Arthritis is a general term that describes inflammation in joints. Osteoarthritis, also called degenerative joint disease, is the most common type of arthritis. It is associated with a breakdown of cartilage in joints and commonly occurs in the hips, knees, and spine. Also, it often affects the finger joints, the joint at the base of the thumb, and the joint at the base of the big toe.
Cartilage is a firm, rubbery material that covers the ends of bones in normal joints.
The shock-absorbing quality of normal cartilage comes from its ability to change shape when compressed. Cartilage can change shape because it contains more than 70 percent water, which can be redistributed with movement. For example, when force is applied to a knee, as in standing or walking, some water from cartilage enters the joint and coats the cartilage. When the force is no longer present, such as when you sit down, the water is reabsorbed and the cartilage regains its normal shape. Because cartilage does not contain nerves, you do not feel pain when these changes in shape occur.