Overview

 Unicameral bone cysts (UBC's) are benign bone lesions that are typically found in the pediatric population. 

Age

85% of UBC's present in the first 2 decades of life.

Gender

There is a 2.5:1 male to female ratio in incidence.

Presentation

UBC's may present as pathologic fractures or as incidental findings.  Approximately 2/3 of patients with a UBC will fracture through it.  Occasionally, a UBC will present as swelling or stiffness of the adjacent joint.

Plain Films

UBC's are typically centrally located, metaphyseal lesions that are often expansile and lytic.  The lesion will be surrounded by an area of thin, expanded cortical bone, which may reveal a nondisplaced or minimally displaced fracture.  The "fallen leaf sign" is associated with UBC's and represents a piece of cortex that has broken off and settled into the fluid filled cavity.  A UBC begins as a metaphyseal lesion that is adjacent to the physis of a growing child.  As the child approaches maturity, the UBC can move to a more diaphyseal location.  

Site

The most common site for UBCs in children is the proximal femur, followed closely by the proximal humerus.  In adults, the most common sites are the ilium and the calcaneus.

Bone Scan

A UBC can show a central cold area (fluid) with peripheral uptake. 

MRI

Differential Diagnosis

Differential includes aneurysmal bone cyst (ABC) and fibrous dysplasia. 

Histopathology

 The lining of a UBC is made up of fibrobasts.

Treatment