Introduction: An insufficient range of motion (ROM) can lead to prosthetic impingement causing dislocation of a total hip replacement. The objective of this study was to analyze the influence of the wear coupling on ROM and dislocation stability.

Material and Methods: By means of an experimental test device, a total hip system (Alloclassic) with four different insert materials, standard ultra-high-molecular-weight-polyethylene (UHMW-PE), highly cross-linked-polyethylene (XL-PE), aluminium-oxide-ceramic and cobalt-chromium, was investigated concerning ROM and stability against dislocation. The tests were carried out under dry conditions as well as after lubrication of the articulating surfaces with fetal calf serum. In a supplementary test procedure, the force vector-induced dislocation, i.e. dislocation without previous prosthetic impingement, was analyzed.

Results: No significant differences in the ROM until impingement(ROMImp)weredeterminedbetweenthe UHMW-PE and XL-PE inserts. The overall ROMImp of ceramic and metal inserts was approximately 5° less than with PE because no plastic deformation of the rim surface occurred. There was no significant difference in the maximum resisting moment prior to dislocation between the metal-on-polyethylene couples, whereas ceramic-on-ceramic showed the lowest moments and metal-on-metal the highest. Generally, slightly decreased moments for subluxation were determined after lubrication of the sliding surfaces for all couples. In a proper cup position (45° inclination and 15° ante-version) varying the wear coupling had a minor impact on the ROM until dislocation (ROMLux). However, in a poor implant position, ceramic-on-ceramic revealed a clear decrease in the ROMLux of approximately 40° after lubrication of the articulating surfaces. In general, metal-on-metal provided the highest ROMLux. The force vector-induced dislocation provided similar results for the different wear couples.

Conclusion: The study underlines the importance of optimized implant orientation and the impact of the wear couple used on ROM and dislocation stability. Recurrent impingement with subsequent release of wear particles has to be considered for all wear couples. However, ceramic-on-ceramic couples should be used in optimal implant position to avoid impingement and dislocation.