Unicameral bone cyst

Original Clinical Contribution: Authored for the OrthopaedicsOne Musculoskeletal Oncology Specialty Section. Preserved, digitized, and updated to modern evidence-based standards.

1. Tumor biology

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

2. Age

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

3. Gender

Slight male preponderance

4. 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

5. Physical findings

Tender if fractured; fractures often incomplete or stable

6. Blood work

Aspiration of cyst yields fluid high in alkaline phosphatase

7. 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

8. Bone scan

Can demonstrate central cold area (fluid) with peripheral uptake

9. 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

10. 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

11. 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

12. 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

13. Complications

Pathologic fracture

14. Evidence-Based Synthesis

Modern clinical decision-making is grounded in high-quality randomized controlled trials and prospective registry analyses, corroborated by seminal biomechanical and anatomical descriptions.

15. References

• Ruiz-Arellanos K, Larios F, Inchaustegui ML et al. Treatment and Outcomes of 4,973 Unicameral Bone Cysts: A Systematic Review and Meta-Analysis. JBJS Rev. 2024. doi:10.2106/JBJS.RVW.23.00159 [Level I: Systematic Review / Meta-Analysis]
• Levy DM, Gross CE, Garras DN. Treatment of Unicameral Bone Cysts of the Calcaneus: A Systematic Review. J Foot Ankle Surg. 2015. doi:10.1053/j.jfas.2014.10.014 [Level I: Systematic Review / Meta-Analysis]
• Zhao JG, Wang J, Huang WJ et al. Interventions for treating simple bone cysts in the long bones of children. Cochrane Database Syst Rev. 2017. doi:10.1002/14651858.CD010847.pub3 [Level I: Systematic Review / Meta-Analysis]
• Zhao JG, Ding N, Huang WJ et al. Interventions for treating simple bone cysts in the long bones of children. Cochrane Database Syst Rev. 2014. doi:10.1002/14651858.CD010847.pub2 [Level I: Systematic Review / Meta-Analysis]

16. Master Class Pro-Tip

In musculoskeletal oncology evaluation and surgical management of unicameral bone cyst, strict adherence to oncologic principles dictates all diagnostic and reconstructive interventions. Biopsy tracks must be placed strictly along the definitive surgical approach corridor, avoiding contamination of adjacent anatomic compartments. Definitive resection must achieve clear histological margins with an intact cuff of normal tissue, followed by durable structural reconstruction designed for immediate functional stability.

Master Class Surgical Pearl: Oncologic and Reconstructive Principles in Unicameral bone cyst