Common causes of ingrown toenails include wearing shoes that are too tight, cutting toenails too short or not straight across, and toe injuries. To prevent ingrown toenails, toenails should be trimmed straight across with no rounded corners and extend past the skin, and shoes and socks should fit properly and feet should be kept clean. An ingrown toenail grows into the skin at the nail borders, causing pain, redness, and possible infection if the skin is broken.