Aneurysmal Bone Cyst

     Aneurysmal bone cyst (ABC) is a solitary, expansile and erosive lesion of bone. It is found most commonly during the second decade and the ratio of female to male is 2:1. ABC's can be found in any bone in the body. The most common location is the metaphysis of the lower extremity long bones, more so than the upper extremity. The vertebral bodies or arches of the spine also may be involved.Approximately one-half of lesions in flat bones occur in the pelvis. One theory of the etiology of primary ABCs is  that these lesions are secondary to increased venous pressure that leads to hemorrhage which causes osteolysis. This osteolysis can in turn promote more hemorrhage causing amplification of the cyst.

More often, ABC's are thought to be a reactive process secondary to trauma or vascular disturbance. ABC's can be secondary to an underlying lesion such as non-ossifying fibroma, chondroblastoma, osteoblastoma, UBC's, chondromyxoid fibroma and fibrous dysplasia. This association is so strong that the lesion should be examined microscopically in several places to eliminate the possibility of a primary lesion. In one report (Kransdorf, Amer J Roentgenol 1995 Mar;164(3):573-80) the authors state that the original lesion can be identified in one-third of cases. The most common precursor lesion was giant cell tumor, (19-39%) of cases, followed by osteoblastoma, angioma, and chondroblastoma. Less common precursor lesions were fibrous dysplasia, non-ossifying fibroma, chondromyxoid fibroma, unicameral bone cyst, fibrous histiocytoma, eosinoplilic granuloma, and osteosarcoma. A translocation involving the 16q22 and 17p13 chromosomes has been identified in the solid variant and extraosseous forms of aneurysmal bone cyst.
   

 The clinical presentation of an ABC is swelling, tenderness and pain. Occasionally there is limited range of motion due to joint obstruction. Spinal lesions can cause neurological symptoms secondary to cord compression. Pathological fractures are rare due to the eccentric location of the lesion. Depending on the location, the differential includes UBC, chondromyxoid fibroma, giant cell tumor, osteoblastoma and the highly malignant telangiectatic osteosarcoma.
     

On plain film, an ABC is normally placed eccentrically in the metaphysis and appears osteolytic. The periosteum is elevated and the cortex is eroded to a thin margin.The expansile nature of the lesion is often reflected by a"blow-out" or "soap bubble" appearance. CT scan can also help delineate lesions in the pelvis or spine where plain film imaging may be inadequate. CT scan can narrow the differential dignosis of ABC by demonstrating multiple fluid-fluid levels within the cystic spaces. MRI can also confirm the multiple fluid-fluid levels and the non-homogeneity of the lesion. ABC appears on both T1 and T2 MRI with a low signal rim encircling the cystic lesion. A careful search for radiological signs of the precursor lesion, if any, is recommended. Some lesions may have a flocculent chondroid matrix that may be a clue to their pathogenesis.
     

On gross examination, an ABC is like a blood filled sponge with a thin periosteal membrane. Soft, fibrous walls separate spaces filled with friable blood clot. Microscopically, the ABC has cystic spaces filled with blood. The fibrous septa have immature woven bone trabeculae as well as I macrophages filled with hemosiderin, fibroblasts, capillaries and giant cells.
The treatment approach will vary depending of the location and aggressiveness of the lesion. A slow growing, indolent ABC has been observed to regress spontaneously. Selective embolectomy of nutrient vessels and percutaneous injection of a fibrosing agent are newer treatment modalities. Percutaneous injection of methylmethacrylate was used successfully by Herve Deramond for an aggressive ABC lesion in the second cervical vertebra.
 

Most lesions can be treated with currettage and application of a high-speed burr. Local recurrence rates vary widely, with one recent report having 4 recurrences in 40 patients (Gibbs JBJS Am 1999 Dec;81(12):1671-8). Recurrence was statistically related to young age and open growth plates, and may be less likely following wide excision than following intralesional treatment by currettage. If a recurrence is detected, a thorough examination of the original radiographs and pathology specimens should be performed to insure that the primary lesion, if any, is discovered, since this may radically alter the treatment plan. Once the precise diagnosis is known, local recurrences may be retreated by appropriate methods. Wide resection and limb-sparing reconstructions are necessary to prevent progressively destructive recurrence. Curettage and bone graft can be complicated by profuse bleeding from the lesion. If bleeding is a concern, preoperative selective embolization can be used. Radiation has been used in some cases where operative treatment is not possible, but this adds the additional risk of malignancy.

Updated 7/10/00 HD
References
Bullough, Peter, Orthopaedic Pathologv (third edition), Times Mirror International Publishers Limited, London, 1997.

Huvos, Andrew. Bone Tumors: Diagnosis. Treatment and Prognosis, W.B. Saunders,  , 1991.

Conway, W., MD, PhD, and C. Hayes, MD. Miscellaneous Lesions of Bone, Radiologic Clinics of North America, Vol. 31, No. 2, p. 339-357, March 1996.

Capanna, et al., Unicameral and Aneursymal Bone Cysts, Orthopedic Clinics of North America, Vol 27, No.3, P. 605-614, July 1996.

1/22/98