Intra-articular changes precede extra-articular changes in the biceps tendon after rotator cuff tears in a rat model.

HYPOTHESIS: Biceps tendon pathology is common with rotator cuff tears. The mechanisms for biceps changes, and therefore its optimal treatment, are unknown. Our objective was to determine the effect of rotator cuff tears on regional biceps tendon pathology. We hypothesized that histologic and compositional changes would appear before organizational changes, both would appear before mechanical changes, and changes would begin at the tendon's insertion site. MATERIALS AND METHODS: Detachment of supraspinatus and infraspinatus tendons or sham surgery was done in 65 Sprague-Dawley rats. Rats were euthanized at 1, 4, or 8 weeks for regional measurements of histologic, compositional, organizational (1, 4 and 8 weeks), or mechanical properties (4 and 8 weeks only). RESULTS: One week after tendon detachments, decreased organization and more rounded cell shape were found in the intra-articular space of the biceps tendon. Aggrecan expression was increased along the entire length of the tendon, whereas all other compositional changes were only at the tendon's proximal insertion into bone. With time, this disorganization and more rounded cell shape extended the length of the tendon. Organizational and cell shape changes also preceded detrimental mechanical changes: decreased modulus in the intra-articular space was found after 8 weeks. CONCLUSIONS: Results support a degenerative component to pathology in the biceps tendon. In addition, changes resembling a tendon exposed to compressive loading occurring first in the intra-articular space indicate that the biceps tendon plays an increased role as a load-bearing structure against the humeral head in the presence of rotator cuff tears.