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

Patients and data collection

The data derive from the ‘Zurzach Osteoarthritis Study’, an observational, prospective cohort study, which examined health and quality of life before and after comprehensive rehabilitation for hip and knee osteoarthritis.14 All patients fulfilled the American College of Rheumatology criteria for knee osteoarthritis and signed written informed consent to participate.15 Clinical outcome data were earlier reported for controlled short-term effects in knee patients.14 16

The rehabilitation intervention consisted chiefly of active therapeutic entities (individual and group, land-based and water-based physiotherapy) and passive modalities (massage, packs, educational measures, coping instructions), which have been previously described in detail.14 For this study, all patients with knee osteoarthritis treated in hospital (2–3 weeks’ inpatient stay) and ambulatory (6–9 weeks’ outpatient sessions) settings were included. Data were collected at the start of the intervention (baseline) and at follow-up 3 months later, when all therapies had been completed (n=190).

Measures

MCIDs were determined on the basis of two instruments: the well-known and best-tested generic Medical Outcomes Study 36-Item Short Form Health Survey (SF-36), version 1, and the condition-specific Western Ontario and McMaster Universities Osteoarthritis Index (WOMAC).17–22 In the medical literature, the SF-36 is the most widely used and most important patient-rated outcome instrument for healthy people and any health-affecting condition.6 10–14 The WOMAC is the most frequently used specific tool for hip and knee.6 9–14 Both instruments require license payments. While the manual of the SF-36 can be obtained for unlimited use for approximately US$300 (as can the German version and manual), the use of the WOMAC is limited by the number of applications (persons and follow-ups).17 19 However, all the WOMAC items are included in the Knee injury and Osteoarthritis Outcome Score (KOOS), which is available free of charge, and the German WOMAC version 1 used in this study is in the public domain.6 20

The SF-36 consists of 36 items distributed over eight scales, four physical and four psychosocial. The WOMAC is a 24-item instrument with three subscales: five items for pain, two for stiffness and 17 for function. By Rasch analysis, four functional factor subdimensions can be further defined.21 22 Of those, the WOMAC standing and walking factor, was included in the present study. The complex rationale for the construction and validation of this factor dimension has been published before.20–22

Table 2 summarises the 11 construct domains of the SF-36 and WOMAC scales used in our analysis and the corresponding transition item anchors, with their specific contents. Seven of the eight scales of the SF-36 were included (role emotional was omitted) together with the three original WOMAC subscales and the additional WOMAC factor standing/walking.

The WOMAC factor standing/walking comprises one pain and one function item for standing together with one pain and one function item for walking on flat surface, that is, four items in total. SF-36 role physical consists of four items: limits in the amount of time spent on work or other activities, accomplished less than would have liked, limits in the kind of work or other activities, and difficulties performing work or other activities. SF-36 social functioning has two items: extent and time of interference of physical or emotional problems with normal social activities.

Descriptive data

Descriptive data at baseline and score differences to 3-month follow-up were compiled using arithmetic means and metric SD. For ease of comparison, all scores were scaled from 0=worst (most pain, no function, worst health) to 100=best (no pain, best function, best health).14 17

Assessment of appropriateness for determining the MCID

As a preliminary analysis to test a scale’s appropriateness for calculation of the MCID, floor and ceiling proportions were determined for each scale, with the floor being the percentage of subjects with the minimum score=0, or worst health, and the ceiling the percentage with the maximum score=100, or best health. A high floor at baseline means that many patients can experience no further deterioration to the follow-up. Vice versa, a high ceiling phenomenon at baseline indicates that many patients cannot further improve. Both the floor and the ceiling affect the ability to calculate changes between baseline and follow-up on a closed scale (0–100) and, consequently, also the quantification of valid MCIDs.

In addition, Spearman rank correlation coefficients between the five transition ratings (much worse, slightly worse, almost equal, slightly better, much better) and the score differences (baseline to follow-up) were calculated. ‘Revicki’s criterion’ was applied1 13: that is, a proposed correlation of ≥0.30 to prove a sufficient level of construct convergence between the scale and the anchor.

Specific anchors

The content of the transition item questions was construct specific for all scales (see table 2). For the constructs of function and role physical (role limitations due to physical health), two transition items were combined (table 2): walking plus dressing and personal hygiene; household chores plus work. At least one of the items had to be rated ‘slightly better’ for the combined item to score ‘slightly better’. The same applied to the rating ‘slightly worse’. However, the response to both items had to be ‘almost equal’ for the rating ‘almost equal’ to be attributed.

Bivariate, unadjusted MCIDs

Bivariate, unadjusted MCIDs were calculated according to the ‘mean change method’.5 8 The mean score change or difference of the transition category ‘slightly better’ minus that of the transition category ‘almost equal’ defined the MCID for improvement. By analogy, the mean score change and difference of the transition category ‘slightly worse’ minus that of the transition category ‘almost equal’ defined the MCID for worsening. A positive MCID reflects improvement, a negative MCID worsening.

The interval for 95% confidence of the MCID was calculated by ±t×square root of (1/n₁ +1/n₂)×pooled SD.23 The pooled SD is equal to   , where the ‘slightly better’ group had n₁=number of patients and s₁=SD of the score changes (baseline to follow-up), and the ‘almost equal group’ had correspondingly n₂ and s₂. The multiplicator ‘square root of (1/n₁ +1/n₂)’ gives the SE of the MCID. The t-value comes from Student’s t distribution with two-sided type I error of 0.05 and df= n₁+ n₂–2 and can be calculated on the internet.24

Multivariate, adjusted MCIDs

In order to obtain a less biased, more generalisable estimate of the MCID, multivariate, confounder-adjusted MCIDs were calculated using the regression model as follows5: score change (baseline to follow-up; in score points)=b₁×transition group (binary coded: 1=‘slightly better’, 0=‘almost equal’)+b₂×baseline score (in score points)+b₃×sex (m/f)+b₄×age (in years)+constant term. The coefficient b₁ is equal to the adjusted MCID and the SE of b₁ is the SE of the MCID. This adjusted, multivariable MCID can also be divided by the specific SD to obtain adjusted effect sizes as described above. The interval for 95% confidence can be calculated by ±t×SE of the MCID.23 24

Standardisation of MCIDs by effect sizes

All MCIDs were further standardised, that is, divided by specific SD to obtain a parameter belonging to the family of ‘effect sizes’.5 23 This results in a dimensionless parameter that is less biassed, especially by different baseline scores, and is more generalisable to the findings of other studies.5 A positive effect size (ES) reflects improvement, a negative ES a worsening of health.

The following three most important and frequently used parameters were calculated: (1) the (baseline) ES according to Kazis et al, which is the crude MCID divided by the baseline SD of the whole sample.25 (2) The standardised response mean (SRM) according to Liang, which is the division of the crude MCID by the SD of the score differences of the whole sample.26 (3) The SMD according to Borenstein, which is the crude MCID divided by the pooled SD of the ‘slightly better’ transition category and the ‘almost equal’ transition category (see above) and multiplied by a correction factor J to reduce bias for small sample sizes.23

MDC95 for comparison with MCID

For each scale, the intraclass correlation coefficients (ICCs) were sourced from the literature.20 21 27 The ICC measures test–retest reliability and is needed to determine the minimum detectable changes with 95% confidence (MDC95). The MDC95 reflects the measurement error of two different measurements on the same scale.25 If an MCID is larger than the MDC95, it reflects a difference (between baseline and follow-up) larger than a difference that may occur due to an error of measurement with 95% confidence. The MDC95 is calculated by the t-values multiplied by the SE of measurement (SEM), which is equal to the SE of the MCID multiplied by the square root of (1−ICC).25 The MDC95 is a CI which applies to single parameter estimates (the MCID) but not to CIs of a parameter.25

Setting and descriptive data

The sample consisted of n=190 patients with knee osteoarthritis with complete baseline and follow-up data (tables 1–3). Of those, 149 (78.1%) were women. The mean age was 66.1 years and the SD 10.2 years (table 1).

The construct overlap (correlation) was ≥0.30 for all scales, with the exception of SF-36 role physical, vitality and social functioning, which failed the ‘Revicki’s criterion’ (table 2).1 However, those three scales were kept in the analysis for the sake of completeness; they are marked in lighter font. The scores at baseline (see also table 3), showed a very high floor rate for SF-36 role physical (65.8%) reflecting n=125/190 subjects with score 0 (worst health); no other scale had a floor phenomenon (table 2). Ceiling (ie, 100=best health) percentage scores were as follows: 23.7% (n=45) for SF-36 social functioning, 5.4% (n=10) for role physical and 3.7% (n=7) for WOMAC stiffness, but were very low or absent on all the other scales. The test–retest ICCs were found in three studies of hip and knee osteoarthritis.20 21 27

Baseline and follow-up scores for the whole sample (n=190) and within the three transition groups relevant for the MCID are shown in table 3. All baseline score levels were around the middle of the scale 0–100, except SF-36 bodily pain, physical functioning and role physical (16.58), reflecting relatively great pain and low physical function and role (table 2). This means that changes between the baseline and follow-up were detectable in both directions, that is, improvements (positive differences) and worsening (negative differences). The mean score at the 3-month follow-up can be calculated by adding the change to the baseline score, example SF-36 general health: 53.49+(–1.80)=51.69.

Consistent with the anchor rating, all changes showed improvements in the ‘slightly better’ category. In the ‘almost equal’ category, on average most changes showed improvements (maximum on SF-36 bodily pain, mean=+8.31), with the exception of WOMAC stiffness and SF-36 general health (table 3). In the ‘slightly worse’ group, 8/11 scales showed worsening, except SF-36 bodily pain, role physical and vitality.

Minimal clinically important differences

In the domain of pain, MCIDs for improvement ranged from 7.09 to 10.41 score points and those for worsening from –4.26 to –7.07; symmetry (improvement and worsening) was higher in the adjusted parameters: 7.09 and 8.19 versus –7.07 and –6.08 (table 4). In the domain of function, WOMAC function showed relatively large MCIDs for improvement (14.48 bivariate and 11.25 multivariate) compared with SF-36 physical functioning (4.23 and 3.81), whereas the levels for worsening were comparable (–5.38 to –7.29). Consistent with these findings, the MCIDs for the WOMAC factor standing/walking were large (improvement: 10.20, 5.93; worsening: –10.62, –12.87). WOMAC stiffness showed very large MCIDs for improvement (20.24 bivariate, 16.24 multivariate) and lower levels for worsening (–6.21, –9.91). Moderate and comparable MCIDs were found for SF-36 mental health (improvement: 5.23, 2.89 vs worsening: –4.07, –8.94) and SF-36 general health (6.00, 7.15 vs –6.53, –5.80).

The three scales with low correlations (<0.30) with the transition item or having high floor/ceiling effects, namely SF-36 role physical, SF-36 vitality and SF-36 social functioning, showed partly lopsided results (eg, SF-36 social functioning, multivariate: +2.57 for improvement, –18.50 for worsening). Moreover, most of those three MCIDs were lower than the corresponding MCD95.

The MCIDs of all the other scales were mostly much higher than the corresponding MDC95, for example, WOMAC function improvement, bivariate: 14.48 versus 3.05; the very few exceptions included the bivariate MCID for worsening of WOMAC stiffness (table 4).

Expressed as ES, SRM and SMD, 12 of 21 of the bivariate and 8/21 of the multivariate MCIDs for improvement were in the range of small effect levels of 0.30–0.50 (table 4). This was also the case for 10/21 (bivariate) and 13/21 (multivariate) MCIDs for worsening. The highest effect sizes were for improvement in WOMAC function (bivariate up to SMD 0.93).