The KOOS‐12 shortform shows no ceiling effect, good responsiveness and construct validity compared to standard outcome measures after total knee arthroplasty

PURPOSE: To investigate the validity, responsiveness and ceiling effect of the recently introduced KOOS-12 and compare its performance to the KOOS, OKS, WOMAC and UCLA activity scales. METHODS: Patients from an independent multicentre study examining a medially stabilized knee system prospectively completed the KOOS, OKS, WOMAC and UCLA preoperatively and at 1 year postoperatively. KOOS-12 scores were calculated from the full length KOOS data. Construct validity was assessed using Spearman's correlation analysis. The ceiling effect was evaluated by calculating the percentage of patients with a maximum score. If the percentage exceeded 15%, a ceiling effect was considered to be present. Responsiveness was evaluated by performing paired t tests on the changes in measures and calculation of Cohen's d. RESULTS: A ceiling effect was present for the KOOS Pain, ADL and QoL subscales and the KOOS-JR at 1 year postoperatively. No ceiling effect was observed for the KOOS-12. Correlation of the KOOS-12 was low (0.3 < r < 0.5) with the UCLA, moderate (0.5 < r < 0.7) with the KOOS symptoms, sports and WOMAC stiffness subscales and high (r > 0.7) with all other scores and subscales. Effect size of the UCLA activity scale was moderate (Cohen's d 0.2-0.8) whereas effect sizes of all other outcome measures were large (d > 0.8). CONCLUSION: The KOOS-12 does not exhibit a ceiling effect, has good convergent construct validity and is responsive to changes in pain, function, QoL and knee impact between preoperatively and 1 year postoperatively. LEVEL OF EVIDENCE: Diagnostic level III.