A loose body is a piece of tissue from within a joint that has somehow become dislodged and floats freely in the joint. It can get caught between the joint when it moves, causing the joint to pop, click, catch, or even lock up.
There are several conditions in the knee that can be sources of a loose body. A loose body can form if a section of the meniscus has torn loose, either from trauma or degeneration. A bone chip from a fracture can also become a loose body. Another possible source for a loose body is osteochondritis dessicans (OCD), a condition in which a piece of cartilage and the underlying bone have been damaged. In some cases, the damaged fragment separates from the bone and floats freely within the joint.