What is Subungual Exostosis ?

Subungal Exostosis


A subungual exostosis is a bony prominence or an outgrowth of bone that can occur under the toenail. This happens below the nail hence the term “subungual”. They generally are a result of some form of trauma to the toe, that results in the formation of a bony irregularity or prominence. Additionally, other small tumors called osteochondromas and enchondroma, can also form in the bone beneath the toenail, as well as in other bones in the body.

Clinical Features

The most common site for the lesion is on the great (big) toe, usually on the inner or medial aspect. It’s not unusual for subungual exostosis to occur on the fingers, especially the index and middle ones.

The condition may occur at any age, but half of the reported cases tend to happen by 20 years of age. Women are reported to be more affected than men.

A firm nodule develops below the nail bed and, as it grows upwards, the nail plate separates from the bed (onycholysis), often causing pain. The lesion usually grows slowly over weeks to months.


The cause is unknown; however, it is thought to occur as a reaction to:

-constant irritation to the bone
-previous trauma
-longstanding or chronic infection
-some inherited conditions such as multiple exostoses syndrome, or multiple exostoses-mental retardation syndrome.

Exostosis under the front of the Nail
Exostosis under the front of the Nail


-Gradual growth 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 as well. Biopsies are rarely required, but may serve as a helpful diagnostic tool.

 Differential diagnosis

Lesions in this region such as subungual verruca (viral wart), pyogenic granuloma (small skingrowths), and other benign skin disorders can appear similar to subungual exostosis. After diagnosis, a decision has to be taken as to whether to remove the exostosis or not.


Spur being removed
Spur being removed

An examination should include careful inspection of the nail plate. If the nail plate is deformed, there may be a history of nail root damage or chronic pressure on the nail plate. Palpation exams should include not only the nail folds where ingrown toenails are common, but also direct pressure on the nail plate itself. When one locates pain at the nail fold, and it is obvious that the nail is intimate with the skin, the clinician can narrow down the diagnosis to either onychocryptosis or paronychia (nail disorders).

In the case of an exostosis, as it 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, with the exostosis being 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.

A partial nail avulsion can be an option too. Dependent on the location of the exostosis, a permanent option is to remove the toenail. If there is a small exostosis and the toenail is thickened or severely deformed, another option is to remove the exostosis. Sometimes, a significant area of the nail bed has to be excised along with the lesion; giving rise to a plastic reconstruction being required later on.                                                                                               In conclusion, consider the possibility of a subungual exostosis in assessing disorders of the toenail. The next “ingrown nail” you see may just be a subungual exostosis in disguise.

 Your feet mirror your general health . . . cherish them!


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