In OA, the cartilage, or soft connective tissue in the joint, deteriorates or breaks down. This causes the joints to lose the cushioning and the joints begin to rub together. OA is often called the wear-and-tear arthritis because it happens most often in the weigh-bearing joints like the hips and knees. RA is caused by the tissue in the joint getting swollen and inflamed. This causes the pain and the redness and swelling that you may see. It can happen in any joint, not just weight-bearing ones.