Exostosis is one of the most common benign bone lesions. It could be a solitary lesion or multiple lesions presented as part of HME. This condition often affects the metaphyseal region of long bones, including femur, tibia, humerus and ribs. Rib exostosis may cause severe complications such as pneumothorax, hemothorax, and diaphragmatic injury. We report two patients with asymptomatic rib exostosis who underwent surgical excision to prevent severe complications. The first patient was a 13-year-old male with HME. At the eight-year follow-up, the size of the exostosis at the left ninth rib was found to have increased to 5 cm in diameter and it compressed the diaphragm. Surgical excision was performed to prevent possible complications. The second patient was a 10-year-old male with HME. At the seven-year follow-up, the size of the exostosis at the right fourth rib was found to have increased to 5 cm in diameter and it protruded into the thoracic cavity. Surgical excision was performed to prevent possible complications. Both patients recovered uneventfully. The periodic evaluation of the size and shape of rib exostosis is important in HME patients, because we must judge the proper timing for surgical excision to prevent severe complications.
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