Abstract
The Kinemax Plus knee replacement has a reported 10 year survival of around 96%. However we found the survival rate of this implant in our cohort to be 75% at 9 years. No abnormalities were found for clinical and radiological parameters. At reoperation the most striking feature was that of significant ultra-high molecular weight polyethylene (UHMWPE) failure. Oxidative and structural analysis of the polyethylene components was therefore undertaken.
Ten Kinemax Plus tibial inserts were analysed; one was a shelf-aged unused implant, the others were explants. An FTIR analysis of the data showed that oxidation is present in all samples. The degree of oxidation however varied with depth and location. Except for a sharp oxidative peak approximately half way into the sample, the shelf aged samples had a fairly constant level of oxidation. The retrieved implants had an overall higher level of oxidation in both bearing and non-bearing regions. The latter had less of a variation in oxidation which implies that in vivo loading exaggerates the degree of oxidation. In the non-articulating regions oxidation of the explants was found to peak often at the region of about 40% from the bottom surface in all retrieved samples. By contrast, most articulating region had two oxidative peaks; one occurring at approximately 1–1.5mm from the surface, which is consistent with findings on subsurface oxidation, and another occurring about 2–3mm from the bottom surface.
SEM imaging provided evidence for the presence of fusion defects by indicating grain boundaries through-out the explants. This indicates a compromised material which is more susceptible to damage. Fatigue loading of the implant has also been seen to produce a subsurface stress maximum at approximately 1 to 2mm below the articulating surface. It is thought that maximum contact stresses within this region cause Type 1 and Type 2 defects to open or become more pronounced. This in turn will increase the local concentration of oxygenating material as it will be present in these defects and voids where surface areas are greater for oxidative reaction. We therefore hypothesise that these fusion defects are the cause for the early failure of the Kinemax implants.
Correspondence should be addressed to: BASK c/o BOA, at the Royal College of Surgeons, 35–43 Lincoln’s Inn Fields, London, WC2A 3PE, England.
