Use of Ultrasound with Motion Capture to Measure Bone Displacement During Movement Made for Functional Hip Joint Center Determination

Abstract

Calculation of hip joint center (HJC) through functional methods with markers placed on skin around thigh and pelvis is a non-invasive method for estimating the center of rotation of a ball and socket joint by recording movements of femur relative to acetabulum. But the HJC through this process suffers from a well- documented source of error known as soft tissue artifacts (STA) which is the major source of error in determining functional HJC. Previous experiments associated with STA determination and compensation for HJC estimation have been invasive such as bone pins and hence are not viable for human based studies. Since STA was found to be subject and task specific, there appeared to be a need for a non-invasive ad-hoc procedure to quantify this error source. We have conducted a set of experiments to see change in thickness of soft tissues from skin surface on thigh up to bone using ultrasound as an ad-hoc to a motion capture system. In this study our hypothesis was that during the movement of thigh the bone moves linearly with respect to the marker on skin in the direction of probe and depth of bone from skin surface changes linearly in the direction of movement. Motion type “Flexion” with bent knee showed a maximum bone displacement of 1.5cm from neutral position with respect to skin with a maximum relative displacement of a virtual skin marker by 27cm in 3D space and a correlation 0.865 in synchronized frames.