Articular cartilage consists of hyaline cartilage. It is composed of a large extracellular matrix with only one cell type, the chondrocyte. Chondrocytes occupy less than 10% of the total volume of the cartilage. The matrix is composed of water, collagens (proteins with a characteristic triple-helical structure), proteoglycans (complex macromolecules that consist of protein and polysaccharides) and non-collagenous proteins and glycoproteins. The chemical composition and physical properties of the cartilage are primarily determined by the extracellular matrix.
Structural heterogeneity is reflected by changes in structure from the joint surface proceeding to the depth. These changes affect collagen fibre and chon-drocyte arrangement. There are four distinct histological zones or layers. There are no blood vessels, lymphatic vessels or nerves. The cartilage obtains nourishment from the synovial cavity by diffusion of fluid.
While it is meant to function over the lifetime of the joint, it only possesses a limited capacity for growth and repair. A limited process of remodelling occurs in articular cartilage in response to injury. This process originates in the chondro-cytes, which are responsible for synthesis and degradation of the extracellular matrix. However, this process does not restore the matrix to normal. Injury can result from acute and/or chronic loading. Acute injury is usually the result of excessive impact loading. Chronic injury results from either interfacial wear caused by impaired lubricating mechanisms, or from repetitive loading over a long period of time.
Cartilage is metabolically active. The chondrocytes secrete matrix molecules and metalloproteases capable of matrix degradation. The breakdown and synthesis of matrix by the chondrocytes is regulated by cytokines, including d Functions of intra-articular fibrocartilaginous menisci (present in the knee, wrist,
1 temporomandibular, acromioclavicular, sternoclavicular and costovertebral joints) e Load bearing, with weight distribution over a large surface;
h Shock absorption;
O Facilitation of rotatory movement;
g Limitation of translatory movement;
Protection of the articular surfaces; Joint lubrication.
interleukin-1 (with catabolic effect) and transforming growth factor-beta (with anabolic effect). Chondrocytes respond to changes in mechanical load by alteration of matrix, probably mediated through cytokine expression.
The functions of articular cartilage include:
Load bearing, with weight transmission and distribution over the underlying bone at high loads; Mechanical shock absorption;
Formation of articular surfaces of diarthrodial joints. It allows relative movements between articular surfaces with minimal friction.
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