Simple/ Unicameral bone cyst

A unicameral bone cyst (UBC), (also known as a simple bone cyst) is a relatively common benign lucent bony lesion, which is seen in childhood and typically remains asymptomatic.


These lesions are usually asymptomatic and found incidentally, although pain, selling and stiffness of the adjacent joint also occur. The most frequent complication is pathological fracture, and this is frequently the cause of presentation 

When uncomplicated by fracture the cysts contain clear serosanguineous fluid surrounded by a fibrous membranous lining. It is thought to arise as a defect during bone growth which fills with fluid, resulting in expansion and thinning of the overlying bone.

During the active phase the cyst remains adjacent to the growth plate. As the lesion becomes inactive it migrates away from the growth plate (normal bone is formed between it and the growth plate) and it gradually resolves 

UBCs are sharply demarcated (narrow transitional zone) lucent lesions with no periosteal reaction. They sometimes expand the bone with thinning of the overlying bone. Prominent ridges of bone can make it appear multiloculated. In less common instances they are truly multiloculated ³.

If there is fracture through this lesion a dependent bony fragment may be seen, and this is known as the fallen fragment sign(arrow head).