The effects of implant position, design and wear on the range of motion after total hip arthroplasty

Impingement of a total hip prosthesis may lead to restricted range of motion (ROM), which may result in subluxation, dislocation and material failure. Due to wear, the polyethylene cups can be penetrated by the femoral head. Wear and design features may restrict the movement of the artificial joint. In a three-dimensional CAD simulation, different positions and designs of acetabular cup and femoral head and the effect of displacement of the centre of rotation were investigated. The results demonstrate the importance of correct positioning and design of the acetabular component to achieve a near physiological ROM. Using ceramic cups, the inclination angle should be approximately 45°, the anteversion angle approximately 15°, and antetorsion of the stem between 0° and 15°. If the cup is too horizontal, flexion and abduction are greatly limited. With steeper cup position (60° inclination) ROM of flexion increases, but risk of dislocation and so do rim fracture (ceramic cups), and penetration and wear rate of polyethylene cups. ROM is clearly reduced by increasing penetration of the femoral head. For example, at an inclination angle of 45°, a depth of penetration of 1 mm reduces flexion by about 7°. Modern total hip prostheses should provide adequate joint movement, precise positioning of components and low wear bearing couples, in particular for the treatment of young and active patients. In addition, design aspects of liner geometry and head size have to be considered to avoid impingement and dislocation.