Aneurysmal Bone Cyst

Aneurysmal bone cysts are benign lesions of unknown origin. They can be classified as primary or secondary, with the latter representing aneurysmal cystic change superimposed on a preexisting bone lesion. In ~30% of cases, a preexisting lesion can be identified. More common preexisting lesions include giant cell tumor (30% of cases), osteoblastoma, angioma, and chondroblastoma. Uncommonly, the precursor lesion is fibrous dysplasia, fibrous histiocytoma, Langerhans cell histiocytosis, osteosarcoma, and even metastasis.

The age distribution of secondary aneurysmal bone cysts is determined by that of the underlying tumor. Primary aneurysmal bone cysts usually occur in patients younger than 20 years of age and tend to occur in the metaphyses of the long bones of the lower limbs. Epiphyseal extension can occur after fusion of the growth plate.

Flat bones of the pelvis and the spine can also be affected. In the spine, both the vertebral body and posterior process are involved in the majority of cases. More than one vertebra may be involved in about 40% of cases, and thoracic spine lesions can be accompanied by rib lesions. Surface aneurysmal bone cysts (those arising from subperiosteal and cortical regions) do not tend to affect the flat bones.

Capanna et al classified aneurysmal bone cysts in the long bones based on their location.

• Type 1: Central with little expansion.
• Type 2: Central with expansion and cortical thinning.
• Type 3: Eccentric with involvement of only one cortex.
• Type 4: Subperiosteal with outward growth and little or no superficial cortical erosion. Usually occur in the diaphysis.
• Type 5: Subperiosteal with both inward and outward growth and cortical destruction. Usually occur in the diaphysis.

Etiology

Various etiologies have been suggested for aneurysmal bone cysts, including 1) reaction to a vascular phenomenon (e.g., arteriovenous malformation, thrombosis of a large vein leading to intramedullary venous engorgement), 2) neoplasm (USP6 and CDH11 oncogenes in primary aneurysmal bone cysts), and 3) trauma (especially surface aneurysmal bone cysts).

Imaging

On radiographs, we usually see a geographic lesion with well-defined borders and a narrow zone of transition, although the radiographic appearance varies with the phase of the lesion (see below). The margin can be sharp but not sclerotic in 55% of cases, sclerotic in 30% of cases, and indistinct in 15%.

• Initial phase: Osteolysis and periosteal elevation.
• Active growth phase: Rapid enlargement and bone destruction. There may be poor demarcation of the lesion with an invisible external border and other aggressive features (Codman triangle, lamellated periosteal reaction). Most patients are symptomatic.
• Stabilization phase: Thin shell of periosteal bone, septal ossification, and mature periosteal reaction (cortical/periosteal buttress at the interface with normal bone). This is the classic appearance of aneurysmal bone cysts.
• Healing phase: Gradual ossification of the aneurysmal bone cyst. Matrix mineralization and trabeculation may be seen. May be the same thing as the solid variant of aneurysmal bone cysts (see below).

CT reveals a lesion with a thin surrounding shell of bone. A thin shell of soft tissue attenuation, representing the fibrous periosteum, can also be seen. An extraosseous mass is rare. About 1/3 of lesions will have fluid-fluid levels on CT, with the higher attenuation material layering dependently.

MRI reveals fluid-fluid levels, septae, a low-signal intensity rim of periosteum, surrounding edema, and peirpheral and septal enhancement.

Primary aneurysmal bone cysts have thin septae and minimal or no solid component. Secondary aneurysmal bone cysts tend to have nodular septae and a larger solid component. In a study of 83 lesions with fluid-fluid levels, when cystic spaces (fluid-fluid levels) occupied less than 1/3 of the lesion, more than 2/3 of the lesions were malignant, and half of these malignant lesions were osteosarcomas. If more than 2/3 of the lesion contained cystic spaces (fluid-fluid levels), 90% of the lesions were benign. If the entirety of the lesion contained cystic spaces (fluid-fluid levels), 100% of the lesions were benign. Biopsy should target the solid component

More recently, it has been suggested that T1 signal higher than skeletal muscle in the antidependent layer may point to malignancy, with the thought that the high T1 signal represents recent hemorrhage and liquefied, necrotic tumor, compared to serous fluid and old blood seen in benign lesions.

Solid Variant

Also known as an extragnathic giant cell reparative granuloma, solid aneurysmal bone cysts are uncommon lesions that are histologically identical to the solid components of aneurysmal bone cysts without the blood-filled cavities.

The affected age group is similar to regular aneurysmal bone cysts, but the distribution is slightly different, with metaphyseal and diaphyseal locations being more equal in incidence. Histologically, the lesions are characterized by intraosseous hemorrhage and may represent the healing stage of a conventional aneurysmal bone cyst.

On MRI, the lesions are predominantly solid with high signal on T1- and T2-weighted images and surrounding edema, which may be striking.

References

• Capanna R, Bettelli G, Biagini R, Ruggieri P, Bertoni F, Campanacci M. Aneurysmal cysts of long bones. Ital J Orthop Traumatol. 1985 Dec;11(4):409-17.
• O'Donnell PG. Chapter 24: Cystic bone lesions. in Imaging of Bone Tumors and Tumor-Like Lesions. Davies AM, Sundaram M, and James SLJ (eds). Springer-Verlag Berlin Heidelberg (2009); pp 430-438.
• O'Donnell P, Saifuddin A. The prevalence and diagnostic significance of fluid-fluid levels in focal lesions of bone. Skeletal Radiol. 2004 Jun;33(6):330-6.
• Parman LM, Murphey MD. Alphabet soup: cystic lesions of bone. Semin Musculoskelet Radiol. 2000;4(1):89-101.
• Rodallec MH, Feydy A, Larousserie F, Anract P, Campagna R, Babinet A, Zins M, Drapé JL. Diagnostic imaging of solitary tumors of the spine: what to do and say. Radiographics. 2008 Jul-Aug;28(4):1019-41.