Hammer toe is a deformity of the second, third or fourth toe, when the toe appears permanently bent at the joint. There are a number of causes of hammer toe. Ill-fitting shoes are a common culprit. Injury, bunions or rheumatoid arthritis are other causes.
Hammer toe can cause other problems. Because the toe joint sticks out, it is more susceptible to shoes rubbing, meaning corns and calluses can develop. Hammer toes can also cause stress on the ball of the foot, which can lead to pain known as metatarsalgia.
Source: NHS choices