Objective. To evaluate histologic, immunohistochemical, and molecular changes in tendon induced by altered strain in a large-animal model.Methods. A full-thickness partial-width laceration of the infraspinatus tendon was created in 5 sheep, while 5 sham-operated sheep were used as controls. Sheep were killed after 4 weeks, and 4 differentially stressed tendon regions (tensile or near bone attachment from overstressed or stress-deprived halves) were evaluated for histopathology, proteoglycan (PG) accumulation, and characterization of glycosaminoglycans and aggrecan catabolites. Gene expression of matrix components, enzymes, and inhibitors was analyzed by reverse transcriptase-polymerase chain reaction.Results. Histopathologic changes were detected in both overstressed and stress-deprived tensile tendon, but only in stress-deprived tendon near bone. In overstressed and stress-deprived tensile tendon, levels of keratan sulfate, chondroitin 4-sulfate, and chondroitin 6-sulfate were increased. In overstressed tensile tendon, levels of ADAMTS-generated aggrecan catabolites were increased. There was increased matrix metalloproteinase 13 (MMP-13) and decreased fibromodulin and decorin expression in all regions. Increased MMP-1, MMP-9, MMP-14, and ADAMTS-1 expression, and decreased type II collagen expression were restricted to stress-deprived tendon. In stress-deprived boneattachment regions, messenger RNA (mRNA) for aggrecan was decreased, and ADAMTS was increased. In overstressed tensile tendon, aggrecan mRNA was increased, and ADAMTS was decreased.Conclusion. The distinct molecular changes in adjacent tissue implicate altered strain rather than humoral factors in controlling abnormal tenocyte metabolism, and highlight the importance of regional sampling. Tendon abnormalities induced by increased strain are accompanied by increased aggrecan, decreased ADAMTS, and low PG expression, which may negatively impact the structural integrity of the tissue and predispose to rupture.Injuries of the shoulder due to overuse are increasingly prevalent in both the sporting arena and the workplace. Occupational injury of the shoulder is second only to neck and low back pain in the frequency of consultation by general practitioners, with the most common injury involving the rotator cuff. (1,2) The prevalence of rotator cuff conditions increases with age, from 30-50% in the seventh decade of life to a staggering 70-80% by the ninth (3,4). Although most cases are treated conservatively, many patients undergo surgical repair with a prolonged convalescence. There are no drugs to specifically treat disorders of the rotator cuff or other tendons, and many surgical repairs fail within 12 months (5). Lack of knowledge of the fundamental mechanisms that underlie tendon breakdown results in limited and inadequate management options for tendon disorders.
