Aneurysmal Bone Cyst

§ overviewOverview

A benign, locally aggressive, blood-filled, expansile lesion of bone characterised by cavernous vascular spaces separated by fibrous septa containing multinucleated giant cells; reclassified by WHO 2020/2022 as a true neoplasm with USP6 gene fusions.

§ icdICD Classification

ICD-10 M85.5

§ etiologyEtiology

• 01Primary ABC (~70%) — USP6 gene rearrangement (17p13) driving neoplastic proliferation
• 02Secondary ABC (~30%) — arises within pre-existing lesion (fibrous dysplasia, ossifying fibroma, central giant cell granuloma, chondroblastoma, osteosarcoma) via altered haemodynamics

§ riskRisk Factors

• 01Young age (< 30 y)
• 02Underlying primary bone lesion for secondary ABC
• 03Prior trauma (occasional trigger)

§ geneticsGenetics & Molecular Biology

• 01USP6 (ubiquitin-specific peptidase 6) gene rearrangements on chromosome 17p13 in ~70% of primary ABC — fusion partners include CDH11, TRAP150, ZNF9, OMD, COL1A1
• 02Absent in secondary ABC — a useful molecular distinguisher

§ epidemiologyEpidemiology

~1.5% of jaw cysts. Peak: 1st–3rd decade (mean 20 y); rare > 30 y. F ≈ M. Mandible > maxilla (3:1); posterior body and ramus commonest.

§ pathogenesisPathogenesis

USP6 gene fusions drive neoplastic proliferation of primitive mesenchymal cells with venous obstruction and repeated microhaemorrhage; osteoclast-rich giant cells resorb bone as blood-filled cavernous spaces expand centrifugally, ballooning the cortex. Secondary ABC arises when a pre-existing lesion alters local haemodynamics leading to similar vascular pooling without USP6 fusion.

§ clinicalClinical Features

• 01Rapidly enlarging, painful bony swelling
• 02Facial asymmetry
• 03Malocclusion / tooth displacement / mobility
• 04Paraesthesia of the inferior alveolar nerve (uncommon)
• 05Egg-shell crackling on palpation over ballooning cortex
• 06Occasional pathological fracture

§ signsSigns & Symptoms

• 01Firm-to-fluctuant expansile mass
• 02Overlying skin/mucosa tense but usually intact
• 03Bruit occasionally audible over lesion (rare)

§ differentialDifferential Diagnosis

• 01Central giant cell granuloma
• 02Solitary bone cyst
• 03Odontogenic keratocyst
• 04Cherubism (bilateral)
• 05Ameloblastoma (multilocular)
• 06Telangiectatic osteosarcoma (mimics on imaging)
• 07Central haemangioma of bone

§ criteriaDiagnostic Criteria

• 01Rapidly expanding, multilocular, blood-filled bone lesion
• 02MRI showing fluid–fluid levels — highly suggestive
• 03Histology: cavernous blood-filled spaces without endothelial lining, fibrous septa with multinucleated giant cells
• 04USP6 rearrangement (FISH/RT-PCR) in primary ABC

§ histopathHistopathology

• 01Cavernous / sinusoidal blood-filled spaces WITHOUT endothelial lining (distinguishes from haemangioma) — walled by fibrous septa
• 02Septa contain plump fibroblasts, multinucleated giant cells, extravasated red cells, haemosiderin and reactive woven bone (rimmed by osteoblasts)
• 03Mitoses may be present but no atypia — exclude telangiectatic osteosarcoma
• 04Solid variant (< 10%) — predominantly fibrous with giant cells and less prominent blood spaces

§ radiographicRadiographic Features

• 01Multilocular, expansile, well-defined radiolucency with 'ballooning' or 'soap-bubble' appearance
• 02Thinned but usually intact cortex ('egg-shell')
• 03Rapid growth on serial films
• 04Root resorption or displacement of adjacent teeth possible

§ opgOPG Findings

• 01Multilocular expansile posterior mandibular lesion with ballooning cortex

§ cbctCBCT Findings

• 013D delineation of septa and cortical thinning; assess IAN canal

§ ctCT Findings

• 01Expansile lesion with internal septa; thinned cortex; occasional fluid–fluid levels visible

§ mriMRI Findings

• 01MODALITY OF CHOICE — multiple fluid–fluid levels (settled blood products) on T2 images are highly suggestive
• 02Multilobulated appearance with variable T1/T2 signals due to blood of different ages
• 03Peripheral and septal enhancement post-contrast

§ usgUltrasonography

• 01Rarely used; may show cavitary lesion with sedimented layers

§ investigationsInvestigations

• 01OPG + CBCT for extent
• 02MRI with contrast for fluid–fluid levels
• 03Angiography selectively for pre-op embolisation of large lesions
• 04Incisional biopsy (with caution — profuse bleeding) for histology
• 05FISH for USP6 gene rearrangement to confirm primary ABC

§ labsLaboratory Findings

• 01Biopsy: blood-filled spaces without endothelial lining, giant cells in septa
• 02FISH: USP6 rearrangement in primary ABC

§ ihcIHC / Special Stains

• 01Giant cells CD68+
• 02Endothelial markers CD31/CD34 NEGATIVE in cavernous spaces (distinguishes from haemangioma)
• 03USP6 break-apart FISH positive in primary ABC

§ whoWHO Classification

WHO 2020 Classification of Soft Tissue and Bone Tumours: Aneurysmal bone cyst — reclassified as a true neoplasm (from pseudocyst) with USP6 gene rearrangement.

§ classificationClassification

• 01Primary ABC (~70%) — de novo, USP6 rearranged
• 02Secondary ABC (~30%) — arising within pre-existing lesion
• 03Solid variant (< 10%) — predominantly fibrous
• 04Extraosseous ABC (rare)

§ planTreatment Planning

• 01MRI to confirm fluid–fluid levels and exclude malignancy
• 02Incisional biopsy with pre-op cross-match (expect brisk bleeding)
• 03Pre-operative selective arterial embolisation for very vascular / large lesions
• 04Choose curettage vs en-bloc resection based on size and cortical involvement

§ treatmentTreatment

• 01Enucleation + aggressive curettage with peripheral ostectomy is the standard treatment for most jaw ABCs
• 02Adjuncts to reduce recurrence: cryotherapy with liquid nitrogen, Carnoy's solution, argon-beam coagulation, or phenol
• 03En-bloc resection with immediate reconstruction for very large or recurrent lesions
• 04Selective arterial embolisation as a pre-op adjunct or standalone therapy for surgically inaccessible sites
• 05Sclerotherapy (percutaneous doxycycline / polidocanol) — emerging minimally invasive option, especially in growing children
• 06Denosumab (anti-RANKL monoclonal antibody) — reported off-label use for refractory/recurrent cases
• 07Radiotherapy — historically used, largely abandoned due to sarcomatous transformation risk

§ medicalMedical Management

• 01Analgesia; antibiotics only if infected
• 02Denosumab in selected refractory cases (specialist care)

§ surgicalSurgical Management

• 01Careful mucoperiosteal exposure → wide bony window → aggressive curettage → adjuvant (cryotherapy / Carnoy) → primary closure or reconstruction
• 02Meticulous haemostasis (brisk bleeding common)
• 03Immediate reconstruction with iliac crest or fibula flap for segmental defects

§ reconstructionReconstruction Options

• 01Autogenous cortico-cancellous graft (iliac crest) for small–moderate defects
• 02Free vascularised fibula for segmental mandibular resections
• 03Titanium reconstruction plate as interim

§ complicationsComplications

• 01Intra-operative haemorrhage (significant risk)
• 02IAN injury with lower lip paraesthesia
• 03Recurrence (10–60% after curettage alone)
• 04Pathological fracture
• 05Malignant transformation (very rare, usually radiation-associated)

§ recurrenceRecurrence Rate

10–60% after simple curettage; < 10% with aggressive curettage + adjuvant; near 0% after en-bloc resection.

§ followupFollow-up Protocol

• 01Clinical + imaging review at 3, 6, 12 months, then annually × 5 y
• 02Beyond 5 y: 2-yearly review
• 03MRI for suspected recurrence

§ prognosisPrognosis

Good with complete excision; recurrence risk high without adjuvant therapy or in very young patients.

§ preventionPrevention

• 01None specific — mostly de novo with USP6 rearrangement

§ examKey Examination Points

• 01Young patient + rapidly expanding jaw swelling = consider ABC
• 02Palpable egg-shell crackling suggests ballooning cortex
• 03Always obtain MRI to look for fluid–fluid levels

§ revisionQuick Revision Summary

• 01Blood-filled multilocular pseudocyst — reclassified as neoplasm (USP6)
• 02Young patients, posterior mandible
• 03Fluid–fluid levels on MRI
• 04Aggressive curettage + adjuvant is standard
• 05Recurrence 10–60% without adjuvant

§ vivaBDS Viva Questions

• 01Define aneurysmal bone cyst.
• 02Why has it been reclassified as a neoplasm?
• 03What is USP6 and its clinical relevance?
• 04Difference between primary and secondary ABC.
• 05Classic MRI finding?
• 06Histological hallmarks?
• 07Differential diagnoses?
• 08Treatment options and adjuvants?
• 09Role of denosumab and embolisation?
• 10Recurrence rates by treatment modality?
• 11How would you differentiate ABC from telangiectatic osteosarcoma?

§ bdsBDS Professional Examination

• 01Long essay: Aneurysmal bone cyst — pathogenesis, features, diagnosis, and management.
• 02Short essay: Fluid–fluid levels on MRI in bone lesions.
• 03Short note: USP6 gene in ABC.

§ fcpsFCPS Residency Questions

• 01Discuss the WHO reclassification of ABC and its clinical implications.
• 02Compare curettage, embolisation, sclerotherapy and denosumab in the management of ABC.
• 03Discuss reconstruction of a segmental mandibular defect following ABC resection.

§ pearlsClinical Pearls

• 01Rapidly expanding, blood-filled, multilocular jaw lesion in a young patient with fluid–fluid levels on MRI is ABC until proven otherwise.
• 02Always request USP6 FISH — it changes diagnosis and, potentially, treatment.
• 03Aggressive curettage + adjuvant beats simple curettage on recurrence.

§ mnemonicsMnemonics

• 01ABC = Angry Ballooning Cavity — Aggressive curettage, Blood-filled, Cortical thinning.

§ readingSuggested Reading

• 01Oliveira AM et al. USP6 and CDH11 oncogenes identify the neoplastic cell in primary aneurysmal bone cysts. Am J Pathol 2004.
• 02WHO Classification of Soft Tissue and Bone Tumours, 5e (2020).
• 03Rapp TB et al. Aneurysmal bone cyst. J Am Acad Orthop Surg 2012.
• 04Lange T et al. Denosumab for treatment of aneurysmal bone cyst. Eur Spine J 2013.

§ differentialDifferential Comparison

§ mcqsMCQs — Assessment (20)

§ relatedRelated Diseases

References

1. WHO Classification of Head and Neck Tumours, 5e (2022)
2. WHO Classification of Soft Tissue and Bone Tumours, 5e (2020)
3. Neville BW et al. Oral and Maxillofacial Pathology, 4e
4. Peterson LJ. Contemporary Oral & Maxillofacial Surgery, 7e