2A-1 A New Method for Measuring Bone Strength using Echo-Tracking

To evaluate bone strength, it is significant to measure the degree of deformation or strain of bone under a certain load. However no method has been available to non-invasively measure bone deformation. To obtain bone mechanical properties such as elasticity, viscoelasticity and plasticity, we need to externally apply a load to a bone and to accurately measure the small displacement of a specific point on the surface of the bone. For the displacement measurements, we improved the echo-tracking (ET) system. The ET system consists of a diagnostic ultrasound system with a 7.5 MHz linear probe and a PC. PZF echo signals were sampled at 50 MHz and interpolated to eight times the sampling frequency. Then the PC calculated and displayed the small displacement. Furthermore we developed a multi-point ET system and defined ET strain: ETS = D/L, where L was the distance from the first tracking point to the last, and D was the maximum distance from the spline fitting curve of the displacement to the straight line connecting the first tracking point to the last. Then we conducted in vitro experiments using three-point bending (TPB) tests of a porcine tibia placed on a testing machine. As a reference of the bone strain, we used strain gauges attached to the surface of the bone and compared the ETS. With respect to the displacement, there was excellent linearity between the data obtained by the ET system and the linear potentiometer (r = 0.999). In the TPB tests of the porcine tibia, the strain gauge readings and the ETS also showed excellent linearity (r = 0.998 for both the proximal and the distal strain gauges). The results suggest that our method has great potential of non-invasive quantitative diagnosis for bone healing and bone strength