Introduction
The healing of a tendon graft in a bone tunnel is a common requirement for reconstructive surgeries. There are two ways that tendons and ligaments can adhere to the bone, directly or via Sharpey's fibers at the fibrocartilaginous junction (indirect insertion). According to studies in basic science, changes in the local mechanical environment and intra- vs. extra-articular position can induce tendon-to-bone healing to create either a direct or indirect insertion. By growing into the interface zone between the tendon and bone, bone and soft tissue help heal wounds.
According to recent study, cytokines such as bone morphogenetic proteins can aid in tendon-to-bone healing. In clinical settings where the tendon and bone healing may be compromised, such as when there is excessive motion at the graft-tunnel interface, when the bone tunnel is enlarged, as is the case during revision cruciate ligament reconstruction, or when the rotator cuff tendon is healing in elderly patients with osteoporosis, augmentation techniques may be especially helpful.
What Is Enthesis?
The enthesis is the term used to describe the area of bone to which a tendon, ligament, or joint capsule is attached. Entheses work to balance the various elastic moduli of tendon and skeletal tissue in order to prevent local peaks in tension, reflecting the general idea that stress concentrates at interfaces between structures with various mechanical properties. Enthesopathies can be brought on by tensile or compressive mechanical stress, a systemic illness, trauma, medication, or age-related degeneration, and they can carry a heavy disease burden.
How Enthesis Get Developed?
In intramembranous ossification, mesenchymal stem cells are converted directly into bone tissue without the need for a cartilage precursor. Fibrous entheses are connected to some of the strongest muscles in the body. Between existing type one collagen bundles, fibroblasts and mesenchymal stem cells develop into osteoblasts and replace the calcified matrix (bundle bone) with an organized lamellar structure. Sharpey's fibers, which are perforating collagen fibers that are anchored straight into bone, are used to attach the enthesis. Sharpey's fibers, which are only seen in areas with interstitial bone lamellae, grow from the tendon and periosteum and penetrate the bone substance. Depending on the tissue type upon insertion, these attachments can be further separated into the two groups of periosteal and bony. Fibrous periosteal entheses attach to the periosteum, which then connects the tendon to the underlying bone in an indirect manner. This reduces stress concentration by dispersing the tendon's force across a larger area.
In addition, fibrous bony entheses insert where the periosteum is lacking, enabling the tendon to enter the bone directly. As the periosteum is removed during bone formation, periosteal fibrous entheses may develop into bony ones with age. Points of tendon implantation via fibrous entheses typically show up as an erratic roughened patch on the surface of a dry bone specimen.
How Tendon to Bone Healing Take Place?
Inflammation, proliferation, and repair or remodeling are usually the order in which tendon healing occurs. As growth factors are released and a blood clot forms around the wound, inflammatory cells begin to invade the area. Neutrophils and macrophages, which are inflammatory cells, release mediators that start the repair process along with tendon-derived cells. There is extracellular matrix deposition, neovascularization, fibroblast proliferation, and chemotaxis. After that, there is remodeling, which results in a decrease in the tissue's vascularity and cellularity and an increase in the morphological organization of the collagen. An integrated collagen bridge gradually re-establishes itself, which promotes healing between the soft tissue and bone. The extracellular matrix's gradual ossification is how this contact's bony integration occurs.
What Are the Stages Involved in Tendon to Bone Healing?
Various animal research has been conducted to better understand the tendon-to-bone repair steps.
Week 2 - The tendon-bone interface was made up of fibrous tissue that was vascular and highly cellular. This granulation tissue had no collagen continuity between the tendon and bone.
Week 4 - The passage was lined with newly produced bone. After that, this new bone began to penetrate the fibrous interface and tendon, as seen by the sporadic continuity of collagen fibers between the fibrous tissue and the new bone.
Week 12 - Collagen fibers, which closely resembled Sharpey's fibers and were aligned in the direction of the pull of the musculotendinous unit, connected the tendon to the nearby bone.
Week 26 - The length of the bone tunnel revealed continuity, resembling a fibrous enthesis, between the tendon's collagen fibers and the surrounding bone.
What Are the Modulation of Tendon to Bone Healing?
Modulation and regulation of collagen production are important factors in ligament healing. Growth factors, such as transforming growth factor, platelet-derived growth factor, vascular endothelial growth factor, and basic fibroblast growth factor, play a major part in this process. They all become more active during the mending of ligaments and tendons, and they cooperate to control the healing process. Moreover, the tumor necrosis factor superfamily member bone morphogenic proteins (BMP) are involved in directing the remodeling of bone at the bone-tendon contact.
Many tissues produce insulin-like growth factors, which function as paracrine or autocrine regulators of development and healing. They are most noticeable during the initial stages of healing when proliferation and inflammation predominate. They specifically stimulate fibroblast migration, proliferation, and the production of collagen and extracellular matrix. The insulin growth factor mediates proliferative effects by working synergistically with other growth factors, such as PDGF (platelet-derived growth factor). The tumor growth factor family performs various anabolic and catabolic activities during the healing process. They include controlling cell motility, collagen synthesis, fibronectin binding, cell proliferation, and proteinase activity.
During the proliferative and remodeling stages of tendon repair, vascular endothelial growth factor induces angiogenesis and is expressed In order to provide a pathway for the influx of cells, cytokines, growth factors, and nutrients; it triggers the ingrowth of neo vasculature from tendon vessels into the injured area.
In addition to osteogenesis, bone morphogenic proteins are crucial for tendon and fracture healing. BMP-2,4 and 7 are involved in osteogenesis, and fracture repair, whereas BMP-12,13 and 14 are expressed during enthesis during development. Matrix metalloproteinases (MMPs) and tissue inhibitors of matrix metalloproteinases are significant additional regulators of tendon-to-bone repair.
Conclusion:
Entheses are intricate structures that act as a crucial connection between soft tissues and bone, lowering pressures and allowing movement. The tendon-to-bone interface needs to be repaired after a number of common injuries, although the repairs are frequently weaker mechanically. The soft tissues around the injured enthesis should be repaired with particular attention. Immobilization after surgical repair, followed by light loading, can speed up the healing process. To know more about the condition, consult the doctor online.
