A Rare Case of Adult Acquired Flatfoot Deformity (AAFD) With Chronic Atraumatic Subtalar Dislocation: Successful Reduction and Fusion of the Talonavicular and Talocalcaneal Joints.

Chronic atraumatic subtalar dislocation is an exceptionally rare and severe manifestation of adult acquired flatfoot deformity (AAFD) secondary to posterior tibial tendon insufficiency (PTTI). Early recognition is critical, as progressive deformity leads to rigid valgus malalignment and functional decline. A 65-year-old male presented with a seven-year history of worsening left foot pain and deformity. Examination showed rigid hindfoot valgus, midfoot collapse, forefoot abduction, and taut skin over the medial talar head. Radiographs revealed chronic lateral subtalar dislocation with talonavicular and talocalcaneal involvement. Surgical reduction and fusion were performed, utilizing three cannulated cancellous screws for the talocalcaneal joint and two bone staples for the talonavicular joint, followed by structured rehabilitation. At six months, the patient demonstrated substantial improvement in pain, function, and radiographic alignment, with stable fusion. This case highlights the importance of recognizing severe AAFD with subtalar dislocation and demonstrates that joint reduction with selective fusion can yield significant functional improvement.