File under "Ewwwww ... but good to know:" physics are partially to blame for ingrown toenails. They also have a hand in determining why ingrowns tend to only affect the big toe ... and why certain folks (teens, pregnant women) are more susceptible to getting them.

A new study published by the University of Nottingham's Cyril Rauch and Mohammed Cherkaoui-Rbati takes on "the physics of nail conditions."

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"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," physicist Rauch told the Smithsonian.

The magazine further explains:

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.

This is why pregnant women and teenagers have a greater risk of getting ingrown toenails: both groups have a surfeit of growth hormones swirling around inside of them, so their nails grow faster than average.

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Of course, physics aren't only to blame. Rushing through one's pedicure can also be a culprit, so be careful how you cut, since "trimming the nail can affect the stresses throughout the nail plate." Stress out the nail, and you affect the way it grows ... and weeks later, it could potentially be spearing straight into your angry big toe. Prevention strategies offered by the Mayo Clinic include trimming nails straight across, keeping them a "moderate" length, and wearing shoes that fit properly.

And while we're at it, why don't our thumbnails ever become ingrown? According to 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.

Read the complete scientific paper in Physical Biology here.

Image via Shutterstock.