Toenails curl when they have reached the end of the nail bed. They begin to grow in the wrong direction when they are no longer connected to a part of the body.
Curling toenails can result in painful ingrown toenails, which can prevent a person from doing daily activities, such as walking normally. These cases should be treated by a doctor, who can also suggest ways to prevent ingrown toenails from reoccurring. Ingrown toenails can penetrate the skin and allow harmful bacteria inside to grow and cause infections. If this happens, the edge of the toenail needs to be removed, or the wound may not heal.