It's easy to take your toes for granted—that is, until your flesh starts swelling, throbbing and burning with the pain of an ingrown toenail. Then the all-to-common condition begins raising some puzzling questions: Why would the side of a toenail grow into the surrounding toe, and why does this happen more often to kids and pregnant women? Why is the big toe particularly prone to ingrown nails?

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Physicist Cyril Rauch tackled these and other thorny toenail mysteries in a recent study on the physics behind nail conditions. “Dermatologists have been dealing with these questions, but we thought physics could be involved in explaining how a toenail, which is essentially a type of plate, can change its curvature,” he says.

Nails are made of keratin, a protein found in the outer layer of human skin. Nails grow as cells from a region inside the finger called the matrix multiply and get pushed outward. Compacted layers of old cells form the hard, translucent nail plate. But the nail must be able to grow smoothly while also adhering to the fingertip. That's why the nail bed has a series of ridges that stretch from the lunula—the white “half moon” shape that is the visible part of the matrix—toward the tip. The underside of the nail plate boasts a similar set of ridges, so that the two fit together like a set of parallel rails on which the nail can slide and grow.

According to Rauch, nail growth is a delicate balancing act between the stresses of growth and adhesion. If the nail grows too quickly or slowly, residual stresses can force the nail's curvature to change, resulting in a painful cut into the surrounding skin. That helps explain why certain people are more prone to ingrown nails.

“We found by looking through the literature that pregnant women tend to get a lot of ingrown nails. Maybe 15 or 19 percent of pregnant women can get them, so it's kind of common,” Rauch says. “When you are pregnant, you have a lot of growth hormones active inside your body. Similarly, in children, there is a stronger element of growth happening.” In elderly people, the growth rate of the nail slows down relative to the adhesion strength, creating a condition called pincer nail where the nail plate forms a tight curl.

The nail's initial curvature also reveals the reason thumbnails don't become ingrown as often as big toenails, says Rauch: “The topography of nails is slightly different, and that will lead to what we call in physics 'boundary conditions' which differ between fingernails and toenails,” he explains. “For example, if you look at the thumbnail and big toenail, what you will see is that they are both quite large, but the toenail will be much flatter at the end compared to the thumbnail.”

Because the toenail is relatively flat, it feels the most stress between growth and adhesion near the tip. When the two forces get out of balance, they are more likely to cause the flat toenail to veer off the rails and become ingrown. With a rounded shape like the thumb, it's more difficult for stress to build up at the tips and promote that painful growth.

At the University of Nottingham's School of Veterinary Medicine and Science, Rauch and coauthor Mohammed Cherkaoui-Rbati are hoping that their research will transcend species and help cure some of the common ailments suffered by hoofed animals as well. Hooves are also made of keratin, and they grow around an animal's toes in a similar fashion.

“Physics doesn't involve any genes, it doesn't differentiate between species, it's all about stress and forces,” Rauch says. Of course, there are some major differences between the human toenail and the typical ungulate hoof. One obvious distinction is that animals tend to walk on their hooves, so there is weight applied to a hoof's free edge. “These differences clearly must be taken into consideration,” Rauch notes. “But essentially speaking, you have many of the same things occurring that we hope can allow us to understand how the cells that divide to form a hoof or a nail are affected by the same physics.”

The growth rates of nails, hooves and claws appear similar across species, the study found. There are also similarities in the kinds of ailments and associated conditions prevalent among people and livestock—though the consequences for livestock can be harsh. Farm animals hobbled by hoof problems may be culled when they are unable to earn their keep, like cows that can't produce their regular milk quota.

“Mares have a tendency to have a hoof-related lameness condition when they are pregnant, just as women suffer from ingrown toenails,” Rauch says. “And when cows begin making milk there are many problems that arise with their hooves. So we are trying to capture something that has value across species to understand why hooves and nails alike can be affected by these conditions.”

Aside from natural causes, people can wind up with troublesome toenails if they forsake nail care for beauty. A simple pedicure is actually an exercise in physics, because trimming the nail changes the boundary conditions and thus alters the stresses throughout the nail plate. Pedicurists who flatten the free edges of toenails may end up creating conditions ripe for ingrown nails, Rauch notes, especially among young people like teenagers whose nails are growing at a faster rate.

So what's the takeaway for pedicure lovers? Make sure the person trimming your nails crafts the right shape. “If you change the shape of the free edge, you affect the stress. In the French manicure they tend to flatten the nail as much as possible, and that has a tendency to buckle the nail. I think that if you repeat this many times, then you may end up with an ingrown condition,” he says.