Interpreting the Knee Osteoarthritis Outcome Score Joint Replacement: Minimum clinically important difference values vary over time within the same patient population

Introduction Total knee arthroplasty (TKA) is the preferred treatment for end-stage osteoarthritis. Minimal clinical important difference (MCID) quantifies if a patient achieves a successful outcome following TKA. This study aims to determine the MCID for Knee Injury Osteoarthritis Outcome Score Joint Replacement (KOOS JR) at two designated time intervals and to compare methodologies for calculating MCID. Methods Patient-Reported Outcome Measures (PROM) were prospectively evaluated for 956 patients undergoing TKA between 2017 and 2018 at a single health care system. Patients who completed baseline, 3-month and 1-year post-operative PROM surveys at clinical follow-up were included in the study. MCID was calculated from baseline to 3-months and baseline to 1-year using anchoring and distribution-based methods. Results Average KOOS JR scores at baseline, 3-month, and 1-year follow-up were 51.7 ± 11.7, 69.2 ± 12.0, and 76.3 ± 14.5. The KOOS JR MCID determined by the PROMIS10 physical health anchoring method was 21.5 ± 14.9 and 27.9 ± 16.0 for the 3-month and 1-year intervals. The KOOS JR MCID determined by the PROMIS10 quality-of-life anchoring method was 21.2 ± 14.9 and 28.9 ± 15.8 for the 3-month and 1-year intervals. The MCID calculated by the distribution method was 7.4 and 8.2 for the 3-month and 1-year intervals. Conclusion KOOS JR MCID varies as a function of time and is variable within the same population depending on MCID calculation methodology. Improved strategy for calculating or standardizing MCID is required to better guide use of KOOS JR and other PROMs in clinical decision-making.