Solitary osteochondromas are thought to be the most common non-cancerous (benign) bone tumor resulting from a developmental abnormality of bone. It occurs when part of the growth plate forms an outgrowth on the surface of the bone. They become cancerous much less than 1% of the time.
Multiple osteochondromatosis is diagnosed when two or more osteochondromas are seen on plain X-rays. About 70 percent of the time, it is inherited. About 30 percent of the time, it occurs randomly. A patient may also have deformities of the forearms, a short stature, and knock-knees and ankles. This is caused by abnormal growth in the bones affected by the osteochondromas. These can become malignant about 1% of the time with large tumors.