An aneurysmal bone cyst is an expansile osteolytic lesion with a thin wall, containing blood-filled cystic cavities. The term aneurysmal is derived from its radiographic appearance.

Trauma is considered an initiating factor in the pathogenesis of some cysts in well-documented cases involving acute fracture. Local hemodynamic alterations related to venous obstruction or arteriovenous fistulae that occur after an injury are important in the pathogenesis of an aneurysmal bone cyst.

The lesion is a component of, or arises within, a preexisting bone tumor in about one third of cases; this finding further substantiates the fact that aneurysmal bone cysts occur in an abnormal bone as a result of associated hemodynamic changes. An aneurysmal bone cyst can arise from a preexisting chondroblastoma, a chondromyxoid fibroma, an osteoblastoma, a giant cell tumor, or fibrous dysplasia. Less frequently, it results from some malignant tumors, such as osteosarcoma, chondrosarcoma, and hemangioendothelioma.

Aneurysmal bone cysts may be purely intraosseous, arising from the bone marrow cavity. In this case, they are primarily cystic and slowly expand into the cortex. They may be extraosseous, arising from surface of bones, eroding adjacent cortex, and extending into the marrow space.

Four phases of pathogenesis are recognized, as follows:

Compared with males, females have an increased incidence.

Aneurysmal bone cysts may occur in patients aged 10-30 years, with a peak incidence in those aged 16 years. About 75% of patients are younger than 20 years.

Regarding the location of the lesions, any bone may be affected. Approximate frequencies by site are shown below:

The most common site is the metaphyseal region of the knee.

Short tubular bones are less frequently affected and are involved in about 10% of cases.

Spinal involvement demonstrates a predilection for the posterior elements. In decreasing order of frequency, the following parts of the spine are involved: cervical, thoracic, lumbar. Contiguous vertebrae may be involved in 25% of cases.

The cyst involves the diaphysis in isolation in about 8% of cases.

Radiographs usually are adequate for diagnosis. Cross-sectional imaging may be required when lesions are in unusual locations, such as the axial skeleton.

The classic description of an aneurysmal bone cyst includes an eccentric radiolucency and a purely lytic or, occasionally, trabecular process, with its epicenter in the metaphysis of an unfused long bone.

The trabeculae in the cyst may create a soap-bubble appearance in the lesion.

The margins of the lesion are well defined, with a smooth inner margin and a rim of bone sclerosis. The tumor does not usually extend into the epiphyseal plate until after complete fusion, when it may occasionally do so. The expansion or ballooning of the cortex occasionally may result in the loss of the sharp definition of its margin. In this case, the finding should correctly be interpreted as an aggressive lesion rather than as solely diagnostic of malignant change.

New bone may horizontally traverse the angle between the original cortex and the expanded part of the bone; this occurs because the periosteum is lifted. No periosteal reaction occurs, except when the periosteum is fractured.

Typically, the spinal lesion is osteolytic, with a predilection for the posterior elements. The lesion may involve the lamina, arches, pedicles, or spinous processes, with or without extension into the vertebral body. The lesion may extend into the adjacent vertebral body, violating the intervertebral disk and causing vertebral collapse and/or extension into the spinal canal, adjacent ribs, and paravertebral soft tissues.

As in the innominate bones, flat bones have osteolysis with an expansile lesion.

Expanded bone may displace the adjacent viscera, such as the urinary bladder, when occurring in the pelvis.

Lesions in the skull have osteolysis, with expansion of both inner and outer tables with intracranial extension.

Mandibular and maxillary lesions are multilocular, expansile, and osteolytic. They predominate in the region of the molar teeth.

Aneurysmal bone cysts are difficult to distinguish from malignant lesions in some locations. It may mimic a sarcoma in the ribs, scapula, or sternum, especially when associated with a large soft-tissue component.

CT may be necessary to define the extent of involvement before surgery or other treatment.

 

Cross-sectional CT is the most useful imaging examination, because it can demonstrate the intraosseous and extraosseous extents of the lesion. CT can be used to determine the nature of the matrix of the tumor, especially when tumors are in complex locations, such as the facial skeleton, spine, thoracic cage, and pelvis.

Fluid-fluid levels may be seen in the cysts. Fluid levels are depicted only when the patient is lying motionless for about 10 minutes and when the scans are obtained in the plane perpendicular to that of the fluid levels. Fluid-fluid levels also are seen in many other bone lesions, and this finding is not specific to aneurysmal bone cysts. These levels may be seen in malignant and benign lesions, such as giant cell tumors, and in telangiectatic osteosarcomas.

CT in the spine can demonstrate stenosis of the spinal canal due to involvement of the posterior elements.

T1-weighted images show predominantly low to intermediate signal intensity with or without fluid levels. Acute hemorrhage into the cyst may have high signal intensity.

T2-weighted images show areas of low to intermediate signal intensity or some areas of heterogeneous high signal intensity, depending on the contents of the cyst. A rim of low signal intensity with internal septa may produce a multicystic appearance.

MRI images of aggressive lesions show tumor enhancement with gadolinium enhancement, especially when they are associated with other tumors.