An accessory navicular is a reasonable common extra bone associated with the navicular bone and posterior tibial tendon. Many authors have associated the presence of an accessory navicular with a flexible flatfoot deformity. The most common proposed reason for this is the close link between the accessory bone and the tendon of the posterior tibial muscle leading to an insufficiency of the posterior tibial tendon (PTT). A larger accessory navicular bone will lead to an elongation of the PTT which may reduce its translation of force to the foot to support it. This makes it reasonable to assume that the problem is associated with a higher prevalence of flat foot or an overpronated foot.
Some studes have reported a link between the presence of an accessory nvaiular and a flatter foot. Other stuides have reported no such link.
Park et al’s study of 196 people found a significantly increased midfoot pronation and forefoot abduction in the presence of an accessory navicular while the hindfoot was not in valgus and only slightly in equinus when compared to a control group of 46 feet that did not have accessory navicular. Sullivan & Miller’s study found no significant difference between the arch in 179 patients without and 49 patients with a symptomatic accessory navicular taht needed surgical with excision, concluding that the accessory navicular played no role in the development of a flat foot. Choi and Lee found that those who had recurrent pain after the surgical excision of an accessory navicular was associated with a flatfoot deformity.
It is probably to safe to say that the results of the research on this question is mixed. There is no clear answer.