Applying the Impairment, Activity Limitation, and Participation Restriction Constructs of the ICF Model to Osteoarthritis and Low Back Pain Trials: A Reanalysis

MATERIALS AND METHODS

This is a secondary analysis of data from 3 previously published randomized controlled trials (RCT) conducted in the UK. Two were done on patients with knee OA and 1 on patients with sciatica. The 3 studies were chosen on the basis of convenience and availability of raw data. In each trial, 1 or 2 of the outcome measures (WOMAC function, SF-36 physical function, and ODQ) were used for the reassessment of the effect of the different interventions. The 3 measures included mostly pure A and P items or a combination of A/P, hence the investigation was confined to these and do not include I. The chosen outcome measures had previously been judged on whether they fit with the ICF structure¹⁵. For example, the 17 items of WOMAC function were reclassified as a combination of 12 pure Activity Limitation (A) items, and 5 questions judged to assess a mixture of Activity Limitation and Participation Restriction (A/P; Table 1). We compared the originally reported outcomes using total WOMAC function (17 items) with the newly defined subscales of 12 pure items on A and 5 mixture items (A/P). Summation over the appropriate items was used. Details of the classification of items for the outcome measures were given in Table 2 of Pollard, et al¹⁵.

The 3 trials examined the efficacy of a range of interventions including a pharmacological package, specific medical procedures and injections, and physiotherapy.

RESULTS

DISCUSSION

We examined the differential effect of a range of interventions on different domains of health as identified by the ICF model¹, using 3 outcome measures: SF-36 physical function, WOMAC function, and ODQ. Each item of these instruments was classified as assessing 1 or more domains, i.e., each item was classified as measuring a pure domain (impairment, activity limitation, or participation restriction), or mixed, such as activity limitation and participation restriction¹⁵. The study supports the hypothesis that different interventions may differentially affect different health domains. Although OA and back pain with sciatica were the focus of this study, the principle of examining different domains of health probably relates to many common musculoskeletal conditions.

The original KIVIS study showed positive effects for TI over corticosteroid injection, with TI significantly improving pain and function. When we examined the subscales of WOMAC function, the effect size for A/P items was larger than that for A only items, suggesting the possible effect of improvement in pain on participation. A differential effect was also noted in the TOPIK study, in which physiotherapy had a larger effect on participation restriction, while enhanced pharmacy seemed to improve activity more. Similarly in the WEST study, using A only items from the SF-36 physical function subscale detected benefit from the epidural steroid injection (ESI) over placebo, which was not apparent when the full physical function scale was used. Although not significant at the 5% level, these results are of interest since the positive effect size for A/P, as opposed to negative effect size for A only items, provides some evidence for a differential effect. The ESI appears to have improved participation but not activity. Thus, combining subscales that measure different domains may result in inconsistencies that disappear once they are treated separately. In such situations, pooling items could result in cancellation of opposite sign estimates (negative and positive) and eventually any effect would disappear or at least get diluted in the final total figure. Thus true treatment effects might be masked by using global outcome measures. The original WEST study showed a significant improvement in pain in the first assessment, and a slightly better score was also reported for the (ESI) group after that, supporting the possibility of better ability to participate. The possible relationships between the interventions investigated and the ICF domains are described in Figure 1.

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Figure 1.

Possible effects of different interventions on individual structures of the International Classification of Functioning, Disability and Health model.

Interest in outcome measures has occupied a central position in many fields of clinical and methodological research, and various issues have been addressed. The contents of measures and their suitability to assess different health conditions has been questioned by Hunt and McKenna²², and their responsiveness to change was highlighted by Ware, et al²³. Methodological problems that arise in attempting to use simple algebraic summation to aggregate items that may measure different domains of health has been explored by many researchers^24,25,26. Methods such as factor analysis and principal components are widely used to identify underlying structures of many health outcome measures. An example is the General Health Questionnaire^27,28. The complexity of these methods, however, and occasional inappropriate use have resulted in much confusion and inconsistency in the inferences drawn²⁹. Latent class models that solve many of the methodological problems of these classical methods were recently developed^30,31.

In addition to methodological issues, however, these statistical methods do not readily match measures to the theoretical domains of the ICF, but attempt to empirically identify dimensionality, and these methods cannot map measurement items to the theoretical domains such as those identified in the ICF. An alternative is the linkage method suggested by Cieza, et al. For example, they have indicated methods of linking OA measures to the ICF categories^2,32,33. However, they have not attempted to separate A and P, finding this too difficult using the linkage method. In our study, the discriminant content validation methods involving experts’ judgment rather than mathematical models, used to identify items as uniquely measuring I, A, or P, or any combination of these (Pollard, et al¹⁵), were numerically investigated. Our results show that it is indeed possible to find significant differences in attributing items to A or P¹⁵, and the results suggest that separating A and P may be important in investigating the effectiveness of different interventions. The view that A and P are combined, suggested by the ICF categories, was often challenged by researchers. Instead of linking items to the ICF categories, the hypothesis that the 2 concepts are distinct was tested. In a study assessing physical function, on community adults, for example, 3 underlying structures were identified, and the authors concluded that combining A and P might not be appropriate³⁴.

Our study addressed the hypothesis that different types of intervention used to treat OA or sciatica might have differential effects on activity limitation and restricted participation. Drugs were found to affect activity limitation more than participation restriction, and exercise and injection have more effect on participation restriction than on activity limitation. Different methods of classification would have obtained different conclusions. For example, based on the ICF categories, A and P would have been treated as 1 component. Based on Badley³⁵, what was defined as acts such as walking and tasks such as dressing would have been considered different, and different conclusions would have been reached. The results from our study, although not conclusive, may help answer interesting questions raised by Johnston and Pollard³⁶ regarding the ability of existing measures to separate individual structures of health, and may provide evidence for the effectiveness of interventions that was not detected by global outcome measures. Such evidence may be of value in advising treatment and rehabilitation programs. Clinical implications, however, are yet to be defined.

Limitations of our study include the fact that we were restricted by the outcome measures used in the original studies. We examined only WOMAC function, SF-36 physical function, and ODQ, and the latter 2 measures were used in just 1 study each. It is important to examine a wider range of outcome measures, especially those that contain many items for each domain including I, and are likely to measure distinctive outcomes, such as the disease-specific Oxford knee and Oxford hip scores. It will also be important to examine studies with longer followup. We examined data from an RCT comparing home exercise with no intervention in men and women aged ≥ 45 years with knee pain³⁷, in which the first assessment took place at 6 months, and no differential effect was found. Another limitation may be that in all the studies examined, there was either no effect or only a very small effect for the intervention. Studies with larger effect would perhaps help to better detect how different interventions might differentially affect health domains.