Inside a joint, a tissue called cartilage cushions the joint and prevents the bones from rubbing against each other. Osteoarthritis occurs when the cartilage of a joint erodes (breaks down). Bones begin to rub against each other, causing pain and difficulty moving the joint. Osteoarthritis also can affect nearby bones, which can become enlarged in places. These enlargements are called bone spurs or osteophytes.
Although the term arthritis means joint inflammation, there is relatively little inflammation in the joints of most people with osteoarthritis. For this reason, and because this type of arthritis seems to be caused by age-related degeneration of the joints, many experts and health care professionals prefer to call it degenerative joint disease.
Osteoarthritis can range from mild to severe. The pain associated with osteoarthritis can be significant and it usually is made worse by movement. Osteoarthritis can be limited to one joint or start in one joint usually the knee, hip, hand, foot or spine or it can involve a number of joints. If the hand is affected, usually many joints of the fingers become arthritic.
Osteoarthritis probably does not have a single cause, and, for most people, no cause can be identified. Age is a leading risk factor, because osteoarthritis usually occurs as people get older. However, research suggests that joints do not always deteriorate as people age. Other factors seem to contribute to osteoarthritis. Sports-related injuries or repeated small injuries caused by repeated movements on the job may increase the risk of developing osteoarthritis. Genetics also plays a role. Obesity seems to increase the risk of developing osteoarthritis of the knees.
Other factors that increase the risk of osteoarthritis include:
Repeated episodes of bleeding into the joint, as may occur in hemophilia or other bleeding disorders
Repeated episodes of gout or pseudogout, in which uric acid or calcium crystals in the joint cause episodes of inflammation
Avascular necrosis, a condition in which the blood supply to the bone near the joint is interrupted, leading to bone death and eventually joint damage – The hip is affected most often.
Chronic (long-lasting) inflammation caused by previous rheumatic illness, such as rheumatoid arthritis
Osteoporosis, which can increase the risk of bone fractures, sometimes leading to osteoarthritis if the fracture is near a joint
Metabolic disorders, such as hemochromatosis, in which a genetic abnormality leads to too much iron in the joints and other parts of the body
One theory is that some people are born with defective cartilage or slight defects in the way joints fit, and as these people age, they are more likely to have cartilage in the joint break down.
Women are affected by osteoarthritis slightly more often than are men.
Osteoarthritis is one of the most common medical conditions, affecting an estimated 15.8 million people in the United States. In many people, it goes unrecognized. It is estimated that as many as half of all those who have osteoarthritis do not know that the pain and stiffness they are experiencing are symptoms of osteoarthritis.