Unicameral bone cysts (simple bone cystss) are tumor-like lesions of unknown etiology that make up about 3% of all primary bone lesions. They are thought to represent a local reactive or developmental growth disturbance. They are more commonly found in boys in the first 2 decades of life, in the proximal diaphyses of the humerus and femur.
Unicameral bone cysts are well-defined, lucent lesions with sclerotic margins that are located centrally in the metaphysis or diaphysis of long bones. There is frequently cortical thinning and expansion, but the width of the cyst is typically less than that of the nearby physis. Epiphyseal extension is not typical, and periosteal reaction is not seen unless the cyst is complicated by a pathologic fracture.
In about 20% of cases of fractures through unicameral bone cysts, a fallen fragment sign is present, representing a fragment of fractured cortex laying in the dependent portion of the fluid-filled lesion. The forme fruste of the fallen fragment sign is the trap door sign, which represents an infolded fragment of cortex that remains attached to the periosteum.
Radiography is usually diagnostic, and CT can be used in equivocal cases. The cyst fluid has attenuation of 15-20 HU and the radiographic findings noted above can be seen to better advantage. Bone scintigraphy may reveal a nonspecific halo of increased uptake around the photopenic cyst, but can also be normal.
MRI reveals a fluid-filled lesion that is low to intermediate signal on T1-weighted images and homogeneously hyperintense on T2-weighted images.
The main differential consideration in the long bones is an aneurysmal bone cyst. Aneurysmal bone cysts almost invariably have some degree of periosteal reaction (usually solid), are eccentrically located, and can have significant cortical expansion.
The radiograph above is from a 4-year-old boy. There is a central, well-defined lucent lesion in the proximal metadiaphysis of the humerus that results in mild cortical thinning and mild expansion. Pseudoseptations can be seen within the lesion. No periosteal reaction is present.