title: 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.
abstract: OBJECTIVE
  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.
  
  
  CONCLUSIONS
  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.
  
  
  METHODS
  Large-scale retrospective observations study, Level II.
authors.affiliation: Oxford University, Oxford, Australia.
authors.forename: Kristina
authors.initials: K
authors.lastname: Harris
authors.affiliation: Oxford University, Oxford, Australia. christopher.lim@ndorms.ox.ac.uk.
authors.forename: Christopher R
authors.identifier: http://orcid.org/0000-0001-7949-0943
authors.initials: CR
authors.lastname: Lim
authors.affiliation: Oxford University, Oxford, Australia.
authors.forename: Jill
authors.initials: J
authors.lastname: Dawson
authors.affiliation: Oxford University, Oxford, Australia.
authors.forename: Ray
authors.initials: R
authors.lastname: Fitzpatrick
authors.affiliation: Oxford University, Oxford, Australia.
authors.forename: David J
authors.initials: DJ
authors.lastname: Beard
authors.affiliation: Oxford University, Oxford, Australia.
authors.forename: Andrew J
authors.initials: AJ
authors.lastname: Price
mesh_terms: D000328:Adult; D000368:Aged; D000369:Aged, 80 and over; D019645:Arthroplasty, Replacement, Knee; D016208:Databases, Factual; D005260:Female; D005500:Follow-Up Studies; D006801:Humans; D008297:Male; D008875:Middle Aged; D000071066:Patient Reported Outcome Measures; D012189:Retrospective Studies; D013222:State Medicine; D006113:United Kingdom
keywords: Arthroplasty; Ceiling; Floor; Function; Knee; OKS; Outcome; Oxford; Pain; Patient; Reported; Score; Subscale