The “extra” bones of the foot

by admin on November 5, 2012

Each human foot contains 28 bones; 14 phalanges, 5 metatarsals, 7 tarsals, and 2 sesamoids. Like other aspects of human anatomy, however, a small percentage of the population may be out there operating with a few more or a few less. Fewer ones usually means a failure of the joints to develop between what would otherwise be two bones. A person with an extra bone usually has what is known as an “accessory ossicle,” a bone that only occurs in a small number of people.
In the human body accessory ossicles can occur almost anywhere, although they usually form in tendons at sites where a tendon may rub against a bone (typically called sesamoid bones, the most famous being the knee-cap.) However, they can also represent a portion of a bone that never fused together in growth and development as a child. There are three (relatively) common accessory ossicles in the foot; the os peroneum, the os tibiale externum, and the os trigonum. They are located (in order) on the outside of the foot, the inside of the foot at the top of the arch, and in the back of the ankle. Most of the time you cannot see an accessory bone from the surface, although an os tibiale externum can sometimes be visible as a bump on the inside of the foot near the ankle (even looking like a second ankle bone.) Between them, the overall incidence of these extra bones in the general population is reported at anywhere from 2-10%.
So what does it mean, then, to have an extra bone in your foot? Most of the time, nothing. They are typically asymptomatic, meaning that you don’t know they are there because they don’t feel any different. The problem is, these accessory bones are usually weaker than their normal anatomic counterparts, and can be injured more easily. Much like an appendix, they have no function but, at any time they can become injured and require attention.
As with any other injury to the extremities, injuries to accessory bones are usually treated with rest, ice, elevation, and usually immobilization in some form. In the foot, this typically ends up being a fracture boot rather than a cast and crutches. Several weeks are given to allow for the injury to heal. Much of the time, this is the extent of inconvenience you may suffer, after which the situation returns to normal. Sometimes, however, the sprain or fracture does not heal, and in that case surgery is typically required.
The good thing about accessory bones is that most people do not have them – therefore, you wouldn’t miss anything if they were gone. Surgical treatment of injuries involving accessory bones is quite simple – you remove them. In the case of the os peroneum or os tibiale externum, they typically involve a tendon, which is slightly more involved than just cutting. But the bottom line is, even if you have an accessory bone, and are unlucky enough to injure it, your chances of a complete recovery are quite high. Online sources of information are hard to find for accessory bones; share your stories here!

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