Aneurysmal bone cysts (ABCs), which are benign lesions of the bone, primarily affect pediatric patients. They were first described by doctors Jaffe and Lichenstein in 1942 during an investigation of spinal and pelvic lesions. The surgeons discovered that the thin-walled lesions housed large, blood-filled cavities.
The initial term, ABC, proved to be a misnomer, as lesions were later shown not to possess endothelial walls, which are present in all true cysts. Moreover, lesions were not aneurysmal in nature either. Further in-depth examination showed ABCs to be protractible benign lesions that form within bone and create cavities that eventually fill with blood. These cavities are lined by trabecular bone, proliferative fibroblasts, and giant cells.
Although ABCs may arise in any bone, 80% of lesions occur in one of three bone types: long bones, spinal bones, and flat bones. ABCs of the spine most commonly affect the posterior segments, where they can cause a wide range of neurologic deficits. ABCs are also locally aggressive. Up to 50% of cases involving flat bones involve the pelvis.
Signs and Symptoms
ABCs usually present with swelling, pain, and a mass proximate to the affected bone. Approximately 1 in 10 affected patients additionally present with pathologic fractures and associated symptoms and bony erosion. Patients may experience signs and symptoms for several weeks or months before a diagnosis of ABC is made. In the case of spinal involvement, secondary neurologic symptoms arise as a result of impingement of adjacent nervous tissue. Other clinical findings may include warmth of the affected area, deformity, weakness, stiffness, and reduced range of motion.
X-rays are often used to establish the initial diagnosis of ABC. Classically, ABCs appear as cystic radiolucent lesions that are eccentrically located and delineated by a thin layer of cortical bone. ABCs have a “soap bubble appearance” owing to the presence of trabeculations that create a multi-locular appearance within the lesion; however, this radiographic finding is not pathognomonic for ABCs. Giant cell tumors, osteoblastoma, and unicameral bone cysts can also have a “soap bubble appearance” on imaging studies. To improve diagnostic specificity and sensitivity, some studies suggest that magnetic resonance imaging be used.
Computed tomography can be used to define a lesion’s osseous borders. However, histologic examination is crucial for establishing an ABC diagnosis. Grossly, ABCs are hemorrhagic, spongy masses that are covered with a thin layer of bone. Histologically, lesions have an abundance of red blood cells and hemosiderin. Within the cyst-like cavities, mitotically active spindle cells, fibroblasts, multi-nucleated giant cells, calcifications, and osteoid may be present. An early, accurate diagnosis of ABC is crucial, as other, more serious conditions are in the differential diagnosis.
Treatment and Prognosis
Generally, ABCs are treated surgically. However, regions that are less accessible may be treated by selective arterial embolization or intralesional injection. In the case of associated pathologic fracture, the patient must undergo curettage or bone scraping with internal fixation. Other adjuvant therapy may also be indicated.
As with any medical or surgical procedure, there are risks inherent in ABC treatment. These complications, although rare, include excessive bleeding, infection, and inadvertent muscle or nerve injury. Most ABC recurrences occur within 2 years. However, patients are typically monitored closely for at least 5 years, as recurrent lesions are much easier to manage when detected early. Children are monitored until they have reached the age of maturity to ensure that any potential recurrence does not interfere with growth and development or lead to deformity.