Osteoarthritis is caused by wear and tear over time or by injury to the joints, which results in the degeneration of the hard, smooth layer of cartilage that normally covers and protects the ends of the bones.
Patients with osteoarthritis may develop bony enlargements of the affected joints, but they do not have the signs of inflammation around the joints, such as warmth, redness, and soft swelling.
Because rheumatoid arthritis is a systemic disease, which means that it spreads freely through the body as it follows the flow of immune cells in the bloodstream, it most commonly affects joints symmetrically on both sides of the body. By contrast, osteoarthritis is often more localized, especially when a joint becomes arthritic secondary to injury, and therefore osteoarthritis is more likely if the arthritis is on only one side of the body, or asymmetric.
Different joints are preferentially affected by the two diseases. Rheumatoid arthritis is most common in the small joints, such as the knuckles, wrists, elbows, ankles, toes, shoulders, and neck. Osteoarthritis very commonly affects the large weight-bearing joints in the hips and knees, as well as the thumb and the joints closest to the tips of the fingers.
Other joint diseases: Like rheumatoid arthritis, other autoimmune diseases can also attack the tissues of the joint.
Infections: If bacteria or viruses get into the joint space, they will initiate a local immune response leading to swelling of the joint and pain. Bacterial infections of joints will cause severe pain and swelling only in the single joint, because the infection tends to be localized. Viruses-especially hepatitis B and C, HIV, and parvovirus-can affect single joints but also can cause a generalized reaction that affects joints all over the body. Infection-associated arthritis generally resolves when the infection is treated.
Gout: Uric acid crystals that form in gout can get into the joint and cause periodic acute joint pain and swelling. When gout is treated with medications, the arthritis that it causes goes away.
Pseudogout: Calcium pyrophosphate crystals get into the joint and cause acute pain and swelling. When treated with medications, the pain disappears.
Polymyalgia rheumatica: This is seen in people older than 50, with a usually abrupt onset of pain and stiffness in the neck, both hips, shoulders, and buttocks.
Fibromyalgia: The fibromyalgia syndrome can cause joint pain but can be distinguished from rheumatoid arthritis by the predominance of pain and tender points in soft tissue and the absence of swollen joints. Fibromyalgia is a poorly understood disorder in which it appears that the body's perception of normal stimuli is altered so that widespread pain occurs, some of which is in the joints.
Examples of causes of joint pain by distribution of affected joints
Type of arthritis
Affects many joints on both sides of the body
Affects a few joints often only on one side of the body