Osteochondroma with large bursa clinically mimicking malignant transformation.

Osteochondroma is a common benign entity. However, symptomatic complications may arise that raise the question of malignant degeneration. Bursa formation about an osteochondroma may clinically and symptomatically mimic malignant degeneration. Magnetic resonance imaging usually permits unequivocal diagnosis distinguishing between secondary bursitis and malignant degeneration. Secondary bursitis can most often be managed conservatively. Magnetic resonance imaging also can differentiate other less common benign complications such as pseudoaneurysm formation caused by the exostosis and fracture at the base of the exostosis.