Unicameral Bone Cyst
Simple Bone Cyst, Solitary Bone Cyst

General Considerations

• Common (3-5% of primary bone tumors)
• Benign
• Solitary lesion
• Most common in proximal humerus and femur
  • Also Iliac bone and calcaneous, especially over age 20
• Most common between 4-10 years old
  • Males:Females 3:1
• Filled with clear, yellowish fluid
  • May contain giant cells and hemosiderin 

Clinical Findings

• Asymptomatic
• If fractured, then
  • Pain
• Limited range of motion 

Imaging Findings

• Conventional radiography is the study of choice
• Most common in proximal humerus (in patients under 20) and femur in skeletally immature male
• Solitary, lytic metaphyseal lesion adjacent to, but not crossing, the epiphyseal plate
  • Migrates towards diaphysis during growth of child
• Well-defined margins with narrow transition zone
• May be slightly expansile
• Long axis of lesion is parallel to long axis of bone
• May have thin sclerotic margin
• Endosteal scalloping and erosion
• Fallen Fragment Sign represents a fragment of bone that falls into the cyst and then into a dependent position
  • Although uncommon, it is pathognomonic for a simple cyst because it indicates the fluid nature of the interior of this lytic lesion
• On CT, they may be shown to contain air-fluid or fluid-fluid levels
• On MRI. They will have low signal intensity on T1 and high on T2
  • Lesions which have fractured will have a heterogeneous signal on T1 and T2 because of the hemosiderin
• Their periphery may enhance with Gadolinium
• Photopenic on bone scan 

Differential Diagnosis

• Fibrous dysplasia
  • Ground-glass; more irregularly shaped
• Eosinophilic granuloma
  • Look for other lesions; beveled-edge; vertebra plana
• Chondroblastoma
  • Extends into epiphysis
• Chondromyxoid fibroma
  • More expansile and eccentric; more rare
• Brown tumor
  • Other signs of hyperparathyroidism
• Aneurysmal bone cyst
  • More expansile
• Enchondroma
  • Smaller bones; internal calcifications 

Treatment

• Curettage and bone grafting, nailing, injection of bone marrow or cryotherapy to prevent pathologic fracture
• Methylprednisolone injections have been used to promote healing 

Complications

• Pathologic fractures in 50-65%
• Growth arrest in affected limb 

Prognosis

• Usually undergo spontaneous regression

Unicameral Bone Cyst. A single frontal radiograph of the shoulder
demonstrates a geographic lytic lesion (yellow arrow) in the metadiaphysis
of the humerus with a sharp zone of transition and inner septations. There is no pathologic fracture through the lesion.
For this same photo without the arrow, click here

There is a pathological fracture (white arrow) through this septated, unicameral bone cyst in the classic location of the proximal humerus.

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Simple Bone Cyst eMedicine Eu-Leong Harvey Teo and Wilfred CG Peh.