exibility 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 attaches a muscle to a bone. Tendons are often the most important structures in stabilizing a joint. A ligament is a similar tissue that attaches one bone to another.
A bursa is a fibrous sac of synovial fluid located between adjacent muscles, where a tendon passes over a bone, or between bone and skin. Bursae cushion muscles, help tendons slide more easily over the joints, and sometimes enhance the mechanical effect of a muscle by modifying the direction in which its tendon pulls. Tendon (synovial) sheaths are elongated cylindrical bursae wrapped around a tendon, seen especially in the hand and foot. They enable tendons to move back and forth more freely in such tight spaces as the wrist and ankle.
Figure 4. Synovial joint
Classes of Synovial Joints
There are six fundamental types of synovial joints, distinguished by the shapes of their articular surfaces and their degrees of freedom. We will begin by looking at these six types in simple terms, but then see that this is an imperfect classification for reasons discussed at the end. All six types can be found in the upper limb. They are listed here in descending order of mobility: one multiaxial type (ball-and-socket), three biaxial types (condylar, saddle, and plane), and two monaxial types (hinge and pivot).
- Ball-and-socket joints. These are the shoulder and hip joints—the only multiaxial joints in the body. In both cases, one bone (the humerus or femur) has a smooth hemispherical head that fits into a cuplike socket on the other (the glenoid cavity of the scapula or the acetabulum of the hip bone).
- Condylar (ellipsoid) joints. These joints exhibit an oval convex surface on one bone that fits into a complementary shaped depression on the other. The radiocarpal joint of the wrist and metacarpophalangeal joints at the bases of the fingers are examples. They are biaxial joints, capable of movement in two planes. To demonstrate this, hold your hand with the palm facing you. Make a fist, and these joints flex in the sagittal plane. Fan your fingers apart, and th