13 yr old female presenting with pain and swelling in proximal part of tibia. No history of trauma, infection or any other swelling.
Ans: aneurysmal bone cyst
ANEURYSMAL BONE CYST (ABC) Definition: - Benign multiloculated , blood-filled cystic mass that is often expansile and destructive. Epidemiology: - Affects all age groups but generally occurs during the first two decades of life (median age approximately 13 years). - No sex predilection.Sites of involvement: - May affect any bone. - Usually arise in the metaphysis of long bone especially the femur, tibia and humerus. Clinical findings: - Pain and swelling which may be secondary to fracture.Imaging: - Usually eccentric, expansile lesion with well defined margins. - Most lesions are completely lytic and often contain a thin shell of reactive bone at the periphery. - CT and MRI may demonstrate internal septa and characteristic fluid-fluid level.Gross: - Well-defined sponge-like mass. - Composed of multiple, blood filled spaces separated by thin, tan-white septa. Histopathology: - Walls of ABC consist of plump uniform fibroblaststs ( which may be mitotically active), multinucleated osteoclast-like giant cells ( sometimes they look like jumping into swimming pool cystic spaces), and thin trabeculae of reactive woven bone. - Surface of reactive woven bone is lined by plump osteoblasts.- 1/3 of cases contain a cartilage-like matrix, called 'blue bone', which is not common in other bone lesions. - Necrosis is uncommon unless there has been a previous pathologic fracture. Genetics: - Rearrangements of chromosome 17q13. Prognosis: - Recurrence rate following curretage is variable (20-70%). - Primary aneurysmal bone cysts account for approximately 70% of all cases. - Majority of secondary ABC arise in association with benign neoplasms, most commonly giant cell tumor of bone (GCT), chondroblastoma, osteoblastoma, and fibrous dysplasia, and less frequently, osteosarcoma.
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