RESUMEN
Os subtibiale is a low prevalence accessory bone of the ankle. This bone is located in the posterior colliculus of the tibial medial malleolus, both in paediatric and adult ages. It can cause pain, redness and/or swelling, which can lead to a mistaken diagnosis of avulsion fracture. Adequate anatomical knowledge is crucial. First, we present the case of a school-aged boy, seen at the outpatient clinic for a 2-month history of pain in both inner ankles after an injury. Second, we present the case of an adult patient with a 3-day history of right medial ankle pain, with no previous injury, evaluated at the Emergency Department. Accurate history-taking and physical examination are essential. The diagnosis is given by conventional radiology of both ankles, in antero-posterior and lateral load views. The initial treatment is conservative (splint or orthesis) to establish and maintain the function of the foot during loading activities. If there is no recovery after 6 months, surgical treatment can be considered.