 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 they move in the frontal plane.
- Saddle joints. Here, both bones have a saddle-shaped surface—concave in one direction (like the front-to-rear curvature of a horse’s saddle) and convex in the other (like the left-to-right curvature of a saddle). The clearest example of this is the trapeziometacarpal joint between the trapezium of the wrist and metacarpal I at the base of the thumb. Saddle joints are biaxial. The thumb, for example, moves in a frontal plane when you spread the fingers apart, and in a sagittal plane when you move it as if to grasp a tool such as a hammer. This range of motion gives us and other primates that invaluable anatomical hallmark, the opposable thumb. Another saddle joint is the sternoclavicular joint, where the clavicle articulates with the sternum. The clavicle moves vertically in the frontal plane at this joint when you lift a suitcase, and moves horizontally in the transverse plane when you reach forward to push open a door.
- Plane (gliding) joints. Here the bone surfaces are flat or only slightly concave and convex. The adjacent bones slide over each other and have relatively limited movement. Plane joints are found between the carpal bones of the wrist, the tarsal bones of the ankle, and the articular processes of the vertebrae. Their movements, although slight, are complex. They are usually biaxial. For example, when the head is tilted forward and back, the articular facets of the vertebrae slide anteriorly and posteriorly; when the head is tilted from side to side, the facets slide laterally. Although any one joint moves only slightly, the combined action of the many joints in the wrist, ankle, and vertebral column allows for a significant amount of overall movement.
- Hinge joints. These are essentially monaxial joints, moving freely in one plane with very little movement in any other, like a door hinge. Some examples are the elbow, knee, and interphalangeal (finger and toe) joints. In these cases, one bone has a convex (but not hemispherical) surface, such as the trochlea of the 