An os tibiale externum, also known as accessory navicular, is an extra bone or piece of cartilage located on the inner side of the foot just above the arch. An accessory bone is a bone that is not normally found in the average human, This condition represents a secondary ossification center (growth center) of the navicular bone and is present from birth. This condition occurs slightly more in females than males.
Over time the navicular bone and the accessory bone may fuse together to form one bone however a problem occurs when the two bones do not fuse together resulting in a non-solid union of bone to bone. This fibrous union is more like scar tissue and can cause pain when excessive strain is placed upon it.
The accessory bone is in the posterior tibial tendon, a tendon, that helps lift the arch of the foot with a significant amount of tension on this tendon when a person walks. People with excessively flat feet tend to put more tension on this tendon and are likely to be more prone to pain from an accessory navicular. Sometimes systemic medical conditions such as certain types of arthritis can aggravate this condition.
Os tibiale externum syndrome can result from the following:
overuse syndrome as seen in an athlete.
trauma to the foot as in an ankle sprain or direct trauma to the navicular bone.
chronic irritation from shoes rubbing against the extra bone, over time, may cause pain.
excessive pronation which strains the attachment of tibialis posterior muscles into the navicular bone.