There are six different types of synovial joints. The types of motion each allow are different, but their underlying physiological structure is essentially the same. Two or more adjoining movable bones, whose adjacent surfaces are covered with a layer of cartilage, are surrounded by a fluid filled capsule comprised of ligaments. A thin membrane, the synovial membrane that lines the inside of the joint capsule, secretes the fluid. The viscous fluid and the cartilage tht covers the ends of the bones allow normal joints to glide smoothly past one another. In healthy joints the synovial membrane is thin, the cartilage that covers the bone is smooth, and a thin layer of synovial fluid covers the bone surfaces. If anything goes wrong with any of these factors, arthritis can result. Arthritis may appear suddenly or come on gradually. Some people may feel a sharp burning or grinding pain, others compare the pain to that of a toothache. Moving the joint usually hurts, although sometimes there is only stiffness. Swelling and deformity that takes place in arthritic joints can result from a thickening of the synovial membrane, and increase in the secretion of synovial fluid, enlargement of the bones, or some combination of these factors.