Multidimensional minimal clinically important differences in knee osteoarthritis after comprehensive rehabilitation: a prospective evaluation from the Bad Zurzach Osteoarthritis Study

OBJECTIVE: To determine minimal clinically important differences (MCIDs) for improvement and worsening in various health dimensions in knee osteoarthritis under conservative therapy. METHODS: Health, symptoms and function were assessed by the generic Short Form 36 and the condition-specific Western Ontario and McMaster Universities Osteoarthritis Index in n=190 patients with knee osteoarthritis before and after comprehensive rehabilitation intervention (3-month follow-up). By means of construct-specific transition questions, MCIDs were defined as the difference between the ‘slightly better/worse’ and the ‘almost equal’ transition response categories according to the ‘mean change method’. The bivariate MCIDs were adjusted for sex, age and baseline score to obtain adjusted MCIDs by multivariate linear regression. They were further standardised as (baseline) effect sizes (ESs), standardised response means (SRMs) and standardised mean differences (SMDs) and compared with the minimal detectable change with 95% confidence (MDC95). RESULTS: Multivariate, adjusted MCIDs for improvement ranged from 2.89 to 16.24 score points (scale 0–100), corresponding to ES=0.14 to 0.63, SRM=0.17 to 0.61 and SMD=0.18 to 0.72. The matching results for worsening were –5.80 to –12.68 score points, ES=–0.30 to –0.56, SRM=–0.35 to –0.52 and SMD=–0.35 to –0.58. Almost all MCIDs were larger than the corresponding MDC95s. CONCLUSIONS: This study presents MCIDs quantified according to different methods over a comprehensive range of health dimensions. In most health dimensions, multivariate adjustment led to higher symmetry between the MCID levels of improvement and worsening. MCIDs expressed as standardised effect sizes (ES, SRM, SMD) and adjusted by potential confounders facilitate generalisation to the results of other studies.