Decoding the Knee Injury and Osteoarthritis Outcome Score for Joint Replacement: A Critical Investigation of a Common...

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 patients. The goal of this study was to evaluate the underlying construct of KOOS JR.

Data were collected from 156 patients (94 women; mean age 68 years [range, 39 to 83]) who were scheduled to undergo primary elective total knee arthroplasty. The KOOS JR and eight physical performance measures 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 were performed to determine the relative contribution of each KOOS JR domain score (i.e., stiffness, pain, and function) to the total score.

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 principal component analysis 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 physical performance measures 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).

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.