Subungual Exostosis

Subungual Exostosis

(Exostosis below the toenails)

An exostosis is a medical term referring to an abnormal bony growth on the surface of a bone.

EX = out of,
OST. (Osteon) = bone,
OSIS = abnormal condition.

Subungual exostosis refers to the abnormal bony projection formed on the bone present under the nail (distal phalanx). This bony projection is covered by a cartilage cap and is categorized as a benign tumour consisting of bone and cartilage.

Subungual exostosis mostly occurs under or beside the big toenail, but may also occur under the small toenails or on the fingers as well.

This condition usually affects teenagers and young adults, and is predominantly seen in females.


The cause of subungual exostosis is unclear; however, it is thought to occur as a reaction to:

– constant irritation

– previous trauma

– chronic infection

– or due to some inherited conditions such as multiple exostoses syndrome.


– Exostosis grows gradually over weeks and months.

– It presents as a hard painful swelling under or beside the toenail, usually on its inner side.

– As it grows it presses against the nail, causing pain, which may be confined to the nail fold.

– With further growth, it separates the nail from the nail bed and erodes the overlying skin (making it prone to infections).


Subungual exostosis is often misdiagnosed for other conditions such as an ingrowing toenail or a malignant tumour.

Clinical examination should be supported by an X-ray to confirm its bony origin. There may be a history of previous trauma. A biopsy may also be a helpful diagnostic tool, but is rarely required.


As an exostosis continues to grow, damaging the surrounding tissues, the best treatment option is to remove the bony projection altogether via surgery.

The procedure is done under local anesthesia. The exostosis is removed, along with the attached nail bed. The surface of the underlying bone is scraped to ensure complete removal of the exostosis and prevent its recurrence in the future. Even then, it is reported to recur in 10% of cases.

Sometimes, a significant area of the nail bed has to be excised along with the lesion; a plastic reconstruction may be required later on.