Radiological Diagnosis

Aneurysmal bone cyst of the first thoracic vertebra.

The plain radiographs show destruction of the spinous process of D1. The CT shows an expansile lesion involving the posterior elements of D1.

Aneurysmal bone cysts (ABCs) are osteolytic lesions of bone, which are capable of very rapid progression. Its name is a misnomer because it is neither an aneurysm nor a cyst [1]. In approximately 80% of patients, ABCs involve the long bones and vertebrae. In the spine, thoracic, lumbar, cervical and sacral are affected in decreasing order of frequency. In the vertebrae the lesion involves the posterior arch, spinous process or transverse process. The vertebral body is not often affected [2].

About 78% of ABCs are seen below the age of 20 years in contrast to giant cell tumors, 85% of which occur in patients who are above the age of 20 years [3]. The sex ratio - male: female is 1:1 [2]. The patient presents with painful swelling, limited range of motion or pathological fracture. Spinal lesions compromise the canal and compress the cord.

The roentgenologic findings are often characteristic, the lesion is expansile and destroys the bone. The cortex is often thin but generally intact and sometimes there is a collapse of the vertebra [4]. The lesions may extend along the spine and several vertebrae may be destroyed [1]. It is the only benign lesion that extends from one bone to another [1],[4]. Radiologically ABCs still cause difficulty in differential diagnosis with respect to giant cell tumors, bone cysts, osteochondroma, osteosarcoma, hemangioma and Ewing's tumor [5].

CT scan demonstrates the expanded and thinned cortex with adjacent calcified or non-calcified periosteal membrane and thick trabeculations crossing the lesion. Extension into soft tissue can be seen. Fluid-fluid levels are usually detected in patients who are scanned after being immobilized for 10 minutes appearing as lower attenuation components in less dependent and dense components in more dependent areas [1]. These levels do not enhance with contrast. The differential diagnosis of fluid-fluid levels are telangiectatic osteosarcoma, simple bone cyst, chondroblastoma, fibrous dysplasia, giant cell tumor, malignant fibrous histiocytoma, synovial sarcoma and soft tissue hemangioma. The solid component of the tumor enhances with contrast.

MR characteristics of ABCs include a low signal rim separating the cyst from the medullary cavity of the affected bone; well defined margins separating the ABC from adjacent soft tissues; generalized inhomogeneity of the lesion with septae showing low signal on T1W and increasing signal on T2W images. The MR signal characteristics of fluid-fluid levels are variable and often differ from cavity to cavity within the same lesion, most likely due to different stages of blood breakdown within the cavities [6].

Aneurysmal bone cysts are benign blood-filled lesions and consist of multiloculated spaces lined by granulation tissue or osteoid. Numerous giant cells are also seen. The cellular component can be a predominant feature forming a substantial area of solid tissue. The etiology of ABC is not clear. It may result from a reactive process with alteration in local hemodynamics related to venous obstruction and arteriovenous fistulas. Trauma may play an important role in the pathogenesis of aneurysmal bone cysts. Thirty to forty percent cysts are associated with other lesions like non-ossifying fibroma, osteoblastoma, osteosarcoma, simple cyst, chondromyxoid fibroma, fibrous dysplasia and others [7].

In the spine, ABC must be differentiated chiefly from osteoblastoma and Langerhan's cell histiocytosis of bone. Infection must be considered if there is involvement of vertebral body or if soft tissue extension is prominent [6].