Histological analysis and biomechanical evaluation of fatty infiltration after rotator cuff tear and suprascapular nerve injury in a rat model.

Background: Rotator cuff tears are the most common shoulder injury, and little is known about the underlying etiology of fatty infiltration after rotator cuff tear. Previous studies speculated that rotator cuff tears lead to neural injury due to tethering of the suprascapular nerve at the notch. This study aimed to evaluate fatty infiltration after suprascapular nerve injury and rotator cuff tears.

Methods: Ninety adult male Sprague-Dawley rats were used and were divided into four groups: sham, tendon transection only, suprascapular nerve ligation, and tendon transection plus suprascapular nerve ligation. The suprascapular nerve injury models were created by tying the suprascapular nerve. At 2, 4, and 8 weeks postoperatively, histological analysis and biomechanical testing were performed to evaluate fatty infiltration and elastic change in the supraspinatus muscles.

Results: The amount of fatty infiltration in the supraspinatus muscle was significantly higher in both the suprascapular nerve ligation and tendon transection plus suprascapular nerve ligation groups than in the tendon transection only group at 2, 4, and 8 weeks. The ultimate failure load and tensile strength were significantly different among the tendon transection only, suprascapular nerve ligation, and tendon transection plus suprascapular nerve ligation groups at 8 weeks postoperatively. Furthermore, the mean Young's modulus of the muscle was significantly greater in the tendon transection plus suprascapular nerve ligation group than in both the tendon transection only and suprascapular nerve ligation groups at 8 weeks postoperatively.

Conclusions: In this study, based on the results of histological and biomechanical examinations in our rat models, the etiology of fatty infiltration after massive rotator cuff tear might be different from the suprascapular nerve injury.