The minimal clinically important difference in the Oxford knee score and Short Form 12 score after total knee arthroplasty

Purpose

The aim of this study was to identify the minimal clinically important difference (MCID) in the Oxford knee score (OKS) and Short Form (SF-) 12 score after total knee arthroplasty (TKA).

Methods

Prospective pre-operative and 1 year post-operative OKS and SF-12 scores for 505 patients undergoing a primary TKA for osteoarthritis were collected during a one-year period. Patient satisfaction with their (1) patient relief and (2) functional outcome was used as the anchor questions. Their response to each question was recorded using a 5-point Likert scale: excellent, very well, well, fair, and poor. Simple linear regression was used to calculate the MCID for improvement in the OKS and physical component of the SF-12 score according to the level of patient satisfaction with their pain relief and function.

Results

The OKS improved by 15.5 (95 % CI 14.7–16.4) points and the SF-12 physical component score improved by 10.1 (95 % CI 9.1–11.2) points for the study cohort. The level of patient satisfaction with their pain relief and function correlated with the improvement in the OKS (r = 0.56; p < 0.001, and r = 0.56; p < 0.001) and the physical component of the SF-12 score (r = 0.51; p < 0.001, and r = 0.60; p < 0.001), respectively. The MCID for the OKS was 5.0 (95 % CI 4.4–5.5) and 4.3 (95 % CI 3.8–4.8) points and for the physical component of the SF-12, it was 4.5 (95 % CI 3.9–5.2) and 4.8 (95 % CI 4.2–5.4) points for pain relief and function, respectively.

Conclusion

The MCID identified for the OKS and SF-12 physical component score after TKA is the best available estimate and can be used to power studies and ensure that a statistical difference is also recognised by a patient.

Level of evidence

Retrospective diagnostic study, Level III.