LEARNING OBJECTIVE: Recognize joint classifications and identify joint movements for the key joints in the body.

Wherever two or more bones meet, a joint is formed. A joint binds various parts of the skeletal system together and enables body parts to move in response to skeletal muscle contractions.

JOINT CLASSIFICATIONS
Joints are classified according to the amount of movement they permit (fig. 1-26). Joint classifications are as follows:

Immovable. Bones of the skull are an example of an immovable joint. Immovable joints are characterized by the bones being in close contact with each other and little or no movement occurring between the bones.

Slightly movable. In slightly movable joints, the bones are held together by broad flattened disks of cartilage and ligaments (e.g., vertebrae and symphysis pubis).

Figure 1-25.-The foot: A. Lateral view of foot; B. Right foot viewed from above.

Freely movable. Most joints in the body are freely movable joints. The joint consists of the joint capsule, articular cartilage, synovial membrane, and synovial (joint) cavity (fig. 1-26). There are six classifications of freely movable joints: ball-in-socket, condyloid, gliding, hinge, pivot, and saddle joints (fig. 1-27). These joints have much more complex structures than the immovable and slightly movable joints. The ends of the bones in this type of joint are covered with a smooth layer of cartilage. The whole joint is enclosed in a watertight sac or membrane containing a small amount of lubricating fluid. This lubrication enables the joint to work with little friction. Ligaments (cords or sheets of connective tissue) reach across the joints from one bone to another and keep the bone stable. When ligaments are torn, we call the injury a sprain; when bones are out of place, we refer to this as a dislocation; and when bones are chipped or broken, the injury is called a fracture.