In the human body, a joint is any area where two or more bones contact each other. The joints, or articulations, connect the skeletal bones together and help the body move. There are three classes of joints in the body. Some move freely, others move slightly and some are fixed in place. Joints are classified by how they allow movement; the major classifications include synathroses, amaphiarthroses and diarthroses.
Synathroses
Synathroses are joints that do not move at all. The bones almost make contact with each other, leaving a thin layer of tissue or cartilage between them. The ones in the skull are examples of this type of joint. The bones appear fused, except for the suture lines running between them.
Amaphiarthroses
The amaphiarthroses are slightly movable joints. The bones of the spine, called vertebrae, are amaphiarthroses. The ribs connect to the breastbone, or sternum, with this same class of joint. Hyaline cartilage connects the bones together in amaphiarthroses joints and allows a slight bit of movement.
Diarthroses
The largest classification of joints in the body is the freely moving joints, called diathroses joints. These joints, connected with ligaments and cushioned with a fluid membrane, are further classified depending on the movement they allow. There are six types of diarthroses joints in the body, including the ball and socket, hinge, pivot, condyloid, saddle and gliding joints.
Ball-and-socket joints allow the greatest range of movement. They allow the body to rotate freely. One bone in the joint is shaped like a ball on the end and the other bone is cup-shaped, in order to receive the ball. Examples of ball-and-socket joints are hips and shoulders.
Hinge joints allow movement in one direction, much like a hinge on a door. The joint allows the bones to move forward and back, but not other directions. Examples of hinge joints include elbows and knees.
Condyloid joints are shaped as ovals and fit into an elliptical cavity. These joints allows the bones to move, but they are not able to rotate. Examples of this joint in the body are the bones in the hand and fingers and between the bones in the foot and toes.
Pivot joints, found in the top of the spine, allow movement in a rotating motion. These joints allow the head to turn side to side.
Saddle joints are only in one location in the body. The thumb has this type of joint. Both of the bones in this joint have convex and concave areas. This allows for all movements except pivoting. The saddle joints help the hand grasp an object.
Gliding joints are joints that allow the bones to slide or twist against one another without the ability to move other directions. The wrist and ankle are areas that have gliding joints.
