When a toenail appears to grow beneath another toenail, the condition is almost always retronychia, a disorder in which a newly forming nail plate pushes backward into the proximal nail fold instead of sliding forward over the old nail as it normally would. The result is two (or sometimes more) stacked nail plates occupying the same nail bed, with the younger nail wedged underneath the older one. Retronychia was only formally described in the medical literature relatively recently, and it is still frequently misdiagnosed as a standard ingrown toenail or a fungal infection, which means many people live with it longer than they need to.
How a Normal Toenail Grows
To understand what goes wrong, it helps to know the basics of how toenails move. A toenail is produced by the nail matrix, a crescent of rapidly dividing cells tucked under the skin at the base of the toe. New nail cells are continuously generated there, hardened into keratin, and pushed forward along the nail bed toward the tip of the toe. Toenails grow slowly compared to fingernails, averaging about 1.6 mm per month, while fingernails average roughly 3.5 mm per month.1PubMed. Growth rate of human fingernails and toenails in healthy American young adults The great toenail grows faster than the smaller toenails, but even so, it takes the better part of a year for a big toenail to fully replace itself. That slow pace matters because any interruption to the matrix’s work affects the nail for months afterward.
What Happens in Retronychia
Retronychia begins when something disrupts the normal forward glide of the nail plate. An episode of trauma, whether a single hard stub or repeated pressure from tight shoes, can temporarily stall or dislodge the nail’s connection to the matrix. When the matrix resumes producing new nail, that fresh plate does not re-engage with the old one and ride it forward. Instead, the new growth pushes backward, embedding itself into the soft tissue of the proximal nail fold, the skin flap that hugs the base of the nail.2PubMed Central. Retronychia Persistent inflammation at the proximal nail fold is the hallmark result of this reverse embedding.3PubMed. Retronychia: proximal ingrowing of the nail plate
Over time, the old nail sits on top while one or more new nail plates stack beneath it, pressing into the fold. Some patients end up with three or even four overlapping plates when the condition goes unrecognized for a long time. The visible clue is a toenail that seems to stop growing forward, often looking thick or ridged at the base, with redness and puffiness around the cuticle area. Because the nail appears stuck rather than ingrown at the sides, people often assume it is just growing strangely rather than recognizing it as a medical issue.
Who Gets It and Why
Retronychia shows a strong preference for the great toenail, which makes sense given that the big toe absorbs the most impact during walking and running. Research on ultrasound-confirmed cases found that about 70% of patients were female, and the big toe was the most commonly affected digit.4PubMed. Ultrasonographic Criteria for Diagnosing Unilateral and Bilateral Retronychia It typically affects younger adults, often women in their twenties and thirties, and is frequently linked to recurrent trauma and footwear-related issues.5PubMed Central. Surgical Management of Retronychia
The footwear connection is worth emphasizing. Tight-fitting shoes, especially narrow-toed styles or athletic shoes worn during high-impact activities, repeatedly compress the big toe and shove the nail plate backward with each step. Runners and hikers are prone to it for the same reason: downhill descents or sudden stops jam the toes against the front of the shoe. A single hard stub or a dropped object can also trigger the chain of events, but repetitive low-grade trauma is the more common culprit because it may go unnoticed until the nail has already stalled.
How Retronychia Differs From an Ingrown Toenail
People naturally assume that any painful, swollen toenail is “ingrown,” but classic ingrown toenails and retronychia are different problems at different ends of the nail. A standard ingrown toenail happens at the distal or lateral edges, where the nail’s corner or side digs into the surrounding skin as it grows forward. Retronychia is a proximal problem: the nail grows backward into the base. The pain and swelling sit at the cuticle rather than along the sides or tip, and the nail typically looks like it has stopped advancing rather than curving into the flesh.
This distinction matters for treatment. Trimming the corners or applying cotton wedges, the usual home remedies for a lateral ingrown nail, does nothing for retronychia because the problem is at the opposite end of the nail. If you have persistent swelling specifically at the base of a toenail, especially after some kind of foot trauma, it is worth considering retronychia rather than assuming the nail is just ingrown in the typical way.
Related Conditions That Can Look Similar
Retronychia is not the only condition that creates a sense of one nail growing under or displacing another. Several other situations produce confusingly similar appearances.
Onychomadesis
In onychomadesis, the nail plate separates from the matrix and nail bed, and eventually the entire nail sheds.6PubMed Central. Recurrent Onychomadesis of the Toenails in Children and Adults: A Case Series As the old nail lifts off, a new one growing from the matrix can be visible underneath, giving the impression that a second nail is pushing the first one out. This is actually the normal replacement process happening in an abnormal context: the matrix temporarily shut down, and when it restarted, the new nail has no attachment to the old one. Viral infections, including hand-foot-and-mouth disease, are an increasingly recognized trigger for onychomadesis. Certain viruses can interfere with the signaling pathways that drive nail stem cell activity, stalling matrix proliferation and arresting nail formation.7PubMed Central. Coxsackievirus A10 impairs nail regeneration and induces onychomadesis by mimicking DKK1 to attenuate Wnt signaling The nail then lifts as it loses its grip on the bed. Unlike retronychia, onychomadesis is usually painless and resolves on its own once the new nail grows out normally.
Onycholysis After Trauma
A hard blow to the toe can cause a subungual hematoma, a pool of blood under the nail. If the blood collection is large enough, the nail can separate from the bed entirely, a process called onycholysis.8PubMed Central. Subungual Hematoma As the detached nail loosens, the new nail growing in from the matrix becomes visible beneath the old one. This looks alarming but is usually straightforward: the dead nail eventually falls off or can be trimmed back, and the replacement nail grows in normally unless the matrix itself was damaged.
Fungal Nails and Psoriasis
Fungal infections of the toenail, known as onychomycosis, can thicken and distort the nail plate enough that it looks layered or doubled. Debris from the fungus builds up between the nail and the bed, giving the appearance of a second structure growing underneath. Psoriasis of the nail can produce similar effects: subungual hyperkeratosis, where excess skin cells accumulate under the nail, lifts and distorts the plate.9PubMed Central. Nail involvement in psoriatic arthritis Neither of these is literally a nail growing under another nail, but both can convincingly mimic that appearance on visual inspection alone.
How Retronychia Is Diagnosed
The condition can be tricky to recognize on sight alone, especially early on. Many cases are initially treated as bacterial paronychia (infection around the nail) or chronic ingrown nails, and the correct diagnosis only emerges when standard treatments fail to resolve the symptoms. The clinical clue is a nail that seems to have stalled in its forward growth, combined with inflammation at the base rather than the sides.
Ultrasound has emerged as the preferred noninvasive way to confirm retronychia. On imaging, doctors look for the presence of two or more overlapping nail plates, a reduced distance between the root of the nail plate and the base of the bone underneath, and swelling of the proximal nail fold.10Actas Dermo-Sifiliográficas. Retronychia: A Case Report Including Ultrasound Imaging and Surgical Treatment Specific ultrasound criteria have been developed, including threshold measurements for nail fold thickness and the distance between the nail plate’s origin and the underlying bone, with different cutoff values depending on whether the affected digit is a big toe or a smaller one.4PubMed. Ultrasonographic Criteria for Diagnosing Unilateral and Bilateral Retronychia Ultrasound also helps rule out other explanations for the swelling, such as tumors or inflammatory joint disease, which can present similarly at the base of the nail.
Treatment Options
How retronychia is treated depends on how far along the condition has progressed. Mild to moderate cases can sometimes be managed conservatively with a combination of topical or injected corticosteroids, strategic taping, clipping back the lifted portion of the nail plate, and orthotic devices to redistribute pressure on the toe. A systematic review of treatment approaches found that conservative management achieved a cure rate of roughly 41%, with no reported side effects.11PubMed. Treatment of retronychia: A systematic review and suggested treatment algorithm That is a respectable success rate for a noninvasive approach, though it also means that more than half of conservatively treated cases need further intervention.
When the nail plates are significantly impacted and stacked, surgery is usually necessary. The procedure involves avulsing, or removing, the old embedded nail plate or plates, which relieves the pressure on the proximal nail fold and allows the newest nail to grow forward normally. Despite sounding drastic, the surgery is straightforward and generally effective. The proximal nail fold inflammation tends to resolve quickly once the offending plates are gone. A new, healthy nail then grows in over the following months, though the slow growth rate of toenails means full cosmetic recovery takes patience.5PubMed Central. Surgical Management of Retronychia
Why It Often Takes So Long to Get the Right Diagnosis
Retronychia is still underdiagnosed, in part because it was only formally described in the dermatology literature relatively recently. Many general practitioners and even some dermatologists do not immediately think of it when they see a swollen, thickened toenail. The condition gets lumped in with more common problems like lateral ingrown nails, paronychia, or fungal infections. Patients are sometimes given repeated courses of antibiotics or antifungal medications that do nothing, because the underlying mechanical problem is never identified.
Awareness is growing, but the practical bottleneck is that ultrasound of the nail unit is not routine in most clinical settings. Without imaging confirmation, the overlapping nail plates are invisible to the naked eye. If you have a toenail that stopped growing forward after trauma, with persistent swelling at the base that antibiotics have not fixed, it is worth specifically asking a dermatologist or podiatrist about retronychia.
Preventing Recurrence
Because retronychia is almost always triggered by mechanical disruption, prevention centers on protecting the toes from repetitive trauma. Properly fitting shoes with adequate room in the toe box are the single most important measure. For runners and hikers, this often means sizing up half a size in athletic shoes to account for foot swelling during activity, and keeping toenails trimmed short enough that they do not catch on the front of the shoe.
After treatment, whether conservative or surgical, recurrence is a real concern if the same biomechanical conditions persist. Someone who returns to the same tight shoes or high-impact sport without any adjustment is at risk of re-triggering the cycle. Orthotic insoles that reduce forward toe pressure, lacing techniques that lock the heel in place, and simply being attentive to any early signs of proximal nail fold swelling can help catch a recurrence before it progresses to the point of needing surgery again.
When Medications Cause Nail Problems
Not all double-nail appearances are from external trauma. Certain medications, particularly chemotherapy drugs, can disrupt the nail matrix from the inside. Because the matrix cells are among the fastest-dividing cells in the body, they are easy targets for drugs designed to stop cell division. Chemotherapy-related nail changes typically affect multiple or all twenty nails and appear in a time pattern that tracks with when the drug was taken.12PubMed Central. Narrative Review of Drug-Associated Nail Toxicities in Oncologic Patients The resulting damage can include onychomadesis, where the nail separates and a new one starts growing underneath, as well as brittleness, discoloration, and ridging. While this is a different mechanism from the mechanical cause of retronychia, the visual result of a “double nail” can be strikingly similar. If you are undergoing chemotherapy and notice nails lifting or stacking, it is worth mentioning to your oncology team, but it is generally expected to resolve once treatment ends and the matrix resumes normal function.
The Emotional Side of Toenail Problems
It is easy to dismiss a toenail issue as cosmetically trivial, but research tells a different story. A hospital-based study of over 500 patients with nail disorders found that quality of life was meaningfully affected, particularly when multiple nails were involved. Women reported a greater impact on social functioning than men did, and the more nails affected, the worse the quality-of-life scores became. For toenail disorders specifically, the social functioning impact was higher among women compared to men. People avoid sandals, swimming pools, and barefoot situations. The discomfort is not just physical: there is genuine self-consciousness and frustration that comes with a visible nail problem that takes months to resolve, especially one that was initially misdiagnosed and mistreated.
Virus-Driven Nail Shedding in Children
A scenario that panics many parents is watching a child’s toenails loosen and fall off weeks after an otherwise mild illness. This is most commonly virus-induced onychomadesis, and hand-foot-and-mouth disease is the classic trigger, though other viral infections can cause it too. The virus temporarily halts nail matrix activity, and the nail that was being produced at the time eventually detaches.13PubMed. Virus-induced onychomadesis: Exploring the role of viral infection in nail shedding Research into the mechanism has shown that certain coxsackieviruses directly interfere with the molecular signaling that keeps nail stem cells proliferating and differentiating properly.7PubMed Central. Coxsackievirus A10 impairs nail regeneration and induces onychomadesis by mimicking DKK1 to attenuate Wnt signaling As the old nail separates, the new one growing in underneath becomes visible, creating the alarming appearance of a nail growing under a nail. In children, this is nearly always self-limiting. The new nail grows in normally, and no treatment is needed beyond keeping the nail bed clean and protected while the replacement grows out. The lag between the illness and the nail shedding, often four to eight weeks, is what confuses parents, since the viral symptoms are long forgotten by the time the nails start falling off.