A black discoloration underneath a toenail is essentially a bruise that forms under the nail plate, caused by blood that has leaked from damaged vessels. This darkening often appears after intense activity or wearing certain shoes. The common assumption that tight or ill-fitting footwear is the culprit is frequently correct. Understanding the specific mechanism behind this injury and how to prevent it is the first step in protecting your foot health.
How Tight Footwear Damages the Nail Bed
Tight or poorly fitting shoes cause a condition known as subungual hematoma, which is the medical term for blood trapped beneath the nail. This happens not from a single crushing blow, but from repeated, low-level trauma to the toe. Repetitive actions like running, hiking, or even brisk walking can cause the toes to slide or jam forward inside the shoe.
Each step drives the end of the toe repeatedly against the inside of the shoe’s toe box, especially when running downhill. This constant impact damages the small capillaries, or blood vessels, in the nail bed underneath the nail plate. The subsequent rupture of these capillaries releases blood into the confined space between the nail and the tissue underneath. Because the blood has nowhere to go, it pools and clots, causing pressure and the characteristic deep purple or black discoloration visible through the translucent nail. This repeated minor injury is why the condition is commonly seen in athletes and is often referred to as “runner’s toe.”
When Is It More Than Just a Bruise?
While a shoe-induced black toenail is a common and usually harmless injury, not all dark spots under the nail are the same, and some require medical attention.
A trauma-related bruise is typically uniform in color, appears only after an injury, and will grow out with the nail over several months. Fungal infections, however, can also cause discoloration, but this usually presents as yellow-brown, green, or black, often accompanied by nail thickening, brittleness, and debris under the nail plate.
A more serious, though rare, possibility is subungual melanoma, a form of skin cancer that develops under the nail. Signs that a dark area may be more than a simple bruise include a streak of discoloration that grows wider or changes shape over time without a known injury. Another warning sign is if the pigment extends into the skin surrounding the nail, which is a feature known as Hutchinson’s sign. If a dark spot appears without trauma, or if the discoloration does not grow out with the nail, it is important to consult a healthcare provider for a proper diagnosis.
Caring for the Injury and Choosing the Right Shoes
Treating the Injury
For a small subungual hematoma that is not causing significant pain, treat it at home by following the RICE protocol: rest, ice, compression, and elevation. Applying a cold compress minimizes swelling and manages initial discomfort. Over-the-counter pain relievers can also ease any throbbing sensation.
If the pain is severe and throbbing, it signals significant pressure building under the nail. A doctor may need to perform a procedure called trephination, which involves creating a tiny, sterile hole in the nail plate. This allows the trapped blood to drain, instantly relieving the pressure. This procedure must always be done by a medical professional to avoid infection or further damage to the nail bed.
Choosing the Right Shoes
Preventing future black toenails starts with selecting properly fitting footwear, especially for activities that involve repetitive impact.
- Ensure there is about a thumb’s width of space between your longest toe and the end of the shoe when trying them on.
- Shop for shoes later in the day when your feet are slightly swollen, ensuring a comfortable fit during exercise.
- Utilize proper lacing techniques to secure the foot in the shoe’s heel cup.
- Securing the foot prevents it from sliding forward and jamming the toes against the front of the shoe.