Congenital Vertical Talus is a severe cause of flat foot (or overpronation) in kids. The talus is almost vertically, pointing plantarly so that the navicular bone is articulating with the top of the head of the talus. Surgery is usually the only satisfactory treatment.

Most Useful Resources:
Congenital Vertical Talus (Foot Health Forum)
Congenital Vertical Talus (Podiatry Arena)
Congenital Vertical Talus (PodiaPaedia)
C is for Congenital Vertical Talus (Podiatry ABC)