There are numerous extra bones that can form in the human foot that are outside the normal number of bones. Called accessory bones, these extra bones are small, often round or egg-shaped, and are usually benign and unnoticed. However, there are certain situations where the presence of these bones can cause problems.

Accessory bones can be found in numerous locations, including the inner side of the foot, the outer side of the foot, under the ball of the foot, under the toes, and behind the ankle. In certain circumstances, the mere presence of these bones can create pain, inflammation, or calluses. For example, one of the most common accessory bones, called an os naviculare externum, is located on the inner side of the foot between the ankle and the big toe joint. This bone often serves as a source of irritation for the posterior tibial tendon, which attaches to the foot near where the accessory bone sits. Sometimes this bone can even form within the tendon itself. The presence of the bone near or in this tendon is one of several factors that eventually leads to tendon inflammation and damage.

When an accessory bone is located under a toe, it will exert pressure on the skin on the bottom surface of the toe. When pressure is exerted on the skin from the ground below and the bone above, a reaction forms in which the skin thickens its outer layer, forming a callus. If the callus becomes thick enough, it will cause pain when stepped on or pressed. In certain situations, such as in those who are diabetic or have poor sensation or circulation, the thick callus can lead to death of the healthy skin underneath, and a chronic wound can form. As is often the case, this accessory bone needs to be removed in order for the skin to heal.

Another common accessory bone that causes symptoms is called the os trigonum. This bone, located behind the ankle joint, can cause pain and swelling on the back of the ankle joint when the site is injured or otherwise becomes inflamed through an ankle sprain, kick to the back of the ankle, or a forceful crushing injury when the ankle bends downward suddenly. This causes an aching pain behind the ankle, which can worsen when the ankle bends downward. Bending of the big toe downward can also create pain, as the tendon that flexes the big toe crosses near this accessory bone.

Treatment of the pain caused by these extra bones typically involves non-surgical measures to reduce the inflammation associated with each condition (with the exception of toe accessory bones causing wounds, which need removal). This varies by each location, but usually involves anti-inflammatory medications, icing, and foot support. When these measures fail, the accessory bones can simply be removed, as this typically resolves all symptoms. An exception is treatment of the os naviculare externum, where the adjacent tendon needs to be repaired and sometimes moved to resolve the symptoms completely.