CROSSLINKED POLYETHYLENE IN KNEE ARTHROPLASTY – SIMULATOR RESULTS OF DIFFERENT MATERIALS

Abstract

Periprosthetic osteolysis, caused in a chronic inflammatory adverse reaction to wear particles in the surrounding tissues, is one of the major reasons for revision arthroplasty so that articulating surfaces with low wear rates are required. Compared with conventional ultra high molecular weight polyethylene (UHMWPE), highly crosslinked polyethylene (HXLPE) shows a reduced wear rate in a hip simulator. The crosslinking process which is achieved by gamma or electronic radiation, followed by heat treatment either above the melting point (remelting) or below (annealing), reduces the mechanical properties of UHMWPE, particularly its fatigue strength. UHMWPE fatigue occurs more frequently in the knee than in the hip due to its higher contact stresses. This is why HXLPE is still controversially discussed for use in total knee prostheses. We have examined the wear behaviour of different HXLPEs [one cruciate-retaining (CR; sequential irradiation and annealing), one ultra-congruent (remelting), one CR (remelting)], compared with conventional UHMWPE in a knee simulator (Stallforth-Ungethuem). In the fixed bearing knee recommended from the manufacturer the wear rates [gravimetric (mg/year); volumetric (mm³/year)] were determined according to the ISO standard and the wear mechanism was analysed by means of a scanning electron microscope.

All insert showed signs of abrasion, scratching and wear polishing, but no traces of fatigue reactions. All HXLPEs produced lower (p< 0.05) wear rates (0.47–3.3 mg/year; 0.5–3.5 mm3/year) than the UHMWPE (8.1–9.1 mg/year; 8.6–9.7 mm3/year), the inserts of HXLPE manufactured by sequential irradiation and annealing showed the lowest wear rates (p< 0.05) overall.

Due to the reduced wear rates without any fatigue symptoms, we conclude that HXLPE is suitable for total knee prostheses and a monitored clinical investigation can be recommended. HXLPE manufactured by sequential irradiation and annealing seems to produce still lower wear rates than those manufactured by remelting, at least when used in total knee prostheses.