Ultrasound and Mocap to improve non invasive hip joint center calculation

The hip joint center (HJC) location is required for calculation of hip kinematics in various applications. The functional method utilizes the movement of femur with respect to acetabulum to find the location of the center. A popular way for measuring this movement is through an optical motion capture system. This method is fast and economical for most applications where we require an instant HJC even though the reconstruction error in bone position calculation exists due to skin artifact. This error is caused by the relative movement of markers placed on skin rather than on actual bone. We observed, while the thigh muscle is contracting and relaxing during the leg movement, the position of the bone relative to the skin is changing. Hence we introduce a novel method of combining ultrasound imaging (to measure the change in soft tissue thickness above Femur bone while leg is flexed) and 3D position of markers placed on thigh. This information can be further used to eliminate the error in estimation of HJC by approximating bone location more correctly than relying just on skin markers. The advantage of using ultrasound machine is its non-invasiveness and portability while the bone position is measured. We determine a position on bone and show that there is a linear relationship between angle of rotation and change in tissue thickness. We use this new recalculated bone position to calculate the center of rotation through a traditional method and compare it with the calculated center through markers on skin. Our new algorithm shows that the residual has been improved by 46%.