Exercise following a short immobilization period is detrimental to tendon properties and joint mechanics in a rat rotator cuff injury model

Rotator cuff tears are a common clinical problem that can result in pain and disability. Previous studies in a rat model showed enhanced tendon to bone healing with postoperative immobilization. The objective of this study was to determine the effect of postimmobilization activity level on insertion site properties and joint mechanics in a rat model. Our hypothesis was that exercise following a short period of immobilization will cause detrimental changes in insertion site properties compared to cage activity following the same period of immobilization, but that passive shoulder mechanics will not be affected. We detached and repaired the supraspinatus tendon of 22 Sprague‐Dawley rats, and the injured shoulder was immobilized postoperatively for 2 weeks. Following immobilization, rats were prescribed cage activity or exercise for 12 weeks. Passive shoulder mechanics were determined, and following euthansia, tendon cross‐sectional area and mechanical properties were measured. Exercise following immobilization resulted in significant decreases compared to cage activity in range of motion, tendon stiffness, modulus, percent relaxation, and several parameters from both a structurally based elastic model and a quasi‐linear viscoelastic model. Therefore, we conclude that after a short period of immobilization, increased activity is detrimental to both tendon mechanical properties and shoulder joint mechanics, presumably due to increased scar production. © 2010 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 28:841–845, 2010     

Biceps tendon properties worsen initially but improve over time following rotator cuff tears in a rat model

Damage to the biceps tendon is often seen in conjunction with rotator cuff tears. However, controversy exists regarding its role in the shoulder and its optimal treatment. A previous study determined that biceps tendons were detrimentally affected in the presence of rotator cuff tears in the rat model and this damage worsened over time. However, whether this damage progresses at later time points to provide a chronic model is unknown. The objective of this study was to determine the changes in the biceps tendon in the presence of a cuff tear over time. Our hypothesis was that histological, compositional, organizational, and mechanical properties would worsen with time. We detached the supraspinatus and infraspinatus tendons of 48 rats and evaluated these properties at 1, 4, 8, and 16 weeks postdetachment. Properties worsened through 8 weeks, but improved between 8 and 16 weeks. We therefore conclude that biceps tendon changes in this model are not truly chronic. Additionally, it has been shown that infraspinatus properties in this model return to normal by 16 weeks, when biceps properties improve, indicating that earlier repair of one or more of the rotator cuff tendons may lead to resolved pathology of the long head of the biceps tendon. © 2011 Orthopaedic Research Society Published by Wiley Periodicals, Inc. J Orthop Res 29:874–879     

Mechanical properties of the long‐head of the biceps tendon are altered in the presence of rotator cuff tears in a rat model

Rotator cuff tears are disabling conditions that result in changes in joint loading and functional deficiencies. Clinically, damage to the long‐head of the biceps tendon has been found in conjunction with rotator cuff tears, and this damage is thought to increase with increasing tear size. Despite its importance, controversy exists regarding the optimal treatment for the biceps. An animal model of this condition would allow for controlled studies to investigate the etiology of this problem and potential treatment strategies. We created rotator cuff tears in the rat model by detaching single (supraspinatus) and multiple (supraspinatus + infraspinatus or supraspinatus + subscapularis) rotator cuff tendons and measured the mechanical properties along the length of the long‐head of the biceps tendon 4 and 8 weeks following injury. Cross‐sectional area of the biceps was increased in the presence of a single rotator cuff tendon tear (by ～150%), with a greater increase in the presence of a multiple rotator cuff tendon tear (by up to 220%). Modulus values decreased as much as 43 and 56% with one and two tendon tears, respectively. Also, multiple tendon tear conditions involving the infraspinatus in addition to the supraspinatus affected the biceps tendon more than those involving the subscapularis and supraspinatus. Finally, biceps tendon mechanical properties worsened over time in multiple rotator cuff tendon tears. Therefore, the rat model correlates well with clinical findings of biceps tendon pathology in the presence of rotator cuff tears, and can be used to evaluate etiology and treatment modalities. © 2008 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 27:416–420, 2009     

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

Rotator cuff tears (RCTs) are among the most common musculoskeletal injuries seen by orthopaedic surgeons. Clinically, massive cuff tears lead to unique pathophysiological changes in rotator cuff muscle, including atrophy, and massive fatty infiltration, which are rarely seen in other skeletal muscles. Studies in a rodent model for RCT have demonstrated that these histologic findings are accompanied by activation of the Akt/mammalian target of rapamycin (mTOR) and transforming growth factor‐β (TGF‐β) pathways following combined tendon‐nerve injury. The purpose of this study was to compare the histologic and molecular features of rotator cuff muscle and gastrocnemius muscle—a major hindlimb muscle, following combined tendon‐nerve injury. Six weeks after injury, the rat gastrocnemius did not exhibit notable fatty infiltration compared to the rotator cuff. Likewise, the adipogenic markers SREBP‐1 and PPARγ as well as the TGF‐β canonical pathway were upregulated in the rotator cuff, but not the gastrocnemius. Our study suggests that the rat rotator cuff and hindlimb muscles differ significantly in their response to a combined tendon‐nerve injury. Clinically, these findings highlight the unique response of the rotator cuff to injury, and may begin to explain the poor outcomes of massive RCTs compared to other muscle‐tendon injuries. © 2015 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 33:1046–1053, 2015.     

Glenoid cartilage mechanical properties decrease after rotator cuff tears in a rat model

Rotator cuff repairs are commonly performed to reduce pain and restore function. Tears are also treated successfully without surgical intervention; however, the effect that a torn tendon has on the glenohumeral cartilage remains unknown. Clinically, a correlation between massive rotator cuff tears and glenohumeral arthritis has often been observed. This may be due to a disruption in the balance of forces at the shoulder, resulting in migration of the humeral head and subsequently, abnormal loading of the glenoid. Our lab previously demonstrated changes in ambulation and intact tendon mechanical properties following supraspinatus and infraspinatus rotator cuff tendon tears in a rat model. Therefore, the purpose of this study was to investigate the effects of supraspinatus and infraspinatus rotator cuff tears on the glenoid cartilage. Nine rats underwent unilateral detachment of the supraspinatus and infraspinatus tendons and were sacrificed after 4 weeks. Cartilage thickness significantly decreased in the antero‐inferior region of injured shoulders. In addition, equilibrium elastic modulus significantly decreased in the center, antero‐superior, antero‐inferior, and superior regions. These results suggest that altered loading after rotator cuff injury may lead to damage to the joint with significant pain and dysfunction. Clinically, understanding the mechanical processes involved with joint damage will allow physicians to better advise patients. © 2012 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 30:1435–1439, 2012     

Rotator cuff repair using cell sheets derived from human rotator cuff in a rat model

To achieve biological regeneration of tendon‐bone junctions, cell sheets of human rotator‐cuff derived cells were used in a rat rotator cuff injury model. Human rotator‐cuff derived cells were isolated, and cell sheets were made using temperature‐responsive culture plates. Infraspinatus tendons in immunodeficient rats were resected bilaterally at the enthesis. In right shoulders, infraspinatus tendons were repaired by the transosseous method and covered with the cell sheet (sheet group), whereas the left infraspinatus tendons were repaired in the same way without the cell sheet (control group). Histological examinations (safranin‐O and fast green staining, isolectin B4, type II collagen, and human‐specific CD31) and mRNA expression (vascular endothelial growth factor; VEGF, type II collagen; Col2, and tenomodulin; TeM) were analyzed 4 weeks after surgery. Biomechanical tests were performed at 8 weeks. In the sheet group, proteoglycan at the enthesis with more type II collagen and isolectin B4 positive cells were seen compared with in the control group. Human specific CD31‐positive cells were detected only in the sheet group. VEGF and Col2 gene expressions were higher and TeM gene expression was lower in the sheet group than in the control group. In mechanical testing, the sheet group showed a significantly higher ultimate failure load than the control group at 8 weeks. Our results indicated that the rotator‐cuff derived cell sheet could promote cartilage regeneration and angiogenesis at the enthesis, with superior mechanical strength compared with the control. Treatment for rotator cuff injury using cell sheets could be a promising strategy for enthesis of tendon tissue engineering. © 2016 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 35:289–296, 2017.

     

Disruption of the anterior–posterior rotator cuff force balance alters joint function and leads to joint damage in a rat model

The rotator cuff assists in shoulder movement and provides dynamic stability to the glenohumeral joint. Specifically, the anterior–posterior (AP) force balance, provided by the subscapularis anteriorly and the infraspinatus and teres minor posteriorly, is critical for joint stability and concentric rotation of the humeral head on the glenoid. However, limited understanding exists of the consequences associated with disruption of the AP force balance (due to tears of both the supraspinatus and infraspinatus tendons) on joint function and joint damage. We investigated the effect of disrupting the APforce balance on joint function and joint damage in an overuse rat model. Twenty‐eight rats underwent 4 weeks of overuse to produce a tendinopathic condition and were then randomized into two surgical groups: Detachment of the supraspinatus only or detachment of the supraspinatus and infraspinatus tendons. Rats were then gradually returned to their overuse protocol. Quantitative ambulatory measures including medial/lateral, propulsion, braking, and vertical forces were significantly different between groups. Additionally, cartilage and adjacent tendon properties were significantly altered. These results identify joint imbalance as a mechanical mechanism for joint damage and demonstrate the importance of preserving rotator cuff balance when treating active cuff tear patients. © 2014 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 32:638–644, 2014.     

Biomechanical comparison of single‐row,double‐row,and transosseous‐equivalent repair techniques after healing in an animal rotator cuff tear model

The transosseous‐equivalent (TOE) rotator cuff repair technique increases failure loads and contact pressure and area between tendon and bone compared to single‐row (SR) and double‐row (DR) repairs, but no study has investigated if this translates into improved healing in vivo. We hypothesized that a TOE repair in a rabbit chronic rotator cuff tear model would demonstrate a better biomechanical profile than SR and DR repairs after 12 weeks of healing. A two‐stage surgical procedure was performed on 21 New Zealand White Rabbits. The right subscapularis tendon was transected and allowed to retract for 6 weeks to simulate a chronic tear. Repair was done with the SR, DR, or TOE technique and allowed to heal for 12 weeks. Cyclic loading and load to failure biomechanical testing was then performed. The TOE repair showed greater biomechanical characteristics than DR, which in turn were greater than SR. These included yield load (p < 0.05), energy absorbed to yield (p < 0.05), and ultimate load (p < 0.05). For repair of a chronic, retracted rotator cuff tear, the TOE technique was the strongest biomechanical construct after healing followed by DR with SR being the weakest. © 2013 Orthopaedic Research Society Published by Wiley Periodicals, Inc. J Orthop Res 31:1254–1260, 2013     

Effects of selective paralysis of the supraspinatus muscle using botulinum neurotoxin a in rotator cuff healing in rats

We hypothesized that a temporary rotator cuff paralysis using botulinum‐neurotoxin A (BoNtA) would lead to an improved tendon‐to‐bone healing after repair of supraspinatus lesions. One hundred sixty Sprague–Dawley rats were randomly assigned to either the BoNtA or the control (saline) group. BoNtA/saline‐solution was injected into the supraspinatus muscle 1 week prior to surgery. A supraspinatus defect was made; we distinguished between a lesion with normal and increased repair load. Furthermore, one subgroup had the operated shoulder immobilized in a cast. Histologic analysis and biomechanical testing followed. Specimens from the BoNtA‐group, which were treated with an increased repair load, showed less cellularity and more organization in the interface tissue compared to the saline control group. In addition, we found that the collagen 1–3 quotient in the BoNtA specimen was significantly (p = 0.0051) higher than in the control group. Ultimate load at failure between the groups was not significantly different (p > 0.05). We did not observe any significant differences between the mobilized and immobilized specimen (p = 0.2079). The study shows that tendon‐to‐bone healing after rotator cuff repair can be altered positively using BoNtA pre‐operatively. Tears with increased repair load seem to benefit the most—at least histologically. © 2012 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 31: 716–723, 2013     

Increased IL‐1β expression and myofibroblast recruitment in subacromial bursa is associated with rotator cuff lesions with shoulder stiffness

We evaluated whether proinflammatory cytokine expression and myofibroblast recruitment in subacromial bursa was linked to rotator cuff lesions with shoulder stiffness. We analyzed expressions of IL‐1β, IL‐6, and TNF‐α in subacromial bursa and joint fluid collected from 14 patients with cuff tears with stiffness as a study group (Group I) and 14 patients with rotator cuff tears without shoulder stiffness as a control group (Group II) using real‐time RT‐PCR, immunohistochemistry, and ELISA. Myofibroblast apoptosis in subacromial bursa was analyzed using terminal deoxynucleotidyl transferase ‐mediated deoxyuridine triphosphate‐biotin nick end‐labeling (TUNEL) and α‐smooth muscle actin immunofluorescence staining. Shoulder function was evaluated using the Constant score. Group I had higher mRNA expression (p < 0.001) and immunoreactivities (p < 0.001) of IL‐1β. They also had higher levels of IL‐1β, IL‐6, and TNF‐α in joint fluid. Increased IL‐1β mRNA expression in the subacromial bursa and IL‐1β levels in joint fluid were correlated with a preoperative deficit in shoulder motion (p < 0.001) and preoperative Constant scores (p < 0.001). Immunofluorescence observations showed that Group I subjects had more myofibroblasts (p < 0.001) than Group II. In Group II, a significant correlation was found between apoptotic myofibroblasts and total myofibroblasts (p = 0.002), but not in Group I (p = 0.510). Increased expression of IL‐1β and myofibroblast recruitment in the subacromial bursa in rotator cuff lesions are linked to shoulder stiffness. © 2008 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 26:1090–1097, 2008     

Expression of interleukin‐1β, cyclooxygenase‐2, and prostaglandin E2 in a rotator cuff tear in rabbits

We investigated the specific factors related to shoulder pain due to a rotator cuff tear using a model in rabbits. A rotator cuff tear was surgically created, and the expression of interleukin‐1β (IL‐1β), prostaglandin E2 (PGE2), and cyclooxygenase‐2 (COX‐2) was analyzed. In the supernatant of the tissue culture of the torn tendon, IL‐1β production was detected. The amount of IL‐1β was highest 1 day after injury, and then decreased gradually to 21 days. PGE2, the mediator of pain and the product of COX‐2, was also detected in the supernatant of the tissue culture. The production of PGE2 significantly increased to 7 days after injury, and then decreased to 21 days. RT‐PCR analysis confirmed the mRNA expression of IL‐1β and COX‐2 in the torn tendon. Immunohistochemical study demonstrated that cells in the tendon stump were immunopositive for IL‐1β and COX‐2. Furthermore, in the affected joint, articular chondrocytes in the remote area from the tear expressed COX‐2 strongly. When the rotator cuff is torn, IL‐1β is produced in the torn tendon, and stimulates the expression of COX‐2 in not only the torn tendon but also in articular chondrocytes. The COX‐2 then produces PGE2, which would mediate shoulder pain. © 2006 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 25:92–97, 2007     

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     

Rotator cuff healing using demineralized cancellous bone matrix sponge interposition compared to standard repair in a preclinical canine model

This Level II study assessed clinically relevant outcomes for repair of large, retracted infraspinatus tendons (ISTs) using a demineralized bone matrix (DBM) sponge (FlexiGraft) hydrated in platelet‐rich plasma (PRP) versus direct repair in a validated canine model. Adult research dogs (n = 10) were used. The IST was transected in each shoulder (n = 20) and randomized to direct repair or repair with DBM‐PRP interposition at 4 weeks posttransection. At 12 weeks postrepair, dogs were sacrificed, and the repair evaluated by magnetic resonance imaging (MRI), histology, and biomechanical testing. MRI and histology scores were significantly (p < 0.05) better in the DBM‐PRP shoulders. Biomechanical testing revealed significantly improved strength of the DBM‐PRP repairs at 5 and 10 mm of displacement, as well as for ultimate failure load. In this canine model of retracted IST repair, DBM‐PRP sponge hydrated in PRP was considered safe and effective. In addition, use of DBM‐PRP was associated with improved MRI and histologic appearance, and improved strength compared to direct repair. Clinical significance: Based on reported failure rates for repair of large, retracted rotator cuff tears, improving tendon‐to‐bone healing is critical. Use of DBM combined with PRP shows potential for addressing this critical clinical need. © 2017 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 36:906–912, 2018.

     

Molecular mechanism of fatty degeneration in rotator cuff muscle with tendon rupture

Fatty degeneration often occurs in rotator cuff muscle with tendon rupture. However, the molecular mechanism underlying this change has not been fully clarified yet. We investigated the gene expression of Wnt10b and adipogenic marker gene, PPARγ and C/EBPα in C2C12 myogenic cell line under inhibition of Wnt10b by adipogenic induction medium, isobutylmethylxanthine, dexamethasone, and insulin (MDI). The role of Wnt‐signal was confirmed by adding Lithium chloride (LiCl), which mimics Wnt signaling to the cultured cell with MDI. We also assessed the expression profiles of same genes in the rat rotator cuff tear model in vivo. MDI induced Oil red‐O staining positive adipocytes and upregulated PPARγ and C/EBPα expression. LiCl inhibited adipogenic induction of MDI. Rotator cuff muscle with tendon rupture showed positive staining for Oil red‐O. Real‐time polymerase chain reaction analyses revealed decreased expression of Wnt10b followed by increased PPARγ and C/EBPα gene expression in the supraspinatus muscle. Fatty degeneration and its molecular events were remarkably seen in the distal one‐third of the detached supraspinatus muscle versus control. Wnt signaling may regulate adipogenic differentiation both in the myoblasts in vitro and the muscle in vivo. Our results indicate that the reduction of Wnt10b in muscle with a rotator cuff tear is a key signal in fatty degeneration of the muscle. © 2011 Orthopaedic Research Society Published by Wiley Periodicals, Inc. J Orthop Res 29:861–866     

应力刺激在肩袖损伤修复中的作用

肩袖损伤是临床常见的肩关节疾患,发病机制比较复杂,治疗效果不甚理想,其难点在于肌腱-骨交界面的修复。目前,已经认识到机械刺激,尤其是应力刺激对肌腱-骨交界面的修复有重要作用。     

Restoration of anterior–posterior rotator cuff force balance improves shoulder function in a rat model of chronic massive tears

The rotator cuff musculature imparts dynamic stability to the glenohumeral joint. In particular, the balance between the subscapularis anteriorly and the infraspinatus posteriorly, often referred to as the rotator cuff “force couple,” is critical for concavity compression and concentric rotation of the humeral head. Restoration of this anterior–posterior force balance after chronic, massive rotator cuff tears may allow for deltoid compensation, but no in vivo studies have quantitatively demonstrated an improvement in shoulder function. Our goal was to determine if restoring this balance of forces improves shoulder function after two‐tendon rotator cuff tears in a rat model. Forty‐eight rats underwent detachment of the supraspinatus and infraspinatus. After four weeks, rats were randomly assigned to three groups: no repair, infraspinatus repair, and two‐tendon repair. Quantitative ambulatory measures including medial/lateral forces, braking, propulsion, and step width were significantly different between the infraspinatus and no repair group and similar between the infraspinatus and two‐tendon repair groups at almost all time points. These results suggest that repairing the infraspinatus back to its insertion site without repair of the supraspinatus can improve shoulder function to a level similar to repairing both the infraspinatus and supraspinatus tendons. Clinically, a partial repair of the posterior cuff after a two‐tendon tear may be sufficient to restore adequate function. An in vivo model system for two‐tendon repair of massive rotator cuff tears is presented. © 2011 Orthopaedic Research Society Published by Wiley Periodicals, Inc. J Orthop Res 29: 1028–1033, 2011