Causes
As you move or put pressure on a joint, cartilage allows bones to slide over one another and acts as a shock absorber. Cartilage itself does not have any nerve cells and therefore cannot sense pain. OA results when the cartilage becomes worn out, allowing the bones underneath to rub against each other and cause pain and swelling. It is not simple wear-and-tear but an actual disease process involving the cells and proteins of the bone and cartilage.
As the condition progresses, the joint may become disfigured and small growths called osteophytes begin to grow inside the joints. Osteophytes are small, irregular, bony growths that are also called bone spurs. Bits of broken-off cartilage or bone are also found floating inside the joint. This causes even more pain, swelling, and immobility of the joint.
The exact cause of these changes is unknown. Scientists believe that the following factors play a role:
- being overweight: Excess weight puts stress on weight-bearing joints such as hips and knees, and increases the risk of cartilage breakdown. This is the most important risk factor for OA affecting hips, and leg and foot joints.
- family genes: Scientists believe that OA may be passed on through families, with the symptoms appearing in middle age.
- injury: Trauma to the joint, such as overuse, can damage the cartilage and lead to OA. This is often the cause of OA in younger people.
- increasing age: The cartilage wears down with time. By age 65, 80% to 90% of people have OA, as shown on X-ray, though a much smaller percentage have symptoms.
- other types of arthritis: For example, rheumatoid arthritis can also damage joints and lead to OA.