CIM Case: Unicameral Bone Cyst - Pathological Fracture

Clinical Scenario

Patient: 12-year-old male Presentation: Right shoulder pain after minor tackle during football, unable to move arm Relevant history: No previous injuries, no prior symptoms, healthy otherwise, no family history of bone disorders Examination findings:

Holding right arm against body, reluctant to move
Localised tenderness over proximal humerus
Swelling around shoulder region
No deformity visible
Intact sensation, moving fingers normally
Radial pulse palpable
No other injuries

Investigations Provided

Laboratory Results

Test	Result	Normal Range	Interpretation
N/A	-	-	Bloods not routinely required for UBC

Imaging

Image 1: AP and Lateral Proximal Humerus Radiographs

Radiological features:

Central lytic lesion in proximal humeral metaphysis
Well-defined margins with thin sclerotic rim
Lesion width less than physeal width
Transverse fracture through the lesion
Cortical fragment visible within the lesion (fallen fragment sign)
No periosteal reaction prior to fracture
Slight expansion of bone (<50% beyond normal diameter)
No soft tissue mass
Physis intact
Lesion extends from physis to mid-diaphysis (~8cm length)

Classification:

Active UBC: Adjacent to physis (as seen here)
Cyst Index: >3.5 (higher recurrence risk)

Questions & Model Answers

What is the diagnosis and what are the characteristic radiographic features?

Describe the classification and natural history of unicameral bone cysts.

How would you manage this pathological fracture initially?

The fracture has healed but the cyst persists. What are your treatment options?

What factors predict recurrence and how would you manage a recurrent UBC?

What if this lesion was in the proximal femur instead? How would your management differ?

Key Teaching Points

Pattern Recognition

This pattern suggests UBC:

Child/adolescent (5-15 years)
Proximal humerus or proximal femur
Central metaphyseal lytic lesion
Well-defined margins
No periosteal reaction (until fracture)
Lesion width < physeal width
Fallen fragment sign on X-ray (pathognomonic)

Distinguish from ABC:

Feature	UBC	ABC
Location	Central	Eccentric
Expansion	<50%	>50% (often "blowout")
Contents	Serous fluid	Blood products
Fluid levels	No	Yes (on MRI)
Age	5-15 years	Any age (peak 10-20)

Critical Management Points

Fallen fragment sign is pathognomonic - confirms diagnosis
Treat the fracture first - allow 6-8 weeks to heal
10-15% heal with fracture - watch before treating cyst
Steroid injection is first-line - 60-70% success, may need repeats
Active cysts recur more - young age, adjacent to physis
Femoral cysts need more aggressive treatment - prophylactic fixation often indicated

Common Examiner Follow-ups

Q: "What is in the cyst fluid and why does it matter?"

UBC fluid characteristics:

Straw-coloured, clear or slightly yellow
Similar to serum (protein content)
Contains elevated prostaglandins, interleukins
These cytokines promote bone resorption
Corticosteroids inhibit this inflammatory cascade - hence their efficacy

ABC fluid is bloody/dark and may have blood-fluid levels on imaging.

Q: "What is the 'cyst index' and how do you use it?"

Cyst Index = Cyst length ÷ Cyst width

Index	Interpretation
<3.5	Lower recurrence risk
>3.5	Higher recurrence risk

Higher index = longer, narrower cyst = higher recurrence. Useful for prognostic counselling and planning frequency of follow-up.

Q: "Can you use flexible nails for proximal humerus UBC?"

Yes, flexible intramedullary nails (TENS) can be used:

Provide continuous decompression ("venting")
Give structural support
Studies show accelerated healing
Retrograde insertion from lateral condyle
Left in place 6-12 months until healed

Increasingly popular approach, especially for recurrent or large cysts.

Aneurysmal Bone Cyst
Pathological Fractures
Fibrous Dysplasia
Paediatric Bone Tumours
Bone Grafting Techniques
Proximal Femur Fractures in Children

Unicameral Bone Cyst - Pathological Fracture