Osteochondroma
Osteochondroma (osteocartilaginous exostosis) is a benign bony outgrowth covered by a cartilage cap, whose cortex and medullary cavity are continuous with those of the parent bone. It is the commonest benign bone tumour, accounting for 35 to 40% of benign and 10 to 15% of all bone tumours.
Most are solitary (about 85%) and found in the first two decades. The remaining 15% occur in multiple hereditary exostoses, which is autosomal dominant. The usual sites are the metaphysis of the distal femur, proximal tibia and proximal humerus, then the pelvis, scapula, ribs and spine. Lesions point away from the nearest joint, grow until skeletal maturity, and stop growing at closure of the physis. The male to female ratio is about 1.5 to 2 to 1.
Types are pedunculated (a stalk, directed away from the joint) and sessile (a broad base). Complications are mechanical irritation, bursa formation (exostosis bursata), fracture of the stalk, tendon or nerve compression (common peroneal nerve at the fibular neck), vascular compression, pseudoaneurysm (popliteal artery), growth deformity, and malignant transformation to a peripheral chondrosarcoma in under 1% of solitary lesions (about 1 to 5% in multiple exostoses).
Pathophysiology.
Loss of function of EXT1 or EXT2 (heparan sulphate synthesis) in a chondrocyte of the growth plate allows a fragment of physeal cartilage to grow out through the periosteal bone collar. The lesion grows by endochondral ossification of its cartilage cap, like a growth plate, and the cap thins with age. Cap thickness above 1.5 cm in an adult (or above 3 cm in a child) suggests malignant change.
