The ankle joint is formed by three bones (Figure 1):
- The Tibia. The large bone forming the top and inside (media aspect) of the ankle joint
- The Fibula. The smaller bone making up the outside (lateral aspect) of the ankle joint.
- The Talus. The dome shaped lower bone of the ankle joint.
An “ankle fracture” (broken ankle) occurs when one or both (fibula or tibia) of the bones making up the top part of the ankle joint are broken. Most commonly this is the bone on the outside of the ankle (the fibula).
Ankle fractures can be broadly divided into two groups:
- Stable Ankle Fractures: The ankle joint remains intact (Figure 2)
- Unstable Ankle Fractures: The ankle joint is displaced (Figure 3)
Figure 1: Normal Ankle
Figure 2: Stable Ankle Fracture
Figure 3: Unstable Ankle Fracture
Most ankle fractures occur following an acute twisting injury where the foot is planted on the ground and the body rotates around it (ex. Figure 4). When the foot is rotated outward, the bone (fibula) on the outside of the ankle (lateral aspect) is broken first. If the rotation continues the structures on the inside of the ankle (medial aspect) are disrupted -either the bone (medial malleolus) is broken or supporting ligament (deltoid ligament) is torn. If the foot rotates inward, the medial malleolus of the tibia can be broken first followed by the fibula if rotation continues. The rotation injury may also cause tearing of the syndesmosis (the strong fibrous tissue that holds the two bones of the lower leg together at the ankle) which can lead to a fibula fracture higher up the leg.
Figure 4: Twisting mechanism causing an Ankle Fracture
A stable ankle fracture (Figure 2) occurs if just the outside bone (fibula) is broken and the syndesomosis structures on the inside of the ankle (medial malleolus and deltoid ligaments) are intact. Stable ankle fractures can be treated non-operatively by limiting activity until the bone has healed (usually 6 weeks).
An unstable ankle fracture (Figure 3) occurs if the injury causes the structures on the inside (medial aspect) of the ankle to be disrupted. This type of injury allows the lower bone of the ankle (the talus) to be loose and potentially poorly positioned within the ankle joint (See Figure 3). These disrupted inside (medial) strutures include either the medial malleolus (the prominent bump on the inside of the ankle) or the deltoid ligament (the strong ligament that supports the inside of the ankle joint). Most unstable ankle fractures benefit from surgery to stabilize the fractures and ensure that the joint heals in an anatomic position (Figure 5). In addition to the potential risks associated with surgery patients are at increased risk of developing ankle arthritis following an ankle fracture.
Figure 5: Unstable Ankle Fracture stabilized with surgery
Edited by John Early MD on May 18th, 2013