The deltoid ligament is the ligament on the medial side of the foot and ankle and is the main stabilizing structure on the medial side of the foot and ankle. The causes of chronic medial deltoid ligament injury in the left ankle mainly include tibialis posterior tendon insufficiency, strain injury, and post ankle arthroplasty. 1. Posterior tibial tendon insufficiency: the main role of the posterior tibial tendon is toe flexion and foot inversion. Insufficiency of the posterior tibial tendon can lead to instability of the arch function, resulting in chronic deltoid ligament injury. 2. Strain injury of deltoid ligament: when the patient’s left ankle joint is repeatedly pulled by force for a long period of time, it will lead to repeated exertion of deltoid ligament, which will cause edema of deltoid ligament and lead to chronic injury of deltoid ligament. 3. After ankle arthroplasty: patients with unstable ankle joint function after ankle arthroplasty are prone to chronic deltoid ligament injury. Patients with chronic deltoid ligament injury on the medial side of the left ankle need to go to the hospital in time and under the guidance of the doctor for regular treatment.