Minimal important differences for the WOMAC osteoarthritis index and the Forgotten Joint Score-12 in total knee arthroplasty patients

N. Holtz, D. F. Hamilton, J. M. Giesinger, B. Jost, K. Giesinger*

*Corresponding author for this work

Research output: Contribution to journalArticlepeer-review

18 Citations (Scopus)
2 Downloads (Pure)


Background: Total knee arthroplasty (TKA) is an effective treatment for end-stage osteoarthritis. Patient reported-outcome measures (PROMs) capture the patients' perception of the success of an intervention. The minimal important difference (MID) is an important characteristic of the PROM, which helps to interpret results. The aim of this study was to identify the MID for the Forgotten Joint Score-12 (FJS-12) and Western Ontario and McMaster Universities (WOMAC) osteoarthritis index. 

Methods: Data were collected in a prospective cohort study. Patients were asked to complete the FJS-12, WOMAC osteoarthritis index and transition items evaluating change over time to determine the MID. We employed an anchor-based methodology relating score change to the response categories of the transition items using both binary logistic regression and receiver operating characteristic (ROC) analysis. 

Results: Data from 199 patients were analysed. Mean age was 72.3 years, 58% were women. Employing binary logistic regression the MID for the FJS-12 was 10.8 points, for the WOMAC pain score 7.5 points and for the WOMAC function score 7.2 points. ROC analyses found a MID of 13.0 points for the FJS-12, 12.5 points for WOMAC pain and 14.7 points for WOMAC function. 

Conclusion: We report MIDs for the FJS-12 and the WOMAC Pain and Function scales in a TKA patient cohort, which can be used to interpret meaningful differences in score. In line with previous research, we found more advanced statistical methods to result in smaller MID estimates for both scores. 

Trial registration: Written consent for this study was obtained from all participants and ethical approval was granted by the local ethics committee (Ethikkommission St. Gallen; EKSG 14/973; Registered 03 July 2014;

Original languageEnglish
Article number401
JournalBMC Musculoskeletal Disorders
Publication statusPublished - 23 Jun 2020


  • Forgotten joint score-12
  • minimal important difference
  • patient-reported outcomes
  • total knee arthroplasty
  • Western Ontario and McMaster universities osteoarthritis index
  • prospective studies
  • follow-up studies
  • humans
  • middle aged
  • male
  • range of motion, articular
  • aged, 80 and over
  • arthroplasty, replacement, knee
  • female
  • knee Joint/physiopathology
  • severity of Illness index
  • osteoarthritis, knee/surgery
  • logistic models
  • treatment outcome
  • Switzerland
  • joint prosthesis
  • ROC curve
  • aged
  • minimal clinically important difference

ASJC Scopus subject areas

  • Rheumatology
  • Orthopedics and Sports Medicine


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