Isolated bone cysts (UBC), also called simple bone cysts, have a lesion cavity filled with fluid and a thin shell membrane on all four walls. Isolated bone cysts often occur in children between the ages of 5 and 15 years and are more common in males than females, with a ratio of approximately 2:1. The lesions are most often found in the epiphysis of the long bones. The most common site is the proximal humerus, followed by the proximal femur. Active Relatively aggressive) bone cysts are located close to the epiphysis and become inactive as the child ages and the lesion gradually moves away from the epiphysis. In patients older than 17 years, lesions occur in non-long canal bones, such as the heel and pelvis. The mechanism of development of simple bone cysts is not known, but is mostly thought to be related to post-traumatic reactions. The most popular theory of occurrence suggests that local venous return is impaired, resulting in elevated pressure and causing local reactive bone resorption. Experimental data demonstrate that both prostaglandins and interleukin IB, which are enriched in the cyst fluid of simple bone cysts, can independently cause bone resorption. Most bone cysts are not clinically symptomatic and are often detected only when pathological fractures occur. Experienced bone oncologists can make the clinical diagnosis of bone cysts based on radiographs alone. The lesion is a well-defined liquid hypodense foci with a thin sclerotic focal shell on all four walls. The lesion is slightly enlarged towards the epiphyseal area near the epiphysis. The lesions are non-eccentric and do not destroy the bone shell, nor do they break through the bone to form extraperiosteal reactive bone, except during the healing phase after a pathologic fracture. Sometimes the dislodged cortical component falls into the cystic cavity, which is shown on x-ray as the Fallen leaf or Fallen fragment sign. When the lesion occurs in the pelvis, CT scan is valuable to determine the location of the lesion and the morphology of the cyst, and MRI can clarify the fluid-rich components of the cyst. Bone ECT scans show a thin peripheral concentration and a cold area in the central lesion. Histological examination will reveal a flattened, cuboidal layer of germ cells in the luminal wall, resembling endothelial-like cells. The composition of the cystic fluid is similar to that of synovial fluid. When fractures occur, they are often associated with hematomas, granulomatous growths, calcified components, and giant cells are seen, which can easily lead to misdiagnosis. Treatment The physician must be completely sure of his diagnosis and should be confident of the treatment chosen. Preoperative consultation and discussion with a bone tumor specialist is necessary, as there are multiple treatment options. When accompanied by pathologic fracture, conservative treatment to allow closed fracture healing is preferred. Sometimes the natural healing of pathologic fractures can also resolve the bone cyst itself. Intracapsular injection of percutaneous desmethylhydrocortisone acetate once/2 months can give satisfactory results after 1-3 treatments. Injection of 2-5 ml of MPA into the cavity of the bone cyst through a double-lumen tube to eliminate intraventricular hematoma and obstruction and to maintain the long-term effect of the drug and eliminate the cause of the disease is the currently advocated method. Clinicians should avoid blind decision of surgical treatment and it is better to transfer to orthopedic oncology center when they encounter recurrent or complex site of bone cysts. Since complete excision of the lesion is too great a loss of bone continuity, simple scraping surgery is more commonly used, but the recurrence rate is as high as 40-45%. In addition, during surgical treatment, it is important to prevent damage to the growth plate near the lesion to avoid affecting bone growth.