A Comparison of Different Patellar Implant Designs in Total Knee Replacement.

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Aim: To compare the intra-operative and post-operative outcome measures between three different patellar implant designs in patients undergoing total knee arthroplasty (TKA) surgery with regards to patellar bone coverage and tracking, patient-reported outcomes and post-operative radiological and scintigraphic analysis. Background: Controversy remains regarding whether the patella should be resurfaced in total knee arthroplasty. However, at present the scientific literature supports patellar resurfacing with regards to reduced re-operation risk and improved long term patient satisfaction. Two basic techniques present for resurfacing the patella; either onlay or inset. In the inset design, a round domed implant is reamed in the patella and fixed with a single central projection (peg). On the other hand, the onlay design has either round (symmetrical) or oval (asymmetrical) shaped patellae with three small peripheral pegs fixed on the cut surface of the patella. When using a round patellar implant, either onlay or inset, some surgeons perform a lateral facetectomy to avoid lateral compression syndrome and enhance patellar tracking. However, neither method has proved to be superior. A wide range of complications are associated with patellar resurfacing. These include patellar fracture, osteonecrosis related to devascularisation, patellar polyethylene (PE) wear, aseptic loosening, instability, dislocation, overstuffing and rupture of the extensor mechanism. Many of these can be catastrophic. One should always consider the peculiar blood supply to the patella during TKA and patellar resurfacing as iatrogenic disruption of supplying vessels has been identified as a major contributing factor in post-operative patellofemoral complications. The hypothesis of this study is that onlay oval design provides optimal bone coverage, better tracking, better patient reported outcome scores and carries the lowest risk of osteonecrosis in comparison with the other two designs. Trial Objectives: To prospectively compare intra-operative observed measures, patient-reported outcome measures and radiological results between inset, onlay round and onlay oval designs of patellar implants in patients undergoing total knee arthroplasty surgery. Specifically, we will compare patellar surface bone coverage, pain and functional scores together with patellar tracking and vascular status of the patella postoperatively.

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