Degeneration of the accessory navicular synchondrosis may be associated with decreased function of the posterior tibial tendon in patients who are middle-aged or older. We investigated the role of ultrasonography in differentiating between degeneration of the accessory navicular synchondrosis with separation of the accessory navicular from the navicular, which has not been previously reported to our knowledge, and a rupture of the posterior tibial tendon.We studied fourteen patients (mean age, fifty-five years; range, forty-one to seventy-two years) who had an operatively confirmed injury of the accessory navicular synchondrosis. The mean duration of follow-up was thirty-nine months (range, twenty-seven to fifty-four months). Preoperative radiographs demonstrated a type-II accessory navicular (an accessory navicular with a synchondrosis) in all fourteen patients. Ultrasonography, which was performed for twelve patients, demonstrated a defect in the synchondrosis in eleven patients and a normal posterior tibial tendon in all twelve. The operative findings included incomplete separation of the synchondrosis in four of the fourteen patients, complete separation of the synchondrosis and the periosteum in eight, and avulsion of the accessory navicular in two.On the basis of our findings, we concluded that post-traumatic degeneration of an accessory navicular synchondrosis may present clinically as a variant type of avulsion or rupture of the posterior tibial tendon in this age-group. Ultrasonography is useful for distinguishing between complete or partial separation through the synchondrosis and rupture or attenuation of the posterior tibial tendon.