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.
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
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.
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.
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