NONINVASIVE DETERMINATION OF BONE MECHANICAL-PROPERTIES USING VIBRATION RESPONSE - A REFINED MODEL AND VALIDATION IN-VIVO

Accurate non-invasive mechanical measurement of long bones is made dif ficult by the masking effect of surrounding soft tissues. Mechanical r esponse tissue analysis (MRTA) offers a method for separating the effe cts of the soft tissue and bone; however, a direct validation has been lacking. A theoretical analysis of wave propagation through the compr essed tissue revealed a strong mass effect dependent on the relative a ccelerations of the probe and bone. The previous mathematical model of the bone and overlying tissue system was reconfigured to incorporate the theoretical finding. This newer model (six-parameter) was used to interpret results using MRTA to determine bone cross-sectional bending stiffness, EI(MRTA). The relationship between EI(MRTA) and theoretica l EI values for padded aluminum rods was R(2) = 0.999. A biological va lidation followed using monkey tibias. Each bone was tested in vivo wi th the MRTA instrument. Postmortem, the same tibias were excised and t ested to failure in three-point bending to determine EI(3-PT) and maxi mum load. Diaphyseal bone mineral density (BMD) measurements were also made. The relationship between EI(3-PT) and in vivo EI(MRTA) using th e six-parameter model is strong (R(2) = 0.947) and better than that us ing the older model(R(2) = 0.645). EI(MRTA) and BMD are also highly co rrelated (R(2) = 0.853). MRTA measurements in vivo and BMD ex vivo are both good predictors of scaled maximum strength (R(2) = 0.915 and R(2 ) = 0.894 respectively). This is the first biological validation of a non-invasive mechanical measurement of bone by comparison to actual va lues. The MRTA technique has potential clinical value for assessing lo ng-bone mechanical properties.