Good validity and reliability of the forgotten joint score in evaluating the outcome of total knee arthroplasty

Background and purpose — When evaluating the outcome after total knee arthroplasty (TKA), increasing emphasis has been put on patient satisfaction and ability to perform activities of daily living. To address this, the forgotten joint score (FJS) for assess- ment of knee awareness has been developed. We investigated the validity and reliability of the FJS. Patients and methods — A Danish version of the FJS ques- tionnaire was created according to internationally accepted standards. 360 participants who underwent primary TKA were invited to participate in the study. Of these, 315 were included in a validity study and 150 in a reliability study. Correlation between the Oxford knee score (OKS) and the FJS was examined and test- retest evaluation was performed. A ceiling effect was defi ned as participants reaching a score within 15% of the maximum achiev- able score. Results — The validity study revealed a strong correlation between the FJS and the OKS (intraclass correlation coeffi cient (ICC) = 0.81, 95% CI: 0.77–0.85; p < 0.001). The test-retest eval- uation showed almost perfect reliability for the FJS total score (ICC = 0.91, 95% CI: 0.88–0.94) and substantial reliability or better for individual items of the FJS (ICC  0.79). We found a high level of internal consistency (Cronbach’s  = 0.96). The ceil- ing effect for the FJS was 16%, as compared to 37% for the OKS. Interpretation — The FJS showed good construct validity and test-retest reliability. It had a lower ceiling effect than the OKS. The FJS appears to be a promising tool for evaluation of small differences in knee performance in groups of patients with good clinical results after TKA.