Age / Sex : 14 / F

Age / Sex : 14/ Female

simple bone cyst

Discussion

Findings: On lateral projection of ankle, a well-defined radiolucent lesion is seen in the base of the calcaneal neck. T1 and T2 weighted sagittal MR images show the cystic nature of the lesion with fluid-fluid level. It is surrounded by thin sclerotic margin and adjacent bone marrow signal change is not associated.

Differential Diagnosis: Diagnostic considerations on plain radiograph are lipoma and thinning of trabeculae.   

Diagnosis: Simple bone cyst (solitary or unicameral bone cyst)

Discussion:
- Simple bone cyst is a common benign lesion of childhood of unknown cause. A cyst occurs mostly frequently in children aged 5-15 years and affects boys with greater frequency than girls (2:1). Most common location for the lesion is the metaphysis of the long bone (proximal humerus, followed by proximal femur), but epiphyseal and diaphyseal location can be rarely observed. Any bone may be affected, with the calcaneus being one of these notable alternate locations. These cysts are usually asymptomatic unless fractured. Cysts in the metaphysis of a tubular bone, occupying more than 85% of the transverse diameter of the bone are vulnerable to fracture.
- On plain radiographs, the lesion is typically an oval radiolucency with cortical thinning and mild osseous expansion. When fractured, a vertical fragment within the lesion migrates to a dependent portion of the fluid-filled cyst. This finding designated the “fallen fragment sign” is the classic radiographic finding for a simple bone cyst.

- Fracture of a simple cyst in the calcaneus is rare. In this site, the cyst almost invariably occurs in the base of the calcaneal neck just inferior to the anterior portion of the posterior facet. Its anterior margin is usually straight and vertical, whereas the posterior border is typically curvilinear and parallels the trabeculae in the posterior portion of the calcaneus. The differential diagnosis includes a lipoma and thinning of trabeculae, which occurs normally in this portion of the calcaneus.

- MR imaging can confirm the fluid content of a simple bone cyst. Internal septations and fluid-fluid levels may be detected. Enhancement of peripheral rim or within the internal septations is evident following intravenous contrast enhancement. After a fracture, the MR findings of a cyst are more complex.

References:
Resnick D. Diagnosis of bone and joint disorders. 4^th ed. Saunders Company 2002:4023