Posterior tibial tendon insufficiency (acquired flat foot syndrome) is a rare disorder in which the tibial tendon fails, causing the foot to gradually deform and "fall." There are approximately 1,000 cases each year in the United States, and it is usually misdiagnosed as a common sprained ankle. Surgery, including tendon harvesting, repositioning of the heal (calcaneal osteotomy), and sometimes bone fusion (arthodesis) are used. For my experiences with this disease, see http://www.cheswick.com/ches/ptti/