Bony impingement affects range of motion after total hip arthroplasty: A subject-specific approach

Hip range of motion after total hip arthroplasty has been shown to be dependent on prosthetic design and component placement. We hypothesized that bony anatomy would significantly affect range of motion. Computer models of a current generation hip arthroplasty design were virtually implanted in a model of pelvis and femur in various orientations ranging from 35 degrees to 55 degrees cup abduction, 0 degrees to 30 degrees cup anteversion, and 0 degrees to 30 degrees femoral anteversion. Four head sizes ranging from 22.2 to 32 mm and two neck sizes ranging from 10-mm and 12-mm diameter were tested. Range of motion was recorded as maximum flexion-extension, abduction-adduction, and axial rotation of the femur before any contact between prosthetic components or bone was detected. Bony impingement preceded component impingement in about 44% of all conditions tested, ranging from 66% in adduction to 22% in extension. Range of motion increased as head size increased. However, increasing head size also increased the propensity for bony impingement, which tended to reduce the beneficial effect of increased head size on range of motion. Reducing neck diameter had a greater effect on prosthetic impingement (mean, 3.5 degrees increase in range of motion) compared to bone impingement (mean, 1.9 degrees ). This model allowed for a clinically relevant assessment of range of motion after total hip arthroplasty and may also be used with patient-specific geometry [such as that obtained from preoperative computed tomography (CT) scans] for more accurate preoperative planning.