A validated outcome categorization of the knee society score for total knee arthroplasty

Purpose To determine cutoff values for the Knee Society Scores (KSS) indicative of a categorical scale of medium-term outcomes. Methods One hundred and fifty-five patients who underwent primary cruciate-retaining TKA with a patellar button for osteoarthritis at a single-centre were assessed prospectively by the KSS and short-form Western Ontario and McMaster Universities Osteoarthritis Index (WOMAC) simultaneously at the 3-year follow-up. A validated categorization of the WOMAC score was used as a standard. The area under the curve (AUC) of receiver-operating characteristic (ROC) was used to assess the discriminative analysis accuracy of the, and the Youden index estimated the optimal cutoff point. Results For the KSS-knee score, the cutoff for an excellent outcome was 90.3 (AUC 0.75, 95% CI 0.71-0.78), 76.6 (AUC 76.6, 95% CI 0.70-076) for good, 64.8 (AUC 0.76, 95% CI 0.72-0.79) for fair, and < 64.8 (AUC 0.69, 95% CI 0.67-0.73) for poor. For the KSS-function score, the cutoff values were 85.2 (AUC 0.71, 95% CI 0.69-0.75), 73.1 (AUC 0.72, 95% CI, 0.70-0.76), 55.7 (AUC 0.70, 95% CI 0.71-0.74), and < 55.7 (AUC 0.68, 95% CI 0.66-0.72), respectively. Conclusion A KSS-knee score >= of 90 was considered an excellent outcome, 77 good, 65 fair, and < 65 poor. For the KSS-function, those values are 85, 73, 56 and < 56, respectively. The treatment outcome's judgement may be clearer for the surgeon concerning a particular patient when using cutoff values for the scoring system employed, such as those determined in the present study.

Automated summary

This study aimed to determine cutoff values for Knee Society Scores for a better assessment of medium-term outcomes after total knee arthroplasty. Different outcome levels for KSS knee and function scores were identified using AUC and Youden index.