A hammertoe is a toe that is bent because of a weakened muscle. The weakened muscle makes the tendons in your toe shorter, causing your toe to curl under your foot. Hammertoes can run in families. They can also be caused by shoes that are too short. Hammertoes can cause problems with walking and can lead to blisters, calluses, and sores. Splints and corrective shoes can treat them. In severe cases, you may need surgery to straighten your toes.