In anatomy, articulation refers to movement of adjacent bones around a joint between them. When used as a noun, articulation is synonymous with the term joint itself. An example can be seen in the acetabulofemoral articulation, or hip joint, where the femur bone in the thigh articulates with the pelvic bones in the hip. Action at a joint as initiated by the muscles that cross it are what make motion possible, and the type of articulation present dictates what types of motion are possible at that joint. Specifically, it is the shape of the articulating surfaces of the adjacent bones at a joint that determine which movements that joint can produce.
There are three categories of articulation in the human body, classified according to their structure and function — synarthroses, which generally do not move; amphiarthroses, which allow a small degree of movement; and diarthroses, which may move in several directions. Synarthroses refer mostly to bones in the head, as in the large flat bones of the skull, which are held together by dense fibrous tissue. Amphiarthroses include the joints between the vertebrae and those between the tibia and fibula bones in the lower leg. These rely on cartilage to hold the bones together.
Synovial joints are considered the “movable” joints. These are distinguished by a sac of lubricating fluid inside of the joint known as synovial fluid that minimizes bone-on-bone friction during movement. The most numerous of the body’s joints, these articulations are classified according to the movements they allow. Hinge or ginglymoid joints only permit flexion and extension, or bending and straightening, as in the elbow joint. Pivot or trochoid joints allow rotation, as when the top two vertebrae, the atlas and axis, turn around each other to turn the head from side to side.
Planar or arthrodial joints permit a slight gliding motion between bones. Examples of this kind of joint include the carpals in the hands and the tarsals in the feet. Condyloid joints, so named for the oval shape of the adjoining bones, allow a wider range of motion, as they feature a bony head that is cupped by the end of another bone, such as in the wrist joint. Movements at this articulation include flexion and extension, adduction and abduction, or the waving of the hand from side to side, and circumduction, or circling the hand on the wrist.
Two additional categories of synovial joint include the saddle joints and ball-and-socket joint. Saddle or sellar joints include the thumb joint and are distinguished by the opposing shapes of the adjacent bones, which resemble two saddles curved around each other in perpendicular directions, as if to form an X. Movements possible at this type of joint include flexion and extension, adduction and abduction, circumduction, and, in the case of the thumb, opposition, or the action of folding the thumb toward the palm of the hand as in touching the tips of the four fingers.
Ball-and-socket joints, seen at the hip and shoulder, allow the fullest range of motion of all of the body’s joints. These allow the limb to circumduct 360 degrees about the joint. All of the above motions are possible at this articulation, as is rotation, or the turning of the limb back and forth within the joint.