Anatomy of Joints
The experience of youthfulness, at any age, is determined by the condition of your joints more than by any other system of the body. When joints are in a healthy condition, the body moves easily. This agility is characteristic of youth and of people who have maintained the health of their joints over their lifetimes.
A joint is present wherever two bones meet each other. There are movable and immovable joints. Immovable joints are not designed to move in relationship to each other. If they do, breakage occurs. Immovable joints are divided into two categories. One is the rigid and absolutely immovable type of joints known as “synarthroses” (literally together joints). For example, the connection between the various bones of the skull and the attachment of the different bones of the pelvis, these are synarthroses. These bones are held together by actual intergrowth of the bones and by strong fibrous cartilage, which cannot be stretched. The other kind of “immovable” joints are called “symphyses,” an example of which is the pubic symphysis, the joint which binds the two pubic bones together in the pubic symphysis, the anterior center part of the pelvis. Symphyses are held together by elastic (stretchable) cartilage.
The rest of the joints are freely movable, according to their design, and are known as “diarthroses” (movable joints). They include the joints of the extremities (arms and legs) and the vertebral column. Diarthroses are lined with a very smooth, glass-like cartilage, which is lubricated by a thick fluid, the synovial fluid, which is produced by the synovial membrane — the lining of the joint. These joints make us what we are — without them we would be rigid like statues. There are several types of movable joints (diarthroses). One type of diarthrosis is called “ball and socket.” These are found in the shoulders and hips where the arms and legs attach to the trunk. These joints allow free movement in all directions. Without them, we would walk and move as the robot C3PO of Star Wars. Hinge joints allow movement in flexion and extension and are found in elbows, knees and fingers. A pivot joint allows only for rotation, and there is only one in the human body: the junction between atlas and axis (the first and second vertebrae of the spinal column). In this joint rests your ability to rotate your head from side to side. Gliding joints, in which the surfaces of the joints slide over each other for short distances, are found in the carpal and tarsal bones of the wrist and ankle respectively.