Unicameral bone cysts (UBC) (also known as a simple bone cysts) are relatively common benign lucent bony lesions that are seen in childhood and typically remain asymptomatic. They account for the S (simple bone cyst) in FEGNOMASHIC the commonly used mnemonic for lytic bone lesions.
They usually found in children in the 1st and 2nd decades (65% in teenagers), and are more common in males (M:F ~ 2-3:1) 2, 6.
These lesions are usually asymptomatic and found incidentally, although pain, swelling 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.
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
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.
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.
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 intralesional steroid injection can be performed 3, 4, 5. If fractured the bone usually heals normally 5. In some instances surgery with curettage and bone grafting is required.
General imaging differential considerations include
- aneurysmal bone cyst (ABC)
- giant cell tumour of bone - usually older, extending to articular surface
- non ossifying fibroma
- eosinophilic granuloma (EG)
- haemophilic pseudotumour (intraosseous)
The differential diagnosis for bone tumours is dependant on the age of the patient, with a very different set of differentials for the paediatric patient.
- bone-forming tumours
- enchondromatosis (Ollier disease)
- Maffucci syndrome
- chondromyxoid fibroma
- fibrous bone lesions
- bone marrow tumours
- other bone tumours or tumour-like lesions
- skeletal metastases
- 1. Hammoud S, Weber K, Mccarthy EF. Unicameral bone cysts of the pelvis: a study of 16 cases. Iowa Orthop J. 2005;25 : 69-74. - Free text at pubmed - Pubmed citation
- 2. Coskun B, Akpek S, Dogulu F et-al. Simple bone cyst in spinous process of the c4 vertebra. AJNR Am J Neuroradiol. 2004;25 (7): 1291-3. AJNR Am J Neuroradiol (full text) - Pubmed citation
- 3. Chew FS, Bui-Mansfield LT, Kline MJ. Musculoskeletal imaging. Lippincott Williams & Wilkins. (2003) ISBN:0781737974. Read it at Google Books - Find it at Amazon
- 4. Lovell WW, Winter RB, Morrissy RT et-al. Lovell and Winter's pediatric orthopaedics. Lippincott Williams & Wilkins. (2006) ISBN:0781753589. Read it at Google Books - Find it at Amazon
- 5. Skinner HB. Current diagnosis & treatment in orthopedics. McGraw-Hill Medical. (2003) ISBN:0071387587. Read it at Google Books - Find it at Amazon
- 6. Greenspan A, Jundt G, Remagen W. Differential diagnosis in orthopaedic oncology. Lippincott Williams & Wilkins. (2006) ISBN:0781779308. Read it at Google Books - Find it at Amazon
- 7.Blumberg ML. CT of iliac unicameral bone cysts. AJR Am J Roentgenol. 1981;136 (6): 1231-2. doi:10.2214/ajr.136.6.1231 - Pubmed citation
Synonyms & Alternative Spellings
|Synonyms or Alternative Spelling||Include in Listings?|
|Unicameral bone cyst||✗|
|Unicameral bone cyst (UBC)||✗|
|Simple bone cyst||✓|
|Simple bone cysts||✗|
|Unicameral bone cysts||✗|