PATELLA RESURFACING AREA RATIO CORRELATES WITH KNEE OUTCOME SCORES WITHIN ONE YEAR AMONG PATIENTS UNDERGOING TOTAL KNEE ARTHROPLASTY

Despite the popularity of total knee arthroplasty (TKA), patella resurfacing remains controversial. It has been shown to reduce the risk of reoperation, however it can be associated with risks such as patella fracture. When resurfacing the patella, some surgeons advocate for using the largest fit component to limit impingement of native patella on the femoral component. In contrast, others advocate a more conservative approach, recognizing that by removing more native bone the patella is weakened and could lead fracture. However, there is a lack of evidence in the literature concerning associations between the proportion of patellar articular surface resurfaced and post operative patient-reported outcomes. The aim of our study was to compare patella coverage of the native patella with the clinical outcome of total knee arthroplasty including the pain scores after surgery as well as the overall knee functional scores, re-operation rate and health resources utilization. We retrospectively reviewed a consecutive series of 419 patients who underwent elective primary total knee replacements performed at a single institution between the years of 2016-2018. We used routine post operative xrays to measure the size of the native patella and the size of the button to obtain percentage coverage of the button to bone. This was then compared to pre and post operative Oxford Knee Injury and Osteoarthritis Score (KOOS) at various time points out to 1 year, obtained through a prospectively collected institutional registry. We also obtained adverse event data and reoperation rates through the same registry. The average age was 67 (Range 44-91), and average BMI was 31.1 (Range 19.4-59). There were 181 males and 238 females. 46 patients were excluded due to the patella not being resurfaced (43 patients), post operative x-rays being unavailable or unreadable (2 patients), or having undergone bilateral joint replacements (1 patient). There were 197 right knee replacements and 176 left knee replacements analysed. The average patella coverage was 44% (26-83%). Patella bone coverage percentage was compared to change in KOOS at the 6 week, 3month, 6month and 1 year mark using the Pearson correlation test. Our results showed that there was no linear relationship between patella button coverage and change in patient KOOS through all time points (correlation coefficient -0.05-0.11). However, when outcomes were stratified into responders and non-responders, responders being a change in KOOS to >10 (Minimum Important Change), then there was a higher proportion of good responders at the one year mark when >50% of the patella was covered with component (84% vs 98%, p=0.027). When analysed in KOOS subscales there was a trend towards higher proportion of patients who achieved clinically significant improvement of symptom subscale (81% vs 94%), which narrowly failed to reach statistical significance (p=0.054). When resurfacing the patella, this study demonstrates that there was a significant increase in patients’ improvement following total knee replacement when more than 50% of the patella is covered with the implant. This can be achieved by a combination of selecting a larger patellar component to maximize coverage, as well as careful resection of any overhanging bone after placement of the trial patellar component.