Posterior tibial tendon dysfunction
Posterior tibial tendon dysfunction is the most common cause of adult acquired flatfoot. It causes a progressive flattening of the arch of the foot and pronation of the rearfoot. Posterior tibial tendon dysfunction can be quite disabling if it not caught and treated early.
Resources:
http://www.podiatry-arena.com/podiatry-forum/showthread.php?t=2518
http://www.podiatry-arena.com/podiatry-forum/showthread.php?t=11187
http://www.podiatry-arena.com/podiatry-forum/showthread.php?t=16852
http://www.podiatry-arena.com/podiatry-forum/showthread.php?t=13775
http://www.runresearchjunkie.com/posterior-tibial-tendon-dysfunction-in-runners/