Progressive flatfoot deformity accompanied with avulsion and dislocation of accessory navicular

The accessory navicular is one of the most common accessory bones in the foot, and it is mostly asymptomatic. Symptomatic cases largely belong to type 2, which is characterized by the presence of a 1-3 mm synchondrosis between the large accessory navicular and the native navicular. Chronic injury to the synchondrosis is considered to be a major cause of symptoms. In this case report, we describe an uncommon case of an avulsed type 2 accessory navicular, which was dislocated to the level of the medial malleolus. A plantar slip existed between the accessory and the native naviculars, which enabled the patient to perform an active inversion of the foot, but not a single heel raise. The collapse of the medial longitudinal arch progressed rapidly over 4 months prior to surgery, with an increased lateral hindfoot pain, suggesting an impingement due to the progression of hindfoot valgus. Surgical treatments, including plication of the ruptured spring ligament, excision of the accessory navicular, and transfer of the flexor digitorum longus tendon, were successful in improving the symptoms and flatfoot deformity. This case represents an uncommon subtype of symptomatic accessory navicular with complete avulsion and dislocation, presenting as a posterior tibial tendon dysfunction disorder and progressive flatfoot deformity