Muscle gene expression patterns in human rotator cuff pathology

Electroconductivity, a regenerative engineering approach to reverse rotator cuff muscle degeneration

Muscle degeneration is one the main factors that lead to the high rate of retear after a successful repair of rotator cuff (RC) tears. The current surgical practices have failed to treat patients with chronic massive rotator cuff tears (RCTs). Therefore, regenerative engineering approaches are being studied to address the challenges. Recent studies showed the promising outcomes of electroactive materials (EAMs) on the regeneration of electrically excitable tissues such as skeletal muscle. Here, we review the most important biological mechanism of RC muscle degeneration. Further, the review covers the recent studies on EAMs for muscle regeneration including RC muscle. Finally, we will discuss the future direction toward the application of EAMs for the augmentation of RCTs.

Transcriptional time course after rotator cuff repair in 6 month old female rabbits

Introduction: Rotator cuff tears are prevalent in the population above the age of 60. The disease progression leads to muscle atrophy, fibrosis, and fatty infiltration, which is not improved upon with surgical repair, highlighting the need to better understand the underlying biology impairing more favorable outcomes. Methods: In this study, we collected supraspinatus muscle tissue from 6 month old female rabbits who had undergone unilateral tenotomy for 8 weeks at 1, 2, 4, or 8 weeks post-repair (n = 4/group). RNA sequencing and enrichment analyses were performed to identify a transcriptional timeline of rotator cuff muscle adaptations and related morphological sequelae. Results: There were differentially expressed (DE) genes at 1 (819 up/210 down), 2 (776/120), and 4 (63/27) weeks post-repair, with none at 8 week post-repair. Of the time points with DE genes, there were 1092 unique DE genes and 442 shared genes, highlighting that there are changing processes in the muscle at each time point. Broadly, 1-week post-repair differentially expressed genes were significantly enriched in pathways of metabolism and energetic activity, binding, and regulation. Many were also significantly enriched at 2 weeks, with the addition of NIF/NF-kappaB signaling, transcription in response to hypoxia, and mRNA stability alongside many additional pathways. There was also a shift in transcriptional activity at 4 weeks post-repair with significantly enriched pathways for lipids, hormones, apoptosis, and cytokine activity, despite an overall decrease in the number of differentially expressed genes. At 8 weeks post-repair there were no DE genes when compared to control. These transcriptional profiles were correlated with the histological findings of increased fat, degeneration, and fibrosis. Specifically, correlated gene sets were enriched for fatty acid metabolism, TGF-B-related, and other pathways. Discussion: This study identifies the timeline of transcriptional changes in muscle after RC repair, which by itself, does not induce a growth/regenerative response as desired. Instead, it is predominately related to metabolism/energetics changes at 1 week post-repair, unclear or asynchronous transcriptional diversity at 2 weeks post-repair, increased adipogenesis at 4 weeks post-repair, and a low transcriptional steady state or a dysregulated stress response at 8 weeks post-repair.

Transcriptional Time Course After Rotator Cuff Tear

Rotator cuff (RC) tears are prevalent in the population above the age of 60. The disease progression leads to muscle atrophy, fibrosis, and fatty infiltration in the chronic state, which is not improved with intervention or surgical repair. This highlights the need to better understand the underlying dysfunction in muscle after RC tendon tear. Contemporary studies aimed at understanding muscle pathobiology after RC tear have considered transcriptional data in mice, rats and sheep models at 2–3 time points (1 to 16 weeks post injury). However, none of these studies observed a transition or resurgence of gene expression after the initial acute time points. In this study, we collected rabbit supraspinatus muscle tissue with high temporal resolution (1, 2, 4, 8, and 16 weeks) post-tenotomy (n = 6/group), to determine if unique, time-dependent transcriptional changes occur. RNA sequencing and analyses were performed to identify a transcriptional timeline of RC muscle changes and related morphological sequelae. At 1-week post-tenotomy, the greatest number of differentially expressed genes was observed (1,069 up/873 down) which decreases through 2 (170/133), 4 (86/41), and 8 weeks (16/18), followed by a resurgence and transition of expression at 16 weeks (1,421/293), a behavior which previously has not been captured or reported. Broadly, 1-week post-tenotomy is an acute time point with expected immune system responses, catabolism, and changes in energy metabolism, which continues into 2 weeks with less intensity and greater contribution from mitochondrial effects. Expression shifts at 4 weeks post-tenotomy to fatty acid oxidation, lipolysis, and general upregulation of adipogenesis related genes. The effects of previous weeks’ transcriptional dysfunction present themselves at 8 weeks post-tenotomy with enriched DNA damage binding, aggresome activity, extracellular matrix-receptor changes, and significant expression of genes known to induce apoptosis. At 16 weeks post-tenotomy, there is a range of enriched pathways including extracellular matrix constituent binding, mitophagy, neuronal activity, immune response, and more, highlighting the chaotic nature of this time point and possibility of a chronic classification. Transcriptional activity correlated significantly with histological changes and were enriched for biologically relevant pathways such as lipid metabolism. These data provide platform for understanding the biological mechanisms of chronic muscle degeneration after RC tears.

The eccentric mechanotransduction, neuro-muscular transmission, and structural reversibility of muscle fatty infiltration. An experimental advanced disuse muscle-wasting model of rabbit supraspinatus

INTRODUCTION

Full-thickness rotator cuff tear is present in almost 50% of patients over age 65 years, and its degree is known to be a good predictor of the severity of muscle-wasting (MW) sarcopaenia, also known as fatty degeneration (FD). A FD CT grade > 2° is recognized as a borderline of its reversibility. A disuse model of supraspinatus FD (grade 2) in rabbits provides clinically relevant data. Therefore, the present study evaluates the correlation between eccentric mechanotransduction, neuromuscular transmission (NT), and reversibility of muscle fatty infiltration (MFI) in rabbit supraspinatus FD > 2°.

MATERIAL AND METHODS

The supraspinatus tendon was detached from the greater tubercle, infraspinatus, and subscapularis in 16 rabbits. The tendon was reinserted after 12 weeks, and the animals were euthanized 24 weeks after reconstruction. MFI was measured in the middle part of the supraspinatus. Single-fibre EMG (SFEMG) examination of the supraspinatus NT was performed on 4 animals.

RESULTS

The power of analysis was 99%. Significant differences in MFI volume were found between the operated (4.6 ±1.1%) and the opposite control sides (2.91 ±0.61%) (p < 0.001). SFEMG revealed no significant differences between the disuse and the control supraspinatus muscles (p > 0.05); however, 6.5% of the examined muscle fibres exhibited NT disorders combined with blockade of conduction in 2.5% of muscle fibres.

CONCLUSIONS

Critical MFI in a disuse model of rabbit supraspinatus FD, CT grade > 2°, is substantially reversible by eccentric training despite subclinical impairment of neuromuscular transmission. In addition, 0.63% reversal of MFI is correlated with 1% hypertrophy of type I and II muscle fibre diameter.

Supraspinatus muscle shear wave elastography (SWE): detection of biomechanical differences with varying tendon quality prior to gray-scale morphologic changes

OBJECTIVE

The purpose of this study was to determine whether SWE can detect biomechanical changes in the supraspinatus muscle that occur with increasing supraspinatus tendon abnormality prior to morphologic gray-scale changes.

MATERIALS AND METHODS

An IRB approved, HIPAA compliant retrospective study of shoulder ultrasounds from 2013-2018 was performed. The cohort consisted of 88 patients (mean age 55 ± 15 years old) with 110 ultrasounds. Images were acquired in longitudinal orientation to the supraspinatus muscle with shear wave velocity (SWV) point quantification. The tendon and muscle were graded in order of increasing tendinosis/tear (1-4 scale) and increasing fatty infiltration (0-3 scale). Mixed model analysis of variance, analysis of covariance, and Spearman rank correlation were used for statistical analysis.

RESULTS

There was no statistically significant age or sex dependence for supraspinatus muscle SWV (p = 0.314, 0.118, respectively). There was no significant correlation between muscle SWV and muscle or tendon grade (p = 0.317, 0.691, respectively). In patients with morphologically normal muscle on gray-scale ultrasound, there were significant differences in muscle SWV when comparing tendon grade 3 with grades 1, 2, and 4 (p = 0.018, 0.025, 0.014, respectively), even when adjusting for gender and age (p = 0.044, 0.028, 0.018, respectively). Pairwise comparison of tendon grades other than those mentioned did not achieve statistical significance (p > 0.05).

CONCLUSION

SWE can detect biomechanical differences within the supraspinatus muscle that are not morphologically evident on gray-scale ultrasound. Specifically, supraspinatus tendon partial tears with moderate to severe tendinosis may correspond to biomechanically distinct muscle properties compared to both lower grades of tendon abnormality and full-thickness tears.

Down-Regulation of Mitochondrial Metabolism after Tendon Release Primes Lipid Accumulation in Rotator Cuff Muscle

Atrophy and fat accumulation are debilitating aspects of muscle diseases and are rarely prevented. Using a vertical approach combining anatomic techniques with omics methodology in a tenotomy-induced sheep model of rotator cuff disease, we tested whether mitochondrial dysfunction is implicated in muscle wasting and perturbed lipid metabolism, speculating that both can be prevented by the stimulation of β-oxidation with l-carnitine. The infraspinatus muscle lost 22% of its volume over the first 6 weeks after tenotomy before the area-percentage of lipid increased from 8% to 18% at week 16. Atrophy was associated with the down-regulation of mitochondrial transcripts and protein and a slow-to-fast shift in muscle composition. Correspondingly, amino acid levels were increased 2 weeks after tendon release, when the levels of high-energy phosphates and glycerophospholipids were lowered. l-Carnitine administration (0.9 g/kg per day) prevented atrophy over the first 2 weeks, and mitigated alterations of glutamate, glycerophospholipids, and carnitine levels in released muscle, but did not prevent the level decrease in high-energy phosphates or protein constituents of mitochondrial respiration, promoting the accumulation of longer lipids with an increasing saturation. We conclude that the early phase of infraspinatus muscle degeneration after tendon release involves the elimination of oxidative characteristics associated with an aberrant accumulation of lipid species but is largely unrelated to the prevention of atrophy with oral l-carnitine administration.

Gene expression profiling of progenitor cells isolated from rat rotator cuff musculotendinous junction

Background

Rotator cuff tendon tears are typically degenerative and usually affect the region of tendon insertion on bone. The remnant torn tendon is degenerative and may not be an ideal source for progenitor cells for cell-based therapies. Therefore, the aim of this study was to determine if musculotendinous junction (MTJ), which is adjacent to tendon would be a viable alternate source of progenitor stem cells. We also sought to study the gene expression profile MTJ progenitors and compare it with progenitors isolated from RC tendon, RC muscle and other existing tissue sources (bone marrow, adipose tissue, and Achilles tendon).

Methods

Rotator cuff tendon (RCT), muscle (RCM), and RCMTJ as well as Achilles tendon (AT) tissues were harvested from healthy male Lewis rats and progenitor cultures were established from these tissues and also from bone marrow and adipose tissue. Quantitative RT-PCR was performed on RNA extracts from intact tissues and progenitor cells using a custom array for the mesenchymal stem cell (MSC) differentiation marker genes. The gene expression profile of MSC differentiation markers within four tissues types, six progenitor cells, and between tissue and their corresponding progenitors were compared.

Results

Progenitors cells can be isolated from rat rotator cuff musculotendinous tissue and their pattern of MSC gene expression was similar to the rotator cuff tendon progenitors for majority of the genes tested. However, there were significant differences between the MSC gene expression patterns of RCMTJ and RCM progenitors. Furthermore, there were differences in gene expression between the RCMTJ tissue and its progenitor cells with respect to MSC differentiation markers. The gene expression pattern of RCMTJ tissue was similar to RCM tissue with respect to markers of chondrogenesis, myogenesis, tenogenesis, and MSC specific markers.

Conclusion

We demonstrate that the musculotendinous junction contains distinct set of progenitor cells and their MSC gene expression pattern is similar to rotator cuff tendon progenitors. RCMTJ progenitors will be an attractive option for cell-based regenerative treatment of chronic rotator cuff tears.

Gene Expression Patterns Analysis in the Supraspinatus Muscle after a Rotator Cuff Tear in a Mouse Model

Rotator cuff tear is a muscle-tendinous injury representative of various musculoskeletal disorders. In general, rotator cuff tear occurs in the tendon, but it causes unloading of the muscle resulting in muscle degeneration including fatty infiltration. These muscle degenerations lead to muscle weakness, pain, and loss of shoulder function and are well known as important factors for poor functional outcome after rotator cuff repair. Given that rotator cuff tear in various animal species results in similar pathological changes seen in humans, the animal model can be considered a good approach to understand the many aspects of the molecular changes in injured muscle. To comprehensively analyze changes in gene expression with time following a rotator cuff tear, we established a rotator cuff tear in mouse supraspinatus tendon of shoulder. At weeks 1 and 4 after the tear, the injured muscles were harvested for RNA isolation, and microarray analysis was performed. Expression patterns of genes belonging to 10 muscle physiology-related categories, including aging, apoptosis, atrophy, and fatty acid transport, were analyzed and further validated using real-time PCR. A total of 39,429 genes were analyzed, and significant changes in expression were observed for 12,178 genes at 1 week and 2,370 genes at 4 weeks after the tear. From the list of top 10 significantly up- and downregulated genes at the 2 time periods and the network evaluation of relevant genes according to the 10 categories, several important genes in each category were observed. In this study, we found that various genes are significantly altered after rotator cuff tear, and these genes may play key roles in controlling muscle degeneration after a rotator cuff tear.

Decreased postoperative gluteus medius muscle cross-sectional area measured by computed tomography scan in patients with intertrochanteric fractures nailing

BACKGROUND

In patients with femoral intertrochanteric fractures treated by cephalomedullary (CM) nailing, abduction force reportedly decreased by 25-30% during the postoperative follow-up period. The purpose of the current study is to evaluate the cross-sectional area (CSA) and adipose tissue ratio (ATR) of the gluteus medius muscle on the postoperative computed tomography (CT) view, expecting this graphic study will support clinical results.

MATERIALS AND METHODS

A total of 27 patients with femoral intertrochanteric fractures treated by CM femoral nail implants completed the study. The mean age at osteosynthesis was 83 years (range: 72-94 years). The mean postoperative follow-up period was 23 months. The three CT axial slice views were defined as slices A, B, and C corresponding to proximal, midway, and distal part of gluteus medius, respectively. The CSA and ATR were assessed bilaterally.

RESULTS

The mean and standard deviation of CSA values (mm²) between the nonoperated/ operated side were as follows: slice A: 2225.8 ± 621.2/1984.5 ± 425.8; slice B: 2145.1 ± 538.3/1854.9 ± 383.9; and slice C: 1711.0 ± 459.0/1434.5 ± 396.9 ( p < 0.01 in slices A, B, and C). The mean and standard deviation of ATR values (%) from the nonoperative/ operative side were as follows: slice A: 2.8 ± 1.7/5.2 ± 3.5; slice B: 2.7 ± 1.9/4.6 ± 3.2; and slice C: 3.6 ± 3.0/4.8 ± 3.2 ( p < 0.01 in slices A and B and p < 0.05 in slice C).

CONCLUSION

Our image findings documented that gluteus medius is significantly changed in CSA and ATR. The damage possibly triggers decrease in muscular strength of hip abduction in the postoperative follow-up period. This measurement is objective, and needed no patient's endurance and cooperation.

The role of mechanobiology in progression of rotator cuff muscle atrophy and degeneration

Rotator cuff (RC) muscles undergo several detrimental changes following mechanical unloading resulting from RC tendon tear. In this review, we highlight the pathological causes and consequences of mechanical alterations at the whole muscle, muscle fiber, and muscle resident cell level as they relate to RC disease progression. In brief, the altered mechanical loads associated with RC tear lead to architectural, structural, and compositional changes at the whole-muscle and muscle fiber level. At the cellular level, these changes equate to direct disruption of mechanobiological signaling, which is exacerbated by mechanically regulated biophysical and biochemical changes to the cellular and extra-cellular environment (also known as the stem cell "niche"). Together, these data have important implications for both pre-clinical models and clinical practice. In pre-clinical models, it is important to recapitulate both the atrophic and degenerative muscle loss found in humans using clinically relevant modes of injury. Clinically, understanding the mechanics and underlying biology of the muscle will impact both surgical decision-making and rehabilitation protocols, as interventions that may be good for atrophic muscle will have a detrimental effect on degenerating muscle, and vice versa. © 2017 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 36:546-556, 2018.

Heterogeneous muscle gene expression patterns in patients with massive rotator cuff tears

Detrimental changes in the composition and function of rotator cuff (RC) muscles are hallmarks of RC disease progression. Previous studies have demonstrated both atrophic and degenerative muscle loss in advanced RC disease. However, the relationship between gene expression and RC muscle pathology remains poorly defined, in large part due to a lack of studies correlating gene expression to tissue composition. Therefore, the purpose of this study was to determine how tissue composition relates to gene expression in muscle biopsies from patients undergoing reverse shoulder arthroplasty (RSA). Gene expression related to myogenesis, atrophy and cell death, adipogenesis and metabolism, inflammation, and fibrosis was measured in 40 RC muscle biopsies, including 31 biopsies from reverse shoulder arthroplasty (RSA) cases that had available histology data and 9 control biopsies from patients with intact RC tendons. After normalization to reference genes, linear regression was used to identify relationships between gene expression and tissue composition. Hierarchical clustering and principal component analysis (PCA) identified unique clusters, and fold-change analysis was used to determine significant differences in expression between clusters. We found that gene expression profiles were largely dependent on muscle presence, with muscle fraction being the only histological parameter that was significantly correlated to gene expression by linear regression. Similarly, samples with histologically-confirmed muscle distinctly segregated from samples without muscle. However, two sub-groups within the muscle-containing RSA biopsies suggest distinct phases of disease, with one group expressing markers of both atrophy and regeneration, and another group not significantly different from either control biopsies or biopsies lacking muscle. In conclusion, this study provides context for the interpretation of gene expression in heterogeneous and degenerating muscle, and provides further evidence for distinct stages of RC disease in humans.

Rotator cuff muscle degeneration and tear severity related to myogenic, adipogenic, and atrophy genes in human muscle

Large rotator cuff tear size and advanced muscle degeneration can affect reparability of tears and compromise tendon healing. Clinicians often rely on direct measures of rotator cuff tear size and muscle degeneration from magnetic resonance imaging (MRI) to determine whether the rotator cuff tear is repairable. The objective of this study was to identify the relationship between gene expression changes in rotator cuff muscle degeneration to standard data available to clinicians. Radiographic assessment of preoperative rotator cuff tear severity was completed for 25 patients with varying magnitudes of rotator cuff tears. Tear width and retraction were measured using MRI, and Goutallier grade, tangent (tan) sign, and Thomazeau grade were determined. Expression of myogenic-, adipogenic-, atrophy-, and metabolism-related genes in biopsied muscles were correlated with tear width, tear retraction, Goutallier grade, tan sign, and Thomazeau grade. Tear width positively correlated with Goutallier grade in both the supraspinatus (r = 0.73) and infraspinatus (r = 0.77), along with tan sign (r = 0.71) and Thomazeau grade (r = 0.68). Decreased myogenesis (Myf5), increased adipogenesis (CEBPα, Lep, Wnt10b), and decreased metabolism (PPARα) correlated with radiographic assessments. Gene expression changes suggest that rotator cuff tears lead to a dramatic molecular response in an attempt to maintain normal muscle tissue, increase adipogenesis, and decrease metabolism. Fat accumulation and muscle atrophy appear to stem from endogenous changes rather than from changes mediated by infiltrating cells. Results suggest that chronic unloading of muscle, induced by rotator cuff tear, disrupts muscle homeostasis. © 2017 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 35:2808-2814, 2017.

Fatty infiltration of stage 1 or higher significantly compromises long-term healing of supraspinatus repairs

BACKGROUND

Fatty infiltration (FI) compromises outcomes of rotator cuff repairs. Most clinicians consider FI of the infraspinatus, whether it is torn or intact, because it is most rapidly affected. The purpose of this study was to report long-term outcomes of isolated supraspinatus repairs and to determine their associations with FI of the infraspinatus and supraspinatus.

METHODS

The records of 182 patients who underwent repair of isolated supraspinatus tears and had preoperative magnetic resonance imaging were retrieved. Of these, 147 patients were evaluated at 10 years' follow-up using the Constant score and magnetic resonance imaging scans.

RESULTS

Preoperative FI was greater in the supraspinatus (52% stage ≥1) than in the infraspinatus (29% stage ≥1). The 10-year Constant scores were influenced by FI of the supraspinatus (P = .006) but not of the infraspinatus (P = .422). Multivariable regression confirmed that Constant scores were significantly lower for female patients, repetitive work, and stage 1 and stage 2 FI of the supraspinatus in addition to open surgery. Retear rates (Sugaya types IV-V) were also influenced by FI of the supraspinatus (P = .001) but not of the infraspinatus (P = .979). Shoulders with supraspinatus FI at stages 0, 1, and 2 had retear rates of 10%, 22%, and 31%, respectively. Multivariable regression affirmed that the odds of retears are significantly increased by both stage 1 and stage 2 FI of the supraspinatus.

CONCLUSIONS

The Constant scores and retear rates were significantly associated with FI of the torn supraspinatus (stage ≥1) but not of the intact infraspinatus. The authors recommend that rapid surgical intervention be considered, before accumulation of fat, especially for young, active patients.

Neer Award 2016: reduced muscle degeneration and decreased fatty infiltration after rotator cuff tear in a poly(ADP-ribose) polymerase 1 (PARP-1) knock-out mouse model

BACKGROUND

Disturbed muscular architecture, atrophy, and fatty infiltration remain irreversible in chronic rotator cuff tears even after repair. Poly (adenosine 5'-diphosphate-ribose) polymerase 1 (PARP-1) is a key regulator of inflammation, apoptosis, muscle atrophy, muscle regeneration, and adipocyte development. We hypothesized that the absence of PARP-1 would lead to a reduction in damage to the muscle subsequent to combined tenotomy and neurectomy in a PARP-1 knockout (KO) mouse model.

METHODS

PARP-1 KO and wild-type C57BL/6 (WT group) mice were analyzed at 1, 6, and 12 weeks (total n = 84). In all mice, the supraspinatus and infraspinatus muscles of the left shoulder were detached and denervated. Macroscopic analysis, magnetic resonance imaging, gene expression analysis, immunohistochemistry, and histology were used to assess the differences in PARP-1 KO and WT mice.

RESULTS

The muscles in the PARP-1 KO group had significantly less retraction, atrophy, and fatty infiltration after 12 weeks than in the WT group. Gene expression of inflammatory, apoptotic, adipogenic, and muscular atrophy genes was significantly decreased in PARP-1 KO mice in the first 6 weeks.

DISCUSSION

Absence of PARP-1 leads to a reduction in muscular architectural damage, early inflammation, apoptosis, atrophy, and fatty infiltration after combined tenotomy and neurectomy of the rotator cuff muscle. Although the macroscopic reaction to injury is similar in the first 6 weeks, the ability of the muscles to regenerate was much greater in the PARP-1 KO group, leading to a near-normalization of the muscle after 12 weeks.

CURRENT CONCEPTS ON THE GENETIC FACTORS IN ROTATOR CUFF PATHOLOGY AND FUTURE IMPLICATIONS FOR SPORTS PHYSICAL THERAPISTS

CONTEXT

Recent advances within the field of genetics are currently changing many of the methodologies in which medicine is practiced. These advances are also beginning to influence the manner in which physical therapy services are rendered. Rotator cuff pathology is one of the most common diagnoses treated by the sports physical therapist. The purpose of this commentary is to educate sports physical therapists on the recent advances regarding how genetics influences rotator cuff pathology, including rotator cuff tears, and provide a perspective on how this information will likely influence post-operative shoulder rehabilitation in the near future.

EVIDENCE ACQUISITION

A comprehensive review of the literature was completed using the Medline database along with individual searches of relevant physical therapy, surgical, cell biology, and sports medicine journals. Search terms included: shoulder, rotator cuff pathology, genetics, apoptosis, and physical therapy. Search results were compiled and evaluated; relevant primary studies and review articles were gathered; the results from this comprehensive review are summarized here.

STUDY DESIGN

Clinical Commentary, Review of the Literature.

RESULTS

Recent advances within the understanding of rotator cuff pathology have further elucidated the cellular and molecular mechanisms associated with rotator cuff tears. There appears to be a hypoxic-induced apoptotic cellular pathway that contributes to rotator cuff tears. Activation of specific proteins termed matrix metalloproteinases appear to be involved in not only primary rotator cuff tears, but also may influence the re-tear rate after surgical intervention. Further advancements in the understanding of the cellular mechanisms contributing to rotator cuff tears and postoperative techniques to help prevent re-tears, may soon influence the methodology in which physical therapy services are provided to patients sustaining a rotator cuff injury.

CONCLUSIONS

At this time continued research is required to more fully develop a comprehensive understanding of the role of genetic variables both within primary rotator cuff tears and their influences on post-operative rehabilitation from rotator cuff repair surgery.

LEVEL OF EVIDENCE

Level 5.

The Rotator Cuff Organ: Integrating Developmental Biology, Tissue Engineering, and Surgical Considerations to Treat Chronic Massive Rotator Cuff Tears

The torn rotator cuff remains a persistent orthopedic challenge, with poor outcomes disproportionately associated with chronic, massive tears. Degenerative changes in the tissues that comprise the rotator cuff organ, including muscle, tendon, and bone, contribute to the poor healing capacity of chronic tears, resulting in poor function and an increased risk for repair failure. Tissue engineering strategies to augment rotator cuff repair have been developed in an effort to improve rotator cuff healing and have focused on three principal aims: (1) immediate mechanical augmentation of the surgical repair, (2) restoration of muscle quality and contractility, and (3) regeneration of native enthesis structure. Work in these areas will be reviewed in sequence, highlighting the relevant pathophysiology, developmental biology, and biomechanics, which must be considered when designing therapeutic applications. While the independent use of these strategies has shown promise, synergistic benefits may emerge from their combined application given the interdependence of the tissues that constitute the rotator cuff organ. Furthermore, controlled mobilization of augmented rotator cuff repairs during postoperative rehabilitation may provide mechanotransductive cues capable of guiding tissue regeneration and restoration of rotator cuff function. Present challenges and future possibilities will be identified, which if realized, may provide solutions to the vexing condition of chronic massive rotator cuff tears.

Muscle architectural changes after massive human rotator cuff tear

Rotator cuff (RC) tendon tears lead to negative structural and functional changes in the associated musculature. The structural features of muscle that predict function are termed "muscle architecture." Although the architectural features of "normal" rotator cuff muscles are known, they are poorly understood in the context of cuff pathology. The purpose of this study was to investigate the effects of tear and repair on RC muscle architecture. To this end thirty cadaveric shoulders were grouped into one of four categories based on tear magnitude: Intact, Full-thickness tear (FTT), Massive tear (MT), or Intervention if sutures or hardware were present, and key parameters of muscle architecture were measured. We found that muscle mass and fiber length decreased proportionally with tear size, with significant differences between all groups. Conversely, sarcomere number was reduced in both FTT and MT with no significant difference between these two groups, in large part because sarcomere length was significantly reduced in MT but not FTT. The loss of muscle mass in FTT is due, in part, to subtraction of serial sarcomeres, which may help preserve sarcomere length. This indicates that function in FTT may be impaired, but there is some remaining mechanical loading to maintain "normal" sarcomere length-tension relationships. However, the changes resulting from MT suggest more severe limitations in force-generating capacity because sarcomere length-tension relationships are no longer normal. The architectural deficits observed in MT muscles may indicate deeper deficiencies in muscle adaptability to length change, which could negatively impact RC function despite successful anatomical repair. © 2016 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 34:2089-2095, 2016.

Gene expression profiles of changes underlying different-sized human rotator cuff tendon tears

BACKGROUND

Progressive cellular and extracellular matrix (ECM) changes related to age and disease severity have been demonstrated in rotator cuff tendon tears. Larger rotator cuff tears demonstrate structural abnormalities that potentially adversely influence healing potential. This study aimed to gain greater insight into the relationship of pathologic changes to tear size by analyzing gene expression profiles from normal rotator cuff tendons, small rotator cuff tears, and large rotator cuff tears.

METHODS

We analyzed gene expression profiles of 28 human rotator cuff tendons using microarrays representing the entire genome; 11 large and 5 small torn rotator cuff tendon specimens were obtained intraoperatively from tear edges, which we compared with 12 age-matched normal controls. We performed real-time polymerase chain reaction and immunohistochemistry for validation.

RESULTS

Torn rotator cuff tendons demonstrated upregulation of a number of key genes, such as matrix metalloproteinase 3, 10, 12, 13, 15, 21, and 25; a disintegrin and metalloproteinase (ADAM) 12, 15, and 22; and aggrecan. Amyloid was downregulated in all tears. Small tears displayed upregulation of bone morphogenetic protein 5. Chemokines and cytokines that may play a role in chemotaxis were altered; interleukins 3, 10, 13, and 15 were upregulated in tears, whereas interleukins 1, 8, 11, 18, and 27 were downregulated.

CONCLUSIONS

The gene expression profiles of normal controls and small and large rotator cuff tear groups differ significantly. Extracellular matrix remodeling genes were found to contribute to rotator cuff tear pathogenesis. Rotator cuff tears displayed upregulation of a number of matrix metalloproteinase (3, 10, 12, 13, 15, 21, and 25), a disintegrin and metalloproteinase (ADAM 12, 15, and 22) genes, and downregulation of some interleukins (1, 8, and 27), which play important roles in chemotaxis. These gene products may potentially have a role as biomarkers of failure of healing or therapeutic targets to improve tendon healing.

Investigating the cellular origin of rotator cuff muscle fatty infiltration and fibrosis after injury

BACKGROUND

rotator cuff muscle atrophy, fibrosis and fatty infiltration are common complications after large and massive rotator cuff tears. Currently, there are no effective treatments for these muscle pathologies after injury. Furthermore, the cellular source for fibrotic and adipose tissues in rotator cuff muscle after injury remains unknown. In this study, we proposed that two groups of muscle resident progenitors, Tie2+ muscle mesenchymal progenitors and PDGFRα(+) fibro/adipogenic progenitor cells (FAPs), contribute significantly to rotator cuff muscle fibrosis and fatty infiltration.

METHODS

we tested our hypothesis using reporter mice. Rotator cuff muscles from Tie2-GFP and PDGFRα-GFP reporter mice were harvested at 2 and 6 weeks after unilateral massive rotator cuff tear surgeries. Immunofluorescent staining for fibroblast and adipocyte markers was conducted.

RESULTS

our results showed significant co-localization of Tie2+ cells with fibrotic markers vimentin and αSMA. In the PDGFRα-GFP reporter mice, GFP signal was seen in only a small fraction of cells staining positive for vimentin and αSMA. However, PDGFRα showed significant co-localization with adipocyte markers, including PPAR-γ, adiponectin, and perilipin A. Oil red O staining confirmed that the mature adipocytes appearing in rotator cuff muscles after injury are also PDGFRα(+).

CONCLUSION

these data demonstrated that the Tie2(+) muscle mesenchymal progenitors are the major source of fibroblasts while PDGFRα(+) FAPs are the major source of adipocytes in rotator cuff muscle fatty infiltration. Basic Science Study.

Rotator cuff tears: An evidence based approach

Lesions of the rotator cuff (RC) are a common occurrence affecting millions of people across all parts of the globe. RC tears are also rampantly prevalent with an age-dependent increase in numbers. Other associated factors include a history of trauma, limb dominance, contralateral shoulder, smoking-status, hypercholesterolemia, posture and occupational dispositions. The challenge lies in early diagnosis since a high proportion of patients are asymptomatic. Pain and decreasing shoulder power and function should alert the heedful practitioner in recognizing promptly the onset or aggravation of existing RC tears. Partial-thickness tears (PTT) can be bursal-sided or articular-sided tears. Over the course of time, PTT enlarge and propagate into full-thickness tears (FTT) and develop distinct chronic pathological changes due to muscle retraction, fatty infiltration and muscle atrophy. These lead to a reduction in tendon elasticity and viability. Eventually, the glenohumeral joint experiences a series of degenerative alterations - cuff tear arthropathy. To avert this, a vigilant clinician must utilize and corroborate clinical skill and radiological findings to identify tear progression. Modern radio-diagnostic means of ultrasonography and magnetic resonance imaging provide excellent visualization of structural details and are crucial in determining further course of action for these patients. Physical therapy along with activity modifications, anti-inflammatory and analgesic medications form the pillars of nonoperative treatment. Elderly patients with minimal functional demands can be managed conservatively and reassessed at frequent intervals. Regular monitoring helps in isolating patients who require surgical interventions. Early surgery should be considered in younger, active and symptomatic, healthy patients. In addition to being cost-effective, this helps in providing a functional shoulder with a stable cuff. An easily reproducible technique of maximal strength and sturdiness should by chosen among the armamentarium of the shoulder surgeon. Grade 1 PTTs do well with debridement while more severe lesions mandate repair either by trans-tendon technique or repair following conversion into FTT. Early repair of repairable FTT can avoid appearance and progression of disability and weakness. The choice of surgery varies from surgeon-to-surgeon with arthroscopy taking the lead in the current scenario. The double-row repairs have an edge over the single-row technique in some patients especially those with massive tears. Stronger, cost-effective and improved functional scores can be obtained by the former. Both early and delayed postoperative rehabilitation programmes have led to comparable outcomes. Guarded results may be anticipated in patients in extremes of age, presence of comorbidities and severe tear patters. Overall, satisfactory results are obtained with timely diagnosis and execution of the appropriate treatment modality.

Quantitative Analysis of Three-Dimensional Distribution and Clustering of Intramuscular Fat in Muscles of the Rotator Cuff

The purpose of this study was to (1) develop and present a technique to quantitatively assess three-dimensional distribution and clustering of intramuscular fat and (2) use the technique to compare spatial characteristics of intramuscular fat in rotator cuff muscles of older adults with and without a supraspinatus tear. Moran's Index (I), an existing quantitative measure of clustering, was extended for use with MRI to allow comparisons across individuals with different size muscles. Sixteen older adults (>60 years) with (N = 6) and without (N = 10) a degenerative supraspinatus tear participated. Following 3D Dixon MRIs of the shoulder, which separates fat from water, rotator cuff muscles were segmented and sectioned and fat% and Moran's I were calculated to assess distribution and clustering, respectively. Moran's I ranged was 0.40-0.92 and 0.39-0.76 for the tear and control subjects, respectively. Compared to uninjured controls, tear subjects demonstrated increased fat distribution (p = 0.036) and clustering (p = 0.020) distally in the supraspinatus. Tear subjects had more pronounced distribution (p < 0.001) and clustering distally (p < 0.001) than proximally. Other rotator cuff muscles exhibited different patterns of fat clustering and distribution. This technique, which we applied to quantify spatial characteristics of intramuscular fat, can be applied to assess clustering of fat in other pathologies and tissues.

Bone morphogenetic protein signaling in rotator cuff muscle atrophy and fatty infiltration

BACKGROUND

reduced mass (atrophy) and increased fat content (fatty infiltration) of rotator cuff muscles are common complications of large or massive rotator cuff (RC) tears, and are believed to be irreversible even after tendon repairs. Clinically, both muscle atrophy and fatty infiltration are important factors contributing to poor functional outcomes after tendon repairs. The molecular mechanism of RC muscle atrophy and FI remains undefined. In this study, we investigated the role of bone morphogenetic proteins (BMP) signaling in RC muscle atrophy and fatty infiltration using a rat model.

METHODS

unilateral massive RC tears was induced in adult rats. RC muscles were harvested at 2 and 6 weeks after injury for BMP signaling analysis. In a separate experiment, BMP inhibitor (LDN-193189) was injected to rats through daily intraperitoneal injection. RC muscles from rats in the treated and control groups were harvested at 6 weeks after injury for biochemistry and histology analysis.

RESULTS

we found significantly increased BMP-14 and BMP-7 expression in rotator cuff muscles after RCT. Inhibiting BMP signaling resulted in increased muscle atrophy and reduced fatty infiltration in rotator cuff muscle after RC tears.

CONCLUSION

this result suggests that BMP signaling inhibits RC muscle atrophy but promotes fatty infiltration.

Muscle atrophy and fatty infiltration after an acute rotator cuff repair in a sheep model

INTRODUCTION

rotator cuff tears (RCTs) are the most common tendon injury seen in orthopedic patients. Muscle atrophy and fatty infiltration of the muscle are crucial factors that dictate the outcome following rotator cuff surgery. Though less studied in humans, rotator cuff muscle fibrosis has been seen in animal models as well and may influence outcomes as well. The purpose of this study was to determine if the rotator cuff would develop muscle changes even in the setting of an acute repair in a sheep model. We hypothesized that fatty infiltration and fibrosis would be present even after an acute repair six months after initial surgery.

METHODS

twelve female adult sheep underwent an acute rotator cuff tear and immediate repair on the right shoulder. The left shoulder served as a control and did not undergo a tear or a repair. Six months following acute rotator cuff repairs, sheep muscles were harvested to study atrophy, fatty infiltration, and fibrosis by histological analysis, western blotting, and reverse transcription polymerase chain reaction (RT-PCR).

RESULTS

the repair group demonstrated an increase expression of muscle atrophy, fatty infiltration, and fibrosis related genes. Significantly increased adipocytes, muscle fatty infiltration, and collagen deposition was observed in rotator cuff muscles in the tendon repair group compared to the control group.

CONCLUSIONS

rotator cuff muscle undergoes degradation changes including fatty infiltration and fibrosis even after the tendons are repair immediately after rupture.

LEVEL OF EVIDENCE

Basic Science Study.