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Invited Commentary

Sargent College of Health & Rehabilitation Sciences
Boston University
635 Commonwealth Ave
Boston, MA 02215
(ajette{at}bu.edu)

	Introduction

 
Dr Hermann and Dr Reese are to be commended for their timely investigation of the widely accepted clinical assumption that variation in cervical impairment is associated with variation in function and disability among patients with cervical spine injuries. I agree with the authors that it is critical that our profession scrutinize the clinical hypotheses and assumptions that guide physical therapists in their diagnosis and treatment planning for patients who seek our services. It is through studies such as theirs that the field of physical therapy will continue to advance toward a more solid foundation of evidence-based practice. The disablement model proposed by Nagi¹ provides a useful framework with which to examine the hypothesized relationships in this investigation, although, as the authors note, other models such as the World Health Organization's International Classification of Impairments, Disabilities, and Handicaps-2 (ICIDH-2) formulation² are available to guide such investigations.

I want to focus my comments on 2 methodological aspects of Hermann and Reese's investigation: a statistical assumption and a measurement concern.

A fundamental analytical assumption in Hermann and Reese's article should be acknowledged explicitly because it affects the interpretation of their data. By using canonical correlational analyses, Hermann and Reese, like many clinical investigators, test the strength of linear associations among specific disablement concepts: impairments, functional limitations, and disability. Although this is a reasonable initial analytical step, analysts need to think beyond linear associations to the examination of more complex relationships among disablement concepts. There is growing evidence that many of the hypothesized relationships among disablement concepts can best be described as curvilinear versus simple linear relationships.³ Several investigators,^4–6 for example, have recently demonstrated that in older adults the relationship between skeletal muscle force and subsequent function and disability can best be characterized as curvilinear, with identifiable clinical thresholds.

The identification of clinical thresholds in the association between specific impairments and function and disability could have important implications for physical therapy practice. Take the example of the relationship between skeletal muscle force and basic physical functions such as walking. If a lower-extremity muscle force threshold could be identified, it would allow for the identification of a minimum level of force needed for improved walking in different patient groups. It could also prove useful in the identification of specific subgroups of patients likely to benefit the most from clinical interventions. Such information could also assist clinicians in identifying evidence on the minimum level of muscle force above which an increase in physiological capacity does not translate into clinically important improvement in a function such as walking. Such data would call into question the common clinical (linear) assumption that improvement in force will logically lead to improvement in function. Such data would guide clinicians into incorporating use of clinical thresholds into their practice. Data such as these will emerge if researchers move beyond the examination of linear models that describe interrelationships among disablement concepts and develop more complex models that describe the precise form of these hypothesized relationships based on our clinical expertise and/or theoretical knowledge of the concepts under investigation.

Some of the measures used by Hermann and Reese illustrate one of the common measurement challenges encountered in conducting conceptually grounded clinical research such as theirs. Simply stated, many clinical measures available to researchers today are not grounded in clearly articulated, distinct clinical concepts such as those included in the disablement model. Without valid measures of distinct clinical concepts such as impairments, functional limitations, and disability, investigators run the risk of using measures that cut across several different clinical concepts, thus confusing the interpretation of their findings.

I believe several of the measures fielded in Hermann and Reese's study displayed confusing conceptual roots and that their questionable validity compromised the authors' ability to interpret their results. Let me illustrate this point with the Neck Disability Index (NDI) used by Hermann and Reese as one of their measures of disability in this study (a similar analysis could be performed on the Medical Outcomes Study 36-Item Short-Form Health Survey [SF-36], which also was used as a measure of disability in this study). No data are presented by the authors to support the validity of the NDI as a distinct measure of disability. If a measure has convincing face validity, this may not be a major flaw. If one examines the face validity of the NDI, however, it appears problematic for use in this study because it appears to encompass concepts other than disability.

Let me illustrate the concern directly. Five of the NDI items—personal care, work, driving, sleeping, and recreation—appear to represent examples of the concept of disability, defined by the authors as performance of socially defined roles accomplished within a cultural and physical environment. The conceptual roots for 5 other NDI items, however, are not so easy to classify and appear to represent concepts other than disability. Items such as lifting, reading, and concentration, for example, appear to better fit the concept of functional limitations, defined in the article as limitation in performance at the level of the whole person. Neck Disability Index items such as pain intensity and headaches may reflect the underlying concept of impairment, defined by the authors as abnormalities of structure or function at the level of organs and/or body systems. Thus, in this study, the use of the NDI as a distinct measure of disability separate from the concepts of functional limitations and impairment is likely flawed. The summary scores would appear to reflect variation in all 3 disablement concepts and not solely disability. This conceptual confusion may explain why the authors found a counterintuitive result. Canonical correlations between impairments and disability (.73) were stronger than observed correlations between the variable sets of functional limitations and disability (.54). The disablement model would clearly suggest that functional limitations should be more strongly related to disability than to impairments.^1,7

One interpretation for this finding can be found in the authors' subanalyses. They report that the greatest contributing variable in the correlation between impairments and disability was found in the correlation between pain intensity and NDI scores. This comes as little surprise when one understands that pain intensity is a component of the NDI. At the very least, the choice of the NDI (or any other instrument) as a measure of disability requires data validating its conceptual foundation. Better yet, researchers need to develop and subsequently use clinical measures designed with a clear underlying conceptual foundation. Such measures are still lacking in disability research; thus, it is understandable why Hermann and Reese made the selections they did in this investigation.

	References

Top Introduction References

 

1. Nagi SZ. Some conceptual issues is disability and rehabilitation. In: Sussman MB, eds. Sociology and Rehabilitation. Washington, DC: American Sociological Association,1965 :100–113.
2. International Classification of Impairments, Disabilities, and Handicaps-2: International Classification of Functioning and Disability. Geneva, Switzerland: World Health Organization,1999 .
3. Buchner D, deLateur B. The importance of skeletal muscle strength to physical function in older adults. Ann Behav Med.1991; 13:95–98.
4. Ferrucci L, Guralnik JM, Buchner D, et al. Departures from linearity in the relationship between measures of muscular strength and physical performance of the lower extremities: the Women's Health and Aging Study. J Gerontol A Biol Sci Med Sci.1997; 52:M275–M285.[Abstract]
5. Jette AM, Jette DU. Functional and behavioral consequences of sarcopenia. Muscle Nerve Suppl.1997; 5:S39–S41.[Medline]
6. Jette AM, Assmann SF, Rooks D, et al. Interrelationships among disablement concepts. J Gerontol A Biol Sci Med Sci.1998; 53:M395–M404.[Abstract]
7. Verbrugge LM, Jette AM. The disablement process. Soc Sci Med.1994; 38:1–14.[Web of Science][Medline]

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