Intra-articular free bodies are caused by trauma to the joint or some specific lesions that cause fragments of cartilage, bone and other tissues to fall out and accumulate in the joint. Intra-articular free bodies are also called joint rats because they can change their position in the joint cavity due to joint movement and move around the joint like a mouse. The most common cause of joint rats is the knee joint. Common causes include exfoliative osteochondritis, synovial osteochondromatosis, osteoarthritis and osteochondral surface fracture and intra-articular hematoma or infection. The main manifestations are the following symptoms: 1. Pain within the knee joint, aggravated by activity, sometimes causing the patient to fall. Repeatedly, the knee joint suddenly locks up and cannot be straightened or flexed. After moving the knee joint slightly, a popping sound often occurs, followed by disappearance of symptoms. After an attack, the joint may become swollen and fluid may accumulate. 2. The site of pain often varies with each attack, and the patient can often find intermittent joint rats, which may be single or several. 3. Sometimes joint rats can be felt superficially in the knee joint. Knee movement may be limited. Atrophy of the quadriceps muscle may also occur. 4.Osteoarthritis may appear if the disease has a long duration. The only method of treatment for arthroplasty is surgical removal, and conservative treatment will not help. Before surgery, under fluoroscopy, the free body will try to push to the joint surface and then removed. If the free body escapes or if several free bodies are hidden during surgery, the free body can be removed by high pressure coal mining type flushing method with patient and repeated impact. After the clinical application of arthroscopy, the free body can be removed microscopically without cutting into the joint, which is less traumatic and more accurate than the original surgery. It should be noted that the joint rat is often repeatedly squeezed between the joint surfaces, which is bound to damage the cartilage and lead to the occurrence of osteoarthritis. Therefore, once an intra-articular free body is present, it should be removed as early as possible.