Does large femoral head reduce the risks of impingement in total hip arthroplasty with optimal and non-optimal cup positioning ?

Abstract: Few studies have analysed the advantage of 36- and 38-mm femoral heads compared to standard (28-32mm) and almost no study assessed whether potential advantages persist, or are even magnified, in the presence of a non-optimal cup positioning. We assessed whether, and to what extent, a large femoral head improves the range of motion in the total hip arthroplasty with different cup orientations. METHODS A mathematical model of the pelvis and the femur was generated by using a laser and ct scanning of a dried cadaveric hip joint. A femoral neck osteotomy was simulated, on the mathematical model, 10 mm proximal to the lesser trochanter. The femoral stem was placed in a neutral position with respect to the native femoral neck. A hemispheric cup of 52 -mm was positioned with an inclination of 45° and an antiversion of 28°. A neutral liner was inserted allowing the positioning of femoral head ranging from 28- to 38-mm. The range of motion was assessed with a cup inclination of 45° and antiversion of 28° (cup positioning of reference). We then repeated the measurements with a non-optimal cup positioning, that is, with a reduced (35°) and increased (55°) cup inclination; with a reduced (5°) and increased (48°) cup antiversion and with a combination of both. SolidEdge 17 UGS Inc software including a “collision check feature”, was used to assess ROM up to the impingement. The type of impingement, component-to-component, component-to-bone or bone-to-bone, was recorded. RESULTS When cup inclination was near to optimal (45°), hip flexion was limited by component-to-component impingement to 98° and 116° with femoral heads of 28 and 32-mm, respectively, and to 125° by bone-to-bone impingement with femoral heads of 36- and 38-mm. When cup inclination was increased (55°), hip flexion was limited by bone-to-bone impingement at 125° with all femoral heads except for the 28-mm femoral head, which caused component-to-component impingement at 107°. When cup inclination was reduced (35°), hip flexion was limited by component –to-component impingement to 90°, 106°, 109°, 110° with a femoral head of 28-mm, 32-mm, 36-mm and 38-mm, respectively. CONCLUSIONS In the presence of a hip prosthesis correctly implanted, large femoral heads increase the ROM compared to the 28-mm femoral head while the advantages seem to be less evident compared to the 32-mm head. In the presence of nonoptimal implant positioning, large femoral heads reduce the risk of impingement compared to the 28-mm but with respect to the 32-mm femoral head the increase in ROM is limited. A major advantage of using a large femoral head in the presence of non-optimal implant positioning seems to be a reduced risk of component-to-component impingement. This could represent a relevant contribution to the durability