FIG. 1.

The effect of chronic, massive tears on the elements of the rotator cuff organ. The native enthesis (A) contains a complex structure that is not restored following surgical repair (B); instead of a gradient in mineral content and interpositional fibrocartilage, the repaired tendon abruptly adjoins bone. The black arrow indicates the suture hole. As shown with micro-computed tomography, the humeral head of uninjured or acutely repaired rotator cuff tears (C) exhibit superior bone quality (e.g., bone mineral density) to the humeral heads of chronically torn rotator cuff tendons (D). Adapted with permission from Killian et al.⁷⁵ Native muscle fibers are polygonal with peripheral nuclei (E) whereas degenerated muscle in the context of chronic tears undergoes atrophy, fibrosis, and fatty infiltration (F). Lastly, uninjured tendon consists of fibroblasts (arrow heads) elongated in the direction of aligned collagen fibrils (G). With chronic degeneration, collagen fibrils become disorganized and delaminated (arrow heads), with concurrent heterotopic cartilage formation (H). Black arrows indicate chondrocyte-like cells. Adapted with permission from Buck et al.²⁴⁴