have one, two, or three degrees of freedom of movement in the cardinal planes.
The number of axes around which the joint move- ment occurs depends on the structure of the bony joint surfaces. Because the surfaces are not pure geometrical shapes, the radius of curvature can vary at different points on the surface. These joint surfaces are described as either ovoid (egg-shaped) or sellar (saddle-like) in shape. Most diarthrodial joints are ovoid. The varying planes of movement and axis around which the movement occurs provide a basis for further subclassifi cation into uniaxial, biaxial, triaxial, and nonaxial joints (Fig. 2.11).
Uniaxial Joints
A uniaxial joint is a synovial joint with one degree of freedom, fl exion, and extension (see Fig 2.11A). The hinge joint is a classic example of a uniaxial joint. The joint moves in one plane around one axis. One of the joint surfaces is concave in shape, and the other surface is shaped like a spool or cylinder. There is a rotary movement of one bone in a direction perpendicular to the axis of rotation, similar to a door rotating around a hinge. There is also a slight sliding or gliding between the joint surfaces as the movement occurs. This accessory
Hinge joint
Atlas Pivot joint
Axis A Figure 2.11 Diarthrodial joints. (A) Uniaxial joints move in one plane around one axis. Continued
