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.
Joints are classified according to the amount of movement they permit (fig. 1-26). Joint classifications are as follows:
Figure 1-26.—Example of immovable, slightly movable, and 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.
Figure 1-27.—Types of joints:
A. Ball-in-socket joint; B. Condyloid joint; C. Gliding joint; D. Hinge joint; E. Pivot joint; F. Saddle joint.
TYPES OF JOINT MOVEMENTS
Joint movements are generally divided into four types: gliding, angular, rotation, and circumduction.
Gliding. Gliding is the simplest type of motion. It is one surface moving over another without any rotary or angular motion. This motion exists between two adjacent surfaces.
Angular .Angular motion decreases or increases the angle between two adjoining bones. The more common types of angular motion are as follows:
Rotation. Rotation is a movement in which the bone moves around a central point without being displaced, such as turning the head from side to side.
Circumduction. Circumduction is the movement of the hips and shoulders.
Other Types of Movement
Other types of movement generally used to indicate specific anatomical positions include the following: