A745. IN VIVO WEAR OF HIGHLY CROSS-LINKED POLYETHYLENE ACETABULAR CUP AGAINST ALUMINA CERAMIC HEAD BY RADIOGRAPHIC AND RETRIEVAL ANALYSIS

Abstract

In total hip arthroplasty (THA), one of concerned issues is osteolysis due to wear debris of ultra-high molecular weight polyethylene (PE) which often leads to aseptic loosening. Reduction of PE wear debris is essential to prevent osteolysis, and different bearing combination as well as improvement of the bearing material itself have been attempted. Hence alumina ceramics was introduced for THA, aiming to reduce PE wear debris. Ceramic on PE couple showed good results in clinical wear compared with metal on PE couples. Highly cross-linked PE (HXLPE) with gamma-ray or electron-beam irradiation followed by thermal treatment has also demonstrated a remarkably low wear in the previous in vitro studies. In in vivo studies, the wear of HXLPE acetabular cups against alumina ceramic femoral head was evaluated to compare with that of conventional PE cups against alumina ceramic femoral head.

The in vivo wear of 61 HXLPE cups (Aeonian; Kyocera Corp., Kyoto, Japan, currently Japan Medical Materials Corp., Osaka, Japan) against alumina ceramic femoral head of 28 mm in diameter with clinical use for 2.1–7.1 years (mean 5.6 years) and eight conventional PE cups against an alumina ceramic femoral head of 28 mm in diameter used for 18.7–23.3 years (mean 20.4 years) were examined by radiographic analysis with Vector Works 10.5. The in vivo wear of eight retrieved HXLPE cups with clinical use for 0.9–6.7 years (mean 2.9 years) and 14 retrieved conventional PE cups used for 16.0–28.0 years (mean 22.0 years) were examined by using a three-dimensional coordinate measuring machine. The worn surfaces of retrieved HXLPE and conventional PE cups were observed by a scanning electron microscope.

In the radiographic study, penetration rate of alumina head into HXLPE and conventional PE for the first 1 year were 0.24 mm/year and 0.34 mm/year respectively. One year later, the HXLPE showed significant lower penetration rate of 0.001 mm/year than the conventional PE penetration rate of 0.12 mm/year (p< 0.01). By the retrieval analysis, the mean penetration of retrieved HXLPE and conventional PE cups were 0.11 and 2.97 mm, and they were similar to the results by radiographic analysis. In the worn surface of the retrieved HXLPE cups used for around 1 year, machine marks were observed. In contrast, the worn surface of the retrieved HXLPE cups used for more than five years were smooth, and furthermore, in high magnification observation they had wear morphology different from conventional PE. These findings from this retrieval study suggest the penetration in the first 1 year detected by radiographic measurement was probably caused by creep deformation in bedding-in stage; and 1 year after, the penetration was probably caused mainly by wear.

By the radiographic analysis, HXLPE cups against alumina ceramic femoral head has a 99 % lower wear rate compared with conventional PE cups. Also, retrieved HXLPE cups against alumina ceramic femoral head exhibited lower wear compared with conventional PE cups. In conclusion, we expect that the HXLPE cup used with alumina ceramic femoral head has favorable wear properties in long-term clinical use.