CTS – carpal tunnel syndrome
MSDs – musculoskeletal disorders
It was not until the 1970s that occupational factors were examined using epidemiologic methods, and the issue of work-relatedness of these conditions began appearing regularly in the international scientific literature. Since then, the literature has increased dramatically with more than 6,000 published scientific articles addressing musculoskeletal disorders (MSDs) and ergonomics in the workplace (NIOSH 1997). Despite the abundant literature, the causal relationships between MSDs and work-related factors remain the subject of considerable debate. Researchers in Australia, Asia, Scandinavia, Western and Central European countries, South America, and North America have been actively studying these relationships in an effort to prevent and reduce MSDs in working populations (Hagberg et al. 1992, Kákosy 1994, Kilbom et al. 1986, Leclerc et al. 1998, Lusa-Moser et al. 1997, Maeda et al. 1982, Muruka 1997, Ohlsson et al. 1995, Pórszász et al. 1997, Silverstein et al. 1987, Yu and Wong 1996). Musculoskeletal disorders are thought to be a major cause of lost time from work and worker disability. Worker compensation claims and health care costs for MSDs has risen rapidly over the last decade in most industrialized countries (Stock 1991).
The World Health Organization has characterized “work-related” diseases as multi-factorial to indicate that a number of risk factors (e.g., physical, work organizational, psychosocial, individual, and sociocultural) contribute to causing these diseases (WHO 1985). There is disagreement, however, on the relative importance of occupational and individual factors in the development of work-related illnesses. The same controversy has been an issue with other medical conditions (occupational and non-occupational) such as certain cancers and lung disorders, both of which have multiple causality. The purpose of this paper is to present an overview of the epidemiologic evidence on the relationship between upper extremity MSDs and workplace factors, and to outline a model for the prevention of these disorders.
In the United States, the only routinely published national source of
information about occupational injuries and illnesses is the Annual Survey
of Occupational Injuries and Illnesses conducted by the Bureau of Labor
Statistics (BLS) of the United States Department of Labor. This survey
is a random sample of about 250,000 private-sector enterprises, but it
excludes self-employed workers, farms with fewer than 11 employees, private
households, and all government agencies. The Annual Survey of Occupational
Injuries and Illnesses data provides estimates of workplace injuries and
illnesses from information that employers provide to the Department of
Labor from their log of recordable injuries and illnesses. In recent years,
the majority (approximately 65%) of the illnesses have been due to repetitive
trauma of the upper extremities (BLS, 1997). The number of repetitive trauma
cases has increased dramatically, rising steadily from 23,800 in 1972 to
332,000 in 1994–a 14-fold increase (Figure 1).
The number of cases decreased by 7% and then again by another 9% in 1995
and 1996, respectively. In the United States, all back disorders are categorized
separately into a single, broad “injury” category rather than as an illness
and are not included in these numbers.
Figure 1. Disorders Associated with Repeated Trauma in the United States from 1980 to 1996.
Not specific to any one type of job, MSDs tend to affect workers in a wide variety of occupations ranging from construction work, assembly line tasks and meat processing jobs, to computer use by newspaper editors. Occupational MSDs usually take months or even years to develop. Thus, it can be difficult to associate specific exposure to defined outcomes. The most common health outcome in the quantification of MSDs has been the occurrence of pain, which is assumed to be the precursor of more severe disease (Riihimaki 1995). Different epidemiologic measures and time scales have been used to quantify MSDs and often include measures of lifetime prevalence, period prevalence, point prevalence, and incidence ratios. Cross-sectional studies usually employ case definitions that take into account prevalent cases at different stages of the disease process. Because of the multi-factorial nature of MSDs, it has been necessary to look at a broad spectrum of outcome measures to assess the effects of these factors.
Even within a geographical region, the definition of a musculoskeletal
disorder (or similar term) often varies depending on the study. Thus, it
is not surprising that controversy has arisen regarding the relative importance
of various risk factors in the etiology of these disorders. Some investigators
restrict themselves to case definitions based on clinical pathology, some
to the presence of symptoms, some to “objectively” demonstrable pathological
processes (e.g., nerve conduction abnormalities), and some to work disability
(e.g., lost work-time status). Although occupational MSDs range from eyestrain
to low back pain, common upper extremity conditions include: carpal tunnel
syndrome, wrist tendinitis, epicondylitis at the elbow, shoulder bursitis
and tendinitis, and myositis in the neck region.
There are several studies that have investigated the specific relationship between tasks involving repetition and force and CTS (Chiang et al. 1993, Moore and Garg 1994, Roquelaure et al. 1997, Silverstein et al. 1987). Silverstein et al. (1987) studied 652 workers in 39 jobs from 7 different plants (electronics, appliance, apparel, and bearing manufacturing; metal casting, and an iron foundry). Investigators classified jobs into 4 groups that accounted for force and repetitiveness: low force/low repetitiveness, high force/low repetitiveness, low force/high repetitiveness, and high force/high repetitiveness. Fourteen cases of CTS were diagnosed based on standardized physical examinations and structured interviews.). There was a statistically significant association between CTS and highly repetitive jobs compared to low repetitive jobs, irrespective of force. Force alone was not statistically associated with CTS. Using multiple logistic analysis a statistically significant odds ratio of 15.5 was computed for jobs with combined exposures to high force and high repetition compared to jobs with low force and low repetition. Age, gender, plant, years on the job, hormonal status, prior health history, and recreational activities were analyzed and determined not to confound the associations.
Chiang et al. (1993) studied 207 workers from 8 fish processing factories in Taiwan. Jobs were divided into 3 groups based on levels of repetitiveness and force. The comparison group (low force/low repetitiveness) was comprised of managers, office staff, and skilled craftsmen (group 1). Fish-processing workers were divided into high repetitiveness or high force (group 2), and high force and high repetitiveness (group 3). CTS was defined on the basis of symptoms and positive physical examination findings, ruling out systemic diseases and injury. CTS prevalence for the overall study group was 14.5%. CTS prevalence increased from group 1, to group 2, and to group 3 (8.2%, 15.3%, and 28.6%, respectively), a statistically significant trend (p<0.01). Although force significantly predicted CTS, repetitiveness did not. Two significant predictors of CTS in female workers were oral contraceptive use and high force work.
In a recent case-control study of French assembly workers, Roquelaure et al. (1997) evaluated non-occupational and occupational factors associated with CTS. Of the 65 cases identified, 55 had been treated surgically. Referents were randomly selected from the same population. The medical history and household activities of the workers and the ergonomic and organizational characteristics of the job were analyzed. Worksite analysis was performed by direct observation, the use of checklists, and by measuring the weight of tools and parts handled. Exertion of force was statistically associated with CTS (odds ratio 9.0). The only personal factor statistically associated with CTS was a parity of at least 3 (odds ratio 3.2). The authors concluded that the number of risk factors accumulated by the workers was a major determinant of CTS.
Moore and Garg (1994) evaluated 32 jobs in an American pork processing plant and then reviewed past plant medical records for CTS cases in these job categories. Exposure assessment included videotape analysis of job tasks for repetitiveness and awkward postures. The force measure was an estimate of the percent maximum voluntary contraction, based on weight of tools and parts and population strength data, adjusted for extreme posture or speed. Jobs were then predicted to be either hazardous or safe, based on exposure data and judgment. The proportion of CTS in the overall study group during the 20 months of case ascertainment was 17.5 per 100 full time equivalents. The hazardous jobs had a significantly higher relative risk for CTS compared to the safe jobs. The authors concluded that the study provided additional epidemiological evidence that upper extremity musculotendinous disorders and some cases of CTS may be causally associated with working tasks.
The relationship between vibration and CTS has been examined in several studies (Bovenzi 1994, Chatterjee 1992, Silverstein et al. 1987). In a study of Italian stone workers Bovenzi (1994) compared 145 quarry drillers and 425 stone carvers exposed to hand vibration to 258 polishers and machine operators who performed manual activity without exposure to hand-transmitted vibration. CTS was assessed by a physician, and exposure was assessed through direct observation of vibrating tools and by interview. Vibration was also measured in a sample of tools. The vibration-exposed stone cutters had a statistically significant odds ratio of 3.4 for CTS relative to the non-exposed group controlling for several confounders. Other studies have associated peripheral neuropathies in the throacic outlet and cubital tunnel with workers exposed to hand and arm vibration (Kákosy 1994).
Luopajarvi and associates (1979) compared the prevalence of hand/wrist tendinitis among 152 female Finnish assembly line packers in a food production factory to 133 female shop assistants in a department store. Exposure to repetitive work, awkward hand/arm postures, and static work were assessed by observation and videotape analysis of factory workers. The health assessment consisted of interviews and physical examinations conducted by a physiotherapist (active and passive motions, grip-strength testing, observation, and palpation). Diagnoses of tenosynovitis and peritendinitis were later determined by medical specialists using these findings and predetermined criteria. The authors determined that the prevalence rate for tendinitis among the assembly line packers was significantly higher than shop assistants.
Kuorinka and Koskinen (1979) studied occupational rheumatic diseases and upper limb strain among 93 Finnish scissors makers and compared them to the same group of department store assistants (n=143) that Luopajarvi et al (1979) used as a comparison group. Exposure assessment included videotape analysis of scissors maker tasks. The time spent in deviated wrist postures per work cycle was multiplied by the number of pieces handled per hour and the number of hours worked to create a workload index. Diagnoses of tenosynovitis and peritendinitis were determined using predetermined criteria (localized tenderness and pain during movement, low-grip force, swelling of wrist tendons). The prevalence rate ratio for muscle-tendon syndrome among the scissors makers was 1.38 compared to the department store assistants. The study group was 99% female. No association was found between age or body-mass index and diagnoses of muscle or tendon syndromes. The number of symptoms increased with the number of parts handled per year. A non-significant trend of increasing prevalence of diagnosed muscle-tendon syndrome with increasing number of pieces handled per year was noted in a nested case-control analysis.
In Japan, Amano and colleagues (1988) conducted a study to investigate cervicobrachial disorders and other MSDs in shoe factory workers. Finger flexor tenosynovitis was reported among 102 assembly line workers in an athletic shoe factory and 102 age and gender-matched non-assembly line workers. Exposure assessment was based on videotape analysis of the tasks of 29 workers on one assembly line. No formal exposure assessment of the comparison group was reported. Diagnoses were determined by physical examination, including palpation for tenderness. The prevalence rates for tenosynovitis among the shoe factory workers were significantly higher than the non-factory workers. Shoe assembly workers held shoe lasts longer in the left hand and had greater frequency of symptoms in the left upper extremity.
In a cross-sectional investigation, Punnett et al. (1985) investigated the prevalence of soft tissue disorders of the hands and arms of female garment workers. The findings were compared with the prevalence of disorders in a group of female hospital employees not required to use repetitive hand motion. One hundred and eighty-eight garment workers and 76 hospital employees were surveyed by questionnaire and physical examination. Workers in hand sewing and trimming had high prevalences of persistent pain in all upper limb sites. Stichers had elevated rates of pain in the shoulders, wrists, and hands. Workers ironing by hand had a significant elevation in elbow pain rates. The authors found a significant prevalence rate ratio of persistent elbow symptoms among garment workers performing repetitive and forceful work compared to hospital employees.
Based on a review of 20 epidemiologic studies that examined physical workplace factors and their relationship to epicondylitis, NIOSH (1997) reported that there is insufficient evidence for support of an association between repetitive work or postural factors alone and elbow MSDs. They did, however, determine that there is “some” evidence for an association with forceful work and epicondylitis and strong evidence for a relationship between exposure to a combination of risk factors (e.g., force and repetition, force and posture) and epicondylitis.
Several studies have reported significant associations between postural variables and neck MSDs (Jonsson et al. 1988, Kilbom et al. 1986, Kilbom and Persson 1987). Kilbom et al. (1986), in a study of electronic workers, reported two significant findings: (1) the more dynamic the working technique, the fewer neck symptoms experienced by electronic workers; and (2) the greater the average time per work cycle spent in neck flexion, the greater the association with symptoms in the neck and neck/shoulder area. A statistically significant association was also obtained from the job analysis variables describing neck forward flexion and upper arm elevation and neck and neck/shoulder disorders.
Shoulder MSDs and their relationship to work risk factors have been reviewed by several authors (Chiang et al. 1993, Hagberg and Wegman 1987, Kuorinka and Forcier 1995, Sommerich et al. 1993). Hagberg and Wegman (1987) attributed a majority of shoulder problems occurring in a variety of occupations to workplace exposure. Kuorinka and Forcier (1995) looked specifically at shoulder tendinitis and stated that the epidemiologic literature is “most convincing” regarding work-relatedness, especially showing an increased risk resulting from overhead and repetitive work.
In a study of fish processing workers in Taiwan, Chiang et al. (1993) studied the relationship between workplace factors and shoulder girdle pain. Shoulder girdle pain was defined as self-assessed symptoms of pain in the neck, shoulder or upper arms, and signs of muscle tender points or palpable hardenings upon physical examination. The force requirements of the jobs were estimated by surface electromyographs in the forearm flexor muscles. Exposure outcome was based on both force and repetitiveness. Using multiple logistic regression analysis with age, gender, and force as co-variates, the authors determined that highly repetitive upper extremity movements were associated with shoulder girdle pain. When tested in the same model with force and repetition, the interaction term for force and repetition was also significant.
In the cross-sectional study by Ohlsson et al. (1995), 85 female industrial assembly-line workers exposed to repetitive tasks with short cycles were compared to 64 referent subjects with no repetitive exposure. Industrial workers performed tasks involving repetitive arm movements with static muscular work of the neck/shoulder muscles. In a multivariate model, there were statistically significant associations between exposure to repetitive work and diagnoses in the neck/shoulders. In addition, age, tendencies towards subjective muscular tension, and stress/worry were also associated with diagnoses in the neck/shoulders. Standardized evaluation of videotape recordings in 74 of the industrial workers revealed significant associations between neck flexion, and elevation and abduction of the arm with the prevalence of neck/shoulder diagnoses. The authors concluded that a substantial prevalence of neck and upper limb disorders are associated with repetitive work performed with a flexed neck and elevated and abducted arms. The authors also suggested that certain personal traits in some workers may potentiate the above association.
The epidemiologic evidence for increased risk of MSDs due to specific shoulder postures is strongest when there is exposure to a combination of risk factors such as force and repetitive work (Baron et al. 1991, Bjelle et al. 1979, English et al. 1995, Herberts et al. 1981, Ohlsson et al. 1994, Ohlsson et al. 1995). An example of this combination would be operating a powered hand tool while working overhead. Although most of the studies that investigate specific shoulder postures as an exposure variable are cross-sectional, prospective studies have revealed that the percent of work cycle spent with the shoulder elevated (Jonsson et al. 1988) or abducted (Kilbom et al. 1986, Kilbom and Persson 1987) predicted change to more severe neck and shoulder disorders.
A number of individual factors that can influence a person’s response
to risk factors for MSDs in the workplace include: age (Biering-Sorensen
1983, English et al. 1995, Ohlsson et al. 1994); gender (Armstrong et al.
1987, Chiang et al. 1993, Hales et al. 1994, Johansson 1994); anthropometry
(Nathan et al. 1992, Werner et al. 1994); cigarette smoking (Svensson and
Andersson 1983); and parity (Roquelaure et al. 1997). There may well be
other individual lifestyle factors such as diet and exercise habits (Nathan
et al. 1992), as well as a whole host of medical conditions, which affect
an individual’s predisposition to developing MSDs. Some, but not all, epidemiologic
studies have used statistical methods to take into account the effects
of these individual factors (e.g., gender, age, body mass index, and parity)
when investigating the associations between job tasks and MSDs. These statistical
methods control for the confounding or modifying effects when determining
the strength of association between MSDs and occupational factors. Studies
that fail to control for the influence of individual factors may either
mask or amplify the effects of work-related factors.
When developing an ergonomics model for the prevention of MSDs it is
useful to employ public health principles. In the context of disease progression
(from no disease to symptomatic invasive disease), public health investigators
recognize a continuum of prevention at three levels, primary, secondary,
and tertiary (Halperin 1996, Last 1988) (Figure
2). Tertiary prevention refers to measures to reduce long-term
impairment and disability. The diagnosis and treatment of an MSD would
constitute tertiary prevention. Secondary prevention is the early detection
of asymptomatic disease and prompt intervention when the disease is preventable
or more easily treatable, such as in screening for breast cancer. An example
of secondary prevention in ergonomics would be the early identification
of carpal tunnel syndrome through nerve conduction screenings (Bingham
et al. 1996). Primary prevention involves the prevention of disease before
its initiation. In pediatric medicine for example, immunization for childhood
diseases is an example of primary prevention. In ergonomics, primary prevention
involves the development of interventions (often engineering changes) to
eliminate the risk factors (i.e. awkward posture, excessive force, and
high repetition) associated with the disorder. The most effective and distributive
prevention strategy for occupational MSDs is through primary prevention.
Figure 2. The three stages of prevention in the disease process.
One of the corner stones of primary prevention strategies within the public health domain are activities involving recognition and evaluation of a health problem followed by intervention programs (Halperin, 1996). This public health paradigm can be easily applied and incorporated into a model for the prevention of MSDs through an ergonomics process. The ergonomic prevention model engaged by the authors is based on “Participatory Action Research” theory developed by Hugentobler et al. 1992 and Israel et al. 1992. According to Israel et al. (1992), participatory action research requires investigators to work collaboratively with the study population (i.e. the workforce). This research process is characterized by participation, cooperation, co-learning, system development, employee empowerment, and balancing research objectives with the objectives of the company (Israel et al. 1992).
As a focus point within the conceptual framework of participatory action
research theory, several investigators have utilized an ergonomics process
to encourage change in occupational settings (Moore and Garg 1996, Rosecrance
and Cook 1996). This process is not unlike the public health paradigm involving
recognition, evaluation, and intervention. The ergonomics process, however,
consists of five specific but overlapping steps: (1) problem identification,
(2) problem analysis, (3) solution development, (4) solution implementation,
and (5) solution evaluation (Figure 3). The
identification of MSDs and their associated risk factors is the first step
in the ergonomics process. Once MSDs are identified, specific work tasks
and methods are analyzed for detecting risk factors and developing potential
solutions. Based on the findings from the analysis, a prioritization for
solution development and implementation is planned. The implemented solutions
are then evaluated. In the majority of cases, the initial solutions are
imperfect, and therefore the situation is reanalyzed and the cycle repeated
until a satisfactory result is obtained.
|Figure 3. The ergonomics process model used in the primary prevention of occupational MSDs. (Original source: Orthopaedic Physical Therapy Clinics of North America, 5/2:274, 1996. With the kind permission of the Publisher)|
The guideing principles of the ergonomic process include structure, scientific approach, participation by management, supervisors, workers, unions, and decision by consensus (Moore and Garg, 1996). A basic tenet of the ergonomics process is employee participation. The focus of employee involvement is to assure workers a sense of personal control over their workplace by educating them about and encouraging them to participate fully in workplace health and safety. This focus is not unlike that of employee empowerment (Wallerstein and Weinger 1992) and has the potential to affect the overall climate of the intervention process (Goldenhar and Schulte 1994). The ergonomics process model is a problem solving process and bears a strong resemblance to continuous learning and improvement which are basic elements in Deming’s philosophy of total quality management (TQM) (Deming 1986, Walton 1986). Among Deming’s (1986) 14 points for TQM are that managers are to provide vision, priorities, and structure but get out of the way and let people “own” their jobs. Experienced workers that perform job tasks involving high force and awkward postures often have ideas on how to make their jobs easier and more efficient. Worker solutions, however, may not come to fruition if they are not encouraged to express their ideas and become involved in the decision making process. In too many situations outside “experts” are called in and impose their solutions on the workers without regard to the people performing the actual tasks. The “expert” model may be inappropriate for many companies attempting to incorporate participatory ergonomic programs. Ergonomic experts will be more successful in conducting ergonomic prevention programs if they include both labor and management in planning and selecting interventions.
Because of the multi-factorial nature of MSDs, an effective approach to dealing with work-related MSDs is a program consisting of primary prevention. Primary prevention programs have included the implementation of a cyclical ergonomics process involving continuous improvement. Key components of the ergonomics process consists of employee participation, management commitment, the identification of ergonomic hazards, the development of solutions and the evaluation of those solutions.
Although workplace prevention programs have been developed to reduce injury and illness, few have undergone systematic evaluation to determine their impact on health and safety outcomes. Consequently, many ergonomic intervention programs have been based on faith and expert judgement without convincing scientific evidence that these approaches are effective. Further research is needed to evaluate the effectiveness of ergonomic intervention efforts in terms of productivity, cost effectiveness, and reductions in occupationally related MSDs. A systematic evaluation can provide crucial guidance and corrective feedback for current and future occupational health and safety efforts. Intervention effectiveness research may provide evidence for effective ergonomic strategies and assure efficient use of limited resources in workplace prevention and intervention programs.
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