It is a joint disease that is accompanied by inflammation of the synovium, the membrane that encloses the joint cavity. It is an autoimmune disease, meaning that the immune system begins to attack certain body structures. Rheumatoid arthritis causes excessive secretion of synovial fluid that will collect in the joint cavity and cause the swelling thereof. The joints mainly affected are the peripheral joints: hands feet shoulders, elbows, wrists and knees.
The affected joints cause intense pain, stiffness and swelling. Simple everyday actions, such as opening a door or write, can become restrictive and painful trials.
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The main difference between rheumatoid arthritis and osteoarthritis, primary rheumatic disease, is its origin. The inflammation in osteoarthritis is mainly due to mechanical wear. Conversely, it is the inflammation that will cause wear of the joint in rheumatoid arthritis.
The affected joints are also different. Osteoarthritis most commonly affects weight-bearing joints (knees, hips, etc.), when rheumatoid arthritis affects more regularly ends of limbs and hands and feet and it may have repercussions on structures other than the joints.
In addition, osteoarthritis is generally seen on the elderly, while the arthritis most often affects people between 40 and 60 years or young adults in rare cases.
You can find more information at the Rheumatoid Arthritis Forum.