Explanation:
Posterior tibial tendon dysfunction (PTTD) is the most common cause of adult acquired flatfoot deformity. The posterior tibial tendon acts as a major dynamic stabilizer of the medial longitudinal arch. When the tendon degenerates or ruptures, it leads to collapse of the medial arch, hindfoot valgus, and forefoot abduction. Early stages typically present with a flexible pes planovalgus deformity, where the hindfoot valgus is correctable. As the condition progresses, secondary ligamentous failure and joint degeneration may lead to rigid deformity. Recognition of this flexible stage is important as it may still be amenable to tendon reconstruction procedures rather than fusion. Reference: Myerson MS, 'Adult acquired flatfoot deformity: treatment of dysfunction of the posterior tibial tendon', Instructional Course Lectures (1997).