The Effec t s of Age and Strain Rate on

t he Mechanica l Prope r t ie s of Bone

J . C . Wal l , W . C . Hut ton and

B . M . Cyron

Med ic a l Engineer ing Unit , Dept . of Enginee r ing & Systems Technology Polytechnic of Cen t r a l London 1 1 5 New Cavend ish St . , London W 1 M 8JS .

185

The increased inc idence of bone fractures in per s ons over 3 5 year s of age (Knowe lden et a l , 1 964 ) could be a t t r ibut ed t o a decrease i n bone s t rength after the fourth decade o f l ife . The re have been several att empts t o show how the st rength of bone var ies with age . ASCENZ I AND BONUCCI ( 1 965 , 1 968) , SEDLIN , and HIRSCH ( 1 966) and EVANS and LEBOW ( 1 95 1 ) c ou l d not show a trend of var iation of bone st rength wit h age , a l t hough HIRSCH and EVANS ( 1 965 ) d id show that the average Ult i mate Tens i le St rength ( UTS) of specimens from infant s was greater than that shown by ot her invest igators for spec imens from adult femora . The? work of VOSE et a l ( 1 96 1 ) tends t o indicate that the int r in ­s ic s t rength o f bone increases with age . MELICK and MILLER ( 1 96 6 ) on the other hand , show a decrease in the UTS with incr easing age . A decrease in both the tens i l e and compr e s s i ve s trengths with age i s demonst rated by LINDAHL and LI NDGREN � 967 , 1 96 8 ) . The results obt ained by YAMADA ( 1 970) indicate that the UTS of bone inc reases up t o t he third decade of l ife and ther e ­after decrease s .

The d i s agreement shown by these worker s would indicate that some systemat ic study , in wh ich a l l t he var iab l e s are s tanda r d ­ised , s hould be undertaken to dete rmine whether age is a dete r ­minant of bone s trength - a v iew also expre s sed b y SEDLIN ( 1 965 � . I t was this that pr ompted t he study of age re lat ed changes in bone s t r ength . The initial work descr ibed here was undertaken at B i rkbeck Co l lege , London , in col laborat ion with Dr . S Chat ter j i and Profe ssor J . W . Jeffer y .

S ince bone is known to be weaker in tens ion than in compre s s ion , it is mor e l ike ly to fail when tens i le forces are appl ied t o it . Mos t acc idental fractures of bone are te ns ion frac tur e s , even t he spiral fractures assoc iated with tors ion , EVANS e t a l ( 1 95 1 ) . For t h i s r eason , it was dec ided t o measure the tens i le s t rength of sma l l s amples of bone and t o see how t he result s var ied with age .

The cent r a l 1 0 cm or so of the femoral d iaphy s is were re ­moved a t post -mor tem and s tored in sealed plas t ic bags at - 20°C . As far as pos s ib le , bone from per s ons known to have had ske letal d isorder s or who had been bed - r idden for a cons iderab l e per iod pr ior to death was not used in this study . A l l bone s had been fr ozen pr ior to test ing , none be ing tes ted fresh .

Par a l le l s t r ips of bone wer e r emoved from the late r a l quad ­r ant . These s t r ips were l apped to remove any c ance l l ous bone or adhe rant t i s s ue inc luding the per iosteum , so tnat only c or t i ­cal bone remained . Between four and s ix s t r ips we re ob ta ined from each bone .

Aft e r determining t he dens ity of these s t r ips of bone by a s imple water d isplacement technique , they were shaped into ten s ­i le test -pieces us ing two hardened - s t ee l j igs . The result ing samples were f l at , dumbe l l shaped test -pieces , SOmm long , with

186

a cen t r a l wai s t ed se ct ion 29 mm in length having a cros s - sect ion­al area of 4mm2 , WALL et al ( 1 972) .

Ten s i le s trength determinat ion was car r ied out us ing a Hounsfield Tensometer , mod if ied to operate in the ve r t ical pos i ­t ion . The bone samples were he ld in spe c ia l ly des igned mould ­type gr ips , WALL & CHATTERJI ( in pr ess ) . Two l inear d isplace ­ment t rans duc ers we re used to measure load and d isplacement and these we re recorded on an X -Y plotter . An extens ion rate of 1 mm per minute was se lected for this work .

The results obt ained for the UTS are shown in fig . 1 . The short hor izontal l ine s indic ate the mean va lue and the l imits of the ve r t ical l ines indicate � 1 SD about t he mean . I t can be seen that the s t rength increases up to the fourth decade of l ife and thereafter decrease s . This t rend is evident again in fig . 2 wh ich shows the mean values of the mechanical propert ies measur�d for each decade age -group plot ted against age . A l l the proper ­t ie s measured , UTS , Elas t ic i t y (E) and Percentage EJ.ongat ion fE) show t he same t rend . - increas ing in val ue up to the fourth decade of l ife and thereaf ter decreasing .

The graph in f ig . 2 also shows the results from two other stud ies of age -change s in the UTS of femoral cortical bone . The common feature of a l l three s tud ies shown i s t hat there is a de­crease in UTS after the third or fouth decade of l ife . The r e s u l t s of LINDAHL and LINDGREN ( 1 967) d iffer from the othe r two studies in that t here is no peak s imply a gradual decrease in s t rength from the second to the fifth decade of l ife and there­after a more not ic eab le decrease . The d ifferences in va lues for UTS obtained in t he three s tudies may pos s ibly be explained by d ifferences in mo i s t ure c ontent of the samples and in s t rain rate .

These three s t ud ies are lacking in data from ' t he lower age r ange of 0- 1 0 years and to a lesser extent from the 1 0 -20 years age group . Thus they shed no l i ght on the f indings of HI RSCH and EVANS ( 1 965 ) and their work on infant bone . The y tes ted very sma l l , f lat , dumb e l l shaped pieces of cor t i ca l bone in t ens ion . I t was found that there was a t endancy towards an increase in UTS with advanc ing age . However , the ave rage va lue of UTS obt ained from infant s was greater than that repor t ed by othe r inve s t igators for fresh adu l t femur s . The t rend demon ­s t r a t ed would fit the find ings of the present study and those of YAMADA ( 1 970) , ie . an inc r ease in UTS wi th age up to the fourth decade of l ife . However , the higher average value of UTS for infant bone with respect t o ad ult bone might better f i t the f ind ings of LINDAHL an d LINDGREN ( 1 9 67) , A l t hough as HIRSCH and EVANS ( 1 96 5 ) point out , these d i fferenc es in value might be due t o d ifferences in sampl ing s i t e and testing technique .

The s i tuation does need c lar if iaat ion and for t h i s re ason we are cont inuing the work and concentrat ing our effor t s on t he younger age groups . However , material is d iff icu lt t o ob tain and t he s izes involved make the pre se nt method of test ing

1 87

uns uitable . We are a t present working on me thods for obt ain ing the mechan ic a l prope r t ie s of smal l bone s ample s . These inc lude miniatur is ing the t en s i le test pr ocedure described above and a penet r at ion t ype t e s t .

I t has already been s t ated t hat the ear l ier work was carr ied out at t he c omparat ively s low extension rate of 1 mm/min . Bone s t rength is known t o vary with s t rain rate �IEKARSKI 1 96 8 ) . I t was s ugge sted b y CURREY ( 1 95 9 ) that the rate of load ing should be as high as pos s ib le , c onsistent wi th accurate reading ; this he reasons is more l ike l y to s imu late the sequence of event s which occur s when bone frac tures in nat ure . We have decided therefore to s t ud y t he effec t s of strain rate on the ten s i le proper t ie s of bone in the hope of predict ing the values that can be expected when bone is frac t ured in vivo under tr auma cond i t ions .

Str ips of bone were removed as prev ious l y descr ibed but this t ime from both the lateral and anter ior quad rants . The s tudy was l imited to ma les from the 70-80 age group . The t e s t pieces were fract ures at rates varying from 3 mnlmin to 3 , 000 ronlmin on a Mayes Universal Hyd ratl l ic test ing machine . The latter rate is t he maximum that can be achieved wit h t h i s par t ic ular machine . Samples tes ted at this rate wi l l frac t ure in about 20 mi l l isec ­ond s . At t he s lowe s t rate used samples are frac t ured in about 30 second s .

The results obt a ined are shown in fig . 3 , in which the var ious mechanical propert ies t e s ted plotted against the extens ion rate , t he rate of extens ion being plot ted on a loga r it hmic scal e . From this graph it can be seen t hat t he UTS does increase wi th increa s ­ing rate of extens ion in the r ange from 3 0 t o 3 , 000 mm per min . The Elas t ic i t y and Percentage Elongat ion do not vary l inearly with the log of extens ion rat e . The Maximum Modulus of Elas t i ­c it y appears a t a rate of 300mnlmin , which coinc ides w ith the minimum percent age e longat ion .

The se pre l iminary r esults do indicate t he importanc e of s t rain rate in the mechan ic a l t e s t ing of bone and a l s o the need for further work in this area . At pre sent we are c arrying out t e s t s varying t he extens ion rates from 1 0 to 3 , 000 ronlmin . The t e s t s are be ing d one for bone samples from each decade age group . I t is hoped that these results w i l l be ava i l able for presentation at the conference where t hey wi l l be d i scussed fur ther .

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REFERENCES

ASCENZI , A and BONUCC I , E { 1 9 65 ) 'The measurement of the ten s i le s t r ength of isolated osteons as an approach to t he prob lem of int imate bone texture . '

I n ' C a lc if ied Tissue s , Proceed ings of the second European sympos ium on C�lcif ied T is s ue s . ' p . 3 2 5 -33 5 . Eds . R ic he l le L . J . and Da l lemagne M . J . , Univer s i t y of Liege .

ASCENZI , A . and BONUCCI , E . { 1 968 ) The c ompress ive prope r t ie s o f s ingle osteons .

Anat . Rec . 1 58 3 7 5 -3 8 6 .

CURREY , J . D . ( 1 95 9 ) . Differences in the tens ile s t r ength of bone of d ifferent histo logic a l t ypes .

J . Anat 93 87 -95 .

EVANS , F . G . and LEBOW , M . { 1 95 1 ) . Regional differenc e s in sone of t he phys ica l proper t ies of the human femur .

J . App l . Phys iol . � 563 -572

EVANS , F . G . PEDERSON , H . E . and LI SSNER , HR { 1 95 1 ) . The role of tens i l e stress in t he mechan ism of femor a l frac t ures .

J . Bone Join t Suxg . 33A : 485-501

HIRSCH C . and EVANS , F . G . { 1 965 ) Stud ie s on some physical prope r t ies of infant compact bone .

Ac t a . Or thop . Scandinav . 3 5 : 300-301

KNOWELDEN , J . , BUHR , A . J . , and DUNBAR , 0 . { 1 964) I nc idence of bone frac ture in persons over 35 years of age .

B r i t . J . Prev . Soc . Med . 1 8 1 3 0- 1 4 1

LINDiAHL , 0 . and LINDGREN , A . G . H . { 1 967) . Cor tical bone in man I I . Variat ion in t ens i le s trength with age and sec .

Ac t a . Orthop . Scandinav . 3 8

LINDAHL , 0 . and LINDGREN , A: G . H . { 1 968 ) . Cor tical bone in man I I I . Var iat ion of c ompr e s s ive s t r ength with age and sec .

Ac t a . Or thop . Scandinav . 3 9 . 1 29 - 1 3 5

MELICK , R .A . and M ILLER , DR . { 1 96 6 ) . Var iat ions of ten s i l e s trength of human c o r t i c a l bone with age .

192

C l i n . Se i . 30 243 - 248

PEKARSKI , K . { 1 968 )

Ph . D . Thes is , Univer s it y of Cambr idge .

SEDLIN , E . D . { 1 965 ) . A Rheo logical Mod e l of c o r t ical bone .

Ac t a . Orthop . Scand inav . Suppl . 83 .

SEDLIN E . D and HIRSCH C . { 1 96 6 ) . Fac tors affect ing the deter ­minat ion of phys ical pr oper t ies of femoral cort ical bone .

Ac t a . Or thop . Scand inav . 3 7 29 -48

VOSE , G . P . , STOVER , B . J . and MACK P . B . { 1 96 1 ) . Quant i t at ive bone s t rength measurements in senile osteoporosis .

J . GERONT . 1 6 1 20 - 1 24 .

WALL , J . C . , CHATTERJI , S . and JEFFERY , J . W . { 1 97 2 ) . Human femoral cor t ical bone . A prel iminary report on t he re lat ion­ship between st rength and dens i ty .

Med . and Bio l . Eng . { I n pre s s ) .

YAMADA , H . ( 1 970 ) . Strength of Bi logical Mat e r i a l s .

Ed . F . G . Evans , The W i l l iams and Wilkins Co . Balt imore .

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