Findings: Large expansile lytic lesion involving the posterior elements of the L3 vertebra. This lesion extends into the posterior 2/3 of the L3 vertebral body with sparing of the anterior aspect. A large expansile component extends into the left paraspinal muscles. The lesion measures 5.0 x 7.5 x 5.6 cm in AP, transverse and craniocaudad dimensions. Fine septations are seen within the expansile portion of the mass. The borders are lobulated with a thin calcific rim. Small flecks of calcification are identified within the lesion. There is mass effect upon the thecal sac.

Diagnosis: Aneurysmal bone cyst.

Discussion: Expansile lesion containing thin-walled, blood-filled cystic cavities.
Types:
1. Primary nonneoplastic lesion (70%)
2. Secondary lesion arising in preexisting bone tumors (30%)

• Giant cell tumor
• Chondroblastoma
• Osteoblastoma
• Fibrous dysplasia

Pathogenesis

1. Trauma possibly related to venous obstruction or AVF.
2. Coexistence of ABC with companion lesion possibly leads to local hemodynamic changes.

Clinical

First to third decade of life.
Pain of relatively acute onset with rapid increase of severity over 6-12 weeks.

Skeletal Location

Most frequent in the long tubular bones and spine.
Order of decreasing frequency - tibia, vertebra, femur, humerus, innominate bone, fibula, ulna, etc.
Within the long tubular bones, aneurysmal bone cysts are seen almost exclusively in the metaphysis.
Vertebral aneurysmal bone cysts generally arise in the posterior elements; however, may extend into
the vertebral body.

Plain Film

Eccentric expansile lytic lesion
Cortex thinned; however, maintained
No periosteal reaction
Respects the epiphysis

MR

Multiple cysts of different signal intensity representing different stages of blood by-products.
Fluid-fluid levels from layering blood are not pathognomonic and can be seen in the other osseous
lesion, i.e., giant cell tumor, simple bone cyst and chondroblastoma.

Ozaki T, et al. Aneurysmal Bone Cysts of the Spine, Arch
Orthop Trauma Surg 1999;119(3-4):159-162.

Paragelopoulos PJ, et al. Aneurysmal Bone Cyst of the Spine.
Management and Outcome, Spine 1998 Mar; 23; (5):621-628.

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