A unicameral (simple) bone cyst is a cavity found within a bone that is filled with straw-colored fluid. It is a benign (non-cancerous) condition.

Unicameral bone cysts (UBC) are usually found in patients younger than 20 years of age. Unicameral bone cysts occur in one bone, in one location. The location of the cysts tends to be in the upper arm (proximal humerus) or thighbone (proximal femur). Less common locations include the pelvis, ankle (talus), or heel (calcaneus).

There is no known cause. Little is known about the development of unicameral bone cysts. There are theories involving a growth defect or disturbance at the epiphyseal plate leading to the formation of a unicameral bone cyst. However, this is not completely understood. No preventive measures can be recommended.

Most unicameral bone cysts have no symptoms. Some are discovered incidentally. Other unicameral bone cysts are not noticed at all.

If a unicameral bone cyst is thinning the bone, there may be pain with weightbearing activities. If there is a pathologic fracture through the cyst, the affected arm or leg may have pain, swelling, and deformity.

A doctor will be able to diagnose a unicameral bone cyst. He or she may use plain X-rays to show a hollow cavity in the metaphyseal portion of the bone.

There is no associated bone expansion. A well-defined cortical rim can be seen. On rare occasions, the cyst may cross the physis of an immature patient and extend into the epiphysis.

If the UBC is not typical in its appearance, a computed tomography scan or magnetic resonance image may help determine the architecture of the bone.