The Independent Effect of Nerve Injury and Tendon Injury on Rotator Cuff Fatty Infiltration

Background: Muscular fatty infiltration (FI), atrophy, and fibrosis commonly occur after chronic rotator cuff tear and are barriers to successful rotator cuff repair. It is unclear if these phenomena result from tendon injury alone, neurological injury, or a combination. Purpose: To determine the role of suprascapular nerve mediated FI with and without chronic rotator cuff tear in a rabbit model. Study Design: Controlled laboratory study. Methods: A total of 22 rabbits underwent bilateral shoulder procedures. Extremities were randomized to surgical procedures: suprascapular neurotomy, supraspinatus/infraspinatus tenotomy, neurotomy and tenotomy combination, or control. After 6 or 12 weeks, shoulder girdles underwent computed tomography (CT) imaging to measure muscle cross-sectional area and muscle density. Histological analysis was performed to assess FI extent, location, and neuromuscular junction morphology. Results: By 6 weeks, all groups showed supraspinatus atrophy and decreased muscle density on CT imaging and increasing adipocytes and fibrosis on histology. Only the isolated neurotomy group showed increasing FI between 6 and 12 weeks ( P < .05). The tenotomy/neurotomy group did not show a statistically significant additional FI at 12 weeks on CT imaging. The majority of adipocytes in all groups localized to the perifascicular space, but tenotomy groups showed more adipocytes along the central tendon. The tenotomy group tended to exhibit concentrated FI near the myotendinous junction compared with a diffuse pattern in neurogenic mediated FI. Compared with the control group, there was no change in the neuromuscular junction of tenotomized rotator cuff tendons. Conclusion: Tenotomy and suprascapular neurotomy are both independently associated with an increased quantity of FI and muscle fibrosis. Although the quantities of FI after isolated tenotomy and neurotomy are similar, the overall pattern of FI is different. Interventions involving tendon transection/retraction provoked the highest percentage of peripennate FI. Results of neuromuscular junction analysis suggest that a subclinical neurological injury is not a requirement for FI. Clinical Relevance: This study supports the notion that subclinical neurological injury is not required for FI and fibrosis to occur. Differences in the pattern of FI were noted in the setting of tenotomy and neurotomy; this knowledge should influence future experimental design when translational animal models for rotator cuff injury are utilized.