Flexor Tendon Injury and Repair. The Influence of Synovial Environment on the Early Healing Response in a Canine Model

Effect of Platelet-Rich Plasma at Different Initiation Times on Healing of the Bone-Tendon Interface of the Rotator Cuff in a Mouse Model

Background

Platelet-rich plasma (PRP) has demonstrated beneficial effects on healing of the bone-tendon interface (BTI).

Purpose

To determine the optimal initiation time for PRP application after rotator cuff repair in an animal model.

Study Design

Controlled laboratory study.

Methods

A total of 136 C57BL/6 mice were included; 40 mice were used to prepare PRP, while 96 mice underwent acute supraspinatus tendon (SST) repair. The animals were randomly divided into 4 groups: a control group and 3 groups in which PRP was injected into the injury interface immediately after surgery, on the 7th postoperative day (PRP-7d), and on the 14th postoperative day. At 4 and 8 weeks postoperatively, the animals were sacrificed, blood was collected by eyeball removal, and samples of the SST-humerus complex were collected. Histological, imaging, immunological, and biomechanical data were compared among the groups using 1-way analysis of variance with the Bonferroni post hoc test.

Results

Histological analysis revealed that the fibrocartilage layer at the BTI was larger in the PRP-7d group compared to the other groups at both 4 and 8 weeks postoperatively. Moreover, the PRP-7d group exhibited improved proteoglycan content and distribution compared to the other groups. Enzyme-linked immunosorbent assay results demonstrated that at 4 weeks postoperatively, higher concentrations of transforming growth factor-β1 and platelet-derived growth factor-BB (PDGF-BB) were seen in the PRP-7d group versus the PRP-14d and control gruops (P < .05), and at 8 weeks postoperatively, the concentration of PDGF-BB was higher in the PRP-7d group versus the control group (P < .05). Biomechanical testing at 4 weeks postoperatively revealed that the failure load and ultimate strength of the SST-humerus complex were superior in the PRP-7d group compared to the other groups (P < .05), at 8 weeks, PRP-7d group was superior to the control group (P < .05). Additionally, at 8 weeks postoperatively, the PRP-7d group exhibited a greater trabecular number and trabecular thickness at the BTI compared to the PRP-14d and control gruops (P < .05).

Conclusion

PRP promoted healing of the BTI after a rotator cuff injury at an early stage.

Clinical Relevance

A PRP injection on the 7th postoperative day demonstrated superior therapeutic effects compared with injections at other time points.

Restoring Function of the Flexor Tendons of the Hand: State of the Science in Kazakhstan

We conducted a literature review to highlight the diagnosis and treatment of hand flexor tendon injuries and discuss the current state of the science of hand surgery in the Republic of Kazakhstan. We reviewed the Google Scholar, PubMed, Web of Science, Elsevier, and National Center for Biotechnology Information databases and other printed sources for open access articles in three languages. We accepted relevant scientific articles that reflected the peculiarities of restoring function of the tendon flexors of the hand. We reviewed articles from the present to the previous 20 years and included 31 of these sources in our literature review. The medical examination of a patient who has sustained a flexor tendon injury should be thorough and gradual, supported by ultrasound and X-ray examination. For effective restoration of hand function following a hand injury, it is important for the surgeon to conduct a step-by-step assessment of the damage that has occurred to soft tissues, superficial and deep tendons, bones, nerves, and blood vessels. The main goal of tendon repair is to preserve tendon function. It is also known that early postoperative tendon movement leads to faster healing. The most frequent complications associated with flexor tendon repair include the development of adhesions, wound infection, tendon rupture, impaired hand function, and scar formation. Relative to the state of the science in the Republic of Kazakhstan, we conclude that hand surgery should be performed by highly skilled specialists in the field of microsurgery in a specialized department with microsurgical equipment.

The effects of NF-κB suppression on the early healing response following intrasynovial tendon repair in a canine model

The highly variable clinical outcomes noted after intrasynovial tendon repair have been associated with an early inflammatory response leading to the development of fibrovascular adhesions. Prior efforts to broadly suppress this inflammatory response have been largely unsuccessful. Recent studies have shown that selective inhibition of IkappaB kinase beta (IKK-β), an upstream activator of nuclear factor kappa-light chain enhancer of activated B cells (NF-κB) signaling, mitigates the early inflammatory response and leads to improved tendon healing outcomes. In the current study, we test the hypothesis that oral treatment with the IKK-β inhibitor ACHP (2-amino-6-[2-(cyclopropylmethoxy)-6-hydroxyphenyl]-4-piperidin-4-yl nicotinenitrile an inhibitor) will modulate the postoperative inflammatory response and improve intrasynovial flexor tendon healing. To test this hypothesis, the flexor digitorum profundus tendon of 21 canines was transected and repaired within the intrasynovial region and assessed after 3 and 14 days. Histomorphometry, gene expression analyses, immunohistochemistry, and quantitative polarized light imaging were used to examine ACHP-mediated changes. ACHP led to reduction in phosphorylated p-65, indicating that NF-κB activity was suppressed. ACHP enhanced expression of inflammation-related genes at 3 days and suppressed expression of these genes at 14 days. Histomorphometry revealed enhanced cellular proliferation and neovascularization in ACHP-treated tendons compared with time-matched controls. These findings demonstrate that ACHP effectively suppressed NF-κB signaling and modulated early inflammation, leading to increased cellular proliferation and neovascularization without stimulating the formation of fibrovascular adhesions. Together, these data suggest that ACHP treatment accelerated the inflammatory and proliferative phases of tendon healing following intrasynovial flexor tendon repair. Clinical Significance: Using a clinically relevant large-animal model, this study revealed that targeted inhibition of nuclear factor kappa-light chain enhancer of activated B cells signaling with ACHP provides a new therapeutic strategy for enhancing the repair of sutured intrasynovial tendons.

Principles of Tendon Structure, Healing, and the Microenvironment

Tendon is a strong viscoelastic tissue, responsible for conducting force from muscle to bone. In the hand, flexor tendons course through fibro-osseous sheaths, composed of an intricate tenosynovium and fibrocartilaginous pulley system. After flexor tendon laceration, changes occur in tendon force transduction as well as vascularity, affecting tendon healing on a tissue and cellular level. Tendon healing occurs through intrinsic and extrinsic mechanisms, which in combination with local anatomy, can predispose to adhesion formation. Understanding the relationship between microenvironmental cues and tendon healing on the cellular and tissue level will improve our knowledge and treatment of flexor tendon injuries.

Metabolic regulation of intrasynovial flexor tendon repair: The effects of dichloroacetate administration on early tendon healing in a canine model

Enriched in glycolytic enzymes, paucicellular and hypovascular intrasynovial flexor tendons fail to mount an effective healing response after injury and repair. In contrast, well-vascularized extrasynovial flexor tendons possess high levels of oxidative phosphorylation (OXPHOS) enzymes and have a markedly improved healing capacity. This study was designed to compare the metabolic profiles of the two types of tendons and to evaluate the impact of metabolic reprogramming on early intrasynovial tendon healing in a clinically relevant canine model. Results showed that healthy intrasynovial tendons expressed higher levels of PDK1 and GAPDH and lower levels of SCX and IGF1 than did extrasynovial tendons. PDK1 encodes a subtype of pyruvate dehydrogenase kinase (PDK) that inhibits OXPHOS. Consistently, ATP production via glycolysis was favored in intrasynovial tendon cells whereas OXPHOS was the preferred pathway in extrasynovial tendon cells. Inhibition of glycolysis in vitro increased SCX expression in intrasynovial tendon cells. Therefore, dichloroacetate (DCA), a PDK1 inhibitor, was used in vivo to shift intrasynovial tendon ATP production from glycolysis to OXPHOS. Oral DCA administration reduced serum lactate concentration and increased acetyl-CoA content in repaired intrasynovial tendons and led to reduced TLR4 and IL1B and increased IGF1, SCX, and TGFB3 expressions in treated intrasynovial tendons compared to controls. Immunohistochemistry staining with anti-Ki67 and anti-CD31 antibodies revealed marked increases in cellularity and neovascularization in treated intrasynovial tendons. Clinical significance: The findings of this experiment indicate that improved gene expression and histological outcomes can be achieved by regulating glucose metabolism in the early stages following intrasynovial tendon repair.

The use of connective tissue growth factor mimics for flexor tendon repair

Intrasynovial flexor tendon lacerations of the hand are clinically problematic, typically requiring operative repair and extensive rehabilitation. The small-molecule connective tissue growth factor (CTGF) mimics, oxotremorine M (Oxo-M) and 4-PPBP maleate (4-PPBP), have been shown to improve tendon healing in small animal models by stimulating the expansion and differentiation of perivascular CD146+ cells. To enhance intrasynovial flexor tendon healing, small-molecule CTGF mimics were delivered to repaired canine flexor tendons via porous sutures. In vitro studies demonstrated that Oxo-M and 4-PPBP retained their bioactivity and could be released from porous sutures in a sustained manner. However, in vivo delivery of the CTGF mimics did not improve intrasynovial tendon healing. Histologic analyses and expression of tenogenic, extracellular matrix, inflammation, and remodeling genes showed similar outcomes in treated and untreated repairs across two time points. Although in vitro experiments revealed that CTGF mimics stimulated robust responses in extrasynovial tendon cells, there was no response in intrasynovial tendon cells, explaining the lack of in vivo effects. The results of the current study indicate that therapeutic strategies for tendon repair must carefully consider the environment and cellular makeup of the particular tendon for improving the healing response.

Intermittent Hydrostatic Pressure Promotes Cartilage Repair in an Inflammatory Environment through Hippo-YAP Signaling In Vitro and In Vivo

The study of chondrogenic progenitor cells (CPCs) as seed cells has become a new focus of cartilage regeneration. The inflammatory environment of osteoarthritis (OA) inhibits the repair ability of CPCs. But the OA patients' CPCs showed an excellent regeneration ability with intermittent hydrostatic pressure (IHP). However, the mechanism is unclear. We compared the expression of the Hippo signaling effect factor YAP between OA and normal cartilages. Then, the relationship between the Kellgren-Lawrence (K-L) score of OA and the rate of YAP-positive cells was analyzed. The changes of CPCs after IHP and IL-1β applications were observed. The OA model was established by cutting the anterior cruciate ligament of rats. The knee joint of the OA rats was distracted by hinged external fixator to create suitable IHP, named as the IHP group. The IHP group plus intra-articular injection of Verteporfin (VP) was named as the IHP+VP group, and the untreated rat group was named as the CON group. Four and 8 weeks after the operation, the reparative effect was evaluated by MASSON staining and immunohistochemical staining. Lower levels of YAP1 and higher expressions of p-YAP1 were found in the OA group as compared to the normal group. IHP inhibited the Hippo signaling in an inflammatory environment and promoted the proliferation of CPCs. The cartilage deterioration in the CON group progressed more significantly than that in the IHP+VP group. The best reparative effect was observed in the IHP group with increased expression of YAP1 and decreased p-YAP1. These results hint that mechanical stress can activate CPCs and promote cartilage repair in an inflammatory environment through inhibiting Hippo signaling.

Neonatal Enthesis Healing Involves Noninflammatory Acellular Scar Formation through Extracellular Matrix Secretion by Resident Cells

Wound healing typically recruits the immune and vascular systems to restore tissue structure and function. However, injuries to the enthesis, a hypocellular and avascular tissue, often result in fibrotic scar formation and loss of mechanical properties, severely affecting musculoskeletal function and life quality. This raises questions about the healing capabilities of the enthesis. Herein, this study established an injury model to the Achilles entheses of neonatal mice to study the effectiveness of early-age enthesis healing. Histology and immunohistochemistry analyses revealed an atypical process that did not involve inflammation or angiogenesis. Instead, healing was mediated by secretion of collagen types I and II by resident cells, which formed a permanent hypocellular and avascular scar. Transmission electron microscopy showed that the cellular response to injury, including endoplasmic reticulum stress, autophagy, and cell death, varied between the tendon and cartilage ends of the enthesis. Single-molecule in situ hybridization, immunostaining, and terminal deoxynucleotidyl transferase-mediated dUTP nick-end labeling assays verified these differences. Finally, gait analysis showed that these processes effectively restored function of the injured leg. These findings reveal a novel healing mechanism in neonatal entheses, whereby local extracellular matrix secretion by resident cells forms an acellular extracellular matrix deposit without inflammation, allowing gait restoration. These insights into the healing mechanism of a complex transitional tissue may lead to new therapeutic strategies for adult enthesis injuries.