Health and disability are multidimensional concepts that can be challenging to describe and even more challenging to communicate to others, as they include many inter-related components. For this reason, conceptual frameworks, or models have been developed to more easily explain, measure, and interpret those factors associated with disability. The development and use of these "disablement models" allows rehabilitation professionals, including the orthotist-prosthetist, to better communicate and participate in clinical care using a common health language.
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World Health Organization
Given the diversity of health care professionals, it is no surprise that a number of very different models of health have been developed and that each represents a fairly unique perspective on health, disability, and disease. This module aims to review the several health models and their recent evolution into the current framework advocated by the World Health Organization (WHO), the International Classification of Function and Disability (ICF). We will also discuss why an understanding of this model is useful to orthotic and prosthetic (O&P) clinician and how it can be used to select appropriate outcome measures.
Just as health care practice has evolved over time, so too have general perceptions of human health and disease. Historically, a disability was considered to be the result of a disease, trauma or some other health condition. Individuals with disabilities were therefore often perceived as anomalies, or deviations from a "normal," healthy state. As such, individuals with a disability were commonly described by the pathological condition they bore (e.g., 'an amputee') rather than a person with a medical condition (e.g., 'an individual with an amputation'). This focus on the medical condition does not include elements related to social factors or the individual's perception of their health.
Figure 2A—The linear, unidirectional biomedical model
This perspective forms the basis of the biomedical model, which describes health as an absence of disease. Using this model, treatments and interventions are a means to move a disabled individual from a diseased or unhealthy state toward a normal or healthy state. In this way, the biomedical model is considered to be a linear, unidirectional model (Figure 2A).
Although the biomedical model is still commonly considered to be the dominant health model, proponents of other health models argue that this framework is limited by its view of disability through changes in pathology, physiology, and biochemistry and that it demonstrates a number of limitations, specifically that it ignores the role of the individual and their society in the description of "disability."
The limitations of the biomedical model are particularly relevant to the selection of an outcome measure. Selecting an instrument that measures only the health condition and not the impact of that condition on the individual's ability (or inability) to perform activities or their desire to participate (or not to participate) in life situations may limit the potential of an outcome measure to detect meaningful change in areas of importance to the individual or the caregiver. The influence of external factors can be quite important to the development of a treatment plan, and therefore, other perspectives on health can be quite informative.
In contrast to the biomedical model, the social model of disability perceives disability as a socially created problem, not a characteristic of the individual. This perspective purports that the limitations caused by the disease or unhealthy state are due to a rigid physical or social environment that creates disability where one may not otherwise exist. The social model describes that these variations from the "normal state" are in fact, normal and that any disability is the result of societal perceptions rather than barriers to participation in life.
The social model approaches disability problems, and therefore solutions, in a very different fashion than the biomedical model. For example, an individual with limited hand and arm strength may have problems opening public doors. The biomedical model would propose a solution that would address physical strength issues in the upper limb, perhaps through the provision of physical therapy. The social model, conversely, would suggest a solution that would address the environmental limitation that imposed this disability and may advocate for automatic doors to replace the manual ones.
The social model advocates equality among individuals, and proposes that everyone can fully and completely participate in life if society's attitudes, information, and physical structures are appropriate for both "normal" and "differently abled" individuals.
The psychological model is not commonly described as a separate model and is instead often incorporated into descriptions of either the biomedical model or the social model. Like the biomedical model, the psychological model describes disability as inherent to the individual. Like social model, the psychological model elevates the role of perception in the description of disability. In the psychological model, the individual, rather the health care provider or society, is enabled to address the restrictions of disability through their perception, beliefs, attitude, and behavior toward the disability.
The biomedical, social, and psychological models converged under the development of psychiatrist George Engel. In 1977, Engel published a description of the basic biophysical model in the esteemed journal, Science. Engel proposed this framework as an alternative to the predominant biomedical model of health and advocated for its use in research, teaching, and the provision of health care. The biopsychosocial model was unique in that it presents a holistic approach to health and therefore health care. This model, like the rehabilitation sciences and services themselves, examines health from a variety of points of view (including the biomedical, the social, and the psychological), and correspondingly interprets "health" as the interaction of these three perspectives.
The biopsychosocial model also advocates that while the three domains of health (i.e., the biomedical, the social, and the psychological) are all interrelated, they are also independent. Thus, if an individual is healthy (by medical standards) and perceives him or herself to be ill, then that individual may be considered unhealthy. Conversely, someone who has a medical condition (i.e., a disease) and perceives them self to be well, may not be considered healthy under the biopsychosocial model. This model of health as a synthesis of body, social, and perceptive factors is at the core of most contemporary models of health and disease.
Two contemporary models of health and disability are commonly used in rehabilitation, the Disablement Model and the WHO models (i.e., the ICIDH and ICF).
The Disablement Model
The Disablement Model was created in the early 1960's by sociologist Saad Nagi as part of efforts to study disability for the United States Social Security Administration (SSA). This model describes four basic phenomena that Nagi considered fundamental to rehabilitation, active pathology, impairment, functional limitations, and disability (Figure 2B).
Figure 2B—The Disablement Model, as proposed by Saad Nagi.
The four elements of Nagi's Disablement Model:
Active pathology describes an interruption in normal body processes that leads to a deviation from the normal state. These include infection, trauma, disease processes, or other degenerative conditions.
Impairment is the complete loss, damage to, or interruption in normal body structures or systems. Interestingly, this model describes how active pathologies commonly result in impairments, but the reverse is not always true. For example, impairment, such as a congenital limb absence, is not the result of active pathology.
Functional Limitations are described as individual performance restrictions. Here, a muscular contraction could cause a functional limitation such as limited range-of-motion in the upper limb.
Disability is described as an express physical and/or mental limitation is the context of a society.
Like the basic biopsychosocial model, the Disablement Model views disability as an interaction between the individual and society. Furthermore, like the biopsychosocial model, Nagi's model suggests that individual and social accommodations to an underlying pathology may mitigate disability for one individual, while emphasizing it in another individual.
In the mid 1990's, several revisions were suggested to the Nagi's original framework. In 1994, Vergbrugge and Jette expanded the Disablement Model to include:
Sociocultural factors: those that pertain to the physical and social environment
Personal factors: those that pertain to the individual (i.e. behaviors, lifestyle attitudes)
This expansion of the Disablement Model was an attempt to develop a complete biopsychosocial definition of disablement, which they define as "the impact that chronic and acute conditions have on functioning of specific body systems and on people's ability to act in necessary, usual, expected and personally desired ways in society." They also proposed that disablement is fluid, that it is capable of change in type, severity, and consequences. Vergbrugge and Jette also tried to separate the "main pathway" (i.e., Nagi's original model of active pathology to disability) from the model variables, which they labeled risk factors, intra-individual factors, and extra-individual factors. It was these variables, they felt, which mitigated or emphasized the intrinsic relationships between the factors in the main pathway (i.e., active pathology, impairment, functional limitations, and disability).
A parallel expansion of the Disablement Model was reported in an Institute of Medicine (IOM) report, Disability in America. In this report, Pope and Tarlov extended Nagi's original model (which they term "the disabling process") by adding two important concepts, risk factors and quality-of-life. Risk factors included biological, environmental (social and physical), and lifestyle/behavior factors capable of interacting with the disabling process. Similarly, quality-of-life was seen to both affect and be affected by each stage of the process (Figure 2C).
Figure 2C—The IOM model of the "Disabling Process" (1991)
The development of the Nagi Model was furthered again in 1997 through a second IOM report, Enabling America. In this report, the IOM made several important changes to the Disablement Model. First, disability was removed from the model, and was instead viewed as an outcome of the individual interacting within the environment. Secondly, in this revision, disability was defined through an "enablement-disablement process." This definition is important because it identifies disability as changeable and reversible, and is therefore defined as bidirectional, as indicated by the bidirectional arrows in the model (Figure 2D). Lastly, the risk factors were renamed to transitional factors, as they were responsible for the transitions between the categories of the enabling-disabling process.
Figure 2D—The revised IOM model of the "Enabling-Disabling Process" (1997)
The modified IOM model also emphasizes the importance of the individual's interaction with the environment, which was changed to include psychological factors in addition to the physical and social factors.
The World Health Organization (WHO) Model(s)
The second contemporary framework to build upon the biopsychosocial model proposed by Engel was started by the World Health Organization (WHO) in the late 1970's. Although developed independently of Nagi's model, this framework addressed many of the same issues as the Disablement Model and its subsequent revisions. The first WHO model, the International Classification of Impairments, Disabilities, and Handicaps (ICIDH) was published in 1980 and is part of the same family of classification systems as the ICD-10 (the International Statistical Classification of Diseases and Related Health Problems). The ICIDH identified three fundamental concepts—impairments, disabilities and handicaps that were suggested to describe disability. This model, like Nagi's original Disablement Model, was a linear model. It showed a directional progression from disease to handicap (Figure 2E).
Figure 2E—ICIDH Model of Disability (1980)
The elements of the original ICIDH Model of Disability:
impairment was defined as an abnormality of a body organ, structure or function (including mental functions)
disability was defined as a restriction in activity that resulted from the impairment
handicap was the disadvantage (in the social context) brought on by the impairment or the disability
The ICIDH also included a taxonomy of impairments, disabilities, and handicaps. Like the historical biomedical models, the original ICIDH was predominantly focused on the disease and related conditions and drew criticism for not adequately describing the role of the individual and the environment in disability.
In 2001, the WHO released a major revision to the ICIDH framework named the International Classification of Function and Disability (ICF). The ICF framework describes changes to health as the dynamic interaction between the health condition and contextual factors (Figure 2F). It is important to note that this is the first time that "health" is a model element in lieu of disability or disablement, suggesting a radical philosophical change in this model when compared to earlier models.
Figure 2F—the ICF model (2001)
In the ICF model, as in the underlying biophsychosocial model, health is described according to three levels which include the body, the individual, and society. Within each level, the ICF model identifies three domains of functioning (i.e., body structures and function) and associated levels of disability (i.e., impairments, limitations, and restrictions).
Definitions for these domains and their associated disabilities reflect the multidimensional nature of this model (Table 2A).
Table 2A—Definitions of ICF domains of functioning and associated disability
Like differences in body structure and function, differences between activity and participation may be subtle. Activity often refers to an individual's ability to perform a task (irrespective of whether or not they would normally do the selected activity), while participation may reflect whether or not that individual chooses to do the task and how they accomplish it. For example, to measure activity, one might ask the questions "how much difficulty do you have in putting on a shirt?" Likewise, to measure participation, the question might instead be "to what extent do you feel limited in getting dressed for work?" The first question is directed at ability while the second targets independence. Figure 7 illustrates offers an example of the distinctions between activity and participation.
Figure 2G—Differentiation of Activity and Participation (adapted from Coster and Khetani)
The WHO provides a hierarchical list of codes for describing specific body functions, body structures, activities and participation via the ICF Browser (Figure 2H). Use of these codes in clinical care provides for a means to classify an individual's health status and assist in diagnoses and/or treatment plans based upon the patient's presentation. Similarly, use of these codes for research purposes allows for a standardization of subject descriptions across individuals, populations, and studies.
Figure 2H—ICF Browser
The ICF framework also allows for the use of two qualifiers, descriptors that record and rate the severity of the problem in the domain of function, to further classify an individual's health status.
The performance qualifier (or "first qualifier") describes the extent of the participation restriction, based upon what the individual does in their current environment. Since the "environment" includes personal and social elements, performance would then be described as how the individual performs the task in their customary manner in the typical environment that it is performed.
Performance qualifiers are rated on a 5-point scale as follows:
The capacity qualifier (or "second qualifier") describes the extent of the activity limitation through the individual's ability to execute the task or action in a "uniform" environment. Thus, the capacity qualifier represents the true ability of the individual to perform the task without assistance or accommodation. The capacity qualifier should be assessed based upon what the individual would be expected to do before the health condition was acquired. Capacity qualifiers are likewise described on the 5-point scale noted above.
Identifying the difference between capacity and performance may provide insight about barriers/facilitators of activity and participation. For example, if performance exceeds the individual's capacity, then some aspect of the environment (such as the dressing stick in the above example) has mitigated the disability with respect to the desired activity. Conversely, if the capacity surpasses the performance, then the environment (or some part of it) has restricted the individual's ability to perform the activity.
For more information on the ICF descriptors and use of the framework in classifying health conditions, readers are referred to the ICF publication, Toward a Common Language for Functioning, Disability, and Health.
Like the foundational biopsychosocial model and the related Disablement Model, the ICF also recognizes the individual and societal components of health. As shown in the model (Figure 5), the ICF framework also includes two categories of contextual factors: environmental factors and personal factors. Environmental factors include elements such as societal attitudes, architectural or physical characteristics, legal systems, climate, terrain, and other characteristics of the psychosocial environment. Personal factors include elements such as age, gender, socioeconomic background, coping styles, vocation, education, experience, behaviors, personal character, and other individual factors. Both contextual and environmental factors impact and/or influence how disability is perceived by the individual.
Given the ability of the ICF framework to evaluate health from these multiple perspectives, the model offers great potential as a multipurpose tool that may be used in policy, planning, clinical care, and research.
Figure 2I—ICF domains measured by various outcome measures (Metcalf, 2007)
One way the ICF framework can be used in clinical practice is in the evaluation of patient outcomes with respect to the domains of functioning addressed by the ICF model. By taking the time to consider outcomes in this context, the clinician gains a comprehensive understanding of the underlying health condition and its associated interaction(s) with the physiology, activities, and environment of the individual. With this knowledge in hand, the clinician may then better select the ideal outcome assessment tools by properly matching the domains of interest with those evaluated by the outcome measure. When selecting an outcome tool, it is not uncommon to find that outcome measures, particularly those with multiple dimensions, or areas of assessment, may address or span multiple ICF domains (Figure 2I).
The great advantage to this systematic approach of outcome assessment is that it promotes a comprehensive view of the patient, and therefore, any treatment or intervention that is considered.
The need for systematic and comprehensive approaches to outcome measurement in acute care and rehabilitation is currently the subject of much discussion and work around the world. There is increasing recognition that rehabilitation practice is most comprehensive when it is grounded in the framework of the ICF, meaning that both interventions and outcomes need to take the ICF into account. In addition to consideration of outcomes related to the ICF components of body structure and functions (as measured by impairments), activity (as measured by limitations) and participation (as measured by restrictions), this also means consideration of contextual factors, such as the characteristics of the client, their family and their environment that contribute to their overall quality of life. The ICF also embraces the notion that interventions can be directed toward making changes in the people, systems and environments that are connected with the individual. With the ICF framework and its associated domains of function considered, a clinician is able to perform a comprehensive assessment of patient health. This, in turn, will enable the clinician to best select those measures suited to evaluating the individual outcomes of interest.
The test questions on the next page will evaluate your comprehension of Module 2. The learning objectives for this module will demonstrate your ability to: