Abstract
BACKGROUD/PURPOSE
Recently, the use of a large diameter femoral head has been discussed as a means to reduce the risk of hip dislocation after total hip arthroplasty (THA). Although it has been clear that increasing the head size increases the oscillation angle and hip stability, a consensus on the usefulness of a larger head size has not been reached due to an increased propensity for bone impingement. We studied the effect of the range of motion (ROM) and bone impingement caused by increasing the femoral head size using a 3D simulation system.
PATIENTS AND METHODS
All patients who had undergone a primary THA in our hospital from October 2010 were selected, and we excluded those with severe osteoarthritis, severe dysplasia (Crowe group), or excessive femoral neck anteversion (35°). This resulted in 60 patients (16 men and 44 women), with a mean age of 66.6 years (range, 47–83 years). The diagnoses were osteoarthritis in 42 hips, osteonecrosis in 11 hips, rheumatoid arthritis in four hips, and femoral neck fracture in three hips. A virtual hip model was generated from the preoperative CT scan and a component was virtually implanted via computer simulation software (Zed Hip, LEXI, Japan). The acetabular cup was implanted with an inclination of 45°, anteversion of 20°, and the femoral stem was implanted into the femur recreating the same head height with an anteversion of 25°. We defined three leg positions: (A) maximum flexion (B) internal rotation with hip in 90°of flexion and 20°of adduction as posterior dislocation, and (C) external rotation with hip in 0°of extension as anterior dislocation. In each leg position, range of motion up to the impingement and the type of impingement (implant or bone) was assessed with 22-, 26-, 28-, 32-, and 36 mm femoral head sizes.
RESULTS
We found a significant increase in ROM between the 22 mm and 26-, 28-, 32-, and 36 mm femoral heads, and between the 26 mm and 36 mm femoral head in each leg position, but no significant difference was found in femoral heads more than 28 mm. Bone impingement preceding implant impingement was increased with larger head sizes more than 50% of the 28 mm head size in each leg position. The 36 mm head size corresponded to (A) 87%, (B) 85%, and (C) 70%, respectively.
CONCLUSIONS
We concluded that increasing the head size increased bone impingement, which led to restricting improvement in ROM. There was no significant increase in ROM for femoral heads more than 28 mm; therefore, excessively large head sizes may result in restrictive ROM due to an increased oscillation angle.
