An accessory navicular is an extra piece of bone on the medial side of the navicular bone in the tendon of the posterior tibial muscle. There are several types of accessory navicular and variations in the shape.
An accessory navicular can be painful, generally for one of two reasons:
One reason is the enlarged piece of bone can become painful becasue of the pressure of the ‘lump’ from the shoe. This will depend on the type of shoe that is worn. For example rigid work boots can be a problem, but softer athletic shoes are often not a problem. Ice skates and ski boots can be a significant problem for someone with an accessory navicular. The treatment for this might involve modifying the shoe or use pads to make an accommodation for the enlarged bone.
The other typical reason for an accessory navicular hurting is that the presence of the extra bit of bone altering the ability of the muscle that it is associated with to be able to do its job. The posterior tibial muscle is an important support mechanism for the arch, so if the muscle can not do this very well then other structures have to help with more effort to support the arch. These other structures can become painful. Typically the treatment for this is the use of foot orthotics to help support the foot so that there is not so much pressure on the other structures that support the foot.