Unicameral Bone Cyst (UBC)
Unicameral bone cyst (UBC) is a benign, fluid-filled intramedullary lesion typically located in the metaphysis or diaphysis of long bones in children and adolescents. It is usually unilocular and adjacent to the cortex. Pathological fracture is the most common presentation.
1.Associated Conditions
UBC may coexist with other lesions:
Secondary aneurysmal bone cyst (ABC)
Fibrous dysplasia
Post-infectious cystic lesions
Post-traumatic intramedullary cysts
2. Epidemiology
Age: 5–15 years
Sex: More common in males (~2:1)
Most common sites: Proximal humerus (50–60%), proximal femur (25–30%)
Rarely in other long bones, pelvis, or calcaneus
3. Pathogenesis
The exact etiology is unclear; proposed mechanisms include intramedullary circulation disturbance and venous obstruction, leading to increased intramedullary pressure and cyst formation. Persistent fluid communication with the growth plate is seen in some cases.
4. Clinical Features
Usually asymptomatic
Most often presents with pathological fracture
Pain typically related to fracture
Cortical thinning in large cysts may cause deformity
5. Imaging
Radiograph:
Metaphyseal or diaphyseal intramedullary location
Unilocular, well-defined, homogeneous radiolucency
Marked cortical thinning
“Fallen fragment sign” (bone fragment within cyst after fracture)
CT: Shows cyst wall and cortical thinning
MRI: T2 hyperintense fluid; thin cyst wall; septa are uncommon
Simple bone cysts typically appear as well-defined, centrally located, intramedullary lucent lesions within the metaphysis of long bones, most often the proximal humerus or femur. They demonstrate a narrow zone of transition, thin sclerotic margins, and may cause mild endosteal expansion or cortical thinning without cortical destruction or periosteal reaction.
On plain radiographs,they are usually unilocular, although pseudotrabeculation may occasionally be seen. In the presence of a fracture, a fallen fragment sign or rising bubble sign may be visible. CT confirms the cystic nature and extent of the lesion but adds little diagnostic advantage.
On MRI,simple bone cysts show low signal intensity on T1-weighted and high signal on T2-weighted sequences, reflecting their fluid content. Post-contrast images demonstrate only thin peripheral enhancement corresponding to the cyst wall, without solid or nodular components. There is typically no marrow or soft-tissue edema, helping distinguish SBCs from more aggressive cystic or neoplastic processes.
6. Histology
Gross:
Thin-walled, unilocular, fluid-filled cyst
Lined by fibrous tissue; inner surface smooth
Microscopic:
Thin cyst wall of fibrovascular connective tissue
Sparse fibroblasts, macrophages, and foam cells within wall
Hemosiderin deposits and cholesterol clefts may be present
Variable inflammatory cell infiltrate
Occasional new bone trabeculae within wall
Differential Diagnosis: Aneurysmal bone cyst, intraosseous ganglion, fibrous dysplasia
7. Treatment and Natural History
Small/asymptomatic: Observation
Large/high fracture risk:
Minimally invasive: Steroid injection, bone marrow aspirate injection
Surgical: Curettage + bone graft ± internal fixation
Recurrence rate: 10–30%; follow-up until growth plate closure recommended
