Unicompartmental knee replacement (UKR) is an alternative to total knee replacement for selected patients with isolated medial or lateral compartment osteoarthritis. One of the most popular UKR implants was introduced as a mobile-bearing design in part to reduce polyethylene wear. However, backside wear of the mobile-bearing implant has not been examined independently from the articular surface.
Sixteen retrieved polyethylene inserts from a medial mobile-bearing UKR from 16 patients were examined after an average of 4.2 years implantation (range 1.5 to 10.0 years). Reasons for revision included aseptic loosening, pain, and progression of osteoarthritis. Each retrieved insert was evaluated using visual damage scoring across the articular and backside surfaces. Inserts were also micro-CT scanned and compared to a reference insert of the same size, to measure wear on the articular and backside surfaces.
The total damage scores were greater (p = 0.01) on the articular surface (27.2 ± 5.7 (standard deviation)) than the backside surface (23.8 ± 6.2). Burnishing, abrasions, and pitting were the most common damage modes on both surfaces, with only pitting greater (p = 0.03) on the articular surface than the backside surface. There was no difference (p = 0.46) in wear rate between the articular surface (0.028 ± 0.025 mm/year) and backside surface (0.029 ± 0.017 mm/year).
The retrieved mobile-bearing UKR polyethylenes demonstrated good overall wear resistance, with no evidence of severe damage. However, backside wear was equal to articular wear, suggesting that the backside surface is a potential source of polyethylene wear debris.