Inlay, Onlay, Oval, or Round, Patellar Implant Choice Outcomes Do Not Confound: Commentary on an article by Monther A. Gharaibeh, MBBS, FJMA, et al.: "Does Choice of Patellar Implant in Total Knee Arthroplasty Matter? A Randomized Comparative Trial of 3 Commonly Used Designs".

Abstract

Commentary Although the prevalence of total knee arthroplasty (TKA) continues to grow, the overall outcomes remain good but still exhibit a subgroup of patients who are not entirely satisfied with their result1. Substantial difficulties in evaluating the differences between satisfied and unsatisfied patients are enhanced by the many choices and alternatives available to knee arthroplasty surgeons. A few notable examples include approach (medial parapatellar compared with other approaches), fixation (cemented, hybrid, or uncemented), posterior cruciate ligament status (cruciate-sacrificing compared with cruciate-sparing), and alignment (kinematic compared with mechanical). Included in the long list of alternatives is the choice of patellar implant type used by the surgeon. Many of us have either attended or participated in professional meetings or symposia in which faculty present their biased rationale for supporting their choice of patellar implant (or not resurfacing). Unfortunately, a paucity of strong evidence usually accompanies these conceivably strongly worded recommendations. Gharaibeh et al. should be congratulated for designing and completing a well-done prospective randomized trial assessing the differences between 3 patellar choices: inlay (IN), onlay round (OR), and onlay oval (OO), or anatomic. Historically, patellar instability and pain were common causes of revision TKA. Although infection has overtaken patellar sources of revision, patellar instability and pain remain among the top reasons for revision2. The percentage of unsatisfied patients who underwent TKA secondary to patellar causes is unknown, but patients who undergo revision TKA secondary to patellar sources of pain and instability are a defined subset of those patients undergoing revision TKA. Evidence-based recommendations with regard to patellar implant choices in TKA have been wanting, and Gharaibeh et al. designed a thoughtful and simple prospective trial to answer the question. Thankfully, the investigators randomized consecutive patients to the trial and measured both clinical and patient-reported outcomes. The Kujala score is likely not recognized by U.S. surgeons, but it is a validated method of assessing patellar pain, including in knee arthroplasty. The authors also included a robust set of radiographic measures, and these would have been strengthened by preoperative measurements as well. Their data would have been bolstered by preoperative alignment indices, as postoperative patellar malalignment and tracking can be attributed to overcorrection or undercorrection of underlying abnormalities. The authors also recorded whether a later facetectomy was performed during the procedure. They reported these findings based on their concern for lateral patellar bone compression syndrome. Not surprisingly, they found that a lateral facetectomy was required less frequently with the OO design patellar implants than the OR design patellar implants. Gharaibeh et al. correlated this finding with improved vascularity of the patella in the single photon emission computed tomography (SPECT) Delayed 4 subset, but differences were not noted otherwise in the vascularity analysis. Although the findings are used to imply that the OO-design patellar implants may improve outcomes, this study was not necessarily powered to assess that over time. With regard to outcomes, the authors showed that, although differences in radiographic indices between implant types did achieve significance for the Caton-Deschamps index, the overall indices for all implants were acceptable. Although the overall radiographic indices skewed slightly in favor of the OO design, these differences did not result in improved patient-reported outcomes. The Kujala scores for all implant types showed improvement and no difference between types, and the Knee injury and Osteoarthritis Outcome Score (KOOS) showed minimal and likely clinically unimportant differences between implant types. In conclusion, this is the type of study that helps to advance the science of knee arthroplasty. Although some will take the minimal radiographic differences as evidence that the OO design is superior, the clinical findings, including a patellar-specific outcome, do not support that observation. As we struggle to identify, predict, and address the factors that contribute to unsatisfactory outcomes in knee arthroplasty, this study helps to eliminate patellar implant design as a factor in those poor outcomes.