Study Design: A randomized controlled prospective experimental study with some repeated measures. Objectives: To characterize behavioral, sensory, motor, and nerve conduction decrements, and histological changes in the median nerve in rats trained to perform a high-force repetitive task. Background: Understanding of work-related carpal tunnel syndrome is hampered by the lack of experimental studies of the causes and mechanisms of nerve compression induced by repetitive motion. Most animal models of nerve compression have not employed voluntary repetitive motion as the stimulus for pathophysiological changes. Methods and Measures: Thirty Sprague-Dawley rats served as controls for 1 or more studies. Ten rats were trained to pull on a bar with 60% maximum force 4 times per minute, 2 h/d, 3 d/wk for 12 weeks. Motor behavior and limb withdrawal threshold force were characterized weekly. Grip strength and median nerve conduction velocity were measured after 12 weeks. Median nerves were examined immunohistochemically for ED1-positive macrophages, collagen, and connective tissue growth factor. Results: Reach rate and duration of task performance declined over 12 weeks. Grip strength and nerve conduction velocity were significantly lower after 12 weeks than in controls. Limb withdrawal threshold increased between weeks 6 and 12. Median nerves at the level of the wrist showed increases in macrophages, collagen, and connective-tissue growth-factor-positive cells. These effects were seen in both the reach and nonreach limbs. Conclusions: This animal model exhibits all the features of human carpal tunnel syndrome, including impaired sensation, motor weakness, and decreased median nerve conduction velocity. It establishes a causal relationship between performance of a repetitive task and development of carpal tunnel syndrome.

J Orthop Sports Phys Ther. 2004; 34(5):244-253. doi:10.2519/jospt.2004.1520

Key Words: cumulative trauma disorder, median nerve, nerve compression, repetitive motion injury, work-related musculoskeletal disorder

The purpose of this commentary is to present recent epidemiological findings regarding work-related musculoskeletal disorders (WMSDs) of the hand and wrist, and to summarize experimental evidence of underlying tissue pathophysiology and sensorimotor changes in WMSDs. Sixty-five percent of the 333,800 newly reported cases of occupational illness in 2001 were attributed to repeated trauma. WMSDs of the hand and wrist are associated with the longest absences from work and are, therefore, associated with greater lost productivity and wages than those of other anatomical regions. Selected epidemiological studies of hand/wrist WMSDs published since 1998 are reviewed and summarized. Results from selected animal studies concerning underlying tissue pathophysiology in response to repetitive movement or tissue loading are reviewed and summarized. To the extent possible, corroborating evidence in human studies for various tissue pathomechanisms suggested in animal models is presented. Repetitive, hand-intensive movements, alone or in combination with other physical, nonphysical, and nonoccupational risk factors, contribute to the development of hand/wrist WMSDs. Possible pathophysiological mechanisms of tissue injury include inflammation followed by repair and/or fibrotic scarring, peripheral nerve injury, and central nervous system reorganization. Clinicians should consider all of these pathomechanisms when examining and treating patients with hand/wrist WMSDs.

J Orthop Sports Phys Ther. 2004;34(10):610-627.  doi:10.2519/jospt.2004.1399

Key Words: carpal tunnel syndrome, hand/wrist tendinitis, inflammation, neuroplasticity, repetitive-motion injury