Effect of simvastatin on rat supraspinatus tendon mechanical and histological properties in a diet‐induced hypercholesterolemia model

Hypercholesterolemia is a common condition and is a risk factor for tendon rupture, specifically in the supraspinatus tendon. In the clinic, statins are commonly prescribed to lower cholesterol, but little information is available examining the effect of statin treatment on the musculoskeletal system. Therefore, the objective of this study was to determine the biomechanical and histological effects of statin treatment in a diet‐induced hypercholesterolemia model. We hypothesized that hypercholesterolemic rats treated with statins would have improved tendon biomechanical and histological properties compared to hypercholesterolemic rats not receiving daily statin treatment. Thirty adult male Sprague‐Dawley rats ate either high‐cholesterol (HC) diet (n = 20) or normal chow (CTL, n = 10). After 6 months, a subset of HC rats began daily oral simvastatin dosing (HC+S) at 20 mg/kg. All rats were sacrificed after a total of 9 months (3 months of statin treatment) and evaluated for histology and mechanics. For mechanics, at the insertion region, HC+S group had increased tendon cross‐sectional area decreased and modulus. No differences were noted in mechanical properties at the midsubstance. For histology, no differences were noted in the insertion region. In the midsubstance region, HC+S group had more spindle shaped cells. Our results suggest that 3 months of simvastatin treatment in a diet‐induced hypercholesterolemia rat model alters some tendon mechanical and histological properties, although a strong conclusion in support of improved parameters cannot be drawn. Therefore, we conclude that simvastatin treatment does not negatively affect tendon properties. © 2016 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 34:2009–2015, 2016.     

Development of a mouse model of supraspinatus tendon insertion site healing

Supraspinatus (SS) tendon tears are common musculoskeletal injuries whose surgical repair exhibits the highest incidence of re‐tear of any tendon. Development of therapeutics for improving SS tendon healing is impaired by the lack of a model that allows biological perturbations to identify mechanisms that underlie ineffective healing. The objective of this study was to develop a mouse model of supraspinatus insertion site healing by creating a reproducible SS tendon detachment and surgical repair which can be applied to a wide array of inbred mouse strains and genetic mutants. Anatomical and structural analyses confirmed that the rotator cuff of the mouse is similar to that of human, including the presence of a coracoacromial (CA) arch and an insertion site that exhibits a fibrocartilagenous transition zone. The surgical repair was successfully conducted on seven strains of mice that are commonly used in Orthopaedic Research suggesting that the procedure can be applied to most inbred strains and genetic mutants. The quality of the repair was confirmed with histology through 14 days after surgery in two mouse strains that represent the variation in mouse strains evaluated. The developed mouse model will allow us to investigate mechanisms involved in insertion site healing. © 2014 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 33:25–32, 2015.     

Pulsed electromagnetic field therapy improves tendon‐to‐bone healing in a rat rotator cuff repair model

Rotator cuff tears are common musculoskeletal injuries often requiring surgical intervention with high failure rates. Currently, pulsed electromagnetic fields (PEMFs) are used for treatment of long‐bone fracture and lumbar and cervical spine fusion surgery. Clinical studies examining the effects of PEMF on soft tissue healing show promising results. Therefore, we investigated the role of PEMF on rotator cuff healing using a rat rotator cuff repair model. We hypothesized that PEMF exposure following rotator cuff repair would improve tendon mechanical properties, tissue morphology, and alter in vivo joint function. Seventy adult male Sprague–Dawley rats were assigned to three groups: bilateral repair with PEMF (n = 30), bilateral repair followed by cage activity (n = 30), and uninjured control with cage activity (n = 10). Rats in the surgical groups were sacrificed at 4, 8, and 16 weeks. Control group was sacrificed at 8 weeks. Passive joint mechanics and gait analysis were assessed over time. Biomechanical analysis and μCT was performed on left shoulders; histological analysis on right shoulders. Results indicate no differences in passive joint mechanics and ambulation. At 4 weeks the PEMF group had decreased cross‐sectional area and increased modulus and maximum stress. At 8 weeks the PEMF group had increased modulus and more rounded cells in the midsubstance. At 16 weeks the PEMF group had improved bone quality. Therefore, results indicate that PEMF improves early tendon healing and does not alter joint function in a rat rotator cuff repair model. © 2016 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 35:902–909, 2017.

     

Overuse of training increases mechanoreceptors in supraspinatus tendon of rats SHR

The presence of mechanoreceptors in tendon after overuse activities can be a further step to learn about tendinopathy and overuse. Studies of tendons mechanoreceptors in rats are rare. We studied 12 isogenic spontaneous hypertensive rats (SHR), which underwent an overuse protocol consisting of an hour per daily session of treadmill running at a speed of 17 m/min, 5 times/week for 4 months. Supraspinatus tendons were evaluated with immunohistochemistry using S100 protein antibodies and histological protocol. Supraspinatus tendons at the end of 4 months of overuse protocol had a high number of media mechanoreceptors (4.3) than controls (0.6). The overexpression of S100 protein antibody in overuse activities maybe could represent a adaptative effort to tendon before the tear. © 2011 Orthopaedic Research Society Published by Wiley Periodicals, Inc. J Orthop Res 29:1771–1774, 2011     

Autologous tendon‐derived cell‐seeded nanofibrous scaffolds improve rotator cuff repair in an age‐dependent fashion

Rotator cuff tendon tears are one of the most common shoulder pathologies, especially in the aging population. Due to a poor healing response and degenerative changes associated with aging, rotator cuff repair failure remains common. Although cell‐based therapies to augment rotator cuff repair appear promising, it is unknown whether the success of such a therapy is age‐dependent. We hypothesized that autologous cell therapy would improve tendon‐to‐bone healing across age groups, with autologous juvenile cells realizing the greatest benefit. In this study, juvenile, adult, and aged rats underwent bilateral supraspinatus tendon repair with augmentation of one shoulder with autologous tendon‐derived cell‐seeded polycaprolactone scaffolds. At 8 weeks, shoulders treated with cells in both juvenile and aged animals exhibited increased cellularity, increased collagen organization, and improved mechanical properties. No changes between treated and control limbs were seen in adult rats. These findings suggest that cell delivery during supraspinatus repair initiates earlier matrix remodeling in juvenile and aged animals. This may be due to the relative “equilibrium” of adult tendon tissue with regards to catabolic and anabolic processes, contrasted with actively growing juvenile tendons and degenerative aged tendons. This study demonstrates the potential for autologous cell‐seeded scaffolds to improve repairs in both the juvenile and aged population. © 2016 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 35:1250–1257, 2017.

     

Arthroscopic repair of partial articular supraspinatus tendon avulsion lesions by conversion to full-thickness tears through a small incision

Purpose: To assess the clinical efficacy of converting partial articular supraspinatus tendon avulsion (PASTA) lesions to full-thickness tears through a small local incision of the bursal-side supraspinatus tendon followed by repair. Methods: We retrospectively analyzed 41 patients with Ellman grade 3 PASTA lesions and an average age of (54.7 ± 11.4) years from March 2013 to July 2017. Patients without regular conservative treatment and concomitant with other shoulder pathologies or previous shoulder surgery were excluded from the study. The tears were confirmed via arthroscopy, and a polydioxanone suture was placed to indicate the position of each tear. A small incision of approximately 6 mm was made using a plasma scalpel on the bursal-side supraspinatus tendon around the positioned suture to convert the partial tear into a fullthickness tear. The torn rotator cuff was sutured through the full thickness using a suture passer after inserting a 4.5-mm double-loaded suture anchor. Data were analyzed using a paired Student’s t-test with statistical significance defined as p <0.05. Results: At the final follow-up of 2 years, the pain-free shoulder joint range of motion and visual analog scale score were significantly improved compared to those before surgery (p < 0.001). The postoperative American Shoulder and Elbow Surgeons shoulder score was (90.6 ± 6.2), which was significantly higher than the preoperative score of (47.9 ± 8.3) (p < 0.001). The University of California at Los Angeles shoulder rating scale score increased from (14.7 ± 4.1) prior to surgery to (32.6 ± 3.4) points after surgery (p < 0.001). No patient had joint stiffness. Conclusion: This modified tear completion repair, by conversion to full-thickness tears through a small incision, has less damage to the supraspinatus tendon on the side of the bursa compared to traditional tear completion repair in the treatment of PASTA lesions. This surgical method is a simple and effective treatment that can effectively alleviate pain and improve shoulder joint function.     

Effect of return to overuse activity following an isolated supraspinatus tendon tear on adjacent intact tendons and glenoid cartilage in a rat model

Rotator cuff tears are common conditions that can alter shoulder mechanics and may lead to damage of intact joint tissues. These injuries are of particular concern in populations who perform tasks requiring repetitive overhead activity (e.g., athletes and laborers) and who are likely to return to aggressive pre‐injury activity levels despite limited understanding of the potentially damaging effects on the remaining tissues. Therefore, we investigated the effect of returning to overuse activity following a supraspinatus tear on shoulder function and the mechanical properties of the remaining intact tendons and glenoid cartilage. Forty rats underwent 4 weeks of overuse activity to create a tendinopathic condition followed by detachment of the supraspinatus tendon and were then randomized into two groups: continued overuse or cage activity. Ambulatory measurements were performed throughout the 8 weeks prior to euthaniasia, and properties of the adjacent tendons and cartilage were evaluated. Results demonstrated that shoulder function was not compromised in the return to overuse group. However, alterations of the glenoid cartilage and biceps tendon properties occurred. Our results help define the contributory roles of common mechanical injury mechanisms and provide a framework by which physicians could better prescribe long‐term treatment strategies for patients. © 2012 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 31: 710–715, 2013     

Effect of timing of surgical SSP tendon repair on muscle alterations

To investigate the impacts of delayed repairs of a supraspinatus tendon tear on the supraspinatus muscle, we used an animal model data from two previously published studies in which one supraspinatus (SSP) tendon was detached. In one cohort, the rabbits were killed in groups of 10 at 4, 8, and 12 weeks. In the other cohort, a repair was done at these time points, 12 rabbits each, and the animals killed were 12 weeks later. SSP fossa volume (Muscle belly plus extramuscular fat [e‐fat] volume), percentage of intramuscular fat (i‐fat), and muscle tissue volume (muscle belly volume minus i‐fat), as well as CT determination of e‐fat and i‐fat of both cohorts, were compared. Fossa volume increased (p < 0.05). Muscle belly and muscle tissue volumes did not increase after repair (p > 0.05), but early repair prevented further volume losses, a fact not seen after 8 and 12 weeks delay of repair. No reversal of e‐fat or of i‐fat occurred, in fact i‐fat almost doubled after 4 weeks delay of repair (p < 0.05). CT studies confirmed the fat results. We conclude that early repair prevented loss of muscle belly and muscle tissue volumes, but that it has no positive influence on fat accumulation. © 2014 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 32:1430–1435, 2014.     

Supraspinatus tendon overuse results in degenerative changes to tendon insertion region and adjacent humeral cartilage in a rat model

The etiology of rotator cuff tendon overuse injuries is still not well understood. Furthermore, how this overuse injury impacts other components of the glenohumeral joint, including nearby articular cartilage, is also unclear. Therefore, this study sought to better understand the time course of tendon protease activity in a rat model of supraspinatus overuse, as well as determine effects of 10 weeks of overuse on humeral head articular cartilage. For these studies, multiplex gelatin zymography was used to characterize protease activity profiles in tendon and cartilage, while histological scoring/mechanical testing and micro‐computed tomography (μCT) imaging were used to quantify structural damage in the supraspinatus tendon insertion and humeral articular cartilage, respectively. Histological scoring of supraspinatus tendon insertions revealed tendinopathic cellular and collagen fiber changes after 10 weeks of overuse when compared to controls, while mechanical testing revealed no significant differences between tensile moduli (overuse: 24.5 ± 11.5 MPa; control: 16.3 ± 8.7 MPa). EPIC‐μCT imaging on humeral articular cartilage demonstrated significant cartilage thinning (overuse: 119.6 ± 6.34 μm; control: 195.4 ± 13.4μm), decreased proteoglycan content (overuse: 2.1 ± 0.18 cm⁻¹; control: 1.65 ± 0.14 cm⁻¹), and increased subchondral bone thickness (overuse: 216.2 ± 10.9 μm; control: 192 ± 17.8μm) in the overuse animals. Zymography results showed no significant upregulation of cathepsins or matrix metalloproteinases in tendon or cartilage at 2 or 10 weeks of overuse compared to controls. These results have further elucidated timing of protease activity over 10 weeks and suggest that damage occurs to other tissues in addition to the supraspinatus tendon in this overuse injury model. © 2016 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 35:1910–1918, 2017.

     

Improving bone density at the rotator cuff footprint increases supraspinatus tendon failure stress in a rat model

The purpose of this study was to investigate whether supraspinatus tendon failure stress at the footprint can increase by improving the bone density at the rotator cuff footprint in a rat model. Bilateral ovariectomies were performed in twenty‐four 4‐month‐old Sprague‐Dawley rats. Half received bisphosphonate (zoledronic acid) and the other half received no treatment (OVX + ZOM and OVX, respectively). Twelve additional rats did not undergo ovariectomy or receive bisphosphonate treatment (CON). All rats were sacrificed at 7 months of age. Quantitative micro‐computed tomography was used to assess bone density in the proximal humerus. A series of stress–relaxation tests were performed to assess stiffness and failure stress of the supraspinatus tendon. Bone density in OVX + ZOM was significantly higher at the rotator cuff footprint when compared to CON and OVX rats (p < 0.0001). The supraspinatus tendons in the OVX group were significantly stiffer when compared to the CON and OVX + ZOM groups (p < 0.05). The failure stress of the OVX + ZOM group was significantly greater than the CON and OVX groups (22.89 ± 4.43 MPa vs. 18.36 ± 3.16 and 17.70 ± 4.92, respectively). In conclusion, improving the bone density at the rotator cuff footprint enhances failure stress of the suprapinatus tendon. © 2009 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 28:308–314, 2010     

Pulsed magnetic field therapy increases tensile strength in a rat Achilles' tendon repair model

PURPOSE: To examine the effect of pulsing electromagnetic fields on the biomechanic strength of rat Achilles' tendons at 3 weeks after transection and repair. METHODS: This noninvasive modality was tested in a prospective, randomized, double-blinded, placebo-controlled study to evaluate the effect of a specific noninvasive radiofrequency pulsed electromagnetic field signal on tendon tensile strength at 21 days post transection in a rat model. RESULTS: In the animals receiving PMF exposure, an increase in tensile strength of up to 69% was noted at the repair site of the rat Achilles' tendon at 3 weeks after transection and repair compared with nonstimulated control animals. CONCLUSIONS: The application of electromagnetic fields, configured to enhance Ca(2+) binding in the growth factor cascades involved in tissue healing, achieved a marked increase of tensile strength at the repair site in this animal model. If similar effects occur in humans, rehabilitation could begin earlier and the risk of developing adhesions or rupturing the tendon in the early postoperative period could be reduced.     

Photochemical repair of Achilles tendon rupture in a rat model

BACKGROUND: Photochemical tissue bonding (PTB) is an emerging technique for bonding or sealing tissue surfaces that requires light and a photoactive dye for its effect. The potential of PTB for tendon repair was assessed in a rat model. MATERIALS AND METHODS: The optical properties of bovine tendon were determined ex vivo to gauge the depth of light penetration as a function of wavelength and dosimetry parameters were established for PTB repair of ruptured tendon. PTB was then tested in vivo to repair transected tendons in Sprague-Dawley rats. Repair strengths were measured using a strain gauge up to 14 days post treatment. RESULTS: The effective penetration depth in tendon was estimated to be 0.68 mm at 514 nm. Following PTB treatment of mechanically ruptured tendon, significant bonding was dependent on the presence of both light and dye and attained a plateau strength at a fluence of 125 J/cm2. In a subsequent in vivo study to investigate PTB for repair of transected rat Achilles tendon, the ultimate stress required to break the repaired tendon was measured immediately after irradiation and at 7 and 14 days post-repair. Results showed that the difference in the ultimate stress between control and PTB treatment groups was statistically significant immediately after treatment and at 7 days (p = 0.04) but not 14 days (p = 0.75) post-repair. CONCLUSIONS: PTB provides a benefit to tendon repair at early stages in repair and is worthy of further investigation as a potential surgical adjunct for tendon repair in orthopedic surgeries.     

Tendon-derived stem cells (TDSCs) promote tendon repair in a rat patellar tendon window defect model

Injured tendons heal slowly and often result in the formation of mechanically and functionally inferior fibrotic scar tissue or fibrous adhesions. This study investigated the use of tendon-derived stem cells (TDSCs) for tendon repair in a rat patellar tendon window defect model. Fibrin glue constructs with or without GFP-TDSCs were transplanted into the window defect. The patellar tendons were harvested for histology, ex vivo fluorescent imaging and biomechanical test at various time points up to week 4. Our results showed that TDSCs significantly enhanced tendon healing as indicated by the increase in collagen production as shown by hematolxylin stain-ability of the tissue, improvement of cell alignment, collagen fiber alignment and collagen birefringence typical of tendon. The labeled cells were observed at weeks 1 and 2 and became almost undetectable at week 4. Both the ultimate stress and Young's modulus were significantly higher in the TDSCs group compared to those in the fibrin glue group at week 4. In conclusion, TDSCs promoted earlier and better repair in a rat patellar tendon window defect model.     

Collagen V expression is crucial in regional development of the supraspinatus tendon

Manipulations in cell culture and mouse models have demonstrated that reduction of collagen V results in altered fibril structure and matrix assembly. A tissue‐dependent role for collagen V in determining mechanical function was recently established, but its role in determining regional properties has not been addressed. The objective of this study was to define the role(s) of collagen V expression in establishing the site‐specific properties of the supraspinatus tendon. The insertion and midsubstance of tendons from wild type, heterozygous and tendon/ligament‐specific null mice were assessed for crimp morphology, fibril morphology, cell morphology, as well as total collagen and pyridinoline cross‐link (PYD) content. Fibril morphology was altered at the midsubstance of both groups with larger, but fewer, fibrils and no change in cell morphology or collagen compared to the wild type controls. In contrast, a significant disruption of fibril assembly was observed at the insertion site of the null group with the presence of structurally aberrant fibrils. Alterations were also present in cell density and PYD content. Altogether, these results demonstrate that collagen V plays a crucial role in determining region‐specific differences in mouse supraspinatus tendon structure. © 2016 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 34:2154–2161, 2016.     

Expression of chondro‐osteogenic BMPs in ossified failed tendon healing model of tendinopathy

Chondrocytes phenotype/markers were expressed in clinical samples of tendinopathy and calcifying tendinopathy. This study examined the spatial‐temporal expression of chondro‐osteogenic Bone Morphogenetic Proteins (BMPs), which might contribute to ectopic chondro‐osteogenesis and failed healing process in tendinopathy. Collagenase was injected into patellar tendon of rats to induce ossified failed tendon healing. At week 2, 4, 8, 12, and 16, the patella tendon was harvested for immunohistochemical staining and analysis of BMP‐2/4/7. BMP‐4/7 showed similar expression patterns, which was different from BMP‐2. The expression of BMP‐2 in the tendon matrix increased at week 2 and was reduced to nearly undetectable level afterwards except at the chondro‐ossification sites. However, the expression of BMP‐4/7 in the healing tendon fibroblast‐like cells and matrix increased at week 2, reduced at week 4 and 8 and increased again at week 12 and 16, consistent with transient healing at week 8 in this animal model. There was increasing strong expression of BMP‐4/7 in the chondrocyte‐like cells in the un‐ossified and ossified areas from week 8–16. BMP‐4/7, besides BMP‐2, might also contribute to ectopic chondro‐osteogenesis and failed healing in tendon injuries. BMP‐4/7, but not BMP‐2, might be involved in regulating late events in ossified failed tendon healing. © 2010 Orthopaedic Research Society Published by Wiley Periodicals, Inc. J Orthop Res 29:816–821     

Histological analysis of achilles tendons in an overuse rat model

The purpose of this study was to design an animal model that induces histological changes in Achilles tendons consistent with those cited in the literature for human Achilles tendon disease. Sprague‐Dawley rats were subjected to 10° uphill treadmill running on a custom‐designed rodent treadmill and at a speed of 17 meters per minute for 1 h, five times per week, over a 12‐week treatment period. Subsequent histological analysis revealed alterations in the rat Achilles tendon that were generally consistent with those described in the literature for diseased human tendon tissues. These features include: decreased collagen fiber organization, more intense collagen staining, and increased cell nuclei numbers. Interestingly, though, immunohistochemical cell typing suggests that the observed increased cellularity does not include a significant inflammatory component but is secondary to increased numbers of endothelial cells (i.e., vascularization) and fibroblasts. These histological features likely represent a biological repair/remodeling response resulting from overuse running. © 2008 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 26:840–846, 2008