A joint is an enclosed space with two corresponding articular surfaces and a surrounding joint capsule. Under normal circumstances, the joint cavity contains a small amount of joint fluid, which lubricates the joint and nourishes the joint cartilage. When there is trauma, degenerative joint degeneration or other pathological changes, the bone, cartilage, or hardened growths run into the joint cavity and form a free body called a joint rat. In general the arthroplasty must be surgically removed to prevent further damage to the joint caused by the arthroplasty.