Abstract
Purpose: The purpose of this study was to evaluate the patterns of wear of patella implants in total knee arthroplasty.
Material and methods: The pattern of polyethylene wear of 17 patella components, 5 kinematic and 12 total condylar, retrieved after an average of 80 month in situ, was studied. The primary diagnosis was osteoarthritis in all cases. The implants were retrieved from 6 men and 11 women with an average age of 64 years at the time of arthroplasty. Their average height was 168 cm and average weight was 68.5 kg.
In all cases, revision was performed for mechanical loosening of one or several components. Using light microscopy, the articulating surface of each patella component was analyzed for six modes of damage: polishing, delaminating, surface deformation due to cold flow, scratching, pitting and abrasion.
To describe the damage, the surface of the patella component was divided into four sections. The contact stresses between the patellar and femoral components were calculated in relation to the areas of wear. Volumetric wear could not be accurately established.
Results: The “Papion” pattern indicated higher rate of wear at the periphery of the patellar implants.
Four modes of damage were observed: polishing in 13, delimitation in 12, cold flow in 6 and scratching in 3. The median total area of polyethylene damage was for polishing 76.5%, delimitation 70.6%, cold flow 35.3% and scratching 17.6%. The average contact stress on the nonconforming Total Condylar patella component was 12.9 Kgf/mm2. It was significantly higher (p< 0.002) than the average contact stress on the conforming Kinematic patellar component – 2.9 Kgf/mm2. The area of wear was smaller, 357.2 mm2 for the nonconforming Total Condylar, than for the conforming Kinematic patella 439.2 mm2. However, this difference was not statistically significant. The average weight of the patients with the Kinematic knee (74.5 kg) was higher as compared to the patients with Total Condylar knee (66 kg) but the difference was not significant.
Conclusions: The high incidence and pattern of wear of the patella components indicates a basic fault in the design of the patellar implants. In our opinion there is insufficient thickness of the polyethylene in most points of contact, specifically at the periphery. We suggest improving two mechanical properties: adequate thickness of the entire polyethylene implant including the periphery and conforming articulating surfaces of the periphery of the dome.
The abstracts were prepared by Orah Naor. Correspondence should be addressed to him at the Israel Orthopaedic Association, PO Box 7845, Haifa 31074, Israel.
