Posterior Tibial Tendon Dysfunction: A condition characterized by a broad range of progressive disorders ranging from TENOSYNOVITIS to tendon rupture with or without hindfoot collapse to a fixed, rigid, FLATFOOT deformity. Pathologic changes can involve associated tendons, ligaments, joint structures of the ANKLE, hindfoot, and midfoot. Posterior tibial tendon dysfunction is the most common cause of acquired flatfoot deformity in adults.Flatfoot: A condition in which one or more of the arches of the foot have flattened out.Tenosynovitis: Inflammation of the synovial lining of a tendon sheath. Causes include trauma, tendon stress, bacterial disease (gonorrhea, tuberculosis), rheumatic disease, and gout. Common sites are the hand, wrist, shoulder capsule, hip capsule, hamstring muscles, and Achilles tendon. The tendon sheaths become inflamed and painful, and accumulate fluid. Joint mobility is usually reduced.Tendons: Fibrous bands or cords of CONNECTIVE TISSUE at the ends of SKELETAL MUSCLE FIBERS that serve to attach the MUSCLES to bones and other structures.Tendon Injuries: Injuries to the fibrous cords of connective tissue which attach muscles to bones or other structures.