Differences between the wear couples metal-on-polyethylene and ceramic-on-ceramic in the stability against dislocation of total hip replacement.

After total hip replacement an insufficient range of motion (ROM) can lead to contact between femoral neck and rim of the cup (= impingement) causing dislocation and consecutive material failure. The purpose of this study was to analyse the influence of different wear couples on the ROM and stability against dislocation. By means of a special testing device the ROM until impingement, the ROM until dislocation as well as the resisting moment against levering the head out of the cup were experimentally determined. Various total hip systems with cup inserts made of ceramic and polyethylene were comparatively examined in different implant positions. Maximum resisting moment as well as the ROM until impingement and dislocation were clearly influenced by the implant position. Furthermore, the stability against dislocation was affected by design parameters, whereas in the case of appropriate implant position differing wear couples (metal-on-polyethylene vs. ceramic-on-ceramic) had a minor impact. However, as shown by tests under lubricant conditions, ceramic-on-ceramic couples provided less dislocation stability in unfavourable implant position in comparison to metal-on-polyethylene. Therefore, ceramic-on-ceramic couples should only be applied in the case of optimised implant orientation preventing impingement and dislocation with subsequent material failure like chipping off or breakage.