Studying Edema Formation After Release of the Infraspinatus Muscle as an Experimental Model of Rotator Cuff Lesions in Sheep: A Histological Analysis

Background: Muscle edema formation and inflammatory processes are early manifestations of acute rotator cuff lesions in sheep. Histological analysis of affected muscles revealed edema formation, inflammatory changes, and muscle tissue disruption in MRs. Hypothesis: Edema contributes to inflammatory reactions and early muscle fiber degeneration before the onset of fatty infiltration. Study Design: Controlled laboratory study. Methods: Osteotomy of the greater tuberosity, including the insertion of the infraspinatus tendon, was performed on 14 sheep. These experimental animal models were divided into 2 groups: a nontrauma group with surgical muscle release alone (7 sheep) and a trauma group with standardized application of additional trauma to the musculotendinous unit (7 sheep). Excisional biopsy specimens of the infraspinatus muscle were taken at 0, 3, and 4 weeks. Results: Edema formation was histologically demonstrated in both groups and peaked at 3 weeks. At 3 weeks, signs of muscle fiber degeneration were observed. At 4 weeks, ingrowth of loose alveolar and fibrotic tissue between fibers was detected. Fatty tissue was absent. The diameter of muscle fibers increased in both groups, albeit to a lesser degree in the trauma group, and practically normalized at 4 weeks. Immunohistology revealed an increase in macrophage types 1 and 2, as well as inflammatory mediators such as prostaglandin E2 and nuclear factor kappa-light-chain-enhancer of activated B cells. Conclusion: Early muscle edema and concomitant inflammation precede muscle fiber degeneration and fibrosis. Edema formation results from tendon release alone and is only slightly intensified by additional trauma. Clinical Relevance: This study illustrates that early edema formation and inflammation elicit muscle fiber degeneration that precedes fatty infiltration. Should this phenomenon be applicable to human traumatic rotator cuff tears, then surgery should be performed as soon as possible, ideally within the first 21 days after injury.