 1 to the sternum. (b) The pubic symphysis. (c) Intervertebral discs, which join adjacent vertebrae to each other by symphyses.
A synchondrosis is a joint in which the bones are bound by hyaline cartilage. An example is the temporary joint between the epiphysis and diaphysis of a long bone in a child, formed by the cartilage of the epiphyseal plate. Another is the attachment of the first rib to the sternum by a hyaline costal cartilage. The other costal cartilages are joined to the sternum by synovial joints.
In a symphysis, two bones are joined by fibrocartilage. One example is the pubic symphysis, in which the right and left pubic bones are joined anteriorly by the cartilaginous interpubic disc. Another is the joint between the bodies of two vertebrae, united by an intervertebral disc. The surface of each vertebral body is covered with hyaline cartilage. Between the vertebrae, this cartilage becomes infiltrated with collagen bundles to form fibrocartilage. Each intervertebral disc permits only slight movement between adjacent vertebrae, but the collective effect of all 23 discs gives the spine considerable flexibility.
The most familiar type of joint is the synovial joint, also called a diarthrosis. Ask most people to point out any joint in the body, and they are likely to point to a synovial joint such as an elbow, knee, or knuckle. Many synovial joints, like these examples, are freely mobile. Others, such as the joints between the wrist and ankle bones and between the articular processes of the vertebrae, have more limited mobility.
Synovial joints are the most structurally complex type of joint and are the type most likely to develop uncomfortable and crippling dysfunctions. They are the most important joints for such professionals as physical and occupational therapists, athletic coaches, nurses, and fitness trainers to understand well. Their mobility makes the synovial joints especially important to the quality of life. Reflect, for example, on the performance extremes of a young athlete, the decline in flexibility that comes with age, and the crippling effect of rheumatoid arthritis.
In synovial joints, the facing surfaces of the two bones are covered with articular cartilage, a layer of hyaline cartilage up to 2 or 3 mm thick. These surfaces are separated by a narrow space, the joint (articular) cavity, containing a slippery lubricant called synovial fluid. This fluid, for which the joint is named, is rich in albumin and hyaluronic acid, which give it a viscous, slippery texture similar to raw egg white. It nourishes the articular cartilages, removes their wastes, and makes movements at synovial joints almost friction-free. A connective tissue joint (articular) capsule encloses the cavity and retains the fluid. It has an outer fibrous capsule continuous with the periosteum of the adjoining bones, and an inner, cellular synovial membrane.
The synovial membrane is composed mainly of fibroblast-like cells that secrete the fluid, and is populated by macrophages that remove debris from the joint cavity. Joint capsules and ligaments are well supplied with lamellar corpuscles and other sensory nerve endings that enable the brain to monitor limb positions and joint movements.
In a few synovial joints, fibrocartilage grows inward from the joint capsule and forms a pad between the articulating bones. In the jaw (temporomandibular) joint, at both ends of the clavicle (sternoclavicular and acromioclavicular joints), and between the ulna and carpal bones, the pad crosses the entire joint capsule and is called an articular disc. In the knee, two cartilages extend inward from the left and right but do not entirely cross the joint. Each is called a meniscus because of its crescent-moon shape. These cartilages absorb shock and pressure, guide the bones across each other, improve the fit between the bones, and stabilize the joint, reducing the chance of dislocation.
Accessory structures associated with a synovial joint include tendons, ligaments, and bursae. A tendon is a strip or sheet of tough collagenous connective tissue that