Joint

Joints are structures in the body that provide movement and mechanical support [3]. Although there are several types of joints in the human body, this article focuses on synovial joints, such as those in the knees, arms, and shoulders. These joints, found at the ends of bones, have a space that allows for a wide range of motion [3]. Formed by endochondral ossification, joints are strengthened by a dense fibrous capsule that is reinforced by ligaments and muscles [3]. The capsule is filled with synovial fluid, a clear liquid that contains hyaluronic acid, a lubricant that also provides nutrients to the joint tissues [3].

The surfaces where two bones meet are covered with articular cartilage. Articular cartilage consists of four layers of tissue (Fig. 1). First, a thin superficial layer provides a smooth surface for two bones to slide against each other. The second layer is very resistant to shear stresses. An intermediate layer is mechanically designed to absorb shock and distribute load or weight efficiently. The fourth or deepest layer is highly calcified and anchors the articular cartilage to the bone.

A unique aspect of articular cartilage is the isolation of its component cells from each other and from other cell types. It is one of the few tissues in the

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3 Superficial zone (fibers parallel to surface) Middle zone (random fibers)

^-Tidemark (calcification line) Calcified zone

Subchondral bone

Cancellous bone

Deep zone

(fibers perpendicular to surface)

Fig. 1. Layers of cartilage in a joint. (Courtesy of Netter Images. Available at: www. NetterImages.com.)

Articular cartilage matrix with regional organization based on chondrocyte proximity and matrix composition (high power)

Collagen fibrils form structural framework for articular cartilage and provide support for chondrocytes and proteoglycan aggregates

3 Superficial zone (fibers parallel to surface) Middle zone (random fibers)

^-Tidemark (calcification line) Calcified zone

Subchondral bone

Cancellous bone

Deep zone

(fibers perpendicular to surface)

Fig. 1. Layers of cartilage in a joint. (Courtesy of Netter Images. Available at: www. NetterImages.com.)

human body that does not have its own blood supply. It obtains its nutrition principally from diffusion of synovial fluid in the synovial cavity [4].

Articular cartilage is able to provide support and flexibility because of the structure of its extracellular matrix [5]. This matrix contains proteoglycans, which are responsible for the compressive stiffness of the tissue and its ability to withstand load, and type 2 collagen, which provides tensile strength and resistance to shear [6], water, chondrocytes, and other molecules [3]. The collagen fibers are arranged in arches, a horizontal orientation near the surface of the cartilage. This orientation allows the cartilage to resist stress and to transmit weight [3].

The water and proteoglycans provide cartilage with elasticity and play a crucial role in reducing friction. Most proteoglycans in articular cartilage are in the form of aggrecan, aggregates of proteoglycan monomers bound to a hyaluronic acid backbone by a noncovalent association with a link glycoprotein. The highly charged, polysulfated glycosaminoglycan components of the aggrecan molecules attract cations and water, resulting in osmotic pressure in the tissue owing to the constraint of the molecular configuration caused by containment within the collagen meshwork [7].

The chondrocytes maintain a balance between production and degradation of cartilage extracellular matrix [3]. Matrix turnover is modulated by chondrocytes that secrete degradative enzymes and enzyme inhibitors [3]. The number and activity of chondrocytes affect the anatomic and tribologic features of cartilage [8]. The chondrocyte itself is regulated by various cytokines and growth factors that can alter the homeostatic balance toward an anabolic or catabolic direction [9,10].

Most load on articular cartilage is produced by contraction of the muscles that stabilize and move the joints [6]. Although cartilage is an excellent shock absorber, it is usually 1 to 2 mm thick in most parts of the joint, which is too thin to serve as the only shock-absorbing tissue in the joint. Subchondral bone and periarticular muscles provide additional protective effects [6].

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