Neathery [8 ] , which rel ate overa l l i nj ury , P/D and age , and of Eppi nger [ 1 5 ] , wh ich rel ate r i b frac­ture s , P/D and age were a l gebra i cal l y sol ved for a rel at i onsh i p among over­a l l i nj ury , ri b fractures and age . The author recogni zes that mathemat i ca l man i pu l a t i ons w i th regress i on rel at ions h i ps can be probl emat i ca l .

** Even though t he a ge range for t h i s severi ty of impact reasonab l y encompasses the s pectrum of current mal e dri v i n g popul at i on , there i s no guarantee that the premortem h i story and/or skel etal cond i t i on of t he cadaver subjects i s not a l so a b i ased subset of t he current ol der · dri ver popu l at i on . From our experi ence , i t wou l d be reasona b l e to expect the s peci men to produce a b i ased esti mate of the i nj ury responses ( i . e . , devel op ing greater i nj uries ) .

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Spec 1men Age

40'

65'

01 fference w1 t h respecl to mean

Response and lnjury Expectat 1ons as Oependent Upon Specimen Aqes of 40 and 65 Years'

Norma l 1Zed Oeflect 1on P/0 Prob. P/0 , 0 . 400

. 374 1 1 . 9

. 397 44 . 6

l njury Classification AIS Prob. AIS 4

f„ 9

46 . 6

orivin9 age 6 . 1 111

lncrease '" e)I,· pected occu rrence for 65 year o l d 'ioec 1men ;2 . 1

Rib fractures Ar Prob . Rr 2 . 7

7 . 3

1 70. 4 H l

20. �

98. 1

•• J

Sase<J on tne ooserveo „esponses for n\d l e c,oec 1meni. 1uOJel.leo tc ··ec;1or · "1odc t cono1t 1ons ' e . . � - \. c 1mpactor niass at 7 . 8 m / s ve l oc 1 ty . dno asc,um1n9 o norma l J , : P c, � r it'uteo resoonse e),oec t a t 1 o n

Prooao 1 1 1 ty actu.J 1 1 , computed f o r t.. ! � ; w n 1 c " ao;us t s ., .: ; • .: •or dl' 1 nteqer ,ca l e

Mean a9e o f ma le <Jriverc.. fr-ow 197i M\'M.A fact ".. .

ApproJ1 1niate mean aqe of c.adaver soec. 1 ... en< o'iec 1r �n1s �tuch

50 A.o• EttM:1s on 12 - Impact fies-10< ilovloo 2 M_ alt Cldlvtr1 ö aF�� ��eg,·:; IAt>tl � � c � · r � 0 9'. s � 1 59 ·" .40 -� -& - ; i: - - - - ! � � :J) � „ /

/ � Lin11r RtQrtsMOn / z P/D • 0.331 • 9. HO'<tAQtl /

��S!'ltl�e::;. (AQI) r • O n S • 0.81

/ / /

.,, / / f • 0.71. s • o.ozz / /

0 20 60 0 80 0 20 '°

0 / 60 80 0 20 '°

/ ,.. / ' /

/ ..., / /

/ / / / •

/ /

60 80 Specamtn Age fYears1 SPtC•men Ao• 1Years1

Fig. 9: Oependencles of biomechantcal responses and resu1tant 1njurfes on the age at death of the cadaver subjects . which were irn·

pacted wtth a 23.4 kg NSS at 7 . 0 m/s.

Occurrence of tionskeletcil lnjuries Oeoendence on Soedmen and Response Charactenst1cs ·

Parameter ano/or Characteri st lc

Average and Standard Oevut i on �itho�t AIS:is

. ·--�ith AIS�s _

Stat15tical S i 9nHicance·

Specimen Age (yr) Weight (kg) Chest Oepth (cm)

Force ( kN ) P/D AIS Rib Fractures ( • )

40.0 ( · 2 . 8 ) 6 3 . 8 ( . 7 . 3 ) 2 0 . 7 ( · 0 . 8 )

3 . 12 ( :0. 63) 365 ( - . 0 1 3 ) 2 . 0 ( · 1 . O ) 2 . 0 ( . ). 5 )

1 1 . 5 1 · 1 3 .0 ) 7 3 . 2 1 · 1 4 . 6 ) 22. 5 1 1 . 9 )

' . 3 5 • ·0 . 3 2 ) 407 i . . 031 ! � . 3 ·C. 5 l

� - p ) . 4 )

Based o n the observed respon')e\ f o r mall' ·.11t:t. 1"'1•n·. "' o �l'( ted t o rr.•911')11 impact cond 1 t ions ( 1 . e . , 23.4 k9 1nio11ctor n•a\· "' "''/' veloc1ty1

Sµec 1mens: 77119 FM, 79/20 FM. 204/64 FM

Specimens: 65/13 FM, 69/15 FM. 83/22 FM, 203/63 f�

S i n 9 l e ta i l t-test� rL S : not s t g n 1 f i cant at tne : e v e i

P 5 . o· � . s . N . S .

N . S . p • 4 . 9 p 0.5 p 2 . 5

1 5 3

In another a na l ys i s of the ma l e reg i on 2 data , the stat i st i cal s i g n i f i cance ( Ta b l e 4 ) of spec imen and response characteri st ics was determined fo r the occurrence a nd/or nonoccurrence of nonskel eta l trauma . Spec i men age i s a s i gn i f i cant factor a s soc i ated with the i nc i dence of potent i al l y 1 1 l i fe ­threaten i ng 11 nonskel etal trauma . Al l of the res ponse parameters i nvesti gated ( s ee Fi g . 9-A I S i s an obv i ou s factor ) are contri but i ng factors that have a s i gn i f i cant i nfl uence on t he l i kel i hood of nonskel etal trauma . Factors wh i c h aggra vate the potent i a l for a r i b cage col l apse are o f utmost concern . As norma l i zed defl ect i ons a pproach the ava l anc he l i mi t P/0 0 . 28-0 . 36 and excess i ve s kel etal damage i s encountered , a pract i cal threat of seri ous non s kel etal i nj ury exi st s . Thorac i c responses i n spec imens bel ow the aval anche l i mi t P/0

� 0 . 32 seem to be wi th i n the l i mit of structural stabi l i ty of the r i b cage so that the s kel eton can prov ide a protecti ve funct i on for the i ntrathorac i c organs a nd vessel s .

SUMMARY

1 . Thorac i c i nj ury i s actual l y composed of two di st i nct modes of trauma : s kel etal ( r i b fractures ) and nonskel etal ( organ and vascul ar l es i ons ) . Even though nons kel etal trauma poses the most s i gn i f i cant 1 1threat-to- l i fe , 11 s k e l etal i nj ur ies have domi nated the response p i cture and have i nfl uenced current op i n i ons a s soci ated wi th thorac ic i nj ury cri teri a .

2 . Oes p i te a demonstrated l i near dependence between overa l l i nj ury severity a nd degree of norma l i zed thorac i c deflect i o n , the potent i a l for seri ous or 1 1 l i fe-threaten i ng 1 1 i nj ury i s more reasonably a ssoci ated with a defl ect i on l imi t ( P/ D � 0 . 32 ) . The defl ect i o n l imit seems to correspond with a s tructural i nstabi l i ty of the ri b cage . Deformation beyond a l i mi t poses a l i kel i hood for a spectrum of serious nons kel etal i nj u ry ( A I S 3-6 ) wh i c h occurs through a col l apse of the ri b cage w ith a subsequent a va l a nc he effect on i ntrathorac ic i nj ury potent i a l .

3 . The defl ect i o n l i mi t ( P/0 � 0 . 32 ) , whi c h a ppears l i n ked to a s tructura l i nsta b i l i ty of the r i b cage , d i v i des moderate and seri ous thoracic i nj ury . Deformati ons w ith i n the stable response reg i on are mostl y revers i bl e w i t h mi nor skel etal i nj u ries and a bra s i ons domi nat i ng the ep i sode . V i ta l o rgans a nd vessel s rema i n wel l protected by the unaffected structura l i ntegr ity of the thorax . Deformati ons beyond the stabl e l i mit seem to i nvol ve a col l apse of the r i b cage and res u l t i n exten s i ve mu l t i p l e r i b fractures a nd a rea l i stic hazard for 11 l i fe-threateni ng 11 i nj ury to the i ntrathorac i c organs a nd vessel s .

4 . I f a dynami c col l apse o f the r i b cage i s a pos s i b l e i nj ury mec han i sm , i t expl a i ns the c l u ster of nons kel eta l i nj ury responses near a norma l i zed defl ect i o n of P/D � 0 . 40 . Deformati ons beyond the s tab l e l im it of t he r i b cage ( P/ 0 � 0 . 32 ) cou l d produce a gross fai l ure of the thorac i c s ke l eton and a j ump i n thorac i c compress ion to P/O � 0 . 40 . At that compress i on l evel the i ntrathorac i c a nter ior ( s ternum ) a nd posteri or ( ve rtebra l ) bony su rfaces woul d be i n near di rect contact . Interesti ngl y , the l i mi t of i ntra­t hora c i c compress i b i l i ty for b l u nt fronta l i mpact has been estimated at P/D � 0 . 40 i n other i nvest i gati ons .

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5 . For b l u nt impact to ma l e cadavers wi th a 2 3 . 4 kg i mpactor mas s and a 7 . 0 m/ s stri k i ng vel oc i ty , the proba bi l i ty of a nons kel eta l i nj ury occurrence i s a stat i st i ca l l y s i gn i fi cant functi on of the spec i men ' s a ge ( P < 5 . 0% ) , l evel of norma l i zed thorac ic defl ect i on ( P < 4 . 9% ) and the extens i veness of resul tant r i b fractures ( P < 2 . 5% ) . Substa nt i a l response and i nj ury di ffer­ences are a l so expected between a 65 year o l d spec imen ( average a ge of a l a boratory cadaver ) and a 40 year o l d spec i men ( average age of current dri vers ) . The o l der specimen i s much more vul nera b l e to i nj u ry w ith the probabi l i ty of A I S > 4, up by 37 . 7% ( 46 . 6% versus 8 . 9% ) a nd r i b fractures > 4 up by 7 7 . 4% ( 9 8 . T% versus 20 . 7% ) .

-

ACKNOWLEDGEMENTS

The author i s greatful for a st imu l at i ng di scu s s i on wi th Dav i d E . Mart i n , Automot i ve Safety Engi neer i n g , Env i ronmental Act i v i t i es Staff , wh ich seeded the i dea for th i s research i nvesti gat i on . The anal ys i s work of Ni cho l as Sargent i s apprec i ated .

REFERENCES

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