12C-4 A Minute Bone Bending Angle Measuring Method Using Echo-Tracking for Assessment of Bone Strength

Abstract

In the previous paper, we reported that an ultrasound system using multi-point echo-tracking (ET) could have a great potential for non-invasive and quantitative diagnosis of bone healing. In this study, the results of a clinical evaluation of the previous method were reviewed and we further improved the ET system to obtain the higher accuracy. The objective of this study was to develop a new ultrasound diagnostic method and to experimentally confirm an improved accuracy of the ET system. First, we investigated the ET system performance by conducting buckling tests using a synthetic bone model which simulated the mechanical properties of the human tibias. To obtain reference data on bone strain, we attached strain gauges to the surface of the bone model. Second, the system was applied to clinically measure the stiffness of the healing position of the tibia in patients in the consolidation period who underwent lengthening of the tibia using an external fixator. To induce an axial load on the tibia, a force of 100-N was applied on the knee of the patients who assumed sitting position and the ET strain (ETS) was measured by detecting small bending curve generated by the bending of the tibial surface. Six tibiae in four patients were measured during the consolidation period. Third, to obtain better accuracy even at a smaller load magnitude, we changed the measurement algorithm for the ET system by using a three-point bending (TPB) test method. Width of the measurement span was increased from 40 mm to 140 mm and a bending angle of the bone fragments generated by loading on the distal part of the proximal fragment was obtained instead of the ETS. Finally, its accuracy was evaluated by measuring the inclination of a metal plate and by conducting a TPB of the bone model. In the buckling test of the bone model, a high correlation coefficient (r=0.978, p<0.01) was obtained between the strain gauge readings and the results obtained with the previous system. In the clinical application, the system was able to measure the ETS change over time until the stiffness at the bone healing position almost reached a level similar to that of an intact tibia. However, it was desirable to have higher accuracy even at a smaller load for patients. From the results of the ET system with the improved algorithm, the standard deviation for the differences between the inclination angles of the metal plate measured by this system and those measured by a potentiometer was 0.005 degree. In the TPB of the bone model, the bending angle measurement was 0.1 degree at an applied load of 25 N. It was concluded that the improved ET system could measure the mechanical property of an intact bone with sufficient accuracy at a smaller load magnitude.