We determined on histologic examination the degree of degeneration at the insertion of 3 rotator cuff tendons in 76 cadaveric shoulders, 17 of which had a partial tear of the supraspinatus. Fiber thinning, the presence of granulation tissue, and incomplete tearing of fibers, all evidence of degeneration, were quantified separately for each tendon. Among the shoulders that were intact on macroscopy, no significant difference in degeneration score could be found. In all 3 tendons degeneration was more prominent on the articular sides compared with the bursal sides (P < .0001). The degeneration score of partially torn supraspinatus was significantly higher than that of the intact tendons (P < .0001). The extent of granulation tissue, 1 criterion of degeneration, seemed to contribute mostly to this difference. Intrinsic degeneration occurred foremost in the articular side of the rotator cuff and might constitute the primary cause of rotator cuff tearing.
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