15-year validation of the forgotten joint score after unicompartmental knee arthroplasty: A reliable indicator of long-term patient satisfaction and joint awareness.

Background: Unicompartmental knee arthroplasty (UKA) offers bone- and ligament-preserving treatment for medial compartment osteoarthritis. The Forgotten Joint Score (FJS) is a modern patient-reported outcome measure (PROM) that assesses joint awareness during daily activity-an ideal benchmark for long-term implant success. However, its utility beyond 10 years postoperatively remains insufficiently studied. Purpose: To evaluate long-term clinical outcomes and validate the Forgotten Joint Score (FJS) 15 years following medial fixed-bearing UKA. Methods: This retrospective cohort study included 54 patients who underwent medial UKA between 2003 and 2007 at a tertiary academic institution. Functional outcomes (Knee Society Knee Score [KSKS], Knee Society Function Score [KSFS], Oxford Knee Score [OKS], and SF-12 Physical and Mental Component Scores [PCS, MCS]) were assessed preoperatively and at 2, 10, and 15 years postoperatively. At 15 years, patients completed the FJS questionnaire unaided. Internal consistency (Cronbach's α), ceiling effects, Pearson correlations, and logistic regression were performed to assess construct validity and patient-perceived outcomes. Multiple linear regression identified preoperative predictors of FJS. Results: Significant and sustained improvements were observed in KSFS, KSKS, OKS, and PCS up to 15 years postoperatively (p < 0.05). The mean FJS at 15 years was 78.6 (SD 23.1); 94.4 % of patients exceeded the PASS threshold, and 50 % achieved "forgotten joint" status (FJS ≥84.38). Internal consistency was excellent (Cronbach's α = 0.88). FJS showed strong correlations with OKS (r = 0.675, p < 0.001), KSFS (r = 0.470), and PCS (r = 0.340). Logistic regression confirmed FJS as a strong predictor of patient satisfaction (classification accuracy 90.7-92.6 %, p < 0.01). Preoperative KSFS was the only significant predictor of long-term FJS (β = 0.6, p = 0.029). Conclusion: The Forgotten Joint Score is a reliable, internally consistent, and valid long-term outcome measure following medial UKA. Its strong associations with functional scores, patient satisfaction, and preoperative function support its incorporation into long-term clinical outcome assessment and highlight UKA's durability and patient acceptability at 15 years. Level of evidence: III, Retrospective Cohort Study.