Tibial post wear in posterior stabilized total knee arthroplasty - An unrecognized source of polyethylene debris

Background: With extensive use of posterior stabilized total knee arthropla sty implants, it is increasingly important to assess the mechanical perform ance of this design alternative. The purpose of this study was to examine t he wear patterns at the femoral cam-tibial post interface in a series of po sterior stabilized prostheses retrieved at revision arthroplasty. Methods: Qualitative and quantitative wear analysis was performed over the surface of the stabilizing posts from twenty-three retrieved total knee com ponents that had been implanted for a mean of 35.6 months (range, 2.3 to 10 7.2 months). The implants were designs from four different manufacturers. D igital images of the anterior, posterior, medial, and lateral surfaces of t he tibial post were made for quantitative analysis and determination of a p ost wear score. Wear was characterized with a grading system that isolates adhesive, abrasive, and fatigue wear, inferring a weighted score from an es timation of generated polyethylene debris. Results: Evidence of wear or damage was observed on all twenty-three of the stabilizing posts, including those revised because of infection. On the av erage, 39.9% (range, 18.5% to 60%) of the post surface demonstrated some fo rm of deformation, with adhesive wear, or burnishing, being the predominant wear mechanism. Seven posts (30%) exhibited severe damage with gross loss of polyethylene. The wear caused premature failure and early revision of tw o components: one of these failures was related to isolated post wear and t he other, to severe post wear and subsequent fracture. Overall, wear was pr imarily posterior, but wear over the anterior, medial, and lateral surfaces was also notable. Conclusions: The cam-post articulation in posterior stabilized implants can be an additional source of polyethylene wear debris. The variability in we ar patterns observed among designs may be due to differences in cam-post me chanics, post location, and post geometry. The surgeon should be aware that the cam-post interface is not an innocuous articulation, and manufacturers should be motivated to produce implants that maintain the function of the post while limiting wear and surface damage.