Aneurysmal bone cyst

An aneurysmal bone cyst (ABC) is a benign expansile tumour-like bone lesion of uncertain aetiology, composed of numerous blood filled channels. 

Epidemiology

Aneurysmal bone cysts are primarily seen in children and adolescents, with 80% occurring in the patients less than 20 years of age ⁸. 

Clinical presentation

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. 

Pathology

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. FNAC is usually non-diagnostic and is dominated by fresh blood ⁷.

Although often primary, up to a third¹¹ of ABCs are secondary to an underlying lesion (e.g chondroblastoma, fibrous dysplasia, giant cell tumour (GCT) ⁴, osteosarcoma). 

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. 

Location

They are typically eccentrically located in the metaphysis of long bones, adjacent to an unfused growth plate. Although they have been described in most bones, the most common locations are ^4,8

• 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%
• spine : 20-30%
  • especially posterior elements, with extension into vertebral body in 40% of cases ⁸
• sacrum

Radiographic features

Plain film and CT

Plain films 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.

Bone scan

Doughnut sign - increased uptake peripherally with a photopenic centre.

MRI

MRI is able to demonstrate the characteristic fluid-fluid levels exquisitely as well as identify the presence of a solid component suggesting the the ABC is secondary. 

The cysts are of variable signal, with 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).

Treatment and prognosis

Traditionally these lesions have been treated operatively (curettage and bone grafting) with a recurrence rate of between 11 and 31%. Percutaneous treatment with fibrosing agents has also been performed, either in isolation as a precursor to surgical excision^1,5.

Differential diagnosis

The differential diagnosis depends on the modality. 

On plain films (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).