Abstract
Purpose of study
To investigate the linear penetration rate of the polyethylene bearing in unicompartmental knee arthroplasty at twenty years.
Introduction
The Phase 1 Oxford medial UKR was introduced in 1978 as a design against wear, with a fully congruous articulation. In 1987 the Phase 2 implant was introduced with new instrumentation and changes to the bearing shape. We have previously shown a linear penetration rate (LPR) of 0.02 mm/year at ten years in Phase 2, but that higher penetration rates can be seen with impingement. The aim of this study was to determine the 20 year in-vivo LPR of the Oxford UKR, using Roentgen Stereophotogrammetric Analysis (RSA).
Patients and Methods
Six Phase 1 (5 patients, mean age 65.3 years) and seven Phase 2 (4 patients, mean age 63.5) Oxford UKR bearings, with an average time since surgery of 22.5 years and 19.5 years respectively, were assessed. Stereoradiographs were taken and penetration was calculated using a model-based RSA system. The penetration for each bearing was calculated by subtracting the measured thickness from the corrected nominal bearing thickness.
Results
The measured LPR was 0.072 mm/year for Phase 1 (S.D. 0.028, range: 0.031 – 0.104 mm/year) and 0.028 mm/year for Phase 2 (S.D. 0.019, range: 0.014 – 0.07 mm/year). This difference in LPR for the two phases was statistically significant (p = 0.0028).
Discussion
The results show that in the knees studied there was a significant difference between the LPR in Phase 1 and 2. This is probably due to the change in instrumentation and bearing shape between Phases. This study demonstrates that very low penetration rates can be maintained to the end of the second decade after implantation. This is of particular importance when the device is used in younger patients.
