Simple bone cysts have been recognized since they were first reported by Virchow in 1876. Simple bone cysts are neoplastic lesions involving tubular and flat bones, resulting in localized swelling and filling of the thin bone cortex with plasma. 80% of cases are found in children aged 3-14 years, with an average age of 9 years. The average age is 9 years old. It accounts for about 3% of skeletal tumors, and the ratio of male to female is about 2-3:1. Pathophysiology: The etiology of simple bone cysts is not clear, and Cohen’s theory of impaired venous return to the bone is a well-recognized theory of the etiology of simple bone cysts, which leads to high pressure in the bone and increased osteoclastic activity, resulting in localized destruction of bone. NATURAL HISTORY: Simple bone cysts initially occur in the metaphysis adjacent to the epiphyseal plate and progressively move away from the epiphyseal plate with growth and development. Lesions close to the epiphyseal plate often show greater mobility and are more likely to progress and recur than lesions farther away from the epiphyseal plate. Active bone cysts near the epiphyseal plate may interfere with the longitudinal growth of long tubular bones; before the age of 10 years, the cysts are closer to the epiphyseal plate and may be active, while after the age of 10 years, the lesions are farther away from the epiphyseal plate and tend to become quiescent, and the bone cysts seldom re-progress after epiphyseal plate closure. Clinical manifestations: Bone cysts are often asymptomatic at the beginning, and pain, swelling and limitation of activity may occur in the adjacent joints. Those occurring in superficial bones may feel localized bone swelling and localized pressure pain, and some cases are discovered by chance when pathological fractures occur or when X-rays are taken. Pathologic fracture is the most common complication of the disease, with an incidence of about 66%. In some cases, limb deformity occurs due to abnormal localized stress of the lesion. Anatomical factors: Simple bone cysts can involve the tubular bone and flat bone, about 94% of simple bone cysts occur in the humerus and proximal femur, and the incidence of humerus is 2-3 times higher than that of femur. Other bones involved include the heel bone (2%), ilium (2%), talus and tibia. Cases occurring in the flat bones are mostly >12-17 years old and are detected late because the lesions are deep and asymptomatic. Diagnostic methods: X-ray: the lesion may involve the metaphysis or diaphysis of the long bones, and rarely involves the epiphysis. Typical cases often show a centrally distended local marrow cavity with increased X-ray transmittance, surrounded by a thin layer of bone, with a clear boundary, and the longitudinal axis of the cyst is often larger than its transverse diameter, and the transverse diameter of the cyst is often smaller than the width of the adjacent epiphyseal plate; there is often no periosteal reaction in the surroundings unless there is a pathologic fracture. The cyst does not break through the cortex, laminar periosteal reaction, or Codman’s triangle. “Fallen leaf sign”: is a special radiographic manifestation of multicompartmental bone cysts. When a pathologic fracture occurs, the thin bone cortex on one side breaks off to form a free bone mass and falls into the cystic cavity (which is often not fluid-filled), where the bone mass floats on the surface of the fluid or may be partially immersed in the fluid. Most often seen in skeletally mature or adjacent mature cases. CT: X-rays usually provide sufficient information to diagnose bone cysts, but CT scans are helpful in evaluating all sites of bone cysts, especially pelvic bone cysts. When the cyst occurs in the middle of the diaphysis or in an atypical area, CT can adequately visualize the extent of lesion involvement. MRI: Typical bone cysts often show homogeneous short T1 and long T2 signals. However, it is difficult to distinguish simple bone cysts from aneurysmal bone cysts in children younger than 8 years of age. Puncture biopsy: In children <10 years of age, both simple bone cysts and aneurysmal bone cysts are associated with hemorrhagic fluid, so puncture is not a reliable means of diagnosing simple bone cysts. In a very small number of younger cases, especially those <5 years of age, simple bone cysts may break through the epiphyseal plate. Cyst visualization: The cyst may contain a single compartment, or multiple incomplete intervals separated by multicompartmental cysts with intervals that prove to be normal bone. The cyst cavity is filled with a yellowish, low viscous slurry. The plasma has been analyzed for the presence of prostaglandins (especially prostaglandin E), interleukin 1β and proteases such as collagenase. These components may mediate increased osteoclastic activity. Differential diagnosis: aneurysmal bone cyst: X-ray film shows the degree of expansion of the lesion and the degree of thinning of the bone cortex and the onset of the site may be conducive to the identification of aneurysmal bone cysts are mostly eccentric, often more pronounced dilatation, and the capsule can be seen as patchy or punctate calcification. Aneurysmal bone cysts are multicompartmental and may show a fluid-fluid plane on MRI. The punctured capsule contains nonclotting blood. Patients often have progressive localized pain and swelling. Giant cell tumor of bone: Most common in adults. A localized painful mass is the main symptom. The lesion is commonly found in the epiphysis of long bones. radiographs show eccentric, expansive osteolytic destruction. Fibrous heteroplasia of bone: It is not uncommon to see bone cysts secondary to fibrous heteroplasia of the femoral neck and proximal humerus. Abnormal swelling of the bone around the cyst, hairy glass-like, sclerotic bone shell around the cyst is wider than that of simple bone cysts to facilitate the identification. Pediatric cases are common in polyostotic FD. Osteosarcoma: Simple osteolytic sarcoma can be misdiagnosed as a bone cyst. Some osteosarcomas mutate or form a pseudocapsule with fewer clinical symptoms and imaging similar to a bone cyst. Capillary-dilated osteosarcoma and Ewing's sarcoma can also resemble bone cysts on imaging, but these malignant tumors often show greater aggressiveness. Treatment: The aim of treatment is to prevent all possible complications and limb dysfunction. There is no single preferred treatment for simple bone cysts. Bone grafting alone: is rarely used due to its high recurrence rate (12%-45%). Scraping and bone grafting: Bone grafting after intracapsular scraping greatly improves the healing rate of the cyst, but the surgery is more traumatic and has more complications, such as intraoperative and postoperative fractures. Percutaneous puncture local injection of hormones: first reported by Scaglietti, the success rate of its cases can reach 90%. This method is widely used because of its considerable effect, relatively simple operation and fewer complications, but most cases require repeated injections until the lesion is completely absorbed or stabilized. This method was the most popular method of treating humeral bone cysts in the 70s-90s. Percutaneous percutaneous autologous bone marrow transplantation: In recent years, some studies have used autologous bone marrow to replace hormones as percutaneous injections, confirming that autologous bone marrow has certain advantages in inducing osteogenesis, shortening the course of the disease to a certain extent and reducing the number of injections. Treatment concept: Analysis of multifactorial studies has confirmed that the key factor influencing the success of treatment is the age of the patient, and that the success rate of patients >10 years of age (90%) is much higher than that of patients <10 years of age (60%), irrespective of the choice of treatment. Regardless of treatment, cysts tend to stabilize after skeletal maturation, but the lesion cannot be converted to imaging normal bone. The goal of treating bone cysts is to abort progressive bone destruction and obtain a normally stressed, functionally stable skeleton, not to pursue a normal radiographic appearance. It has been suggested that 90% of simple bone cysts can be treated conservatively. On the contrary, if the cyst involves key weight-bearing areas (e.g., femoral neck, heel bone), and there is a high risk of pathological fracture, or if an impending pathological fracture is confirmed by serial X-ray follow-up, aggressive surgical treatment is warranted. It should be emphasized that healing of the pathologic fracture should be ensured before any surgical or perforating injection treatment is attempted; however, in displaced pathologic fractures of the femoral neck or posterior column of the heel, internal fixation with incision and reduction is feasible to restore normal blood flow and anatomic integrity to reduce long-term function.