Articular cartilage, which makes possible the painless, low-friction movement of synovial joints, consists of a sparsely distributed population of highly specialized cells called chondrocytes that are embedded within a matrix and provide articular cartilage with remarkable mechanical properties. Dendrite: Definition, Function, and Malfunction, Cholinergic: Definition, Effects, and Function, Temporal Bone: Definition, Anatomy, and Fracture, Spongy Bone(Cancellous Bone): Definition & Function. Articular cartilage functions as a shock absorber and also reduces friction between bones where they meet at joints. The articular cartilage function is dependent on the molecular composition of the extracellular matrix (ECM). decreases friction and distributes loads. 1. This type of joint has a very limited range of motion. This layer protects the deeper layers from sheer stresses, but is very thin. Tiny vessels penetrate the cortical end plate and can invade the calcified cartilage zone up to the tidemark. So, you can find hyaline cartilage in the larynx and trachea in the throat portion of the body, and then also in all of the joints where the surfaces of bones are articulating each other. The two major problems that need to be addressed in repair of articular cartilage are the filling of the defect void with a tissue that has the same mechanical properties as articular cartilage and the promotion of successful integration between the repair tissue and the native articular cartilage and calcified cartilage. Hyaline cartilage appears slightly glassy under a microscope. This so-called molecular marker technology (biomarkers) is intended to provide new means of assaying ongoing active processes in articular cartilage. The articular cartilage _____ surface area and _____ stress, is nearly frictionless aiding lubrication and minimizing wear of opposing bone surfaces Increasing the surface area and decreasing stress allows the articular cartilage to function in Raleigh, ... R.L. The main cell types in cartilage are chondrocytes, the ground substance is chondroitin sulfate, and the fibrous sheath is called perichondriu… articular cartilage is 2 to 4 mm thick in length. The cartilage provides a smooth surface that facilitates easy movement. During motion, one bone remains in place and the other moves. Air-tent analogy with articular cartilage. Examples of planar joints are the carpals of the hand and the tarsals of the foot, and in the temporomandibular joint as shown below. They are categorized according to structure and type of motion. Articular Cartilage Structure and Function Articular cartilage is composed of chondrocytes embedded within an exquisitely organized extracellular matrix of collagen and proteoglycans. The x-ray featured below shows the healthy, undamaged sternoclavicular joint of a young male on the left-hand side, and the same joint after trauma and subsequent osteoarthritis to the right. The image below shows the characteristic ball and socket shape of the prosthetic hip joint to the left, and an arthritic natural hip joint to the right. Download as PDF. Cartilage is a flexible connective tissue that differs from bone in several ways; it is avascular and its microarchitecture is less organized than bone. All of these structures of the articular capsule of the joint (outer layer, inner layer, synovial fluid, bursae, fat pad) all have specific functions in the knee. The air-tent system requires a pump that must be constantly operating to maintain inflation of the tent because of leaks in the fabric. To further reduce friction between the articulating surfaces of the bones, the knee joint is lined by a synovi… The knee, with a wider range of motion, is considered to be a modified hinge joint. Articular cartilage is a white elastic tissue that allows bones to glide smoothly against each other. The main function of the articular cartilage is to provide a smooth surface where the bones connect and touch. The upper portion of the middle zone contains randomly (isometrically) arranged collagen fibrils, and rounded chondrocytes that are sparsely but fairly evenly distributed. In this regard, the preservation and health of articular cartilage are paramount to joint health. Articular cartilage is the highly specialized connective tissue of diarthrodial joints. The unique structure of articular cartilage provides the intrinsic functional properties that enable the knee joint to withstand and resist excessive compressive and biomechanical loads, facilitating almost frictionless knee movement. Ultra-structure of articular cartilage Chondrocytes organize the collagen, proteoglycans and non-collagenous proteins into a unique and highly specialized tissue, suitable for carrying out the functions stated above. Additionally, each patient is unique and must be evaluated individually to determine the best course of treatment. With further development such techniques may be used in diagnosis, for estimation of the activity of the tissue-destroying process, to document effects of therapeutic intervention, and most importantly, to discover processes during the early stage before clinical symptoms become apparent. Normal human articular cartilage is 2–4 mm thick (Shepherd and Seedhom, 1999); 10% of the tissue is cells (chondrocytes), 10%–25% is the extracellular matrix the chondrocytes synthesize (Eyre, 2002; Archer and Francis-West, 2003; Quinn et al., 2013; Martel-Pelletier et al., 2016), and 65%–80% of the tissue is water (Fox et al., 2009). Articular cartilage lies at the end of bones at the joint. The knee joint can undergo damage through excessive rotation; a common football injury is the dreaded meniscus tear. The basic science of articular cartilage: structure, composition, and function. Functions of Cartilage Cartilage holds some bones together, for instance, rib cartilage. In the lower limbs, the weight-bearing properties of articular cartilage are unable to properly distribute weight and the shock-absorbant characteristics of cartilage are lost. Dick HeinegÃ¥rd, ... Tore Saxne, in Rheumatology (Sixth Edition), 2015. Recent studies suggest that the mammalian (or mechanistic) target of rapamycin (mTOR) is crucial for maintenance of articular cartilage homeostasis and could be a potential therapeutic target in delaying the degenerative process. The COL network resists the swelling tendency of PG molecules and provides cartilage functional integrity, tensile and shear stiffness and strength. The term ‘articular cartilage’ does not refer to the type of cartilage structure, but to its location. This hydration gives the cartilage springiness needed for shock absorbency. Also called rotary or trochoid joints, pivot joints have a circular range of motion on a single axis due to one bone surface having a ring-like shape. Articular cartilage (cartilage that covers the articulating part of a bone) is of the type called hyaline (glasslike) because thin sections of it are translucent, even transparent. This causes it to heal very slowly. Many arterial terminal branches, probably end arteries, are present in the subchondral bone plate and end in sinusoids of uneven caliber and an irregular distribution. In the embryo, it provides support and is a precursor to bone. The layered structure of articular cartilage is diagrammed in Fig. 1. Joint - Joint - Articular nerves: The sources of nerve fibres to a joint conform well to Hilton’s law—the nerves to the muscles acting on a joint give branches to that joint as well as to the skin over the area of action of these muscles. The junction where cartilage and bone meet is called the chondro-osseous junction or osteochondral interface. Some of the fragments formed will no longer be retained in the tissue but are released to surrounding body fluids. To reduce this friction, all articulating surfaces involved in movement are covered with a white, shiny, slippery layer called articular cartilage. Danila, in Pathobiology of Human Disease, 2014. As further advances are made in this exciting area of orthopedics, promising new methods will evolve to improve the treatment of articular cartilage lesions and narrow the use of total joint arthroplasty. It helps in the formation of bones in growing children. Very few chondrocytes are found in this layer. Here, proteoglycans are more prevalent than in any of the other layers, while collagen fibrils are relatively disorganized in comparison to the layer above. The mineralized matrix of the deep zone provides a gradual transition in mechanical stiffness of the cartilage tissue as it merges with the supporting underlying subchondral bone. When articulation is smooth, less stress is exercised on the cartilage surface and the tissue is more resistant to wear, in the same way oil added to a squeaky door hinge prevents the erosion of the touching surfaces. The function of articular cartilage is dependent on the molecular composition of the extracellular matrix (ECM), which consists mainly of proteoglycans and collagen. At the base, horizontal fibers are firmly adherent to the subchondral bone. It is therefore correct to say that articular cartilage occurs only in the presence of a synovium. Specifically, autophagy is now emerging as a crucial homeostatic process responsible for the maintenance of chondrocyte survival; suppression of autophagy results in decreased chondroprotection, enhanced apoptosis, and cartilage degeneration. A ‘tidemark’ distinguishes between the non-calcified and calcified layers. It is believed, although research is still in its infancy, that the PCM enables cellular signaling, with cartilage homeostasis as a result. Articular cartilage is devoid of blood vessels, nerves, and lymphatics cells. Low levels of oxygen mean chondrocytes primarily depend upon anaerobic metabolism. In joint: Articular cartilage. 2. 3. As mentioned previously, the main proteoglycan in cartilage is aggrecan , which forms large aggregates with hyaluronan and are negatively charged to hold water in the tissue. In response to injury, aging, or long-term stressful stimuli, chondrocytes—and therefore articular cartilage—become less able to withstand harsh conditions over time. Kyle R. Flik, ... Brian J. Cole, in Clinical Sports Medicine, 2006. Articular (hyaline) Cartilage Components. Articular Cartilage Function. Most important, articular cartilage has a limited capacity for intrinsic healing and repair. The elbow is the best example of a simple hinge joint. The basic science of articular cartilage: structure, composition, and … It is deformable but elastic, and…. Mechanical injury to the articular cartilage is a key risk factor in joint damage and predisposition to osteoarthritis. provide lubrication of articular surfaces. The first three layers of articular cartilage are shown in the simple diagram below. This itself is a result of its isolation from the systemic regulation, and lack of blood vessels and nerve supply (Mankin, 1982). Cartilaginous joints (growth plates, the symphysis, the spine, and the ribs) have very little movement and no synovial membrane. Articular cartilage can be damaged by injury or normal wear and tear. Full-thickness defects induce mesenchymal chondroprogenitor cells to differentiate into repair tissue, but this is predominantly fibrous in nature and degenerates with time. Movement of the bones causes friction between the articulating surfaces. In processes resulting in destruction of cartilage tissue, proteolytic enzymes degrade ECM proteins. So this is called articular cartilage. This may allow the withstanding of pressure and weight-bearing brought a… The articulating surface of the femoral condyles, tibial plateaus and the back of the patella are covered with this cartilage. This layer acts as a bridge between the superficial and deep zones, and is the thickest layer. mTOR signaling is implicated in cell growth, proliferation, as well as its ability to negatively regulate autophagy—a cell survival and adaptation process. The unique structure of articular cartilage provides the intrinsic functional properties that enable the knee joint to withstand and resist excessive compressive and biomechanical loads, facilitating almost frictionless knee movement. Joints between the bones, knee, elbow, and rib cage are some typical locations. One of the mechanisms through which autophagy is suppressed in the articular chondrocytes is through increased mTOR signaling. Articular cartilage does not usually regenerate (the process of repair by formation of the same type of tissue) after injury or disease leading to loss of tissue and formation of a defect. This chapter outlines the architectural and compositional features of … Johanne Martel-Pelletier, ... Jean-Pierre Pelletier, in Osteoarthritis, 2007. Articular cartilage is devoid of blood vessels, lymphatics, and nerves and is subject to a harsh biomechanical environment. In adults, perichondrium tissue does not cover articular cartilage in the joints or where the ligaments attach to bone. Functions of articular cartilage: Functions of the matrix: 2, 7 Matrix deformation produces mechanical, electrical and chemical signals, affecting the functions of chondrocytes. Articular cartilage damage most commonly occurs in the knee, but the elbow, wrist, ankle, shoulder, and hip joint can also be affected. Indeed, articular cartilage deforms less over a stiff and dense bone structure (whereas inhomogeneities in stiffness or in density will deform it more). It is caused by the nerves surrounding the bony layers which are no longer protected by the cushioning effect of cartilage. As a person ages, this cartilage can wear away, leading to joint pain and swelling that is sometimes only alleviated by surgery. The subchondral bone contains blood vessels and nerves; while the articular cartilage itself is avascular and aneural (Fox et al., 2009). Planar joints do not rotate, but allow two, relatively flat bone surfaces to glide across one another. This so-called molecular marker technology (biomarkers) is intended to provide new means of assaying ongoing active processes in, Biology of Normal Joint and Evaluation of the Joint Including Clinical, Imaging, and Pathologic Evaluation, Melanie Fisher, ... Caroline N. Dealy, in, The Role of Subchondral Bone in Osteoarthritis, Johanne Martel-Pelletier, ... Jean-Pierre Pelletier, in, Postoperative Cartilage Repair Rehabilitation, Tissue Engineering and Regenerative Medicine: Applications, Arthroscopy: The Journal of Arthroscopic & Related Surgery. What are the (4) functions of articular cartilage? It covers the end of the femur and acetabulum. Conventional surgical techniques of cartilage repair are partially successful in alleviating symptoms, but fail to regenerate tissue anywhere similar in nature to native articular cartilage. Articular cartilage locations are found throughout the body. Chondrocytes are rounder in this second layer. Copyright © 2020 Elsevier B.V. or its licensors or contributors. Hypertrophic chondrocytes, which secrete mineralized matrix, are present in the deepest region of the deep zone, where it meets the subchondral bone. If a portion of ‘wet’ articular cartilage with all of its layers was separated into the most important individual elements, water would provide 65–80% of its weight, type II collagen fibrils would account for 10–20% (along with very small percentages of other collagen types), and 10–15% would be made up primarily of Aggrecan, but also other proteoglycans. All diarthroidal joints, in which articular cartilage is found, have certain characteristics. allow movement of bony surfaces with minimal friction (minimal wear) improve fit of articular surfaces. Sah, in Comprehensive Biomaterials II, 2017. One is rounded, the other hollowed. Thus, the knee joint is supplied by branches from the femoral, sciatic, and obturator nerves, which among them supply the various muscles moving the joint. The joint range of movement depends on the specific type of diarthroidal joint. articular cartilage is 2 to 4 mm thick in length. What is Articular Cartilage? At this level there are fewer proteoglycans and higher numbers of organized collagen fibrils. The cartilage is structurally divided into three zones, each with a unique cellular morphology and arrangement of the type II collagen fibers ( Fig. This chapter outlines the architectural and compositional features of articular cartilage. Here, the chondrocytes begin to form columns along an axis of collagen fibrils. The first type is called hyaline cartilage. The successful management of articular cartilage lesions is challenging for both the surgeon and patient. This venous plexus is very vulnerable to compressive or shearing forces. Animal studies using mTOR inhibitors or genetic deletion show protection from cartilage-degenerative mechanisms in mouse models of OA, suggesting the therapeutic potential of inhibiting the mTOR signaling pathway to achieve articular cartilage homeostasis [1,3]. It is also in direct contact with the synovial fluid of the joint capsule. As discussed earlier, some of the fragments released will activate immune responses with the potential to further enhance disease activity. The subchondral bone plate possesses an irregular surface to which the articular cartilage is keyed in. While the PCM was first observed nearly a century ago, its role is still under investigation. distribute loads over a wide area. Thus, matrix also plays a role in recording a loading history of the articular cartilage. It can absorb compressive forces, but not to the same degree as the underlying radial zone. The structure and cellular features of adult articular cartilage are uniquely designed to create and maintain a tissue with maximal durability and stability. Melanie Fisher, ... Caroline N. Dealy, in Encyclopedia of Biomedical Engineering, 2019. Articular cartilage is the hyaline cartilage. Is healthy articular cartilage vascularized? Cartilage is a non-vascular type of supporting connective tissue that is found throughout the body . With the ability to move along two axes (up and down, side to side), ellipse-shaped bone surfaces – one concave and one convex – are found primarily in the hand and wrist, and the foot. The articular cartilage covers the articulating bony surfaces and is composed of hyaline cartilage. The typical response to cartilage injury in which the subchondral plate is fractured is the formation of fibrocartilage, a scarlike tissue unsuited to… The remaining 30% of the articular cartilage is the deep zone. By using sensitive immunoassays such fragments can be quantified in synovial fluid or serum. In ball and socket joints, one bone surface is almost spherical in form and the other distinctly and deeply concave, providing a connection which provides maximum range of movement with a lower risk of dislocation. This structure is at right angles to the underlying bone – an arrangement that provides the most resistance to compressive forces. From: Comprehensive Biomaterials II, 2017. Therefore, therapies that can promote chondrocyte survival and stop or delay the process of cartilage degeneration are not available clinically. The articular cartilage allows low-friction joint movement, absorbs the biomechanical forces, and stabilizes the joint. Articular cartilage is a pressurized structure that is similar conceptually to an air tent. Although bone is a harder tissue than articular cartilage it is actually a better shock absorber and hence the normal subchondral bone tissue protects articular cartilage against damage caused by excessive loads. AC is a connective tissue that covers the ends of long bones and functions as a load-bearing, low-friction, and wear-resistant surface to facilitate joint movement. Since articular cartilage is a postmitotic tissue, it depends on processes such as autophagy to remove damaged organelles, misfolded or aggregated proteins, as well as eliminating pathogens in response to stress. While the intricate ultrastructure of articular cartilage is critical for proper function, these same properties create challenges for cartilage restoration. It also allows gliding motions. Articular cartilage’s role is often better understood in patients in which it has degenerated (usually due to the ageing process or intensive sports). ScienceDirect ® is a registered trademark of Elsevier B.V. ScienceDirect ® is a registered trademark of Elsevier B.V. URL: https://www.sciencedirect.com/science/article/pii/B9780123814227100422, URL: https://www.sciencedirect.com/science/article/pii/B978032302588150052X, URL: https://www.sciencedirect.com/science/article/pii/B978032309138100005X, URL: https://www.sciencedirect.com/science/article/pii/B978012386456704301X, URL: https://www.sciencedirect.com/science/article/pii/B9780128027332000190, URL: https://www.sciencedirect.com/science/article/pii/B9780128012383641193, URL: https://www.sciencedirect.com/science/article/pii/B9780323039291500070, URL: https://www.sciencedirect.com/science/article/pii/B9781437708783100245, URL: https://www.sciencedirect.com/science/article/pii/B9780128035818093048, Principles of Regenerative Medicine (Second Edition), In processes resulting in destruction of cartilage tissue, proteolytic enzymes degrade ECM proteins. A lesser-known example is the sternoclavicular joint. coefficient (Figure 1). Thus, it should be possible to identify the activity of a process in the meniscus on the one hand and activity in cartilage on the other. Articular Capsule: Function. Chondrocytes in articular cartilage are surrounded by a narrow pericellular matrix (PCM) that is both biochemically and biomechanically distinct from the extracellular matrix (ECM) of the tissue. Articular cartilage serves a vital role in maintaining joint function by minimising friction and distributing load. The main function of the middle zone is shock absorption. cartilage exhibits stress-shielding of the solid matrix components due to its high water content, the incompressibility of water, and the structural organization of the proteoglycan and collagen molecules. These sinusoids terminate in venous radicles forming a transverse sinus, perpendicular to the tiny venous branches. Note the shiny hyaline layer in the humeroulnar joint of the elbow below. The ECM consists mainly of proteoglycan and collagens.The main proteoglycan in cartilage is aggrecan, which, as its name suggests, forms large aggregates with hyaluronan.These aggregates are negatively charged and hold water in the tissue. The trabecular subchondral bone is situated under this thin end-plate zone. The deep zone is divided by the tidemark into an upper hyaline cartilage portion that is continuous with the lower middle zone, and extends its linear arrangement of collagen fibrils and chondrocyte stacks; and a lower region which is calcified (mineralized). Articular cartilage is a white, smooth tissue which covers the ends of bones in joints. The structure of articular cartilage into three zones with different characteristics allows for an efficient, load-bearing surface which distributes compressive forces generated during diarthroidal joint loading and diarthroidal joint motion. Articular cartilage has a limited capacity for self-regeneration after injury. This hyaline cartilage is known as articular cartilage. 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. Cartilage is not innervated and therefore relies on diffusion to obtain nutrients. Articular cartilage is usually found in layers of between 2 and 4 mm thick. introduced a cell-based therapy in which culture-expanded chondrocytes were transplanted into defects, raising the expectations of a breakthrough in repairing damaged articular cartilage (Brittberg et al., 1994). Ball and socket joints are found in the hips and shoulders. Some of the fragments formed will no longer be retained in the tissue but are released to surrounding body fluids. The term hyaline comes from the Greek word “hyalos,” which means glassy. Any pain is not caused by damage to the cartilage itself, as cartilaginous connective tissues are aneural (without a nerve network). 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