Muscle Degeneration Associated With Rotator Cuff Tendon Release and/or Denervation in Sheep

Background: The effect of an additional neurological injury (suprascapular nerve traction injury) to a chronically retracted rotator cuff muscle is incompletely understood and warrants clarification. Purpose: To investigate the microscopic and macroscopic muscle degeneration patterns caused by tendon release and/or muscle denervation in a sheep rotator cuff model. Study Design: Controlled laboratory study. Methods: Infraspinatus muscle biopsy specimens (for histological analysis) were obtained from 18 Swiss alpine sheep before and 16 weeks after release of the infraspinatus tendon (tenotomy [T] group; n = 6), transection of the suprascapular nerve (neurectomy [N] group; n = 6), or tendon release plus nerve transection (tenotomy + neurectomy [T&N] group; n = 6). Magnetic resonance imaging (MRI) and computed tomography (CT) were used to assess retraction (CT), muscle density (CT), volume (MRI T2), and fat fraction (MRI Dixon). Stiffness of the infraspinatus was measured with a spring scale. Results: At 16 weeks postoperatively, the mean infraspinatus muscle volume had decreased significantly more after neurectomy (to 47% ± 7% of the original volume; P = .001) and tenotomy plus neurectomy (48% ± 13%; P = .005) than after tenotomy alone (78% ± 11%). Conversely, the mean amount of intramuscular fat (CT/MRI Dixon) was not significantly different in the 3 groups (T group: 50% ± 9%; N group: 40% ± 11%; T&N group: 46% ± 10%) after 16 weeks. The mean myotendinous retraction (CT) was not significantly different in the T and T&N groups (5.8 ± 1.0 cm and 6.4 ± 0.4 cm, respectively; P = .26). Stiffness was, however, most increased after additional neurectomy. In contrast to muscle changes after tendon release, denervation of the muscle led to a decrease in the pennation angle of lengthened muscle fibers, with a reduced mean cross-sectional area of pooled muscle fibers, a slow- to fast-type transformation, and an increase in the area percentage of hybrid fibers, leading to overall significantly greater atrophy of the corresponding muscle. Conclusion: Although it is unclear which experimental group (T or T&N) most accurately reflects the clinical scenario in a given case, these findings provide baseline information for clinical differentiation between muscle changes caused by denervation or rotator cuff tendon lesions. Clinical Relevance: The findings of this study help to understand how and to which extent a neurological lesion of the supplying suprascapular nerve could influence the pattern of anatomic-physiological muscular changes after rotator cuff tendon tears.