An aneurysmal bone cyst (ABC) is a benign expansile lesion consisting of multiple blood-filled cavities separated by bony septae. They commonly present in long bones, including the femur and tibia, but may occur in other locations, such as the pelvis, vertebral body, and posterior elements of the spine. Typical presentation includes males aged less than 20 years who complain of pain and swelling at the affected site. ABCs in the spine, which account for 25% of all ABCs, may present with neurologic deficits.
ABCs are classified by tumor aggression, as growth rate can be variable. Low-grade tumors may be well-contained and centrally located and have limited expansion. More aggressive ABCs can expand quickly around or through cortical bone.
X-ray characteristics include a radiolucent lesion arising from the medullary canal or metaphysis. The tumor can expand through cortical bone but typically has a thin rim of new bone surrounding the lesion. A bubbly appearance created by bony septation is seen within the lesion. A computed tomography scan helps define the outer cortex and can aid in planning for the biopsy. Magnetic resonance imaging will best show multiple fluid-filled septations with characteristic fluid-fluid levels created by layering of blood and serum.
ABCs may arise or grow adjacent to other bone tumors, such as giant cell tumors, fibrous dysplasia, nonossifying fibroma, chondroblastoma, osteoblastoma, and osteosarcoma. These secondary ABCs represent 30% of all lesions.
Differentiating ABCs from other similar presenting bone tumors can be difficult. A simple bone cyst is centrally located and does not expand through the cortex. Giant cell tumors rarely occur in patients aged less than 20 years and typically begin at the epiphysis and extend to the metaphysis. Fever, weight loss, and fatigue are associated with malignancies and should not be present with benign tumors such as ABCs.
Incisional biopsy is the standard approach for histologic evaluation to confirm the diagnosis. Multiple samples must be taken to rule out other tumors that demonstrate an aneurysmal component, such as giant cell tumors, telangiectatic osteosarcoma, and fracture through a simple cyst. Fine-needle biopsy alone has an unacceptably high missed diagnosis rate.
Treatment consists of aggressive intralesional curettage and bone grafting once the definitive diagnosis has been established. However, curettage and bone grafting alone are associated with recurrence rates for ABCs of up to 20% to 40%. Using adjunct therapies, including phenol, polymethylmethacrylate cement, liquid nitrogen, radionuclide ablation, and/or high-speed burr, can significantly lower recurrence rates. Wide excision with en bloc resection has the lowest recurrence rates but will negatively affect the function of involved extremities in many cases. ABCs that present with acute fracture should be allowed to heal before definitive treatment is performed.
Dagan Cloutier, MPAS, PA-C, practices in a multispeciality orthopedic group in the southern New Hampshire region and is editor in chief of the Journal of Orthopedics for Physician Assistants (JOPA).
- Rapp TB, Ward JP, Alaia MJ. Aneurysmal bone cyst. J Am Acad Orthop Surg. 2012;20(4):233-241.
- McKean J. Aneurysmal bone cyst. Updated August 22, 2014. Available at orthobullets.com/pathology/8036/aneurysmal-bone-cyst