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Atrophy of the rotator cuff muscles is a factor that complicates the treatment of a massive rotator cuff tear (RCT). However, the molecular mechanisms that govern the development of muscle atrophy after RCTs have not been well defined. The Akt/mammalian target of rapamycin (mTOR) signaling pathway plays a central role in maintaining muscle mass in response to mechanical loading. The role of this pathway in the development of muscle atrophy after a massive RCT remains unknown. The purpose of this study was to investigate the regulation of the Akt/mTOR pathway in the development of muscle atrophy after a RCT and suprascapular nerve (SSN) injury. We evaluated the activity of the Akt/mTOR signaling pathway and how this pathway interacts with two atrophy-related genes, MuRF-1 and MAFbx, in supraspinatus muscles of rats that underwent unilateral complete rotator cuff tendon transection or SSN transection. Akt/mTOR activity was significantly reduced after tendon rupture, but increased after nerve injury. MuRF-1 and MAFbx were only up-regulated following denervation. These results suggest that tendon transection leads to a decrease in protein synthesis with down-regulation of the Akt/mTOR signaling pathway, whereas denervation leads to an increase in protein degradation via upregulation of expression of MuRF-1 and MAFbx. ß

Rotator cuff tears represent a large burden of muscle-tendon injuries in our aging population. While small tears can be repaired surgically with good outcomes, critical size tears are marked by muscle atrophy, fibrosis, and fatty infiltration, which can lead to failed repair, frequent re-injury, and chronic disability. Previous animal studies have indicated that Transforming Growth Factor-β (TGF-β) signaling may play an important role in the development of these muscle pathologies after injury. Here, we demonstrated that inhibition of TGF-β1 signaling with the small molecule inhibitor SB431542 in a mouse model of massive rotator cuff tear results in decreased fibrosis, fatty infiltration, and muscle weight loss. These observed phenotypic changes were accompanied by decreased fibrotic, adipogenic, and atrophy-related gene expression in the injured muscle of mice treated with SB431542. We further demonstrated that treatment with SB431542 reduces the number of fibro/adipogenic progenitor (FAP) cells—an important cellular origin of rotator cuff muscle fibrosis and fatty infiltration, in injured muscle by promoting apoptosis of FAPs. Together, these data indicate that the TGF-β pathway is a critical regulator of the degenerative muscle changes seen after massive rotator cuff tears. TGF-β promotes rotator cuff muscle fibrosis and fatty infiltration by preventing FAP apoptosis. TGF-β regulated FAP apoptosis may serve as an important target pathway in the future development of novel therapeutics to improve muscle outcomes following rotator cuff tear.

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Background Muscle atrophy, fatty infiltration, and fibrosis of the muscle have been described as important factors governing outcome after rotator cuff injury and repair. Muscle fibrosis is also thought to have a role in determining muscle compliance at the time of surgery. The transforming growth factor-β (TGF-β) pathways are highly conserved pathways that exert a potent level of control over muscle gene expression and are critical regulators of fibrosis in multiple organ systems. It has been shown that TGF-β can regulate important pathways of muscle atrophy, including the Akt/mammalian target of rapamycin pathway. The purpose of this study was to evaluate the expression of TGF-β and its downstream effectors of fibrosis after a massive rotator cuff tear (RCT) in a previously established rat model. Methods To simulate a massive RCT, infraspinatus and supraspinatus tenotomy and suprascapular nerve transection were performed on Sprague-Dawley rats with use of a validated model. Two and 6 weeks after surgery, supraspinatus muscles were harvested to study alterations in TGF-β signaling by Western blotting, quantitative polymerase chain reaction, and histologic analysis. Results There was a significant increase in fibrosis in the rotator cuff muscle after RCT in our animal model. There was a concomitant increase in TGF-β gene and protein expression at both 2 and 6 weeks after RCT. Evaluation of the TGF-β signaling pathway revealed an increase in SMAD2 activation but not in SMAD3. There was an increase in profibrotic markers collagen I, collagen III, and α-smooth muscle actin. Conclusions TGF-β signaling is significantly upregulated in rat supraspinatus muscles after RCTs.

Peri-tendinous injection of local anaesthetic, both alone and in combination with corticosteroids, is commonly performed in the treatment of tendinopathies. Previous studies have shown that local anaesthetics and corticosteroids are chondrotoxic, but their effect on tenocytes remains unknown. We compared the effects of lidocaine and ropivacaine, alone or combined with dexamethasone, on the viability of cultured bovine tenocytes. Tenocytes were exposed to ten different conditions: 1) normal saline; 2) 1% lidocaine; 3) 2% lidocaine; 4) 0.2% ropivacaine; 5) 0.5% ropivacaine; 6) dexamethasone (dex); 7) 1% lidocaine+dex; 8) 2% lidocaine+dex; 9) 0.2% ropivacaine+dex; and 10) 0.5% ropivacaine+dex, for 30 minutes. After a 24-hour recovery period, the viability of the tenocytes was quantified using the CellTiter-Glo viability assay and fluorescence-activated cell sorting (FACS) for live/dead cell counts. A 30-minute exposure to lidocaine alone was significantly toxic to the tenocytes in a dose-dependent manner, but a 30-minute exposure to ropivacaine or dexamethasone alone was not significantly toxic. Dexamethasone potentiated ropivacaine tenocyte toxicity at higher doses of ropivacaine, but did not potentiate lidocaine tenocyte toxicity. As seen in other cell types, lidocaine has a dose-dependent toxicity to tenocytes but ropivacaine is not significantly toxic. Although dexamethasone alone is not toxic, its combination with 0.5% ropivacaine significantly increased its toxicity to tenocytes. These findings might be relevant to clinical practice and warrant further investigation.

Rotator cuff tears (RCTs) are among the most common musculoskeletal injuries seen by orthopaedic surgeons. Clinically, massive cuff tears lead to unique pathophysiological changes in rotator cuff muscle, including atrophy, and massive fatty infiltration, which are rarely seen in other skeletal muscles. Studies in a rodent model for RCT have demonstrated that these histologic findings are accompanied by activation of the Akt/mammalian target of rapamycin (mTOR) and transforming growth factor-b (TGF-b) pathways following combined tendon-nerve injury. The purpose of this study was to compare the histologic and molecular features of rotator cuff muscle and gastrocnemius muscle-a major hindlimb muscle, following combined tendon-nerve injury. Six weeks after injury, the rat gastrocnemius did not exhibit notable fatty infiltration compared to the rotator cuff. Likewise, the adipogenic markers SREBP-1 and PPARg as well as the TGF-b canonical pathway were upregulated in the rotator cuff, but not the gastrocnemius. Our study suggests that the rat rotator cuff and hindlimb muscles differ significantly in their response to a combined tendon-nerve injury. Clinically, these findings highlight the unique response of the rotator cuff to injury, and may begin to explain the poor outcomes of massive RCTs compared to other muscle-tendon injuries. ß

Introduction Rotator cuff tears are the most common injury seen by shoulder surgeons. Many late stage rotator cuff tear patients develop glenohumeral osteoarthritis as a result of torn cuff tendons, termed cuff tear arthropathy. However, the mechanisms of cuff tear arthropathy have not been fully established. It has been hypothesized that a combination of synovial and mechanical factors contribute equally to the development of cuff tear arthropathy. The goal of this study was to assess the utility of this model in investigating cuff-tear arthropathy. Methods We utilized a rat model which accurately reflects rotator cuff muscle degradation after massive rotator cuff tears through either infraspinatus and supraspinatus tenotomy or suprascapular nerve transection. Using a Modified-Mankin Scoring System (MMS), we found significant glenohumeral cartilage damage following both rotator cuff tenotomy and suprascapular nerve transection after only 12 weeks. Results Cartilage degeneration was similar between groups, and was present on both the humeral head and the glenoid. Denervation of the supraspinatus and infraspinatus muscles without opening the joint capsule caused cartilage degeneration similar to that found in the tendon transection group. Conclusions These results suggest that altered mechanical loading after rotator cuff tears is the primary factor in cartilage degeneration after rotator cuff tears. Clinically, understanding the process of cartilage degeneration after rotator cuff injury will help guide treatment decisions in the setting of rotator cuff tears. Level of evidence Basic Science Study, Animal Model