The bones of the skeleton support us and protect our internal organs. The skeleton also allows us to move by serving as the framework to which our muscles are attached. However, movement would not be possible if bones were rigidly attached to each other. A joint, or articulation, is the point at which the ends of two bones meet in the body.
Types
Although there are many different joints, they are divided into three basic types based on how much or how little they let the bones move. The three types are: fixed, or immovable; slightly movable; and freely movable.
Immovable Joints
Immovable, or fixed, joints, are connected by fibrous connective tissue. Their name is misleading because immovable joints do allow for a tiny amount of movement. Although the skull looks like one big piece of bone, it is actually pieced together by several immovable joints called "sutures." In infants, these joints allow the skull to go through the birthing process without being crushed and are necessary for growth of the skull. Once we have finished growing and have become adults, the sutures fuse together but still remain slightly flexible. There are two other types of immovable joints: syndesmoses, those between two bones that are close together, such as the borders between your arm and leg bones, and gomphoses, which hold your teeth in their sockets.
Slightly Movable Joints
Slightly movable joints are joined together by cartilage and allow for more movement than immovable joints. There are two types: synchondroses and symphyses. Synchondroses are temporary joints that allow the long bones of our bodies to grow. Once the bones are at their final length, the cartilage in these joints turns to bone (ossifies). Symphyses are permanent joints that are composed of disks joined together by cartilage--your vertebral column and the point where your pubic bones meet in the middle are examples of this kind of slightly movable joint.
Freely Movable Joints
Most of the familiar joints are freely movable, joined by cartilage and cushioned cavities filled with synovial fluid, which provides lubrication and nourishment. Within this category are six types that are very different based on their function. The six types of freely movable joints, along with an example to illustrate each, are pivot joints (neck), hinge joints (elbow), ellipsoidal joints (finger bones), ball-and-socket joints (shoulder), saddle joints (thumb), and gliding joints (wrist).
Arthritis
Nearly every joint in the body can be afflicted with arthritis, which is defined as inflammation of a joint. The three most common types of arthritis are osteoarthritis, rheumatoid arthritis, and gout. Osteoarthritis is caused by a wearing away of the cartilage cushions around joints, which is most commonly caused by aging, but can also be due to injury, heredity, or obesity. Rheumatoid arthritis is an autoimmune disease, which means that the body's immune system attacks its own tissues (in this case those surrounding the joints) as if they were foreign substances. Gout is a very painful form of arthritis that often begins in the big toe; this disorder can be caused by diet, alcohol, obesity, and certain medications.
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