Functional morphology of the supraspinatus tendon.

Grossly normal supraspinatus tendons were analyzed by stereomicroscope dissection and three-dimensional serial-section reconstruction. Four structurally independent subunits were identified: the tendon proper extended from the musculotendinous junction to approximately 2.0 cm medial to the greater tuberosity. It was composed of parallel collagen fascicles oriented along the tensional axis and separated by a prominent endotenon region. There was no interdigitation of fascicles, and an 18% incidence of fascicle convergence as the fascicles coursed from muscle toward greater tuberosity. The attachment fibrocartilage extended from the tendon proper to the greater tuberosity, consisted of a complex basket-weave of collagen fibers, and stained diffusely with alcian blue. The densely packed unidirectional collagen fibers of the rotator cable extended from the coracohumeral (CH) ligament posteriorly to the infraspinatus, coursing both superficial and deep to the tendon proper. The capsule was composed of thin collagen sheets each with uniform fiber alignment that differed slightly between sheets. These data describe a specialized tendon capable of internally compensating for changing joint angles through fascicles which are structurally independent and can slide past one another. The tendon attachment exhibits a structure adapted to tensional load dispersion and resistance to compression.