The os navicularis is an accessory ossicle or small bone that is a normal variant however may predispose to certain tendon injuries and may itself be a source of pain. The os navicularis (or accessory navicular) is found alongside the navicular bone, the boney prominence on your inside foot just above the arch of the foot.
This condition represents an anatomic variant that you are born with.
Symptoms of an accessory navicular typically include medial ankle pain. The posterior tibial tendon may become irritated as it inserts along the navicular and accessory navicular.
If the accessory ossicle is determined to be the true, singular source of pain, it may require surgical removal. If the accessory ossicle has led to irritation of a nearby tendon, then there are non-surgical options for that tendinopathic condition including tendon debridement with a TENEX procedure, needling under ultrasound, and/or injecting platelet-rich plasma (PRP).