The body is designed to move, and movement is critical to good health. To accomplish movement, the body is made up of joints where two bones come together and articulate. The joint allows for motion in one or more planes, and all of the body's typical 206 bones articulate with another bone, with the sole exception of the hyoid bone, which is located in the neck. Some joints in the body move very little or not at all, but the joints most easily recognized are synovial joints. These synovial joints are characterized by a fibrous capsule that surrounds the joint articulation enclosing the rich synovial fluid, which is designed to maintain the health of the joint. There are at least five types of synovial joints.
Ball and Socket
The ball and socket joint is the most obvious joint type because it allows for the most amount of movement in the greatest number of directions. Ball and socket joints are formed by a ball-shaped end of one bone fitting into a socket on another bone with a tough labrum of connective tissue that helps keep the ball in the socket. Examples of a ball and socket joint include the hip and shoulder, both of which are capable of rotation, flexion and extension and lateral movement. These joints are extremely mobile but also have the least amount of stability, causing them to be susceptible to injury.
Hinge
The hinge joint is formed by one bone having a concave surface and the adjoining bone having a complementary convex surface. These types of joints are able to move in only one plane of motion-flexion/extension. One example of the hinge joint is the elbow, which is controlled by the biceps muscle on one side, creating flexion, and the triceps on the other, moving the elbow into extension.
Pivot
The pivot joint also is able to move in only one direction, such as rotation. At the top of the spine, the atlas and axis vertebrae meet, and the axis has a point of bone that sticks up, called the dens. The dens forms a joint with the main body of the atlas and is held in place by a fibrous ring. The atlas pivots around the dens allowing the head to rotate 90 degrees in each direction.
Condyloid
The condyloid synovial joint is characterized by one side of the joint being in the shape of an oval and the other side being a small open socket or cavity. This joint allows motion in several planes, but is unable to accomplish rotation. Examples of this joint in the human skeleton are those found in the palm of the hand, called metacarpal-phalangeal joints, and those in the foot, called metatarsal-phalangeal joints.
Saddle
A saddle joint is named appropriately as it is shaped like a saddle, allowing motion over a wide range. According to the Minnesota State University Anatomy Department, the only saddle joint in the human body is found in the thumb.
