A trimalleolar fracture happens once the 3 malleolar bones that frame the ankle joint break at the same time. The malleolar are specific elements of the tibia and fibula that form the ankle joint. These comprise the lateral malleolus, which is at the tip of the calf bone, the medial malleolus that is on inside of the leg bone (shinbone) and also the posterior malleolus, that is at the rear of the leg bone.