Knee Injury and Osteoarthritis Outcome Score Interpretation Thresholds at 3 and 12 Months After Arthroscopic Meniscal Surgery. What Changes, and What Stays the Same?

OBJECTIVE: To evaluate the change in minimal important change (MIC), patient acceptable symptom state (PASS), and treatment failure (TF) thresholds for the Knee injury and Osteoarthritis Outcome Score (KOOS) from 3 to 12 months following arthroscopic meniscus surgery. DESIGN: Retrospective cohort study. METHODS: We included patients from the Knee Arthroscopy Cohort Southern Denmark who had meniscus surgery between 2013 and 2015. We calculated the interpretation threshold values for MIC, PASS, and TF using an anchor-based adjusted predictive modeling method. Thresholds at 3 and 12 months postoperatively were compared to evaluate changes over time. RESULTS: The proportions of people who reported a clinically relevant improvement, or their symptoms being acceptable, increased by 10% to 15% points from 3 to 12 months after surgery. MIC thresholds for the 5 KOOS subscales remained stable from 3 to 12 months with statistically nonsignificant differences (95% confidence intervals) ranging from -0.3 (-3.0, 2.6) to -2.4 (-6.1, 1.3). All PASS thresholds except for quality of life (QOL) decreased by -6.5 (-11.8, -1.5) to -3.7 (-7.1, -0.4) points, indicating that higher symptom levels were accepted at 12 months. In contrast, the proportion reporting their treatment to have failed remained stable over time (19% and 17%). For QOL, a 7.4 (2.0, 13.2) higher TF threshold at 12 months suggested that improved QOL was required to not consider that the treatment had failed. CONCLUSION: More patients reported being improved at 1 year compared to at 3 months following meniscus surgery. The KOOS MIC thresholds were stable over time, whereas time-specific PASS values should be applied after meniscus surgery.