The effect of tear size and nerve injury on rotator cuff muscle fatty degeneration in a rodent animal model

Background: Irreversible muscle changes after rotator cuff tears is a well-known negative prognostic factor after shoulder surgery. Currently, little is known about the pathomechanism of fatty degeneration of the rotator cuff muscles after chronic cuff tears. The purposes of this study were to (1) develop a rodent animal model of chronic rotator cuff tears that can reproduce fatty degeneration of the cuff muscles seen clinically, (2) describe the effects of tear size and concomitant nerve injury on muscle degeneration, and (3) evaluate the changes in gene expression of relevant myogenic and adipogenic factors after rotator cuff tears using the animal model. Materials and methods: Rotator cuff tears were created in rodents with and without transection of the suprascapular nerve. The supraspinatus and infraspinatus muscles were examined at 2, 8, and 16 weeks after injury for histologic evidence of fatty degeneration and expression of myogenic and adipogenic genes. Results: Histologic analysis revealed adipocytes, intramuscular fat globules, and intramyocellular fat droplets in the tenotomized and neurotomized supraspinatus and infraspinatus muscles. Changes increased with time and were most severe in the muscles with combined tenotomy and neurotomy. Adipogenic and myogenic transcription factors and markers were upregulated in muscles treated with tenotomy or tenotomy combined with neurotomy compared with normal muscles. Conclusions: The rodent animal model described in this study produces fatty degeneration of the rotator cuff muscles similar to human muscles after chronic cuff tears. The severity of changes was associated with tear size and concomitant nerve injury.