An ingrown toenail (also known as onychocryptosis) occurs when the nail grows sideways into the toe, and not outward. The result is pain in the side of the toe. The big toe is most commonly affected. It can be caused by any situation that interferes with the proper outward growth of the nail, such as improper trimming of the nail or incorrect footwear. Very often it is also the result of severe toenail fungal infection that weakens the nail structure and due to tight footwear, an ingrown toenail appears.