Giant Osteochondroma of Tibia in a Case of Hereditary Multiple Exostoses: A Case Report

Osteochondromas, aka exostoses, are benign neoplasms of the cartilage. Genetic mutations of the EXT1 and EXT2 genes leads to a syndrome causing multiple osteochondromas, namely the hereditary multiple exostoses. Among the complications related to multiple osteochondromas, malignant degeneration into chondrosarcoma has the worst prognosis. Sudden enlargement of size of an osteochondroma or sudden onset of pain at the site of previous osteochondroma in a skeletally mature patient is concerning for malignant transformation. However, the size criterion does not apply for skeletally immature individuals, as observed in the case report here. The imaging of an uncommonly large osteochondroma of the extremity in a skeletally immature patient with hereditary multiple exostoses has been described here. Normal thickness of the cartilage cap (<3cm) proved the benignity of the lesion on imaging, thereby nullifying the size criterion in the skeletally immature. Keywords: Hereditary multiple exostoses, Osteochondroma, Cartilage cap, Heparan sulphate, Bayonet deformity, Chondrosarcoma.