Measurement

Dynamic hip kinematics during gait, squatting, chair-rising, and twisting were analyzed for six healthy subjects and eleven patients with osteoarthritis (OA). Continuous anteroposterior radiographic images were recorded using a flat panel X-ray detector (Fig. 1), and each hip joint was scanned by computed tomography (CT). The 3D positions and orientations of the pelvis and femur in movement cycle were determined using a 3D-to-2D model-to-image registration technique. A matching algorithm maximizing correlations between density-based digitally reconstructed radiographs from CT data and the radiographic images was applied (Fig. 2). The relative positions and orientations of the pelvis with respect to the world coordinate systems were defined as pelvic movements (anterior-posterior tilt, contralateral-ipsilateral rotation, Fig. 3b and c), and those of the femur with respect to the world coordinate systems were defined as femoral movements (flexion-extension, internal-external rotation, Fig. 3d). We also defined the relative positions and orientations of the femur for the pelvis as hip movements (flexion-extension, internal-external rotation, Fig. 3e and f).