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hand or wrist motion was 2.4 percent compared with 2.7 percent for nonrepetitive motion (95 percent CI for the difference, −2 percent, −1.5 percent; P = .69).6 Reports from workers and survey data tend to overestimate the prevalence of a disorder because social, cultural, and medicolegal factors have a major influence. In 1988 the National Health Interview Survey showed that 1.4 percent (1.87 million) of working adults in the United States reported that they had a “condition affecting the wrist and hand called carpal tunnel syndrome.” Only 675,000 indicated that a health care provider had made this diagnosis.66 Carpal tunnel syndrome must be distinguished from the vast array of upper extremity musculoskeletal complaints collectively called repetitive motion disorders. Many of these conditions are the product of somatization, the reporting of somatic symptoms that have no pathophysiological explanation, amplified by medicalization, whereby uncomfortable bodily states and isolated symptoms are reclassified as diseases for which medical treatment is sought.8,9
The association of carpal tunnel syndrome with work-related risk factors is a recurring theme of causation among workers, ergonomists, lawyers, and physicians. The majority of the literature that tries to establish this as a causal association fails to meet the appropriate standards of epidemiological validity.63,68 To conclude that carpal tunnel syndrome is a repetitive motion disorder, one must ask the question, “How significant a risk factor is repetition for the development of carpal tunnel syndrome?” To answer this question, one must consider the interaction of job exposures (extrinsic risk factors) with various innate anatomic, physiological, or behavioral characteristics of the worker (intrinsic risk factors) that render him or her more likely to develop the disorder. Occupational risk factors alone do not explain its occurrence; rather, it is the culmination of many distinct converging causal links. The majority of cases are likely due to intrinsic risk factors. One investigation concluding that carpal tunnel syndrome is closely correlated with health habits and life-style49 is supported by an analysis showing that 81.52 percent of the explainable variation in electrophysiologically defined carpal tunnel syndrome was due to body mass index, age, and wrist depth/width ratio, whereas only 8.29 percent was due to job-related factors.24 There may be important interactions between extrinsic and intrinsic risk factors that are yet to be understood. While there is a biologically plausible mechanism to relate forceful grip to compression of the median nerve,64,65 there is no such correlate to postulate biological plausibility with regard to repetition.
In reviewing the published literature on work-related repetitive hand injuries, Hagberg et al. estimated the attributable fraction by (OR − 1)/OR (where OR was the estimated odds ratio) and concluded that exposure to physical workload factors, such as repetitive and forceful gripping, is probably a major risk factor for at least 50 percent, and as much as 90