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