Review Question - QID 220384

This patient has an incidentally noted, lytic lesion in the proximal femur. A biopsy confirmed the diagnosis of an aneurysmal bone cyst (ABC) involving the right femoral neck. Based on the location and history of antecedent hip pain, curettage with bone grafting to limit recurrence and provide structural support is indicated (Answer 4).

ABCs are benign cystic lesions that most commonly occur in the metaphysis of long bones, including the proximal femur. On x-ray, these lesions are often lytic, geographic lesions with a thin sclerotic margin. In contrast to unicameral bone cysts, ABCs typically are often expansive, with a diameter greater than the epiphysis. ABCs should be differentiated from other lesions which can have fluid-fluid levels on imaging, such as giant cell tumors (GCTs), which are more common in older patients, and malignant lesions, such as telangiectatic osteosarcoma. While both GCTs and ABCs can be treated with curettage, medical therapies such as denosumab are reserved for GCTs.

Rapp et al. reviewed the pathophysiology, diagnosis, and treatment options of ABCs in their 2012 review article. They note that curettage with bone grafting is the preferred treatment strategy. However, other techniques involving ablation, sclerotherapy, cryotherapy, and en bloc resection may also be considered depending on the clinical scenario. Recurrence after curettage has been estimated to be 20% within 20 years. The authors conclude that the risk factors for recurrence include younger age, open physes, and histological characteristics.

Li et al. performed a retrospective cohort study of 41 pediatric patients with proximal femoral bone cysts, 32% of which were ABCs. They found a 17% recurrence rate after surgery, which was significantly associated with a cyst location between the epiphysis and intertrochanteric ridge. They reported a 20% complication rate which included limb length discrepancy, residual proximal femoral deformities, and premature physeal closure, and complications were associated with the presence of a pathological fracture. They concluded that early surgical treatment minimizes complications and the risk of subsequent fracture.

Figure A shows an AP pelvis radiograph demonstrating a geographic, lytic lesion with a thin sclerotic margin involving >50% of the right femoral neck.

Illustration A shows an intraoperative fluoroscopic image demonstrating curettage of the lesion. Illustration B shows the lesion after bone grafting.

Incorrect Answers:
Answer 1: The patient has hip pain and a lytic lesion involving a significant portion of the proximal femur. She would be at risk for a pathologic fracture with continued observation.
Answer 2: Radiation therapy alone is not indicated for ABCs.
Answer 3: Denosumab is a monoclonal antibody to RANK-ligand, which is useful for the treatment of GCTs, not ABCs.
Answer 5: ABCs are benign and this lesion is amenable to curettage and grafting. Proximal femoral replacement is not indicated in this patient.