Polyethylene wear in a non-congruous unicompartmental knee replacement: a retrieval analysis.

Polyethylene wear is considered a threat to the long-term survival of unicompartmental knee replacement (UKR). This study aims to determine the linear penetration and volumetric wear associated with a non-congruent commonly used UKR, the St Georg Sled. This prosthesis has a biconvex femoral component which articulates with a flat polyethylene tibial component resulting in point contact. Components were retrieved from the knees of 19 patients undergoing revision for a failed UKR after primary replacement. Linear penetration and volumetric wear was measured using a coordinate measuring machine. Using an un-used implant, a three-dimensional computer model of the surface was generated and compared to the explanted specimen. Most patients had revisions because of disease progression to other compartments rather than specific wear related pathology. The mean linear wear for the sample was 0.89 mm (S.D. 0.59 mm) over an average period of 5.6 years giving the mean linear penetration rate of 0.15 mm/year. The mean volumetric wear was 99.7 mm3 (S.D. 79.3 mm3) over the same period giving a mean annual volumetric wear rate of 17.3 mm3/year. The results in this study indicate that clinically significant wear is not necessarily a feature of this fixed bearing unicompartmental knee replacement. The pattern of wear is consistent with clinical findings and indicates excavation of the polyethylene after implantation but at a slower rate than would be anticipated.