Abstract
Malpositioning still occurs in total hip arthroplasty (THA). As a result of mal-orientation, THA bearing can be subjected to edge loading. The main objective of the study was to assess if the wear rate of ceramic-on-ceramic and metal-on-polyethylene increases under edge loading conditions and to determine which of the most commonly used hip bearings is the most forgiving to implant mal-orientation.
Materials and methods
Two different polyethylenes (UHMWPE and vitamin E blended HXLPE) and ceramics (pure aluminum PAL and alumina-toughened zirconia ATZ) were tested with a hip simulator and compared to metal-on-metal results. The inclination angle was selected at 45°, 65° and 80°. In addition, the ceramic-on-ceramic barings were tested at conditions that produced microseparation.
Results
Contrary to metal-on-metal that is highly susceptible to edge loading, the wear rate of ceramic-on-ceramic and metal-on-polyethylene articulations does not increase with increasing cup inclination. In fact, the polyethylenes showed a contra-intuitive behaviour as its wear rate decreased slightly but significantly with increasing inclination angle. This behaviour can be explained when looking closely at the contact stresses and areas. (Figure 1 shows the wear area of the vitamin E blended HXLPE at 45° and figure 2 at 80° cup inclination).
The newest biomaterials, vitamin E blended HXLPE and ATZ, showed markedly lower wear rates compared to their conventional counterparts, UHMWPE and PAL. The ATZ ceramic-on-ceramic articulation showed the lowest wear rate (even when microseparation is included) of all tested pairings, but the new vitamin-doped HXLPE seems to be the most forgiving materials when it comes to implant mal-orientation. It shows low wear rate even at an extremely high cup inclination angle. Therefore, a surgeon that discovers a mal-positioned polyethylene cup at the first post-op X-ray will not need to worry unduly about increased wear (but “only” about a potential dislocation).
