A role for TGFβ signaling in Gli1+ tendon and enthesis cells

The development of musculoskeletal tissues such as tendon, enthesis, and bone relies on proliferation and differentiation of mesenchymal progenitor cells. Gli1+ cells have been described as putative stem cells in several tissues and are presumed to play critical roles in tissue formation and maintenance. For example, the enthesis, a fibrocartilage tissue that connects tendon to bone, is mineralized postnatally by a pool of Gli1+ progenitor cells. These cells are regulated by hedgehog signaling, but it is unclear if TGFβ signaling, necessary for tenogenesis, also plays a role in their behavior. To examine the role of TGFβ signaling in Gli1+ cell function, the receptor for TGFβ, TbR2, was deleted in Gli1-lineage cells in mice at P5. Decreased TGFβ signaling in these cells led to defects in tendon enthesis formation by P56, including defective bone morphometry underlying the enthesis and decreased mechanical properties. Immunohistochemical staining of these Gli1+ cells showed that loss of TGFβ signaling reduced proliferation and increased apoptosis. In vitro experiments using Gli1+ cells isolated from mouse tail tendons demonstrated that TGFβ controls cell proliferation and differentiation through canonical and non-canonical pathways and that TGFβ directly controls the tendon transcription factor scleraxis by binding to its distant enhancer. These results have implications in the development of treatments for tendon and enthesis pathologies.

Toward the Development of a Tissue Engineered Gradient Utilizing CRISPR-Guided Gene Modulation

Cellular, compositional, and mechanical gradients are found throughout biological tissues, especially in transition zones between tissue types. Yet, strategies to engineer such gradients have proven difficult due to the complex nature of these tissues. Current strategies for tissue engineering complex gradients often utilize stem cells; however, these multipotent cells require direction from environmental cues, which can be difficult to control both in vitro and in vivo. In this study, we utilize clustered regularly-interspaced short palindromic repeats (CRISPR)-guided gene modulation to direct the differentiation of multipotent adipose-derived stem cells (ASCs) to demonstrate the effectiveness of CRISPR-engineered cells in tissue engineering applications. Specifically, we screen CRISPR-interference (CRISPRi) constructs targeting the promotors of selected osteogenic inhibitors and demonstrate that ASC osteogenic differentiation and mineral deposition can be regulated with CRISPRi targeting of Noggin without the use of exogenous growth factors in tissue engineered constructs. As a proof of concept, we combine three technologies developed out of our laboratories to demonstrate the controlled deposition of these engineered cells in a gradient with CRISPR-activation multiplex-engineered aggrecan/collagen type-II-chondrogenic ASCs on a high density anisotropic type I collagen construct to create a cell and tissue gradient similar to the fibrocartilage-to-mineralized-fibrocartilage gradient in the enthesis. Our results display the promise of CRISPR-engineered ASCs to produce tissue gradients, similar to what is observed in native tissue.

The Structure, Biology, and Mechanical Function of Tendon/Ligament-Bone Interfaces

After tendon or ligament reconstruction, the interface between the hard bone and soft connective tissue is considerably weakened and is difficult to restore through healing. The tendon/ligament-bone interface is mechanically the weakest point under tensile loading and is often the source of various postoperative complications, such as bone resorption and graft laxity. A comprehensive understanding of the macro- and microfeatures of the native tendon/ligament-bone interface would be beneficial for developing strategies for regenerating the tissue. This article discusses the structural, biological, and mechanical features of the tendon/ligament-bone interfaces and how these can be affected by aging and loading conditions.

Anatomical design and production of a novel three-dimensional co-culture system replicating the human flexor digitorum profundus enthesis

The enthesis, the specialized junction between tendon and bone, is a common site of injury. Although notoriously difficult to repair, advances in interfacial tissue engineering techniques are being developed for restorative function. Most notably are 3D in vitro co-culture models, built to recreate the complex heterogeneity of the native enthesis. While cell and matrix properties are often considered, there has been little attention given to native enthesis anatomical morphometrics and replicating these to enhance clinical relevance. This study focuses on the flexor digitorum profundus (FDP) tendon enthesis and, by combining anatomical morphometrics with computer-aided design, demonstrates the design and construction of an accurate and scalable model of the FDP enthesis. Bespoke 3D-printed mould inserts were fabricated based on the size, shape and insertion angle of the FDP enthesis. Then, silicone culture moulds were created, enabling the production of bespoke anatomical culture zones for an in vitro FDP enthesis model. The validity of the model has been confirmed using brushite cement scaffolds seeded with osteoblasts (bone) and fibrin hydrogel scaffolds seeded with fibroblasts (tendon) in individual studies with cells from either human or rat origin. This novel approach allows a bespoke anatomical design for enthesis repair and should be applied to future studies in this area.

Novel bisphosphonate-based cathepsin K-triggered compound targets the enthesis without impairing soft tissue-to-bone healing

Background: Osteoadsorptive fluorogenic sentinel 3 (OFS-3) is a recently described compound that contains a bone-targeting bisphosphonate (BP) and cathepsin K (Ctsk)-triggered fluorescence signal. A prior study in a murine Achilles repair model demonstrated its effectiveness at targeting the site of tendon-to-bone repair, but the intrinsic effect of this novel bisphosphonate chaperone on tendon-to-bone healing has not been previously explored. We hypothesized that application of this bisphosphonate-fluorophore cargo conjugate would not affect the biomechanical properties or histologic appearance of tendon-bone repairs. Materials and Methods: Right hindlimb Achilles tendon-to-bone repair was performed on 12-week old male mice. Animals were divided into 2 groups of 18 each: 1) Achilles repair with OFS-3 applied directly to the repair site prior to closure, and 2) Achilles repair with saline applied prior to closure. Repaired hindlimbs from 12 animals per group were harvested at 6 weeks for biomechanical analysis with a custom 3D-printed jig. At 4 and 6 weeks, repaired hindlimbs from the remaining animals were assessed histologically using H&E, immunohistochemistry (IHC) staining for the presence of Ctsk, and second harmonic generation (SHG) imaging to evaluate collagen fibers. Results: At 6 weeks, there was no significant difference in failure load, stiffness, toughness, or displacement to failure between repaired hindlimbs that received OFS-3 versus saline. There was no difference in tissue healing on H&E or Ctsk staining on immunohistochemistry between animals that received OFS-3 versus saline. Finally, second harmonic generation imaging demonstrated no difference in collagen fiber parameters between the two groups. Conclusion: OFS-3 did not significantly affect the biomechanical properties or histologic appearance of murine Achilles tendon-to-bone repairs. This study demonstrates that OFS-3 can target the site of tendon-to-bone repair without causing intrinsic negative effects on healing. Further development of this drug delivery platform to target growth factors to the site of tendon-bone repair is warranted.

Hot spots and frontiers in bone-tendon interface research: a bibliometric analysis and visualization from 2000 to 2023

Objective

In this research, we investigated the current status, hotspots, frontiers, and trends of research in the field of bone-tendon interface (BTI) from 2000 to 2023, based on bibliometrics and visualization and analysis in CiteSpace, VOSviewer, and a bibliometric package in R software.

Methods

We collected and organized the papers in the Web of Science core collection (WoSCC) for the past 23 years (2000-2023), and extracted and analyzed the papers related to BTI. The extracted papers were bibliometrically analyzed using CiteSpace for overall publication trends, authors, countries/regions, journals, keywords, research hotspots, and frontiers.

Results

A total of 1,995 papers met the inclusion criteria. The number of papers published and the number of citations in the field of BTI have continued to grow steadily over the past 23 years. In terms of research contribution, the United States leads in terms of the number and quality of publications, number of citations, and collaborations with other countries, while the United Kingdom and the Netherlands lead in terms of the average number of citations. The University of Leeds publishes the largest number of papers, and among the institutions hosting the 100 most cited papers Hospital for Special Surgery takes the top spot. MCGONAGLE D has published the highest number of papers (73) in the last 10 years. The top three clusters include #0 "psoriatic arthritis", #1 "rotator cuff repair", and #2 "tissue engineering". The structure and function of the BTI and its key mechanisms in the healing process are the key to research, while new therapies such as mechanical stimulation, platelet-rich plasma, mesenchymal stem cells, and biological scaffolds are hot topics and trends in research.

Conclusion

Over the past 23 years, global research on the BTI has expanded in both breadth and depth. The focus of research has shifted from studies concentrating on the structure of the BTI and the disease itself to new therapies such as biomaterial-based alternative treatments, mechanical stimulation, platelet-rich plasma, etc.

Achilles tendon and enthesis assessment using ultrashort echo time magnetic resonance imaging (UTE-MRI) T1 and magnetization transfer (MT) modeling in psoriatic arthritis

The purpose of this study is to investigate the use of ultrashort echo time (UTE) magnetic resonance imaging (MRI) techniques (T1 and magnetization transfer [MT] modeling) for imaging of the Achilles tendons and entheses in patients with psoriatic arthritis (PsA) compared with asymptomatic volunteers. The heels of twenty-six PsA patients (age 59 ± 15 years, 41% female) and twenty-seven asymptomatic volunteers (age 33 ± 11 years, 47% female) were scanned in the sagittal plane with UTE-T1 and UTE-MT modeling sequences on a 3-T clinical scanner. UTE-T1 and macromolecular proton fraction (MMF; the main outcome of MT modeling) were calculated in the tensile portions of the Achilles tendon and at the enthesis (close to the calcaneus bone). Mann-Whitney-U tests were used to examine statistically significant differences between the two cohorts. UTE-T1 in the entheses was significantly higher for the PsA group compared with the asymptomatic group (967 ± 145 vs. 872 ± 133 ms, p < 0.01). UTE-T1 in the tendons was also significantly higher for the PsA group (950 ± 145 vs. 850 ± 138 ms, p < 0.01). MMF in the entheses was significantly lower in the PsA group compared with the asymptomatic group (15% ± 3% vs. 18% ± 3%, p < 0.01). MMF in the tendons was also significantly lower in the PsA group compared with the asymptomatic group (17% ± 4% vs. 20% ± 5%, p < 0.01). Percentage differences in MMF between the asymptomatic and PsA groups (-16.6% and -15.0% for the enthesis and tendon, respectively) were higher than the T1 differences (10.8% and 11.7% for the enthesis and tendon, respectively). The results suggest higher T1 and lower MMF in the Achilles tendons and entheses in PsA patients compared with the asymptomatic group. This study highlights the potential of UTE-T1 and UTE-MT modeling for quantitative evaluation of entheses and tendons in PsA patients.

Hedgehog Activation for Enhanced Rotator Cuff Tendon-to-Bone Healing

BACKGROUND

Rotator cuff repair is a common orthopaedic procedure, yet the rate of failure to heal after surgery is high. Repair site rupture is due to poor tendon-to-bone healing and lack of regeneration of the native fibrocartilaginous enthesis. During development, the enthesis is formed and mineralized by a pool of progenitors activated by hedgehog signaling. Furthermore, hedgehog signaling drives regenerative enthesis healing in young animals, in contrast to older animals, in which enthesis injuries heal via fibrovascular scar and without participation of hedgehog signaling.

HYPOTHESIS

Hedgehog activation improves tendon-to-bone healing in an animal model of rotator cuff repair.

STUDY DESIGN

Controlled laboratory study.

METHODS

A total of 78 adult Sprague-Dawley rats were used. Supraspinatus tendon injury and repair were completed bilaterally, with microsphere-encapsulated hedgehog agonist administered to right shoulders and control microspheres administered to left shoulders. Animals were sacrificed after 3, 14, 28, or 56 days. Gene expression and histological, biomechanical, and bone morphometric analyses were conducted.

RESULTS

At 3 days, hedgehog signaling pathway genes Gli1 (1.70; P = .029) and Smo (2.06; P = .0173), as well as Runx2 (1.69; P = .0386), a transcription factor of osteogenesis, were upregulated in treated relative to control repairs. At 14 days, transcription factors of tenogenesis, Scx (4.00; P = .041), and chondrogenesis, Sox9 (2.95; P = .010), and mineralized fibrocartilage genes Col2 (3.18; P = .031) and Colx (1.85; P = .006), were upregulated in treated relative to control repairs. Treatment promoted fibrocartilage formation at the healing interface by 28 days, with improvements in tendon-bone maturity, organization, and continuity. Treatment led to improved biomechanical properties. The material property strength (2.43 vs 1.89 N/m2; P = .046) and the structural property work to failure (29.01 vs 18.09 mJ; P = .030) were increased in treated relative to control repairs at 28 days and 56 days, respectively. Treatment had a marginal effect on bone morphometry underlying the repair. Trabecular thickness (0.08 vs 0.07 mm; P = .035) was increased at 28 days.

CONCLUSION

Hedgehog agonist treatment activated hedgehog signaling at the tendon-to-bone repair site and prompted increased mineralized fibrocartilage production. This extracellular matrix production and mineralization resulted in improved biomechanical properties, demonstrating the therapeutic potential of hedgehog agonism for improving tendon-to-bone healing after rotator cuff repair.

CLINICAL RELEVANCE

This study demonstrates the therapeutic potential of hedgehog agonist treatment for improving tendon-to-bone healing after rotator cuff injury and repair.

The plantar proximal cortex of the third metatarsal bone shows raised longitudinal ridges at the suspensory ligament enthesis in normal equine isolated limbs - a radiographic, computed tomography, and MRI study

Introduction

Knowledge of normal radiographic appearance is essential to avoid misinterpretation of radiographs. This study aimed to assess the computed tomographic (CT) appearance of the plantar surface of the proximal metatarsus and evaluate the influence of the radiographic angle on the trabecular/cortical interface of the proximal plantar metatarsal cortex on lateromedial and slightly oblique radiographs.

Methods

Eight hindlimbs were collected from six horses with no known history of lameness and euthanized for reasons unrelated to the study. Limbs underwent computed tomographic (CT) and radiographic examination (dorsoplantar, lateromedial, and slightly oblique radiographic views obtained by angling the beam dorsally and plantarly from the plane used for the lateromedial projection). Standing magnetic resonance (MR) imaging and computed tomography (CT) were used to confirm normalcy. Images were compared side-by-side by two experienced readers.

Results

Limbs were normal at MR imaging. Longitudinal linear ridges were present on the proximal plantar metatarsal surface in all limbs (1-2 sagittal ridges and 1 ridge located at the medial or lateral margin of the suspensory ligament). Longitudinal ridges were positioned facing an adipose-muscular bundle of the suspensory ligament on CT images and were visible as linearly increased opacities on dorsoplantar radiographs. The delineation of the trabecular/cortical interface of the proximal metatarsus changed with radiographic projection and was the sharpest on the plantaro 85° lateral to the dorsomedial oblique view.

Conclusion

The proximal third metatarsal bone shows individual morphological variations, with longitudinal linear ridges that alter the bone homogeneity on dorsoplantar radiographs. An oblique plantaro 85° lateral to the dorsomedial view is suggested to better assess the presence of subcortical sclerosis when proximal suspensory enthesopathy is suspected.

Blocking CCN2 Reduces Established Palmar Neuromuscular Fibrosis and Improves Function Following Repetitive Overuse Injury

The matricellular protein cell communication factor 2/connective tissue growth factor (CCN2/CTGF) is critical to development of neuromuscular fibrosis. Here, we tested whether anti-CCN2 antibody treatment will reduce established forepaw fibro-degenerative changes and improve function in a rat model of overuse injury. Adult female rats performed a high repetition high force (HRHF) task for 18 weeks. Tissues were collected from one subset after 18 wks (HRHF-Untreated). Two subsets were provided 6 wks of rest with concurrent treatment with anti-CCN2 (HRHF-Rest/anti-CCN2) or IgG (HRHF-Rest/IgG). Results were compared to IgG-treated Controls. Forepaw muscle fibrosis, neural fibrosis and entheseal damage were increased in HRHF-Untreated rats, compared to Controls, and changes were ameliorated in HRHF-Rest/anti-CCN2 rats. Anti-CCN2 treatment also reduced phosphorylated-β-catenin (pro-fibrotic protein) in muscles and distal bone/entheses complex, and increased CCN3 (anti-fibrotic) in the same tissues, compared to HRHF-Untreated rats. Grip strength declines and mechanical sensitivity observed in HRHF-Untreated improved with rest; grip strength improved further in HRHF-Rest/anti-CCN2. Grip strength declines correlated with muscle fibrosis, entheseal damage, extraneural fibrosis, and decreased nerve conduction velocity, while enhanced mechanical sensitivity (a pain-related behavior) correlated with extraneural fibrosis. These studies demonstrate that blocking CCN2 signaling reduces established forepaw neuromuscular fibrosis and entheseal damage, which improves forepaw function, following overuse injury.

Loss of Fgfr1 and Fgfr2 in Scleraxis-lineage cells leads to enlarged bone eminences and attachment cell death

BACKGROUND

Tendons and ligaments attach to bone are essential for joint mobility and stability in vertebrates. Tendon and ligament attachments (ie, entheses) are found at bony protrusions (ie, eminences), and the shape and size of these protrusions depend on both mechanical forces and cellular cues during growth. Tendon eminences also contribute to mechanical leverage for skeletal muscle. Fibroblast growth factor receptor (FGFR) signaling plays a critical role in bone development, and Fgfr1 and Fgfr2 are highly expressed in the perichondrium and periosteum of bone where entheses can be found.

RESULTS AND CONCLUSIONS

We used transgenic mice for combinatorial knockout of Fgfr1 and/or Fgfr2 in tendon/attachment progenitors (ScxCre) and measured eminence size and shape. Conditional deletion of both, but not individual, Fgfr1 and Fgfr2 in Scx progenitors led to enlarged eminences in the postnatal skeleton and shortening of long bones. In addition, Fgfr1/Fgfr2 double conditional knockout mice had more variation collagen fibril size in tendon, decreased tibial slope, and increased cell death at ligament attachments. These findings identify a role for FGFR signaling in regulating growth and maintenance of tendon/ligament attachments and the size and shape of bony eminences.

Biological approaches to the repair and regeneration of the rotator cuff tendon-bone enthesis: a literature review

Entheses are highly specialised organs connecting ligaments and tendons to bones, facilitating force transmission, and providing mechanical strengths to absorb forces encountered. Two types of entheses, fibrocartilaginous and fibrous, exist in interfaces. The gradual fibrocartilaginous type is in rotator cuff tendons and is more frequently injured due to the poor healing capacity that leads to loss of the original structural and biomechanical properties and is attributed to the high prevalence of retears. Fluctuating methodologies and outcomes of biological approaches are challenges to overcome for them to be routinely used in clinics. Therefore, stratifying the existing literature according to different categories (chronicity, extent of tear, and studied population) would effectively guide repair approaches. This literature review supports tissue engineering approaches to promote rotator cuff enthesis healing employing cells, growth factors, and scaffolds period. Outcomes suggest its promising role in animal studies as well as some clinical trials and that combination therapies are more beneficial than individualized ones. It then highlights the importance of tailoring interventions according to the tear extent, chronicity, and the population being treated. Contributing factors such as loading, deficiencies, and lifestyle habits should also be taken into consideration. Optimum results can be achieved if biological, mechanical, and environmental factors are approached. It is challenging to determine whether variations are due to the interventions themselves, the animal models, loading regimen, materials, or tear mechanisms. Future research should focus on tailoring interventions for different categories to formulate protocols, which would best guide regenerative medicine decision making.

Three-Dimensional Bioprinting of a Structure-, Composition-, and Mechanics-Graded Biomimetic Scaffold Coated with Specific Decellularized Extracellular Matrix to Improve the Tendon-to-Bone Healing

Healing of a damaged tendon-to-bone enthesis occurs through the formation of fibrovascular scar tissue with greatly compromised histological and biomechanical properties instead of the regeneration of a new enthesis due to the lack of graded tissue-engineering zones in the interface during the healing process. In the present study, a structure-, composition-, and mechanics-graded biomimetic scaffold (GBS) coated with specific decellularized extracellular matrix (dECM) (GBS-E) aimed to enhance its cellular differentiation inducibilities was fabricated using a three-dimensional (3-D) bioprinting technique. In vitro cellular differentiation studies showed that from the tendon-engineering zone to the bone-engineering zone in the GBS, the tenogenic differentiation inducibility decreased in correspondence with an increase in the osteogenic differentiation inducibility. The chondrogenic differentiation inducibility peaked in the middle, which was in consistent with the graded cellular phenotypes observed in a native tendon-to-bone enthesis, while specific dECM coating from the tendon-engineering zone to the bone-engineering zone (tendon-, cartilage-, and bone-derived dECM, respectively) further enhanced its cellular differentiation inducibilities (GBS-E). In a rabbit rotator cuff tear model, histological analysis showed that the GBS-E group exhibited well-graded tendon-to-bone differentiated properties in the repaired interface that was similar to a native tendon-to-bone enthesis at 16 weeks. Moreover, the biomechanical properties in the GBS-E group were also significantly higher than those in other groups at 16 weeks. Therefore, our findings suggested a promising tissue-engineering strategy for the regeneration of a complex enthesis using a three-dimensional bioprinting technique.

Anatomical Tissue Engineering of the Anterior Cruciate Ligament Entheses

The firm integration of anterior cruciate ligament (ACL) grafts into bones remains the most demanding challenge in ACL reconstruction, since graft loosening means graft failure. For a functional-tissue-engineered ACL substitute to be realized in future, robust bone attachment sites (entheses) have to be re-established. The latter comprise four tissue compartments (ligament, non-calcified and calcified fibrocartilage, separated by the tidemark, bone) forming a histological and biomechanical gradient at the attachment interface between the ACL and bone. The ACL enthesis is surrounded by the synovium and exposed to the intra-articular micromilieu. This review will picture and explain the peculiarities of these synovioentheseal complexes at the femoral and tibial attachment sites based on published data. Using this, emerging tissue engineering (TE) strategies addressing them will be discussed. Several material composites (e.g., polycaprolactone and silk fibroin) and manufacturing techniques (e.g., three-dimensional-/bio-printing, electrospinning, braiding and embroidering) have been applied to create zonal cell carriers (bi- or triphasic scaffolds) mimicking the ACL enthesis tissue gradients with appropriate topological parameters for zones. Functionalized or bioactive materials (e.g., collagen, tricalcium phosphate, hydroxyapatite and bioactive glass (BG)) or growth factors (e.g., bone morphogenetic proteins [BMP]-2) have been integrated to achieve the zone-dependent differentiation of precursor cells. However, the ACL entheses comprise individual (loading history) asymmetric and polar histoarchitectures. They result from the unique biomechanical microenvironment of overlapping tensile, compressive and shear forces involved in enthesis formation, maturation and maintenance. This review should provide a road map of key parameters to be considered in future in ACL interface TE approaches.

Menstrual Blood-Derived Mesenchymal Stem Cells Encapsulated in Autologous Platelet-Rich Gel Facilitate Rotator Cuff Healing in a Rabbit Model of Chronic Tears

BACKGROUND

Successful management of chronic rotator cuff (RC) tears remains a challenge owing to its limited intrinsic healing capacity and unsatisfactory failure rate. Menstrual blood-derived mesenchymal stem cells (MenSCs) have the potential to differentiate into the chondrogenic or osteogenic lineage. Autologous platelet-rich gel (APG), a gel material derived from platelet-rich plasma (PRP), can be applied as a carrier system for cell delivery and also as a releasing system for endogenous growth factors.

PURPOSE

To investigate the effect of human MenSCs encapsulated in APG (MenSCs@APG) on the healing of chronic RC tears in a rabbit model.

STUDY DESIGN

Controlled laboratory study.

METHODS

After evaluation of the effect of PRP on MenSC proliferation or differentiation, the stem cells were encapsulated in APG for in vivo injection. Supraspinatus tenotomy from the right greater tuberosity was performed on 45 New Zealand White rabbits. After 6 weeks, these rabbits were randomly allocated to 3 supplemental treatments during supraspinatus repair: saline injection (control [CTL] group), APG injection (APG group), and MenSCs@APG injection (MenSCs@APG group). At week 18, these rabbits were sacrificed to harvest the humerus-supraspinatus tendon complexes for micro-computed tomography (CT), histological evaluation, tensile test, and MenSC tracking.

RESULTS

In vitro results showed that APG can stimulate MenSC proliferation and enhance chondrogenic or osteogenic differentiation. In vivo results showed that APG can act as a carrier for delivering MenSCs into the healing site, and also as a stimulator for enhancing the in vivo performance of MenSCs. Micro-CT showed that bone volume/total volume and trabecular thickness of the new bone in the MenSCs@APG group presented significantly larger values than those of the APG or CTL group (P < .05 for all). Histologically, compared with the CTL or APG group, significantly more mature fibrocartilage regenerated at the healing site in the MenSCs@APG group. A large number of human nuclei-stained cells were observed in the MenSCs@APG group, presenting a similar appearance to fibrochondrocytes or osteocytes. Biomechanically, the MenSCs@APG group showed significantly higher failure load and stiffness than the APG or CTL group (P < .05 for all).

CONCLUSION

Human MenSCs@APG facilitated RC healing in a rabbit model of chronic tears.

CLINICAL RELEVANCE

Autogenous MenSCs@APG may be a new stem cell-based therapy for augmenting RC healing in the clinic.

MiRNAs as Potential Regulators of Enthesis Healing: Findings in a Rodent Injury Model

MicroRNAs (miRNAs) are short non-coding RNA sequences with the ability to inhibit the expression of a target mRNA at the post-transcriptional level, acting as modulators of both the degenerative and regenerative processes. Therefore, these molecules constitute a potential source of novel therapeutic tools. In this study, we investigated the miRNA expression profile that presented in enthesis tissue upon injury. For this, a rodent enthesis injury model was developed by creating a defect at a rat's patellar enthesis. Following injury, explants were collected on days 1 (n = 10) and 10 (n = 10). Contra lateral samples (n = 10) were harvested to be used for normalization. The expression of miRNAs was investigated using a "Fibrosis" pathway-focused miScript qPCR array. Later, target prediction for the aberrantly expressed miRNAs was performed by means of the Ingenuity Pathway Analysis, and the expression of mRNA targets relevant for enthesis healing was confirmed using qPCRs. Additionally, the protein expression levels of collagens I, II, III, and X were investigated using Western blotting. The mRNA expression pattern of EGR1, COL2A1, RUNX2, SMAD1, and SMAD3 in the injured samples indicated their possible regulation by their respective targeting miRNA, which included miR-16, -17, -100, -124, -133a, -155 and -182. Furthermore, the protein levels of collagens I and II were reduced directly after the injury (i.e., day 1) and increased 10 days post-injury, while collagens III and X showed the opposite pattern of expression.

Optogenetic-Induced Muscle Loading Leads to Mechanical Adaptation of the Achilles Tendon Enthesis in Mice

The growth of the skeleton depends on the transmission of contractile muscle forces from tendon to bone across the extracellular matrix-rich enthesis. Loss of muscle loading leads to significant impairments in enthesis development. However, little is known about how the enthesis responds to increased loading during postnatal growth. To study the cellular and matrix adaptations of the enthesis in response to increased muscle loading, we used optogenetics to induce skeletal muscle contraction and unilaterally load the Achilles tendon and enthesis in young (i.e., during growth) and adult (i.e., mature) mice. In young mice, daily bouts of unilateral optogenetic loading led to expansion of the calcaneal apophysis and growth plate, as well as increased vascularization of the normally avascular enthesis. Daily loading bouts, delivered for 3 weeks, also led to a mechanically weaker enthesis with increased molecular-level accumulation of collagen damage in young mice. However, adult mice did not exhibit impaired mechanical properties or noticeable structural adaptations to the enthesis. We then focused on the transcriptional response of the young tendon and bone following optogenetic-induced loading. After 1 or 2 weeks of loading, we identified, in tendon, transcriptional activation of canonical pathways related to glucose metabolism (glycolysis) and inhibited pathways associated with cytoskeletal remodeling (e.g., RHOA and CREB signaling). In bone, we identified activation of inflammatory signaling (e.g., NFkB and STAT3 signaling) and inhibition of ERK/MAPK and PTEN signaling. Thus, we have demonstrated the utility of optogenetic-induced skeletal muscle contraction to elicit structural, functional, and molecular adaptation of the enthesis in vivo especially during growth.

Hic1 identifies a specialized mesenchymal progenitor population in the embryonic limb responsible for bone superstructure formation

The musculoskeletal system relies on the integration of multiple components with diverse physical properties, such as striated muscle, tendon, and bone, that enable locomotion and structural stability. This relies on the emergence of specialized, but poorly characterized, interfaces between these various elements during embryonic development. Within the appendicular skeleton, we show that a subset of mesenchymal progenitors (MPs), identified by Hic1, do not contribute to the primary cartilaginous anlagen but represent the MP population, whose progeny directly contribute to the interfaces that connect bone to tendon (entheses), tendon to muscle (myotendinous junctions), and the associated superstructures. Furthermore, deletion of Hic1 leads to skeletal defects reflective of deficient muscle-bone coupling and, consequently, perturbation of ambulation. Collectively, these findings show that Hic1 identifies a unique MP population that contributes to a secondary wave of bone sculpting critical to skeletal morphogenesis.

Ultrasonographic Evaluation of Entheseal Fibrocartilage in Patients with Psoriatic Arthritis, Athletes and Healthy Controls: A Comparison Study

The aims of this study were as follows: (1) To evaluate the entheseal fibrocartilage (EF) during Achilles tendon insertion in patients with Psoriatic Arthritis (PsA) by using power Doppler ultrasound (PDUS), (2) to assess the intra and inter-reader reliability of the evaluation of EF thickness, (3) to compare the EF thickness of PsA patients, athletes and healthy controls (HCs), and (4) to evaluate the correlations between EF abnormalities, disease activity and functional indices in PsA.

METHODS

Consecutive PsA patients attending our unit were asked to participate. HCs and agonist athletes were enrolled as a control group. A bilateral PDUS evaluation of Achilles tendons was performed in order to evaluate the EF in all patients and controls.

RESULTS

In total, 30 PsA patients, 40 athletes and 20 HCs were enrolled. The median (IQR) EF thickness among the PsA patients, athletes and HCs was 0.035 cm (0.028-0.04) cm, 0.036 (0.025-0.043) cm and 0.030 (0.020-0.038) cm, respectively (p = 0.05 between PsA patients and HCs). The intra-reader reliability was excellent [ICC (95% CI) of 0.91 (0.88-0.95)] and the inter-reader reliability was good (0.80 (0.71-0.86). The assessment of EF was feasible, with a mean time of 2 min. No correlations were found with disease activity indices in PsA patients.

CONCLUSION

The assessment of EF is a feasible and reproducible test and may be explored as a potential imaging biomarker.

Targeted deletion of Fgf9 in tendon disrupts mineralization of the developing enthesis

The enthesis is a transitional tissue between tendon and bone that matures postnatally. The development and maturation of the enthesis involve cellular processes likened to an arrested growth plate. In this study, we explored the role of fibroblast growth factor 9 (Fgf9), a known regulator of chondrogenesis and vascularization during bone development, on the structure and function of the postnatal enthesis. First, we confirmed spatial expression of Fgf9 in the tendon and enthesis using in situ hybridization. We then used Cre-lox recombinase to conditionally knockout Fgf9 in mouse tendon and enthesis (Scx-Cre) and characterized enthesis morphology as well as mechanical properties in Fgf9^ScxCre and wild-type (WT) entheses. Fgf9^ScxCre mice had smaller calcaneal and humeral apophyses, thinner cortical bone at the attachment, increased cellularity, and reduced failure load in mature entheses compared to WT littermates. During postnatal development, we found reduced chondrocyte hypertrophy and disrupted type X collagen (Col X) in Fgf9^ScxCre entheses. These findings support that tendon-derived Fgf9 is important for functional development of the enthesis, including its postnatal mineralization. Our findings suggest the potential role of FGF signaling during enthesis development.

Polysaccharide-Based Composite Scaffolds for Osteochondral and Enthesis Regeneration

The rotator cuff and Achilles tendons along with the anterior cruciate ligament (ACL) are frequently injured with limited healing capacity. At the soft-hard tissue interface, enthesis is prone to get damaged and its regeneration in osteochondral defects is essential for complete healing. The current clinical techniques used in suturing procedures to reattach tendons to bones need much improvement for the generation of the native interface tissue, that is, enthesis, for patients to regain their full functions. Recently, inspired by the composite native tissue, much effort has been made to fabricate composite scaffolds for enthesis tissue regeneration. This review first focuses on the studies that used composite scaffolds for the regeneration of enthesis. Then, the use of polysaccharides for osteochondral tissue engineering is reviewed and their potential for enthesis regeneration is presented based on their supporting effects on osteogenesis and chondrogenesis. Gellan gum (GG) is selected and reviewed as a promising polysaccharide due to its unique osteogenic and chondrogenic activities that help avoid the inherent weakness of dissimilar materials in composite scaffolds. In addition, original preliminary results showed that GG supports collagen type I production and upregulation of osteogenic marker genes. Impact Statement Enthesis regeneration is essential for complete and functional healing of tendon and ligament tissues. Current suturing techniques to reattach the tendon/ligament to bones have high failure rates. This review highlights the studies on biomimetic scaffolds aimed to regenerate enthesis. In addition, the potential of using polysaccharides to regenerate enthesis is discussed based on their ability to regenerate osteochondral tissues. Gellan gum is presented as a promising biopolymer that can be modified to simultaneously support bone and cartilage regeneration by providing structural continuity for the scaffold.

Evaluation of Standard and Proposed Reference Values for Entheseal Thickening by Using Musculoskeletal Ultrasound

OBJECTIVE

Ultrasound (US) is increasingly used to evaluate enthesitis. One of the US features of enthesitis is thickening. However, there is no consensus on how the entheseal thickening needs to be defined, and existing cut-off levels have been criticized for being frequently positive in healthy controls (HCs). Our objective was to determine the frequency of thickening of entheses on US using the existing cut-off values in HCs and in patients with axial spondyloarthritis (axSpA) and propose new values to improve discriminative value.

METHODS

Eighty HCs and 100 patients with axSpA had US scans of 2160 entheses. Sensitivity, specificity, odds ratio (OR), and accuracy were calculated according to accepted cut-off levels from the literature and proposed cut-offs were calculated as the mean ± 2 SD.

RESULTS

Thickening according to current cut-off levels was found in 20.4% (196/960) of healthy participants' entheses and 33% (396/1200) of entheses of patients with axSpA. Thickening according to proposed cut-off levels decreased frequency of thickening in both groups, and therefore increased specificity at the cost of decreasing sensitivity. The only anatomical site where the thickness had a value to discriminate disease from health was seen at the triceps tendon enthesis with an OR of 13.4 (95% CI 4.0-44.8) according to the current cut-offs compared to 10.3 (95% CI 4.0-26.6) with the proposed cut-off levels.

CONCLUSION

Although using cut-offs appears to be an appealing method to evaluate entheseal thickness, the measurements may be affected by several confounding factors, leading to a low discriminative value, except for at the triceps tendon enthesis.

Scaffold-based tissue engineering strategies for soft-hard interface regeneration

Repairing injured tendon or ligament attachments to bones (enthesis) remains costly and challenging. Despite superb surgical management, the disorganized enthesis newly formed after surgery accounts for high recurrence rates after operations. Tissue engineering offers efficient alternatives to promote healing and regeneration of the specialized enthesis tissue. Load-transmitting functions thus can be restored with appropriate biomaterials and engineering strategies. Interestingly, recent studies have focused more on microstructure especially the arrangement of fibers since Rossetti successfully demonstrated the variability of fiber underspecific external force. In this review, we provide an important update on the current strategies for scaffold-based tissue engineering of enthesis when natural structure and properties are equally emphasized. We firstly described compositions, structures and features of natural enthesis with their special mechanical properties highlighted. Stimuli for growth, development and healing of enthesis widely used in popular strategies are systematically summarized. We discuss the fabrication of engineering scaffolds from the aspects of biomaterials, techniques and design strategies and comprehensively evaluate the advantages and disadvantages of each strategy. At last, this review pinpoints the remaining challenges and research directions to make breakthroughs in further studies.

Tendon progenitor cells as biological augmentation improve functional gait and reduce scar formation after rotator cuff repair

BACKGROUND

High rates of structural failure are reported after rotator cuff repairs due to inability to recreate the native enthesis during healing. The development of biological augmentation methods that mitigate scar formation and regenerate the enthesis is still an unmet need. Since neonatal enthesis is capable of regeneration after injury, this study tested whether delivery of neonatal tendon progenitor cells (TPCs) into the adult injured environment can enhance functional and structural supraspinatus enthesis and tendon healing.

METHODS

TPCs were isolated from Ai14 Rosa26-TdTomato mouse Achilles tendons and labeled using adenovirus-Cre. Fifty-two CB57BL/6J mice underwent detachment and acute repair of the supraspinatus tendon and received either a fibrin-only or TPC-fibrin gel. Immunofluorescence analysis was carried out to determine cellularity (DAPI), fibrocartilage (SOX9), macrophages (F4/80), myofibroblasts (α-smooth muscle actin), and scar (laminin). Assays for function (gait and biomechanical testing) and structure (micro-computed tomography imaging, picrosirius red/Alcian Blue staining, type I and III collagen staining) were carried out.

RESULTS

Analysis of TdTomato cells after injury showed minimal retention of TPCs by day 7 and day 14, with detected cells localized near the bursa and deltoid rather than the enthesis/tendon. However, TPC delivery led to significantly increased %Sox9+ cells in the enthesis at day 7 after injury and decreased laminin intensity across almost all time points compared to fibrin-only treatment. Similarly, TPC-treated mice showed gait recovery by day 14 (paw area and stride length) and day 28 (stance time), while fibrin-treated mice failed to recover gait parameters. Despite improved gait, biomechanical testing showed no differences between groups. Structural analysis by micro-computed tomography suggests that TPC application improves cortical thickness after surgery compared to fibrin. Superior collagen alignment at the neo-enthesis was also observed in the TPC-augmented group at day 28, but no difference was detected in type I and III collagen intensity.

CONCLUSION

We found that neonatal TPCs improved and restored functional gait by reducing overall scar formation, improving enthesis collagen alignment, and altering bony composition response after supraspinatus tendon repair. TPCs did not appear to integrate into the healing tissue, suggesting improved healing may be due to paracrine effects at early stages. Future work will determine the factors secreted by TPCs to develop translational targets.

ACL Graft Matching: Cadaveric Comparison of Microscopic Anatomy of Quadriceps and Patellar Tendon Grafts and the Femoral ACL Insertion Site

BACKGROUND

The optimal graft choice between the bone-patellar tendon-bone (BPTB) and the quadriceps tendon remains controversial. Studies evaluating the microscopic anatomy of the quadriceps tendon-patellar bone (QTB) and BPTB grafts for anterior cruciate ligament (ACL) reconstruction are currently lacking.

HYPOTHESIS

The relationship between post-ACL reconstruction graft bending angle (GBA) and the angle corresponding to the GBA (cGBA) would indicate that the BPTB can bend more than the QTB at the femoral tunnel aperture.

STUDY DESIGN

Controlled laboratory study.

METHODS

Twenty paired human cadaveric knees fixed at <10° of knee joint flexion (mean age, 82.5 years) underwent histological sectioning and staining with Masson trichrome and toluidine blue. The femoral ACL insertion, QTB graft, and BPTB graft were microscopically analyzed. The width of the direct insertion, thickness of the uncalcified fibrocartilage and calcified fibrocartilage, ligament attachment angle, and cGBA for each group were measured. Eighteen patients who underwent ACL reconstruction with QTB or BPTB autograft were included for the evaluation of GBA using computed tomography images at 1 week postoperatively.

RESULTS

The mean insertion widths of the femoral ACL, QTB, and BPTB were 7.81, 9.07, and 6.54 mm, respectively. The QTB was 16% wider than the ACL, while the BPTB was 16% narrower than the ACL. The mean insertion thicknesses of the femoral ACL, QTB, and BPTB were 0.53, 0.94, and 0.38 mm, respectively. The QTB was 77% thicker than the ACL (P < .001), while the BPTB was 28% thinner than the ACL (P = .017). The mean ligament attachment angles of the femoral ACL, QTB, and BPTB were 20.3°, 30.2°, and 33.3°, respectively, and the QTB and the BPTB were 49% and 64% larger, respectively, than the ACL. The mean cGBAs of the femoral ACL, QTB, and BPTB were 33.9°, 35.1°, and 12.3°, respectively. The BPTB was 64% smaller than the ACL, while there was no significant difference between the QTB and the ACL. The mean GBA was 57.7°.

CONCLUSION

The insertion width and thickness were significantly greater and smaller in the QTB and BPTB grafts, respectively, than in the ACL. The relationship between GBA after ACL reconstruction and cGBA in knee extension indicates that at the femoral tunnel aperture, the BPTB can bend more than the QTB.

CLINICAL RELEVANCE

QTB graft may allow more anatomic ACL reconstruction to be performed.

Structural and pathological changes in the enthesis are influenced by the muscle contraction type during exercise

Mechanical stress is involved in the onset of sports-related enthesopathy. Although the amount of exercise undertaken is a recognized problem during disease onset, changes in muscle contraction type are also involved in the increase in mechanical stress during exercise. This study aimed to clarify the effects of increased mechanical stress associated with muscle contraction type and amount of exercise on enthesis. Twenty mice underwent treadmill exercise, and the muscle contraction type and overall load during exercise were adjusted by varying the angle and speed conditions. Histological analysis was used to the cross-sectional area of the muscle; area of the enthesis fibrocartilage (FC), and expression of inflammation-, degeneration-, and calcification-related factors in the FC area. In addition, the volume and structure of the bone and FC area were examined using microcomputer imaging. Molecular biological analysis was conducted to compare relative expression levels of inflammation and cytokine-related factors in tendons. The Overuse group, which increased the amount of exercise, showed no significant differences in parameters compared to the sedentary mice (Control group). The mice subjected to slow-speed downhill running (Misuse group) showed pathological changes compared to the Control and Overuse groups, despite the small amount of exercise. Thus, the enthesis FC area may be altered by local mechanical stress that would be increased by eccentric muscle contraction rather than by mechanical stress that increases with the overall amount of exercise. Clinical Significance: The muscle contraction type might be more involved in the onset of sports-related enthesopathy rather than the amount of exercise.

The effect of mechanical stress on enthesis homeostasis in a rat Achilles enthesis organ culture model

Tendons and ligaments are jointed to bones via an enthesis that is essential to the proper function of the muscular and skeletal structures. The aim of the study is to investigate the effect of mechanical stress on the enthesis. We used ex vivo models in organ cultures of rat Achilles tendons with calcaneus including the enthesis. The organ was attached to a mechanical stretching apparatus that can conduct cyclic tensile strain. We made the models of 1-mm elongation (0.5 Hz, 3% elongation), 2-mm elongation (0.5 Hz, 5% elongation), and no stress. Histological evaluation by Safranin O staining and Toluidin Blue and Picro Sirius red staining was conducted. Expression of sex-determining region Y-box 9 (Sox9), scleraxis (Scx), Runt-related transcription factor 2 (Runx2), and matrix metalloproteinase 13 (Mmp13) were examined by real-time polymerase chain reaction and immunocytochemistry. Terminal deoxynucleotidyl transferase-mediated deoxyuridine triphosphate biotin nick end-labeling and live/dead staining and was conducted for evaluation of the apoptosis and cell viability. The structure of the enthesis was most maintained in the model of 1-mm elongation. The electronic microscope showed that the enthesis of the no stress model had ill-defined borders between fibrocartilage and mineralized fibrocartilage, and that calcification of mineralized fibrocartilage occurred in the model of 2-mm elongation. Sox9 and Scx was upregulated by 1-mm elongation, whereas Runx2 and Mmp13 were upregulated by 2-mm elongation. Apoptosis was inhibited by low stress. The results of this study suggested that 1-mm elongation can maintain the structure of the enthesis, while 2-mm elongation promotes degenerative changes.

Age-related cellular and microstructural changes in the rotator cuff enthesis

Rotator cuff injuries increase with age. The enthesis is the most frequent site of rotator cuff injury and degeneration. Understanding age-related changes of the enthesis are essential to determine the mechanism of rotator cuff injuries, degeneration, and to guide mechanistically driven therapies. In this study, we explored age-related cellular changes of the rotator cuff enthesis in young, mature, and aged rats. Here we found that the aged enthesis is typified by an increased mineralized zone and decreased nonmineralized zone. Proliferation, migration, and colony-forming potential of rotator cuff derived cells (RCECs) was attenuated with aging. The tenogenic and chondrogenic potential were significantly reduced, while the osteogenic potential increased in aged RCECs. The adipogenic potential increased in RCECs with age. This study explores the cellular differences found between young, mature, and aged rotator cuff enthesis cells and highlights the importance of using age-appropriate models, as well as provides a basis for further delineation of mechanisms and potential therapeutics for rotator cuff injuries.

Mesenchymal Stem Cell Sheets for Engineering of the Tendon-Bone Interface

Failure to regenerate the gradient tendon-bone interface of the enthesis results in poor clinical outcomes for surgical repair. The goal of this study was to evaluate the potential of composite cell sheets for engineering of the tendon-bone interface to improve regeneration of the functionally graded tissue. We hypothesize that stacking cell sheets at early stages of differentiation into tenogenic and osteogenic progenitors will create a composite structure with integrated layers. Cell sheets were fabricated on methyl cellulose and poly(N-isopropylacrylamide) thermally reversible polymers with human adipose-derived stem cells and differentiated into progenitors of tendon and bone with chemical induction media. Tenogenic and osteogenic cell sheets were stacked, and the engineered tendon-bone interface (TM-OM) was characterized in vitro in comparison to stacked cell sheet controls cultured in basal growth medium (GM-GM), osteogenic medium (OM-OM), and tenogenic medium (TM-TM). Samples were characterized by histology, quantitative real-time polymerase chain reaction, and immunofluorescent staining for markers of tendon, fibrocartilage, and bone including mineralization, scleraxis, tenomodulin, COL2, COLX, RUNX2, osteonectin, and osterix. After 1 week co-culture in basal growth medium, TM-OM cell sheets formed a tissue construct with integrated layers expressing markers of tendon, mineralized fibrocartilage, and bone with a spatial gradient in RUNX2 expression. Tenogenic cell sheets had increased expression of scleraxis and tenomodulin. Osteogenic cell sheets exhibited mineralization 1 week after stacking and upregulation of osterix and osteonectin. Additionally, in the engineered interface, there was significantly increased gene expression of IHH and COLX, indicative of endochondral ossification. These results highlight the potential for composite cell sheets fabricated with adipose-derived stem cells for engineering of the tendon-bone interface. Impact statement This study presents a method for fabrication of the tendon-bone interface using stacked cell sheets of tenogenic and osteogenic progenitors differentiated from human adipose-derived mesenchymal stem cells, resulting in a composite structure expressing markers of tendon, mineralized fibrocartilage, and bone. This work is an important step toward regeneration of the biological gradient of the enthesis and demonstrates the potential for engineering complex tissue interfaces from a single autologous cell source to facilitate clinical translation.

Pegylated insulin-like growth factor-1 biotherapeutic delivery promotes rotator cuff regeneration in a rat model

Tears in the rotator cuff are challenging to repair because of the complex, hypocellular, hypovascular, and movement-active nature of the tendon and its enthesis. Insulin-like Growth Factor-1 (IGF-1) is a promising therapeutic for this repair. However, its unstable nature, short half-life, and ability to disrupt homeostasis has limited its clinical translation. Pegylation has been shown to improve the stability and sustain IGF-1 levels in the systemic circulation without disrupting homeostasis. To provide localized delivery of IGF-1 in the repaired tendons, we encapsulated pegylated IGF-1 mimic and its controls (unpegylated IGF-1 mimic and recombinant human IGF-1) in polycaprolactone-based matrices and evaluated them in a pre-clinical rodent model of rotator cuff repair. Pegylated-IGF-1 mimic delivery reestablished the characteristic tendon-to-bone enthesis structure and improved tendon tensile properties within 8 weeks of repair compared to controls, signifying the importance of pegylation in this complex tissue regeneration. These results demonstrate a simple and scalable biologic delivery technology alternative to tissue-derived grafts for soft tissue repair.

Osteoarthritic meniscal entheses exhibit altered collagen fiber orientation

PURPOSE/AIM

The knee menisci are vital for maintaining the stability of the joint, allowing for force distribution, and protecting the underlying articular cartilage during loading. Each meniscus is attached to the underlying bone via two ligamentous entheses composed of collagen fibers that are continuous throughout all four zones of the attachment: ligament, uncalcified fibrocartilage, calcified fibrocartilage, and subchondral bone. The collagen fibers of the meniscal entheses are important for proper functionality of the entheses, particularly in preventing meniscal extrusion which is a common hallmark of osteoarthritis. The goal of this work was to assess changes in collagen fiber orientation present in osteoarthritic knee joints.

MATERIALS AND METHODS

Entheses were harvested from patients undergoing total knee arthroplasties and prepared histological sections were stained with picrosirius red to identify collagen fiber angle and fiber deviation.

RESULTS

In the calcified fibrocartilage the collagen fibers of the lateral anterior enthesis inserted at significantly (p < 0.1) shallower angles, and the fiber deviation was significantly (p < 0.1) less compared to the lateral posterior enthesis. These differences in the calcified fibrocartilage may occur as an adaptation to loading regimes of the osteoarthritic joint. When compared to the collagen fiber orientation of healthy entheses, collagen fibers in osteoarthritic tissue inserted at shallower insertion angles and demonstrated higher levels of deviation.

CONCLUSIONS

Changes to meniscal enthesis collagen fiber orientation with end stage osteoarthritis could offer an explanation for the change in functionality of diseased tissue and may contribute to meniscal extrusion and ultimately the degeneration of articular cartilage.

Effects of selective breeding for voluntary exercise, chronic exercise, and their interaction on muscle attachment site morphology in house mice

Skeletal muscles attach to bone at their origins and insertions, and the interface where tendon meets bone is termed the attachment site or enthesis. Mechanical stresses at the muscle/tendon-bone interface are proportional to the surface area of the bony attachment sites, such that a larger attachment site will distribute loads over a wider area. Muscles that are frequently active and/or are of larger size should cause attachment sites to hypertrophy (training effect); however, experimental studies of animals subjected to exercise have provided mixed results. To enhance our ability to detect training effects (a type of phenotypic plasticity), we studied a mouse model in which 4 replicate lines of High Runner (HR) mice have been selectively bred for 57 generations. Selection is based on the average number of wheel revolutions on days 5 & 6 of a 6-day period of wheel access as young adults (6-8 weeks old). Four additional lines are bred without regard to running and serve as non-selected controls (C). On average, mice from HR lines voluntarily run ~3 times more than C mice on a daily basis. For this study, we housed 50 females (half HR, half C) with wheels (Active group) and 50 (half HR, half C) without wheels (Sedentary group) for 12 weeks starting at weaning (~3 weeks old). We tested for evolved differences in muscle attachment site surface area between HR and C mice, plastic changes resulting from chronic exercise, and their interaction. We used a precise, highly repeatable method for quantifying the three-dimensional (3D) surface area of four muscle attachment sites: the humerus deltoid tuberosity (the insertion point for the spinodeltoideus, superficial pectoralis, and acromiodeltoideus), the femoral third trochanter (the insertion point for the quadratus femoris), the femoral lesser trochanter (the insertion point for the iliacus muscle), and the femoral greater trochanter (insertion point for the middle gluteal muscles). In univariate analyses, with body mass as a covariate, mice in the Active group had significantly larger humerus deltoid tuberosities than Sedentary mice, with no significant difference between HR and C mice and no interaction between exercise treatment and linetype. These differences between Active and Sedentary mice were also apparent in the multivariate analyses. Surface areas of the femoral third trochanter, femoral lesser trochanter, and femoral greater trochanter were unaffected by either chronic wheel access or selective breeding. Our results, which used robust measurement protocols and relatively large sample sizes, demonstrate that muscle attachment site morphology can be (but is not always) affected by chronic exercise experienced during ontogeny. However, contrary to previous results for other aspects of long bone morphology, we did not find evidence for evolutionary coadaptation of muscle attachments with voluntary exercise behavior in the HR mice.

Muscle attachment sites and behavioral reconstruction: An experimental test of muscle-bone structural response to habitual activity

OBJECTIVE

Behavioral reconstruction from muscle attachment sites (entheses) is a common practice in anthropology. However, experimental evidence provides mixed support for the assumed association between enthesis size and shape with changes in habitual activity. In this study, a laboratory mouse model was used to experimentally test whether activity level and type alters muscle architecture and the underlying bone cross-sectional geometry of entheses in order to assess the underlying assumption that behavioral changes lead to quantifiable differences in both muscle and enthesis morphology.

MATERIALS AND METHODS

Female wild-type mice were separated into one control group and two experimentally increased activity groups (running, climbing) over an 11-week study period. At the start of the experiment, half of the mice were 4 weeks and half were 7 weeks of age. The postmortem deltoideus and biceps brachii muscles were measured for potential force production (physiological cross-sectional area) and potential muscle excursion (fiber length). Bone cross-sectional geometry variables were measured from microCT scans of the humerus and radius at the enthesis and non-enthesis regions of interest across activity groups.

RESULTS

Activity level and type altered potential force production and potential muscle excursion of both muscles in the younger cohort. We observed differences in cortical bone geometry in both the humerus enthesis and radius non-enthesis region driven exclusively among the younger wheel-running mice.

DISCUSSION

These results indicate that in addition to muscle architectural changes, bone structural properties at the enthesis do show an adaptive response to increased activity, such as running but only during earlier development. However, further research is required in order to apply these findings to the reconstruction of living behavior from anthropological specimens.

Histomorphology of the Subregions of the Scapholunate Interosseous Ligament and Its Enthesis

Background  The scapholunate interosseous ligament (SLIL) has three subregions: dorsal, proximal, and volar. The SLIL enthesis has not previously been studied despite its important mechanical function in wrist joint biomechanics. Questions/Purposes  This study aims to compare the histomorphological differences between the SLIL subregions, including at their entheses. Three questions are explored: Do the gross dimensions differ between SLIL subregions? Does the enthesis qualitatively, and its calcified fibrocartilage (CF) quantitatively, differ between (a) SLIL subregions and (b) scaphoid and lunate attachments? Methods  Twelve fresh-frozen human cadaveric wrists were dissected and the gross dimensions of the SLIL subregions measured. Subregions were histologically processed for morphological and compositional analyses, including quantification of enthesis CF area. Results  The dorsal subregion was the thickest. The dorsal and volar subregions had fibrocartilaginous entheses, while the proximal subregion was attached to articular cartilage. The dorsal subregion had significantly more CF than the volar subregion. There was no significant difference in the enthesis CF between scaphoid and lunate attachments in the three subregions. Conclusions  There are significant morphological differences between the SLIL subregions. The dorsal subregion has the largest amount of CF, which is consistent with the greater biomechanical force subjected to this subregion. The similar histomorphology of the ligament at the scaphoid and lunate entheses suggests that similar biomechanical forces are applied to both attachments. Clinical Relevance  The histomorphological results confirm that the dorsal subregion is the strongest of the three subregions. The results from the entheseal region may have important implications in the study of graft incorporation during SLIL reconstruction.

The effects of maturation and aging on the rotator cuff tendon-to-bone interface

Rotator cuff tendon injuries often occur at the tendon-to-bone interface (i.e., enthesis) area, with a high prevalence for the elderly population, but the underlying reason for this phenomenon is still unknown. The objective of this study is to identify the histological, molecular, and biomechanical alterations of the rotator cuff enthesis with maturation and aging in a mouse model. Four different age groups of mice (newborn, young, adult, and old) were studied. Striking variations of the entheses were observed between the newborn and other matured groups, with collagen content, proteoglycan deposition, collagen fiber dispersion was significantly higher in the newborn group. The compositional and histological features of young, adult, and old groups did not show significant differences, except having increased proteoglycan deposition and thinner collagen fibers at the insertion sites in the old group. Nanoindentation testing showed that the old group had a smaller compressive modulus at the insertion site when compared with other groups. However, tensile mechanical testing reported that the old group demonstrated a significantly higher failure stress when compared with the young and adult groups. The proteomics analysis detected dramatic differences in protein content between newborn and young groups but minor changes among young, adult, and old groups. These results demonstrated: (1) the significant alterations of the enthesis composition and structure occur from the newborn to the young time period; (2) the increased risk of rotator cuff tendon injuries in the elderly population is not solely because of old age alone in the rodent model.

Factors Involved in Morphogenesis in the Muscle-Tendon-Bone Complex

A decline in the body's motor functions has been linked to decreased muscle mass and function in the oral cavity and throat; however, aging of the junctions of the muscles and bones has also been identified as an associated factor. Basic and clinical studies on the muscles, tendons and bones, each considered independently, have been published. In recent years, however, research has focused on muscle attachment as the muscle-tendon-bone complex from various perspectives, and there is a growing body of knowledge on SRY-box9 (Sox9) and Mohawk(Mkx), which has been identified as a common controlling factor and a key element. Myostatin, a factor that inhibits muscle growth, has been identified as a potential key element in the mechanisms of lifetime structural maintenance of the muscle-tendon-bone complex. Findings in recent studies have also uncovered aspects of the mechanisms of motor organ complex morphostasis in the superaged society of today and will lay the groundwork for treatments to prevent motor function decline in older adults.

Deletion of Fibroblast growth factor 9 globally and in skeletal muscle results in enlarged tuberosities at sites of deltoid tendon attachments

BACKGROUND

The growth of most bony tuberosities, like the deltoid tuberosity (DT), rely on the transmission of muscle forces at the tendon-bone attachment during skeletal growth. Tuberosities distribute muscle forces and provide mechanical leverage at attachment sites for joint stability and mobility. The genetic factors that regulate tuberosity growth remain largely unknown. In mouse embryos with global deletion of fibroblast growth factor 9 (Fgf9), the DT size is notably enlarged. In this study, we explored the tissue-specific regulation of DT size using both global and targeted deletion of Fgf9.

RESULTS

We showed that cell hypertrophy and mineralization dynamics of the DT, as well as transcriptional signatures from skeletal muscle but not bone, were influenced by the global loss of Fgf9. Loss of Fgf9 during embryonic growth led to increased chondrocyte hypertrophy and reduced cell proliferation at the DT attachment site. This endured hypertrophy and limited proliferation may explain the abnormal mineralization patterns and locally dysregulated expression of markers of endochondral development in Fgf9^null attachments. We then showed that targeted deletion of Fgf9 in skeletal muscle leads to postnatal enlargement of the DT.

CONCLUSION

Taken together, we discovered that Fgf9 may play an influential role in muscle-bone cross-talk during embryonic and postnatal development.

Surface metrology of bone surface attachments of knee ligaments

OBJECTIVES

Textural differences between entheses reflect biomechanical activities of the musculoskeletal system. Methods used to measure these surfaces have limitations. Here, the surface metrology of roughness of articular and entheseal surfaces of the knee are investigated with an optical profiler.

METHODS

Osteological specimens of six femora and seven tibiae were prepared from cadavers. Measurements were obtained to surrogate body mass. Specimens were molded with polyvinylsiloxane and casts prepared with resin, which were scanned using a white light optical profiler. Scans were processed by a computer program. Each scan produced 32 variables, categorized into 6 groups for each location.

RESULTS

The distribution of data was mostly normal. Analysis of variance (ANOVA) identified Ssk significant (p-value .002); post hoc Tukey testing indicated significance between femoral PCL and tibial ACL entheses groups (p-value .007), and between tibial ACL and tibial entheses groups (p-value .002) suggesting the ability to differentiate anterior and posterior cruciate ligament entheses. Sku was found significant with a t test between articular and entheseal surfaces. Correlation coefficients were significant between surface metrology parameters and measurements related to body mass.

CONCLUSIONS

This study distinguished differences between entheses of the anterior and posterior cruciate ligaments, with the Ssk parameter most useful. Differences in articular and entheseal surfaces were found with the Sku parameter most useful. Correlations indicated a relationship between body mass and surface metrology parameters. Finally, these findings suggest this method can be used for further investigation of spondyloarthropathies.

Enthesis lesions are associated with X-ray progression in psoriatic arthritis

OBJECTIVE

To analyze the relationship among enthesis ultrasound (US) lesions and radiological structural damage in psoriatic arthritis (PsA) patients.

METHODS

Consecutive PsA patients with swelling of at least 1 of the 2nd to 5th metacarpophalangeal joints were included. Clinical and demographic data were collected. The Madrid Sonographic Enthesitis Index (MASEI) was selected to evaluate the enthesis, with its total score and MASEI-activity and MASEI-structural damage subscores. The modified Sharp van der Heijde method for PsA and the New York criteria for sacroiliitis were selected to evaluate cumulative bone damage on X-rays.

RESULTS

Twenty-seven patients were included. Male gender, older age, longer PsA duration and acute reactant factors were associated with greater bone cumulative damage. Enthesis tendon thickening, enthesophytes, total MASEI and the MASEI-structural damage subscore showed significant correlations with radiographic peripheral and sacroiliac damage scores. Tendon thickening and enthesophytes were the enthesis lesions more frequently associated with radiographic damage in PsA.

CONCLUSION

The enthesis MASEI score was associated with axial and articular radiographic structural damage in PsA patients. The MASEI-structural damage subscore correlated better with cumulative bone damage in PsA than the MASEI-activity subscore.

SARS-CoV-2 Infection Induces Psoriatic Arthritis Flares and Enthesis Resident Plasmacytoid Dendritic Cell Type-1 Interferon Inhibition by JAK Antagonism Offer Novel Spondyloarthritis Pathogenesis Insights

Objective

Bacterial and viral infectious triggers are linked to spondyloarthritis (SpA) including psoriatic arthritis (PsA) development, likely via dendritic cell activation. We investigated spinal entheseal plasmacytoid dendritic cells (pDCs) toll-like receptor (TLR)-7 and 9 activation and therapeutic modulation, including JAK inhibition. We also investigated if COVID-19 infection, a potent TLR-7 stimulator triggered PsA flares.

Methods

Normal entheseal pDCs were characterized and stimulated with imiquimod and CpG oligodeoxynucleotides (ODN) to evaluate TNF and IFNα production. NanoString gene expression assay of total pDCs RNA was performed pre- and post- ODN stimulation. Pharmacological inhibition of induced IFNα protein was performed with Tofacitinib and PDE4 inhibition. The impact of SARS-CoV2 viral infection on PsA flares was evaluated.

Results

CD45+HLA-DR+CD123+CD303+CD11c- entheseal pDCs were more numerous than blood pDCs (1.9 ± 0.8% vs 0.2 ± 0.07% of CD45+ cells, p=0.008) and showed inducible IFNα and TNF protein following ODN/imiquimod stimulation and were the sole entheseal IFNα producers. NanoString data identified 11 significantly upregulated differentially expressed genes (DEGs) including TNF in stimulated pDCs. Canonical pathway analysis revealed activation of dendritic cell maturation, NF-κB signaling, toll-like receptor signaling and JAK/STAT signaling pathways following ODN stimulation. Both tofacitinib and PDE4i strongly attenuated ODN induced IFNα. DAPSA scores elevations occurred in 18 PsA cases with SARS-CoV2 infection (9.7 ± 4 pre-infection and 35.3 ± 7.5 during infection).

Conclusion

Entheseal pDCs link microbes to TNF/IFNα production. SARS-CoV-2 infection is associated with PsA Flares and JAK inhibition suppressed activated entheseal plasmacytoid dendritic Type-1 interferon responses as pointers towards a novel mechanism of PsA and SpA-related arthropathy.

Why Inhibition of IL-23 Lacked Efficacy in Ankylosing Spondylitis

The term spondyloarthritis pertains to both axial and peripheral arthritis including ankylosing spondylitis (AS) and psoriatic arthritis (PsA), which is strongly linked to psoriasis and also the arthritis associated with inflammatory bowel disease. The argument supporting the role for IL-23 across the spectrum of SpA comes from 4 sources. First, genome wide associated studies (GWAS) have shown that all the aforementioned disorders exhibit IL-23R pathway SNPs, whereas HLA-B27 is not linked to all of these diseases-hence the IL-23 pathway represents the common genetic denominator. Secondly, experimental animal models have demonstrated a pivotal role for the IL-23/IL-17 axis in SpA related arthropathy that initially manifests as enthesitis, but also synovitis and axial inflammation and also associated aortic root and cutaneous inflammation. Thirdly, the emergent immunology of the human enthesis also supports the presence of IL-23 producing myeloid cells, not just at the enthesis but in other SpA associated sites including skin and gut. Finally, drugs that target the IL-23 pathway show excellent efficacy for skin disease, efficacy for IBD and also in peripheral arthropathy associated with SpA. The apparent failure of IL-23 blockade in the AS which is effectively a spinal polyenthesitis but evidence for efficacy of IL-23 inhibition for peripheral enthesitis in PsA and preliminary suggestions for benefit in axial PsA, raises many questions. Key amongst these is whether spinal inflammation may exhibit entheseal IL-17A production independent of IL-23 but peripheral enthesitis is largely dependent on IL-23 driven IL-17 production. Furthermore, IL-23 blocking strategies in animal models may prevent experimental SpA evolution but not prevent established disease, perhaps pointing towards a role for IL-23 in innate immune disease initiation whereas persistent disease is dependent on memory T-cell responses that drive IL-17A production independently of IL-23, but this needs further study. Furthermore, IL-12/23 posology in inflammatory bowel disease is substantially higher than that used in AS trials which merits consideration. Therefore, the IL-23 pathway is centrally involved in the SpA concept but the nuances and intricacies in axial inflammation that suggest non-response to IL-23 antagonism await formal definition. The absence of comparative immunology between the different skeletal sites renders explanations purely hypothetical at this juncture.

Tissue Engineering for the Insertions of Tendons and Ligaments: An Overview of Electrospun Biomaterials and Structures

The musculoskeletal system is composed by hard and soft tissue. These tissues are characterized by a wide range of mechanical properties that cause a progressive transition from one to the other. These material gradients are mandatory to reduce stress concentrations at the junction site. Nature has answered to this topic developing optimized interfaces, which enable a physiological transmission of load in a wide area over the junction. The interfaces connecting tendons and ligaments to bones are called entheses, while the ones between tendons and muscles are named myotendinous junctions. Several injuries can affect muscles, bones, tendons, or ligaments, and they often occur at the junction sites. For this reason, the main aim of the innovative field of the interfacial tissue engineering is to produce scaffolds with biomaterial gradients and mechanical properties to guide the cell growth and differentiation. Among the several strategies explored to mimic these tissues, the electrospinning technique is one of the most promising, allowing to generate polymeric nanofibers similar to the musculoskeletal extracellular matrix. Thanks to its extreme versatility, electrospinning has allowed the production of sophisticated scaffolds suitable for the regeneration of both the entheses and the myotendinous junctions. The aim of this review is to analyze the most relevant studies that applied electrospinning to produce scaffolds for the regeneration of the enthesis and the myotendinous junction, giving a comprehensive overview on the progress made in the field, in particular focusing on the electrospinning strategies to produce these scaffolds and their mechanical, in vitro, and in vivo outcomes.

Effect of kartogenin-loaded gelatin methacryloyl hydrogel scaffold with bone marrow stimulation for enthesis healing in rotator cuff repair

BACKGROUND

Strategies involving microfracture, biomaterials, growth factors, and chemical agents have been evaluated for improving enthesis healing. Kartogenin (KGN) promotes selective differentiation of bone marrow mesenchymal stem cells (BMSCs) into chondrocytes. Gelatin methacryloyl (GelMA) is a promising biomaterial for engineering scaffolds and drug carriers. Herein, we investigated KGN-loaded GelMA hydrogel scaffolds with a bone marrow-stimulating technique for the repair of rotator cuff tear.

METHODS

KGN-loaded GelMA hydrogel scaffolds were obtained by ultraviolet GelMA crosslinking and vacuum freeze-drying. Fifty-four New Zealand rabbits were randomly divided into (1) repair only (control), (2) microfracture + repair (BMS), and (3) microfracture + repair augmentation with a KGN-loaded GelMA hydrogel scaffold (combined) groups. Tendons were repaired by transosseous sutures. The structure, degradation, and in vitro KGN release of the scaffolds were characterized. Animals were euthanized 4, 8, and 12 weeks after repair. Enthesis healing was evaluated by macroscopy, microcomputed tomography, histology, and biomechanical tests.

RESULTS

The KGN-loaded GelMA hydrogel scaffolds are porous with a 60.4 ± 28.2-μm average pore size, and they degrade quickly in 2.5 units/mL collagenase solution. Nearly 81% of KGN was released into phosphate-buffered saline within 12 hours, whereas the remaining KGN was released in 7 days. Macroscopically, the repaired tendons were attached to the footprint. No differences were detected postoperatively in microcomputed tomography analysis among groups. Fibrous scar tissue was the main component at the tendon-to-bone interface in the control group. Disorderly arranged cartilage formation was observed at the tendon-to-bone interface in the BMS and combined groups 4 weeks after repair; the combined group exhibited relatively more cartilage. The combined group showed improved cartilage regeneration 8 and 12 weeks after repair. Similar results were found in tendon maturation scores. The ultimate load to failure and stiffness of the repaired tendon increased in all 3 groups. At 4 weeks after repair, the BMS and combined groups exhibited greater ultimate load to failure than the control group, although there was no difference in stiffness among groups. The BMS and combined groups exhibited greater ultimate load to failure and stiffness than the control group, and the combined group exhibited better values than the BMS group at 8 and 12 weeks after repair.

CONCLUSION

Compared with the bone marrow-stimulating technique, the KGN-loaded GelMA hydrogel scaffold with bone marrow stimulation improved enthesis healing by promoting fibrocartilage formation and improving the mechanical properties.

IL-17A and TNF Modulate Normal Human Spinal Entheseal Bone and Soft Tissue Mesenchymal Stem Cell Osteogenesis, Adipogenesis, and Stromal Function

Objective: The spondylarthritides (SpA) are intimately linked to new bone formation and IL-17A and TNF pathways. We investigated spinal soft tissue and bone mesenchymal stem cell (MSC) responses to IL-17A and TNF, including their osteogenesis, adipogenesis, and stromal supportive function and ability to support lymphocyte recruitment. Methods: Normal spinal peri-entheseal bone (PEB) and entheseal soft tissue (EST) were characterized for MSCs by immunophenotypic, osteogenic, chondrogenic, and adipogenic differentiation criteria. Functional and gene transcriptomic analysis was carried out on undifferentiated, adipo- differentiated, and osteo-differentiated MSCs. The enthesis C-C Motif Chemokine Ligand 20-C-C Motif Chemokine Receptor 6 (CCL20-CCR6) axis was investigated at transcript and protein levels to ascertain whether entheseal MSCs influence local immune cell populations. Results: Cultured MSCs from both PEB and EST displayed a tri-lineage differentiation ability. EST MSCs exhibited 4.9-fold greater adipogenesis (p < 0.001) and a 3-fold lower osteogenic capacity (p < 0.05). IL-17A induced greater osteogenesis in PEB MSCs compared to EST MSCs. IL-17A suppressed adipogenic differentiation, with a significant decrease in fatty acid-binding protein 4 (FABP4), peroxisome proliferator-activated receptor gamma (PPARγ), Cell Death Inducing DFFA Like Effector C (CIDEC), and Perilipin-1 (PLIN1). IL-17A significantly increased the CCL20 transcript (p < 0.01) and protein expression (p < 0.001) in MSCs supporting a role in type 17 lymphocyte recruitment. Conclusions: Normal spinal enthesis harbors resident MSCs with different in vitro functionalities in bone and soft tissue, especially in response to IL-17A, which enhanced osteogenesis and CCL20 production and reduced adipogenesis compared to unstimulated MSCs. This MSC-stromal-enthesis immune system may be a hitherto unappreciated mechanism of “fine tuning” tissue repair responses at the enthesis in health and could be relevant for SpA understanding.

Bi-fated tendon-to-bone attachment cells are regulated by shared enhancers and KLF transcription factors

The mechanical challenge of attaching elastic tendons to stiff bones is solved by the formation of a unique transitional tissue. Here, we show that murine tendon-to-bone attachment cells are bi-fated, activating a mixture of chondrocyte and tenocyte transcriptomes, under regulation of shared regulatory elements and Krüppel-like factors (KLFs) transcription factors. High-throughput bulk and single-cell RNA sequencing of humeral attachment cells revealed expression of hundreds of chondrogenic and tenogenic genes, which was validated by in situ hybridization and single-molecule ISH. ATAC sequencing showed that attachment cells share accessible intergenic chromatin areas with either tenocytes or chondrocytes. Epigenomic analysis revealed enhancer signatures for most of these regions. Transgenic mouse enhancer reporter assays verified the shared activity of some of these enhancers. Finally, integrative chromatin and motif analyses and transcriptomic data implicated KLFs as regulators of attachment cells. Indeed, blocking expression of both Klf2 and Klf4 in developing limb mesenchyme impaired their differentiation.

Updating the use of the Madrid Sonographic Enthesis Index (MASEI): a systematic review of the literature

OBJECTIVE

To perform a systematic review of the literature to evaluate the use of the enthesis ultrasound Madrid Sonographic Entesis Index (MASEI) from its publication.

METHODS

A systematic search of MEDLINE, EMBASE, and Cochrane Central Register databases was performed. The search strategy was constructed to identify publications containing terms related to enthesis and ultrasound. The only applied filter was studies conducted in humans. One reviewer systematically screened the search. A second reviewer verified the selection. The data extraction was focused on study characteristics, including population and components of the OMERACT filter.

RESULTS

Sixty-eight of the 1581 identified studies had used MASEI, including 41 (60%) abstracts and 27 (40%) articles. Of the 27 articles, MASEI was mainly used for spondyloarthritis and related diseases in 12 (44%) articles, followed by both psoriatic arthritis and rheumatoid arthritis in five (19%) articles; however, it was also used in diseases such as Behçet disease, FM, familiar Mediterranean fever, SS, crystal arthropathies and systemic sclerosis. The feasibility of MASEI was reported in three (11%) articles, and the reliability in 12 (44%) with good to excellent values. No article evaluated the responsiveness to treatment. The construct validity of MASEI was assessed using biomarkers in seven (26%) articles, clinical examination in 13 (48%) and imaging procedures (only X-rays) in two (7%). The discriminative validity was assessed in 16 (59%) articles, not only in SpAs.

CONCLUSION

MASEI is a feasible, reliable and valid ultrasound score for the study of enthesis in spondyloarthritis, psoriatic arthritis and other diseases.

Effect of suprascapular nerve injury on rotator cuff enthesis

BACKGROUND

Numerous reports have shown that retracted rotator cuff tears may cause suprascapular nerve injury, and nerve injury causes atrophy and fat accumulation in the rotator cuff muscles. However, the effect of suprascapular nerve injury on rotator cuff enthesis has not been directly defined. This study aimed to investigate the effect of suprascapular nerve injury on rotator cuff enthesis.

METHODS

Twenty-four Wistar albino rats underwent bilateral transection of the suprascapular nerve. Additional 6 rats were used as the sham group. Bilateral supraspinatus and infraspinatus entheses were examined after 1, 4, 8, and 12 weeks of nerve transection. Histomorphometric analyses were performed for each zone of enthesis.

RESULTS

Compared with normal enthesis, significant and consistent decrease in cellularity were observed in the tendon and bone at all time points (P < .001). Collagen bundle diameter in the tendon also decreased in a similar manner (P < .001). Apart from the tendon and bone zones, fibrocartilage and calcified fibrocartilage zones showed similar response, and significant decrease in cellularity was observed 8 weeks after nerve transection (P < .001).

CONCLUSION

This study identifies suprascapular nerve injury as an underlying mechanism leading to compromise of the rotator cuff enthesis structure. Suprascapular nerve injury may be considered as an etiologic factor for the impaired healing after repair of a massive tear.

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.

Adhesive-based tendon-to-bone repair: failure modelling and materials selection

Surgical reattachment of tendon to bone is a procedure marked by high failure rates. For example, nearly all rotator cuff repairs performed on elderly patients with massive tears ultimately result in recurrence of tearing. These high failure rates have been attributed to stress concentrations that arise due to the mechanical mismatch between tendon and bone. Although recent studies have identified potential adhesives with mechanical properties tuned to alleviate these stress concentrations, and thereby delay the onset of failure, resistance to the progression of failure has not been studied. Here, we refined the space of adhesive material properties that can improve surgical attachment by considering the fracture process. Using cohesive zone modelling and physiologically relevant values of mode I and mode II adhesive fracture toughnesses, we predicted the maximum displacement and strength at failure of idealized, adhesively bonded tendon-to-bone repairs. Repair failure occurred due to excessive relative displacement of the tendon and bone tissues for strong and compliant adhesives. The failure mechanism shifted to rupture of the entire repair for stiffer adhesives below a critical shear strength. Results identified a narrow range of materials on an Ashby chart that are suitable for adhesive repair of tendon to bone, including a range of elastomers and porous solids.

Interleukin-23 pathway at the enthesis: The emerging story of enthesitis in spondyloarthropathy

The inflammatory disorders collectively termed the seronegative spondyloarthropathies (SpA) include ankylosing spondylitis (AS), psoriatic arthritis (PsA), reactive arthritis, the arthritis associated with inflammatory bowel disease including Crohn's disease and ulcerative colitis, the arthritis related to anterior uveitis, and finally, somewhat controversially Behcet's disease. All of these diseases are associated with SNPs in the IL-23R or the interleukin-23 (IL-23) cytokine itself and related downstream signaling JAK pathway genes and the interleukin-17 (IL-17) pathway. In rheumatoid arthritis, the target of the immune response is the synovium but the SpA disorders target the tendon, ligament, and joint capsule skeletal anchorage points that are termed entheses. The discovery that IL-23R-expressing cells were ensconced in healthy murine enthesis, and other extraskeletal anchorage points including the aortic root and the ciliary body of the eye and that systemic overexpression of IL-23 resulted in a severe experimental SpA, confirmed a fundamentally different immunobiology to rheumatoid arthritis. Recently, IL-23R-expressing myeloid cells and various innate and adaptive T cells that produce IL-17 family cytokines have also been described in the human enthesis. Blockade of IL-23 pathway with either anti-p40 or anti-p19 subunits has resulted in some spectacular therapeutic successes in psoriasis and PsA including improvement in enthesitis in the peripheral skeleton but has failed to demonstrate efficacy in AS that is largely a spinal polyenthesitis. Herein, we discuss the known biology of IL-23 at the human enthesis and highlight the remarkable emerging story of this unique skeletal tissue.