Collagen V haploinsufficiency in female murine patellar tendons results in altered matrix engagement and cellular density, demonstrating decreased healing.

Collagen V (Col5) is a quantitatively minor component of collagen fibrils comprising tendon, however, plays a crucial role in regulation of development and dynamic healing processes. Clinically, patients with COL5a1 haploinsufficiency, known as classic Ehlers-Danlos Syndrome (cEDS), present with hyperextensible skin, joint instability and laxity, with females more likely to be affected. Previous studies in Col5-deficient mice indicated that reduced Col5a1 expression leads to a reduction in stiffness, fibril deposition, and altered fibril structure. Additionally, Col5-deficient male tendons demonstrated altered healing compared to wild-type tendons, however female mice have not yet been studied utilizing this model. Along with clinical differences between sexes in cEDS patient populations, differences in hormone physiology may be a factor influencing tendon health. Therefore, the objective of this study was to utilize a Col5a1+/ - female mouse model, to determine the effect of Col5 on tendon cell morphology, cell density, tissue composition, and mechanical properties throughout healing. We hypothesized that reduction in Col5 expression would result in an abnormal wound matrix post-injury, resulting in reduced mechanical properties compared to normal tendons. Following patellar tendon surgery, mice were euthanized at 1, 3, and 6-week post-injury. Col5-deficient tendons demonstrated altered and decreased healing compared to WT tendons. The lack of resolution in cellularity by 6-week post-injury in Col5-deficient tendons influenced the decreased mechanical properties. Stiffness did not increase post-injury in Col5-deficient mice, and collagen fiber realignment was delayed during mechanical loading. Therefore, increased Col5a1 expression post-injury is necessary to re-establish matrix engagement and cellularity throughout tendon healing.

Novel application of in vivo microdialysis in a rat Achilles tendon acute injury model.

Tendon injury and healing involve intricate changes to tissue metabolism, biology, and inflammation. Current techniques often require animal euthanasia or tissue destruction, limiting assessment of dynamic changes in tendon, including treatment response, disease development, rupture risk, and healing progression. Microdialysis, a minimally invasive technique, offers potential for longitudinal assessment, yet it has not been applied to rat tendon models. Therefore, the objective of this study is to adapt a novel application of an in vivo assay, microdialysis, using acute injury as a model for extreme disruption of the tendon homeostasis. We hypothesize that microdialysis will be able to detect measurable differences in the healing responses of acute injury with high specificity and sensitivity. Overall results suggest that microdialysis is a promising in vivo technique for longitudinal assessment for this system with strong correlations between extracellular fluid (ECF) and dialysate concentrations and reasonable recovery rates considering the limitations of this model. Strong positive correlations were found between dialysate and extracellular fluid (ECF) concentration for each target molecule of interest including metabolites, inflammatory mediators, and collagen synthesis and degradation byproducts. These results suggest that microdialysis is capable of detecting changes in tendon healing following acute tendon injury with high specificity and sensitivity. In summary, this is the first study to apply microdialysis to a rat tendon model and assess its efficacy as a direct measurement of tendon metabolism, biology, and inflammation.NEW & NOTEWORTHY This study adapts a novel application of microdialysis to rat tendon models, offering a minimally invasive avenue for longitudinal tendon assessment. Successfully detecting changes in tendon healing after acute injury, it showcases strong correlations between extracellular fluid and dialysate concentrations. The results highlight the potential of microdialysis as a direct measure of tendon metabolism, biology, and inflammation, bypassing the need for animal euthanasia and tissue destruction.

Neonatal Achilles Tendon Microstructure is Negatively Impacted by Decorin and Biglycan Knockdown After Injury and During Development.

Interest in studying neonatal development and the improved healing response observed in neonates is increasing, with the goal of using this work to create better therapeutics for tendon injury. Decorin and biglycan are two small leucine-rich proteoglycans that play important roles in collagen fibrillogenesis to develop, maintain, and repair tendon structure. However, little is known about the roles of decorin and biglycan in early neonatal development and healing. The goal of this study was to determine the effects of decorin and biglycan knockdown on Achilles tendon structure and mechanics during neonatal development and recovery of these properties after injury of the neonatal tendon. We hypothesized that knockdown of decorin and biglycan would disrupt the neonatal tendon developmental process and produce tendons with impaired mechanical and structural properties. We found that knockdown of decorin and biglycan in an inducible, compound decorin/biglycan knockdown model, both during development and after injury, in neonatal mice produced tendons with reduced mechanical properties. Additionally, the collagen fibril microstructure resembled an immature tendon with a large population of small diameter fibrils and an absence of larger diameter fibrils. Overall, this study demonstrates the importance of decorin and biglycan in facilitating tendon growth and maturation during neonatal development.

Investigating the temporal roles of decorin and biglycan in tendon healing.

The small leucine-rich proteoglycans, decorin and biglycan, are minor components of the tendon extracellular matrix that regulate fibrillogenesis and matrix assembly. Our study objective was to define the temporal roles of decorin and biglycan during tendon healing using inducible knockout mice to include genetic knockdown at specific phases of healing: time of injury, the proliferative phase, and the remodeling phase. We hypothesized that knockdown of decorin or biglycan would adversely affect tendon healing, and that by prescribing the timing of knockdown, we could elucidate the temporal roles of these proteins during healing. Contrary to our hypothesis, decorin knockdown did not affect tendon healing. However, when biglycan was knocked down, either alone or coupled with decorin, tendon modulus was increased relative to wild-type mice, and this finding was consistent among all induction timepoints. At 6 weeks postinjury, we observed increased expression of genes associated with the extracellular matrix and growth factor signaling in the biglycan knockdown and compound decorin-biglycan knockdown tendons. Interestingly, these groups demonstrated opposing trends in gene expression as a function of knockdown-induction timepoint, highlighting distinct temporal roles for decorin and biglycan. In summary, this study finds that biglycan plays multiple functions throughout tendon healing, with the most impactful, detrimental role likely occurring during late-stage healing. Statement of clinical importance: This study helps to define the molecular factors that regulate tendon healing, which may aid in the development of new clinical therapies.

Knockdown of biglycan reveals an important role in maintenance of structural and mechanical properties during tendon aging.

Biglycan, a small leucine-rich proteoglycan (SLRP), is involved in collagen fibrillogenesis and also acts as a signaling molecule. Although decorin has been considered as the primary SLRP in developing and maintaining tendon structure and mechanics, more recent work using inducible knockdown models suggests that biglycan is involved in tendon homeostasis. The purpose of the study was to determine the role of biglycan in tendon homeostasis to maintain mechanical and structural integrity in aged mice. Aged (485 days old) female Bgn+/+ control (wild type [WT], n = 16) and 16 bitransgenic conditional Bgnflox/flox mice (I-Bgn-/- , n = 16) with a tamoxifen-inducible Cre (driven by ROSA) were utilized. After biglycan knockdown, the transgenic model demonstrated effective knockdown of the target gene without any compensation from other SLRPs or type I collagen. Patellar tendon cellularity was not modified after biglycan knockdown. However, biglycan knockdown had an impact on collagen fibrillogenesis with a higher percentage of small diameter fibrils (25-45 nm) and a lower percentage of medium size fibrils (150-165 nm) in I-Bgn-/- tendons. Biglycan knockdown also induced a reduction in the midsubstance modulus and maximum stress compared to WT. Stress relaxation was reduced at 4% strain in I-Bgn-/- tendons but no changes were observed in dynamic modulus and tan delta. As in mature tendons (120 days old), this study showed significant effects of biglycan knockdown on mechanical and structural properties of aged tendons only 30 days after knockdown. These data suggest that biglycan has a major role in maintaining homeostasis in aged tendon.

Post-injury tendon mechanics are not affected by tamoxifen treatment.

PURPOSE: A growing interest in the mechanisms that govern tendon healing has resulted in the develop-ment of tools, such as the tamoxifen-inducible mouse knockdown model, to address these questions. However, tamoxifen is a selective estrogen receptor modulator and may interfere with the tendon healing process. The objective of this study was to evaluate the effects of tamoxifen on post-injury tendon mechanics in wild-type mice. METHODS: The mice underwent treatment at the time of injury using an established mouse injury model and the injured tendons were evaluated 3 weeks post-injury. The treatment contained tamoxifen suspended in corn oil and was compared to a treatment with only corn oil, as well as mice with no treatment. Tendons were evaluated by measuring the quasi-static and viscoelastic mechanics, collagen fiber realignment, cellularity, and nuclear morphology. RESULTS: Mechanical testing of the tendons post-injury revealed no changes to viscoelastic mechanics, quasi-static mechanics, or collagen realignment during loading after tamoxifen treatment with the dosage regimen utilized (three daily injections of 4.5 mg/40 g body weight). Additionally, histological analysis revealed no changes to cellularity or cell nuclear shape. CONCLUSION: Overall, this study revealed that tamoxifen treatment at the time of tendon injury did not result in changes to tendon mechanics or the histological parameters at 3 weeks post-injury.

Decorin knockdown is beneficial for aged tendons in the presence of biglycan expression.

Decorin and biglycan are two major small leucine-rich proteoglycans (SLRPs) present in the tendon extracellular matrix that facilitate collagen fibrillogenesis, tissue turnover, and cell signal transduction. Previously, we demonstrated that knockout of decorin prevented the decline of tendon mechanical properties that are associated with aging. The objective of this study was to determine the effects of decorin and biglycan knockdown on tendon structure and mechanics in aged tendons using tamoxifen-inducible knockdown models. We hypothesized that the knockdown of decorin and compound knockdown of decorin and biglycan would prevent age-related declines in tendon mechanics and structure compared to biglycan knockdown and wild-type controls, and that these changes would be exacerbated as the tendons progress towards geriatric ages. To achieve this objective, we created tamoxifen-inducible mouse knockdown models to target decorin and biglycan gene inactivation without the abnormal tendon development associated with traditional knockout models. Knockdown of decorin led to increased midsubstance modulus and decreased stress relaxation in aged tendons. However, these changes were not sustained in the geriatric tendons. Knockdown in biglycan led to no changes in mechanics in the aged or geriatric tendons. Contrary to our hypothesis, the compound decorin/biglycan knockdown tendons did not resemble the decorin knockdown tendons, but resulted in increased viscoelastic properties in the aged and geriatric tendons. Structurally, knockdown of SLRPs, except for the 570d I-Dcn -/- /Bgn -/- group, resulted in alterations to the collagen fibril diameter relative to wild-type controls. Overall, this study identified the differential roles of decorin and biglycan throughout tendon aging in the maintenance of tendon structural and mechanical properties and revealed that the compound decorin and biglycan knockdown phenotype did not resemble the single gene decorin or biglycan models and was detrimental to tendon properties throughout aging.

Increasing Vascular Response to Injury Improves Tendon Early Healing Outcome in Aged Rats.

Tendon injuries positively correlate with patient age, as aging has significant effects on tendon homeostatic maintenance and healing potential after injury. Vascularity is also influenced by age, with both clinical and animal studies demonstrating reduced blood flow in aged tissues. However, it is unknown how aging effects vascularity following tendon injury, and if this vascular response can be modulated through the delivery of angiogenic factors. Therefore, the objective of this study is to evaluate the vascular response following Achilles tendon injury in adult and aged rats, and to define the alterations to tendon healing in an aged model following injection of angiogenic factors. It was determined that aged rat Achilles tendons have a reduced angiogenesis following injury. Further, the delivery of vascular endothelial growth factor, VEGF, caused an increase in vascular response to tendon injury and improved mechanical outcome in this aged population. This work suggests that reduced angiogenic potential with aging may be contributing to impaired tendon healing response and that the delivery of angiogenic factors can rescue this impaired response. This study was also the first to relate changes in vascular response in an aged model using in vivo measures of blood perfusion to alterations in healing properties.

Biglycan has a major role in maintenance of mature tendon mechanics.

Decorin and biglycan are two small leucine-rich proteoglycans (SLRPs) that regulate collagen fibrillogenesis and extracellular matrix assembly in tendon. The objective of this study was to determine the individual roles of these molecules in maintaining the structural and mechanical properties of tendon during homeostasis in mature mice. We hypothesized that knockdown of decorin in mature tendons would result in detrimental changes to tendon structure and mechanics while knockdown of biglycan would have a minor effect on these parameters. To achieve this objective, we created tamoxifen-inducible mouse knockdown models targeting decorin or biglycan inactivation. This enables the evaluation of the roles of these SLRPs in mature tendon without the abnormal tendon development caused by conventional knockout models. Contrary to our hypothesis, knockdown of decorin resulted in minor alterations to tendon structure and no changes to mechanics while knockdown of biglycan resulted in broad changes to tendon structure and mechanics. Specifically, knockdown of biglycan resulted in reduced insertion modulus, maximum stress, dynamic modulus, stress relaxation, and increased collagen fiber realignment during loading. Knockdown of decorin and biglycan produced similar changes to tendon microstructure by increasing the collagen fibril diameter relative to wild-type controls. Biglycan knockdown also decreased the cell nuclear aspect ratio, indicating a more spindle-like nuclear shape. Overall, the extensive changes to tendon structure and mechanics after knockdown of biglycan, but not decorin, provides evidence that biglycan plays a major role in the maintenance of tendon structure and mechanics in mature mice during homeostasis.

Coordinate roles for collagen VI and biglycan in regulating tendon collagen fibril structure and function.

Tendon is a vital musculoskeletal tissue that is prone to degeneration. Proper tendon maintenance requires complex interactions between extracellular matrix components that remain poorly understood. Collagen VI and biglycan are two matrix molecules that localize pericellularly within tendon and are critical regulators of tissue properties. While evidence suggests that collagen VI and biglycan interact within the tendon matrix, the relationship between the two molecules and its impact on tendon function remains unknown. We sought to elucidate potential coordinate roles of collagen VI and biglycan within tendon by defining tendon properties in knockout models of collagen VI, biglycan, or both molecules. We first demonstrated co-expression and co-localization of collagen VI and biglycan within the healing tendon, providing further evidence of cooperation between the two molecules during nascent tendon matrix formation. Deficiency in collagen VI and/or biglycan led to significant reductions in collagen fibril size and tendon mechanical properties. However, collagen VI-null tendons displayed larger reductions in fibril size and mechanics than seen in biglycan-null tendons. Interestingly, knockout of both molecules resulted in similar properties to collagen VI knockout alone. These results indicate distinct and non-additive roles for collagen VI and biglycan within tendon. This work provides better understanding of regulatory interactions between two critical tendon matrix molecules.

Pulsed electromagnetic field therapy alters early healing in a rat model of rotator cuff injury and repair: Potential mechanisms.

Rotator cuff repair failure remains common due to poor tendon healing, particularly at the enthesis. We previously showed that pulsed electromagnetic field (PEMF) therapy improved the mechanical properties of the rat supraspinatus tendon postoperatively. However, little is known about the mechanisms behind PEMF-dependent contributions to improved healing in this injury model. The objective of this study was to determine the influence of PEMF treatment on tendon gene expression and cell composition, as well as bone microarchitecture and dynamic bone metabolism during early stages of healing. We hypothesized that PEMF treatment would amplify tendon-healing related signaling pathways while mitigating inflammation and improve bone metabolism at the repair site. Rats underwent rotator cuff injury and repair followed by assignment to either control (non-PEMF) or PEMF treatment groups. Gene and protein expression as well as tendon and bone histological assessments were performed 3, 7, 14, 21, and 28 days after injury. Gene expression data demonstrated an upregulation in the bone morphogenetic protein 2 signaling pathway and increases in pro-osteogenic genes at the insertion, supporting important processes to re-establish the tendon-bone interface. PEMF also downregulated genes related to a fibrotic healing response. Anti-inflammatory effects were demonstrated by both gene expression and macrophage phenotype. PEMF significantly increased the rate of kinetic bone formation directly adjacent to the tendon enthesis as well as the number of cuboidal surface osteoblasts (active osteoblasts) in the humeral head. This study has provided insight into how PEMF affects cellular and molecular processes in the supraspinatus tendon and adjacent bone after injury and repair.

Modulation of vascular response after injury in the rat Achilles tendon alters healing capacity.

Tendons are relatively hypovascular but become hypervascular during both injury and degeneration. This is due to the angiogenic response, or the formation of new blood vessels, to tissue injury. The objective of this study was to evaluate the effect of vascular modulation in the rat Achilles tendons during healing. Fischer rats received a bilateral Achilles incisional injury followed by local injections of vascular endothelial growth factor (VEGF), anti-VEGF antibody (B20.4-1-1), or saline either early or late during the healing process. Vascular modulation and healing were evaluated using multiple in vivo ultrasound imaging modalities, in vivo functional assessment, and ex vivo measures of tendon compositional and mechanical properties. The late delivery of anti-VEGF antibody, B20, caused a temporary reduction in healing capacity during a time point where vascularity was also decreased, and then an improvement during a later time point where vascularity was increased relative to control. However, VEGF delivery had a minimal impact on healing and vascular changes in both early and late delivery times. This study was the first to evaluate vascular changes using both in vivo imaging methods and ex vivo histological methods, as well as functional and mechanical outcomes associated with these vascular changes. Clinical significance: this study demonstrates that the alteration of vascular response through the delivery of angiogenic growth factors has the ability to alter tendon healing properties.

Induced Knockdown of Decorin, Alone and in Tandem With Biglycan Knockdown, Directly Increases Aged Murine Patellar Tendon Viscoelastic Properties.

Tendon injuries increase with age, yet the age-associated changes in tendon properties remain unexplained. Decorin and biglycan are two matrix proteoglycans that play complex roles in regulating tendon formation, maturation, and aging, most notably in extracellular matrix assembly and maintenance. However, the roles of decorin and biglycan have not been temporally isolated in a homeostatic aged context. The goal of this work was to temporally isolate and define the roles of decorin and biglycan in regulating aged murine patellar tendon mechanical properties. We hypothesized that decorin would have a larger influence than biglycan on aged tendon mechanical properties and that biglycan would have an additive role in this regulation. When decorin and biglycan were knocked down in aged tendons, minimal changes in gene expression were observed, implying that these models directly define the roles of decorin and biglycan in regulating tendon mechanical properties. Knockdown of decorin or biglycan led to minimal changes in quasi-static mechanical properties. However, decorin deficiency led to increases in stress relaxation and phase shift that were exacerbated when coupled with biglycan deficiency. This study highlights an important role for decorin, alone and in tandem with biglycan, in regulating aged tendon viscoelastic properties.

Biocompatibility and bioactivity of an FGF-loaded microsphere-based bilayer delivery system.

Many drug delivery systems rely on degradation or dissolution of the carrier material to regulate release. In cases where mechanical support is required during regeneration, this necessitates composite systems in which the mechanics of the implant are decoupled from the drug release profile. To address this need, we developed a system in which microspheres (MS) were sequestered in a defined location between two nanofibrous layers. This bilayer delivery system (BiLDS) enables simultaneous structural support and decoupled release profiles. To test this new system, PLGA (poly-lactide-co-glycolic acid) microspheres were prepared using a water-in-oil-in-water (w/o/w) emulsion technique and incorporated Alexa Fluor-tagged bovine serum albumin (BSA) and basic fibroblast growth factor (bFGF). These MS were secured in a defined pocket between two polycaprolactone (PCL) nanofibrous scaffolds, where the layered scaffolds provide a template for new tissue formation while enabling independent and local release from the co-delivered MS. Scanning electron microscopy (SEM) images showed that the assembled BiLDS could localize and retain MS in the central pocket that was surrounded by a continuous seal formed along the margin. Cell viability and proliferation assays showed enhanced cell activity when exposed to BiLDS containing Alexa Fluor-BSA/bFGF-loaded MS, both in vitro and in vivo. MS delivered via the BiLDS system persisted in a localized area after subcutaneous implantation for at least 4 weeks, and bFGF release increased colonization of the implant. These data establish the BiLDS technology as a sustained in vivo drug delivery platform that can localize protein and other growth factor release to a surgical site while providing a structural template for new tissue formation. STATEMENT OF SIGNIFICANCE: Localized and controlled delivery systems for the sustained release of drugs are essential. Many strategies have been developed for this purpose, but most rely on degradation (and loss of material properties) for delivery. Here, we developed a bilayer delivery system (BiLDS) that decouples the physical properties of a scaffold from its delivery kinetics. For this, biodegradable PLGA microspheres were sequestered within a central pocket of a slowly degrading nanofibrous bilayer. Using this device, we show enhanced cell activity with FGF delivery from the BiLDS both in vitro and in vivo. These data support that BiLDS can localize sustained protein and biofactor delivery to a surgical site while also serving as a mechanical scaffold for tissue repair and regeneration.

Localized delivery of ibuprofen via a bilayer delivery system (BiLDS) for supraspinatus tendon healing in a rat model.

The high prevalence of tendon retear following rotator cuff repair motivates the development of new therapeutics to promote improved tendon healing. Controlled delivery of non-steroidal anti-inflammatory drugs to the repair site via an implanted scaffold is a promising option for modulating inflammation in the healing environment. Furthermore, biodegradable nanofibrous delivery systems offer an optimized architecture and surface area for cellular attachment, proliferation, and infiltration while releasing soluble factors to promote tendon regeneration. To this end, we developed a bilayer delivery system (BiLDS) for localized and controlled release of ibuprofen (IBP) to temporally mitigate inflammation and enhance tendon remodeling following surgical repair by promoting organized tissue formation. In vitro evaluation confirmed the delayed and sustained release of IBP from Labrafil-modified poly(lactic-co-glycolic) acid microspheres within sintered poly(ε-caprolactone) electrospun scaffolds. Biocompatibility of the BiLDS was demonstrated with primary Achilles tendon cells in vitro. Implantation of the IBP-releasing BiLDS at the repair site in a rat rotator cuff injury and repair model led to decreased expression of proinflammatory cytokine, tumor necrotic factor-α, and increased anti-inflammatory cytokine, transforming growth factor-ß1. The BiLDS remained intact for mechanical reinforcement and recovered the tendon structural properties by 8 weeks. These results suggest the therapeutic potential of a novel biocompatible nanofibrous BiLDS for localized and tailored delivery of IBP to mitigate tendon inflammation and improve repair outcomes. Future studies are required to define the mechanical implications of an optimized BiLDS in a rat model beyond 8 weeks or in a larger animal model.

Effects of immobilization angle on tendon healing after achilles rupture in a rat model.

Conservative (non-operative) treatment of Achilles tendon ruptures is a common alternative to operative treatment. Following rupture, ankle immobilization in plantarflexion is thought to aid healing by restoring tendon end-to-end apposition. However, early activity may improve limb function, challenging the role of immobilization position on tendon healing, as it may affect loading across the injury site. This study investigated the effects of ankle immobilization angle in a rat model of Achilles tendon rupture. We hypothesized that manipulating the ankle from full plantarflexion into a more dorsiflexed position during the immobilization period would result in superior hindlimb function and tendon properties, but that prolonged casting in dorsiflexion would result in inferior outcomes. After Achilles tendon transection, animals were randomized into eight immobilization groups ranging from full plantarflexion (160°) to mid-point (90°) to full dorsiflexion (20°), with or without angle manipulation. Tendon properties and ankle function were influenced by ankle immobilization position and time. Tendon lengthening occurred after 1 week at 20° compared to more plantarflexed angles, and was associated with loss of propulsion force. Dorsiflexing the ankle during immobilization from 160° to 90° produced a stiffer, more aligned tendon, but did not lead to functional changes compared to immobilization at 160°. Although more dorsiflexed immobilization can enhance tissue properties and function of healing Achilles tendon following rupture, full dorsiflexion creates significant tendon elongation regardless of application time. This study suggests that the use of moderate plantarflexion and earlier return to activity can provide improved clinical outcomes. © 2019 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res.

Nicotine impairs intra-substance tendon healing after full thickness injury in a rat model.

Nicotine is harmful to many bodily systems; however, the effects of nicotine on intra-substance tendon healing remain largely unexplored. The purpose of this study was to examine the functional, structural, and biomechanical effects of nicotine on the healing of Achilles tendons in rats after an acute full-thickness injury. Sixty Sprague-Dawley rats were enrolled in this study. Half were exposed to 0.9% saline and half to 61 ng/mL of nicotine for 3 months via subcutaneous osmotic pumps. At 3 months, all rats underwent blunt full thickness transection of the left Achilles tendon and were immobilized for one week in plantarflexion. In-vivo assays were conducted prior to injury, at 21 days, and at 42 days post-injury and included the following: Functional limb assessment, passive joint mechanics, and vascular evaluation. Rats were sacrificed at 21 and 42 days for biomechanical testing and histologic evaluation. Rats exposed to nicotine demonstrated decreased vascularity, greater alteration in gait mechanics, and increased passive ROM of the ankle joint. Biomechanically, the nicotine tendons failed at lower maximum loads, were less stiff, had smaller cross-sectional areas and had altered viscoelastic properties. Histologically, nicotine tendons demonstrated decreased vessel density at the injury site. This study demonstrates that nicotine leads to worse functional outcomes and biomechanical properties in tendons. The decreased vascularity in the nicotine group may suggest an underlying mechanism for inferior tendon healing. Patients should be counseled that using nicotine products increase their risk of poor tendon healing and may predispose them to tendon re-rupture. © 2018 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res.

Biceps Detachment Preserves Joint Function in a Chronic Massive Rotator Cuff Tear Rat Model.

BACKGROUND: Lesions of the long head of the biceps tendon are often associated with massive rotator cuff tears (MRCTs), and biceps tenotomy is frequently performed for pain relief and functional reservation. However, the efficacy and safety of biceps tenotomy regarding the effects on the surrounding tissues in chronic MRCT are unclear. HYPOTHESIS: Biceps tenotomy would result in improved mechanical and histological properties of the intact subscapularis tendon and improved in vivo shoulder function while not compromising glenoid cartilage properties. STUDY DESIGN: Controlled laboratory study. METHODS: Right supraspinatus and infraspinatus tendons were detached in 25 male Sprague-Dawley rats, followed by 4 weeks of cage activity to create a chronic MRCT condition. Animals were randomly divided into 2 groups and received either biceps tenotomy (n = 11) or sham surgery (n = 14) and were sacrificed 4 weeks thereafter. Forelimb gait and ground-reaction forces were recorded 1 day before the tendon detachment (baseline), 1 day before the surgical intervention (biceps tenotomy or sham), and 3, 7, 10, 14, 21, and 28 days after the intervention to assess in vivo shoulder joint function. The subscapularis tendon and glenoid cartilage were randomly allocated for mechanical testing or histologic assessment after the sacrifice. RESULTS: Compared with sham surgery, biceps tenotomy partially restored the in vivo shoulder joint function, with several gait and ground-reaction force parameters returning closer to preinjury baseline values at 4 weeks. With biceps tenotomy, mechanical properties of the subscapularis tendons were improved, while mechanical properties and histological Mankin scores of the glenoid cartilage were not diminished when compared with the sham group. CONCLUSION: Biceps tenotomy in the presence of chronic MRCT partially preserves overall shoulder function and potentially restores subscapularis tendon health without causing detrimental effects to joint cartilage. This laboratory study adds to the growing literature regarding the protective effects of biceps tenotomy on the shoulder joint in a chronic MRCT model. CLINICAL RELEVANCE: This study provides important basic science evidence supporting the use of biceps tenotomy in patients with massive rotator cuff tears.

Tendon healing affects the multiscale mechanical, structural and compositional response of tendon to quasi-static tensile loading.

Tendon experiences a variety of multiscale changes to its extracellular matrix during mechanical loading at the fascicle, fibre and fibril levels. For example, tensile loading of tendon increases its stiffness, with organization of collagen fibres, and increases cell strain in the direction of loading. Although applied macroscale strains correlate to cell and nuclear strains in uninjured tendon, the multiscale response during tendon healing remains unknown and may affect cell mechanosensing and response. Therefore, this study evaluated multiscale structure-function mechanisms in response to quasi-static tensile loading in uninjured and healing tendons. We found that tendon healing affected the macroscale mechanical and structural response to mechanical loading, evidenced by decreases in strain stiffening and collagen fibre realignment. At the micro- and nanoscales, healing resulted in increased collagen fibre disorganization, nuclear disorganization, decreased change in nuclear aspect ratio with loading, and decreased indentation modulus compared to uninjured tendons. Taken together, this work supports a new concept of nuclear strain transfer attenuation during tendon healing and identifies several multiscale properties that may contribute. Our work also provides benchmarks for the biomechanical microenvironments that tendon cells may experience following cell delivery therapies.