Aneurysmal Bone Cyst

Aneurysmal Bone Cyst
• Nonneoplastic vasocystic tumour
• Age group: 10-20 years old(peak age: 11 years)
• Can occur in any decade of adult life, but nearly 80% occur in the 2nd decade
• Represent 1.5% of all primary bone tumors
• Maybe primary or secondary( arising in other tumours)
• Frequency: distal femur> proximal tibia> proximal humerus> distal radius.
• The vertebrae are involved in 12% to 27% of patients
• Primary ABC maybe neoplastic. Chromosomal rearrangements suggestive of cancer like the USP6 and CDH11 are seen.
• Spontaneous regression may be seen in active primary ABCs. But rare in aggressive or secondary lesions
• May be of hydraulic pressure origin, secondary to haemorrhage and could be traumatically induced.
• Are not true cysts but rather sponge-like collections of interconnected fibrous tissue and blood-filled spaces
• Are destructive lesions and replace bone and thin the cortices of the host bone.
• In the spine, two-thirds of aneurysmal bone cysts will arise from posterior elements and one-third will arise from the vertebral body and present as an aggressive osteolytic lesion with extensive permeative cortical destruction.
• Children with open physes are much more prone to local recurrence (up to 50%)
• Pathological fractures are common and more frequent in humerus and femur


histology and classification

Campanacci’s classification of ABC (1)
1. Aggressive cyst: signs of reparative osteogenesis with ill-defined margins and no periosteal shell.
2. Active cyst: has an incomplete periosteal shell and a defined margin between the lesion and the host bone.
3. Inactive cyst: has a complete periosteal shell and a sclerotic margin between the cyst and the long bone.


Histology:

• Composed of blood filled spaces with intervening fibrous septae. True endothelial cells are not found.
• The tissue is sponge like with cavernous spaces filled with blood.
• Cavities surrounded by gray or brownish tissue with an osseous component
• At the periphery of the lesion is an eggshell-like layer of periosteal bone around the lesion
• Solid variant of ABC consist of fibrous or granular tissue with local haemorrhages and a layer of reactive bone.
• There are skeletal tumours that may demonstrate an aneurysmal component: Giant cell tumour, chondroblastoma, osteoblastoma, chondromyxoid fibroma, fibrous dysplasia, and malignant haemorrhagic osteosarcoma

Imaging
X rays:
– Eccentrically (maybe central also) located lytic lesion, expansile with septations.
– Classically, blown out cortex with egg-shell thin rim of reactive bone
– Sites: metaphysis of long bone, posterior elements of spine, distal phalanx of fingers
– Fluid levels can be seen on MRI /CT scans, which may also be seen with telengiectatic osteosarcoma

CT:
Useful to assess lesions of the pelvis or vertebral column more precisely than radiography
Helps to assess carefully the presence of the periosteal rim of bone around a lesion

MRI
‣ can evaluate soft tissue involvement
‣ On T2-weighted images, the lesions have high signal levels, and layering in the blood (fluid-fluid levels) often can be seen
‣ more accurate assessment than CT or radiography of the extent of an aneurysmal bone cyst

DD
Differential Diagnosis for Holes in Bone: (Mnemonic: FOGMACHINE)
Fibrous dysplasia
Osteoid osteoma, Osteoblastoma,Osteosarcoma, Osteofibrous dysplasia
Giant cell tumor
Myeloma
Aneurysmal bone cyst, adamantinoma
Chondromyxoid fibroma, chondroblastoma, chondrosarcoma
Histiocytosis
Infection
Nonossifying fibroma
Enchondroma, Ewing sarcoma


Treatment
– Augmented curettage and bone grafting.
– Repeated embolisation to reduce the rate of haemorrhagic expansion especially in difficult areas like the pelvis
– Pathologic fractures: require curettage , bone grafting +/- stabilisation
– Pelvic ABCs: spontaneous regression after biopsy has been noted. Consider observation after biopsy. If regression occurs no need for further surgery. If progression occurs, perform curettage
– Spinal ABCs: preoperative selective arterial embolisation, extended intralesional curettage with grafting +/- limited fusion. Embolisation though poses risk of cerebral emboli and also ischemic damage to the cord
– Incompletely resectable, recurrent or aggressive ABCs: low dose radiation(26 to 30 cGy)
– If a patient does have a local recurrence, repeat surgical excision can be performed.
– Selective arterial embolisation as a definitive procedure can be used in locations where a tourniquet cannot be used and control of bleeding can become difficult (e.g., spine, pelvis, and the proximal portions of the extremities)(2)


Ref:
1. Campanacci M, Capanna R, Picci P. Unicameral and aneurysmal bone cysts. Clin Orthop 1986; 204:25-36.
2. Green JA, Bellemore MC, Marsden FW. Embolization in the treatment of aneurysmal bone cysts. J Pediatr Orthop 1997;17(4):440-443.