Aneurysmal Bone Cyst
CIM Case: Aneurysmal Bone Cyst
Clinical Scenario
Patient: 13-year-old female Presentation: Gradual onset pain in the right knee over 8 weeks, worsening with activity, no history of trauma, no night sweats or weight loss Relevant history: Previously healthy, active in netball, no previous fractures, no family history of bone tumours Examination findings:
- Tender fullness over the proximal tibial metaphysis, medial aspect
- No warmth or erythema
- Full range of motion at knee (slight discomfort at end-range)
- No effusion
- No lymphadenopathy
- Normal neurovascular examination
- Antalgic gait
Investigations Provided
Laboratory Results
| Test | Result | Normal Range | Interpretation |
|---|---|---|---|
| Haemoglobin | 132 g/L | 115-145 | Normal |
| WCC | 7.5 x 10ā¹/L | 5-13 | Normal |
| Platelets | 290 x 10ā¹/L | 150-400 | Normal |
| ESR | 12 mm/hr | 0-15 | Normal |
| CRP | 3 mg/L | <5 | Normal |
| ALP | 180 U/L | 150-400 | Normal (child) |
| Calcium | 2.35 mmol/L | 2.15-2.55 | Normal |
| Phosphate | 1.4 mmol/L | 1.0-1.8 | Normal |
| LDH | 165 U/L | 120-300 | Normal |
Imaging
Image 1: AP and Lateral Knee X-rays
Radiological features:
- Eccentric, expansile lytic lesion in the proximal tibial metaphysis
- Size approximately 4 x 3 cm
- Well-defined margins with thin cortical shell ("blown out" appearance)
- Multiple thin internal septations
- No periosteal reaction
- No soft tissue mass
- No matrix mineralisation
- Adjacent physis appears intact
Image 2: MRI Knee with Contrast
MRI findings:
- Well-defined multiloculated cystic lesion
- Multiple fluid-fluid levels (pathognomonic)
- T1: Hypointense with variable signal (blood products)
- T2: Hyperintense with characteristic layering
- Thin peripheral enhancement
- No solid enhancing component (important)
- Intact overlying cortex
- No adjacent marrow oedema
- No soft tissue extension
Image 3: CT Scan
CT findings:
- Thinned but intact cortical shell
- Internal septations visible
- No cortical destruction
- No matrix mineralisation
- Confirms geographic bone destruction
Questions & Model Answers
What is the likely diagnosis and what are the typical imaging features?
What is the difference between primary and secondary ABC?
What is the differential diagnosis for this lesion?
How would you manage this patient? Describe your surgical technique.
What are the recurrence risk factors and how would you follow up?
How would management differ if this was in the spine?
Key Teaching Points
Pattern Recognition
This pattern suggests Aneurysmal Bone Cyst:
- Adolescent (10-20 years)
- Eccentric metaphyseal location
- Expansile lytic lesion with "blown out" cortex
- Fluid-fluid levels on MRI
- No solid enhancing component
- No matrix mineralisation
Distinguish from Telangiectatic Osteosarcoma:
| Feature | ABC | Telangiectatic Osteosarcoma |
|---|---|---|
| Solid component | None | Thick nodular enhancement |
| Cortex | Thin but intact | Destroyed |
| Periosteal reaction | None | Aggressive |
| LDH/ALP | Normal | Often elevated |
Critical Management Points
- Biopsy is ESSENTIAL - exclude telangiectatic osteosarcoma
- Sample the wall - secondary ABC is found in rim, not cyst
- Adjuvant therapy reduces recurrence - phenol, cryotherapy
- 35% are secondary - look for underlying lesion
- Fluid-fluid levels are NOT pathognomonic - other lesions can have them
- Most recurrences within 2 years - close follow-up essential
Common Examiner Follow-ups
Q: "What is the USP6 gene rearrangement?"
USP6 (ubiquitin-specific peptidase 6) gene on chromosome 17p13:
- Rearranged in 65-70% of primary ABC
- Most common partner: CDH11 (t(16;17))
- Results in overexpression of USP6
- Promotes osteoclast activation and bone destruction
- NOT present in secondary ABC
- Can help distinguish primary from secondary lesions
Q: "When would you use selective arterial embolisation?"
| Indication | Notes |
|---|---|
| Large/vascular lesions | Reduce surgical bleeding |
| Spinal ABC | May be definitive or preoperative |
| Pelvic ABC | Difficult surgical access |
| Recurrent lesions | Before repeat surgery |
| Patient unfit for surgery | Palliative/temporising |
Agents used: Polyvinyl alcohol (PVA), Gelfoam, coils
Q: "Can you use radiation for ABC?"
Radiation is generally AVOIDED due to:
- Risk of secondary sarcoma (1-3% in irradiated bone tumours)
- Most ABCs can be managed with surgery/embolisation
- Young patient population (long life expectancy)
Only considered for:
- Spine lesions inaccessible to surgery
- Multiple failed surgical treatments
- Life-threatening location (skull base)
- Dose: 20-30 Gy (low dose)
Related Topics
- Unicameral Bone Cyst
- Giant Cell Tumour
- Telangiectatic Osteosarcoma
- Chondroblastoma
- Fibrous Dysplasia
- Spinal Bone Tumours