A free body is a free object, more commonly referred to as a free object within a joint, also known as a joint rat, which is a hard, rounded object of free bone, cartilage, or cartilage protruding from the synovial membrane within a joint. The free body can easily cause some mechanical damage to the joint. It is easy to move back and forth within the joint, and it is not easy to determine the exact location, so the free body is tricky to treat clinically until minimally invasive surgery is performed.