The accessory navicular, (os naviculare accessorium, os tibiale externum) is an anatomical variation that involves an additional bone on the medial side or the foot near the navicular bone. This accessory bone is not common (affects few percent of the population) and when it does occur, the enlarged area can be painful from shoe pressure or affecting foot biomechanics due to a difference where tendons attach.
The accessory navicular can be classified into three different types:
Type I: The accessory navicular is a small, rounded bone located proximal to the navicular bone, connected to it by a fibrous band or synchondrosis (cartilage joint).
Type II: The accessory navicular is larger and separated from the navicular bone by a synchondrosis. It may have its own ossification center.
Type III: The accessory navicular is a fragment of bone within the navicular bone itself.
Treatment for an accessory navicular if it is painful usually involves conservative measures such as rest, immobilization, foot orthotic devices or foot supports, and physical therapy to relieve symptoms. In cases where conservative treatments are ineffective, surgical removal of the accessory navicular may be considered and is typically a good long term option.