Unicameral Bone Cyst (UBC)

Modern Study Review (AI-Generated)

High-Yield Summary

Unicameral Bone Cysts (UBCs) are benign, fluid-filled lesions predominantly affecting the metaphyseal region of long bones in children and adolescents, most commonly the proximal humerus and femur. They are often asymptomatic but may present with pathologic fractures after minimal trauma. Early diagnosis and appropriate management are essential to prevent fracture-related morbidity and optimize bone healing.

Key Diagnostic Findings

Anatomy

• Location: Metaphysis of long bones near the growth plate (physis).
• Common sites: Proximal humerus (most frequent), proximal femur.

Clinical Presentation

• Usually asymptomatic until a pathologic fracture occurs.
• Pathologic fractures often result from minimal trauma.
• Occasionally, mild localized pain or swelling may be present.

Imaging

• X-rays:
• Well-defined, radiolucent, trabeculated cystic lesion with a thin sclerotic margin.
• Classic “fallen leaf” sign: a fragment of bone seen within the cyst cavity, indicative of a pathologic fracture.
• MRI/CT: Useful for surgical planning and assessing cyst extent but not routinely required.

Histology

• Cyst lined by a thin membrane of loose connective tissue.
• Contains fibroblasts, multinucleated giant cells, and hemosiderin-laden macrophages.
• No true epithelial lining (distinguishes from other cystic lesions).

Classification Systems

• No universally accepted formal classification system for UBCs; however, cysts are often described by activity:
• Active cyst: Adjacent to the physis, higher risk of growth and fracture.
• Latent cyst: Located away from the physis, lower risk of progression.

Current Gold Standard Treatment

– Non-operative management is first-line for asymptomatic cysts or minimally displaced fractures.

• Steroid injections reduce cyst fluid production and promote healing but may require multiple treatments.
• Surgery is reserved for large cysts, recurrent fractures, or failure of less invasive methods.

Modern Complications & Outcomes

Complications

• Pathologic fracture (most common initial complication).
• Recurrence of cyst after treatment, especially with aspiration alone.
• Growth disturbance is rare but possible if the physis is involved.
• Surgical risks: infection, graft failure, neurovascular injury.

Outcomes

• Overall prognosis is excellent with appropriate treatment.
• Healing rates improve significantly with curettage and bone grafting compared to steroid injections alone.
• Functional outcomes focus on pain resolution and return to full activity without fracture risk.
• Long-term follow-up is recommended until cyst resolution or skeletal maturity.

Classic Clinical Notes

Unicameral Bone Cyst (UBC)

• Solitary, fluid-filled cystic lesions located in metaphysis of long bones near physis in kids & adolescents
• Predominantly proximal femur & proximal humerus

Signs & Symptoms:

• Usually asymptomatic
• Pathologic fracture associated with minimal trauma

X-rays:

• Radiolucent, trabeculated lesion with well-defined borders surrounded by sclerotic margin
• ‘Fallen leaf’ sign in pathologic fracture

Histology:

• Lined with loose connective tissue membrane containing small numbers of fibroblasts, giant cells & hemosiderin-laden macrophages

Treatment:

• 15% will heal after fracture with observation alone
• Treat fracture as you would for most fractures of that region
• Once fracture healed, aspiration & steroid injections (x 3)
• Curettage & bone grafting improves healing rate & provides mechanical support

Last Updated on January 25, 2026 by orthonet