Synovial Joint - an overview (2024)

Regeneration of Entire Articular Surface by Endogenous Cell Homing

Besides the aforementioned focal defect regenerations, cell homing-based approaches have been applied for a regeneration of entire synovial joint condyle. We have recently demonstrated that functional entire articular surface of a synovial joint can be regenerated without cell transplantation using a layer-deposited PCL-HA scaffold with two distinct inner microstructures: 400 and 200μm repeats of interlaid strands and microchannels for cartilage and bone regions, respectively [121]. TGFβ3-loaded collagen gel was infused into the microchannels of the cartilage layer. The bioengineered grafts surgically replaced the entire rabbit humeral condyles and induced promising regeneration of hyaline cartilage and vascularized subchondral bone, fully integrated with host tissues [121]. Analysis of recruited cells in scaffolds revealed that regenerated cartilage and subchondral bone were from synovial cells and bone marrow MSCs, respectively. Fully layered articular surface and functional restoration of the operated joints were obtained in TGFβ3-infused grafts, lending support to the idea that precisely controlled scaffold design combined with biological cues can lead to the functional regeneration of large complex tissues invivo [121,123]. Nonetheless, long-term functionality and homeostasis of the regenerating cartilage, especially in larger animal models or patients, may warrant a prolonged release of the biological cues to maintain the tissue maturity and perhaps cell transplantation. Delivery vehicles made of biodegradable synthetic or biological polymers are incorporable in cell-free scaffold systems [87,130,131]. Whether this approach will be realized in preclinical animal models and in OA patients warrants separate investigations.

Abstract

Synovial joints allow for the articulation of long bones within a fluid-filled cavity. Articular cartilage is a connective tissue covering the ends of bones that functions as a load-bearing, low-friction, and wear-resistant surface to facilitate joint movement. Synovial fluid (SF) is the viscous liquid found inside synovial joints that functions as a biomechanical lubricant. The extracellular matrix of cartilage allows for load-bearing during joint motion, while the viscous and lubricating properties of SF reduce cartilage wear. The homeostasis of these tissues is dependent on mechanobiology – cell populations receiving mechanical cues to guide biosynthesis and thereby alter their microenvironment and tissue-scale mechanical properties. This chapter includes tissue engineering approaches to the in vitro development of articular cartilage and SF for the treatment and study of damaged synovial joints. First, the composition, structure, and function of cartilage and SF are discussed, followed by the mechanical properties of these tissues and the theoretical models that have been used to describe them. Next, the effects of mechanical stimuli on chondrocyte biosynthesis and pathological changes in SF composition are covered. Finally, advances in the tissue engineering of cartilaginous tissue and SF are reviewed, focusing on the mechanobiology of chondrocytes in tissue-engineered constructs.

Introduction

Synovial joints, the most mobile type of joints in the body, are susceptible to inflammatory injury leading to arthritis. The synovium is a common target of a variety of insults including direct microbial infection, crystal deposition and autoimmune attack, e.g. in rheumatoid arthritis (RA). This chapter will review normal synovial joint structure and function, the processes that lead to inflammatory arthritis, an approach to differential diagnosis, and the principles of treatment of RA. The topic and discussion will be illustrated by a patient with inflammatory arthritis found to have RA. It is the commonest chronic inflammatory rheumatic disease, affecting 1–2% of the population. RA not only produces extensive morbidity, but also is associated with a reduction in life expectancy.

Zygapophysial Joint Synovial Folds

Z joint synovial folds are synovium-lined extensions of the capsule that protrude into the joint space to cover part of the hyaline cartilage. Although the function of the synovial folds has not been definitively determined, they are thought to provide lubrication to the Z joints, through the secretion of synovial fluid, and also to protect the margins of the articular cartilage (Uhrenholt etal., 2008). The synovial folds vary in size and shape in the different regions of the spine. Figure 2-8 shows a photomicrograph by Singer and colleagues (1990) demonstrating a large Z joint synovial fold. Chapters 5, 6, and 7Chapter 5Chapter 6Chapter 7 discuss the unique characteristics of Z joint synovial folds in the cervical, thoracic, and lumbar regions, respectively.

Kos in 1969 described the typical intraarticular fold (meniscus) (Fig. 2-9) as being attached to the capsule by loose connective tissue. Synovial tissue and blood vessels were distal to the attachment, followed by dense connective tissue (Bogduk & Engel, 1984).

In 1982 Engel and Bogduk reported on a study of 82 lumbar Z joints. They found at least one intraarticular fold within each joint. The intraarticular structures were categorized into three types. The first was described as a connective tissue rim found running along the most peripheral edge of the entire joint. This connective tissue rim was lined by a synovial membrane. The second type of meniscus was described as an adipose tissue pad, and the third type was identified as a distinct, well-defined, fibroadipose meniscoid. This latter type of meniscus usually was found entering the joint from either the superior or the inferior pole or both poles of the joint.

Giles and Taylor (1987) studied 30 lumbar Z joints, all of which were found to have menisci. The menisci were renamed zygapophysial joint synovial folds because of their histologic composition. Free nerve endings were found within the folds, and the nerve endings met the criteria necessary for classification as pain receptors (nociceptors). That is, they were distant from blood vessels and were of proper diameter (0.6 to 12 µm). Therefore the synovial folds (menisci) themselves were found to be pain sensitive. This meant that if the Z joint synovial fold became compressed by, or trapped between, the articular facets making up the Z joint, back pain could result (see Fig. 2-9). Other investigators have confirmed the presence of sensory fibers in the Z joint synovial folds (Ahmed etal., 1993; Vandenabeele, Creemers, & Lambrichts, 1996).

Abstract

Synovial joints allow for the articulation of long bones within a fluid-filled cavity. Articular cartilage (AC) is a connective tissue covering the ends of bones that functions as a load-bearing, low-friction, and wear-resistant surface to facilitate joint movement. Synovial fluid (SF) is the viscous liquid found inside synovial joints that functions as a biomechanical lubricant. The SF is encapsulated within the joint space by the synovium (SYN), a thin membrane-like tissue consisting of cells and extracellular matrix. The extracellular matrix of cartilage allows for load-bearing during joint motion, while the viscous and lubricating properties of SF reduce cartilage wear. The homeostasis of these tissues is dependent on mechanobiology – cell populations receiving mechanical cues to guide biosynthesis and thereby alter their microenvironment and tissue-scale mechanical properties. This chapter includes Tissue engineering (TE) and regenerative medicine (RM) approaches to the in vitro and in vivo development of articular cartilage and SF for the treatment and study of damaged synovial joints. First, the composition, structure, and function of cartilage and SF are discussed, followed by the mechanical properties of these tissues and the theoretical models that have been used to describe them. Next, the effects of mechanical stimuli on chondrocyte biosynthesis and pathological changes in SF composition are covered. Finally, advances in the fields of TE and RM applied to cartilaginous tissue and SF are reviewed, focusing on the mechanobiology of chondrocytes in Tissue engineered constructs.

Structure and Components of the Synovial Joints

The synovial joint is characterized by its mobility, as these joints are able to move freely in multiple planes. A synovial joint consists of two bony surfaces that are encompassed by a fibrous capsule with a synovial lining. The joint contains synovial fluid, which allows the bony surfaces to articulate with each other. The extracellular matrix consists of water and proteoglycans (glycosaminoglycan and hyaluronic acid). The viscoelastic properties of synovial fluid and its inherent function as a lubricant and shock absorber are largely attributed to hyaluronic acid. The fibrous capsule has a rich network of substance P (a neurotransmitter for pain) nociceptive nerve fibers that can potentially generate the sensation of pain. Limb joints are typically synovial joints; these include hip, knee, and shoulder joints.

As inflammation ensues, fluid and polymorphonuclear leukocytes infiltrate the joint space of synovial joints. Vasodilation and venous congestion contribute to pain-provoking capsular distention and neuronal sensitization of substance P nerve fibers. If left untreated, the inflammatory cascade can destroy the integrity of the joint and permanently impair its function. This can lead to chronic pain and disability.

Bursae, tendon sheaths, and entheses are soft tissues commonly located surrounding synovial joints. Bursae are flattened fibrous sacs lined with a synovial membrane, containing a thin film of synovial fluid. Tendon sheaths are elongated bursae that wrap around a tendon that is subject to friction. Entheses are the insertion sites of tendons or ligaments into bones. They are functionally integrated with the synovial joint and can be involved in the rheumatic disease processes.

Abstract

Synovial joints permit the distinct movement and functioning of the different skeletal elements in the limbs but share essential structural features, including articular cartilage, intra/perijoint ligaments, synovial lining, and lubricating fluids. Here, we review most recent advances in the developmental biology of synovial joints, paying particular attention to the following: the origin and diversification of joint progenitor cells; roles of signaling proteins in joint genesis and morphogenesis, including GDF5; mechanisms by which articular cartilage acquires its mature postnatal multizone organization; and defects in pediatric congenital joint conditions such as symphalangism. We include novel data on hip joint development that surprisingly has been understudied. At variance with their developmental prowess and dynamism, joint tissues have a frustratingly poor capacity to self-repair and regenerate after trauma or chronic disease. Thus, we discuss implications and insights from developmental studies that could suggest ways to design more effective bioengineered strategies for tissue repair and could also lead to means by which the regenerative activities of resident embryonic-lineage progenitors recently identified in adult joint tissues could be boosted.

Abstract

Synovial joints enable movement and protect the integrity of the articular cartilage. Joints form within skeletal condensations destined to undergo chondrogenesis. The suppression of this chondrogenic program in the interzone is the first morphological sign of joint formation. While we have a fairly good understanding of the essential roles of BMP and TGFβ family members in promoting chondrogenic differentiation in developing skeletal elements, we know very little about how BMP activity is suppressed specifically within the interzone, a crucial step in joint development. The function of the BMP ligand Gdf5 has been especially difficult to decipher. On the one hand, Gdf5 is required to promote chondrogenesis of articular elements. On the other hand, Gdf5 is highly expressed in the joint interzone where chondrogenesis must be suppressed for the formation of many joints. Here we review the evidence that BMP signaling must be suppressed within the joint interzone for joint morphogenesis to progress, and consider how Gdf5 exerts its divergent effects on chondrogenesis and joint formation. We also consider how TGFβ signaling impacts formation of the interzone. Finally, we propose a model whereby Gdf5 exerts distinct effects in the interzone vs. surrounding cartilage based on the repertoire of BMP receptors available in these tissues. Understanding how BMP antagonists and counteracting TGFβ signals intersect with Gdf5 to sculpt the joint interzone is essential for understanding the origin of osteoarthritis and other diseases of joint tissues.

TYPES OF JOINTS

Skeletal joints, the sites of articulation between one bone or cartilage and another, are generally of three types: fibrous joints (skull-type sutures), cartilaginous and fibrocartilaginous joints (discovertebral joints), and synovial joints (most limb joints).

Fibrous joints, or skull-type articulations, are called synarthroses: one bone is joined to another by an unossified fibrous membrane or residual plate of cartilage. Fibrous joints, such as skull sutures, allow little or no movement. Bones united by ligamentous attachments, such as the proximal and distal tibiofibular joints and the superior sacroiliac joints, are called syndesmoses.

Cartilaginous joints are amphiarthroses and are of two types: fibrocartilaginous and cartilaginous. Bony surfaces coated with hyaline cartilage and united by fibrocartilaginous disks are symphyses (fibrocartilaginous joints). Examples include discovertebral, manubriosternal, xiphisternal, and costosternal joints and symphysis pubis. United epiphyseal hyaline cartilage is an example of a cartilaginous amphiarthroidal joint. Amphiarthroses allow a limited range of movement but provide considerable stability.

Synovial joints or diarthroses are freely movable joints. The bony surfaces are coated with hyaline cartilage and united by a fibrous articular capsule. The synovial membrane lines the inner surface of the capsule but does not cover the articular cartilage (Figure 1-1). The capsule is strengthened by collateral ligaments. Synovial joints comprise most of the joints of the extremities and are the most accessible joints to direct inspection and palpation. Synovial joints share important structural components: subchondral bone, hyaline cartilage, a joint cavity, synovial lining, articular capsule, and supporting ligaments.

Spine

Facet joint synovial cysts are diagnosed with greater frequency with the increasing use of CT and MRI for the evaluation back pain.⁶⁰ These cysts are associated with facet joint osteoarthritis, spondylolisthesis, and facet instability^61,62 and are encountered in the lumbar and cervical spine (Figure 18-23). Facet synovial cysts in the thoracic spine are quite rare.⁶³

Ganglion cysts are felt to arise from spinal ligaments, frequently the ligamentum flavum in the lumbar spine, or rarely, in the cervical spine.^64,65 These lesions are often in close proximity to the facet joints, making differentiation from synovial cysts difficult on imaging studies alone.⁶⁶