Aneurysmal Bone Cyst

Definition

  • A benign cystic lesion of bone that expands the cortex.
  • It is the only bone lesion that derives its name from its X-ray appearance rather than its histology.
  • Term first used by Jaffe & Lichtenstein in 1942

Classification

TypeDescription
PrimaryArise de novo
SecondaryArise in pre-existing tumours, frequently GCTs. More than 50% of ABCs arise from preexisting lesions

Incidence

  • 2.5% of all primary bone tumours.
  • Half as frequent as giant cell tumours
  • M=F
  • 80% in the first two decades of life
  • In contrast, only 15% of patients with GCT are in the first two decades of life

Aetiology

  • Unknown
  • Trauma may be related
  • nearly 1/2 are seen to occur in conjunction with another benign tumour and may represent a breakdown in the body’s reaction to the other tumour

Localisation

  • Can involve any bone.
    • 1. Proximal end of long bones
      • In long bones involves the metaphysis but can cross the growth plate.
    • 2. Spine
      • In the spine involve the posterior elements (Differential Diagnosis here is osteoblastoma).
  • ABC is the commonest benign tumour of the clavicle

History

  • Pain
    • usually mild and intermittent
    • may be Rapid ↑ in pain
  • Swelling
    • Swelling will tend to increase until the lesion is treated
  • Rarely, a pathologic fracture
  • Neurological
    • If the lesion is located in the vertebral column, it may cause signs and symptoms of spinal cord compression (leg weakness, bowel or bladder dysfunction).

Examination

  • May be a mass or neurological symptoms if the spinal cord is compressed

Investigations

Aneurysmal Bone Cyst, imaging of an aneurysmal bone cyst
Imaging of an aneurysmal bone cyst. Image by Meryem Boubbou, Karima Atarraf, Lamiae Chater, Abderrahmane Afifi, Siham Tizniti, Wikimedia Commons, CC BY 2.0.

X-ray

  • Expansile, multilocular balloon disrupting the adjacent bone & elevating the periosteum
    • Most commonly, an area of lucency situated eccentrically in the medullary cavity of a long bone.
    • Less commonly, may be situated centrally within the medulla.
    • Much less commonly, may arise in the cortex or periosteum.
  • No matrix mineralisation is present in the lesion
  • Most ABCs are completely lytic
  • ABCs may cross joints & involve several bones
    • particularly in the spine where several adjacent vertebrae & ribs may be involved.
  • lesion tends to involve the cortex & may destroy it completely, when it may bulge out into the soft tissue where it usually forms a thin rim of calcification.
  • margins can be poorly or well defined; in half of cases the X-ray appearances suggest a benign process, & in a small number of cases they may suggest malignancy.
  • Periosteal new bone formation causing a buttress effect is characteristic.
  • There is no matrix calcification

CT & MRI

  • internal septae & fluid levels with a layering effect.
  • Fluid levels (T2 weighted scans)
  • Finger in balloon sign
    • preservation of a cortical cuff extending for a short distance into the expanded area of destructive blowout

Pathology

Aneurysmal Bone Cyst, histology of an aneurysmal bone cyst at intermediate magnification
Histology of an aneurysmal bone cyst at intermediate magnification. Image by Nephron, Wikimedia Commons, CC BY-SA 3.0.

Gross Pathology

  • Red brown granular material (haemosiderin deposition)
  • “a hole containing blood”
  • blood in the lesion is not clotted
  • no endothelial lining
  • pressure in an ABC may be elevated to arteriolar levels

Histology

  • Essential feature is cavernomatous spaces,
  • with walls that lack the normal features of blood vessels, such as muscle or elastic lamina
  • Thin strands of bone are often present in the fibrous tissues of the walls
  • septae almost invariably contain giant cells; this helps to distinguish from unicameral bone cyst
  • Solid areas contain spindle cells that are loosely arranged
  • Chondroid like zones of calcification in solid portions of septae are commonly found & are relatively specific

Behaviour

  • ABCs are usually aggressive lesions associated with major bone destruction, pathological fractures & local recurrence
  • Spontaneous malignant transformation not recorded in the Mayo files but occurs very occasionally

Differential Diagnosis (Histology)

  • Giant cell tumour
  • Giant cell reparative granuloma
  • Low-grade osteosarcoma
  • Telangiectatic osteosarcoma – probably most difficult Differential Diagnosis
    • This disease is uncommon & rarely involves the vertebrae or small bones of the hands or feet
  • Renal cell carcinoma metastasis

Associated Conditions

May occur in other benign bone tumours or processes

Treatment

  • Intralesional curettage & bone grafting & chemical cauterization of cyst walls
    • treatment of choice
  • Lesions in expendable bones are excised
  • Preoperative Embolisation
    • If the lesion has a large soft tissue component or is large (>5-6cm) should consider preoperative embolisation
    • Especially pelvis, sacrum, vertebral bodies
  • Even incomplete resection may be followed by regression of the lesion
  • High recurrence rate
    • Recurrence tends to occur within 6-18 months. It is very rare after 2 years.
    • Recurrence rate is higher in spinal tumours
  • Radiotherapy
    • has no role unless surgery is impossible.
    • It used to be used adjuvantly but no longer is used to minimise the risk of sarcomatous transformation

Prognosis

  • 95% of patients cured
  • ↑ rate of recurrence in younger patients
  • Children with open physes are much more prone to local recurrence (up to 50%)

Reviewed by Professor Phong Tran, Head of Orthopaedic Surgery, Western Health. Last updated 10 October 2026.