Answers to Questions Posed by Congress
The questions below provided the impetus for the study. The charge to the panel, prepared by the NRC and the IOM, was to conduct a comprehensive review of the science base and to address the issues outlined in the questions. The panel's responses to the questions follow.
1. What are the conditions affecting humans that are considered to be work-related musculoskeletal disorders?
The disorders of particular interest to the panel, in light of its charge, focus on the low back and upper extremities. With regard to the upper extremities, these include rotator cuff injuries (lateral and medial), epicondylitis, carpal tunnel syndrome, tendinitis, tenosynovitis of the hand and wrist (including DeQuervains' stenosing tenosynovitis, trigger finger, and others) and a variety of nonspecific wrist complaints, syndromes, and regional discomforts lacking clinical specificity. With regard to the low back, there are many disabling syndromes that occur in the absence of defined radiographic abnormalities or commonly occur in the presence of unrelated radiographic abnormalities. Thus, the most common syndrome is nonspecific backache. Other disorders of interest include back pain and sciatica due to displacement and degeneration of lumbar intervertebral discs with radiculopathy, spondylolysis, and spondylolisthesis, and spinal stenosis (ICD 9 categories 353-357, 722-724, and 726-729).
2. What is the status of medical science with respect to the diagnosis and classification of such conditions?
Diagnostic criteria for some of the musculoskeletal disorders considered to be work-related and considered in this report are clear-cut, espe-
cially those that can be supported by objective ancillary diagnostic tests, such as carpal tunnel syndrome. Others, such as work-related low back pain, are in some instances supported by objective change, which must be considered in concert with the history and physical findings. In the case of radicular syndromes associated with lumbar intervertebral disc herniation, for example, clinical and X-ray findings tend to support each other. In other instances, in the absence of objective support for a specific clinical entity, diagnostic certainty varies but may nevertheless be substantial. The clinical picture of low back strain, for example, while varying to some degree, is reasonably characteristic.
Epidemiologic definitions for musculoskeletal disorders, as for infectious and other reportable diseases, are based on simple, unambiguous criteria. While these are suitable for data collection and analysis of disease occurrence and patterns, they are not appropriate for clinical decisions, which must also take into account personal, patient-specific information, which is not routinely available in epidemiologic databases.
3. What is the state of scientific knowledge, characterized by the degree of certainty or lack thereof, with regard to occupational and non-occupational activities causing such conditions?
The panel has considered the contributions of occupational and non-occupational activities to the development of musculoskeletal disorders via independent literature reviews based in observational epidemiology, biomechanics, and basic science. As noted in the chapter on epidemiology, when studies meeting stringent quality criteria are used, there are significant data to show that both low back and upper extremity musculoskeletal disorders can be attributed to workplace exposures. Across the epidemiologic studies, the review has shown both consistency and strength of association. Concerns about whether the associations might be spurious have been considered and reviewed. Biological plausibility for the work-relatedness of these disorders has been demonstrated in biomechanical and basic science studies, and further evidence to build causal inferences has been demonstrated in intervention studies that show reduction in occurrence of musculoskeletal disorders following implementation of interventions. The findings suggest strongly that there is an occupational component to musculoskeletal disorders. Each set of studies has inherent strengths and limitations that affect confidence in the conclusions; as discussed in Chapter 3 (methodology), when the pattern of evidence is considered across the various types of studies, complementary strengths are demonstrated. These findings were considered collectively through integration of the information across the relevant bodies of scientific evidence. Based on this approach, the panel concludes, with a high
degree of confidence, that there is a strong relationship between certain work tasks and the risk of musculoskeletal disorders.
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?
A. Individual Risk Factors
Because 80 percent of the American adult population works, it is difficult to define a “general population” that is different from the working population as a whole. The known risk factors for musculoskeletal disorders include the following:
Age—Advancing age is associated with more spinal complaints, hand pain, and other upper extremity pain, e.g., shoulder pain. Beyond the age of 60, these complaints increase more rapidly in women than men. The explanation for spinal pain is probably the greater frequency of osteoporosis in women than in men. The explanation for hand pain is probably the greater prevalence of osteoarthritis affecting women. However, other specific musculoskeletal syndromes do not show this trend. For example, the mean age for symptomatic presentation of lumbar disc herniation is 42 years; thereafter, there is a fairly rapid decline in symptoms of that disorder.
Gender—As noted above, there are gender differences in some musculoskeletal disorders, most particularly spinal pain due to osteoporosis, which is more commonly found in women than in men, and hand pain due to osteoarthritis, for which there appears to be a genetic determinant with increased incidence in daughters of affected mothers.
Healthy lifestyles—There is a general belief that the physically fit are at lower risk for musculoskeletal disorders; there are few studies, however, that have shown a scientific basis for that assertion. There is evidence that reduced aerobic capacity is associated with some musculoskeletal disorders, specifically low back pain and, possibly, lumbar disc herniations are more common in cigarette smokers. Obesity, defined as the top fifth quintile of weight, is also associated with a greater risk of back pain. There currently is little evidence that reduction of smoking or weight reduction reduces the risk.
Other exposures—Whole-body vibration from motor vehicles has been associated with an increase in risk for low back pain and lumbar disc
herniation. There is also evidence that suboptimal body posture in the seated position can increase back pain. Some evidence suggests that altering vibrational exposure through seating and improved seating designs to optimize body posture (i.e., reduce intradiscal pressure) can be beneficial.
Other diseases—There is a variety of specific diseases found in the population that predispose to certain musculoskeletal disorders. Among the more common are diabetes and hypothyroidism, both associated with carpal tunnel syndrome.
B. Work-Related Risk Factors
Chapter 4 of this report explores the enormous body of peer-reviewed data on epidemiologic studies relevant to this question. Detailed reviews were conducted of those studies judged to be of the highest quality based on the panel's screening criteria (presented in the introduction and in Chapter 4). The vast majority of these studies have been performed on populations of workers in particular industries in which workers exposed to various biomechanical factors were compared with those not exposed for evidence of symptoms, signs, laboratory abnormalities, or clinical diagnoses of musculoskeletal disorders. A small number of studies have been performed in sample groups in the general population, comparing individuals who report various exposures with those who do not.
The principal findings with regard to the roles of work and physical risk factors are:
Lifting, bending, and twisting and whole-body vibration have been consistently associated with excess risk for low back disorders, with relative risks of 1.2 to 9.0 compared with workers in the same industries without these factors.
Awkward static postures and frequent repetitive movements have been less consistently associated with excess risk. For disorders of the upper extremity, vibration, force, and repetition have been most strongly and consistently associated with relative risks ranging from 2.3 to 84.5.
The principal findings with regard to the roles of work and psychosocial risk factors are:
High job demand, low job satisfaction, monotony, low social support, and high perceived stress are important predictors of low back musculoskeletal disorders.
High job demand and low decision latitude are the most consistent of these factors associated with increased risk for musculoskeletal disorders of the upper extremities.
In addition, in well-studied workforces, there is evidence that individual psychological factors may also predispose to risk, including anxiety and depression, psychological distress, and certain coping styles. Relative risks for these factors have been generally less than 2.0.
5. What is the incidence of such conditions in (a) the general population, (b) specific industries, and (c) specific occupational groups?
There are no comprehensive national data sources capturing medically defined musculoskeletal disorders, and data available regarding them are based on individual self-reports in surveys. Explicitly, these reports include work as well as nonwork-related musculoskeletal disorders without distinction; therefore, rates derived from these general population sources cannot be considered in any sense equivalent to rates for background, reference, or unexposed groups, nor conversely, as rates for musculoskeletal disorders associated with any specific work or activity. There are no comprehensive data available on occupationally unexposed groups and, given the proportion of adults now in the active U.S. workforce, any such nonemployed group would be unrepresentative of the general adult population. According the 1997 report from the National Arthritis Data Workgroup (Lawrence, 1998), a working group of the National Institute of Arthritis and Musculoskeletal and Skin Diseases, 37.9 million Americans, or 15 percent of the entire U.S. population, suffered from one or more chronic musculoskeletal disorders in 1990 (these data cover all musculoskeletal disorders). Moreover, given the increase in disease rates and the projected demographic shifts, they estimate a rate of 18.4 percent or 59.4 million by the year 2020. In summary, data from the general population of workers and nonworkers together suggest that the musculoskeletal disorders problem is a major source of short- and long-term disability, with economic losses in the range of 1 percent of gross domestic product. A substantial portion of these are disorders of the low back and upper extremities.
The Bureau of Labor Statistics (BLS) data, while suffering a number of limitations, are sufficient to confirm that the magnitude of work-related musculoskeletal disorders is very large and that rates differ substantially among industries and occupations, consistent with the assumption that work-related risks are important predictors of musculoskeletal disorders. BLS recently estimated 846,000 lost-workday cases of musculoskeletal disorders in private industry. Manufacturing was responsible for 22 percent of sprains/strains, carpal tunnel syndrome, or tendinitis, while the ser-
vice industry accounted for 26 percent. Examining carpal tunnel syndrome alone, manufacturing, transportation, and finance all exceeded the national average, while for the most common but less specific sprains and strains, the transportation sector was highest, with construction, mining, agriculture, and wholesale trade all higher than average. These data suggest that musculoskeletal disorders are a problem in several industrial sectors, that is, the problems are not limited to the traditional heavy labor environments represented by agriculture, mining, and manufacturing.
The National Center for Health Statistics (NCHS) survey data provide added information on self-reported health conditions of the back and the hand. This survey presents estimates for back pain among those whose pain occurred at work (approximately 11.7 million) and for those who specifically reported that their pain was work-related back pain (5.6 million).
The highest-risk occupations among men were construction laborers, carpenters, and industrial truck and tractor equipment operators, and among women the highest-risk occupations were nursing aides/order-lies/attendants, licensed practical nurses, maids, and janitor/cleaners. Other high-risk occupations were hairdressers and automobile mechanics, often employed in small businesses or self-employed.
Among men, the highest-risk industries were lumber and building material retailing, crude petroleum and natural gas extraction, and saw-mills/planing mills/millwork. Among women, the highest-risk industries were nursing and personal care facilities, beauty shops, and motor vehicle equipment manufacturing.
Questions from the NCHS survey on upper-extremity discomfort elicited information about carpal tunnel syndrome, tendinitis and related syndromes, and arthritis. Carpal tunnel syndrome was reported by 1.87 million people; over one-third of these were diagnosed as carpal tunnel syndrome by a health care provider and half were believed to be work-related. Tendinitis was reported by 588,00 people, and 28 percent of these were determined to be work-related by a health care provider. Over 2 million active or recent workers were estimated to have hand/wrist arthritis. The survey did not report these conditions by either occupation or industry.
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?
A. Development and Prevention in Working Populations
Because the majority of the U.S. population works, the data for the population as a whole apply to the 80 percent who are working. There is
substantial evidence that psychosocial factors, in addition to the physical factors cited above (see response to Question 4), are significant contributors to musculoskeletal disorders. Relevant factors are repetitive, boring jobs, a high degree of perceived psychosocial stress, and suboptimal relationships between worker and supervisor.
The weight and pattern of both the scientific evidence and the very practical quality improvement data support the conclusion that primary and secondary prevention interventions to reduce the incidence, severity, and consequences of musculoskeletal injuries in the workplace are effective when properly implemented. The evidence suggests that the most effective strategies involve a combined approach that takes into account the complex interplay between physical stressors and the policies and procedures of industries.
The complexity of musculoskeletal disorders in the workplace requires a variety of strategies that may involve the worker, the workforce, and management. These strategies fall within the categories of engineering controls, administrative controls, and worker-focused modifiers. The literature shows that no single strategy is or will be effective for all types of industry; interventions are best tailored to the individual situation. However, there are some program elements that consistently recur in successful programs:
1. Interventions must mediate physical stressors, largely through the application of ergonomic principles.
2. Employee involvement is essential to successful implementation.
3. Employer commitment, demonstrated by an integrated program and supported by best practices review, is important for success.
Although generic guidelines have been developed and successfully applied in intervention programs, no single specific design, restriction, or practice for universal application is supported by the existing scientific literature. Because of limitations in the scientific literature, a comprehensive and systematic research program is needed to further clarify and distinguish the features that make interventions effective for specific musculoskeletal disorders.
B. Development and Prevention in Specific Occupations
Occupations that involve repetitive lifting, e.g., warehouse work, construction, and pipe fitting, particularly when that activity involves twisting postures, are associated with an increased risk for the complaint of low back pain and, in a few studies, an increased risk for lumbar disc herniation.
The prevalence of osteoarthritic changes in the lumbar spine (disc space narrowing and spinal osteophytes) is significantly greater in those whose occupations require heavy and repetitive lifting compared with age-matched controls whose occupations are more sedentary. Despite these radiographical differences, most of the studies show little or no difference in the prevalence of low back pain or sciatica between those with radiological changes of osteoarthritis and those with no radiological changes. Based on the current evidence, modification of the lifting can reduce symptoms and complaints. Specific successful strategies, which include ergonomic interventions (such as the use of lift tables and other devices and matching the worker's capacity to the lifting tasks), administrative controls (such as job rotation), and team lifting, appear successful. Despite enthusiasm for their use, there is marginal or conflicting evidence about lifting belts and educational programs in reducing low back pain in the population with heavy lifting requirements. Some examples of positive interventions include:
Truck drivers—Vibration exposure is thought to be the dominant cause for the increased risk for low back pain and lumbar disc herniation. There are some data to support the efficacy of vibrational dampening seating devices.
Hand-held tool operators—Occupations that involve the use of hand-held tools, particularly those with vibration, are associated with the general complaints of hand pain, a greater risk of carpal tunnel syndrome, and some tenosynovitis. Redesign of tools is associated with reduced risks.
Food processing—Food processing, e.g., meat cutting, is associated with a greater risk of shoulder and elbow complaints. Job redesign appears to reduce this risk, but this information is largely based on best practices and case reports.
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?
The panel's recommended research agenda is provided in Chapter 12 of the report.