A Critical Review of Epidemiologic Evidence for Work-Related Musculoskeletal Disorders of the Neck, Upper Extremity, and Low Back

Chapter 5. Hand/Wrist Musculoskeletal Disorders (Carpal Tunnel Syndrome, Hand/Wrist Tendinitis, and Hand-Arm Vibration Syndrome): Evidence for Work-Relatedness

Musculoskeletal disorders (MSDs) of the hand/wrist region have been separated into three components for the purpose of this review: (a) Carpal Tunnel Syndrome (CTS), (b) Hand/Wrist Tendinitis, and (c) Hand-Arm Vibration Syndrome (HAVS). Each of these are described with regard to the evidence for causality between workplace risk factors and development of MSDs.

Summary

Over 30 epidemiologic studies have examined physical workplace factors and their relationship to carpal tunnel syndrome (CTS). Several studies fulfill the four epidemiologic criteria that were used in this review, and appropriately address important methodologic issues. The studies generally involved populations exposed to a combination of work factors, but a few assessed single work factors such as repetitive motions of the hand. We examined each of these studies, whether the findings were positive, negative, or equivocal, to evaluate the strength of work-relatedness using causal inference.

There is evidence of a positive association between highly repetitive work alone or in combination with other factors and CTS based on currently available epidemiologic data. There is also evidence of a positive association between forceful work and CTS. There is insufficient evidence of an association between CTS and extreme postures. Individual variability in work methods among workers in similar jobs and the influence of differing anthropometry on posture are among the difficulties noted in measuring postural characteristics of jobs in field studies. Findings from laboratory-based studies of extreme postural factors support a positive association with CTS. There is evidence of a positive association between work involving hand/wrist vibration and CTS.

There is strong evidence of a positive association between exposure to a combination of risk factors (e.g., force and repetition, force and posture) and CTS. Based on the epidemiologic studies reviewed above, especially those with quantitative evaluation of the risk factors, the evidence is

clear that exposure to a combination of the job factors studied (repetition, force, posture, etc.) increases the risk for CTS. This is consistent with the evidence that is found in the biomechanical, physiological, and psychosocial literature. Epidemiologic surveillance data, both nationally and internationally, have also consistently indicated that the highest rates of CTS occur in occupations and job tasks with high work demands for intensive manual exertion–for example, in meatpackers, poultry processors, and automobile assembly workers.

Introduction

In 1988, CTS had an estimated population prevalence of 53 cases per 10,000 current workers [Tanaka et al. (in press)]. Twenty percent of these individuals reported absence from work because of CTS. In 1994, the Bureau of Labor Statistics (BLS) reported that the rate of CTS cases that result in “days away from work” was 4.8 cases per 10,000 workers. The agency also reported that the median number of days away from work for CTS was 30, which is even greater than the median reported for back pain cases [BLS 1995]. In 1993, the incidence rate (IR) for CTS workers’ compensation cases was 31.7 cases per 10,000 workers; only a minority of these cases involved time off of work [Washington State Department of Labor and Industry 1996]. These data suggest that about 5 to 10 workers per 10,000 workers will miss work each year due to work-related CTS.

In recent years, the literature relating occupational factors to the development of CTS has been extensively reviewed by numerous authors [Moore 1992; Stock 1991; Gerr et al. 1991; Hagberg et al. 1992; Armstrong et al. 1993; Kuorinka and Forcier 1995; Viikari-Juntura 1995]. Most of these reviews reach a similar conclusion—work factors are one of the important causes of CTS. One review [Moore 1992] found the evidence more equivocal, but stated that the epidemiologic studies revealed a fairly consistent pattern of observations regarding the spectrum and relative frequency of CTS [among other musculoskeletal disorders (MSDs)] among jobs believed to be hazardous. The epidemiologic studies which form the basis for these reviews are outlined in Tables 5a–1 to 5a–4 of this chapter.

Thirty studies of occupational CTS are listed on Tables 5a–5. Twenty-one are cross-sectional studies, six are case-control, and three involve a longitudinal phase; all have been published since 1979. We included one surveillance study [Franklin et al. 1991] because it has been included in many of the earlier reviews. The few earlier studies of CTS identified were clinical case series, or did not identify work place risk factors and were not included in the tables related to CTS.

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Outcome and Exposure Measures

In four of 30 studies listed in Tables 5a–1 to 5a–4, CTS was assessed based on symptoms alone; in another nine studies, the case definition was based on a combination of symptoms and physical findings. Electrophysiological tests of nerve function were completed in 14 studies. Electrodiagnostic testing (nerve conduction studies) has been considered by some to be a requirement for a valid case definition of CTS, as is similarly used for a clinical diagnosis in individuals with CTS. A few studies which have looked at the relationship of occupational factors to CTS have used a health outcome based on electrodiagnostic testing alone [Nathan et al. 1988; Schottland et al. 1991; Radecki 1995.] However, some authors [Nilsson 1995; Werner et al. 1997] have discouraged the use of labeling workers as having “CTS” or “median nerve mononeuropathy” based on abnormal sensory nerve conduction alone (without symptoms). The reason for this view is illustrated in a recent prospective study by Werner et al. [1997]. On follow-up six to eighteen months after initial evaluation, they found that asymptomatic active workers with abnormal sensory median nerve function (by Nerve Conduction Studies [NCS]) were no more likely to develop symptoms consistent with CTS than those with normal nerve function. Studies which have used nerve conduction tests for epidemiologic field studies have employed a variety of evaluation methods and techniques [Nathan et al. 1988, 1994b; Bernard et al. 1993; Osorio et al. 1994]. Normal values for nerve conduction studies have also varied from laboratory to laboratory. NCS results have been found to vary with electrode placement, temperature, as well as age, height, finger circumference and wrist ratio [Stetson 1993], suggesting that “normal” values may need to be corrected for those factors.

Several epidemiologic studies have used a surveillance case definition of CTS based on symptoms in the median nerve distribution and abnormal physical examination findings using Phalen’s test and Tinel’s sign, and have not included NCS. Two recent studies [Bernard et al. 1993; Atterbury et al. 1996] looked at CTS diagnosis based on questionnaire and physical examination findings and its association with the “gold standard” of nerve conduction diagnosed median mononeuropathy. Both studies found statistically significant evidence to support the use of an epidemiologic CTS case definition based on symptoms and physical examination (not requiring NCS) for epidemiologic surveillance studies. Nathan [1992a] also found a strong relationship between symptoms and prolonged sensory median nerve conduction. (It is important to note here that a case definition used for epidemiologic purposes usually differs from one used for medical diagnosis and therapeutic intervention.)

Researchers have relied on a variety of methods to assess exposure to suspected occupational risk factors for CTS. These methods include direct measurement, observation, self-reports, and categorization by job titles. Most investigators agree that use of observational or direct measurement methods increases the quality (both the precision and accuracy) of ergonomic exposure assessments, but these methods also tend to be costly and time consuming. In general, misclassification errors tend to dilute the observed associations between disease and physical workload [Viikari-Juntura 1995].

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Repetition

Definition of Repetition for CTS

For our review, we identified studies that examined repetition or repetitive work for the hand and wrist for CTS as cyclical or repetitive work activities that involved either 1) repetitive hand/finger or wrist movements such as hand gripping or wrist extension/flexion, ulnar/radial deviation, and supination or pronation. Most of the studies that examined repetition or repetitive work as a risk factor for CTS had several concurrent or interacting physical workload factors. Therefore, repetitive work should be considered in this context, with repetition as only one exposure factor, accompanied by others such as force, extreme posture, and, less commonly, vibration.

Studies Reporting on the Association of Repetition and CTS

Nineteen studies reported on the results of the association between repetition and CTS. Several studies in Table 5a-1 quantitatively measured [Moore 1992; Chiang et al. 1990, 1993; Silverstein et al. 1987] or observed [Stetson et al. 1993; Nathan et al. 1988, 1992a; Barnhart et al. 1991; Osorio et al. 1994] and categorized repetitive hand and wrist movements in terms of: a) the frequency or duration of tasks pertaining to the hand/wrist, b) the ratio of work-time to recovery time, c) the percentage of the workday spent on repetitive activities, or d) the quantity of work performed in a given time. The rest of the studies generally used job titles or questionnaires to characterize exposure.

Studies Meeting the Four Evaluation Criteria
Five epidemiologic studies of the hand/wrist area addressing repetitiveness and CTS [Chiang et al. 1990, 1993; Moore and Garg 1994; Osorio et al. 1994; Silverstein et al. 1987] met the four criteria. Chiang et al. [1990] studied 207 workers from 2 frozen food processing plants. Investigators observed job tasks and divided them into low or high repetitiveness categories of wrist movement based on cycle time, as previously described by Silverstein et al. [1987]. Jobs were also classified according to whether or not workers’ hands were exposed to cold work conditions. The resulting exposure groups were: Group 1–Not Cold, Low Repetitiveness (mainly office staff and technicians); Group 2–Cold Exposure or High Repetitiveness; and Group 3–Cold Exposure and High Repetitiveness. CTS diagnosis was based on abnormal clinical examination and nerve conduction studies. Prevalence of CTS was 3% in Group 1, 15% in Group 2, and 37% in Group 3. Statistical modeling that also included gender, age, length of employment, and cold resulted in an odds ratio (OR) of 1.87 (p=0.02) for CTS among those with highly repetitive jobs. The OR for CTS among those exposed to cold conditions and high repetitiveness was 3.32 (p=0.03). The authors cautioned that cold exposure may have at least partially acted as a proxy for forceful hand/wrist exertion in this study group.

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). The fish-processing workers were divided into high repetitiveness or high force (group 2), and high force and high repetitiveness (group 3). Repetition of upper limb movements (not specifically the wrist) was defined based on observed cycle time [Silverstein et al. 1987]. 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). Repetitiveness alone was not a significant predictor of CTS (OR 1.1). Statistical modeling showed that women in this study group had a higher prevalence of CTS than men (OR 2.6, 95% confidence interval [CI] 1.3–5.2). Because the proportion of women varied by exposure group (48%, 75%, and 79% from group 1 to 3), further analyses were limited to females. The OR for repetitiveness was 1.5 (95% CI 0.8–2.8), con-trolling for oral contraceptive use and force.

Moore and Garg [1994] evaluated 32 jobs in a pork processing plant and then reviewed past OSHA illness and injury logs and plant medical records for CTS cases in these job categories. A CTS case required the recording of suggestive symptoms (numbness and tingling) combined with electrodiagnostic confirmation (as reported by the attending electromyographers) of a case. Incidence ratios (IRs) were calculated using the full-time equivalent number of hours worked reported on the logs. The exact number of workers was not reported. 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 (%MVC) based on weight of tools, and parts and population strength data adjusted for extreme posture or speed. Jobs were then categorized as hazardous or safe (for all upper extremity MSDs, not for CTS), based on exposure data and the judgment of the investigators. The hazardous jobs had a relative risk (RR) for CTS of 2.8 (95% CI 0.2–36.7) compared to the safe jobs. Due to the lack of data from individual workers, the study was unable to control for common confounders. Potential for survivor effect (79% of the workforce was laid off the year prior to the study), a limited latency period (8–32 months), and the potential for incomplete case ascertainment (underreporting is known to be a problem with OSHA illness and injury logs) limit confidence in this estimate. This study did not specifically address the relationship between repetitiveness and CTS. No significant association was identified between repetitiveness and the grouped “upper extremity musculoskeletal disorders,” but there was very little variability in repetitiveness (31 of the 32 jobs had a cycle time less than 30 seconds).

Osorio et al. [1994] studied 56 supermarket workers. Exposure to repetitive and forceful wrist motions was rated as high, moderate, or low, following observation of job tasks. The CTS case definition was based on symptoms and nerve conduction studies. CTS-like symptoms occurred more often (OR 8.3, 95% CI 2.6–26.4) among workers in the high exposure group compared to the low exposed group. The odds of meeting the symptom and NCS-based CTS case definition among the high exposure group were 6.7 (95% CI 0.8–52.9), compared to the low exposure group.

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 divided jobs into high or low repetitiveness categories, based on analysis of videotaped job tasks of 3 representative workers in each job. High repetitiveness was defined as cycle time less than 30 seconds or at least 50% of the work cycle spent performing the same fundamental movements. Jobs were also divided into high or low force categories based on EMGs of representative workers’ forearm flexor muscles while they performed their usual tasks. EMG measurements were averaged within each work group to characterize the force requirements of the job. High force was defined as a mean adjusted force >6 kg. Jobs were then classified into 4 groups: low force/low repetitiveness, high force/low repetitiveness, low force/high repetitiveness, and high force/high repetitiveness. Fourteen cases (2.1% prevalence) of CTS were diagnosed based on standardized physical examinations and structured interviews.

The OR for CTS in highly repetitive jobs compared to low repetitive jobs, irrespective of force, was 5.5 (p<0.05) in a statistical model that also included age, gender, years on the job, and plant. The OR for CTS in jobs with combined exposures to high force and high repetition was 15.5 (p<0.05), 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 identified.

Studies Meeting at Least One Criterion
Fourteen additional studies met at least one of the criteria.

Barnhart et al. [1991] studied ski manufacturing workers categorized as having repetitive or nonrepetitive jobs based on observational exposure methods for hand/wrist exposure. The participation rate for this study was below 70%. Three different case definitions were used for CTS based on symptoms, physical exam findings, and NCS using the mean median-ulnar difference in each group. Each case definition used the NCS results. The authors reported a significant prevalence ratio (PR) of 2.3 for the mean median-ulnar sensory latency nerve difference among those in repetitive jobs compared to those in non-repetitive jobs. However, the difference was found in the ulnar rather than in the median nerve. The median nerve latencies were not statistically different between the two groups. Baron et al. [1991] studied CTS in 124 grocery store checkers and 157 other grocery store workers who were not checkers. The CTS case definition required symptoms that met pre-determined criteria on a standardized questionnaire and physical examinations. The OR for CTS among checkers was 3.7 (95% CI0.7–16.7), in a model that included age, hobbies, second jobs, systemic disease, and obesity. Participation rates at the work sites were higher among the exposed group (checkers: 85% participation, non-checkers: 55% participation). After telephone interviews in which 85% of the non-checkers completed questionnaires, investigators reported that the proportion of non-checkers meeting the case definition did not increase.

Cannon et al. [1981] in a case-control study of aircraft engine workers did not find a significant association with the performance of repetitive motion tasks (OR 2.1, 95%CI 0.9–5.3), but found a significant association with self-reported use of vibrating hand tools, history of gynecologic surgery, and an inverse relationship with years on the job. One must assume from the article that “repetitive motion tasks” were defined by job title. The diagnosis of CTS was based on medical and workers’ compensation records.

In English et al.’s [1995] case-control study of upper limb disorders diagnosed in orthopedic clinics, the case series included 171 cases of CTS and 996 controls. Exposure was based on self-reports; repetitiveness was defined as a motion occurring more than once per minute. The logistic regression model of CTS found significant associations with height (negative), weight (positive), presentation at the clinic as a result of an accident (negative), and two occupational factors: 1) uninterrupted shoulder rotation with elevated arm (OR 1.8, 95% CI 1.2–2.8) and 2) protection from repeated finger tapping (OR 0.4, 95% CI 0.2–0.7). The authors note that the latter observation presented “difficulties of interpretation.” Limitations of this study concern the lack of exposure assessment for repetition, and the questionable reliability for reported limb movements as an accurate measure of repetition.

Feldman et al. [1987] studied electronic workers at a large manufacturing firm using a questionnaire survey and biomechanical job analysis. Four work areas with 84 workers were identified as “high risk” with highly repetitive and forceful tasks. Workers in these high risk areas had physical examinations and NCS. Sixty-two workers from the high risk area had repeat NCS one year later. Comparing these high risk workers to the others, one can calculate ORs for symptoms of numbness and tingling [OR 2.26 (p<0.05)] and a positive Phalen’s sign [2.7 (p<0.05)]. Longitudinal NCS of workers in the high risk area showed significant worsening in the median motor latency and sensory conduction velocity in the left hand, and motor changes over a year’s period, which the authors attributed to work exposure. A limitation of this study concerns inadequate exposure information about the extent of worker exposure to repetitive and forceful work.

McCormack et al. [1990] studied 1,579 textile production workers and compared them to 468 other nonoffice workers, a comparison group that included machine maintenance workers, transportation workers, cleaners, and sweepers. The textile production workers were divided into four broad job categories based on similarity of upper extremity exertions. No formal exposure assessment was conducted. Health assessment included a questionnaire and screening physical examination followed by a diagnostic physical examination. CTS was diagnosed using predetermined clinical criteria. The severity of cases was also reported as mild, moderate, or severe. The overall prevalence for CTS was 1.1%, with 0.7% in boarding, 1.2% in sewing, 0.9% in knitting, 0.5% in packaging/folding, and 1.3% in the comparison group. None of the differences were statistically significant. A statistical model that also included age, gender, race, and years of employment showed that CTS occurred more often among women in this study (p<0.05). Interpretation of these data, especially with a low prevalence disorder like CTS, is difficult since gender varied with job (94% of boarding workers were female, compared to 56% in the comparison group), and the comparison group (machine maintenance workers, transportation workers, cleaners and sweepers) may have also been exposed to upper extremity exertions. Interactions among potential confounders were not addressed, but they are suspected because of significant associations between race and three MSDs.

Morgenstern et al. [1991] mailed questionnaires to 1,345 union grocery checkers and a general population group. Exposure was based on self-reported time working as a checker. Symptoms of CTS were significantly associated with age and the use of diuretics, and nonsignificantly associated with average hours worked per week, and years worked as a checker. A positive CTS outcome was based on the presence of all four symptoms: pain in the hands or wrist, nocturnal pain, tingling in the hands or fingers, or numbness. The estimated attributable fraction of CTS symptoms to working as a checker was about 60%, using both a general population comparison group and a low exposed checker group. The limitations of this study are: 1) the use of an overly sensitive health outcome measure, for example, 32% of the surveyed population reported numbness; and 2) the use of self-reported exposure.

Nathan et al. [1988] studied median nerve conduction of 471 randomly selected workers from four industries (steel mill, meat/food packaging, electronics, and plastics manufacturing). Median nerve sensory latency values were adjusted for age for statistical analyses. Thirty-nine percent of the study subjects had impaired sensory nerve conduction, or “slowing” of the median nerve. The five exposure groups were defined as follows: Group 1 is very low force, low repetition (VLF/LR); Group 2 is low force, very high repetition (LF/VHR); Group 3 is moderate force, moderate repetition (MF/MR); Group 4 is high force/moderate repetition (HF/MR); and Group 5 is very high force/high repetition (VHF/HR). There was no significant difference between Group 1 and Group 2, the groups that had the greatest differences in repetition. The authors reported a significantly higher number of subjects with median nerve slowing in Group 5 (VHF/HR) compared to Group 1 (VLF/LR), but not in other groups, using a statistical method described as a “pairwise unplanned simultaneous test procedure” [Sokal and Rohlf 1981]. The authors also reported that when individual hands were the basis of calculations rather than subjects, Group 3 had a significantly higher prevalence of median nerve slowing. Calculations of the data using PRs and chi-squares [Kleinbaum et al. 1982] resulted in significantly higher prevalences of median nerve slowing in each of Groups 3, 4, and 5 (moderate to high repetition, with moderate to very high force) compared to Group 1 (VLR/LF). PRs are 1.9 (95% CI 1.3–2.7), 1.7 (95% CI 1.1–2.5), and 2.0 (95% CI 1.1–3.4) for Groups 3, 4, and 5, respectively. A conservative (Bonferroni) adjustment of the significance level to 0.0125 for multiple comparisons [Kleinbaum et al. 1982] would result in Group 5 no longer being statistically significantly different from Group 1 (p=0.019), but Group 4 (p=0.009), and Group 3 (p=0.000) remain statistically significantly higher than Group 1 in prevalence of median nerve slowing.

In 1992, Nathan et al. [1992a] reported on a follow-up evaluation in the same study group. Sixty-seven percent of the original study subjects were included. Hands (630), rather than subjects, were the basis of analysis in this study. Novice workers (those employed less than 2 years in 1984) were less likely to return than non-novice workers (56% compared to 69%, p=.004). Maximum latency differences in median nerve sensory conduction were determined as in the Nathan et al. [1988] study. The authors state that there was no significant difference in the prevalence of median nerve slowing between any of the exposure categories in Nathan et al. [1988] using the same statistical method described in the Nathan et al. 1988 study. However, calculations using common statistical methods result in the following PRs for slowing: Group 3–1.5 (95% CI 1.0–2.2), Group 4–1.4 (95% CI 0.9–2.1), and Group 5–1.0 (95% CI 0.5–2.2), compared to Group 1. Group 5 had the same prevalence of slowing (18%) as Group 1 in 1989. In 1984 the prevalence of slowing was 29% in Group 5, and 15% in Group 1. The drop in prevalence of median nerve slowing in Group 5 between 1984 and 1989 might be explained by the higher drop-out rate among cases in Group 5 compared to Group 1 (PR 2.9, 95% CI 1.3–6.6). This was not addressed by the authors.

Punnett et al. [1985] compared the symptoms and physical findings of CTS in 162 women garment workers and 76 women hospital workers such as nurses, laboratory technicians, and laundry workers. Eighty-six percent of the garment workers were sewing machine operators and finishers (sewing and trimming by hand). The sewing machine operators were described as using highly repetitive, low force wrist and finger motions, whereas finishing work also involved shoulder and elbow motions. The exposed garment workers probably had more repetitive jobs than most of the hospital workers. CTS symptoms occurred more often among the garment workers (OR 2.7, 95% CI 1.2–7.6) compared to the hospital workers. There was a low participation rate (40%) among the hospital workers.

Schottland et al. [1991] carried out a comparison of NCS findings in poultry workers and job applicants as referents. No exposure assessment was performed, and applicants were not excluded if they had prior employment in the plant. Results indicated that the right median nerve sensory latency was significantly longer in 66 female poultry workers compared to 41 female job applicants. In these two groups of women there were less pronounced differences in the left median sensory latency. The latencies in the 27 male poultry workers did not differ significantly from the 44 male job applicants, although the power calculations presented in the paper noted limited power to detect differences among male participants. The OR for percentage of female poultry workers who exceeded the criteria value for the right median sensory latency is 2.86 (95% CI 1.1–7.9). The major limitations of this study are the absence of detailed information on exposure and the inclusion of former poultry workers into the applicant group, as well as the inadequate sample size, and the personal characteristics of these workers. This study found a significant association between highly repetitive, highly forceful work and abnormal NSC consistent with CTS. It does not allow analysis of repetition alone.

Stetson et al. [1993] used measurements of sensory nerve conduction velocity of the median nerve as indicators of nerve impairment or CTS; clinical examination results were not reported in this article. Three groups were studied: a reference group of 105 workers without occupational exposure to highly forceful or repetitive hand exertions, 103 industrial workers with hand/wrist symptoms, and 137 asymptomatic industrial workers. Exposure was assessed with a checklist by trained workers. Factors considered included repetitiveness (Silverstein criteria), force defined by the weight of an object that is carried or held, localized mechanical stress, and posture. Exposure assessments were available on 80% of the industrial workers. Most of the industrial workers were on repetitive jobs (76%), a minority carried more than ten pounds some of the time (32%), and gripped more than six pounds at least some of the time (44%). The analysis controlled for several confounders including age, gender, finger circumference, height, weight, and a square-shaped wrist. In the comparison of the asymptomatic to symptomatic industrial workers, the mean exposure for the symptomatic industrial workers was nonsignificantly slightly greater for all exposure factors except for repetitiveness. The median sensory amplitudes were significantly smaller (p<0.01) and latencies longer (p<0.05 ) for industrial workers with exposure to high grip forces compared to those without. Mean sensory amplitudes were significantly smaller (p<0.05) and motor and sensory latencies were significantly longer (p<0.01) in the industrial asymptomatic workers compared to the control group. These findings for the motor latencies are similar to Feldman et al. [1987]. Since most of the industrial workers were exposed to repetitive work, it is not clear whether this study population allowed a comparison between repetitive and non-repetitive work. Overall this study suggests that repetitive work combined with other risk factors is associated with slowing of median nerve conduction.

The Wieslander et al. [1989] case-control study used self-reported information collected via telephone interview about the duration of exposure (number of years and hours per week) to several work attributes including repetitive work. Definitions for these work attributes were not provided. Three categories of duration of exposure were defined for each attribute (<1 year, 1–20 years, and >20 years), but the asymmetry of the categories was not explained. A significant OR for reporting repetitive movements of the wrist comparing CTS patients to hospital referents (OR 4.6) and general population referents (OR 9.6) was reported, but only among those employed greater than 20 years. Those employed from 1–20 years compared to the referent population had elevated ORs for repetitive movements of the wrist (1.5 for CTS patients compared to hospital referents, and 2.3 compared to population referents), but these were not significant. Jobs with increasing numbers of work risk factors gave increasing ORs (from 1.7 to 7.1) among CTS cases when compared to referents; these were statistically significant when there were two or more risk factors. Given the limited quality of the exposure data and findings (repetition is a significant risk factor only after 20 years of exposure), this is only suggestive of a relationship between repetition alone and CTS.

Studies Not Meeting Any of the Criteria
Liss et al. [1995] conducted a mail survey concerning CTS among 2,124 Ontario dental hygienists compared to 305 dental assistants who do not scale teeth. Both groups had a low response rate (50%). The age adjusted OR was 5.2 (95% CI 0.9–32) for being told by a physician that you had CTS and 3.7 (95% CI 1.1–1.9) using a questionnaire-based definition of CTS. The major limitations of this study are the low participation rate, the lack of a detailed exposure assessment for repetitiveness, and self-reported health outcome.

Strength of Association—Repetition and CTS

Three of the five studies that met all four criteria evaluated the effect of repetitiveness alone on CTS: Chiang et al. [1990], Silverstein et al. [1987], and Chiang et al. [1993].

Chiang et al. [1990] reported an OR of 1.9 (p<0.05) for CTS among those with highly repetitive jobs. The OR for CTS among those exposed to high repetitiveness and cold was 3.32 (p<0.05). The additional effect attributed to cold may be at least partially explained by forceful motions among workers who were also exposed to cold. Force was not evaluated in this study.

Silverstein et al. [1987] reported an OR of 5.5 (p<0.05) for repetition as a single predictor of CTS. Among workers exposed to high repetition and high force, the OR was 15.5 (p<0.05).

Chiang et al. [1993] reported a significant trend of increasing prevalence of CTS with increasing exposure to repetition and/or force (8.2%, 15.3%, and 28.6%, p<0.05). Repetition (of the whole upper limb, not the wrist) alone did not significantly predict CTS (OR 1.1).

In summary, three studies that met all four criteria reported ORs for CTS associated with repetition. The statistically significant ORs for CTS attributed to repetition alone ranged from 1.9 to 5.5. The statistically significant ORs for CTS attributed to repetition in combination with force or cold ranged from 3.3 to 15.5. Gender, age, and other potential confounders were addressed and are unlikely to account for the associations reported.

Five other studies observed job tasks, then grouped them into categories according to estimated levels of repetitiveness combined with other risk factors [Feldman et al. 1987; Moore and Garg 1994; Nathan et al. 1988, 1992a; and Osorio et al. 1994]. CTS case definitions reported here required more than symptom-defined criteria. Moore and Garg [1994] reviewed medical records; Nathan et al. [1988] and Osorio et al. [1994] performed nerve conduction studies.

Feldman et al. [1987] reported an OR of 2.7 (p<0.05) for a positive Phalen’s test among workers in high exposure jobs, compared to low exposure jobs.

Moore and Garg [1994] reported an OR of 2.8 (0.2, 36.7) for CTS among workers in “hazardous” jobs compared to workers in “nonhazardous” jobs.

Nathan et al.’s [1988] data result in Prs for four groups with varying levels of repetitiveness and force from very low (VL) to very high (VH), compared to a very low force, low repetition group (VLF/LR):

LF/VHR versus VLF/LR: 1.0 (95% CI 0.5–2.0)

MF/MR versus VLF/LR: 1.9 (95% CI 1.3–2.7)

HF/MR versus VLF/LR: 1.7 (95% CI 1.1– 2.5)

VHF/HR versus VLF/LR: 2.0 (95% CI 1.1–3.4).

Nathan et al. [1992a] data, a 5-year follow-up of the 1988 study, result in PRs for the following groups:

LF/VHR versus VLF/LR: 1.0 (95% CI 0.6– 1.9)

MF/MR versus VLF/LR: 1.5 (95% CI 1.0– 2.2)

HF/MR versus VLF/LR: 1.4 (95% CI 0.9– 2.1)

VHF/HR versus VLF/LR: 1.0 (95% CI 0.5– 2.2).

Osorio et al. [1994] reported an OR of 6.7 (95% CI 0.8–52.9) for CTS among workers in high exposure jobs, compared to workers in low exposure jobs. Using a symptom-based case definition, the OR for the same comparison groups was 8.3 (95% CI 2.6– 26.4).

To summarize, three of the five studies reviewed resulted in statistically significant positive findings for CTS associated with combined exposures. Feldman et al. [1987] reported an elevated OR for CTS with high combined exposure. Nathan et al.’s [1988] data resulted in elevated PRs for CTS among the three highest combined exposure groups. Nathan et al.’s [1992a] data resulted in an elevated PR for CTS among one of the high combined exposure groups. There was evidence of survivor bias in the highest exposure group.

The following studies used job title or job category to represent exposure to repetitiveness combined with other exposures and defined CTS based on physical examination [Baron et al. 1991, McCormack et al. 1990, Punnett et al. 1985] or nerve conduction studies [Schottland et al. 1991].

Baron et al. [1991] reported an OR of 3.7 (95% CI 0.7–16.7) for CTS, defined by symptoms and physical examination, among grocery checkers compared to other grocery workers.

McCormack et al. [1990] reported the following ORs for CTS among workers in each of four broad job categories that were considered exposed, compared to a comparison group of maintenance workers and cleaners that was considered to have low exposure:

Boarding versus Low: 0.5 (95% CI 0.1–2.9)

Sewing versus Low: 0.9 (95% CI 0.3–2.9)

Packaging versus Low: 0.4 (95% CI 0.0–2.4)

Knitting versus Low: 0.6 (95% CI 0.1–3.1)

Punnett et al. [1985] reported an OR of 2.7 (95% CI 1.2–7.6) for CTS among garment workers versus hospital workers.

Schottland et al. [1991] reported an OR of 2.86 (95% CI 1.1–7.9) for prolonged right median sensory latency among female poultry workers, compared to female applicants for the same jobs. No significant differences were identified among males.

In summary, two of the four studies reviewed above reported significantly elevated ORs for CTS or median sensory nerve conduction slowing.

Wieslander et al. [1989] reported an OR for CTS (surgical cases, confirmed by NCS) of 2.7 (95% CI 1.3–5.4) among those with self-reported exposure to repetitive wrist movement >20 years, compared to hospital referents, and 4.5 (95% CI 2.0–10.4), compared to population referents. Significant OR s for CTS among those with combined job risk factors ranged from 3.3 to 7.1.

The remaining two studies relied on self-reported symptoms and self-reported exposures from mail [Morgenstern et al. 1991] or telephone surveys [Liss et al. 1995]. Data quality and response rates limit interpretation of findings.

In conclusion, among the studies that measured repetition alone, there is evidence that repetition is positively associated with CTS. The majority of studies provide evidence of a stronger positive association between repetition combined with other job risk factors and CTS.

Temporal Relationship: Repetition and CTS

The question of which occurs first, exposure or disease, can be addressed most directly in prospective studies. However, study limitations such as survivor bias can cloud the interpretation of findings. In our analysis of Nathan et al.’s [1992a] data, 2 of 3 groups that were exposed to forceful hand/wrist exertions were more likely to have median nerve slowing when nerve conduction testing was repeated 5 years later. The highest exposure group had the same prevalence of slowing as the lowest exposure group in 1989, whereas they had a higher prevalence rate in 1984. As discussed above, this apparent decrease in prevalence over 5 years can probably be explained by a higher drop-out rate among cases in the highest exposure group, compared to the lowest exposure group. These interpretations of the data differ from those of the authors. Further study is needed to clarify these issues. However, to our knowledge, there is no evidence demonstrating that those with CTS would be more likely to be hired in jobs that involve high exposure to repetitive hand/wrist exertions and combined job risk factors, compared to those without CTS. In fact, employment practices tend to exclude new workers with CTS from jobs that require repetitive and intensive hand/wrist exertion.

Feldman et al. [1987] reported longer median motor (but not sensory) latencies among workers with combined exposure to hand/wrist exertion, compared to nerve conduction findings in the same group one year earlier.

Cross-sectional studies provide evidence that exposure occurred before CTS, by using case definitions that exclude pre-existing cases, and by excluding recently hired workers from the study. The studies that provide evidence that repetitive and combined job exposures are associated with CTS followed these practices, therefore the associations identified cannot be explained by disease occurring before exposure.

Consistency in Association for Repetition and CTS

One study [English et al. 1995] reported a statistically significant negative association between repetitive work and CTS. The specific exposure was self-reported repeated finger tapping; the investigators stated that they had difficulty interpreting this finding. All of the other statistically significant findings pointed to a positive association between repetitive work and CTS. The non-significant estimates of RR were also mostly greater than one.

Coherence of Evidence for Repetition

One of the most plausible ways that repetitive hand activities may be associated with CTS is thorough causing a substantial increase in the pressure in the carpal tunnel. This in turn can initiate a process which results in either reversible or irreversible damage to the median nerve [Rempel 1995]. The increase in pressure, if it is of sufficient duration and intensity, may reduce the flow of blood in the epineural venules. If prolonged, this reduction in flow may affect flow in the capillary circulation, resulting in greater vascular permeability and endoneural and synovial edema. Because of the structure of the median nerve and the carpal tunnel, this increase in fluid and resulting increase in pressure may persist for a long period of time. If the edema becomes chronic, then it may trigger a fibrosis which damages the function of the nerve. The interplay between acute increases in pressure and chronic changes to the nerve could partially explain why there is not a stronger correlation between symptoms of CTS and slowing of the median nerve. Both symptoms and slowing of the median nerve are likely to have both acute and chronic components in many cases of CTS.

The work determinants of pressure in the carpal tunnel are wrist posture and load on the tendons in the carpal tunnel. For example, the normal resting pressure in the carpal tunnel with the wrist in a neutral posture is about 5 millimeters of mercury (mmHg), and typing with the wrist in 45° of extension can result in an acute pressure of 60 mmHg. Substantial load on the fingertip with the wrist in a neutral posture can increase the pressure to 50 mmHg. A parabolic relationship between wrist posture and pressure in the carpal tunnel has been found. In laboratory studies of normal subjects, elevated carpal tunnel pressures quickly return to normal once the repetitive activity stops; patients with CTS take a long time for the pressure to return to their baseline values. One of the supporting observations for this model is that at surgery for CTS, edema and vascular sclerosis (fibrosis due to ischemia) are common [Rempel 1995].

This model of the etiology of work-related CTS is consistent with two observations from the epidemiological literature. First, it illustrates why both work and nonwork factors such as obesity may be important because anything that increases pressure in the carpal tunnel may contribute to CTS. Second, it explains why repetitiveness independent of wrist posture and load on the flexor tendons may not be a major risk factor for CTS.

Exposure-Response Relationship for Repetition

Evidence of an exposure-response relationship is provided by studies that show a correlation between the level or duration of exposure and either the number of cases, the illness severity, or the time to onset of the illness. Silverstein et al. [1987] showed an increasing prevalence of CTS signs and symptoms among industrial workers exposed to increasing levels of repetition and forceful exertion. This relationship was not seen when repetition alone was assessed. Similar findings on an exposure-response relationship were reported by Chiang et al. [1993], Osorio et al. [1994], Wieslander et al. [1989], and by Stock [1991] in her reanalysis of the Nathan et al. [1988]data.

Morgenstern et al. [1991] and Baron et al. [1991] reported increased prevalence of CTS with increasing length of time working as a grocery cashier.

Conclusions Regarding Repetition

Based on the epidemiologic studies noted above, especially those with quantitative evaluation of repetitive work, the strength of association for CTS and repetition has been shown to range from an OR of 2 to 15. The higher ORs are found when contrasting highly repetitive jobs to low repetitive jobs, and when repetition occurred in combination with high levels of forceful exertion. Those studies with certain epidemiologic limitations have also been fairly consistent in showing a relationship between repetition and CTS. The evidence from those studies which defined CTS based on symptoms, physical findings, and NCS is limited, due to the variety of methods used [Nathan et al. 1988; Stetson et al. 1993; Barnhart et al. 1991].

There is evidence of a positive association between highly repetitive work alone and CTS. There is strong evidence of a positive association between highly repetitive work in combination with other job factors and CTS, based on currently available epidemiologic data.

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Force and CTS

Definition of force for CTS

The studies reviewed in this section determined hand/wrist force exposure by a variety of methods. Some investigators [Armstrong and Chaffin 1979; Chiang et al. 1993; Silverstein et al. 1987] measured force by EMGs of representative workers’ forearm flexor muscles while they performed their usual tasks. EMG measurements were averaged within each work group to characterize the force requirements of the job; jobs were then divided into low or high categories if the average force was above or below a cutoff point. Moore and Garg [1994] estimated force as %MVC, 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 (for any upper extremity musculoskeletal disorder), based on exposure data and judgment. Stetson et al. [1993] estimated manipulation forces based on weights of tools and parts and systematically recorded observations of one or more workers on each job. Jobs were then ranked according to grip force cutoffs. Nathan et al. [1988, 1992a] and Osorio et al. [1994] estimated relative levels of force (e.g., low, moderate, high) after observation of job tasks. McCormack et al. [1990] grouped jobs into broad job categories based on similarity of observed job tasks; one job group (boarding) required forceful hand/wrist exertions. Baron et al. [1991] and Punnett et al. [1985] used job title as a surrogate for exposure to forceful hand/wrist exertions.

Much of the epidemiologic data on CTS and force overlaps with those studies discussed in the above section on repetition. Repetitive work is frequently performed in combination with external forces, and much of the epidemiologic literature has combined these two factors when determining association with CTS.

Studies Reporting on the Association of Force and CTS

Eleven studies reported results on the association between force and CTS. The epidemiologic studies that addressed forceful work and CTS tended to compare working groups by classifying them into broad categories based on estimates of the forcefulness of hand/wrist exertions in combination with estimated repetitiveness. In most studies the exposure classification was an ordinal rating (e.g., low, moderate, or high); in some studies job categories or titles were used as surrogates for exposure to force exertions.

Studies Meeting the Four Evaluation Criteria
Four studies that evaluated the relationship between forceful hand/wrist exertion and CTS met all four criteria: Chiang et al. [1993], Moore and Garg [1994], Osorio et al. [1994], Silverstein et al. [1987]. Chiang et al. [1993] studied 207 workers from 8 fish-processing factories in Taiwan. Jobs were divided into 3 groups based on levels of force and repetitiveness. The comparison group (low force/low repetitiveness) was managers, office staff, and skilled craftsmen. The fish-processing workers were divided into high force or high repetitiveness (group 2), and high force and high repetitiveness (group 3). Hand force requirements of jobs were estimated by electromyographs of forearm flexor muscles of a representative worker from each group performing usual job tasks. High force was defined as an average hand force of >3 kg repetition of the upper limb (not specifically the wrist) was defined based on observed cycle time [Silverstein et al. 1987]. 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 3 (8.2%, 15.3%, and 28.6%), a statistically significant trend p<0.01). Statistical modeling showed that women in this study group had a higher prevalence of CTS than men (OR 2.6, 95% CI 1.3–5.2). Force also significantly predicted CTS (OR 1.8, 95% CI 1.1–2.9), but not repetitiveness. Because the proportion of women varied by exposure group (48%, 75%, and 79% from groups 1 to 3), the possibility of an interaction between gender and job exposure exists, but this was not statistically examined. In an analysis limited to females, the 2 significant predictors of CTS were oral contraceptive use (OR 2.0, 95% CI 1.2–5.4), and force (OR 1.6, 95% CI 1.1–3.0). Concern over interpretation of these findings is raised because oral contraceptive use varies with age, and age may vary with job exposures. These potential interactions were not examined, and women’s ages by job group were not reported.

Moore and Garg [1994] evaluated 32 jobs in a pork processing plant and then reviewed past OSHA 200 logs and plant medical records for CTS cases in these job categories. IRs were calculated using the full-time equivalent (FTE) number of hours worked as reported on the logs. The exact number of workers was not reported. Exposure assessment included videotape analysis of job tasks for repetitiveness and awkward postures. The force measure was an estimate of the %MVC, 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 (for all Upper Extremity MSDs), based on exposure data and judgment. CTS was determined by reviewing OSHA 200 logs and plant medical records. The proportion of CTS in the overall study group during the 20 months of case ascertainment was 17.5 per 100 FTEs. If the occurrence of CTS did not vary over this period, the proportion of CTS in a 12-month period would be 10.5 per 100 FTEs. The hazardous jobs had a RR for CTS of 2.8 (0.2, 36.7) compared to the safe jobs. Potential for survivor effect (79% of the workforce was laid off the year before the study), limited latency period (8-32 months), and the potential for incomplete case ascertainment (underreporting is common on OSHA 200 logs, and logs were not reviewed for the first 12 months of the study) limit confidence in this estimate. One of the more hazardous jobs, the Ham Loaders, required extreme wrist, shoulder and elbow posture and was rated 4 on a 5-point scale for force, yet there was no observed morbidity. Since this job did not start until 1989, the period of observation for musculoskeletal disorders for this job was only 8 months. Other jobs studied allowed for up to a 32-month latency period. The possibility of differential case ascertainment between exposed and unexposed jobs exists, both because of different observation periods, as well as the likelihood that turnover may have been greater in the exposed jobs. It is also unclear whether employees worked full-time or part-time hours.

Osorio et al. [1994] studied 56 supermarket workers. Exposure to repetitive and forceful wrist motions was rated as high, moderate, or low, following observation of job tasks (97% initial concordance with 2 independent observers). The CTS case definition was based on symptoms and nerve conduction studies. CTS-like symptoms occurred more often (OR 8.3, 95% CI 2.6–26.4) among workers in the high exposure group compared to the low exposed group. The odds of meeting the symptom and NCS-based CTS case definition among the high exposure group were 6.7 (95% CI 0.8–52.9), compared to the low exposure group.

Silverstein et al. [1987] measured force by electromyographs of representative workers’ forearm flexor muscles while they performed their usual tasks. EMG measurements were averaged within each work group to characterize the force requirements of the job; jobs were then divided into high or low categories if the mean adjusted force was above or below 4 kg. Jobs were then classified into 4 groups that also accounted for repetitiveness: low force/low repetitiveness, high force/low repetitiveness, low force/high repetitiveness, and high force/high repetitiveness. Fourteen cases (2.1% prevalence) of CTS were diagnosed based on standardized physical examinations and structured interviews.

The OR for CTS in high force jobs compared to low force jobs, irrespective of repetitiveness, was 2.9 (p>0.05). The plant- adjusted OR for CTS in jobs with combined exposures to high force and high repetition was 14.3 (p<0.05), 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 identified. The OR for CTS in jobs with combined exposure from the multiple logistic analysis was 15.5 (95% CI 1.7–142.)

Studies Meeting at Least One Criterion
Baron et al. [1991] studied CTS in 124 grocery store checkers and 157 other grocery store workers who were not checkers. The CTS case definition required symptoms that met pre-determined criteria on a standardized questionnaire. Physical examinations were also performed, but participation rates at the work sites were higher among the exposed group (checkers: 85% participation, non-checkers: 55% participation). Telephone interviews to non-checkers resulted in questionnaire completion by 85% of the non-checkers. Based on a questionnaire case definition,the OR for CTS among checkers was 3.7 (95% CI 0.7–16.7), in a model that included age, hobbies, second jobs, systemic disease, and obesity.

McCormack et al. [1990] studied 1,579 textile production workers compared to 468 other nonoffice workers, a comparison group that included machine maintenance workers, transportation workers, cleaners, and sweepers. The textile production workers were divided into four broad job categories based on similarity of upper extremity exertions. The Boarding group required the most physical exertion. No formal exposure assessment was conducted. Health assessment included a questionnaire and screening physical examination followed by a diagnostic physical examination. CTS was diagnosed using predetermined clinical criteria. The severity of cases was also reported as mild, moderate or severe. The overall prevalence for CTS was 1.1%, with 0.7% in Boarding, 1.2% in Sewing, 0.9% in Knitting, 0.5% in Packaging/Folding, and 1.3% in the comparison group. None of the differences were statistically significant. A statistical model that also included age, gender, race, and years of employment showed that CTS occurred more often among women in this study (p<0.05). Interpretation of these data, especially with a low prevalence disorder like carpal tunnel syndrome, is difficult since gender varied with job (e.g., 94% of Boarding workers were female, compared to 56% in the comparison group), and the comparison group may have also been exposed to upper extremity exertions (machine maintenance workers, transportation workers, cleaners and sweepers). Interactions among potential confounders were not addressed, but they are suspected because of significant associations between race and three musculoskeletal disorders.

Nathan et al. [1988] studied median nerve conduction of 471 randomly selected workers from four industries (steel mill, meat/food packaging, electronics, and plastics manufacturing). Jobs were grouped into 5 relative levels of force (from very light to very high) after observation of job tasks. Jobs were also rated for repetitiveness (5 levels). Thirty-nine percent of the study subjects had impaired sensory conduction, or “slowing” of the median nerve. The 5 exposure groups were defined as follows: Group 1 is very low force, low repetition (VLF/LR); Group 2 is low force, very high repetition (LF/VHR); Group 3 is moderate force, moderate repetition (MF/MR); Group 4 is high force/moderate repetition (HF/MR); and Group 5 is very high force/high repetition (VHF/HR). The most logical comparisons to evaluate the effect of force would be Groups 3, 4, and 5 (moderate, high, and very high force) compared to Group 1 (low force). Group 2 jobs are not a good comparison because they are very highly repetitive, which may confound the comparisons. The authors reported a significantly higher number of subjects with median nerve slowing in Group 5 (VHF/HR) compared to Group 1 (VLF/LR), but not in other groups, using an uncommon statistical method (pairwise unplanned simultaneous test procedure [Sokal and Rohlf 1981]). The authors also reported that when individual hands were the basis of calculations rather than subjects, Group 3 had a significantly higher prevalence of median nerve slowing. Calculations of the more familiar PRs and chi-squares [Kleinbaum et al. 1982], using the published data, result in higher prevalences of median nerve slowing in each of Groups 3, 4, and 5, compared to Group 1 (PRs: 1.9, 95% CI 1.3–2.7; 1.7, 95% CI 1.1–2.5; and 2.0, 95% CI 1.1–3.4, respectively). A conservative adjustment (Bonferroni) of the significance level to 0.0125 for multiple comparisons [Kleinbaum et al. 1982] would result in Group 5 no longer being statistically significantly different from Group 1 (p=0.019), but Group 4 (p=0.009) and Group 3 (p=0.000) remain statistically significantly higher than Group 1 in prevalence of median nerve slowing.

In 1992 Nathan et al. [1992a] reported on a follow-up evaluation in the same study group. Sixty-seven per cent of the original study subjects were included. Hands (630), rather than subjects, were the basis of analysis in this study. Novice workers (those employed less than 2 years in 1984) were less likely to return than non-novice workers (56% compared to 69%, p=.004). Probable CTS was defined on the basis of symptoms reported during a structured interview and a positive Phalen’s or Tinel’s test. Maximum latency differences in median nerve sensory conduction were determined as in the 1984 study. The authors state that there was no significant difference in the prevalence of slowing between any of the exposure categories in 1989. However, calculations using common statistical methods show significantly higher prevalences of slowing in Group 4 (PR 1.4, 95% CI 0.9–2.1) compared to Group 1. Group 3's prevalence of slowing was 26% compared to Group 1's 18%, but this difference was not statistically significant (p=0.07). Group 5 had the same prevalence of slowing (18%) as Group 1 in 1989; the prevalence of slowing in Group 5 was 29% in 1984. The drop in prevalence of slowing in Group 5 between 1984 and 1989 might be explained by the higher drop-out rate among cases in Group 5 compared to Group 1 (PR 2.9, 95% CI 1.3–6.6). This was not addressed by the authors.

Punnett et al. [1985] compared the symptoms and physical findings of CTS in 162 women garment workers and 76 women hospital workers such as nurses, laboratory technicians, and laundry workers. Eighty-six percent of the garment workers were sewing machine operators and finishers (sewing and trimming by hand). The sewing machine operators were described as using highly repetitive, low force wrist and finger motions, whereas finishing work also involved shoulder and elbow motions. The exposed garment workers likely had more repetitive jobs than most of the hospital workers. CTS symptoms occurred more often among the garment workers (OR 2.7, 95% CI 1.2–7.6) compared to the hospital workers. There was a low participation rate (40%) among the hospital workers.

Stetson et al. [1993] conducted nerve conduction studies on 105 administrative and professional workers, and 240 automotive workers. Hand/wrist forces were estimated based on weights of tools and parts and systematically recorded observations of one or more workers on each job. Jobs were then ranked according to grip force cutoffs: <6 lb, >6 lb, >10 lb. Median nerve measures differed among the groups: index finger sensory amplitudes were lower and distal sensory latencies were longer among automotive workers in jobs requiring grip force >6 lb and >10 lb, compared to those requiring less than 6 lb (p<0.05 for all). At the wrist, median sensory amplitudes were also lower and distal median sensory latencies were also longer among the >6 lb, and the >10 lb exposure groups (p<0.05 for 3 of 4 differences). Age, height, and finger circumference were included in statistical models. The automotive workers were then divided into two groups, symptomatic (n=103) and asymptomatic (n=137), based on whether or not they met standard interview criteria for CTS symptoms. When comparisons were made to the administrative and professional workers, 15 of 16 measures of median and ulnar nerve function showed lower amplitudes and longer latencies (p<0.05) among the asymptomatic automotive workers; differences were greater between the symptomatic automotive workers and the white collar workers. The symptomatic automotive workers had lower amplitudes and longer latencies for 5 of 6 median sensory measures (p<0.05), compared to the asymptomatic automotive workers; there were no significant differences in ulnar nerve function between these two groups. Asymptomatic automotive workers had “healthier” median nerves than automotive workers with CTS symptoms, but there were no differences between these 2 groups in ulnar nerve function, suggesting that the case definition was specific for CTS.

Of the studies that addressed CTS, almost all examined occupations and jobs in which force was combined with another exposure factor (such as repetition or awkward postures). Chiang et al. [1993] estimated exposure to hand/wrist force independent of repetitiveness and found statistically significant RRs for CTS ranging from 1.6 to 1.8. Estimates of RR that were not statistically significant ranged from 0.4 to 6.7 [McCormack et al. 1990; Osorio et al. 1994]. Relative risk estimates for CTS among workers exposed to a combination of forceful and repetitive hand/wrist exertions ranged from 1.0 to 15.5 [Nathan et al. 1988, 1992a; Silverstein et al. 1987].

Study limitations may impact the interpretation of findings. One limitation to consider is gender effect. Of the studies listed above reporting statistically significant associations between forceful hand/wrist exertions and CTS, gender effect was controlled for in the analyses. Other potential limitations such as selection factors impact the interpretation of the studies reviewed. Survivor bias can be a concern. If workers with CTS are more likely to leave jobs that require forceful and repetitive hand/wrist exertions than jobs without those demands, then the workers in the highest risk jobs may be “survivors” (those who did not get CTS). Our analysis of Nathan’s [1992a] data from a follow up of industrial workers shows that cases (with median nerve slowing) were more likely to drop out of the most highly exposed group than the unexposed group, which might explain why the RR for high exposure decreased from 2.0 to 1.0 over a 5-year period. Survivor bias results in an underestimate of the RR.

Refined or exact measures of exposure to forceful hand/wrist exertions are not always used in epidemiologic studies (e.g., sometimes exposure is based on job category and not actual forceful measurements); this can result in some study subjects being assigned to the wrong exposure category. When this occurs, the usual effect is again to underestimate the RR between exposure groups.

Stetson et al. [1993] did not report RR estimates for exposure variables, but they reported that median sensory amplitudes were significantly smaller and distal sensory latencies were significantly longer in groups with forceful hand exertions (p<0.05). Age, height, and finger circumference were included in statistical models.

Temporality, Force and CTS

Temporal issues can usually best be addressed using longitudinal studies. However, study limitations, such as survivor bias, can cloud the findings of even prospective studies. In our re-analysis of Nathan et al.’s [1992a] data, 2 of 3 groups exposed to forceful hand/wrist exertions were more likely to have median nerve slowing when nerve conduction testing was repeated 5 years later. The highest exposure group had the same prevalence of slowing as the lowest exposure group in 1989, whereas there had been a higher prevalence rate in 1984. As discussed above, this apparent decrease in prevalence over 5 years can likely be explained by survivor bias. Our interpretations of the data differ from those of the author. Further study is needed to clarify these issues. To our knowledge, there is no evidence that workers with preexisting CTS are more likely to seek or to be employed in jobs with high force requirements. We believe that employment practices would, if they had any influence, tend to exclude new hires with CTS from jobs with high force requirements for the hand/wrist.

Case definitions in most of the cross sectional studies excluded cases that occurred before working on the current job. This limits CTS cases studied to those that occurred following current exposure. Several of the studies reviewed also required a minimum time period of working on the job before counting CTS cases. This increases the likelihood that exposure to forceful hand/wrist exertion occurred for a sufficient length of time to develop CTS.

There is evidence that CTS is also attributable to nonwork causes (hobbies, sports, other medical conditions, and hormonal status in women, etc.). One issue which deals with temporality is whether those with nonwork related CTS would be more likely to be hired into jobs requiring more forceful hand/wrist exertions than those without CTS. Again, it seems unlikely that those with preexisting CTS would be preferentially hired into jobs requiring highly forceful hand/wrist exertions.

Consistency of Association for Force and CTS

Most of the statistically significant estimates of RR for CTS among workers with exposure to forceful hand/wrist exertions were positive. No studies found statistically significant negative associations between forceful hand/wrist exertions and CTS. One study reported ORs that were less than one among the groups that were described as exposed to repetitive hand movements; chance and study limitations cannot be ruled out as possible explanations for this finding. The other non significant estimates of RR were, with one exception, greater than one.

Statistical significance can be a function of power (the ability of a study to detect an association when one does exist). In general, larger studies are necessary in order to have sufficient power to detect associations with rare diseases. CTS is a less frequently observed disorder than tendinitis, for example, and so larger studies are required to detect associations with confidence.

Coherence of Evidence, Force and CTS

Please refer to the Repetition and CTS Section.

ExposureResponse Relationship, Force and CTS

None of the studies reviewed demonstrated that increasing levels of force alone resulted in increased risk for CTS. The only evidence for an increasing risk for CTS that can be attributed to increasing levels of force alone is from a comparison across 2 studies that used the same methods. Chiang et al. [1993] and Silverstein et al. [1987] used the same methods to measure hand/wrist force requirements and repetitiveness of jobs. Chiang et al. [1993] used a lower cutoff point (3 kg compared to 4 kg) in Silverstein et al.’s [1987] study for classifying jobs as “high force”; these investigators used identical definitions of repetitiveness. Therefore, a comparison of the RR estimates between the 2 studies provides some information about the level of risk associated with different levels of force. Chiang et al. [1993] reported an OR of 2.6 (95% CI 1.0–7.3) for the high force and repetitive (HF/HR) (>3 kg) group (limited to females to avoid confounding) compared to the low force and repetitive (LF/LR) group; whereas Silverstein et al. [1987] reported an OR of 15.5 (95% CI 1.7–142) for the HF/HR group (in a statistical model that included gender, age, years on the job, plant and exposure level) compared to the LF/LR group. This comparison provides limited evidence of an increased RR for CTS with increasing level of hand/wrist force.

There is more evidence of a dose response relationship for CTS with increasing levels of force and repetition combined. Chiang et al. [1993] reported a statistically significant trend of increasing prevalence of CTS with increasing exposure level (8.2% [LF/LR], 15.3% [HF or HR], and 28.6% [HF/HR], p<0.01). Silverstein et al. [1987] suggested a multiplicative effect when exposure to high force and high repetitiveness were combined (15.5), compared to high force (1.8) or high repetitiveness (2.7) alone.

Of the remaining nine studies, seven are consistent with the combined effect of force and repetition [Stetson et al. 1993; Moore and Garg 1994; Osorio et al. 1994; Armstrong and Chaffin 1979; Nathan et al. 1988; Punnett et al. 1985; Baron et al. 1991], one is not [McCormack et al. 1990]; and one is equivocal [Nathan et al. 1992a].

In conclusion, there is evidence that force alone is associated with CTS. There is strong evidence that a combination of forceful hand/wrist exertion and repetitiveness are associated with CTS.

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Posture and CTS

Definition of Extreme Postures For CTS

We selected those studies which addressed posture of the hand/wrist area including those addressing pinch grip, ulnar deviation, wrist flexion/extension. Posture is a difficult variable to examine in ergonomic epidemiologic studies. It is hypothesized that extreme or awkward postures increase the required force necessary to complete a task. Posture may increase or decrease forceful effort; its impact on MSDs may not be accurately reflected in measurement of posture alone. Reasons that the variable “extreme posture” has not been measured or analyzed in many epidemiologic studies are: 1) because of the extreme variability of postures used in different jobs as well as the extreme variability of postures between workers performing the same job tasks, 2) because several studies have taken into account the effects of posture when determining other measured variables such as force [Silverstein et al. 1987; Moore and Garg 1994]; and 3) stature often has a major impact on postures assumed by individual workers during job activities.

Studies Meeting the Four Evaluation Criteria
Two studies fulfilled the four criteria for posture and CTS: Moore and Garg [1994], Silverstein et al. [1987]. The overall study designs are mentioned above; the following section will cover the posture assessment.

For the exposure assessment of the posture variables in the Silverstein et al. [1987] study, three representative workers from each selected job performing the jobs for at least three cycles were videotaped using two cameras. The authors then extrapolated the posture data to non-observed workers.

Moore and Garg [1994] used a wrist classification system similar to that used by Stetson et al. [1993], classifying the wrist angle estimated from videotape as neutral, non-neutral or extreme if the flexion/extension angle was 0° to 25°, 25° to 45° and greater than 45°, respectively; or if ulnar deviation was less than 10°, 10° to 20°, and greater than 20°, respectively.

Strength of Association: Posture and CTS

Silverstein found no significant association between percentages of cycle time observed in extreme wrist postures or pinch grip and CTS. “CTS jobs” had slightly more ulnar deviation and pinching but these differences were not statistically significant. The authors noted that among all the postural variables recorded, the variability between individuals with similar or identical jobs was probably the greatest for wrist postural variables. This individual variation within jobs was not taken into account in the analysis, creating a potential for misclassification of individuals by using the variable “job category” in the analysis. The effect of exposure misclassification is usually to decrease differences between exposure groups and decrease the magnitude of association.

Moore and Garg’s [1994] classification of jobs did not separate the posture variables from other work factors, and used posture along with other variables to classify jobs into “hazardous” and “safe” categories. The RR of CTS occurring in hazardous jobs was 2.8 but not statistically significant (p=0.44).

Studies Not Meeting All Four Evaluation Criteria
deKrom et al. [1990] compared certain exposure factors between 28 CTS cases from a community sample and 128 CTS cases from a hospital (a total of 156 CTS cases) to 473 community “non-cases” (n=473). The authors relied on self-reported information about duration of exposure (hours per week) to CTS risk factors (flexed wrist, extended wrist, extended and flexed wrists combined; pinch grasp and typing), with respondents recalling exposure from the present to 5 years prior from the questionnaire date. Four groups of duration were used in the analyses (0; 1–7; 896-19, 20–40 hours/week). In this study, the selection process of cases was not consistent. Initially, a random population sample was used, then hospital outpatients were used to supplement the number of CTS cases when numbers were found to be insufficient. This may be a problem when estimating the etiologic role of workload, as cases seeking medical care may cause a referral bias. However, the authors stated that they came up with the same relationship between flexed and extended wrist using only CTS cases from the population-based data. The risk of CTS was found to increase with the reported duration of activities with flexed wrist (RRs from 1.5 to 8.7, with increasing hours) or activities with extended wrist (RR from 1.4 to 5.4 with increasing hours) over the past 5 years, but not for working with a flexed or extended wrist in combination,or working with a pinched grasp. Given the period of recall for self-reported exposure (0–5 years), and no independent observation or attributes of exposure, these results must be interpreted with caution (meaning that within the limitations of the data and conclusions, when considered with other studies that have more stringent methods, the RRs seem consistent and supportive and do not offer alternate conclusions).

Armstrong and Chaffin’s [1979] pilot study of female sewing machine operators with symptoms and/or signs for CTS compared to controls found that pinch force exertion (exposure measurements estimated from EMG, film analysis) was significantly associated (OR 2.0). Pinch force was a combination of factors—posture and forceful exertion. The authors reported that CTS-diagnosed subjects used deviated wrist postures more frequently than non diseased, particularly during forceful exertions. What is unable to be answered due to the study design, was whether the deviated postures were necessitated due to symptoms and signs of CTS, or the deviated postures caused or exacerbated the symptoms and signs.

Stetson et al. [1993] found that “gripping greater than 6 pounds” per hand was a significant risk factor for median distal sensory dysfunction (an indicator of CTS) when the study population was divided into exposed and non-exposed groups. “Gripping greater than 6 pounds” is a variable which combines two work-related variables, posture and forceful exertion. As seen with other studies referenced above, the single work-related variable was not found to be associated with median nerve dysfunction, but the combination of variables was significant. Looking specifically at wrist deviation in the Stetson et al. [1993] study, the mid palm to wrist sensory amplitude was smaller in the group not exposed to wrist deviation (p=0.04) compared to those exposed to wrist deviation (contrary to what was expected). Also, no significant differences were found in the mean measurements between non exposed and exposed groups for use of pinch grip.

Tanaka et al. [1995] analysis of the Occupational Health Supplement of the NHIS population survey depended on self-reported CTS, self-reported exposure factors, and occupation of the respondent for analysis. Self-reported bending and twisting of the hand and wrist (OR 5.9) was found to be the strongest variable associated with “medically-called CTS” among recent workers, followed by race, gender, vibration and age (repetition and force were not included in the logistic models). Limitations of self-reported health outcome and exposure do not allow the conclusions of this study to stand alone; however, when examined with the other studies, it suggests a relationship between posture and CTS.

The two other studies which examined posture and its relationship to CTS did not focus on the hand and wrist. English et al. [1995] found a relationship between self-reported rotation of the shoulder and elevated arm and CTS, an OR of 1.8. Liss et al. [1995] found an OR of 3.7 for self-reported CTS comparing risk factors from dental hygienists to dental assistants, with self-reported percent of time the trunk was in a rotated position relative to the lower body as one of the factors.

Given these limitations of categorizing posture, three studies [Stetson et al. 1993; Loslever and Ranaivosoa 1993; Armstrong and Chaffin 1979] using different methods to measure posture and estimate force, found that the combination of significant force and posture was significantly related to CTS. Marras and Shoenmarklin [1993] also found posture to be significantly associated with CTS when comparing jobs where grip strength was three times greater than in the low risk jobs. In those studies which used self-reports for categorizing posture, the associations were also positive.

Temporal Relationship

There were no longitudinal studies which examined the relationship between extreme posture and CTS. Two cross-sectional studies that met the evaluation criteria addressed the association between posture and CTS. Silverstein et al. [1987] did not find a significant relationship between CTS and extreme posture, but exposure assessment was limited to representative workers; inter-individual variability limited the ability to identify actual relationships between postures and CTS. In the Stetson et al. [1993] study, the authors mentioned the limitations of interpretation of their posture results due to misclassification of workers. They extrapolated exposure data to non-observed workers, so individual variability in work methods and differing anthropometry are not accounted for. These limitations all influence outcome, and the conclusions must be interpreted with caution, and considered along with biomechanical and laboratory studies.

Coherence of Evidence

Flexed wrist postures may reduce the area of the carpal tunnel thus potentially increasing the pressure in the tunnel with a concomitant increase in the risk of CTS [Skie et al. 1990; Armstrong et al. 1991]. Marras and Shoenmarklin [1993] found that the variables of wrist flexion, extension, angular velocity, and wrist flexion, extension, angular acceleration discriminated between jobs with a high versus a low risk of having an upper extremity reportable injury (an OSHA recordable disorder due to repetitive trauma). The authors suggested that this result was due to high accelerations requiring high forces in tendons. Szabo and Chidgey [1989] showed that repetitive flexion and extension of the wrist created elevated pressures in the carpal tunnel compared to normal subjects, and that these pressures took longer to dissipate than in normal subjects. Observed repetitive passive flexion and extension appeared to “pump up” the carpal tunnel pressure; active motion of the wrist and fingers also had an effect over and above that of the passive motions tested. Laboratory studies demonstrate that carpal canal pressure is increased from less than 5mmHg to more than 30 mmHg during wrist flexion and extension [Gelberman et al. 1981].

ExposureResponse Relationship, CTS and Posture

Few studies address exposure-response relationship between CTS and extreme posture. deKrom et al. [1990] reported an increased risk of CTS with workers reporting increasing weekly hours of exposure to wrist flexion or extension (but not a combination of flexion/extension). Laboratory studies also support a dose-response relationship of increased carpal tunnel pressure due to increasing wrist deviation from neutral [Weiss et al. 1995] and pinch force [Rempel 1995].

In conclusion, there is insufficient evidence in the current epidemiologic literature to demonstrate that awkward postures alone are associated with CTS.

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Vibration and CTS

Definition of Vibration for CTS

We selected studies that addressed manual work involving vibrating power tools and CTS specifically.

Studies Meeting the Four Evaluation Criteria
Two studies examining the association between vibration and CTS fulfilled the four criteria [Chatterjee 1992; Silverstein et al. 1987]. Chatterjee et al. [1982] performed independent exposure assessment of the vibrating tools, and found the rock drillers to be exposed to vibration between the frequencies of 31.5 and 62 Hertz.

Silverstein et al. [1987] is discussed above. Silverstein [1987] had no quantitative measures of vibration, but observed exposure from videotapes and found all jobs with vibration exposure to be highly repetitive and mostly forceful jobs.

Studies Not Meeting the Evaluation Criteria
There are seven studies on Table 5a–4 that meet at least one of the four criteria.

In addition, there are 2 clinical case studies of vibration and CTS [Rothfleish and Sherman 1978; Lukas 1970] that were not controlled for confounders and not referenced in Table 5a–4. Rothfleisch and Sherman [1978] found an excess of power hand tool users among CTS patients. Lucas [1970] examined workers using vibrating hand tools including stone cutters, tunnelers, coal miners, forest workers and grinders (all with a mean of 14 years exposure to vibration) and found CTS in 21%. He found that the prevalence of CTS in some groups was as high as 33% (neither study had a referent group.)

Cannon et al. [1981] found that the self-reported use of vibrating tools, in combination with reported forceful and repetitive hand motions, was associated with a greater incidence of CTS than was repetitive motion alone.

Bovenzi’s study in 1994 compared stone workers (145 quarry drillers and 425 stone carvers) exposed to hand-transmitted vibration to 258 polishers and machine operators who performed manual activity only not exposed to hand-transmitted vibration. CTS was assessed by a physician, and exposure was assessed through direct observation to vibrating tools and by interview. Vibration was also measured in a sample of tools.

Strength of Association: Vibration and CTS

Chatterjee et al. [1982] found a significant difference between rock drillers with symptoms and signs of CTS and the controls using the following NCS measurements: median motor latency, median sensory latency, median sensory amplitude, and median sensory duration, all at the p<0.05 level. Based on nerve conduction measurements, they also found an OR of 10.9 for rock drillers having abnormal NCS amplitudes in the median and ulnar nerves compared to controls. Bovenzi et al. [1991] found an OR of 21.3 for CTS based on symptoms and physical exam comparing vibration-exposed forestry operators using chain-saws to maintenance workers performing manual tasks. Bovenzi’s study in 1994 found an OR of 0.43 for CTS defined by signs and symptoms, controlling for several confounders. In the Silverstein et al. [1987] study the crude OR for high force/high repetition jobs with vibration compared to high force/high repetition without vibration was 1.9, but not statistically significant. This suggested that there may have been confounding (the OR was not statistically significant) between high force/high repetition and vibration. Nilsson et al. [1990] found that platers operating tools such as grinders and chipping hammers had a CTS prevalence of 14% compared to 1.7% among office workers. Nathan et al. [1988] found a PR of 2.0 (95% CI 1.3–3.4) for slowing of nerve conduction velocity when grinders were compared to administrative and clerical workers. Cannon et al. [1981] found an OR of 7.0 for CTS with the use of vibrating hand tools, although there was a strong potential for confounding by hand or wrist posture and forceful exertion.

Temporal Relationship

There were no longitudinal studies which examined the relationship between vibration and CTS.

Consistency in Association

All studies on Table 5a–4 examining vibration and CTS found a significantly positive relationship between CTS and vibration exposure. Most studies had ORs greater than 3.0, so that results were less likely to be due to confounding.

Coherence of Evidence and Vibration

The mechanism by which vibration contributes to CTS and tendinitis development is not well understood, probably because vibration exposure is usually accompanied by exposure to forceful and repetitive movements. Muscles exposed to vibration exhibit a tonic vibration reflex that leads to increasing involuntary muscle contraction. Vibration has also been shown to produce short-term tactility impairments which can lead to an increase in the amount of force exerted during manipulative tasks. Vibration can also lead to mechanical abrasion of tendon sheaths. Neurological and circulatory disturbances probably occur

independently by unrelated mechanisms. Vibration may directly injure the peripheral nerves, nerve endings, and mechanoreceptors, producing symptoms of numbness, tingling, pain, and loss of sensitivity. It has been found in rats that vibration has caused epineural edema in the sciatic nerve [Lundborg et al. 1987]. Vibration may also have direct effects on the digital arteries. The innermost layer of cells in the blood vessel walls appears especially susceptible to mechanical injury by vibration. If damaged, these vessels may become less sensitive to the actions of certain vasodilators that require an intact endothelium. The NIOSH Criteria Document on exposure to hand-arm vibration NIOSH [1989] quoted Taylor [1982] as follows: “ It is not known whether vibration directly injures the peripheral nerves thereby causing numbness and subsequent sensory loss, or whether the para-anaesthesia of the hands is secondary to the vascular constriction of the blood vessels causing ischemia . . . in the nerve organs.”

Exposure-Response Relationship, CTS and Vibration

In the studies examined, only dichotomous categorizations were made, so conclusions concerning an exposure-response relationship cannot be drawn. However, we can see significantly contrasting rates of CTS between high and low exposure groups. Wieslander et al. [1989] found that based on exposure information obtained from telephone interviews, CTS surgery was significantly associated with vibration exposure. Exposure for 1–20 years gave an OR of 2.7, more than 20 years gave an OR of 4.8.

Conclusion

In conclusion, there is evidence supporting an association between exposure to vibration and CTS.

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Confounding and CTS

It is clear that CTS has several non-occupational causes. When examining the relationship of occupational factors to CTS, it is important to take into account the effects of these individual factors; that is, to control for their confounding or modifying effects. Studies that fail to control for the influence of individual factors may either mask or amplify the effects of work-related factors. Most of the epidemiologic studies of CTS that address work factors also take into account potential confounders.

Almost all of the studies reviewed controlled for the effects of age in their analysis [Chiang et al. 1990, 1993; Stetson et al. 1993; Silverstein et al. 1987; Wieslander et al. 1989; Baron et al. 1991; Tanaka et al. 1995, In Press; McCormack et al. 1990]. Likewise, most studies included gender in their analysis, either by stratifying [Schottland et al. 1991; Chiang et al. 1993], by selection of single gender study groups [Morganstern et al. 1991; Punnett et al. 1985] or by including the variable in the logistic regression model [Silverstein et al. 1987; Stetson et al. 1991; Baron et al. 1991]. Through selection of the study population and exclusion of those with metabolic diseases, most studies were able to eliminate the effects from these conditions. Other studies did control for systemic disease [Chiang et al. 1993; Baron et al. 1991]. Anthropometric factors have also been addressed in several studies [Stetson et al. 1993; Nathan et al. 1997; 1992b; Werner et al. 1997]. As more is learned about confounding, more variables tend to be addressed in more recent studies (smoking, caffeine, alcohol, hobbies). In those older studies which may not have controlled for multiple confounders, it is unlikely that they are highly correlated with exposure, especially those with ORs above 3.0. When examining those studies that have good exposure assessment, widely contrasting levels of exposure, and that control for multiple confounders, the evidence supports a positive association between occupational factors and CTS.

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Conclusions

There are over 30 epidemiologic studies which have examined workplace factors and their relationship to CTS. These studies generally compared workers in jobs with higher levels of exposure to workers with lower levels of exposure, following observation or measurement of job characteristics. Using epidemiologic criteria to examine these studies, and taking into account issues of confounding, bias, and strengths and limitations of the studies, we conclude the following:

There is evidence for a positive association between highly repetitive work and CTS. Studies that based exposure assessment on quantitative or semi quantitative data tended to show a stronger relationship for CTS and repetition. The higher estimates of RR were found when contrasting highly repetitive jobs to low repetitive jobs, and when repetition is in combination with high levels of forceful exertion. There is evidence for a positive association between force and CTS based on currently available epidemiologic data. There is insufficient evidence for a positive association between posture and CTS. There is evidence for a positive association between jobs with exposure to vibration and CTS. There is strong evidence for a relationship between exposure to a combination of risk factors (e.g., force and repetition, force and posture) and CTS. Ten studies allowed a comparison of the effect of individual versus combined work risk factors [Chiang et al. 1990, 1993; Moore and Garg 1994; Nathan et al. 1988, 1992a; Silverstein et al. 1987; Schottland et al. 1991; McCormack et al. 1990; Stetson et al. 1993; Tanaka et al. [In Press]. Nine of these studies demonstrated higher estimates of RR when exposure was to a combination of risk factors,compared to the effect of individual risk factors. Based on the epidemiologic studies reviewed above, especially those with quantitative evaluation of the risk factors, the evidence is clear that exposure to a combination of job factors studied (repetition, force, posture, etc.) increases the risk for CTS. This is consistent with the evidence that is found in the biomechanical, physiologic, and psychosocial literature.

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