Porth's Essentials of Pathophysiology, 4e

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Musculoskeletal Function

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Fibrous Joints (Synarthroses) Fibrous joints, or synarthroses, are joints in which the bones are joined by dense fibrous tissue. They have little or no movement, depending on the length of the fibers connecting the articulating bone. The sutures of the skull are examples of fibrous joints. In children and young adults the bones of the skull are joined by dense connec- tive tissue; and in older persons, by bone. Another type of fibrous joint that is partially moveable unites bones with a sheet of fibrous tissue, either a ligament or fibrous mem- brane. The interosseous membrane that joins the radius and the ulna of the forearm is an example of this type of joint. A unique type of fibrous joint is the one between the root of a tooth and the alveolar processes of the jaw. The fibrous tissue between the tooth and alveolar bone of the tooth socket is called the periodental ligament. Cartilaginous Joints (Amphiarthoses) Cartilaginous joints, or amphiarthroses, are connected by hyaline cartilage or fibrocartilage and have limited motion. In primary cartilaginous joints, the bones are joined by hyaline cartilage, which permits slight bending during early childhood. Primary cartilaginous joints are usually temporary articulations, such as those present during early development, where the epiphysis and the shaft are joined by cartilage or growth plate. Secondary cartilagi- nous joints are strong, slightly moveable joints united by fibrocartilage. The fibrocartilaginous intervertebral disks between the vertebrae consist of connective tissue that holds the vertebrae together. These joints provide strength and shock absorption as well as considerable flexibility. Synovial Joints (Diarthroses) Synovial joints, or diarthroses , are freely moveable joints in which the articulating bone ends are separated by a joint cavity containing synovial fluid. Most joints in the body are of this type. Although as a group they are classified as freely moveable, synovial joints include pla- nar joints, which allow almost no movement (e.g., ver- tebrocostal joint), and hinge joints, which allow angular movement in one plane (e.g., interphalangeal, knee, and ankle joints). Only the ball-and-socket joints permit movement in all directions (e.g., shoulder and hip joint). All synovial joints are covered with articular cartilage, a feature that allows the surfaces of these joints to slide freely past each other during movement. The articular cartilage, which is typically hyaline cartilage, is unique in that its free surface is not covered with perichon- drium. It has only a peripheral rim of perichondrium, and calcification of the portion of cartilage abutting the bone may limit or preclude diffusion from blood vessels supplying the subchondral bone. Regeneration of most cartilage is slow; it is accomplished primarily by growth that requires the activity of perichondrium cells. In synovial joints, the articulating ends of the bones are not connected directly but are indirectly linked by a strong fibrous capsule (i.e., joint capsule) that sur- rounds the joint and is continuous with the periosteum

Articular cartilage Fibrous capsule Joint cavity

Synovial membrane

Articular cartilage

FIGURE 42-7. Synovial (diarthrodial) joint, showing the articular cartilage, fibrous joint capsule, joint cavity, and synovial membrane.

(Fig. 42-7). The joint capsule consists of two layers: an outer fibrous layer and an inner synovial membrane. The outer fibrous layer of the capsule supports the joint and helps to hold the bones in place. Additional sup- port is provided by ligaments that extend between the bones and by tendons that attach to muscles supplying the joint. The synovium secretes a slippery fluid with the consistency of egg white called synovial fluid. In addition to supplying nutrients and oxygen to the chondrocytes in the articular cartilage, this fluid serves as a lubricant for the joint. Moreover, macrophages in the synovial fluid act to phagocytize debris in the joint space. The collagen fibers in the articular surface cartilage are arranged to evenly distribute the forces generated by pressure to this tissue. The proteoglycan molecules of the cartilage, found isolated or aggregated in a network, contain large amounts of water. These matrix compo- nents, rich in hydrophylic glycosaminoglycans, function as a mechanical spring. When pressure is applied, water is forced out of the cartilage matrix into the synovial fluid. When the pressure is released, water is attracted back into the collagen matrix. These water movements, which are brought about by the use of a joint, are essen- tial for the delivery of nutrients and the exchange of car- bon dioxide, oxygen, and other molecules between the synovial fluid and articular cartilage. Ligaments andTendons Ligaments and tendons are dense connective tissue structures that connect muscles and bone and support synovial joint structures. The dense connective tissue found in tendons and ligaments has a limited blood sup- ply and is composed largely of intercellular bundles of collagen fibers arranged in the same direction and plane. Collagen is an inelastic and insoluble fibrous protein. Because of its molecular configuration, collagen has great tensile strength; the breaking point of collagenous fibers found in human tendons is reached with a force of several hundred kilograms per square centimeter.