A rat model of massive rotator cuff tears

Effect of polydeoxyribonucleotide and polynucleotide on rotator cuff healing and fatty infiltration in a diabetic rat model

Failure rate after chronic rotator cuff repair is considerably high. Moreover, diabetes mellitus is known as a compromising factor of rotator cuff tear. The effect of Polydeoxyribonucleotide (PDRN) and polynucleotide (PN) on tendon healing and fatty infiltration is unclear as tissue regeneration activator in diabetic state. Therefore, a diabetic rat model with chronic rotator cuff tear was made for mechanical, histologic and blood tests. In the animal study using a diabetic rat cuff repair model, the administration of PDRN and PN increased the load to failure of repaired cuffs and improved tendon healing and decreased fatty infiltration. Also, the plasma levels of vascular endothelial growth factor and fibroblast growth factor were elevated in PDRN and PN administrated groups. We concluded that PDRN and PN appear to boost tendon recovery and reduce the presence of fatty infiltration following cuff repair in diabetic state. Also, PN showed a later onset and a longer duration than PDRN associated with the mean plasma growth factors.

The rat as a novel model for chronic rotator cuff injuries

Chronic rotator cuff injuries (CRCIs) still present a great challenge for orthopaedics surgeons. Many new therapeutic strategies are developed to facilitate repair and improve the healing process. However, there is no reliable animal model for chronic rotator cuff injury research. To present a new valuable rat model for future chronic rotator cuff injuries (CRCIs) repair studies, and describe the changes of CRCIs on the perspectives of histology, behavior and MRI. Sixty male Wistar rats were enrolled and underwent surgery of the left shoulder joint for persistent subacromial impingement. They were randomly divided into experimental group (n = 30, a 3D printed PEEK implant shuttled into the lower surface of the acromion) and sham operation group (n = 30, insert the same implant, but remove it immediately). Analyses of histology, behavior, MRI and inflammatory pain-related genes expression profiles were performed to evaluate the changes of CRCIs. After 2-weeks running, the rats in the experimental group exhibited compensatory gait patterns to protect the injured forelimb from loading after 2-weeks running. After 8-weeks running, the rats in the experimental group showed obvious CRCIs pathological changes: (1) acromion bone hyperplasia and thickening of the cortical bone; (2) supraspinatus muscle tendon of the humeral head: the bursal-side tendon was torn and layered with disordered structure, forming obvious gaps; the humeral-side tendon is partially broken, and has a neatly arranged collagen. Partial fat infiltration is found. The coronal T2-weighted images showed that abnormal tendon-to-bone junctions of the supraspinatus tendon. The signal intensity and continuity were destroyed with contracted tendon. At the nighttime, compared with the sham operation group, the expression level of IL-1β and COX-2 increased significantly (P = 0063, 0.0005) in the experimental group. The expression of COX-2 in experimental group is up-regulated about 1.5 times than that of daytime (P = 0.0011), but the expression of IL-1β, TNF-a, and NGF are all down-regulated (P = 0.0146, 0.0232, 0.0161). This novel rat model of chronic rotator cuff injuries has the similar characteristics with that of human shoulders. And it supplies a cost-effective, reliable animal model for advanced tissue engineered strategies and future therapeutic strategies.

Rotator cuff muscle fibrosis can be assessed using ultrashort echo time magnetization transfer MRI with fat suppression

Muscle degeneration following rotator cuff tendon tearing is characterized by fatty infiltration and fibrosis. While tools exist for the characterization of fat, the ability to noninvasively assess muscle fibrosis is limited. The purpose of this study was to evaluate the capability of quantitative ultrashort echo time T1 (UTE-T1) and UTE magnetization transfer (UTE-MT) mapping with and without fat suppression (FS) for the differentiation of injured and control rotator cuff muscles and for the detection of fibrosis. A rat model of chronic massive rotator cuff tearing (n = 12) was used with tenotomy of the right supraspinatus and infraspinatus tendons and silicone implants to prevent healing. Imaging was performed on a 3-T scanner, and UTE-T1 mapping with and without FS and UTE-MT with and without FS for macromolecular fraction (MMF) mapping was performed. At 20 weeks postinjury, T1 and MMF were measured in the supraspinatus and infraspinatus muscles of the injured and contralateral, internal control sides. Histology was performed and connective tissue fraction (CTF) was measured, defined as the area of collagen-rich extracellular matrix divided by the total muscle area. Paired t-tests and correlation analyses were performed. Significant differences between injured and control sides were found for CTF in the supraspinatus (mean ± SD, 14.5% ± 3.9% vs. 11.3% ± 3.7%, p = 0.01) and infraspinatus (17.0% ± 5.4% vs. 12.5% ± 4.6%, p < 0.01) muscles, as well as for MMF using UTE-MT FS in the supraspinatus (9.7% ± 0.3% vs. 9.5% ± 0.2%, p = 0.04) and infraspinatus (10.9% ± 0.8% vs. 10.1% ± 0.5%, p < 0.01) muscles. No significant differences between sides were evident for T1 without or with FS or for MMF using UTE-MT. Only MMF using UTE-MT FS was significantly correlated with CTF for both supraspinatus (r = 0.46, p = 0.03) and infraspinatus (r = 0.51, p = 0.01) muscles. Fibrosis occurs in rotator cuff muscle degeneration, and the UTE-MT FS technique may be helpful to evaluate the fibrosis component, independent from the fatty infiltration process.

Decellularized biological matrices for the repair of rotator cuff lesions: a systematic review of preclinical in vivo studies

Background: Rotator cuff tears (RCTs), resulting from degeneration or trauma of the shoulder tendons, are one of the main causes of shoulder pain. In particular, massive RCTs represent 40% of all injuries, require surgical treatment, and are characterized by poor clinical outcomes and a high rate of failure. In recent years, the use of biological decellularized patches for augmentation procedures has received great interest owing to their excellent self-integration properties, improving healing and, thus, presenting an innovative therapeutic option. However, the findings from clinical studies have emerged with conflicting viewpoints regarding the benefits of this procedure, as an excessive tension load might compromise the integrity of the tendon-to-bone connection when the patch exhibits low elasticity or insufficient strength. This could prevent the healing process, leading to unpredictable results in clinical practice. Methods: This systematic review was conducted following Preferred Reporting Items for Systematic reviews and Meta-Analyses (PRISMA) guidelines across three databases (PubMed, Scopus, and Web of Knowledge) to underline the results obtained in preclinical studies involving animal models of RCT surgeries that utilized the biological decellularized matrix augmentation technique in the last 5 years. Results: Thirteen articles were included after the screening, and the SYRCLE tools were applied to assess the risk of bias in in vivo studies. Open-surgery techniques were conducted to create tendon defects or detachment in different animal models: rat (31%), rabbit (46%), dog (15%), and sheep (8%). Patches decellularized with non-standardized protocols were used in 77% of studies, while commercially available matrices were used in 15%. Of the studies, 31% used allogenic patches, 61% used xenogenic patches, and 8% utilized both xenogenic and autologous patches. Conclusion: Overall, this review provides a comprehensive overview of the use of acellular patches and their effective therapeutic potential in rotator cuff (RC) repair at the preclinical level with the aim of expanding the strategies and matrices available for surgeons. Systematic review registration: https://www.crd.york.ac.uk/prospero/, identifier CRD42023468716.

Evaluation of articular changes using a rat mono-iodoacetate-induced shoulder arthritis model by histology and radiology

The rat mono-iodoacetate (MIA) arthritis model has been used in studies on the hip, knee, and ankle joints. Few studies have explored its utility in shoulder arthritis research, and none have evaluated the effects of time and different MIA doses on arthritis progression. Therefore, we developed a rat MIA shoulder arthritis model to evaluate articular changes through radiological and histological analyses. Sprague-Dawley rats (n = 108) were equally divided into groups that were intra-articularly injected with 0.5 mg of MIA (in 50 µL of purified water), 2.0 mg of MIA (in 50 µL of purified water), or purified water (50 µL; sham group). Throughout the study period, 18 rats (six per group) were evaluated by computed tomography and assessed using the Larsen's classification system; 90 rats were further evaluated histologically using the Osteoarthritis Research Society International scoring system. Computed tomography revealed that the groups injected with MIA developed arthritis and osteophytes 14 days after injection, which progressed temporally. The Larsen's grades worsened over time; at all time points, the scores were higher in the group injected with 2.0 mg of MIA than in the group injected with 0.5 mg of MIA. Furthermore, concurrent with the worsening Larsen's grades, the Osteoarthritis Research Society International scores also significantly increased over time; at all time points, they were higher in the group injected with 2.0 mg of MIA than in the group injected with 0.5 mg of MIA. Our rat MIA shoulder arthritis model revealed radiologically and histologically confirmed temporal and MIA dose-dependent arthritic changes.

Intact revision rotator cuff repair stabilizes muscle atrophy and fatty infiltration after minimum follow up of two years

BACKGROUND

The extent of fatty infiltration and rotator cuff (RC) atrophy is crucial for the clinical results after rotator cuff repair (RCR). The purpose of this study was to evaluate changes in fatty infiltration and RC atrophy after revision RCR and to correlate them with functional outcome parameters.

METHODS

Patients who underwent arthroscopic revision RCR for symptomatic recurrent full-thickness tear of the supraspinatus tendon between 2008 and 2014 and were retrospectively reviewed with a minimum follow up of 2 years. Magnetic resonance imaging (MRI) was performed pre- and postoperatively to assess 1) tendon integrity after revision RCR according to Sugaya classification, (2) RC atrophy according to Thomazeau classification, and (3) fatty infiltration according to Fuchs MRI classification. Constant score (CS) and the American Shoulder and Elbow Surgeon (ASES) score were used to correlate functional outcome, tendon integrity, and muscle degeneration.

RESULTS

19 patients (17 males and 2 females) with a mean age of 57.5 years (range, 34 to 72) were included into the study at a mean follow-up of 50.3 months (range, 24 - 101). At final evaluation, 9 patients (47%) presented with intact RCR and 10 patients (53%) suffered a re-tear after revision repair. No progress of fatty infiltration was observed postoperatively in the group with intact RC, atrophy progressed in only 1 out of 9 patient (11%). Fatty infiltration progressed in 5/10 patients (50%) and RC atrophy increased in 2/10 patients (20%) within the re-tear group. CS (42.7 ± 17.7 preop, 65.2 ± 20.1 postop) and ASES (47.7 ± 17.2 preop, 75.4 ± 23.7 postop) improved significantly from pre- to postoperatively (p < 0.001). A positive correlation between fatty infiltration and RC integrity was detected (r = 0.77, p < 0.01). No correlation between clinical outcome and tendon integrity or RC atrophy was observed.

CONCLUSION

Arthroscopic revision RCR leads to reliable functional outcomes even in case of a recurrent RC retear. An intact RCR maintains the preoperative state of fatty infiltration and muscle atrophy but does not lead to muscle regeneration.

LEVEL OF EVIDENCE

Level IV; Therapeutic study.

Effect of suprascapular nerve injury on muscle and regenerated enthesis in a rat rotator cuff tear model

BACKGROUND

Massive rotator cuff tears (RCTs) are complicated by muscle atrophy, fibrosis, and intramuscular fatty degeneration, which are associated with postoperative tendon-to-bone healing failure and poor clinical outcomes. We evaluated muscle and enthesis changes in large tears with or without suprascapular nerve (SN) injury in a rat model.

METHODS

Sixty-two adult Sprague-Dawley rats were divided into SN injury (+) and SN injury (-) groups (n=31 each), comprising tendon (supraspinatus [SSP]/infraspinatus [ISP]) and nerve resection and tendon resection only cases, respectively. Muscle weight measurement, histological evaluation, and biomechanical testing were performed 4, 8, and 12 weeks postoperatively. Ultrastructural analysis with block face imaging was performed 8 weeks postoperatively.

RESULTS

SSP/ISP muscles in the SN injury (+) group appeared atrophic, with increased fatty tissue and decreased muscle weight, compared to those in the control and SN injury (-) groups. Immunoreactivity was only positive in the SN injury (+) group. Myofibril arrangement irregularity and mitochondrial swelling severity, along with number of fatty cells, were higher in the SN injury (+) group than in the SN injury (-) group. The bone-tendon junction enthesis was firm in the SN injury (-) group; this was atrophic and thinner in the SN injury (+) group, with decreased cell density and immature fibrocartilage. Mechanically, the tendon-bone insertion was significantly weaker in the SN injury (+) group than in the control and SN injury (+) groups.

CONCLUSIONS

In clinical settings, SN injury may cause severe fatty changes and inhibition of postoperative tendon healing in large RCTs. Level of evidence: Basic research, controlled laboratory study.

Animal Models of Rotator Cuff Injury and Repair: A Systematic Review

There are a large number of animal studies on rotator cuff injury and repair, but a lack of detailed research and evaluation on the animal models. This systematic review aims to provide a framework for animal studies and repair patches for rotator cuff injury. Four hundred nine animal studies were included, of which the most common animal model of rotator cuff injury is rat (53.56%), the most common site of rotator cuff injury is the supraspinatus tendon (62.10%), and the most common injury type (degree) is acute tear (full thickness) (48.41%). The most common research purpose is to evaluate the repair effect of the patch (24.94%), followed by the observation of pathophysiological changes after rotator cuff injury (20.87%). Among the five types of repair patch materials including nondegradable and degradable synthetic materials, autologous and allogeneic tissues, and naturally derived biomaterial, the last one is the mostly used (52.74%). For different animal models, the rodent models (rat and mouse) are the most commonly used and probably the most suitable species for preliminary studies of rotator cuff injury; the rabbit, canine, sheep, and goat models are more suitable for biomechanical performance testing, rehabilitation training, and validation of surgical methods; and the nonhuman primate models (monkey and baboon) are the closest to human, but it is more difficult to carry out the animal studies on them because of ethical issues, high feeding cost, and management difficulties.

Decellularized Bovine Pericardial Patch Loaded With Mesenchymal Stromal Cells Enhance the Mechanical Strength and Biological Healing of Large-to-Massive Rotator Cuff Tear in a Rat Model

PURPOSE

The purpose of this study was to determine whether the addition of decellularized bovine pericardial patch loaded with mesenchymal stromal cells enhanced bone-to-tendon healing and improved the biomechanical strength of large-to-massive rotator cuff tears in a small animal model.

METHODS

Adipose-derived mesenchymal stromal cells (MSCs) from rat inguinal fat were isolated, cultured, and loaded onto decellularized bovine pericardium patches. To simulate large-to-massive tears, rats were managed with free cage activity for 6 weeks after tear creation. A total of 18 rats were randomly allocated to repair-only (control), repair with pericardial patch augmentation (patch), or repair with MSC loaded pericardial patch augmentation (patch-MSC). Each group had 6 rats (one shoulder of each rat was used for histological evaluation and another for biomechanical evaluation). MSCs seeded on the pericardial patches were traced on four shoulders from 2 other rats at 4 weeks after surgery. Histological evaluation for bone-to-tendon healing and biomechanical testing was carried out at 8 weeks after repair.

RESULTS

MSCs tagged with a green fluorescent protein were observed in the repair site 4 weeks after the repair. One shoulder each in the control and patch groups showed complete discontinuity between the bone and tendon. One shoulder in the control group showed attenuation with only a tenuous connection. Fibrocartilage and tidemark formation at the bone-to-tendon interface (P = .002) and collagen fiber density (P = .040) and orientation (P = .003) were better in the patch-MSC group than in the control or patch group. Load-to-failure in the patch-MSC and patch groups was higher than that in the control group (P = .001 and .009, respectively).

CONCLUSION

Decellularized bovine pericardial patches loaded with adipose-derived and cultured mesenchymal stromal cells enhanced healing in terms of both histology and mechanical strength at 8 weeks following rotator cuff repair in a rat model.

CLINICAL RELEVANCE

Large-to-massive rotator tears need a strategy to prevent retear and enhance healing. The addition of decellularized bovine pericardial patch loaded with MSCs can enhance bone-to-tendon healing and improve biomechanical healing of large-to-massive rotator cuff tears following repair.

Role of animal models in biomedical research: a review

The animal model deals with the species other than the human, as it can imitate the disease progression, its’ diagnosis as well as a treatment similar to human. Discovery of a drug and/or component, equipment, their toxicological studies, dose, side effects are in vivo studied for future use in humans considering its’ ethical issues. Here lies the importance of the animal model for its enormous use in biomedical research. Animal models have many facets that mimic various disease conditions in humans like systemic autoimmune diseases, rheumatoid arthritis, epilepsy, Alzheimer’s disease, cardiovascular diseases, Atherosclerosis, diabetes, etc., and many more. Besides, the model has tremendous importance in drug development, development of medical devices, tissue engineering, wound healing, and bone and cartilage regeneration studies, as a model in vascular surgeries as well as the model for vertebral disc regeneration surgery. Though, all the models have some advantages as well as challenges, but, present review has emphasized the importance of various small and large animal models in pharmaceutical drug development, transgenic animal models, models for medical device developments, studies for various human diseases, bone and cartilage regeneration model, diabetic and burn wound model as well as surgical models like vascular surgeries and surgeries for intervertebral disc degeneration considering all the ethical issues of that specific animal model. Despite, the process of using the animal model has facilitated researchers to carry out the researches that would have been impossible to accomplish in human considering the ethical prohibitions.

Anti-inflammatory and Tendon-Protective Effects of Adipose Stem Cell-Derived Exosomes with Concomitant Use of Glucocorticoids

Glucocorticoid (GC) injections are commonly used in clinical practice to relieve pain and improve function in patients with multiple shoulder disabilities but cause detrimental effects on rotator cuff tendons. Adipose stem cell-derived exosomes (ASC-Exos) reportedly recover impaired tendon matrix metabolism by maintaining tissue homeostasis. However, it is unclear whether additional treatment with ASC-Exos overrides the detrimental effects of GCs without interfering with their anti-inflammatory effects. Thus, we aimed to investigate the anti-inflammatory effect of ASC-Exos with GCs and protective effect of ASC-Exos against GC-induced detriments. The present study comprised in vitro and in vivo studies. In vitro inflammatory analysis revealed that ASC-Exos exerted a synergic anti-inflammatory effect with GCs by significantly decreasing secretion of proinflammatory cytokines by RAW cells and increasing secretion of anti-inflammatory cytokines. In vitro cytoprotective analysis showed that ASC-Exos overrode GC-induced detrimental effects on tenocytes by significantly improving GC-suppressed cellular proliferation, migration, and transcription of tenocytic matrix molecules and degradative enzyme inhibitors and significantly decreasing GC-induced cell senescence, apoptosis, and transcription of ROS and tenocytic degradative enzymes. In vivo studies revealed that additional ASC-Exo injection restored impairments in histological and biomechanical properties owing to GC administration. Collectively, these results suggest that ASC-Exos exert a stronger anti-inflammatory effect in combination with GCs, overriding their detrimental effects on rotator cuff tendons.

Progression of muscle loss and fat accumulation in a rabbit model of rotator cuff tear

Rotator cuff (RC) tears present a treatment challenge due to muscle atrophy and degeneration, fatty infiltration, and fibrosis. The purpose of this study was to generate a high time-resolution model of RC tear in rabbits and to characterize the progression of architectural and histological changes. Thirty-five female New Zealand White rabbits (age: 6 months) underwent left supraspinatus tenotomy. Five rabbits were used to evaluate immediate muscle architectural changes. The remaining 30 rabbits underwent right shoulder sham surgery and sacrifice at 1, 2, 4, 8, or 16 weeks. Histology was used to quantify muscle fiber cross-sectional area (CSA), muscle degeneration and regeneration, and fat localized to inter- versus intrafascicular regions. Muscle fiber CSA decreased by 26.5% compared to sham at 16 weeks (effect of treatment, p < 0.0001). Muscle degeneration increased after tenotomy (effect of treatment, p = 0.0006) without any change in regeneration. Collagen and fat content increased by 4 weeks and persisted through 16 weeks. Interfascicular fat was increased at all time points, but intrafascicular fat was increased only at 1, 4, and 16 weeks posttenotomy. Intrafascicular fat adjacent to degenerating muscle fibers increased as well (effect of treatment, p < 0.0001; effect of time, p = 0.0102). Statement of clinical relevance: Rabbit supraspinatus tenotomy recapitulates key features of the pathophysiology of human RC tears, including muscle atrophy and degeneration, lack of regeneration, fat accumulation, and fibrosis.

Translational therapy from preclinical animal models for muscle degeneration after rotator cuff injury

Chronic rotator cuff tears are debilitating diseases which significantly affect patients’ quality of life and pose substantial financial burden to the society. The intraoperative reparability of injured tendon and postoperative probability of tendon retear are highly associated with the quality of torn muscles, specifically, the severity of muscle atrophy and fatty infiltration. Animal models that reproduce the characteristic muscle pathology after rotator cuff injury have been developed and used to provide insight into the underlying biology and pathophysiology. In this review, we briefly summarize the current information obtained from preclinical animal studies regarding the degenerative change of cuff muscle subsequent to tendon release and/or suprascapular nerve denervation. Importantly, we focus on the potential translational therapeutic targets or agents for the prevention or reversal of muscle atrophy and fatty infiltration. While further studies are warranted to assess the safety and efficacy of novel therapies derived from these preclinical animal research, we believe that their clinical translation for the treatment of rotator cuff disorders is on the horizon.

The Translational potential of this article

Novel therapeutic strategies described in this review from preclinical animal studies hold a great translational potential for preventing or reversing rotator cuff muscle pathology, while further assessments on their safety and efficacy are warranted.

The “Second Hit” of Repair in a Rabbit Model of Chronic Rotator Cuff Tear

The rabbit supraspinatus is a useful translational model for rotator cuff (RC) repair because it recapitulates muscle atrophy and fat accumulation observed in humans after a chronic tear (the “first hit”). However, a timeline of RC tissue response after repair, especially with regard to recent evidence of muscle degeneration and lack of regeneration, is currently unavailable. Thus, the purpose of this study was to characterize the progression of muscle and fat changes over time after the repair of a chronic RC tear in the rabbit model. Two rounds of experiments were conducted in 2017–2018 and 2019–2020 with N = 18 and 16 skeletally mature New Zealand White rabbits, respectively. Animals underwent left supraspinatus tenotomy with repair 8 weeks later. The unoperated right shoulder served as control. The rabbits were sacrificed at 1-, 2-, 4-, and 8-weeks post-repair for histological and biochemical analysis. Atrophy, measured by fiber cross-sectional area and muscle mass, was greatest around 2 weeks after repair. Active muscle degeneration peaked at the same time, involving 8% of slide areas. There was no significant regeneration at any timepoint. Fat accumulation and fibrosis were significantly increased across all time points compared to contralateral. Statement of Clinical Significance: These results demonstrate model reproducibility and a “second hit” phenomenon of repair-induced muscle atrophy and degeneration which partially recovers after a short time, while increased fat and fibrosis persist.

Aging Aggravates the Progression of Muscle Degeneration After Rotator Cuff Tears in Mice

PURPOSE

The purpose of this study was to evaluate the impact of aging on muscle degeneration after rotator cuff tear (RCT) in mice.

METHODS

Young (12-week-old) and aged (50-to-60-week-old) female C57BL/6 mice were used (n = 29 for each group). The rotator cuff was transected, and the proximal humerus was removed to induce degeneration of the rotator cuff muscles. The mice were euthanized 4 and 12 weeks after the procedure (referred to as RCT-4wk mice and RCT-12wk mice, respectively) and compared with the sham-treated mice. The supraspinatus muscles were collected for histology, Western blot analysis, and gene expression analyses.

RESULTS

There was a significant increase in fat tissue in aged RCT-4wk mice (P = .001) and aged RCT-12wk mice (P < .001) compared with sham-treated aged mice, and aged RCT-12wk mice had a significantly increased fat area ratio compared with aged RCT-4wk mice (P < .001). The fat area was significantly larger in both the aged RCT-4wk (P = .002) and RCT-12wk mice (P < .001) than in the corresponding young mice. Muscular fibrosis was significantly increased in aged RCT-12wk mice compared with aged sham-treated mice (P = .005) and young RCT-12wk mice (P = .016). There were also significant increases in the expression of perilipin and transcripts of adipogenic and fibrogenic differentiation markers in aged RCT mice compared with young RCT mice.

CONCLUSION

The present results show that aging is critically involved in the pathology of muscular fatty infiltration and fibrosis after RCT, and muscular degeneration progresses over time in aged mice.

CLINICAL RELEVANCE

Aging promotes the progression of muscle degeneration in a mouse RCT model. Furthermore, this study shows that muscle degeneration occurs in aged mice even without denervation and that the model described in the present study is a useful tool for studying the pathology of muscle degeneration.

Identification of Diagnostic Biomarkers Associated with Stromal and Immune Cell Infiltration in Fatty Infiltration After Rotator Cuff Tear by Integrating Bioinformatic Analysis and Machine-Learning

Purpose

The present study aimed to explore potential diagnostic biomarkers for fatty infiltration (FI) of the rotator cuff muscles after rotator cuff tear (RCT) and investigate the influence of stromal and immune cell infiltration on this pathology.

Methods

The GSE130447 and GSE103266 datasets were downloaded from the Gene Expression Omnibus database. Differentially expressed genes (DEGs) were identified, and gene set enrichment analyses were performed by R software. Two machine learning algorithms, random forest and multiple support vector machine recursive feature elimination (mSVM-RFE), were used to screen candidate biomarkers. The diagnostic value of the screened biomarkers was further validated by the area under the ROC curve (AUC) in the GSE103266 dataset. Murine microenvironment cell population counter (mMCP-counter) method was employed to estimate stromal and immune cell infiltration of FI. The correlation between biomarkers and infiltrated immune and stromal cell subsets was further analyzed.

Results

A total of 2123 DEGs were identified. The identified DEGs were predominantly linked to immune system process, extracellular matrix organization and PPAR signalling pathway. FABP5 (AUC = 0.958) and MGP (AUC = 1) were screened as diagnostic biomarkers of FI. Stromal and immune cell infiltration analysis showed that monocytes, mast cells, vessels, endothelial cells and fibroblasts may be related to the process of FI. FABP5 and MGP were positively correlated with vessels whereas negatively correlated with monocytes and mast cells.

Conclusion

FABP5 and MGP can serve as diagnostic biomarkers of FI after RCT, and stromal and immune cell infiltration may play a crucial role in this pathology.

Cervical Spine Pathology Increases the Risk of Rotator Cuff Tear: A Population-Based Cohort Study

Background:

Patients with cervical radiculopathy typically present with shoulder pain and weakness; these symptoms are similar to those of rotator cuff disease. Studies investigating cervical spine pathology (CSP) as an independent risk factor for rotator cuff tear (RCT) are lacking in the literature.

Purpose:

To investigate the risk of RCT among patients with CSP who have undergone cervical diskectomy (CD) and to determine whether CD reduces this risk.

Study Design:

Cohort study; Level of evidence, 3.

Methods:

The authors queried the Taiwan National Health Insurance Research Database for patients diagnosed with CSP between 2004 and 2008 and followed up until the end of 2010. A control cohort comprised patients without CSP who were age- and sex-matched in a 4-to-1 ratio with patients with CSP through propensity score matching. A Cox multivariate proportional hazards model was applied to analyze the risk factors for RCT. After adjustment for confounders, the authors calculated the hazard ratio (HR) and adjusted HR (aHR) between the study and control cohorts. The effects of CD on the risk of RCT were also analyzed.

Results:

The study included 3245 patients and 12,980 matched controls. A higher RCT incidence rate was found in the CSP cohort, with an aHR of 1.52 (95% CI, 1.22-1.89; P < .001). Patients with CSP who underwent CD had a risk of RCT similar to that of the controls, with an aHR of 1.65 (95% CI, 0.90-3.03; P > .05).

Conclusion:

Patients with CSP had a 1.52-fold higher risk of RCT than healthy controls. Patients with CSP with CD did not have a high risk of RCT, possibly indicating a protective effect of diskectomy against RCT.

Injection of Micronized Human Amnion/Chorion Membrane Results in Increased Early Supraspinatus Muscle Regeneration in a Chronic Model of Rotator Cuff Tear

Surgical repair of severe rotator cuff tear often results in retear due to unaddressed muscle degeneration. The objective of this study was to test the regenerative potential of micronized dehydrated Human Amnion/Chorion Membrane (dHACM), in a clinically relevant delayed reattachment model of rotator cuff repair. Micronized dHACM was injected into rat supraspinatus muscle during tendon re-attachment surgery, three weeks after original tendon injury. One week after material injection, inflammatory and mesenchymal stem cell infiltration into supraspinatus muscles was assessed via flow cytometry. Histological methods were utilized to assess structural and regenerative changes in muscle one and three weeks after material injection. Micronized dHACM injection resulted in increased M1-like macrophages (17.1 [Formula: see text] fold change over contralateral controls) and regenerating muscle fibers (4.3% vs 1.7% in saline treated muscles) one week after injection compared to saline treated muscles. Tendon reattachment itself exhibited intrinsic healing in this model, demonstrated by a general return of muscle weight and reduced fibrosis. Our results indicate that injection of micronized dHACM may initiate an inflammatory response in degenerated muscle that promotes early muscle regeneration, and that our animal model may be a suitable platform for studying treatments in muscle at early timepoints, before intrinsic healing occurs.

Polydeoxyribonucleotide and Polynucleotide Improve Tendon Healing and Decrease Fatty Degeneration in a Rat Cuff Repair Model

BACKGROUND

After surgical repair of chronic rotator cuff tears, healing of the repaired tendons often fails and is accompanied by high-level fatty degeneration. Our purpose was to explore the effects of polydeoxyribonucleotide (PDRN) and polynucleotide (PN) on tendon healing and the reversal of fatty degeneration in a chronic rotator cuff tear model using a rat infraspinatus.

METHODS

Sixty rats were randomly assigned to the following three groups (20 rats per group: 12 for histological evaluation and 8 for mechanical testing): saline + repair (SR), PDRN + repair (PR), and PN + repair (PNR). The right shoulder was used for experimental intervention, and the left served as a control. Four weeks after detaching the infraspinatus, the torn tendon was repaired. Saline, PDRN, and PN were applied to the repair sites. Histological evaluation was performed 3 and 6 weeks after repair and biomechanical analysis was performed at 6 weeks.

RESULTS

Three weeks after repair, the PR and PNR groups had more CD168-stained cells than the SR group. The PR group showed a larger cross-sectional area (CSA) of muscle fibers than the SR and PNR groups. Six weeks after repair, the PR and PNR groups showed more adipose cells, less CD68-stained cells, and more parallel tendon collagen fibers than the SR group. The PR group had more CD 68-stained cells than the PNR group. The PR group showed a larger CSA than the SR group. The mean load-to-failure values of the PR and PNR groups were higher than that of the SR group, although these differences were not significant.

CONCLUSION

PDRN and PN may improve tendon healing and decrease fatty degeneration after cuff repair.

Arthroscopic Tenotomy of the Long Head of the Biceps Tendon and Section of the Anterior Joint Capsule Produce Moderate Osteoarthritic Changes in an Experimental Sheep Model

Osteoarthritis (OA) of the glenohumeral (GH) joint is a common cause of shoulder pain, resulting in considerable invalidity. Unfortunately, the study of its pathogenesis is challenging. Models of OA are necessary to identify specific targets for therapy and to be able to interfere with the development and evolution of OA. This study aims to assess the effect of an arthroscopic tenotomy of the long head of the biceps tendon (LHBT) and section of the anterior glenohumeral joint capsule on the ovine glenohumeral joint. In addition, the authors aim to validate and evaluate the reliability of a modified semi-quantitative MRI score to assess joint degeneration in a sheep’s shoulder. Eight skeletally mature sheep received an arthroscopic tenotomy of the LHBT and section of the anterior joint capsule and were euthanized four months after surgery. All animals tolerated the surgery well, and no complication was recorded for six weeks. Moderate degenerative changes to the ovine shoulder joint were found on MRI and histological evaluation. The arthroscopic tenotomy of the LHBT and the anterior glenohumeral joint capsule section caused moderate degenerative changes to the ovine shoulder joint.

In vivo supraspinatus muscle contractility and architecture in rabbit

The rotator cuff (RC) muscles are crucial in moving and stabilizing the glenohumeral joint, and tears can be functionally devastating. Chronic fatty and fibrotic muscle changes, which are nonresponsive to surgical tendon repair, are a focus of contemporary research. The rabbit model recapitulates key biological features of human RC tears, but function and physiology are poorly characterized; limited force and stress data are inconsistent with literature norms in other mammalian species. Here, we present an improved method to assess the physiology of the rabbit supraspinatus muscle (SSP), and we report values for healthy SSP architecture and physiology. Using female New Zealand White Rabbits (n = 6) under 2% isoflurane anesthesia, we surgically isolated the SSP and maximum isometric force measured at 4-6 muscle lengths. Architectural analysis was performed, and maximum isometric stress was computed. Whole muscle length-tension curves were generated using architectural measurements to compare experimental physiology to theoretical predictions. Maximum isometric force (80.87 ± 5.58 N) was dramatically greater than previous reports (11.06 and 16.1 N; P < 0.05). Architectural measurement of fiber length (34.25 ± 7.18 mm), muscle mass (9.9 ± 0.93 g), pennation angle (23.67 ± 8.32°), and PCSA (2.57 ± 0.20 cm²) were consistent with prior literature. Isometric stress (30.5 ± 3.07 N/cm²) was greater than previous reports of rabbit SSP (3.10 and 4.51 N/cm²), but similar to mammalian skeletal muscles (15.7-30.13 N/cm²). Previous studies underestimated peak force by ∼90%, which has profound implications for interpreting physiological changes as a function of disease state. The data that are presented here enable understanding the physiological implications of disease and repair in the RC of the rabbit.NEW & NOTEWORTHY We introduce an improved method to assess rabbit supraspinatus muscle physiology. Maximum isometric force measured for the rabbit supraspinatus was dramatically greater than previous reports in the literature. Consequently, the isometric contractile stress reported is almost 10 times greater than previous reports of rabbit supraspinatus, but similar to available literature of other mammalian skeletal muscle. We show that previous reports of peak supraspinatus isometric force were subphysiological by ∼90.

Engineered Cell Sheets for the Effective Delivery of Adipose-Derived Stem Cells for Tendon-to-Bone Healing

BACKGROUND

Efforts are being made to treat rotator cuff tears (RCTs) that exhibit poor healing and high retear rates. Tendon-to-bone healing using mesenchymal stem cells is being explored, but research is needed to establish effective delivery options.

PURPOSE

To evaluate the effects of an adipose-derived stem cell (ADSC) sheet on mesenchymal stem cell delivery for tendon-to-bone healing of a chronic RCT in rats and to demonstrate that ADSC sheets enhance tendon-to-bone healing.

STUDY DESIGN

Controlled laboratory study.

METHODS

Mesenchymal stem cells were obtained from rat adipose tissue, and a cell sheet was prepared using a temperature-responsive dish. To evaluate the efficacy of stem cells produced in a sheet for the lesion, the experiment was conducted with 3 groups: repair group, cell sheet transplantation after repair group, and cell sheet-only group. Histological, biomechanical, and micro-computed tomography (micro-CT) results were compared among the groups.

RESULTS

Hematoxylin and eosin staining for histomorphological analysis revealed that the cell sheet transplantation after repair group (5.75 ± 0.95) showed statistically significant higher scores than the repair (2.75 ± 0.50) and cell sheet-only (3.25 ± 0.50) groups (P < .001). On safranin O staining, the cell sheet transplantation after repair group (0.51 ± 0.04 mm²) had a larger fibrocartilage area than the repair (0.31 ± 0.06 mm²) and cell sheet-only (0.32 ± 0.03 mm²) groups (P = .001). On micro-CT, bone volume/total volume values were significantly higher in the cell sheet transplantation after repair group (23.98% ± 1.75%) than in the other groups (P < .039); there was no significant difference in the other values. On the biomechanical test, the cell sheet transplantation after repair group (4 weeks after repair) showed significantly higher results than the other groups (P < .005).

CONCLUSION

Our study shows that engineered stem cells are a clinically feasible stem cell delivery tool for rotator cuff repair.

CLINICAL RELEVANCE

This laboratory study provides evidence that ADSCs are effective in repairing RCTs, which are common sports injuries.

Assessment of whether the rabbit subscapularis tendon model is suitable for studying the human chronic rotator cuff pathology: Discovery of a new ligament connecting the glenoid and subscapularis tendon

OBJECTIVE

This study aimed to investigate the anatomical relationship between the subscapularis tendon and glenosubscapularal ligament (GSL) that we accidentally identified from our previous study on a rabbit shoulder model and to determine whether this anatomical relationship has an impact on the rabbit shoulder model for studying the human chronic rotator cuff pathology.

METHODS

In this study, 15 male New Zealand rabbits aged 12 weeks and weighing 2.5 kg were used. Moreover, 3 rabbits were sacrificed for the anatomical and histological investigation of the relationship between the subscapularis tendon and GSL at baseline. The remaining 12 rabbits underwent the subscapularis tendon tenotomy from the lesser tuberosity using a standardized procedure. The GSL was cut on the left side and preserved on the right side. For histomorphometric analysis, 6 rabbits were first sacrificed at 6 weeks and then the remaining 6 rabbits at 12 weeks.

RESULTS

In all the rabbits, GSL was identified, connecting the upper portion of glenoid and subscapularis muscle-tendon junction. The mean thickness of the middle portion of GSL was 1.1±0.2 mm; the mean length of GSL was 8.4±2.3 mm. The mean widths of the proximal and distal attachments were 2.4±0.3 and 4.2±0.5 mm, respectively. The mean size of the native subscapularis muscle fibers was 122.6±4.3 μm2. The mean size of the muscle fibers in shoulders with tenotomy alone was 112.6±6.2 and 102.6±4.8 μm2 at 6 and 12 weeks, respectively. The mean size of the muscle fibers in shoulders with tenotomy plus GSL cut severing was 88.3±9.7 and 56.4±5.2 μm2 at 6 and 12 weeks, respectively. The significant muscle atrophy was observed both at 6 and 12 weeks in the shoulders with tenotomy plus GSL cut compared with those with tenotomy alone as well as those with the native subscapularis. However, the muscle atrophy was not significantly different in the shoulders with tenotomy alone at different time points.

CONCLUSION

Because GSL may prevent the subscapularis retraction, the rabbit subscapularis tendon model may not be suitable for studying the human chronic rotator cuff pathology if GSL is neglected or preserved.

A Synthetic Graft With Multilayered Co-Electrospinning Nanoscaffolds for Bridging Massive Rotator Cuff Tear in a Rat Model

BACKGROUND

Graft bridging is used in massive rotator cuff tear (MRCT); however, the integration of graft-tendon and graft-bone is still a challenge.

HYPOTHESIS

A co-electrospinning nanoscaffold of polycaprolactone (PCL) with an "enthesis-mimicking" (EM) structure could bridge MRCT, facilitate tendon regeneration, and improve graft-bone healing.

STUDY DESIGN

Controlled laboratory study.

METHODS

First, we analyzed the cytocompatibility of the electrospinning nanoscaffolds, including aligned PCL (aPCL), nonaligned PCL (nPCL), aPCL-collagen I, nPCL-collagen II, and nPCL-nanohydroxyapatite (nHA). Second, for the EM condition, nPCL-collagen II and nPCL-nHA were electrospun layer by layer at one end of the aPCL-collagen I; for the control condition, the nPCL was electrospun on the aPCL. In 40 mature male rats, resection of both the supraspinatus and infraspinatus tendons was performed to create MRCT, and the animals were divided randomly into EM and control groups. In both groups, one end of the layered structure was fixed on the footprint of the rotator cuff, whereas the other end of the layered structure was sutured with the tendon stump. The animals were euthanized for harvesting of tissues for histologic and biomechanical analysis at 4 weeks or 8 weeks postoperatively.

RESULTS

All scaffolds showed good cytocompatibility in vitro. The graft-tendon tissue in the EM group had more regularly arranged cells, denser tissue, a significantly higher tendon maturing score, and more birefringence compared with the control group at 8 weeks after operation. Newly formed fibrocartilage could be observed at the graft-bone interface in both groups by 8 weeks, but the EM group had a higher graft-bone healing score and significantly more newly formed fibrocartilage than the control group. An enthesis-like structure with transitional layers was observed in the EM group at 8 weeks. Biomechanically, the values for maximum failure load and stiffness of the tendon-graft-bone complex were significantly higher in the EM group than in the control group at 8 weeks.

CONCLUSION

The co-electrospinning nanoscaffold of aPCL-collagen I could be used as a bridging graft to improve early graft-tendon healing for MRCT in a rat model and enhance early enthesis reconstruction in combination with a multilayered structure of nPCL-collagen II and nPCL-nHA.

CLINICAL RELEVANCE

We constructed a graft to bridge MRCT, enhance graft-tendon healing and graft-bone healing, and reconstruct the enthesis structure.

Adipose Stem Cell-Derived Exosomes Decrease Fatty Infiltration and Enhance Rotator Cuff Healing in a Rabbit Model of Chronic Tears

BACKGROUND

Fatty infiltration and poor tendon-bone healing in chronic rotator cuff tears (RCTs) are associated with unsatisfactory prognosis. Adipose stem cell-derived exosomes (ASC-Exos), having multiple biological effects, can prevent muscle degeneration in acute RCTs. However, the effects of ASC-Exos on fatty infiltration and tendon-bone healing in chronic RCTs remain unknown.

PURPOSE

To study the effects of ASC-Exos on fatty infiltration and tendon-bone healing in a chronic RCT rabbit model.

STUDY DESIGN

Controlled laboratory study.

METHODS

At week 0, we randomly allocated 35 rabbits to receive sham surgery (14 rabbits) or establish a bilateral RCT model (21 rabbits, detachment of the supraspinatus tendon). At week 6, a total of 7 rabbits received sham surgery, and 7 rabbits with RCT were sacrificed for fatty infiltration assay. The remaining 14 rabbits with bilateral RCTs were randomly assigned to a saline group (7 rabbits that received local saline injection and rotator cuff repair) or an ASC-Exos group (7 rabbits that received local ASC-Exos injection and rotator cuff repair). At week 18, all rabbits were sacrificed for histological examination and biomechanical testing.

RESULTS

At week 18, the ASC-Exos group showed significantly lower fatty infiltration (14.01% ± 2.85%) compared with the saline group (21.79% ± 3.07%) (P < .001), and no statistical difference compared with the time of repair (10.88% ± 2.64%) (P = .127). For tendon-bone healing, the ASC-Exos group showed a higher histological score and more newly regenerated fibrocartilage at the repair site than did the saline group. Regarding biomechanical testing, the ASC-Exos group showed significantly higher ultimate load to failure, stiffness, and stress than the saline group.

CONCLUSION

Local injection of ASC-Exos in chronic RCTs at the time of repair could prevent the progress of fatty infiltration, promote tendon-bone healing, and improve biomechanical properties.

CLINICAL RELEVANCE

ASC-Exos injection may be used as a cell-free adjunctive therapy to inhibit fatty infiltration and improve rotator cuff healing in the repair of chronic RCTs.

Partial Infraspinatus Tendon Transection as a Means for the Development of a Translational Ovine Chronic Rotator Cuff Disease Model

OBJECTIVE

 Rotator cuff tendon tears are the most common soft tissue injuries in the shoulder joint. Various animal models have been described for this condition, but all current translational animal models have inherent weaknesses in their ability to generate chronically degenerated rotator cuff tendons. The objective of this study was to evaluate a partial infraspinatus tendon transection model as a means of creating a chronically degenerated rotator cuff tendon in an ovine model and compare the injury characteristics of this model to those observed in human patients with severe chronic rotator cuff tendon injuries.

STUDY DESIGN

 The infraspinatus tendons of six sheep were partially detached followed by capping of the detached medial section of the tendon with Gore-Tex. Human tissue samples of the supraspinatus tendon were harvested from patients undergoing primary reverse shoulder arthroplasty and served as positive controls of chronic rotator cuff tendinopathy.

RESULTS

 Transected sheep tendons were characterized predominantly by an acute reactive and reparative pathological process as compared with the chronic degenerative changes observed in the human tendons. In contrast, the non-transected portion of the ovine tendon showed histological changes, which were more chronic and degenerative in nature when compared with the transected tendon.

CONCLUSION

 Overall, histological features of the non-transected portion of ovine tendon were more similar to those observed in the chronic degenerated human tendon.

Suprascapular nerve injury affects rotator cuff healing: A paired controlled study in a rat model

Purpose

We designed a paired controlled study to investigate the role of the suprascapular nerve (SSN) in rotator cuff healing using a rat tear model, and we hypothesised that rotator cuff healing is impaired in the absence of a healthy SSN.

Methods

Bilateral supraspinatus tenotomy from the great tuberosity was performed for 36 Wistar rats, which was then repaired immediately. A defect on the SSN was made on the right side, and a sham surgery was performed on the SSN at the left side. Twelve rats were sacrificed for biomechanical (six rats) and histological (six rats) testing, evaluated at 3, 6, and 9 weeks after surgery.

Results

The bone–tendon junction on the nerve-intact side showed significantly better biomechanical characteristics than the nerve-injured side in terms of maximum load, maximum stress over time, stiffness at 9 weeks, and Young's modulus at 3 and 9 weeks. On the nerve-injured side, significantly smaller fibrocartilage layers and muscle fibres could be obtained over time. In addition, on the nerve-injured side, inferior bone–tendon interface formation was obtained in terms of cell maturity, cell alignment, collagen orientation, and the occurrence of tidemark and Sharpey's fibres through 9 weeks. In addition, neuropeptide Y was secreted in the nerve-intact group at 6 and 9 weeks.

Conclusion

This study showed the inferior healing of the bone–tendon junction on the nerve-injured side compared with the nerve-intact side, which indicates that the SSN plays an important role in rotator cuff healing. Surgeons should pay more attention to SSN injury when treating patients with rotator cuff tear.

Influence of a nerve injury proximal to the suprascapular nerve on healing of repaired rotator cuff tear

BACKGROUND

Massive rotator cuff tears have a high rate of re-injury because of severe fatty infiltration. Our data showed that injuries proximal to the suprascapular nerve may be one cause of massive rotator cuff tears. The purpose of this study was to evaluate, using a rat model, how brachial plexus injury associated with a massive rotator cuff tear influences healing of the rotator cuff repair.

METHODS

Seventy Sprague-Dawley rats were divided into three groups: rotator cuff tear with BP injury (DT group) (n = 28), rotator cuff tear without brachial plexus injury (T group) (n = 28), and a sham-operated group (n = 14). In the DT group, the rotator cuff tear was made and repaired 4 weeks after brachial plexus ligation. The gross assessment (evaluated the wet weight), biomechanical testing (evaluated the yield stress and the Young's modulus) and histological analyses (using the Bonar scale) were performed at baseline in the sham group, and at 4 and 12 weeks postoperatively in the DT and T groups (n = 7/group/time).

RESULTS

Mean wet weight and yield stress were significantly lower in the DT group than in the T group. Additionally, the mean Young's modulus was significantly higher in the DT group than in the T group. Histologically, greater tendon degeneration was observed around the musculotendinous junction in the DT group than in the T group.

CONCLUSION

The gross, biomechanical and histological data show that the repaired rotator cuff tendon with brachial plexus injury in rats does not heal as well as a repaired tendon without an accompanying brachial plexus injury. This suggests that more proximal neuropathy is one risk factor for re-tear of a repaired rotator cuff tendon.

Non-fibro-adipogenic pericytes from human embryonic stem cells attenuate degeneration of the chronically injured mouse muscle

Massive tears of the rotator cuff (RC) are associated with chronic muscle degeneration due to fibrosis, fatty infiltration, and muscle atrophy. The microenvironment of diseased muscle often impairs efficient engraftment and regenerative activity of transplanted myogenic precursors. Accumulating myofibroblasts and fat cells disrupt the muscle stem cell niche and myogenic cell signaling and deposit excess disorganized connective tissue. Therefore, restoration of the damaged stromal niche with non-fibro-adipogenic cells is a prerequisite to successful repair of an injured RC. We generated from human embryonic stem cells (hES) a potentially novel subset of PDGFR-β+CD146+CD34-CD56- pericytes that lack expression of the fibro-adipogenic cell marker PDGFR-α. Accordingly, the PDGFR-β+PDGFR-α- phenotype typified non-fibro-adipogenic, non-myogenic, pericyte-like derivatives that maintained non-fibro-adipogenic properties when transplanted into chronically injured murine RCs. Although administered hES pericytes inhibited developing fibrosis at early and late stages of progressive muscle degeneration, transplanted PDGFR-β+PDGFR-α+ human muscle-derived fibro-adipogenic progenitors contributed to adipogenesis and greater fibrosis. Additionally, transplanted hES pericytes substantially attenuated muscle atrophy at all tested injection time points after injury. Coinciding with this observation, conditioned medium from cultured hES pericytes rescued atrophic myotubes in vitro. These findings imply that non-fibro-adipogenic hES pericytes recapitulate the myogenic stromal niche and may be used to improve cell-based treatments for chronic muscle disorders.

Adipocyte hyperplasia: the primary mechanism of supraspinatus intramuscular fat accumulation after a complete rotator cuff tendon tear: a study in the rabbit

Intramuscular fat (IMF) accumulates in muscles of the rotator cuff after tendon tear. The number and cross-sectional area of fat clumps and of adipocytes were quantified on osmium tetroxide stained sections of the proximal, middle and distal quarters of SSP muscles 4, 8 and 12 weeks after SSP tendon division in a rabbit model. Linear mixed-effects models were fitted to the data and statistical significance was evaluated by ANOVA. Both the number (P<0.001) and cross-sectional area (P<0.0005) of fat clumps increased after tendon detachment while time had no significant effect (both at P>0.01). IMF accumulation was more important in the distal quarter of detached SSP muscle near tendon sectioning and characterized by increases of the number (P<0.0005) and cross-sectional area of fat clumps (P<0.0005) compared to the proximal quarter. Adipocyte number increased after tendon detachment (P<0.0005) and over time (P<0.01). The cross-sectional area of adipocytes increased in the detached group compared to controls (P<0.01) while time had no significant effect (P>0.01). Interestingly, the number of adipocytes in the distal quarter increased (P<0.0005) but the cross-sectional area was smaller (P<0.0005) compared to adipocytes in the proximal quarter. Adipocyte hyperplasia localized near tendon sectioning was the main contributor to fat accumulation in the detached SSP muscles.

Exosomes Isolated From Adipose-Derived Stem Cells: A New Cell-Free Approach to Prevent the Muscle Degeneration Associated With Torn Rotator Cuffs

BACKGROUND

Fatty infiltration, inflammation, and apoptosis are common degenerative changes in patients with chronic rotator cuff tears that can lead to muscle atrophy and can even result in massive irreparable rotator cuff tears. Some data have demonstrated the proregenerative, anti-inflammatory, and anti-apoptotic properties of stem cell-derived exosomes in some orthopaedic disorders, but their effect on torn rotator cuff muscles has never been investigated.

PURPOSE

To study the effect of exosomes isolated from human adipose-derived stem cells (ASCs-Exos) on muscle degeneration, regeneration, and biomechanical properties in a rat model of a massive rotator cuff tear (MRCT).

STUDY DESIGN

Controlled laboratory study.

METHODS

A bilateral supraspinatus and infraspinatus tenotomy was performed on rats to create an MRCT model. Forty-two rats were randomly assigned to 3 groups: the sham surgery group, the saline group (lesions treated with a saline injection), and the ASCs-Exos group (lesions treated with an ASCs-Exos injection). Wet muscle weight, fatty infiltration, inflammation, vascularization, regeneration, and biomechanical properties were evaluated at 8 and 16 weeks after surgery.

RESULTS

The results revealed that the ASCs-Exos treatment could prevent the atrophy, fatty infiltration, inflammation, and vascularization of muscles in the MRCT model (P < .001). Additionally, the myofiber regeneration and biomechanical properties of ASCs-Exos-treated rotator cuffs were significantly elevated compared with those in the saline-treated group (P < .001).

CONCLUSION

This study demonstrates that ASCs-Exos can effectively decrease atrophy and degeneration and improve muscle regeneration and biomechanical properties in torn rotator cuff muscles.

CLINICAL RELEVANCE

ASCs-Exos can be used as a new cell-free approach to prevent the muscle degeneration associated with torn rotator cuffs and may be helpful to repair torn rotator cuffs. Nevertheless, further work needs to be done in a large animal model owing to the inherent regenerative potential possessed by rodents.

Human Tendon-Derived Collagen Hydrogel Significantly Improves Biomechanical Properties of the Tendon-Bone Interface in a Chronic Rotator Cuff Injury Model

PURPOSE

Poor healing of the tendon-bone interface (TBI) after rotator cuff (RTC) tears leads to high rates of recurrent tear following repair. Previously, we demonstrated that an injectable, thermoresponsive, type I collagen-rich, decellularized human tendon-derived hydrogel (tHG) improved healing in an acute rat Achilles tendon injury model. The purpose of this study was to investigate whether tHG enhances the biomechanical properties of the regenerated TBI in a rat model of chronic RTC injury and repair.

METHODS

Tendon hydrogel was prepared from chemically decellularized human cadaveric flexor tendons. Eight weeks after bilateral resection of supraspinatus tendons, repair of both shoulders was performed. One shoulder was treated with a transosseous suture (control group) and the other was treated with a transosseous suture plus tHG injection at the repair site (tHG group). Eight weeks after repair, the TBIs were evaluated biomechanically, histologically, and via micro-computed tomography (CT).

RESULTS

Biomechanical testing revealed a larger load to failure, higher stiffness, higher energy to failure, larger strain at failure, and higher toughness in the tHG group versus control. The area of new cartilage formation was significantly larger in the tHG group. Micro-CT revealed no significant difference between groups in bone morphometry at the supraspinatus tendon insertion, although the tHG group was superior to the control.

CONCLUSIONS

Injection of tHG at the RTC repair site enhanced biomechanical properties and increased fibrocartilage formation at the TBI in a chronic injury model.

CLINICAL RELEVANCE

Treatment of chronic RTC injuries with tHG at the time of surgical treatment may improve outcomes after surgical repair.

Leucine attenuates muscle atrophy and autophagosome formation by activating PI3K/AKT/mTOR signaling pathway in rotator cuff tears

Rotator cuff tears (RCTs), the most common tendon injury, are always accompanied by muscle atrophy, which is characterized by excessive protein degradation. Autophagy-lysosome systems are the crucial proteolytic pathways and are activated in atrophying muscle. Thus, inhibition of the autophagy-lysosome pathway might be an alternative way to minimize skeletal muscle atrophy. In this present study, combined with a tendon transection-induced rat model of massive rotator cuff tears, HE staining and transmission electron microscopy methods, we found leucine supplementation effectively prevented muscle atrophy, muscle injury and autophagosome formation. Utilizing immunoblotting, we discovered that leucine supplementation is able to inhibit the rise in autophagy-related protein expression (including LC3, Atrogin-1, MuRF1, Bnip3 and FoxO3) driven by tendon transection. The PI3K/AKT/mTOR pathway that was essential in autophagosome formation and autophagy was blocked in atrophying muscle and reactivated in the presence of leucine. Once administrated with LY294002 (PI3K inhibitor) and Rapamycin (mTOR inhibitor), leucine mediated by the anti-atrophic effects was nearly blunted. These results suggest that leucine potentially attenuates tendon transection-induced muscle atrophy through autophagy inhibition via activating the PI3K/AKT/mTOR pathway.

Skeletal Muscle Regenerative Engineering

Skeletal muscles have the intrinsic ability to regenerate after minor injury, but under certain circumstances such as severe trauma from accidents, chronic diseases or battlefield injuries the regeneration process is limited. Skeletal muscle regenerative engineering has emerged as a promising approach to address this clinical issue. The regenerative engineering approach involves the convergence of advanced materials science, stem cell science, physical forces, insights from developmental biology, and clinical translation. This article reviews recent studies showing the potential of the convergences of technologies involving biomaterials, stem cells and bioactive factors in concert with clinical translation, in promoting skeletal muscle regeneration. Several types of biomaterials such as electrospun nanofibers, hydrogels, patterned scaffolds, decellularized tissues, and conductive matrices are being investigated. Detailed discussions are given on how these biomaterials can interact with cells and modulate their behavior through physical, chemical and mechanical cues. In addition, the application of physical forces such as mechanical and electrical stimulation are reviewed as strategies that can further enhance muscle contractility and functionality. The review also discusses established animal models to evaluate regeneration in two clinically relevant muscle injuries; volumetric muscle loss (VML) and muscle atrophy upon rotator cuff injury. Regenerative engineering approaches using advanced biomaterials, cells, and physical forces, developmental cues along with insights from immunology, genetics and other aspects of clinical translation hold significant potential to develop promising strategies to support skeletal muscle regeneration.

How Long Should We Wait to Create the Goutallier Stage 2 Fatty Infiltrations in the Rabbit Shoulder for Repairable Rotator Cuff Tear Model?

Background

Significant proportion of rotator cuff tears (RCTs) in clinical field are of a kind of repairable tear wherein the degree of fatty infiltration is of Goutallier stage 1 or stage 2. Therefore, the animal model, showing similar fatty infiltration, seems preferable for researches. The purpose of this study is to find out the proper time frame in which there is Goutallier stage 1 or stage 2 fatty infiltration in the rabbit RCT model for the research of repairable RCT in humans.

Methods

Supraspinatus tendon tears were created in forty male New Zealand white rabbits at their right shoulder (n= 8 for each group), and a sham operation on the left shoulder. Rabbits were divided into five groups (2nd, 4th, 6th, 8th, and 12th weeks). Specimens were harvested from the central portion of the supraspinatus muscle for haematoxylin and eosin (H &E) staining, followed by histological and Goutallier grading evaluation. Results are expressed as mean ± standard deviation by Sigma Plot software (version 7.0).

Results

At two weeks, mainly lipoblasts were observed around the muscle fibers, and at four weeks these lipoblasts were replaced by mature adipocytes with fatty infiltration amount (2.13 ± 0.35). The degree of muscle atrophy was (1.50 ± 0.53) at four weeks compared to sham group (0.88 ± 0.64) with significant difference (p < 0.05). The inflammatory process appeared as two phases. At two weeks, it was increased with grading value (1.88 ± 0.35). However, in the four-week group, it showed a sharp decrease (0.50 ± 0.53). At six weeks, inflammation reappeared to increase (1.13 ± 0.83). Then, a gradual decline appeared at eight weeks (0.88 ± 0.83) and at 12 weeks (0.50 ± 0.92).

Conclusions

At two and four weeks, both fat distribution in rabbit supraspinatus muscles and Goutallier grading scale mostly appeared as grade 2. Therefore, we can consider four weeks to be a suitable period for making a repairable RCT animal model for the human research, considering the early acute tissue reaction at 2 weeks after the tendon tears.

Reduced mitochondrial lipid oxidation leads to fat accumulation in myosteatosis

Myosteatosis is the pathologic accumulation of lipid that can occur in conjunction with atrophy and fibrosis following skeletal muscle injury. Little is known about the mechanisms by which lipid accumulates in myosteatosis, but many clinical studies have demonstrated that the degree of lipid infiltration negatively correlates with muscle function and regeneration. Our objective was to determine the pathologic changes that result in lipid accumulation in injured muscle fibers. We used a rat model of rotator cuff injury in this study because the rotator cuff muscle group is particularly prone to the development of myosteatosis after injury. Muscles were collected from uninjured controls or 10, 30, or 60 d after injury and analyzed using a combination of muscle fiber contractility assessments, RNA sequencing, and undirected metabolomics, lipidomics, and proteomics, along with bioinformatics techniques to identify potential pathways and cellular processes that are dysregulated after rotator cuff tear. Bioinformatics analyses indicated that mitochondrial function was likely disrupted after injury. Based on these findings and given the role that mitochondria play in lipid metabolism, we then performed targeted biochemical and imaging studies and determined that mitochondrial dysfunction and reduced fatty acid oxidation likely leads to the accumulation of lipid in myosteatosis.-Gumucio, J. P., Qasawa, A. H., Ferrara, P. J., Malik, A. N., Funai, K., McDonagh, B., Mendias, C. L. Reduced mitochondrial lipid oxidation leads to fat accumulation in myosteatosis.

The effect of adipose-derived stem cells on enthesis healing after repair of acute and chronic massive rotator cuff tears in rats

BACKGROUND

Chronic massive rotator cuff tears heal poorly and often retear. This study investigated the effect of adipose-derived stem cells (ADSCs) and transforming growth factor-β3 (TGF-β3) delivered in 1 of 2 hydrogels (fibrin or gelatin methacrylate [GelMA]) on enthesis healing after repair of acute or chronic massive rotator cuff tears in rats.

METHODS

Adult male Lewis rats underwent bilateral transection of the supraspinatus and infraspinatus tendons with intramuscular injection of botulinum toxin A (n = 48 rats). After 8 weeks, animals received 1 of 8 interventions (n = 12 shoulders/group): (1) no repair, (2) repair only, or repair augmented with (3) fibrin, (4) GelMA, (5) fibrin + ADSCs, (6) GelMA + ADSCs, (7) fibrin + ADSCs + TGF-β3, or (8) GelMA + ADSCs + TGF-β3. An equal number of animals underwent acute tendon transection and immediate application of 1 of 8 interventions. Enthesis healing was evaluated 4 weeks after the repair by microcomputed tomography, histology, and mechanical testing.

RESULTS

Increased bone loss and reduced structural properties were seen in chronic compared with acute tears. Bone mineral density of the proximal humerus was higher in repairs of chronic tears augmented with fibrin + ADSCs and GelMA + ADSCs than in unrepaired chronic tears. Similar improvement was not seen in acute tears. No intervention enhanced histologic appearance or structural properties in acute or chronic tears.

CONCLUSIONS

Surgical repair augmented with ADSCs may provide more benefit in chronic tears compared with acute tears, although there was no added benefit to supplementing ADSCs with TGF-β3.

Full-thickness rotator cuff tear in rat results in distinct temporal expression of multiple proteases in tendon, muscle, and cartilage

The etiology of joint tissue degeneration following rotator cuff tear remains unclear. Thus, the purpose of this study was to understand the timeline of protease activity in the soft tissues of the shoulder (tendon, muscle, and cartilage) that may lead to down-stream degeneration following rotator cuff tear. A well-established rat model involving suprascapular nerve denervation and supraspinatus/infraspinatus tendon transection was employed. Histological staining and/or micro-computed tomography (µCT) were used to observe structural damage in the supraspinatus tendon and muscle, humeral head cartilage, and subchondral bone. Multiplex gelatin zymography was utilized to assess protease activity in the supraspinatus tendon and muscle, and humeral head cartilage. Zymography analysis demonstrated that cathepsins were upregulated in the first week in all tissues, while MMP-2 maintained prolonged activity in supraspinatus tendon between 1 and 3 weeks and increased only at 3 weeks in supraspinatus muscle. In supraspinatus tendon, increased cathepsin L and MMP-2 activity in the first week was concurrent with matrix disorganization and infiltration of inflammatory cells. In contrast, significant upregulation of cathepsin L and K activity in supraspinatus muscle and humeral head cartilage did not correspond to any visible tissue damage at 1 week. However, focal defects developed in half of all animals' humeral head cartilage by 12 weeks (volume: 0.12 ± 0.09 mm³ ). This work provides a more comprehensive understanding of biochemical changes to joint tissue over time following rotator cuff tear. Overall, this provides insight into potential therapeutic targets and will better inform ideal intervention times and treatments for each tissue. © 2018 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 37:490-502, 2019.

Amplification of Mitochondrial Activity in the Healing Response Following Rotator Cuff Tendon Injury

Mitochondrial function following rotator cuff tendon injury (RCI) influences the tendon healing. We examined the mitochondrial morphology and function under hypoxia in the shoulder tendon tissue from surgically-induced tenotomy-RCI rat model and cultured swine tenocytes. The tendon tissue was collected post-injury on 3-5 (Group-A), 10-12 (Group-B), and 22-24 (Group-C), days and the corresponding contralateral tendons were used as control for each group. There was higher protein expression of citrate synthase (P < 0.0001) [10.22 MFI (mean fluorescent intensity)] and complex-1 (P = 0.0008) (7.86 MFI) in Group-A and Group-B that decreased in Group-C [(P = 0.0201) (5.78 MFI and (P = 0.7915) (2.32 MFI), respectively] compared to control tendons. The ratio of BAX:Bcl2 (Bcl2 associated x protein:B cell lymphoma 2) in RCI tendons increased by 50.5% (Group-A) and 68.4% (Group-B) and decreased by 25.8% (Group-C) compared to normoxic controls. Hypoxia increased β-tubulin expression (P = 0067) and reduced PGC1-α (P = 0412) expression in the isolated swine tenocytes with no effect on the protein expression of Complex-1 (P = 7409) and citrate synthase (P = 0.3290). Also, the hypoxic tenocytes exhibited about 4-fold increase in mitochondrial superoxide (P < 0.0001), altered morphology and mitochondrial pore integrity, and increase in mitochondrial density compared to normoxic controls. These findings suggest the critical role of mitochondria in the RCI healing response.

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.

Reversal of Fatty Infiltration After Suprascapular Nerve Compression Release Is Dependent on UCP1 Expression in Mice

BACKGROUND

In large rotator cuff tears, retraction of the supraspinatus muscle creates suprascapular nerve traction and compression. However, suprascapular nerve transection, when used in previous models, is different from chronic compression of the suprascapular nerve in patients. To define the role of suprascapular nerve chronic injury in rotator cuff muscle atrophy and fatty infiltration, we developed a novel reversible suprascapular nerve compression mouse model.

QUESTIONS/PURPOSES

We asked: (1) Can suprascapular nerve injury be induced by compression but reversed after compression release? (2) Can muscle fatty infiltration be induced by suprascapular nerve compression and reversed after compression release? (3) Is white fat browning involved in fatty infiltration resorption?

METHODS

Mice in a common strain of C57BL/6J were randomly assigned to suprascapular nerve transection (n = 10), nerve compression (n = 10), nerve compression and release (n = 10), or sham control (n = 10) groups. To study the role or white fat browning on muscle fatty infiltration, additional UCP1 reporter mice (n = 4 for nerve compression and n = 4 for nerve compression release) and knockout mice (n = 4 for nerve compression and n = 4 for nerve compression release) were used. Nerve injury was testified using osmium tetroxide staining and neural muscular junction staining and then semiquantified by counting the degenerating axons and disrupted junctions. Muscle fatty infiltration was evaluated using Oil Red O staining and then semiquantified by measuring the area fraction of fat. Immunofluorescent and Oil Red O staining on UCP1 transgenic mice was conducted to testify whether white fat browning was involved in fatty infiltration resorption. Ratios of UCP1 positively stained area and fat area to muscle cross-section area were measured to semiquantify UCP1 expression and fatty infiltration in muscle by blinded reviewers. Analysis of variance with Tukey post hoc comparisons was used for statistical analysis between groups.

RESULTS

Suprascapular nerve injury was induced by compression but reversed after release. The ratios of degenerating axons were: sham control: 6% ± 3% (95% confidence interval [CI], 3%-10%); nerve compression: 58% ± 10% (95% CI, 45%-70% versus sham, p < 0.001); and nerve compression and release: 15% ± 9% (95% CI, 5%-26% versus sham, p = 0.050). The supraspinatus muscle percentage area of fatty infiltration increased after 6 weeks of nerve compression (19% ± 1%; 95% CI, 18%-20%; p < 0.001) but showed no difference after compression release for 6 weeks (5% ± 3%; 95% CI, 1%-10%; p = 0.054) compared with sham (2% ± 1%; 95% CI, 1%-3%). However, the fat area fraction in UCP1 knockout mice did not change after nerve compression release (6% ± 1%; 95% CI, 4%-8% at 2 weeks after compression and 5% ± 0.32%; 95% CI, 4%-6% after 2 weeks of release; p = 0.1095).

CONCLUSIONS

We developed a clinically relevant, reversible suprascapular nerve compression mouse model. Fatty infiltration resorption after compression release was mediated through white fat browning.

CLINICAL RELEVANCE

If the mechanism of browning of white fat in rotator cuff muscle fatty infiltration can be confirmed in humans, a UCP1 agonist may be an effective treatment for patients with suprascapular nerve injury.

Biologics and stem cell-based therapies for rotator cuff repair

The rotator cuff is composed of several distinct muscles and tendons that function in concert to coordinate shoulder motion. Injuries to these tendons frequently result in permanent dysfunction and persistent pain. Despite considerable advances in operation techniques, surgical repair alone still does not fully restore rotator cuff function. This review focuses on recent research in the use of biologics and stem cell-based therapies to augment repair, highlighting promising avenues for future work and remaining challenges. While a number of animal models are used for rotator cuff studies, the anatomy of the rotator cuff varies dramatically between species. Since the rodent rotator cuff shares the most anatomical features with the human, this review will focus primarily on rodent models to enable consistent interpretation of outcome measures.

Fatty Infiltration Is a Prognostic Marker of Muscle Function After Rotator Cuff Tear

BACKGROUND

Massive rotator cuff tears (RCTs) begin as primary tendon injuries and cause a myriad of changes in the muscle, including atrophy, fatty infiltration (FI), and fibrosis. However, it is unclear which changes are most closely associated with muscle function.

PURPOSE

To determine if FI of the supraspinatus muscle after acute RCT relates to short-term changes in muscle function.

STUDY DESIGN

Controlled laboratory study.

METHODS

Unilateral RCTs were induced in female rabbits via tenotomy of the supraspinatus and infraspinatus. Maximal isometric force and rate of fatigue were measured in the supraspinatus in vivo at 6 and 12 weeks after tenotomy. Computed tomography scanning was performed, followed by histologic analysis of myofiber size, FI, and fibrosis.

RESULTS

Tenotomy resulted in supraspinatus weakness, reduced myofiber size, FI, and fibrosis, but no differences were evident between 6 and 12 weeks after tenotomy except for increased collagen content at 12 weeks. FI was a predictor of supraspinatus weakness and was strongly correlated to force, even after accounting for muscle cross-sectional area. While muscle atrophy accounted for the loss in force in tenotomized muscles with minimal FI, it did not account for the greater loss in force in tenotomized muscles with the most FI. Collagen content was not strongly correlated with maximal isometric force, even when normalized to muscle size.

CONCLUSION

After RCT, muscle atrophy results in the loss of contractile force from the supraspinatus, but exacerbated weakness is observed with increased FI. Therefore, the level of FI can help predict contractile function of torn rotator cuff muscles.

CLINICAL RELEVANCE

Markers to predict contractile function of RCTs will help determine the appropriate treatment to improve functional recovery after RCTs.

HMGB2 is a novel adipogenic factor that regulates ectopic fat infiltration in skeletal muscles

Although various surgical procedures have been developed for chronic rotator cuff tear repair, the re-tear rate remains high with severe fat infiltration. However, little is known about the molecular regulation of this process. Mesenchymal stem cells (MSCs) in the intra-muscular space are origin of ectopic fat cells in skeletal muscle. We have previously shown that high-mobility group box 2 (HMGB2), which is a nuclear protein commonly associated with mesenchymal differentiation, is involved in the early articular cartilage degeneration. In this study, we addressed the role of HMGB2 in adipogenesis of MSCs and fat infiltration into skeletal muscles. HMGB2 was highly expressed in undifferentiated MSCs and co-localized with platelet-derived growth factor receptor α (PDGFRA) known as an MSC-specific marker, while their expressions were decreased during adipocytic differentiation. Under the deficiency of HMGB2, the expressions of adipogenesis-related molecules were reduced, and adipogenic differentiation is substantially impaired in MSCs. Moreover, HMGB2⁺ cells were generated in the muscle belly of rat supraspinatus muscles after rotator cuff transection, and some of these cells expressed PDGFRA in intra-muscular spaces. Thus, our findings suggest that the enhance expression of HMGB2 induces the adipogenesis of MSCs and the fat infiltration into skeletal muscles through the cascade of HMGB2-PDGFRA.

Association of Inflammatory Responses and ECM Disorganization with HMGB1 Upregulation and NLRP3 Inflammasome Activation in the Injured Rotator Cuff Tendon

Inflammation and extracellular matrix (ECM) disorganization following the rotator cuff tendon injuries (RCTI) delay the repair and healing process and the molecular mechanisms underlying RCTI pathology are largely unknown. Here, we examined the role of HMGB1 and NLRP3 inflammasome pathway in the inflammation and ECM disorganization in RCTI. This hypothesis was tested in a tenotomy-RCTI rat model by transecting the RC tendon from the humerus. H&E and pentachrome staining revealed significant changes in the morphology, architecture and ECM organization in RC tendon tissues following RCTI when compared with contralateral control. Severity of the injury was high in the first two weeks with improvement in 3–4 weeks following RCTI, and this correlated with the healing response. The expression of proteins associated with increased HMGB-1 and upregulation of NLRP3 inflammasome pathway, TLR4, TLR2, TREM-1, RAGE, ASC, Caspase-1, and IL-1β, in the first two weeks following RCTI followed by decline in 3–4 weeks. These results suggest the association of inflammatory responses and ECM disorganization with HMGB1 upregulation and NLRP3 inflammasome activation in the RC tendons and could provide novel target(s) for development of better therapeutic strategies in the management of RCTI.

Localized SDF-1α Delivery Increases Pro-Healing Bone Marrow-Derived Cells in the Supraspinatus Muscle Following Severe Rotator Cuff Injury

To examine how the chemotactic agent stromal cell-derived factor-1alpha (SDF-1α) modulates the unique cellular milieu within rotator cuff muscle following tendon injury, we developed an injectable, heparin-based microparticle platform to locally present SDF-1α within the supraspinatus muscle following severe rotator cuff injury. SDF-1α loaded, degradable, N-desulfated heparin-based microparticles were fabricated, injected into a rat model of severe rotator cuff injury, and were retained for up to 7 days at the site. The resultant inflammatory cell and mesenchymal stem cell populations were analyzed compared to uninjured contralateral controls and, after 7 days, the fold-change in anti-inflammatory, M2-like macrophages (CD11b+CD68+CD163+, 4.3X fold-change) and mesenchymal stem cells (CD29+CD44+CD90+, 3.0X, respectively) was significantly greater in muscles treated with SDF-1α loaded microparticles than unloaded microparticles or injury alone. Our results indicate that SDF-1α loaded microparticles may be a novel approach to shift the cellular composition within the supraspinatus muscle and create a more pro-regenerative milieu, which may provide a platform to improve muscle repair following rotator cuff injury in the future.

Muscle stem cell activation in a mouse model of rotator cuff injury

Rotator cuff (RC) tears are frequently complicated by muscle atrophy. Muscle stem cells (MuSCs) repair damaged myofibers following injury, but their role in the prevention or pathogenesis of atrophy following RC tears remains undefined. We hypothesized that the RC MuSC population would be affected by supraspinatus (SS) and infraspinatus (IS) tendon transection (TT) compared to uninjured muscle in a mouse model of RC tear. C57BL6/J mice underwent unilateral SS and IS TT and contralateral sham surgery. At 3, 8, or 14 weeks after injury, mice were euthanized, and SS and IS were harvested for FACS sorting of CD31-/CD45-/Sca1-/ITGa7+/VCAM+ MuSCs or histological analysis. Ki-67+ MuSCs from injured muscle increased 3.4-fold at 3 weeks (p = 0.03) and 8.1-fold at 8 weeks (p = 0.04) following TT injury, but returned to baseline by 14 weeks (p = 0.91). Myod1 remained upregulated 3.3-fold at 3 weeks (p = 0.03) and 2.0-fold at 14 weeks (p = 0.0003), respectively. Myofiber cross-sectional area was decreased at both 3 and 14 weeks after injury, but the number of MuSCs per fiber remained relatively constant at 3 (p = 0.3) and 14 (p = 0.6) weeks after TT. In this study, we characterized the longitudinal effect of RC tendon injury on the MuSC population in supraspinatus and infraspinatus muscles. MuSCs are transiently activated, and are not depleted, in spite of persistent muscle atrophy. © 2017 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 36:1370-1376, 2018.

Early versus late repair of rotator cuff tears in rats

BACKGROUND

In the event of a traumatic rotator cuff tear, patients are routinely advised that early surgical intervention produces an optimal repair, despite a lack of direct evidence to support this recommendation. To address this knowledge gap, massive rotator cuff tears in rats were assessed by biomechanical and bone morphometric analyses after early or late repair.

METHODS

Combined supraspinatus and infraspinatus tendon tears of the left shoulder were created in 21 adult Wistar rats, which were divided into 2 groups. The tendons of the injured shoulder in the animals in group I were surgically repaired 8 weeks after the injury. Under the same anesthesia, the same injury was created on the right shoulder, which was immediately repaired. The rats from group I were euthanized 8 weeks after the repairs. No repair was performed in the rats from group II, which were euthanized 8 weeks after the injury. Tissues from both groups were harvested and biomechanically tested for supraspinatus tendon and bone morphometry analysis of the humeral head.

RESULTS

All biomechanical properties were significantly increased in the early repair group compared with the late repair group. No significant differences were observed in bone morphometry of the humeral head when early and late repair groups were compared.

CONCLUSION

Early surgical repair of a massive rotator cuff tear leads to improved biomechanical properties of the tissue after healing. Proximal humerus bone morphometry was unaffected by surgical repair timing.

Relationship between neuropathy proximal to the suprascapular nerve and rotator cuff tear in a rodent model

BACKGROUND

Rotator cuff tears are believed to coexist with cervical spine lesions. In cases of preexisting neuropathy, such as cervical spine lesions, fatty degeneration has likely already occurred due to the neuropathy. In these cases, rotator cuff tear is thought to occur easily because of preexisting extensive fatty degeneration and degeneration of the tendons due to neuropathy. This study aimed to evaluate the effects of paralysis due to neuropathy proximal to the suprascapular nerve on the supraspinatus and infraspinatus tendons using a rat model of brachial plexus paralysis.

METHODS

This study included fifteen, 8-week-old Sprague-Dawley rats. The left shoulder was included in the paralysis group and the contralateral shoulder constituted the sham group. Biomechanical testing (evaluated maximum tear force, maximum displacement and Young's modulus) (n = 10) and histological analyses (n = 5) (using the Bonar scale) were performed at 12 weeks postoperatively to confirm the degeneration of the tendon.

RESULTS

The mean maximum tear force was significantly lower in the paralysis group than in the sham group (P = 0.008), indicating that rotator cuff tears occurred with a lower force in the paralysis group. Additionally, the average Young's modulus was significantly greater in the paralysis group than in the sham group (P = 0.003), indicating that the rotator cuff muscle became hard and inflexible in the paralysis group. The Bonar scales of the histological analyses were significantly higher in the paralysis group (total score = 7.04 ± 0.61) than the sham group (total score = 0) (p < 0.0001).

CONCLUSIONS

If neuropathy of proximal to the suprascapular nerve, such as cervical spine or brachial plexus lesion, exists, weakness and degeneration of the rotator cuff tendon and stiffness of the rotator cuff muscle develop. Neuropathy is likely a cause of rotator cuff tears.