Dislocation after total hip arthroplasty: implant design and orientation.

Implant design and positioning are important factors in maintaining stability and minimizing dislocation after total hip arthroplasty. Although the advent of modular femoral stems and acetabular implants increased the number of head, neck, and liner designs, the features of recent designs can cause intra-articular prosthetic impingement within the arc of motion required for normal daily activities and thus lead to limited motion, increased wear, osteolysis, and subluxation or dislocation. Minimizing impingement involves avoiding skirted heads, matching a 22-mm head with an appropriate acetabular implant, maximizing the head-to-neck ratio, and, when possible, using a chamfered acetabular liner and a trapezoidal, rather than circular, neck cross-section. Computer modeling studies indicate the optimal cup position is 45 degrees to 55 degrees abduction. Angles <55 degrees require anteversion of 10 degrees to 20 degrees of both the stem and cup to minimize the risk of impingement and dislocation.