Unicameral Bone Cyst

Key Points:


Unicameral bone cysts (UBCs) are benign intramedullary lytic lesions that occur mainly in the metaphysis of long bones.  The most common locations are the proximal femur and proximal humerus (Wilkins, 2000).  UBCs that occur in the diaphysis are rare and known as latent cysts (Pretell-Mazzini, 2014).  The bone surrounding a UBC is non-reactive.  There are various theories on the cause of UBC, but currently there is no known cause.  The most widely accepted theory is that there is a defect in metaphyseal remodeling that blocks interstitial fluid drainage which increases pressure and leads to bone necrosis and fluid accumulation (Canale, 2013). 


UBCs account for 3% of all bone tumors. They commonly occur in patients age 4-10 years old (Herring, 2014).  Males are twice as likely to have a UBC as females.

Clinical Findings:

UBCs are usually asymptomatic and are often recognized on radiographs obtained for other reasons.  Therefore, many go unrecognized until a traumatic event causes a pathologic fracture at the site of the lesion.  Once this occurs, presenting symptoms are pain, swelling, erythema and deformity of the affected bone (Pretell-Mazzini, 2014). 

Imaging Studies:

Radiographic images of the affected area show a metaphyseal lesion that is symmetrically expansile, lytic and radiolucent.  There is a thin cortical rim that is usually not compromised if no fracture is present.  There is no periosteal reaction unless there is a fracture. The “fallen leaf” sign is pathognomonic for UBC and represents the part of the fractured cortex that settles to the most dependent part of the cyst (Herring, 2014). 
CT scans are usually not obtained for UBC evaluation except when the lesion cannot easily be seen on radiographs. MRI shows a cystic lesion with low signal intensity on T1 weighted images and high signal intensity on T2-weighted images (Pretell-Mazzini, 2014). 


Non-operative treatment is the choice for most UBCs. Most UBCs in the upper extremity and those in the lower extremity with a low risk of pathologic fracture are treated with observation and serial radiographs. Surgical treatment is indicated when UBCs are symptomatic, rapidly enlarging and/or are at risk for pathologic fracture, such as in the proximal femur.  Curettage with or without bone grafting shows modest healing rates and high recurrence rates.  Other treatment options include aspiration and injection of the lesion with corticosteroids, bone marrow aspirates or demineralized bone matrix, all with questionable superiority over other surgical options.  In locations at risk for pathologic fracture, prophylactic internal fixation can be considered (Pretell-Mazzini, 2014). 


Recurrence is a common complication of treatment of UBCs. The recurrence rate after curettage and bone grafting can be as high as 50% (Canale, 2013).

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  1. Canale, S. Campbell’s Operative Orthopaedics. Philadelphia, PA: Elsevier: 2013
  2. Herring, J. A., & Texas Scottish Rite Hospital for Children, (2014). Tachdjian's pediatric orthopaedics: From the Texas Scottish Rite Hospital for Children.
  3. Pretell-Mazzini J, Murphy RF, Kushare I, Dormans JP:  Unicameral Bone Cysts: General Characteristics and Management Controversies. J Am Acad Orthop Surg 2014; 22: 295-303. 
  4. Wilkins RM: Unicameral Bone Cysts. J Am Acad Orthop Surg 2000; 8(4): 217- 222.

Top Contributors:

Christine Stairs MD
Janay Mckie MD