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. It accounts for the S (simple bone cyst) in FEGNOMASHIC the commonly used mnemonic for benign lytic bone lesions. 

Epidemiology

They usually found in children in the 1^st and 2^nd decades (65% in teenagers), and are more common in males (M : F ~ 2 - 3 : 1) ^2,6 .

Clinical presentation

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 ^1-2,6.

Location

They are typically intramedullary and are most frequently found in the metaphysis of long bones, abutting the growth plate ¹. Locations include ^1-2,5:

• proximal humerus : most common ~ 50 - 60%
• proximal femur : ~ 30%
• other long bones
• everywhere else is relatively uncommon
  • spine : usually posterior elements
  • pelvis : only 2% of UBC ¹

Pathology

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 ^3,5.

Radiographic features

Plain film 

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.

CT & MRI

CT and MRI add little to the diagnosis, but are however helpful in eliminating other entities that can potentially mimic a simple bone cyst (see differential diagnosis below).

MR signal characteristics for an uncomplicated lesion include

• T1 : low signal 
• T2 : high signal 

Usually there no fluid-fluid levels unless there has been a complication with haemorrhage.

Scintigraphy

Tends to be photopaenic (cold spot). 

Treatment and prognosis

Intervention is usually not required for asymptomatic lesion. If large and threatening to fracture, or causing deformity then intra-lesional steroid injection can be performed ^3-5. If fractured the bone usually heals normally ⁵. In some instances surgery with curettage and bone grafting is required.

Differential diagnosis

• aneurysmal bone cyst (ABC)
• giant cell tumour : usually older, extending to articular surface
• non ossifying fibroma
• eosinophilic granuloma (EG)