Structural changes of the rotator cuff caused by experimental subacromial impingement in the rat.

Subacromial impingement of the infraspinatus tendon was experimentally created in 28 young adult rats by thickening the undersurface of the acromion with either one or two platelike bony transplants of the ipsilateral scapular spine. Nine nonoperated and eight shoulders that had undergone a sham operation served as control groups. The rats were killed after 2 days and after 1, 2, 4, 8, 16, and 32 weeks for histologic evaluation. All rats with experimental subacromial impingement showed an infraspinatus tear on the bursal side of the tendon. An isolated tear on the articular side or within the tendon was not seen. Two plates caused larger tears than one (P = .04), and more long-standing impingement was associated with larger lesions (P = .002). Multiple chondrocytes were observed within the tendon adjacent to the bony transplants. No calcium deposits were found. In the subacromial space rapid thickening of the bursa was observed. The undersurfaces of the bony transplants showed no evidence of abrasion or remodeling caused by the tendon. The shoulders of the control groups were found intact without any alteration. Experimental subacromial impingement in the rat caused bursal side rotator cuff tears. The type of partial tears that are most frequently observed in clinical practice, that is, intratendinous and articular side tears, were not seen in this experimental model.