Articular Cartilage Structure and Function

Articular cartilage is the layer covering the joint surfaces. Its main function is to produce smooth articulations among the joint surfaces.

In most joints, adult human articular cartilage is of the hyaline type. The thickness of human articular cartilage varies from area to area within a single joint and from joint to joint. In large joints, the thickness is 2 mm to 4 mm. Knowing the structure of articular cartilage is important so that abnormality could be noted.

Structure of Articular Cartilage

Articular cartilage structure  consists of different molecular components consisting of  four zones include:

Superficial or tangential zone

This is the smallest zone that constitutes 10 percent of the cartilage. Collagen fibers are thin and are oriented horizontally to subchondral bone. Low glycan content. This zone is called the lamina splendens.

Middle or transitional zone

it is the largest zone that constitutes 50% of the cartilage. Collagen fibers are thicker and start to be arranged into radial bundles. It has got High proteoglycan and water content.

Deep or radial zone

It constitutes 20% of the cartilage. it carries largest collagen fibers arranged perpendicular to subchondral bone and possesses many chondrocytes.

Calcified zone

This zone separates cartilage from the subchondral bone. Collagen fibers penetrate into this zone and anchor the cartilage to the bone.

Tidemark is a basophilic line which straddles the boundary between calcified and uncalcified cartilage;

Articular cartilage is avascular and lacks nerve structures.

Microscopic Composition of Articular Cartilage

The tissue is composed of a relatively small number of cells and abundant extracellular matrix. Chondrocyte activity is necessary for the synthesis of the matrix and probably its physiologic degradation and removal.

Extracellular Matrix

The matrix is responsible for maintaining the homeostasis of the environment and is the main component providing the biomechanical properties of the articular cartilage.

The matrix contains a large amount of water, a network of collagen fibers, and a ground substance composed mainly of carbohydrate and noncollagenous protein. A small amount of lipid and inorganic compounds is also present

• Water makes up 65% to 80% of the mass of the cartilage and accounts for 80% of the weight near the surface. In the deep zone, it is about 65%.
  Water content decreases with normal aging and increases with osteoarthritis. Increased water content leads to increased permeability and decreased strength
• Collagen makes up 10 to 20% of total cartilage mass. Type II is most abundant, collagen accounting for 90% to 95% of the total collagen content. Small amounts of types V, VI, IX, X, and XI collagen are also present.
  Collagen provides the framework and tensile strength.
• Proteoglycans make up 10 to 15% of cartilage and provide compressive strength. Proteoglycans also retain water and aggrecan is the most responsible for hydrophilic behavior.
  Proteoglycans are produced by chondrocytes and are composed of glycoaminoglycans subunits which are mainly chondroitin sulfate and keratin sulfate.

Noncollagenous protein is also present in the matrix

The pericellular matrix contains a delicate meshwork of fine filaments. In mature adult articular cartilage, there is a sharply defined boundary between the pericellular and the intercellular matrix. This zone contains a high concentration of protein and glycoprotein. The collagen fibers at this site are arranged circumferentially to form a strong enclosure about the chondrocyte.

Cells

Cells of the cartilage are chondrocytes which are derived from their precursor cells called chondroblasts. These cells produce collagen, proteoglycans, and enzymes.

They respond to mechanical stimuli like load and chemical stimuli like growth factors.

Immature articular cartilage also has stem cells but adult articular does not.

Chondrocytes are involved with the physiologic turnover of extracellular ground substance, which is responsible for the biomechanical and biologic properties of this tissue. Chondrocytes exhibit different properties in different zones.

Lubrication of Articular Surfaces

Joint cartilage is lubricated by various mechanisms

Synovial joints serve as mechanical bearings with low coefficients of friction. Their three major sources of lubrication are:

Hydrodynamic Lubrication

Loading of the articular cartilage causes compression that forces water out of the cartilage. This fluid forms an aqueous layer that separates and protects the opposing surfaces.

Boundary-Layer Lubrication

A small glycoprotein called lubricin, which is produced by synovial lining cells, binds to articular cartilage where it retains a protective layer of water molecules.

Hyaluronic Acid

It is produced by synovial lining cells and lubricates the contact surface between synovium and cartilage.

Nutrition of Articular Cartilage

Adult cartilage is avascular, and chondrocytes obtain nutrients through diffusion. The nutrients are derived from the synovial fluid whose diffusion is facilitated during joint loading. With joint loading, some of the water in cartilage is squeezed out into the synovial space. When the joint is unloaded, the hydrophilic properties of the cartilage proteoglycans cause the water to be sucked back into the cartilage. As the water returns to the cartilage, diffusion of nutrients from the synovial fluid is facilitated.

In normal articular cartilage, chondrocytes rarely divide. Chondrocytes synthesize and replace the extracellular matrix components. Proteoglycans have a faster turnover rate compared with collagen. The degradation of these macromolecules is accomplished by proteolytic enzymes like Metalloproteases, such as collagenases , stromelysins  and gelatinases.

Cytokines such as interleukin-1 and tumor necrosis factor can increase or upregulate the degradative process, whereas transforming growth factor and insulin-like growth factor-1 have an opposite effect on chondrocyte metabolism.

Articular cartilage breakdown can be detected by assays using monoclonal antibodies to measure type II collagen and proteoglycans in bodily fluids.

In osteoarthritis, the patients experience pain due to irritation of the subchondral bone, which gets exposed as the cartilage degenerates. Additionally, accumulation of synovial fluid can cause pain through distention of the innervated joint capsule and synovium. Accompanying mild synovial inflammation also causes pain.