The influence of implant position on the wear of alumina-on-alumina studied in a hip simulator

The most important purpose of total hip arthroplasty is to restore function, relieve pain and obtain stability of the hip joint. The major long term problem in hip replacement surgery is wear of bearing surfaces, the second dislocation of the implant. Besides implant and patient related factors, inappropriate tensional forces due to malposition of acetabular cup, possibly leading to prosthesis impingement, is a main cause for these complications. The incidence of early dislocation following primary total hip arthroplasty is on average between 3 and 5%. Cup malposition is major cause of dislocation, in better cases implant impingement leads to restricted range of motion and higher stresses on bearing surfaces. Therefore having a correct position in the acetabular cups is necessary to reduce dislocation, subluxation, and material failure. Accurate placement of acetabular cups minimizes the risk of implant early dislocations, of high wear rate and the risk of late dislocations. Alternative bearing surfaces, other than polyethylene, have been introduced in the clinical practice to minimize the negative effects of wear on hip arthroplasty survival; an alumina coupling with better tribological properties could be particularly useful when a perfect orientation of implant components is not achievable. In order to evaluate the effect of cup positioning on alumina bearing surfaces, 12 ceramic acetabular cups were tested in a hip joint simulator with bovine calf serum as lubricant. Three configurations were fixed: 23°, 45°, and 63°. After five million cycles, no significant differences were observed among the three tested configurations.