Simple bone cyst (Unicameral bone cyst)

• 3% of primary bone tumors. 
• Nearly always in first 2 decades.
• Most often between 4-10years.
• 2:1 male female ratio.

Pathology

• Common sites are proximal humerus – 50%, proximal femur- 18-27% and proximal tibia, distal tibia and calcaneum.
• Cyst cavity may become multiloculated after fracture healing.
• Categorized as latent and active.
  • Lesions <0.5cm from physis are considered active.
  • Epiphyseal involvement rare but seen in aggressive lesions.
  • Latent cysts also have growth potential.
• Recurrence and worsening rare after skeletal maturity.
• >75% detected in childhood.
• >95% involve metaphysis.
• Cyst is filled with fluid rich in protein content. Shown to have elevated levels of IL-1, IL-6, prostaglandins, lysosomal enzymes, IL1B, TNF-A, Nitric oxide production thought to be important.
• Oxygen free radicals thought to be important.
• Fluid shows increased intraosseous pressure of >30cm H2O.
• Fluid may be straw coloured or serosanguinous.
• Most characteristic histopathologic finding is thin membrane like lining of epithelium like cells.
• Osteoclasts, cholesterol cells and fat cells also seen.
• Genetic analysis shows single and multifocal cytogenetic rearrangements.

Theories about causation

Mirra- Intraosseous entrapment of synovium.

Jaffe and Lichtenstein- Localized failure of ossification during periods of rapid growth.

Cohen- Blockage of venous drainage.

Current evidence supports blockage of intraosseous venous circulation. 

Clinical features

Fractures occur in up to 90%.

Physeal closure occur after fracture in 10%.

Fracture readily heals but cyst does not.

Spontaneous healing of cyst after fracture occurs in <5%.

Radiological features

Symmetrically expansive, radiolucent with a thin shell of cortical bone.

Fallen fragment sign seen in less than 10%.

MRI shows complex appearance with heterogenous fluid signals with nodular and thick peripheral enhancements.

Double density fluid levels, septation and low signal on T1 with high signal on T2 suggest ABC.

Differential diagnosis

ABC

Fibrous dysplasia

Atypical eosinophilia

Treatment

Treatment if necessary should be undertaken immediately after the fracture ha healed.

Treatment more aggressive in younger children and in older individuals with lesions in weight bearing bones.

Treatment options

Corticosteroid injection

Injection of autologous marrow.

Multiple drilling and drainage

Curretage and bone grafting

Corticosteoid injection

Reported by Scaglietti in 1979.

Healing occurs in 90%.

Thickening of cortex, increasing opacity, remodelling and bone scar are signs of healing.

Cysts with rapid venous flow shown by radiografin instillation show higher rates of failure.

Look for loculation.

Each locule must be injected.

40-120mg of methylprednisolone injected.

Repeated every 2 months for 2-5 dosestill complete healing occurs.

Follow-up xrays taken every 2-3 months.

Temporary Cushing syndrome may be seen.

Other materials injected

Autologous bone marrow- Single injection recurrence rate of 12%

Demineralized bone matrix

Bioabsorbable calcium phosphate paste

Multiple drilling

Percutaneous drilling of multiple holes.

Lavage with saline

Multiple K wires, flexible nails or cannulated screws fixation

Displaced pathological fractures of femur needs curettage, bone  grafting and internal fixation.