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.

Level of evidence

II.