Osteoarthritis (OA) is the most common form of arthritis.

It is a 'non-inflammatory' type of arthritis, which means that inflammation is not the key component. It is completely different from the less common rheumatoid arthritis, which is an inflammatory arthritis in which the body's immune system attacks its own tissues, causing joint damage.

• The causes and treatments of these other arthritis-related diseases differ from OA.
• While OA sometimes may be painful, it is not always disabling, and unlike rheumatoid arthritis, is unlikely to produce severe deformity of the joints.
• Osteoarthritis can involve a number of joints, but it is not a disease that spreads to involve other tissues or joints throughout the body

Osteoarthritis (OA) has been called a "degenerative" condition because it is caused, in part, by wear-and-tear of a joint over time. Its impact is significant:

• OA occurs in both men and women and usually develops after age 45.
• More than 16 million Americans, including over 50 percent of people over 65, have some degree of osteoarthritis.
• Older people often don't realize that they have OA if they are free of pain and other symptoms. However, x-rays often reveal some OA of the spine or fingers in elderly individuals.

Osteoarthritis develops in a joint when cartilage - the smooth shiny tissue that lines and cushions the ends of the bones inside a joint, -begins to break down. This can happen from:

• Prolonged "wear-and-tear" as we age
• Prior injury or damage to the joint from trauma or infection
• Cartilage that is altered by other disease or is genetically weak.

But in most cases we simply don't know what causes it.

Unfortunately, damaged cartilage cannot heal to become normal again, though tremendous research is underway developing methods to restore damaged cartilage.

Here is how OA progresses:

• The smooth cartilage that lines and protects the bone ends begins to retain water, and changes occur in some of the chemical substances that make up the cartilage
• Tiny cracks develop in the cartilage, which then splits further forming clefts and fissures
• The ends of the bones begin to thicken and grow out from the joint margin. These small bone growths are called osteophytes or "spurs". Actually, these spurs are nature's way of trying to help the damaged joint by allowing the load through the joint to be redistributed. But these bony outgrowths often interfere with the mechanism of joint movement.
• Cysts, which are small cavities, develop in the bone just beneath the damaged cartilage
• Fragments of damaged cartilage or bone may break off and float around freely in the joint as 'loose bodies' and may cause additional problems.
• As the cartilage becomes more damaged, the joint space becomes narrower and narrower.
• The damage to cartilage within a joint can irritate and inflame the inner lining of the joint called the synovial membrane and cause it to produce excess fluid. It normally produces a lubricant called synovial fluid, which helps to lessen friction in the joint.

The fluid may then build up within the joint and lead to detectable swelling.

When the cushioning system of the joint is lost, the bones may grind painfully against each other. The joint can begin to stiffen, and movement is impaired.

Some people are fortunate in that despite having these rather severe osteoarthritis changes in a joint, they experience very little, or no pain at all.

The knee is the most commonly affected joint.

If osteoarthritis develops in the hips or knees, it often occurs in only one joint but may affect any number of joints.

If the hands are affected, a few finger joints may become arthritic at the same time.

Rheumatoid arthritis is not just a disease of the joints, it is a systemic disease, meaning it affects the whole body. Abnormalities occur in the blood vessels, circulating cells and proteins, as well as connective tissue. Not surprisingly, RA is associated with more generalized disturbances - such as anemia (low red blood cell count) - which are proportional to the activity of the arthritis. Usually more than one joint is involved in RA, with the hands almost always affected.