Dominique Laron

Muscle degeneration in rotator cuff tears

Rotator cuff tears are among the most common injuries seen by orthopedic surgeons. Although small- and medium-sized tears do well after arthroscopic and open repair, large and massive tears have been shown to develop marked muscle atrophy and fatty infiltration within the rotator cuff muscles. These pathologic changes have been found to be independent predictors of failed surgical repair with poor functional outcomes. To understand the pathophysiology of rotator cuff disease, we must first develop an understanding of the changes that occur within the cuff muscles themselves. The purpose of this review is to summarize the molecular pathways behind muscular degeneration and emphasize new findings related to the clinical relevance of muscle atrophy and fatty infiltration seen with rotator cuff tears. Understanding these molecular pathways will help guide further research and treatment options that can aim to alter expression of these pathways and improve outcomes after surgical repair of massive rotator cuff tears.

A Mouse Model of Massive Rotator Cuff Tears

BACKGROUND Rotator cuff tears are the most common tendon injury seen in orthopaedic patients. Muscle atrophy and fatty infiltration in rotator cuff muscles are considered among the key factors responsible for the failure of attempted repair of a massive rotator cuff tear. However, the pathophysiology of rotator cuff muscle atrophy and fatty infiltration remains largely unknown, partly because of the lack of appropriate small animal models. The goal of this study was to develop a mouse model of muscle atrophy and fatty infiltration after a rotator cuff tear. We also sought to study the role of denervation on muscle atrophy and fatty infiltration after a rotator cuff tear. METHODS Adult wild-type FVB/N mice were randomly divided into three groups. Mice in different groups received unilateral complete supraspinatus and infraspinatus tendon transection, suprascapular nerve transection, or both procedures. Sham surgery was performed on the contralateral shoulder to serve as a control. Mice were killed twelve weeks after surgery. Histological analysis and high-resolution magnetic resonance imaging were used to evaluate muscle atrophy and fat infiltration after a rotator cuff tear. RESULTS Significant and consistent muscle atrophy and fatty infiltration were observed in the rotator cuff muscles after rotator cuff tendon transection. We further found that denervation significantly increases the amount of muscle atrophy and fatty infiltration after a rotator cuff tear. CONCLUSIONS We successfully developed a novel mouse model of a massive rotator cuff tear, which simulates major pathological changes, including muscle atrophy and fatty infiltration after massive rotator cuff tears seen in patients.

Evaluation of Akt/mTOR activity in muscle atrophy after rotator cuff tears in a rat model.

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 up‐regulation of expression of MuRF‐1 and MAFbx.

MRI quantification of fatty infiltration and muscle atrophy in a mouse model of rotator cuff tears

Rotator cuff pathology is the most common shoulder problem seen by orthopedic surgeons. Rotator cuff muscle fatty infiltration and muscle atrophy are common in larger tears and are considered predicting factors for the prognosis of cuff repair. Clinically, MRI is the gold standard in determining fatty infiltration and muscle atrophy; however, analysis for MRI imaging is primarily qualitative in nature with the results lacking further validation. We have recently developed a mouse model of rotator cuff tears. The goal of this study is to quantify and verify rotator cuff muscle atrophy and fatty infiltration using high‐resolution MRI in our mouse model. The rotator cuff muscles were analyzed for fat using a triglyceride quantification assay (TQA), muscle volume was measured through water displacement (WD), and histology. The study revealed that MRI had a high correlation with fat as measured with histology and TQA (R2 = 098). MRI also correlated well with atrophy measured with WD and wet weight. This suggests that MRI is a reliable modality in evaluating the progression of fatty infiltration and muscle atrophy following rotator cuff tears in a small animal model.

A comparison of lidocaine, ropivacaine and dexamethasone toxicity on bovine tenocytes in culture

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.

TGF-β Small Molecule Inhibitor SB431542 Reduces Rotator Cuff Muscle Fibrosis and Fatty Infiltration By Promoting Fibro/Adipogenic Progenitor Apoptosis

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.

Rat rotator cuff muscle responds differently from hindlimb muscle to a combined tendon-nerve injury

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‐β (TGF‐β) 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 PPARγ as well as the TGF‐β 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.

Upregulation of transforming growth factor-β signaling in a rat model of rotator cuff tears.

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.

Advances in the Orthopedic Management of Osteogenesis Imperfecta

The long-established study of osteogenesis imperfecta (OI) has opened a realm of scientific research surrounding connective tissue disorders. Over the past decade alone there have been vast advancements in the understanding of the underlying genetic variations of this disease, pharmacologic treatments, and the technological and surgical options for fracture deformity. It is important to appreciate the progressive nature of the advances concerning OI. This article aims to synthesize the expanding evolution of the field surrounding OI over the past decade.

Role of pulsed electromagnetic fields (PEMF) on tenocytes and myoblasts—potential application for treating rotator cuff tears

The post‐surgery integrity of the tendons and muscle quality are the two major factors in success of rotator cuff (RC) repair. Though surgical techniques for rotator cuff repair have significantly improved in the past two decades, there are no effective treatments to improve tendon‐to‐bone healing and muscle quality after repair at this point in time. Pulsed electromagnetic fields (PEMF) have previously been used for promoting fracture healing. Previous studies have shown that PEMF has a positive role in promoting osteoblast precursors proliferation and differentiation. However, PEMFs effect on tenocytes and muscle cells has not been determined fully yet. The purpose of this study is to define the role of a commercially available PEMF on tenocytes and myoblasts growth and differentiation in vitro. Human rotator cuff tenocytes and C2C12 murine myoblasts were cultured and treated with PEMF for 2 weeks under regular and inflammatory conditions. Our results showed that 2 weeks treatment of PEMF enhanced gene expressions of growth factors in human rotator cuff tenocytes under inflammatory conditions. PEMF significantly enhanced C2C12 myotube formation under normal and inflammatory conditions. Results from this study suggest that PEMF has a positive role in promoting tenocyte gene expression and myoblast differentiation. Therefore, PEMF may potentially serve as a non‐operative treatment to improve clinical incomes rotator cuff tendon repairs. Results