An exostosis in simple terms is an extra piece of formed bone. Generally this is an outgrowth of an existing bone of the foot or ankle. The exostosis may also break off the existing bone to become a free fragment or a loose body within or around a joint.
Some of the more common areas of an exostosis formation is underneath a toenail as a subungual exostosis. An exostosis of a bone in the toe can form a digital corn. Any traumatic event that leads to degenerative changes around a joint will commonly form an exostosis. A true bone spur of the heel is nothing more than an exostosis.
If an exostosis is thought to be present your podiatrist will most likely have an x-ray taken of your foot to evaluate it. The underlying cause of forming the exostosis needs to be addressed. An exostosis can be treated conservatively or surgically depending on location and symptoms. If a surgery is performed where the exostosis is removed this is termed an exostectomy.