Pathology of Aneurysmal bone cyst Dr Ijeghede Osaletin Senior Registrar (Pink Team).

Outline Introduction Epidemiology Aetiology Pathogenesis Pathology Staging Complication.

Introduction Aneurysmal bone cyst (ABC) is a misnomer as the lesion is neither an anuerysm nor a cyst. As defined by WHO, ABC is a benign tumour like lesion. It is described as ‘’an expanding osteolytic lesion consisting of blood filled spaces of variable sizes separated by CT septa containing trabeculae or osteoid tissue and osteoclast giant cells.”

Incidence ABC represent 1% of all primary bone tumours sampled for biopsy. 70% of affected patient are between 5 and 20yrs of age, approx half occurs in the 2nd decade of life. Although the lesion has been reported in infants. There is no sex predilection.

ABC can be found in any part of the body ABC can be found in any part of the body. They are eccentrically located in long bones. Long bones( 50-60%) typically metaphysis. -lower limbs 40% (prox tibia/fibula 24% and prox femur 13%) -Upper limbs 20% Spine and sacrum 20-30%( esp posterior elements).

Aetiology ABC represent either a primary neoplastic condition or a secondary response (arteriovenous malformation) to the destructive effects of an underlying primary tumour. Primary (65%) linked to genetic abnormalities, most commonly described translocation t(16;17) (q22;p13)

Secondary (35%) to other lesions- NOF, fibromyxomas, fibrous dysplasia, chondroblastomas, giant cell tumour, simple bone cyst, talengiectatic osteosarcomas, chondrosarcomas and metastatic disease.

Pathogenesis unknown ??. Arteriovenous fistulas and venous blockage- increase pressure –expansion –erosion and resorption of the surrounding bone. The malformation also cause local haemorrhage –reactive osteolytic tissue. Most primary ABC demonstrate a translocation of t(16;17) (q22;p13) with up regulation of Ubiquitin specific protease

USP 6(Tre 2) gene on 17q13 leading to juxta position of CDH 11 on 16q22. This fusion leads to increase cellular Cadherin 11 activity that seems to arrest osteoblastic maturation.

Pathology Gross- Consists of an encapsulated mass of soft, friable, reddish brown tissue. Usually contained within a thin subperiosteal shell of newborn. Microscopy- Variable number of vascular spaces whose walls are lined with tissue composed of fibroblastic cells with collagen, giant cells, hemosiderin and osteoid (secondary to microfractures).

Staging Buraczewski and Dabska Stage 1 (initial phase)- Osteolysis without peculiar findings, there is rapid increase in size of osseous erosion. Stage 11( Growth phase)- Enlargement of involved bone, formation of shell arround cental part of lesion. Stage 111 (stabilization phase)- fully developed radiological pattern.

Enneking protocol for staging of Benign musculoskeletal neoplasm. Stage 1 (Latent)- Remains static or heals spontaneously Stage 2 (Active)- Progressive growth but limited by natural barriers. Stage 3 (Locally aggressive)- progressive growth, not limited by natural barriers.

Complications Deformity. Disruption of growth plate Neurologic symptoms Pathologic fracture. Re occurrence. Joint stiffness