Visual diagnosis: an 11-year-old girl who has left leg pain.

Unicameral bone cysts (UBCs) in children usually are asymptomatic. Most UBCs are discovered when a radiograph is performed on a child who has had accidental trauma to a limb. Symptomatic cysts typically present with pain, often the result of pathologic fracture through a large cyst or occult stress fracture within the thinned cortex around the cyst. Simple radiography is the best method for detecting such cysts, which typically are located within the long bone (femur, tibia, fibula, humerus), but can appear elsewhere. Cysts typically appear in the proximal metaphysis, but some involve the epiphysis and growth plate, thereby affecting bone growth. If clinically necessary to confirm the diagnosis, computed tomography or magnetic resonance imaging can delineate the cyst better or demonstrate an occult fracture. For the asymptomatic UBC, close follow-up is the recommended course of action. However, surgical intervention by corticosteroid or autogenous bone marrow injection or open curettage with bone grafting is recommended if the cyst is symptomatic, carries an increased risk for pathologic fracture (weight-bearing bone or dominant arm of a throwing athlete), or shows signs of an impending pathologic fracture. Clinical and radiographic follow-up is recommended after surgical intervention, because UBC recurrence after initial surgery is reported to occur in 18% to 88% of patients.