Biomechanics of the accessory navicular synchondrosis.

The accessory navicular is commonly considered an asymptomatic variant, but when traumatized, it can become the source of clinical symptoms. The accessory naviculars were divided into Types I, IIa and b, and III based on their appearance and location with relationship to their parent navicular. Only Type IIa and b accessory naviculars have a synchondrosis. The synchondrosis of Type IIa and b can undergo tension, shear, and compression forces causing avulsion or a painful pseudarthrosis to develop.