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Work-Related Musculoskeletal Disorders: A Review of the Evidence 3 Seven Questions Posed by Congressman Robert Livingston The material in Section 2 puts in perspective the state of the evidence with respect to the range of factors that could be contributing to musculoskeletal disorders and describes the extent to which there is a scientific basis for concluding that such disorders originate in the workplace and can be reduced through programmatic interventions. This section presents the steering committee's response to the seven questions posed by Representative Livingston. As our analysis has been at the level of the family of musculoskeletal disorders, our response will be provided across disorders. Question 1: What are the conditions affecting humans that are considered to be work-related musculoskeletal disorders? The musculoskeletal conditions that may be caused by (non-accidental) physical work activities include disorders of inflammation, degeneration, and physiological disruption of muscles, tendons, ligaments, nerves, synovia, and cartilage involving limbs and trunk. These entities are included in categories 353-355, 722-724, and 726-729 of the International Classification of Diseases (commonly referred to as ICD-9) (World Health Organization, 1977). Not every disorder in these categories may be caused by mechanical stressors, but all the major musculoskeletal disorders of interest are included in these groupings. Common examples are low back strain, tenosynovitis, and carpal tunnel syndrome. Question 2: What is the status of medical science with respect to the diagnosis and classification of such disorders? There is great variation in the diagnostic criteria for the many musculoskeletal disorders, ranging from clinical diagnoses based on symptoms and signs for some, to diagnoses based on structural and functional criteria for others. The diagnostic criteria used in epidemiological studies are often different from those used to make treatment decisions. This difference can lead to classification of cases for research that would be unacceptable when invasive treatment alternatives are considered, but it has little influence on the conclusion of appropriately designed epidemiological surveys. In the classification of back pain, a symptom classification is often used because the precise etiology of the painful process often cannot be identified. For specific disease entities, such as herniated disks, there are accepted diagnostic criteria that are based on clinical symptoms and signs, as well as imaging information. Similarly, there are disease entities for upper extremities for which there are accepted diagnostic criteria; for other disease entities, they are classified broadly on the basis of clinical symptoms and signs.
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Work-Related Musculoskeletal Disorders: A Review of the Evidence Question 3: What is the state of scientific knowledge, characterized by the degree of certainty or lack thereof, with regard to occupational and nonoccupational activities causing such conditions? The relationships among work factors, biomechanical loads, and responses are supported by mathematical models and direct measurements. The mathematical models are widely accepted and applied to design mechanical structures in aircraft and automotive design. Direct measurements have been used to a lesser extent than modeling because they are potentially injurious to human subjects; however, when they have been used they generally support the biomechanical models. It has been shown that the load forces encountered over time in normal work activities often approach the physiological and mechanical tissue limits. Limits may be exceeded as a result of a single high force or as a result of repeated loads over time. Some tissues have a greater ability to adapt to repeated loads if there is sufficient recovery time between successive loads, while other tissues, e.g., nerves, are less able to adapt. Biomechanical loads are encountered in activities of work, daily living, and recreation. The contribution of these activities to tissue response is related to their relative duration and intensity. For most people, their main exposure is at work. There is a substantial body of epidemiological literature that shows a disproportionately high incidence of musculoskeletal disorders of all types among persons exposed to high biomechanical loads. Although there can be debate about acceptable exposure limits, there can be little disagreement about the fundamental relationship between extreme work exposures and musculoskeletal morbidity. Question 4: What is the relative contribution of any causal factors identified in the literature to the development of such conditions in (a) the general population; (b) specific industries; and (c) specific occupational groups? Data at the population and industry level have been collected for a variety of purposes by different groups using different methods. Some data are based on survey results, some on clinical or medical diagnoses, and some on compensation claims. In the judgment of the steering committee, it is not possible to make useful comparisons on the basis of these data. The incidence of musculoskeletal disorders in any specific occupational group can be expected to reflect the tissue loads imposed by the work, the tissue tolerances of the mix of individuals doing it, and the other activities in their lives imposing related loads. The relative contribution of the different factors in any occupational group depends on (1) the strength of these relationships across the ranges of individuals and activities in that group and (2) the variability of the individuals and activities. For example, gender will not be an important predictive factor if men and women do not, on average, have different tissue tolerances for the loads imposed in that occupational group or if workers in the group are overwhelmingly men or women. The relative contribution in a specific industry will, similarly, depend on the mix of individuals and tasks in it. Unfortunately, measurements of the relevant features of individuals and tasks are typically unavailable, limiting our ability to assess the relative contribution of different factors across groups. The evidence shows that the incidence of musculoskeletal
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Work-Related Musculoskeletal Disorders: A Review of the Evidence disorders is higher among individuals who perform activities that exceed tissue tolerances. Generally speaking, the more that they perform those activities, the greater are their risks for such disorders. Question 5: What is the incidence of such conditions in (a) the general population; (b) specific industries; and (c) specific occupational groups? Current knowledge about the incidence of each of the conditions described in Question 1 in the general adult population is limited because (1) the conditions are clinically diagnosed, typically in doctors' offices; (2) diagnostic criteria for these conditions are not uniformly applied; (3) there are no data collection systems to capture such diagnoses in the health care system; and (4) two-thirds of the adults in the general population are employed, with variable occupational risks. Those who are not employed, including those with various chronic conditions and disabilities, are not a suitable reference population. Data on industry and occupational groups are based on a wide variety of methods that have been collected for different purposes. It is the steering committee's judgment that it is not possible to make useful comparisons on the basis of these data. Question 6: Does the literature reveal any specific guidance to prevent the development of such conditions in (a) the general population; (b) specific industries; and (c) specific occupational groups? Specific interventions can affect the reported rate of musculoskeletal disorders in specific industries and for specific occupations. Interventions can also reduce reports of musculoskeletal disorders, the presence of risk factors, and the reporting of comfort and pain associated with work. It is also clear that the effectiveness of interventions can be improved if they are tailored to specific occupations and work settings. There is a dearth of data on interventions in the general population. Question 7: What scientific questions remain unanswered, and may require further research, to determine which occupational activities in which specific industries cause or contribute to work-related musculoskeletal disorders? Looking at this web of evidence, we have reached three major conclusions: Musculoskeletal disorders are a serious national problem: estimates of costs range from $13 to $20 billion annually. These problems are caused by work and non-work activities. There are interventions that can reduce the problems. We have also identified some focused research projects, the results of which could increase the efficacy of interventions. Some of these projects would produce useful results
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Work-Related Musculoskeletal Disorders: A Review of the Evidence within specific research areas; others would increase the connections among the areas. It is a strength of the science that it points to these specific opportunities (see the discussion of future research above).
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