Development of Fatty Atrophy After Neurologic and Rotator Cuff Injuries in an Animal Model of Rotator Cuff Pathology

Background: Detachment of a tendon from its osseous insertion, as can be the case with severe rotator cuff injuries, leads to atrophy of and increased fat in the corresponding muscle. We sought to validate a rotator cuff injury model in the rabbit and to test the hypothesis that tenotomy of a rotator cuff tendon would consistently create muscle atrophy and fatty degeneration analogous to the changes that occur after injury to a nerve innervating the same muscle. Methods: New Zealand white rabbits were divided into three groups: (1) partial rotator cuff tear without retraction of the muscle, (2) complete rotator cuff tear with retraction of the muscle, and (3) nerve transection of the subscapular nerve. Animals were killed at two or six weeks after injury, and the muscles were analyzed for weight, cross-sectional area, myosin fiber-type composition, and fat content. In addition, the subscapular nerve was harvested at two weeks and evaluated for neuronal injury. Results: At six weeks after injury, the rabbit muscles in the complete tenotomy and nerve transection groups had significant decreases in wet mass and increases in fat content relative to the control groups. Fat accumulation had a similar spatial pattern at six weeks in both the nerve transection and complete tenotomy groups. Such changes were not seen in the partial tenotomy group. No change was found in muscle myosin fiber-type composition. At two weeks after injury, subscapular nerves in the complete tenotomy group showed gross evidence of neuronal injury. Conclusions: This study establishes the rabbit subscapularis muscle as a valid model to study the muscular changes associated with rotator cuff tears. Our data suggest that the muscular changes associated with complete tenotomy are comparable with those seen with denervation of the muscle and suggest that chronic rotator cuff tears may induce a neurologic injury. Clinical Relevance: Chronic rotator cuff injuries are associated with neuronal injury of the affected muscle. As such, neuronal injury may explain the histopathologic changes that have been observed following chronic rotator cuff tears.