RETRIEVAL STUDY OF IN VIVO WEAR AND OXIDATION OF HIGHLY CROSS-LINKED POLYETHYLENE ACETABULAR CUP AGAINST CERAMIC HEAD

Abstract

One of important issues of concern in total hip arthroplasty (THA) is osteolysis due to wear debris of ultra-high molecular weight polyethylene (PE), and it often leads to aseptic loosening. Reduction of PE wear debris is essential to prevent osteolysis, and different bearing interfaces as well as improvement of the bearing material itself have been attempted. Alumina ceramics as the bearing material for THA was introduced in Europe and Japan in the 1970s in aim to reduce the PE wear debris. The clinical results have proved the superiority of ceramic on PE couples to metal on PE couples in wear resistance. PE materials cross-liked by irradiation have also demonstrated a significant low wear by in vitro studies. Several types of highly cross-linked polyethylene (CLPE), with the irradiation dose of 50 to 105 kGy, have been developed and extensively used since 1998. In this study, the in vivo wear and oxidation of CLPE acetabular cup combined with ceramic femoral head were evaluated using retrieved cups.

Eight retrieved CLPE acetabular cups (Aeonian; Kyocera Corp., Kyoto, Japan, currently Japan Medical Materials Corp., Osaka, Japan) with clinical use for 3–80 months (mean 34 months) were examined. All cups were used against alumina or zirconia ceramic femoral heads. The linear wear of the retrieved CLPE cups was measured using a three-dimensional coordinate measurement machine. The worn surfaces of retrieved CLPE cups were observed by a scanning electron microscope (SEM). Oxidative degradation of the retrieved CLPE cups was expressed in terms of an oxidation index which was calculated from microscopic Fourier transformed infrared spectroscopy analysis, according to ASTM F2102.

The linear wear rate of retrieved CLPE cups was in 0.006–0.08 mm/year range, which was similar to the results reported by the previous radiographic study. In the worn surface of the CLPE cup retrieved after clinical use shorter than 39 months, machine marks were observed. In contrast, those retrieved after clinical use of 70 and 80 months were smooth. Oxidation indices of retrieved CLPE cups were: 0.12–0.37 in worn surface and 0.13–0.34 in unworn surface, respectively. There was no difference in the oxidation indices between the worn surface and unworn surface.

The retrieved CLPE acetabular cups in this study showed low and stable wear rates. The results showed a notable reduction in wear of the CLPE cups compared to that of conventional PE cups in the previous studies. And also, the oxidation indices of the retrieved CLPE cups were the same level as conventional PE cups. These findings from this retrieval study showed that there is neither progressive wear in the clinical use for 3–80 months, material failures due to wear, delamination nor cracks. The lower wear rate and smooth surface of the CLPE acetabular cup suggest the possibility of reduced wear debris from those cups articulated against the ceramic femoral head. We expect that the CLPE acetabular cup has favorable wear properties in long-term clinical use.