[Osteocartilaginous exostoses (osteochondromas)]

Osteochondroma is the most common true bone neoplasm formed by a cartilage capped bony projection arising on the external surface of a bone containing a marrow cavity that is continuous with that of the underlying bone. Osteochondromas generally arise in the bones preformed by cartilage. Osteochondromas may be solitary and multiple. Most reported cases have been in the first 3 decades with no known sex predilection. The lesion has three layers - perichondrium, cartilage and bone. The cartilage cap is usually less than 2.0 cm thick, superficial chondrocytes are clustered, the ones close to the transition to the bone resemble a growth plate, they are organized into chords and undergo enchondrial ossification. Loss of the architecture of cartilage, wide fibrous bands, myxoid change, increased chondrocyte cellularity, mitotic activity, significant chondrocyte atypia and necrosis are all features that may indicate secondary malignant transformation.