Range of movement correlates with the Oxford knee score after total knee replacement: A prediction model and validation

- This study has identified and quantified the relationship between the OKS and ROM after TKR.
- The OKS is an independent predictor of ROM after TKR (p < 0.001). An increasing (better) score correlates with a greater ROM.
- Validated regression models can be used to predict probable ROM and reliability increases when confounders are accounted for.
- Total OKS correlates more strongly than individual component questions, reflecting its usefulness as a one dimensional score.
- This study aids in reconciliation of subjective PROMs and objective measures of joint function when evaluating TKR outcome.

BackgroundPatient reported outcome measures are widely used in the evaluation of outcomes after Total Knee Replacement (TKR) in joint registries and large studies. The aim of this study was to assess the relationship between the Oxford knee score (OKS) and range of motion (ROM) after TKR, and to construct and validate prediction models of ROM from the measured OKS.MethodsEight hundred sixty patients reviewed five years postoperatively and 273 patients reviewed nine to 10 years postoperatively completed an OKS. Of these, 808 (94%) and 226 (83%) patients, respectively, had a complete dataset (knee extension and ROM) and formed the study cohort.ResultsRegression analysis demonstrated a significant correlation between the OKS and ROM (r = 0.38, p < 0.001) after adjusting for other confounding variables (age, sex, body mass index, and knee extension). A prediction model was constructed and validated using a second cohort of 226 patients at nine to 10 years after their TKR. Intraclass correlation demonstrated good reliability (r = 0.60, 95% CI 0.47 to 0.69) between predicted and actual measured ROM for this group. However, when the OKS is used in isolation the reliability of the predicted ROM is diminished (intraclass correlation r = 0.41, 95% CI 0.24 to 0.55).ConclusionsThe OKS is an independent predictor of ROM after TKR. It is also possible to predict ROM from the OKS, but the reliability of this is improved when other independent predictors such as age, gender, body mass index (BMI) and degree of knee extension are also acknowledged.