Hellhake, P. R.

Pratt Jr., G. W.

Rabin, D.

Seder, J. A.

English

ISBS (International Society of Biomechanics in Sports): Conference Proceedings Archive

ANALYSIS OF BONE STRENGTH USING ULTRASOUND SYSTEM P. R. Hellhake The Association f o r  the Advancement of  Sports Potent i a1 Doe Run Road, Box 185, Unionvil le, Pa. 19375 Dm Rabin M t .  Sinai Hospital 0. W. Pratt,  Jr. Massachusetts I n s t i t u t e  of Technology J. A. Seder The Association f o r  the Advancement of  Sports Potent ia l  INTRODUCTION There i s  current ly  no acceptable diagnostic technique t o  accurately separate pat ients  having high r i s k  of  f rac tu re  from normal age-matched controls. A1 1 avai l a b l e  noninvasive, i n  vivo, techniques t o  assess f rac tu re  threshold measure e i ther  bone density or bone mineral content. Studies aimed a t  predic t ing f rac tu re  r i s k  have re la ted  the breaking strength of bone evaluated i n  v i t r o  t o  i t ' s  mass and have shown good corre la t ion (Smith, 1976;Dalen, 1976;Smi th, 1972). However, i n  vivo, while some ind iv idua ls  w i th  f rac tu re  had lower bone mass than those without fracture, considerable overlap between f rac tu re  and control  groups was found when bone mass was measured i n  the  radius of  pat ients  w i th  and without vertebral col lapse f rac tu re  (Dalen, 1976); i n  the femoral neck of pat ients with and without f rac tu re  of  the proximal femur (Smith, 1972); i n  vertebrae removed postmortem from those w i th  and without collapse f rac tu re  (Arnold, 1965); and i n  the opposite radius of  pat ients  wi th  and without co l l es  f racture.  Blacks i n  South A f r i ca  have been found t o  have a t  least  a 10 f o l d  lower incidence of femoral neck f rac tu re  than South Afr ican whites, yet the bone density i n  these whites was greater, as determined by measuring c o r t i c a l  thickness (Solomon, 1979). Clear ly these data could r e s u l t  from i n s u f f i c i e n t l y  sensi t ive methods or measures not taken a t  the f rac tu re  s i te ,  however, a more l i k e l y  explanation i s  tha t  bone density i s  not the so le determinant of  f racture r i s k  (Hayes, 1983;Solomon, 1979;Johnston, 1981). Current c l i n i c a l  methods f o r  measuring bone mineral content are: radiogrammetry, s ing le  photon absorptiometry, dual photon absorptiometry, CAT scan, and neutron ac t i va t ion  analysis (which actual ly  measures whole body calcium). Single photon absorptiometry, o f f e r i ng  2-4% error  w i th  good reproduc ib i l i t y ,  does not provide information about the  ax ia l  skeleton, which i s  80% t r r b r c u l r r  bonr and can not be assemred with single photon techniques (Dalen, 1974;Seeman,1981;Riqgs, 1980). The value of CAT scanning fo r  measurement of  vertebral bone density has been qubstioned because of potent ia l  inaccuracies caused by interspersed marrow f a t  i n  areas of trabecular bone, Dual energy CAT scanning i s  generally not available, and i n  general CAT scanning i s  expensive and associated with a r e l a t i v e l y  large radiat ion exposure. Dual photon absorptiometry, which uses a """Gd source, r e l i e s  on estimation of the two separate energy levels  and subtracts t h e i r  difference t o  el iminate so f t  t issue error providing trabccular (spine) bone mineral content. Error of measurement i s  2-4X and reproduc ib i l i t y  i s  2X, however, the instrumentation i s  expensive and does del iver low doses of radiation. Biomechanical research has demonstrated tha t  the e las t ic  properties of bone o f fe r  a more e f fec t ive  measure of bone strength than bone mineral content (Abendschein, 1970;Craven, 1973;Greenf ie ld ,  1975;Reilly, 1974;Schryver, 1978;Andre, 1981; Pratt, 198l;Carter, 1977;Carter, 1976). E last ic  modulus which re lates stress t o  s t r a i n  i n  a viscoelastic, anisotropic material, such as bone, i s  a measure of these e las t ic  properties. Both e las t ic  modulus and the compressive strength of bone are proportional t o  the s t r a i n  r a t e  raised t o  the 0.06 power (Carter, 1977;Carter, 1976). Thus the veloc i ty  of ultrasound transmission i n  bone, which provides a measure of e las t ic  modulus, may determine f racture threshold. The f 01 lowing relat ionship applies: V, = Elast ic  modulus (En)/(P) density En = Elast ic  modulus of bone P = Bone density Ve  = Velocity of sound transmission A s  i t  can be seen the e las t i c  modulus re la tes  d i r e c t l y  t o  the veloc i ty  of sound squared. Further work has shown a d i rec t  relat ionship between V-2 and compressive strength, which fo r  our purposes empir ical ly re lates t o  breaking strength (Pratt, 1981). Abendrchien and Hyatt (1970) have demonstrated a high degree of corre lat ion between the ultrasound transmission veloc i ty  and the measurement of mass density, the modulus of e las t ic i ty ,  and the mathematical formulation of the breaking strength of human bone. Their work established that  the mechanical modulus, determined by s t a t i c  loading, correlated t o  ul t rasonic ve loc i ty  (r=.88!3, p<.OOl); tha t  the mechanical modulus correlated t o  density (rz.9376, p<.0001); and f i n a l l y  that  the mechanical modulus correlated t o  e las t i c  modulus (r=. 9093, p<,OOl), showing a high corre lat ion of the veloc i ty  of sound transmission i n  bone t o  the e las t i c  modulus and ul t imate breaking strength. A r e l i a b l e  non-invasive method f o r  monitoring f rac tu re  threshold, i n  vivo, i s  of considerable s igni f icance t o  the sportsmedicine p rac t i t ioner  involved i n  the study and treatment of stress re la ted i n j u r i e s  t o  bone, as wel l  as t o  a l l  areas of medicine concerned wi th  bone diseases. More c l i n i c a l  study i s  necessary t o  evaluate the r o l e  of ultrasound ve loc i ty  measurement i n  t h i s  area, however, the equipment necessary t o  conduct such study requires operation by technicians t ra ined i n  the use of electronic instrumentation f o r  time measurement. Our goal i s  t o  develop equipment t o  make c l i n i c a l  study of u l t rasonic  ve loc i ty  measurement more pract ica l .  EQU I PMENT an i ndus t r i a l  qua l i t y  e lect ron ic  d i g i t a l  ca l iper  accurate t o  0.025mm was used t o  measure the transmission path distance. The ca l iper  incorportated a push button zeroing funct ion making i t  unnecessary t o  read the distance measurement during actual ultrasound transmission. TWO 1.25 MHz quartz p iezoelect r ic  ultrasound transducers were mounted on the d i g i t a l  ca l iper  f o r  transmission and reception of the ultrasound signal through the bone. Signal generation and ampl i f icat ion of  the received signal  was accomplished through the use of a broadband u l t rasonic  pulser/receiver producing large amplitude e l e c t r i c a l  pulses of  one micro second duration. The signal processing por t ion  of  the system consisted of an 8 b i t  analog t o  d i g i t a l  converter interfaced t o  an 8 b i t  micro computer. The 8 b i t  resolut ion of  the analog t o  d i g i t a l  converter allowed the continous analog signal t o  be sampled and quantized i n t o  a s t r i n g  of 256 binary values. Since the signal presented by the u l t rasonic  pulser/receiver was both r e p e t i t i v e  and per iodic and was followed by a t r i gge r  point, equivalent t ime sampling was possible. Equivalent time sampling makes i t  possible f o r  an analog t o  d i g i t a l  converter t o  appear t o  record information a t  a higher sampling r a t e  then designed by acquir ing only one binary sample a f t e r  each t r igger .  Following each subsequent t r i gge r  the time between the t r i gge r  and the  sample point  i s  incremented a precise amount and t h i s  w i l l  continue u n t i l  a l l  256 sample points  have been acquired. Under normal condit ions the approximate range f o r  time measurements was determined t o  be IQ-30 microseconds. A time delay was b u i l t  i n  so tha t  sampling began a t  10 microseconds and with an equivalent time sampling r a t e  of 12.5 MHz the l a s t  acquired sample point  i n  a sweep would be 30.48 microseconds. The 12.5 MHz equivalent time sampling r a t e  was acceptable according t o  the Nyquist c r i t e r i o n  (Heyberger, 1983). Waveform averaging was performed t o  reduce the signal  t o  noise r a t i o  and software was developed t o  mathematically determine the time of the f i r s t  a r r i v i n g  signal  t o  the  receiv ing transducer. Addit ional features included 5.25 inch f loppy disk Fig. 1. Instrumentation setup f o r  measurement of  u l t rasonic  ve loc i t y  through bone. ~ t o r a g e  of  measurement data, ve loc i ty ,  date and c l i n i c a l  comments. Sof tware was a1 so avai l a b l e  f o r  s t a t i s t i c a l  analysis and p l o t t i n g  o f  data and pa t ien t  h is to r ies .  TESTING R prel iminary t es t i ng  procedure was arranged i n  which the analog data s ignal  from the  ultrasound pulser/receiver was presented s i  mu1 taneousl y t o  both the  m i  c ro  computer s ignal  anal y s i  s system and a 15 MHz osci l loscope operated by a t ra ined technician. Each of  the  42 d i f f e r e n t  sample s i t e s  were analyzed during four separate t r i a l s  by both systems. Considerably more t ime and e f f o r t  was required f o r  the  technician-osci l loscope system t o  recognize and record the  po in t  of  the f i r s t  a r r i v i n g  ultrasound signal. Rddi t ional ly,  data gathered using the technician- osci l loscope system had t o  be a t  a l a t e r  t ime entered i n t o  a computer f o r  ve loc i t y  ca lcu la t ion  and data reduction. S t a t i s t i c a l  analysis was performed on the range of  readings f o r  each s i t e .  The micro computer s ignal  analysis system showed an average 40.89% increase i n  data rep roduc ib i l i t y  over the  technician-oscil loscope system. No t e s t s  have yet been performed t o  determine accuracy of the systems, however, before use the micro computer ve loc i t y  measurement system w a s  ca l ibra ted t o  ve loc i t y  measurements made i n  d i s t i l l e d  water. CONCLUSION Experimental i n  v i t r o  studies ind ica te  t ha t  an empir ical re la t ionsh ip  ex i s t s  between the  ve loc i t y  of  sound and f r ac tu re  threshold o f  the  bone. C l i n i c a l  study i s  necessary t o  f u l l y  evaluate the useful lness of ultrasound ve loc i ty  measurement f o r  purposes of diagnosis of f racture threshold. The instrumentation presented here i s  intended t o  a id  such c l i n i c a l  study by s impl i fy ing data co l lec t ion  and reduction, Evaluation of the instrumention i s  current ly  underway a t  a number of i n s t i t u t i o n s  involved i n  research of  diseases and stress re la ted i n j u r i e s  of bone. ACKNOWLEDGMENT This research was supported by a grant from EQB, Inc. Acknowledgment i s  also due Ms. P. M i l l e r  and her s t a f f  f o r  help with data co l lec t ion  and f i e l d  tes t ing  of equipment. REFERENCES Abendschein, W. and Hyatt, (3. W. (1970) Ul trasonics and selected physical propert ies of  bone. Clin. Orthop. Rel. Res. 69, 294-301. Andre, M. P. , Craven, J. Dm and Greenfield, M.A. (1980) Measurement of the ve loc i ty  of ultrasound i n  the human femur i n  vivo. Medical Physics 7(4):324-330. Arnold, J.S. (1965) Amount and qua l i t y  of  trabecular bone i n  osteoporotic vertebral fractures. Clin. Endocrinol. Metab. 2: 221-238. Craven, J.D., Constantini, M.A., Greenfield, M.A., e t  a l .  (1973) Measurement of  the ve loc i ty  of  ultrasound i n  human c o r t i c a l  bone and i t s  potent ia l  c l i n i c a l  importance. Invest, Radiol. 8:72-77. Carter, D.R, and Hayes, W, (1977) The compressive behavior of bone as a two-phase porous structure. J. Bone & Jo in t  Surg. 59-A (7 : 954-962. Carter, D.R. and Hayes, W. (1976) Bone compressive strength: the i n f  luence of density and s t r a i n  rate. Science 18: 1174-1176. Dalen, N., Hellstron, L. and Jacobson, B. (1976) Bone mineral content and mechanical strength of the femoral neck. Acta. Orthop. Scand. 47: 503-508. Dalen, N.,Lambe, B. and Wallfren (1974) Bone mineral content losses i n  oophorectomized women. J. Bone Jo in t  Surg. 56A: 1235- 1238. Greenfield, M.A., Craven, D.S., Wishko, c t  al.  (1975) The modulus of e l a s t i c i t y  of human c o r t i c a l  bone: an in-vivo measurement and i t ' s  c l i n i c a l  implications. Radiology 115: 163-166. Hayes, Wilson, C. (1983) Biomechanics of bone: Impl icat ion f o r  assessment of  bone strength. ASMBR workshop Dm Heyberger, C. (1983) D i g i t a l  storage osci 1 loscopes. Medical Electronics 82:62-68. Johnston, C.C., Jr. (1981) Bone mass and r i s k  of  f racture.  Rheumatol ogy 5: 247-250. Pratt,  G.W., Jr. (1981) An i n  v ivo method of  u l t rasonic  evaluating bone strength, AAEP proceedings. Rei l ly ,  D.T., Burstein, A.H. and Frankel, B.H. (1974) The e l a s t i c  modul us f o r  bone. J. B i  omech. 7: 271-275. Riggs, L.B., Seeman, Em, Hodgson, S,F. andTaves, D. (1980) The f loride/calcium regimen f o r  primary osteoporosis, In: 'OTrT-9ZST :Z qa3ueq -SUP yqe lndod ue>lJ ju pue uerse3ne3 6ura6e ut Aqrsuap auoa (6~6~) -1 6uou!o~o~ -62%-szs :&TS vwwr -s rsorodoaqso se q3ns saqeqs pazl lelaurwap ur asn s Lq~ :sseu auoq jo quaualnseetu OA~A UI (2~6~) -i=d 'n~ pue -~r ''3-3 rurqsuqoy l -w-a 'qqrws 'ZOG-96* :L* -pue>s -doqqro -eqe3~ -eq>edwox IeJoue j uewnq jo sa lq~edo~d le3 rueq>atu pue Aq rsuap lelau rtu 'a6e uaanqaq sdrqsuorqelaa (9~6~) -vga 'qqrws pue -8-3 'qqrws '8ZZ :L9 -2saAuI 'u? I3 -r -uoqalays ~elnarpuadde aqq pue letxe aqq uo uoyq~unjsAp aurmopue jo sq>a j ja lerquara j jra (~86~) -1-8 *s66~~ pue =r-~ Cuosuqoy C-a C~ewny 6-d=y 6p~ojj~ *-M-H Irauqem I-3 'urwaas -sz:(T)~s -sau =qan -r -ww -as~oq aqq jo auoq Ie3tqAoD jo sarq~ado~d 6urpuag (8~6~) = j-H 'ra~Alq3~ galOtu rq lea 'SsaAd qled -Arun -(sp3) qq~jred pue uoqsuqoy 'ny 'qso~j Le>nlaa -quawqea~l pue srsauaaoqqed ur saxueApw quaxau :srsoAoaoaqso 

ANALYSIS OF BONE STRENGTH USING ULTRASOUND SYSTEM

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ANALYSIS OF BONE STRENGTH USING ULTRASOUND SYSTEM

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ISBS (International Society of Biomechanics in Sports): Conference Proceedings Archive