Medial ankle sprains have a very low incidence (5% of ankle ligament injuries), and most of the time, the first structure involved is the deltoid ligament complex. However, other structures, such as the spring ligament, posterior tibial tendon, and flexor retinaculum, can also get damaged. A 32-year-old man who had an ankle sprain while performing sports activities showed magnetic resonance imaging findings compatible with a detachment of the flexor retinaculum and medial dislocation of the posterior tibial tendon. Surgical intervention was suggested to this patient; deepening of the medial retromalleolar groove and reattaching of the retinaculum were performed. It is very important to consider this rare injury after a medial ankle sprain and to think out of the box, not always assuming a deltoid ligament lesion. Level of Evidence V; Therapeutic Studies; Expert Opinion.