Synovial Joint Classification
Plane (gliding) joints
Major characteristics: 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 vertbral column allows for a significant amount of overall movement.
Saddle joint
Major characteristics: 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 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.
Hinge joint
Major characteristics: 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 humerus and the condyles of the femur. This fits into a concave depression on the other bone, such as the trochlear notch of the ulna and the condyles of the tibia.
Pivot joint
Major characteristics: These are monoaxial joints in which the bone spins on its longitudinal axis. There are two principal examples: the atlanoaxial joint between the first two vertabrae, and the radioulnar joint at the elbow. At the atlantoaxial joint, the dens of the axis projects into the vertabral foramen of the atlas and is held against the anterior arch of the atlas by the trnasverse ligament. As the head rotates left and right, the skull and atlas pivot around the dens. At the radioulnar joint, the anular ligament of the ulna wraps around the neck of the radius. During pronation and supination of the forearm, the disclike radial head pivots like a wheel turning on its axle. The edge of the wheel spins against the radial notch of the ulna like a car tire spinning in the snow.
Ball-and-Socket joint
Major characteristics: 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) joint
Major characteristics: 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.
