Exfoliative osteochondritis, referred to as OCD, is not uncommon in clinical practice and is distinguished between primary and secondary. Most believe that it is caused by traumatic osteochondral fractures or repeated mild trauma resulting in impaired blood flow and necrotic detachment of osteochondral bone, while bacterial emboli or fat embolism of the terminal artery and familial inheritance have also been proposed. The fragment consists of cartilage and the bone beneath it. The fragment is attached to the parent bone with or without a fibrous tip. There is fibrous tissue or fibrocartilage covering the severed surface of the parent bone and fragment and a small amount of new bone formation. Completely free fragments are growing in size as the free body absorbs synovial nutrients. The free body varies in size and number, and joint strangulation may occur. Tip fracture leads to intra-articular hematoma, and joint wear produces proliferative arthritis. The cause of this disease is still unclear, there are two theories as follows: (1) Trauma theory: frequent and continuous trauma can cause irreparable damage to bone and cartilage, resulting in osteochondral degeneration, exfoliation or free. This understanding can explain why the disease is more likely to occur in athletes or people with high activity. (2) Endocrine and genetic factors theory: The disease can also occur in people who do not exercise much and do not suffer from frequent trauma, obviously trauma theory is difficult to explain this group of cases, so some scholars suggest that the disease may be related to endocrine or genetic factors. 2, clinical manifestations Prevalent in males between the ages of 16-25, common in the knee and elbow joints, but also in the hip, shoulder, ankle or metatarsophalangeal joints. It usually erodes one joint and has no systemic symptoms. There may be dull pain in the joint, aggravated by activity, reduced by rest, and light swelling of the joint. Free bodies may present with joint locking, hematoma, and traumatic arthritis. Joint swelling, effusion, pressure pain, and palpable masses may be detected, and twisting sounds may be heard with restricted movement. Muscle atrophy. The medial and lateral femoral condyles, patellofemoral articular surface, humeral epicondyle, radial tuberosity, and the internal superior talus of the ankle joint can be developed, and pressure pain can be induced. 3, examination and diagnosis (1) X-ray examination: the typical injury shows a limited subchondral bone sclerosis with a clear outline, completely detached and displaced from the surrounding normal bone. A translucent defect area can be seen in the femoral condyles, and free bodies can be seen in the joint cavity. Although X-rays are commonly used for this disease, they are of limited diagnostic value for OCD because they do not directly show cartilage and often miss small intraosseous lesions or bony lesions that have not yet been stripped, i.e., they do not allow early detection of the lesion and do not facilitate staging of the lesion. （MRI is an effective method for early diagnosis and staging of exfoliative osteochondritis, as it can clearly show the morphology and signal changes of the articular cartilage and subchondral bone without damage to the joint. (3) Arthroscopy: As a less invasive surgical method, it has been considered the “gold standard” for evaluating articular cartilage, but in clinical use, arthroscopy is found to have certain shortcomings compared to MRI. Arthroscopy cannot detect early osteochondral lesions that have not undergone gross morphologic changes, thus causing a discrepancy in the recognition of MRI versus arthroscopy. This is particularly true in type I OCD lesions, and the ability of MRI to reflect cartilage surface contours and thickness challenges the “gold standard” status of arthroscopy. 4. Staging of exfoliative osteochondritis Stage I: Subchondral bone necrosis followed by involvement of the articular manifestations of the cartilage. The cartilage becomes slightly softer and loses its luster. Stage II: A part of the cartilage of the articular surface together with a small piece of cancellous bone under it gradually separates from the surrounding normal tissue due to ischemia and necrosis. Stage III: further cartilage loss, bone depression at the exfoliation, fibrous tissue attached at the bottom, and crater-like changes with irregular edges.