Upper extremity tendinopathies are associated with performance of forceful repetitive tasks. We used our rat model of repetitive strain injury to study changes induced in forelimb flexor digitorum tendons. Rats were trained to perform a high repetition high force (HRHF) handle-pulling task (12 reaches/min at 60 AE 5% maximum pulling force [MPF]), or a low repetition negligible force (LRNF) reaching and food retrieval task (three reaches/min at 5 AE 5% MPF), for 2 h/day in 30 min sessions, 3 days/week for 3-12 weeks. Forelimb grip strength was tested. Flexor digitorum tendons were examined at midtendon at the level of the carpal tunnel for interleukin (IL)-1b, neutrophil, and macrophage influx, Substance P, connective tissue growth factor (CTGF), and periostin-like factor (PLF) immunoexpression, and histopathological changes. In HRHF rats, grip strength progressively decreased, while IL-1b levels progressively increased in the flexor digitorum peritendon (para-and epitendon combined) and endotendon with task performance. Macrophage invasion was evident in week 6 and 12 HRHF peritendon but not endotendon. Also in HRHF rats, Substance P immunoexpression increased in week 12 peritendon as did CTGF-and PLF-immunopositive fibroblasts, the increased fibroblasts contributing greatly to peritendon thickening. Endotendon collagen disorganization was evident in week 12 HRHF tendons. LRNF tendons did not differ from controls, even at 12 weeks. Thus, we observed exposure-dependent changes in flexor digitorum tendons within the carpal tunnel, including increased inflammation, nociceptor-related neuropeptide immunoexpression, and fibrotic histopathology, changes associated with grip strength decline. Keywords: cytokines; inflammation; flexor digitorum tendon; repetitive task; PLF; CTGF; WMSD Tendinopathies of the hand and wrist tendons are associated with forceful repetition in the workplace.
1-3The incidence of flexor tenosynovitis is significantly higher in strenuous meat processing jobs: 25.3% for female packers, 16.8% for female sausage makers, and 12.5% for male meat cutters. The incidence in nonstrenuous jobs was less than 1% during a 31 month study period.4,5 Manufacturing workers performing highly repetitive and forceful jobs are 29 times more likely to develop wrist and hand tendonitis than workers performing low repetition and low force jobs.
5,6The etiology and pathophysiology of overuse-induced tendinopathies are still under investigation. Although the presence of an inflammatory component has not been identified by all investigators, 7-9 increased inflammatory molecules, for example, PGE 2 , have been found in tenosynovium of patients diagnosed with carpal tunnel syndrome (CTS), especially during the intermediate phase.10,11 However, PGE 2 was not found in tendon biopsies collected during the chronic painful tendinosis stage, although increased glutamate neurotransmitter and its receptor were evident. 8,9 The neurochemical Substance P is associated with chronic pain mediation 12 and has also been identif...
