[Computer-based motion simulation of total hip prostheses with ceramic-on-ceramic wear couple. Analysis of implant design andorientation as influence parameters].

OBJECTIVE: In THA, ceramic-on-ceramic wear couples are increasingly used. A restricted range of motion (ROM) due to unfavourable implant design or positioning may cause impingement or dislocation, which can result in failure of ceramic inserts.

METHODS: By means of a 3-D CAD program different hip joint movements were simulated and the effects of ceramic hip implant design and position on the range of motion were analysed.

RESULTS: To offer sufficient ROM and to minimise risk of impingement and dislocation, inclination angle of the acetabular cup should be 45 degrees, cup anteversion 15 degrees and stem antetorsion 0 degrees to 10 degrees. In regard to implant design, acetabular cups with slightly- recessed ceramic inserts should be used. Prosthetic systems with an elevated liner or with a mushroom-shaped femoral head are associated with limited ROM and increased risk of mechanical failure. The ratio of head to neck diameter should never be less than 2 : 1. Larger heads not only increase ROM, but also the stability of the prosthesis against dislocation. Thereby, the wear rate of ceramic-on-ceramic couples is not increased, in contrast to polyethylene.

CONCLUSION: Considering certain criteria for ceramic hip implants regarding implant positioning, design and handling, ceramic-on-ceramic couples can be used with low risk of revision surgery and they can also reduce the prosthesis loosening associated with wear in young and active patients.