Aneurysmal bone cyst
Aneurysmal bone cysts (ABC) are benign expansile tumour-like bone lesions of uncertain aetiology, composed of numerous blood filled channels, and mostly diagnosed in children and adolescents.
Aneurysmal bone cysts are primarily seen in children and adolescents, with 80% occurring in the patients less than 20 years of age 8.
Patients may present with pain, which may be of insidious onset or abrupt due to pathological fracture, with a palpable lump or with restricted movement.
ABCs consist of blood-filled spaces of variable size that are separated by connective tissue containing trabeculae of bone or osteoid tissue and osteoclast giant cells. They are not lined by endothelium. A fine needle aspiration cytology is usually non-diagnostic often dominated by fresh blood 7.
A variant of ABCs is the giant cell reparative granuloma which is usually seen in the tubular bones of the hands and feet as well as in the craniofacial skeleton. Occasionally they are also seen in appendicular long bones where they are known as solid aneurysmal bone cysts. Histologically these two entities are identical 6.
- long bones: 50-60%, typically of the metaphysis
- lower limb: 40%
- tibia and fibula: 24%, especially proximal tibia
- femur: 13%, especially proximally
- upper limb: 20%
- lower limb: 40%
- spine and sacrum: 20-30%
- especially posterior elements, with extension into the vertebral body in 40% of cases 8
- craniofacial: jaw, basisphenoid, and paranasal sinuses
- epiphysis, epiphyseal equivalent, or apophysis: rare but important
Plain radiograph and CT
Radiographs demonstrate sharply defined, expansile osteolytic lesions, with thin sclerotic margins. CT will demonstrate these findings to a greater degree, and is also better at assessing cortical breach and extension into soft tissues.
Additionally, CT can demonstrate fluid-fluid levels, which are harder to appreciate than on MRI and require viewing with narrow window width 3.
Doughnut sign: increased uptake peripherally with a photopenic centre.
MRI is able to demonstrate the characteristic fluid-fluid levels exquisitely as well as identify the presence of a solid component suggesting the ABC is secondary.
The cysts are of a variable signal, with a surrounding rim of low T1 and T2 signal. Focal areas of high T1 and T2 signal 8 are also seen presumably representing areas of blood of variable age (see ageing blood on MRI).
It is important to remember that the presence of fluid-fluid levels, although characteristic of ABC is by no means unique to it, and is also seen in both benign and malignant lesions (e.g. giant cell tumours (GCT), chondroblastoma, simple bone cysts and telangiectatic osteosarcomas).
- T1 (C+): septations may enhance 12
- ABCs are poorly vascular 15
Treatment and prognosis
Traditionally these lesions have been treated operatively (curettage and bone grafting) with a recurrence rate of ~20% (range 11-31%). Percutaneous treatment with fibrosing agents has also been performed, either in isolation as a precursor to surgical excision 1,5.
Spontaneous regression may occur, including following partial removal, but this is not the typical natural history 13,14.
The differential diagnosis depends on the modality.
On plain radiography (and to a lesser degree CT) the diagnosis includes most of the lesions included in the mnemonic FEGNOMASHIC.
On MRI the differential is much shorter, especially when age, location and plain film appearance is taken into account. The main differential includes both lesions that intrinsically have fluid-fluid levels (thus see fluid-fluid level containing bone lesions) and those from which an ABC may arise (chondroblastoma, fibrous dysplasia, giant cell tumour (GCT) 4, osteosarcoma (especially telangiectatic osteosarcoma).
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The differential diagnosis for bone tumours is dependent on the age of the patient, with a very different set of differentials for the paediatric patient.
- bone-forming tumours
- cartilage-forming tumours
- chondromyxoid fibroma
- fibrous bone lesions
- bone marrow tumours
- other bone tumours or tumour-like lesions
- aneurysmal bone cyst
- benign fibrous histiocytoma
- giant cell tumour of bone
- Gorham massive osteolysis
- haemophilic pseudotumour
- intradiploic epidermoid cyst
- intraosseous lipoma
- musculoskeletal angiosarcoma
- musculoskeletal haemangiopericytoma
- primary intraosseous haemangioma
- simple bone cyst
- impending fracture risk