Synovial osteochondromatosis is a monoarticular synovial proliferative disease that is characterized by cartilaginous, fibrocartilaginous, or osteochondral microsomes that develop in the synovial membrane within the joints, synovium, and tendon sheaths. It is commonly seen in the knee and hip joints, but can develop throughout the body in joints, bursae, and tendon sheaths. Milgram divides it into three stages: 1. early stage: synovial chondrogenesis without free body formation; 2. migratory stage: active synovial lesions with free body formation; 3. late stage: presence of free bodies without synovial lesions. Etiology: The etiology of this disease is unknown, and the factors involved are trauma, inflammation, cartilage remnants, osteochondral metaplasia as tumor, etc. It is thought that the disease arises due to inflammatory irritation of the synovium. It is also believed that a portion of mesenchymal cells are hindered in the process of differentiation into synovial membrane or cartilage, and that this portion of embryonic cells, later develops into synovial chondrosarcoma. Some authors believe that the synovial connective tissue is transformed into cartilage nodules through chemotaxis, and these cartilage nodules are shed and enter the joint cavity, where they are nourished by the joint fluid and gradually grow, and later most of them become calcified or even ossified. Clinical manifestations: The disease is most common in men aged 30-40 years old. The joints may present with symptoms such as pain, swelling, stiffness, foreign body sensation, twisting pronation, playing soft leg or repeated interlocking. Physical examination may include widespread joint pain, synovial thickening, and nodules may be visualized. Ancillary examinations: If the cartilage nodes are ossified or calcified, multiple free bodies may be found on the X-ray. If the x-ray is normal, arthrography, MRI, and arthroscopy may be required for clarification, especially in the hip joint. On arthroscopic examination, the synovial membrane of the diseased joint is congested and hypertrophied, and villi are formed. Also visible are free bodies of varying extent and number, which may be limited or extensive. The free bodies are attached to the synovium and may fuse into substantial masses in large numbers. Diagnosis: Diagnosis can only be made with medical history, physical examination, and ancillary tests. In some patients, recurrent interlocking of the joints with normal radiographic findings may alert the patient to the disease. Treatment: The principle of treatment is arthroscopic free body removal. If the synovial membrane is hypertrophic and edematous with multiple nodules attached, synovectomy should be performed at the same time, but it is difficult to restore normal joint function. Some scholars believe that synovectomy is not better than free body removal alone. There may be recurrence after surgery, and in rare cases, the disease may turn into chondrosarcoma. The prognosis of this disease is related to the severity of the destruction of the articular cartilage surface.