Should the acetabular cup be medialised in total hip arthroplasty.

BACKGROUND: During total hip arthroplasty (THA), medialisation of the cup decreases the lever arm of body weight and is considered to be beneficial. Theoretically it should be compensated by an equivalent increase of the femoral offset in order to maintain global offset. GOAL OF THE STUDY: We investigated via a numerical model the forces on each bundle of the abductor muscles and the loading forces at the head-cup interface. We compared THA with and without medialisation of the cup and with and without restoration of the global offset.
METHOD: A digital finite element model of the hip joint with THA was constructed. Monopodal weightbearing was simulated with the cup positioned so that the hip centre of rotation was left unchanged. Forces in the abductors and pressure on the prosthetic head were measured and then compared with three other configurations: 1. Medialisation of the cup by 5 to 15 mm with reproduction of the native femoral offset or; 2. with an increased femoral offset compensating for the cup medialisation and; 3. no medialisation of the cup with a femoral offset increased.
RESULTS: Medialisation of the cup decreased stresses on the head-cup interface and on the abductor muscles, even when the global offset was not restored. Anatomical placement of the cup did not give the best results. The optimal outcome in terms of stress was observed when the cup was medialised but global offset restored. Any increase of global offset significantly increases the length and the tension in the anterior bundles of the abductor muscles.