summary Aneurysmal Bone Cysts are benign, non-neoplastic, reactive bone lesions that most commonly occur in the femur and tibia. The condition typically presents in patients less than 20 years of age with focal pain and swelling. Diagnosis is made with radiographs showing an expansile, eccentric, and lytic lesion with bony septae and a biopsy showing blood-filled spaces without endothelial lining. Treatment is usually nonoperative for lesions associated with a fracture. Aggressive curettage, with adjuvant treatment and bone grafting, is indicated for symptomatic lesions without associated fracture. Epidemiology Demographics 75% of patients are < 20 yrs. Anatomic location 15% in spine >60% in long bones (Femur and tibia being most common) 51% occured in the lower extremities, 22.5% in upper extremities usually in metaphysis metatarsal and calcaneus are the most common locations in the foot posterior elements of pelvis may be found in similar location as telangiectatic osteosarcomas Etiology Pathophysiology primary and secondary forms primary ABC driven by upregulation of the ubiquitin-specific protease USP6 (Tre2) gene on 17p13 when combined by a translocation with a promoter pairing most commonly described translocation t(16;17)(q22;p13) leading to juxtaposition of promoter region CDH11 on 16q22 secondary ABC not considered a neoplasm because no known translocation has been identified Associated conditions associated with other tumors 30% of time giant cell tumor chondroblastoma fibrous dysplasia chondromyxoid fibroma NOF Presentation Symptoms pain and swelling may present with pathologic fracture Physical exam neurologic deficits possible with spine lesions Imaging Radiographs expansile, eccentric and lytic lesion with bony septae ("bubbly appearance") usually in metaphyseal classic cases have thin rim of periosteal new bone surrounding lesion no matrix mineralization MRI or CT scan will show multiple fluid lines lesion can expand into soft tissue Studies Histology Characteristic findings cavernous space blood-filled spaces without endothelial lining cavity lining numerous benign giant cells spindle cells thin strands of woven (new) bone present Differential Radiographic differential includes UBC telangiectatic osteosarcoma Histologic differential includes telangiectatic osteosarcoma giant cell tumor Differential of Aneurysmal Bone Cyst "Bubbly" lytic lesion on xray "Lakes of Blood" on histology Treatment is curettage and bone grafting Aneurysmal Bone cyst o o o UBC o NOF o Giant Cell Tumor o Chondroblastoma o Chondromyoid fibroma o Osteoblastoma o Telangiectatic osteosarcoma o Treatment Nonoperative nonoperative fracture management indications ABC with acute fracture indicated until fracture has healed. Once healed, treat as an ABC without fracture unless the fracture has led to spontaneous healing of the ABC Operative aggressive curettage (+/- adjuvant) and bone grafting indications symptomatic ABC without acute fracture technique possible adjuvants phenol argon beam liquid nitrogen outcomes local recurrence in up to 25% and more common in children with open physes