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This study investigated the construct validity of the KOOS JR in 156 patients undergoing primary TKA by correlating KOOS JR scores with physical performance measures (PPMs) at 2, 6, and 12 weeks post-op. The KOOS JR total score was most strongly correlated with the pain subscore, and principal component analysis revealed that pain was the dominant contributor to the total score's variance. The authors conclude that KOOS JR primarily reflects perceived pain rather than overall knee health.
The KOOS JR score after TKA appears to be driven primarily by pain, suggesting it may not accurately reflect overall knee function or stiffness as intended.
BACKGROUND The Knee Injury and Osteoarthritis Outcome Score for Joint Replacement (KOOS JR) is a composite patient-reported outcome measure intended to represent overall knee health. Limited investigations exist to determine the validity of KOOS JR as a marker of joint function among total knee arthroplasty (TKA) patients. The goal of this study was to evaluate the underlying construct of KOOS JR. METHODS Data were collected from 156 patients (94 women; mean age 68 years (range, 39 to 83)) who were scheduled to undergo primary elective TKA. The KOOS JR and eight physical performance measures (PPMs) were postoperatively administered at two, six, and 12 weeks. Spearman's rank correlation coefficients (rho) were used to assess the association among our variables. Principal component analyses (PCAs) were performed to determine the relative contribution of each KOOS JR domain score (i.e., stiffness, pain, and function) to the total score. RESULTS The KOOS JR total score showed the strongest correlation (P < 0.001) with the pain score (rho = 0.94 to 0.96), followed by the function (rho = 0.80 to 0.85) and stiffness (rho = 0.59 to 0.65) scores. The PCA indicated that one dominant component explained 69 to 74% of the total variance across the three domain scores, with the pain score contributing the most. There were no PPMs that were correlated with the KOOS JR function score (P ≥ 0.059), except the Step Up measure that was weakly correlated at six weeks (rho = -0.33, P = 0.003). CONCLUSIONS The KOOS JR appears to be a unidimensional construct that largely reflects perceived pain, despite its intention to represent overall knee health. Appropriate outcome metrics should be selected based on the specific goals of assessment and tailored to reflect what matters most to patients, whether that is reducing pain, addressing stiffness, or achieving functional recovery.
