Bony lump under the toenail

What is a bony lump under the toenail, and how common is it?

The clinical name is subungual exostosis (also called Dupuytren's exostosis). It's a benign (non-cancerous) growth of bone from the tip of the end bone of a toe, or less often a finger¹. It grows under the nail and can cause pain and a misshapen nail¹.

It's described as uncommon or rare¹². Most lumps are on the big toe³. It's diagnosed most often in children and young adults⁴.

What causes it, and who gets it?

The cause isn't known for certain. Most clinicians think it's a reaction to repeated minor injury or ongoing irritation¹⁵. An infection, an inherited tendency and a cartilage cyst have also been suggested¹.

An earlier injury to the toe is the best-recorded link, but it's reported in only about 30% of cases⁶. So there's no reason to assume you caused it. Children, teenagers and young adults are the people most often affected⁴.

What does it feel like, how is it diagnosed, and what else could it be?

It usually starts as a firm lump under the end of the nail that doesn't move, with thick, hard skin on top. It grows slowly over weeks to months and can lift the nail off its bed⁷. Typical symptoms are pain, redness, swelling, a misshapen nail and shoes rubbing on it³¹. As it grows, the toe can swell, the skin can break down into a sore (an ulcer) and the nail bed can become infected⁴.

An X-ray is the key test to confirm it⁴. Looking at a tissue sample under the microscope can help tell it apart from other lumps⁸.

It's often mistaken for a wart, an ingrown nail, an infection of the skin beside the nail (paronychia) or a fungal nail⁹⁸. Missed or late diagnosis is common, and it can lead to lasting pain and nail changes¹⁹. A lump that doesn't respond to wart treatment is a reason to look further⁸¹⁰.

Some features can mark a nail lump as more serious:

• A lump that doesn't improve with treatment, looks unusual or ulcerates can need imaging and a tissue sample (biopsy) to rule out cancers such as melanoma or squamous cell carcinoma¹⁰⁵.
• A dark area under a nail that wasn't caused by an injury can be a sign of melanoma¹¹.
• Signs of infection are a high temperature, hot, red, swollen and painful skin around the nail, and pus. A toe that hasn't improved after 3 weeks of treatment, or is getting worse, is also a warning sign¹².
• Diabetes or poor circulation make any nail problem more serious, because diabetes can affect healing⁷¹³.

What helps first?

Once an X-ray confirms it, the usual treatment is surgery to remove the lump together with the cartilage cap that covers it⁴.

Not every lump needs surgery. These lumps rarely go away on their own and can grow, so the decision depends on how much the lump affects your comfort and quality of life¹⁴. Padding or changing your shoes isn't a reliable way to deal with it, especially if the lump is getting worse or causing a sore or infection¹⁴.

What helps if that isn't enough?

A lump is more likely to come back if it wasn't completely removed. Being younger, especially under 18, is also linked with it returning, possibly because bones are growing quickly and the edge between normal bone and the lump is hard to see¹⁵.

In children and teenagers, missing the diagnosis or treating it inappropriately can cause harm, through overtreatment or recurrence¹⁶. A lump that regrows or looks different from the original can need more imaging or a biopsy¹⁰.

What's the outlook, and can it come back?

After removal the outlook is generally good. Changes to the nail are the most common complication, while recurrence and infection are less common¹. The nail can be permanently lost or misshapen¹⁴. It may also regrow thickened, and the skin can be slow to heal¹⁷.

Because the cause isn't known, there's no known way to prevent a subungual exostosis. Complete removal is what lowers the chance of it returning¹⁵. Comfortable, well-fitting shoes are sensible general advice. After surgery, regrowth of the lump, increasing pain or the signs of infection described above are the things to watch for.