A visible second nail plate forming beneath your existing toenail is usually the result of your nail matrix, the tissue responsible for producing the nail, continuing to generate new growth while the old nail plate above it has stalled or detached. The two most common explanations are retronychia, where a new nail grows forward and stacks beneath an older nail that has become embedded, and onychomadesis, where the matrix temporarily shuts down, then restarts, pushing out a fresh plate underneath the old one. Both look alarming but are treatable, and understanding which is happening changes what you should do about it.
How a Second Nail Plate Forms
Your toenail is produced by the nail matrix, a crescent of tissue tucked under the skin at the base of the nail. The matrix works continuously, pushing new cells forward to form the hard plate you trim. The bulk of the nail plate comes from the deeper part of the matrix, while the top surface comes from the portion closest to the skin fold at the base of the nail.
1PubMed. Nail biology and nail science
When something disrupts this process, the old nail plate and the new growth can end up occupying the same space. If the old plate sticks in place while the matrix keeps producing fresh nail beneath it, you get what looks like a nail growing under a nail.
Retronychia and Nail Plate Stacking
Retronychia is the condition most likely to produce a dramatic-looking double nail. It was first described in 1999, and it occurs when the newest nail plate, instead of sliding forward smoothly, embeds itself backward into the proximal nail fold (the strip of skin at the base of the nail). Older nail generations that never fully shed can stack on top of the new plate, creating multiple layers visible under a single nail fold.
2PubMed. Retronychia of the toenails: a review with emphasis on pathogenesis, new diagnostic and management trends
The big toe is the most frequently affected, though any toenail can develop it.
The usual trigger is some form of trauma. A stubbed toe, tight shoes, repetitive pressure from running, or a single hard impact can cause the growing nail plate to lose its normal forward trajectory. Once the plate starts embedding into the fold instead of growing outward, the body responds with inflammation, pain, and sometimes granulation tissue, the raw, reddish tissue that forms around a wound.
3PubMed. Retronychia: proximal ingrowing of the nail plate
People often mistake it for a fungal infection or a standard ingrown toenail, which delays proper treatment. A key difference: a regular ingrown nail digs into the skin at the side, while retronychia involves the base of the nail pushing backward.
Onychomadesis and Nail Shedding
Onychomadesis looks different from retronychia but can also create the appearance of a new nail growing beneath the old one. In this case, the nail matrix temporarily stops working altogether. When it resumes, a new plate begins forming at the base while the old plate, now disconnected, lifts and eventually sheds. During the transition, both the old plate and the emerging new plate are visible at the same time.
4PubMed Central. Differential Diagnosis of Retronychia, Onychomycosis and Onychomadesis
Onychomadesis can happen after physical trauma, but it also commonly follows systemic illness. High fever, severe infection, or chemotherapy can shock the matrix into pausing. One well-documented trigger is hand, foot, and mouth disease (HFMD), the childhood illness caused by enteroviruses. Nail shedding was recognized as a complication of HFMD and is probably underreported because it occurs weeks after the rash has cleared, when families have stopped thinking about the infection.
5PubMed. Nail matrix arrest in the course of hand, foot and mouth disease
A series of pediatric cases in a hospital emergency department documented onychomadesis appearing as a late complication of HFMD, prompting the authors to emphasize that clinicians should warn parents about the possibility so they are not alarmed weeks later.
6Pediatric Emergency Care. Onychomadesis as a Late Complication of Hand-Foot-Mouth Disease
Specific viral strains seem to matter. During a 2010 outbreak in Taiwan involving coxsackievirus A6, about 37% of those infected with that strain developed onychomadesis, compared with only 5% of patients infected with other enterovirus strains.
7PubMed Central. An outbreak of coxsackievirus A6 hand, foot, and mouth disease associated with onychomadesis in Taiwan, 2010
The good news is that onychomadesis from a viral illness is almost always self-limiting. The new nail grows in normally, and the old one falls off on its own. No treatment is usually needed beyond patience and keeping the area clean.
Telling Retronychia and Onychomadesis Apart
Because both conditions involve a visible new nail plate underneath an old one, mixing them up is easy. The practical clues that separate them come down to pain and timing. Retronychia tends to cause persistent inflammation and swelling at the base of the nail, and the area is tender to touch. Onychomadesis, by contrast, is usually painless. The old nail simply lifts or detaches without much redness or discomfort at the fold. If you can wiggle the old nail without pain and there is no angry-looking tissue at the base, onychomadesis is more likely.
History matters, too. If you remember a specific toe injury or you wear tight athletic shoes, retronychia is high on the list. If your child recently had a viral rash, or you went through a period of illness or high fever a month or two ago, onychomadesis fits better. An ultrasound exam at a dermatologist’s office can confirm retronychia by showing specific features at the nail’s origin point, including a characteristic halo around the embedded plate and measurable thickening of the proximal nail fold.
8PubMed. Ultrasonographic Criteria for Diagnosing Unilateral and Bilateral Retronychia
Growths That Mimic a Second Nail
Not everything that looks like a nail growing under a nail is actually a second nail plate. A few other conditions can produce a hard bump or raised structure under the toenail that feels like new nail tissue but is something else entirely.
Subungual Exostosis
A subungual exostosis is a benign bony outgrowth from the tip of the toe bone, right beneath the nail bed. It is slow-growing and typically appears as a hard, painless-to-moderately-painful lump that lifts or distorts the nail from underneath.
9PubMed Central. Subungual exostoses of the toes: a systematic review
It can sometimes poke through the nail bed and become visible as a firm nodule under or at the edge of the nail. A case report described an 18-year-old male presenting with a protruding papule on his big toenail caused by this type of growth.
10PubMed. Subungual exostosis treated with an in situ thin split-thickness toenail bed graft
Unlike retronychia or onychomadesis, which involve actual nail plates, an exostosis is bone or cartilage. An X-ray usually confirms it quickly. Surgical removal is the standard treatment, and recurrence is uncommon.
Glomus Tumors
Glomus tumors are another benign growth that can form beneath the toenail, arising from specialized structures in the nail bed involved in temperature regulation. They tend to cause intense, sometimes throbbing pain, particularly with cold exposure or direct pressure. A glomus tumor can also cause visible nail changes, including splitting of the nail plate.
11PubMed Central. Glomus tumor of the nail unit
If you have a small, extremely painful spot under your toenail that worsens with cold temperatures, a glomus tumor is worth investigating. MRI is the preferred imaging tool for these.
Accessory Nails on the Little Toe
If the doubled nail is specifically on your fifth (pinky) toe, there is a good chance you are looking at an accessory nail rather than any disease process. An accessory nail of the fifth toe is a common anatomical variant where a smaller, separate nail plate forms beside or beneath the main nail. It can be congenital or develop over time from chronic shoe pressure. One case report described bilateral “petaloid nails,” where the lateral portion of the fifth toenail splits into its own distinct smaller nail unit.
12PubMed Central. A Curious Case of Bilateral Double Fifth Toenails: Petaloid Nails
This variant is usually painless and cosmetic. Many people live with it their entire lives without it causing problems, though tight footwear can make the extra nail edge irritating.
Treatment for Retronychia
Treatment depends on how early the condition is caught and how many nail layers have stacked up. In the early stages, conservative approaches can work. These include topical steroid application, taping the nail fold away from the plate, and using concentrated urea dressings overnight to soften thickened nail tissue, followed by careful debridement.
13International Journal of Surgery Case Reports. Trauma a major contributing factor of retronychia: Case series
However, recurrence is common with conservative treatment alone, and antibiotics do not resolve the underlying problem even when a superficial infection is present.
A non-surgical approach that showed more promising results involved mechanically eliminating the sharp proximal edge of the uppermost nail layer and thinning the stacked plates using specialized drills. In a case series of 20 patients treated this way every one to two weeks, all achieved completely healthy nail growth with no relapse over an 18-month follow-up. Treatment duration ranged from 2 to 14 months, depending on how much healthy nail was left beneath the damaged portion and patient factors like age and diabetes.
14PubMed Central. Chronic Retronychia: Nonsurgical Treatment
When conservative options fail or the condition is advanced, nail plate avulsion, the removal of the old stacked plates, is the standard approach. A systematic review of retronychia treatment found that surgical avulsion produced the highest cure rate at about 78%, though roughly one in ten patients experienced long-term changes to the nail’s appearance afterward, mainly in the form of nail dystrophies.
15PubMed. Treatment of retronychia: A systematic review and suggested treatment algorithm
The procedure itself is straightforward for a dermatologist or podiatrist. The stacked layers are carefully peeled away, the embedded portion freed from the nail fold, and anti-inflammatory treatment applied. Once the old plates are removed, the newest nail plate underneath typically resumes growing normally.
What Makes Retronychia Come Back
Retronychia has a tendency to recur if the original trigger is not addressed. Runners, hikers, and people who spend long hours in tight-fitting shoes are repeat offenders. If ill-fitting footwear caused the initial episode, switching to shoes with a roomy toe box can prevent a second round. Keeping toenails trimmed straight across, not too short, also helps the nail plate maintain its forward trajectory instead of catching on the skin fold.
Certain medical conditions increase risk for recurrence. Diabetes and obesity were flagged in the non-surgical treatment study as factors that slowed healthy nail regrowth and could complicate healing.
14PubMed Central. Chronic Retronychia: Nonsurgical Treatment
People with circulatory problems or peripheral neuropathy should be especially attentive to changes at the nail base, since reduced sensation means they may not notice early inflammation that would prompt others to seek care sooner.
When Children Lose Nails After Being Sick
Parents sometimes panic when a child’s toenails start peeling off weeks after a bout of hand, foot, and mouth disease. The lag between the illness and the nail shedding, often four to eight weeks, makes the connection easy to miss. By the time nails start separating, the fever, mouth sores, and rash are long gone, so the nail loss seems to come from nowhere.
The process is painless in the vast majority of cases. The old nail lifts from its base, and if you look carefully, you can see a thin new nail already forming underneath. Because the matrix has restarted on its own, the new nail will grow in normally over the next several months. There is no need for antifungal medications, and no topical treatment speeds the process. The main practical concern is protecting the exposed nail bed during the transition, since young children are not always careful about banging their toes. A loose adhesive bandage over the affected toe can help during the vulnerable weeks when the old plate has mostly detached but the new one has not fully hardened.
Fungal Infections and How They Differ
Onychomycosis, a fungal nail infection, is probably the most common misdiagnosis for both retronychia and onychomadesis. Fungal infections can cause thickened, discolored nails that lift from the nail bed, and at first glance a layered or separating nail might look fungal. The distinction matters because treating a non-fungal condition with antifungal medication wastes months and delays the right intervention.
A few features help separate fungal infections from the conditions discussed above. Fungal nails tend to thicken gradually over months or years, usually start at the free edge (the tip you trim), and produce crumbly, yellowish debris under the plate. Retronychia involves swelling and tenderness at the base, with a relatively normal-looking nail tip. Onychomadesis involves clean separation near the base without the crumbly texture. If there is any doubt, a nail clipping sent for fungal culture or a simple in-office test can rule fungal infection in or out before committing to a treatment plan.
Seeing a Specialist
A doubled toenail does not always require a doctor’s visit. If you recently had a viral illness and a nail is painlessly lifting while a new one appears beneath it, you are almost certainly dealing with onychomadesis, and time will resolve it. But several signs should prompt a trip to a dermatologist or podiatrist:
- Pain and swelling at the base: persistent redness, tenderness, or puffy tissue at the proximal nail fold suggests retronychia or infection and benefits from professional assessment.
- A hard lump under the nail: a firm, non-compressible bump that distorts the nail shape could be a subungual exostosis or another growth that needs imaging.
- Intense pain with cold: sharp, localized pain triggered by temperature changes points toward a glomus tumor.
- No obvious trigger: if you have not had trauma, tight shoes, or recent illness and a second nail structure is appearing, a specialist can rule out less common causes.
- Recurrence: if the same toe keeps developing stacked nails, the underlying mechanical cause likely needs to be identified and corrected.
Ultrasound is increasingly used in nail clinics to visualize what is happening beneath the fold without needing to remove the nail for diagnosis. Specific measurements of nail fold thickness and the position of the nail plate’s origin can confirm retronychia with high accuracy, sparing patients unnecessary exploratory procedures.
8PubMed. Ultrasonographic Criteria for Diagnosing Unilateral and Bilateral Retronychia
For suspected bony growths, a plain X-ray of the toe is usually the first step, with MRI reserved for soft-tissue masses like glomus tumors.