Non-invasive assessment of soft-tissue artefacts in hip joint kinematics using motion capture data and ultrasound depth measurements.

In human movement analysis, the hip joint centre can be estimated using a functional method based on the relative motion of the femur to the pelvis measured using reflective markers attached to the skin surface by an optical motion capture system. The principal source of errors in estimating the hip joint centre location using functional methods is soft tissue artefacts due to the relative motion between the markers and bone; one of the main objectives in human movement analysis is the assessment of soft tissue artefact. Various studies have described the movement of soft tissue artefact and compensated for it invasively; examples include: intra-cortical pins, external fixators, percutaneous skeletal trackers, and Roentgen photogrammetry. The goal of this study is to present a non-invasive method to assess soft tissue artefact using optical motion capture data and tissue thickness from ultrasound measurements during flexion and abduction (both with the knee extended) of the hip joint. Results showed that the skin marker displacements caused by the artefact are non-linear and larger in areas closer to the hip joint. It was also found that the marker displacements are dependent on the movement type and relatively larger in abduction movement.