STUDIES OF CONTEMPORARY POLYETHYLENES IN DIFFERENT TOTAL KNEE DESIGNS

Abstract

Osteolsyis is one of the main reasons for revision of total joint replacements. The osteolytic reaction is influenced by dose, size (particles < 1μm are believed to be biologically more active) and shape of wear particles, so that low wear rates and biologically less active particles are required.

In addition, in the knee a range of design and kinematic variables have to be considered as they can markedly influence wear regardless of the type of polyethylene used. Furthermore, UHMWPE (ultra-high-molecular-weight-polyethylene) fatigue occurs more frequently in the knee joint than in the hip joint due to its changing tribocontact areas combined with high weight bearing. This is why crosslinked polyethylene (XPE) is still controversially discussed for use in total knee prostheses.

Question: Is XPE appropriate for both fixed- and mobile-bearing knee prostheses? Are XPE-particles different from UHMWPE-particles?

Method: In a knee-joint-simulator (Stallforth-Ungeth-uem) 4 XPE (1 sequential irradiated/annealed, 3 different remelted, fixed- and mobile-bearing-types) and 2 UHMWPE-inserts (fixed- and mobile-bearing) were tested (ISO). The gravimetric wear rates (mg/year) were measured (5mill. cycles), the wear mechanism was analysed by scanning electron microscope (SEM). Furthermore, 100.000 particles were analysed by SEM (20nm-nucleo-pore-filter;acid digestion method;ISO) in size and shape.

Results: All the inserts showed traces of abrasion, scratching and wear polishing. XPEs produced lower wear rates (range 0,6–4,3mg/year;p< 0,05) than UHM-WPEs (range 8,4–8,5mg/year) in fixed- as well as mobile-bearing knee prostheses without fatigue reactions. The sequentially irradiated and annealed insert showed the lowest wear rate (0,6mg/year;p< 0,05) overall. For all the groups the particles were smooth, granular, irregular and less fibrillar and more than 85% of the particles were submicron. The particle size was independent of the radiation dose. Fixed-bearings produced a larger amount of particles < 1μm (93,2–96,3%;p< 0,05) than mobile-bearings (85,5–89,5%).

Conclusion: All crosslinked tibial inserts, fixed- as well as mobile-bearings, showed statistically significant (p< 0.05) reduced wear rates without any fatigue reactions. The fixed-bearing sequential irradiated and annealed insert had the lowest wear rate (p< 0.05). XPEs (fixed- and mobile-bearings) and UHMWPEs have similar wear particles in shape. The particle size was independent of the radiation dose, but most of the particles are in the biologically more active range of size (fixed> mobile-bearing). Due to this further investigation with in-vivo-models is required.