IS case 445: Unicameral bone cyst

Meena Moorthy, MD, MBA

University of Rochester

Imaging Sciences URMC 2010
Publication Date: 2010-08-30


Patient is a 5-year-old male who sustained an injury during a soccer game four months ago. He was seen at an outside hospital, where radiographs of his left humerus showed a fracture through a cystic structure in the proximal diaphysis. He presented for follow-up of the cystic lesion two months later, when radiographs again show the lesion. Further evaluation was done with an MR a month later.


Initial radiograph of the left humerus showed a lucent lesion in the proximal diaphysis, with areas of sclerosis likely secondary to healing fracture. Follow-up imaging with MR revealed a large centrally located, slightly expansile lesion with internal septations in the proximal diaphysis. It demonstrated high T2 signal and low T1 signal intensity. Post-contrast images showed mild rim enhancement. There was some dependent sedimentation.


Unicameral bone cyst (UBC), or simple bone cyst, is a lesion of bone that is tumor-like and thought to be related to a local disturbance of bone growth. UBC is always central in location, and while approximately 60-75% are found in the proximal diaphysis of the humerus or of the femur, they can occur in any bone. Simple bone cysts occur almost exclusively in a younger population.

On plain radiographs, the UBC is seen as a well-circumscribed, radiolucent lesion with sclerotic margins. A characteristic feature is that there is NO periosteal reaction. An UBC on MRI demonstrates fluid characteristics on all sequences. UBCs are generally asymptomatic, however they are commonly fractured, which is what leads to their discovery. Fracture through a UBC leads to the classic radiographic finding of the " fallen fragment sign"; this is when a piece of the fractured cortex falls into the dependent portion of the cyst. This happens because an UBC is fluid-filled, not matrix filled.

Treatment used to be limited to curettage of the cyst, followed by grafting with small pieces of donor bone; however this can lead to damage to the growth plate as many of these lesions are periphyseal A newer treatment is injection of methylprednisolone acetate into the lesion to induce bone repair.


  1. Greenspan A. Orthopedic Imaging, 4th ed. Philadelphia: Lippincott, Williams & Wilkins, 2004.
  2. Helms CA, Brant WE. Fundamentals of Diagnostic Radiology, 3rd ed. Philadelphia: Lippincott, Williams & Wilkins, 2007: 1078-1079

2 images