Aneurysmal Bone Cyst - Calcaneus
Aneurysmal Bone Cyst - Calcaneus
Clinical Scenario
A 20-year-old male presents with progressive right heel pain over 4 months, worse with weight-bearing. He denies any history of trauma. The pain has been gradually worsening and now limits his ability to stand for prolonged periods at work (he is a chef).
History:
- 4-month progressive heel pain
- Weight-bearing pain, worse with prolonged standing
- No trauma history
- No night pain or constitutional symptoms
- No previous bone lesions
- No family history of bone tumours
- Works as a chef, on feet all day
Examination Findings:
- Localised tenderness over lateral calcaneus
- Mild swelling, no erythema or warmth
- No palpable mass
- Full ankle and subtalar range of motion
- Pain on heel loading
- Normal neurovascular examination
- Mild antalgic gait (off-loads right heel)
Investigations
Laboratory Results
Imaging
Plain X-ray Right Foot (Lateral and Axial Calcaneus):
- Expansile, eccentric lytic lesion in calcaneus body
- Well-defined with thin cortical margins
- 'Blown-out' appearance with egg-shell thin cortex
- Internal septations visible
- No matrix calcification
- No periosteal reaction
- No pathological fracture
MRI Right Calcaneus with Gadolinium:
- 3.5 x 2.8 x 2.5 cm multiloculated cystic lesion
- Multiple fluid-fluid levels (pathognomonic feature)
- T1: Low signal
- T2: Very high signal with internal septations
- Thin rim enhancement with gadolinium
- Surrounding bone marrow oedema
- No solid enhancing components
- Intact cortex (thinned but not breached)
- Subtalar joint not involved
CT Chest (Staging):
- No pulmonary metastases
- Normal lung parenchyma
Questions & Model Answers
What is your differential diagnosis for this lesion?
What is the role of USP6 gene testing and how does it help in diagnosis?
How would you approach the biopsy and what are your histological expectations?
What are the treatment options for this aneurysmal bone cyst?
What are the specific considerations for treating bone tumours in the calcaneus?
What is the prognosis and follow-up protocol for this patient?
Key Teaching Points
| Concept | Detail |
|---|---|
| Classic Finding | Fluid-fluid levels on MRI (blood sedimentation) |
| Critical Differential | Telangiectatic osteosarcoma - MUST biopsy |
| Genetic Marker | USP6 gene rearrangement (70% of primary ABC) |
| Secondary ABC | 30% arise within another tumour (GCT, chondroblastoma) |
| Treatment | Extended curettage with adjuvant + bone graft |
| Recurrence | 15-25% - repeat curettage usually successful |
| Embolisation | Useful adjunct or for difficult locations |
| Prognosis | Excellent - malignant transformation extremely rare |
Common Examiner Follow-up Questions
-
"What is a secondary ABC and how do you identify it?"
- ABC arising within a pre-existing bone tumour
- 30% of all ABCs are secondary
- Underlying tumours: GCT, chondroblastoma, osteoblastoma, fibrous dysplasia
- USP6 negative (primary ABC is USP6 positive)
- Histology shows features of underlying tumour
-
"How do you distinguish ABC from telangiectatic osteosarcoma?"
- Both have fluid-fluid levels
- ABC: bland fibroblasts, low mitoses, USP6+
- Tel-OS: atypical cells in septa, high mitoses, aggressive features
- BIOPSY IS ESSENTIAL - never treat without histology
- Any doubt → refer to sarcoma pathologist
-
"What if the lesion recurs after treatment?"
- Re-image to confirm recurrence
- Consider re-biopsy (exclude malignancy)
- Repeat curettage with more aggressive adjuvant
- Consider embolisation before surgery
- Denosumab may have role in recurrent disease
- En bloc resection rarely needed
-
"What is the role of denosumab in ABC?"
- ABCs contain osteoclast-like giant cells (RANK-L mediated)
- Denosumab is RANK-L inhibitor
- May shrink lesion (neoadjuvant)
- Used for recurrent or difficult-to-treat lesions
- Limited evidence base but promising results
- Discontinuation may lead to regrowth