The Oxford knee score and its subscales do not exhibit a ceiling or a floor effect in knee arthroplasty patients: an analysis of the National Health Service PROMs data set.

PURPOSE: In this study, we examined whether the OKS demonstrated a floor or a ceiling effect when used to measure the outcome of knee replacement surgery in a large national cohort. METHODS: NHS PROMs database, containing pre- to 6 month post-operative OKS on 72,154 patients, mean age 69 (SD 9.4), undergoing knee replacement surgery, was examined to establish the proportion of patients achieving top or bottom OKS values pre- and post-operatively. RESULTS: Pre-operatively, none of patients achieved the maximum/'best' (48) and minimum (0) scores. Post-operatively, no patients (0 %) achieved the minimum/'worst' score, but the percentage achieving the maximum score increased to 2.7 %. Subgroup analyses demonstrated that the highest post-operative overall ceiling percentage was 3 %, in a subgroup of patients between 60 and 79 years of age and 13.7 % in a group of patients who had a pre-operative OKS above 41. Furthermore, 10.8 % of patients achieved the top post-operative OKS-PCS and 4.7 % top post-operative OKS-FCS. CONCLUSION: Based on NHS PROMs data, the OKS does not exhibit a ceiling or floor effect overall, or for both its pain and function subscales, and remains a valid measure of outcomes for patients undergoing TKA. LEVEL OF EVIDENCE: Large-scale retrospective observations study, Level II.