Multi-group invariance testing of the knee injury osteoarthritis outcome score for joint replacement scale

Objective: The Knee Osteoarthritis Outcome Score for Joint Replacement (KOOS-JR) scale is commonly used to assess patient progress. Scale structural validity has not been completely assessed. The purpose of this study was to assess the internal consistency, structural validity, and multi-group invariance properties of the KOOS-JR in a large sample of patients receiving knee arthroplasty or non-operative care. Methods: A cross-sectional study using the Surgical Outcome System (SOS) database. Patients receiving care for degenerative knee conditions were included in the study. Internal consistency was assessed using Cronbach's alpha and McDonald's Omega. A confirmatory factor analysis was conducted to confirm scale structure of the KOOS-JR using a priori cut-off values (Comparative Fit Index [CFI], Tucker-Lewis Index [TLI], Incremental Fit Index [IFI] >= 0.95, Root Mean Square Error of Approximation [RMSEA] <= 0.06 preferred and <= 0.08 acceptable). Multigroup invariance testing was conducted across sex, age, and intervention groups. Results: Internal consistency was acceptable (alpha = 0.83; omega = 0.83). The unidimensional structure of the KOOS-JR exceeded most contemporary model fit recommendations (CFI = 0.976, TLI = 0.964, IFI = 0.976, RMSEA = 0.067). The KOOS-JR was invariant across groups, allowing for comparison of variances and means between sex, age, and intervention groups. Conclusion: The KOOS-JR met or exceeded most of the recommendations for model fit. The scale can be used to assess differences between males and females, middle and older aged adults, and between baseline measures of patients who received total knee arthroplasty or non-operative care.