The posterior tibial tendon is the tendon that holds up the arch. If the foot is flat or pronated this may cause the tendon to have to work harder. Like a rubber band the tendon can stretch out and fail to function well. This can lead to further collapse of the mid- and rearfoot, and subsequent osteoarthritis. The earlier the pronation is diagnosed and treated the better, to save the tendon. Orthotic therapy is very important in this case. For more information, please click this link.