Unicameral Bone Cyst (UBC)

Unicameral Bone Cyst (UBC)

Prepared by Dr. Enes KANAY

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

Fallen leaf sign on Xray