Simple Bone Cyst (Unicameral Bone Cyst)
A benign, fluid-filled cavity in bone, most often found in the upper arm of children and teenagers. With the right treatment plan, the long-term outlook is excellent and full return to normal activity is the rule.
What is a simple bone cyst?
A Simple Bone Cyst (SBC), also called a Unicameral Bone Cyst (UBC), is a benign (non-cancerous) cavity inside the bone lined by a thin fibrous membrane and filled with clear to straw-coloured serous fluid. It accounts for around 3% of all bone tumours. Although its name means "single-chambered," older cysts — particularly after a fracture — can develop internal compartments.2,8
Who gets simple bone cyst and where does it occur?
65% in teenagers; mean age around 9 years
Slight male predominance
50–60% in the upper arm; ~30% in the proximal femur
What causes a simple bone cyst?
The exact cause is unknown. The most widely accepted explanation (Cohen's theory) suggests:
- • A focal defect in metaphyseal remodelling blocks drainage of interstitial fluid
- • Venous obstruction raises pressure inside the bone
- • Prostaglandins in the cyst fluid switch on bone-resorbing cells (osteoclasts)
What are the symptoms of simple bone cyst?
About 50% of patients sustain a fracture at some point. Risk factors include male gender, very thin cortex, multilocular cysts and a "ballooned" bone contour.9
How is simple bone cyst diagnosed?
- • Well-defined, central, lucent lesion in the metaphysis
- • Narrow zone of transition with a thin sclerotic margin
- • Fallen fragment sign — a bone fragment settling in the dependent part after fracture (pathognomonic)
- • Rising bubble sign — gas bubble at the non-dependent part (pathognomonic)
- • MRI: only if the diagnosis is uncertain on plain films
- • CT: helpful to confirm a fracture and measure cyst density (~10–15 HU)
- • Bone scan: 'doughnut sign' — peripheral uptake with central photopenia
- • Biopsy: rarely required; reserved for atypical lesions
How does a simple bone cyst behave over time?
Around 8–20% heal after a fracture with adequate immobilisation, and the majority resolve by the time the skeleton matures.2,10
Simple Bone Cyst — patient education videos
Two short videos walk you through what a simple bone cyst is, how we diagnose it, and how a young patient was treated and returned to full activity.
Current treatment options for simple bone cyst
Treatment is tailored to age, cyst size, location, fracture risk and whether the lesion is active or latent. The aim is healing with the least invasive option that will reliably work.3,10
Conservative observation
- • Best for: asymptomatic, latent cysts away from the growth plate
- • Plan: serial X-rays every 6 months until skeletal maturity
- • Healing after fracture: ~30% with observation alone (up to ~50% in recent series) — the fracture itself mechanically disrupts the cyst
Minimally invasive treatments
- • Two-needle technique with deliberate disruption of the cyst wall
- • Usually 2–4 sessions, 8 weeks apart
- • Best for unilocular, small, humeral cysts in the latent stage
- • Uses the osteogenic potential of red bone marrow
- • Combined with demineralised bone matrix improves results
- • Day-care procedure with rapid recovery
- • Current gold standard for long-bone cysts (humerus, femur)
- • Provides continuous decompression and immediate stability
- • Average healing time ~5.8 months — the fastest of all options
- • Cannulated screws or hydroxyapatite pins allow ongoing drainage
- • Multiple drilling: close to 100% healing in small series
- • Useful when nailing is not anatomically suitable
Curettage and bone grafting
- • Healing rate: ~90% with autograft or allograft
- • Modern enhancement: curettage + calcium sulfate + drainage achieves 92–100% healing
- • Adjuvants: ethanol or calcium sulfate to reduce recurrence
- • Large lesions / proximal femur: may need plate fixation in addition to grafting
- • Revision: open revision is superior to percutaneous in complex recurrent cases
Healing and recurrence rates
What's the outlook with simple bone cyst?
The outlook is excellent. A simple bone cyst never becomes cancer, and cure rates range from 77% with conservative care to close to 100% with flexible intramedullary nailing. Most children return to full sport and activity with no long-term limitation.3,8
- • Conservative treatment: ~64% healing
- • Steroid or marrow injection: ~77–78% healing
- • Curettage with grafting: ~90% healing
- • Flexible intramedullary nailing: ~100% healing in long bones
Which approach is best for me?
- • The cyst is asymptomatic and found by chance
- • It is small, latent and away from the growth plate
- • There has been a fracture with good healing potential
- • The cyst is causing pain or limiting function
- • Fracture risk is high (thin cortex, large active cyst)
- • There have been recurrent fractures
- • Minimally invasive treatments have failed
- • There is a displaced fracture needing fixation
- • The lesion is very large or the diagnosis is uncertain
Surveillance protocol
- • Serial X-rays of the affected bone every 6 months until skeletal maturity
- • Clinical review at each visit for pain, function and growth
- • Continue surveillance after any treatment to detect recurrence early
- • Sudden severe pain or suspected fracture
- • Inability to use the affected limb
- • Significant swelling or deformity
- • Recurrent pain after previous treatment
Talk to a specialist
References
- 1PubMed — Comparative study of minimally invasive treatments for simple bone cysts, 2010
- 2Radiopaedia — Simple bone cyst: imaging features and natural history review
- 3JBJS — Large-scale analysis of 4,973 unicameral bone cyst treatments and outcomes, 2024
- 4Bezmialem Science — Local methylprednisolone injection results and technique, 2020
- 5ScienceDirect — Current concepts in pathophysiology and treatment of unicameral bone cysts, 2018
- 6Journal of Korean Orthopaedic Association — Flexible intramedullary nailing outcomes for SBC, 2012
- 7NCBI PMC — Predictive factors for success of steroid therapy in unicameral bone cysts, 2013
- 8NCBI PMC — Comprehensive review of etiopathogenesis and current treatments, 2022
- 9NCBI PMC — Clinical factors affecting pathological fracture and healing in SBC, 2014
- 10NCBI PMC — Systematic review and meta-analysis of SBC treatment modalities, 2014
- 11NCBI PMC — Immediate vs. delayed surgery for pathological fractures through SBC, 2020
- 12NCBI PMC — Open surgical treatment outcomes and revision rates for SBC, 2023
Important Notice: This information is provided for educational purposes only and is not a substitute for professional medical advice, diagnosis, or treatment. The content on this page should not be used to diagnose or treat any health condition.
Always seek the advice of a qualified orthopaedic oncologist or healthcare provider with any questions about simple bone cyst or any other medical condition.
Treatment decisions are individualised and depend on patient age, cyst size and location, activity level, fracture risk, previous treatments and family preferences. While many simple bone cysts are managed conservatively with excellent outcomes, some require minimally invasive or surgical intervention for the best result.
In case of emergency — severe pain, suspected fracture, inability to use the limb, or significant functional impairment — seek immediate medical attention or contact your local emergency services.
The Orthoncology Clinic is committed to providing current, evidence-based information while emphasising the importance of professional medical consultation for all health-related decisions.