DOES GRADE OF PATELLOFEMORAL CARTILAGE WEAR INFLUENCE OUTCOME IN MOBILE-BEARING UNICOMPARTMENTAL KNEE ARTHROPLASTY?

Aims Patellofemoral joint (PFJ) cartilage wear remains among the most cited reasons for withholding medial unicompartmental knee arthroplasty (UKA). Asian patients who sit cross-legged or on the floor impose greater patellofemoral loads. Most series grade the PFJ as a single unit, or as present vs. absent, and seldom test whether the differences exceed the minimal clinically important difference (MCID). We mapped the PFJ zone by zone to determine whether any zone, at any grade, in any combination, predicts outcome. Methods Of 1,393 eligible Oxford Phase III mobile-bearing medial UKAs performed by one surgeon between January 2016 and February 2022, 989 performed in 727 patients had complete data. Outerbridge grades were recorded for trochlea, patellar ridge, medial and lateral facets. Mean follow-up was 7.3 y (4.4 to 10.5). Forgotten Joint Score (FJS-12), Oxford Knee Score (OKS) and WOMAC were compared across grades against MCID thresholds (14, 5 and 10) and tested for equivalence. Logistic regression was used to determine the patient-acceptable symptom state (OKS PASS >= 30). Results Mean preoperative varus was 170.9 o (SD 3.7), postop was 176.5 o (SD 2.9). Mean FJS-12 was 86.2 (SD 15.0), OKS 42.8 (SD 6.4) and WOMAC 17.7 (SD 6.0). A normal PFJ was seen in 240 knees (24.3%); wear in the medial facet was in 64.6%, trochlea in 55.7% and ridge in 50.6%. Mean PROMs were unaffected by grade. Mean scores showed a tail effect: ridge Grade 3 or worse tripled the risk of an unacceptable PASS (13.2% vs 4.5%; odds ratio 0.09 (95% CI 0.01 to 0.54); p = 0.009). Conclusion PFJ wear is common and does not influence UKA outcome at any grade or zone. Results are not influenced by trochlear and medial facet wear, however severe. Grade 3 or worse ridge wear triples the risk of an unacceptable result.