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Unicameral bone cyst

Tumor biology

  • Benign cystic lesion of bone.
  • Lesion is filled with yellow-green fluid (may be blood-tinged if pathologic fracture has occurred)

Age

  • 80-90% of patients are younger than 20 years of age
  • Younger patients tend to have more active lesions

Gender

Slight male preponderance

Presentation

  • Lesions may be asymptomatic and identified incidentally
  • 50% of patients present with pathologic fracture, often due to minimal trauma
  • Some patients present with swelling or stiffness of adjacent joint

Physical findings

Tender if fractured; fractures often incomplete or stable

Blood work

Aspiration of cyst yields fluid high in alkaline phosphatase

Plain films

Conventional radiographs usually diagnostic; other radiographic tests not generally required

Site
  • Central medullary lesion
  • Thought to begin in metaphysis and migrate into diaphysis with bone growth
    • Lesions are metaphyseal or metaphyseal/diaphyseal; can transverse physis and extend into epiphysis (very rare)
  • 80% of cases involve of proximal humerus or proximal femur
  • Other sites of involvement include ilium, calcaneus, and talus; usually found in older patients
Size

Lesions can grow to be quite large

Tumor effect on bone
  • Lytic lesions; borders somewhat lobulated
  • If patient has pathologic fracture, look for "fallen fragment" or "fallen leaf" sign; caused by cortical fracture fragment displaced centrally into fluid-filled cyst
Bone response to tumor
  • May have thin sclerotic margin
  • May cause bony expansion
  • Generally geographic lesions
  • May be new periosteal reaction in response to pathologic fracture, even if undisplaced
Matrix

None

Cortex
  • Not unusual for cortex to undergo endosteal erosion circumferentially
  • Periosteum usually not involved; however, will thicken following pathologic fracture
Soft tissue mass

None

Bone scan

Can demonstrate central cold area (fluid) with peripheral uptake

MRI: Unicameral bone cyst versus aneurysmal bone cyst

Aneurysmal bone cyst

  • Often multicystic
  • Fluid/fluid level in a cystic lesion generally aneurysmal bone cyst
  • Markedly expansile.

Unicameral bone cyst

  • Will have a fluid/ fluid level only if hemorrhage has occurred secondary to fracture
  • Only minimal bony expansion

Differential diagnosis

Based on expansile metaphyseal lesion in a child

  • Aneurysmal bone cyst
  • Generally eccentric with cortical erosion and neo-corticalization
  • Fibrous dysplasia
  • Chondromyxoid fibroma
  • Telangiectatic osteosarcoma
  • Infection
  • Nonossifying fibroma

Pathology

  • Gross: Cyst that may be fluid filled
  • Microscopic: If cyst wall obtained, will see fibrous tissue that may be lined by flattened spindle cells
  • Reactive bony changes may be present

Diagnosis and treatment

  • Plain films and aspiration usually diagnostic
  • Aspiration generally returns green-yellow fluid
  • If no fluid is obtained, or it is hemorrhagic, move to open biopsy
  • Repeated aspirations and injections of steroid or autologous bone marrow are treatments of choice in lesion without pathologic fracture
    • Technique may take 6-12 months to yield partially healed lesion
  • Pathologic fractures heal without undue delay and are unaffected by steroid injection
    • Generally one will delay treatment of a fracture until it is healed to prevent leakage of steroid
  • Curettage with bone graft may be indicated if structural integrity of bone is at risk
  • Local recurrence rate is high and increases with incomplete excision, however

Complications

Pathologic fracture

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