Synovial joints allow considerable degree of movement. Explain the features of this joint that facilitate this.
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A synovial joint is the most common joint in mammals, and it allows for more movement of articulating bones than other joints such as synarthroses, sutures, syndesmoses, or gomphoses. Anatomically, a synovial joint is a solid organ consisting of articular cartilage, synovium, perichondrium, and subchondral bone, including all cell types in bone marrow. The smooth and slippery lubricated surface of the articular cartilage covers the ends of bones, allowing the sliding of bones at the joint without rubbing each other. Load transfer with a minimum frictional coefficient and shock absorption is the principal function of the articular cartilage during movement [1]. Articular cartilage is hyaline cartilage, which is avascular, aneural, and alymphatic. The dense extracellular matrix (ECM) is sparsely populated by highly specialized cells known as chondrocytes. ECM is largely composed of water (up to 80% of the wet weight) and collagen (up to 60% of the dry weight, of which 90% is type II collagen) with other noncollagenous proteins and glycoproteins (10–15% of the wet weight) [1]. The fibrils and fibers formed by type II collagen are intertwined with proteoglycan aggregates.
Osteoarthritis (OA) is one of the most common forms of arthritis and represents chronic degeneration of the synovial joints, which is the leading cause of chronic disabilities worldwide [2–4]. OA is also known as degenerative joint disease or degenerative arthritis [5,6]. It is a chronic condition that breaks down cartilage, the material that cushions the joints. Cartilage breakdown causes bones to rub against each other, causing joint stiffness, pain, swelling, and reduced movement [5,7]. Multiple tissues including articular cartilage, subchondral bone, synovium, meniscus, and joint capsule can be involved in OA, although articular cartilage is frequently the focus of attention due to its breakdown and loss of function [8]. It is estimated that about 27 million people in the United States have OA afflicting about 14% of the US adult population aged 25 years and older, and about 34% of those adults are aged over 65 years [6]. Common symptoms of OA include pain, swelling, sore or stiff joints, and decreased mobility.
A healthy articular cartilage tolerates intensive and repetitive physical stress, but surprisingly it cannot self-heal or repair, even with the most minor injury. When damaged, the articular cartilage does not spontaneously repair due to a paucity of progenitor cells [9], a lack of blood vessels [9], a low cell-to-matrix ratio (∼5%) [10,11], and low metabolic activity of mature chondrocytes [10,11]. The limited spontaneous repair of cartilage lesions leads to a progressive loss in joint function, together with the continuing destabilization of tissue matrix [8,12].
Total joint arthroplasty (TJA) is the predominant therapy for late stage OA using synthetic prosthesis of metal, plastic, and ceramics. Despite its reported clinical success, TJA has significant limitations, including wear debris, aseptic loosening, metal allergy, and infections. With an average service life of 10–15 years, TJA necessitates an inevitable revision in many patients; revision surgeries are technically challenging and costly [13–16]. Since current synthetic prosthesis cannot remodel with native bone that undergoes physiologically necessary remodeling, there has been a great deal of effort to develop biological therapies for cartilage and synovial joint regeneration, including abrasion arthroplasty, autologous chondrocyte implantation (ACI), and mosaicplasty [17]. However, few therapies are effective or commercially viable; they have drawbacks such as limited integration, suboptimal conformity of cartilage surface, altered cartilage phenotype, and donor site morbidity [13–16]. Additionally, the existing biological therapies are only applicable to focal cartilage lesions but not the entire articular surface of the synovial joint. To date, no biological therapies exist for resurfacing of the entire articular cartilage of the synovial joint.
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