A bimalleolar fracture is a type of ankle injury in which the bones forming the upper portion of the joint break off in two places simultaneously. Also known as a Pott’s fracture for an 18th-century physician who published an article on the injury after suffering it himself, a bimalleolar fracture involves the lateral and medial malleoli, the pair of bony projections that form the rounded bumps felt to either side of the ankle. This injury occurs when a powerful lateral force rolls the sole of the foot to the outside in a motion known as eversion and at the same time turns the foot out at the ankle in a motion known as external rotation. A forceful enough movement can cause both the medial malleolus, a prominence on the inside of the tibia bone in the lower leg, and the lateral malleolus, a similar prominence on the outside of the fibula bone alongside it, to tear away from their respective bones.
The bones of the ankle joint are the parallel tibia and fibula bones of the shin and the talus bone of the hindfoot, which sits atop the calcaneus or heel bone. These bones are connected to one another by several ligaments, including the large deltoid ligament linking the medial malleolus of the tibia along the inside of the ankle to the talus below, and the anterior and posterior talofibular ligaments joining the lateral malleolus of the fibula to the talus along the outside of the ankle. When excessive forces are placed upon the joint, as in an athlete colliding with another athlete in such a way that the ankle abruptly rolls in one direction, damage to one or more of these ligaments and/or the bones to which they attach may occur.
In the case of a bimalleolar fracture, the blow comes from the outside of the ankle, making contact with the lateral malleolus in a horizontal direction. This causes the foot to evert and externally rotate so powerfully that the deltoid ligament along the inside of the ankle is overstretched to the point of tearing the medial malleolus to which it attaches away from the body of the tibia. Simply rolling the ankle is not typically a forceful enough injury to damage the bone. Normally, the ligament itself is sprained or torn, although the majority of the time it affects the talofibular ligaments on the outside of the ankle rather than the tough deltoid ligament.
As the deltoid ligament yanks downward on the medial malleolus, the talus bone below — to which the ligament’s other end attaches — abruptly shifts laterally. This action by the talus either causes the lateral malleolus to tear away as the talus makes contact with the distal or bottom end of the fibula or causes the fibula bone to snap at the bottom of its shaft. Either way the injury may be referred to as a bimalleolar fracture, even if both malleoli are not affected. To repair a bimalleolar fracture, surgeons will realign the bones and reattach any broken pieces using surgical screws or other implants.