Tibialis posterior tendon abnormalities in feet with accessory navicular bone and flatfoot.

To assess tibialis posterior tendon (TPT) pathology, we investigated 27 feet with the accessory navicular bone and 22 normal feet by MRI. We found two major anatomical differences in the feet with the accessory navicular bone; the TPT directly inserted in the accessory navicular bone, without any continuity to the sole of the foot or with a slip, less than 1 mm in thickness, and there was a mass with the density of fibrocartilage tissue, between the tendon and the bone in 20/27 feet. These abnormalities were not detected in the control group. 3 patients in the study group were operated on and the MRI findings were confirmed. These findings suggest that patients with the accessory navicular bone and flatfoot should be examined by MRI for insertion abnormalities of the TPT.