Bimalleolar and Trimalleolar Ankle Fractures

Bimalleolar and trimalleolar ankle fractures are specific types of injuries that commonly occur because of trauma, such as slips on the ice, a fall down stairs, sports injuries, and car crashes.

With a bimalleolar fracture, bones on both the inner and outer side of the ankle are injured. The same is true of a trimalleolar ankle fracture, but with a third injury to the back of the tibia bone at the ankle joint.

This article explains the anatomy, structure, and function of the bones involved in bimalleolar and trimalleolar ankle fractures. It discusses how these fractures occur, and why these more complex ankle fractures typically require surgery.

Anatomy of the Ankle

The ankle joint is where the bones of the leg (the tibia and fibula) meet with the bones on the hindfoot (talus) to form a joint that allows the foot to bend up and down. The ankle joint is susceptible to injury and one common type of injury is called an ankle fracture.

Other bones around the ankle joint can be injured as well, but an ankle fracture usually involves the bones of the tibia and fibula. The ends of these bones, with the bony bumps you see on the inner and outer side of the ankle, are called the medial malleolus (tibia) and lateral malleolus (fibula).

Bimalleolar and trimalleolar ankle fractures are considered more serious, or complex, fractures because they involve the ends of both the tibia and fibula bones. This leads to increased instability that, in most cases, will need surgical repair.

Causes of Ankle Fractures

Most bimalleolar and trimalleolar ankle fractures are serious injuries. They often occur from high energy injuries such as falls and motor vehicle collisions.

In addition to the force of impact, there is often a significant force of rotation on the ankle seen in sports injuries and serious falls.

Sometimes these fractures can occur due to a weakening of the bone, most commonly in people with osteoporosis. In people with underlying conditions that weaken the bone, these bimalleolar and trimalleolar ankle fractures can occur with less force than in other people.

These ankle fractures are more complicated than injuries that, for example, may be treated by simply wearing a brace for several weeks. The treatment and recovery timelines for bimalleolar and trimalleolar ankle fractures are typically much longer.

Symptoms of an Ankle Fracture

Common symptoms of an ankle fracture include pain and swelling. These symptoms may be just in the ankle itself, or they may be felt in other parts of the foot and up toward the knee.

Putting pressure on the injury can make pain worse if you have a trimalleolar or bimalleolar fracture.

Bimalleolar Ankle Fractures

When a bimalleolar ankle fracture occurs, there is an injury to both the medial malleolus (inner side of the ankle) and the lateral malleolus (outer side of the ankle). Because both sides are injured, the ankle joint becomes unstable. 

This means that the fracture disrupts the structural integrity of the ankle (joint). Because the joint is unstable, it is susceptible to damage and early ankle arthritis if left untreated. Therefore, the typical treatment is to surgically repair the fracture to stabilize the ankle joint.

Bimalleolar Equivalent Ankle Fractures

One special subset of these fractures is called a bimalleolar equivalent fracture. This typically occurs when there is a fracture of the lateral malleolus and a ligament injury on the inner side of the ankle (the deltoid ligament). 

While this particular injury does not involve a bone injury on the inner side of the ankle, the ligament injury that has occurred causes the ankle joint to become unstable and requires surgical treatment to stabilize the joint.

Bimalleolar equivalent fractures need to be considered any time a lateral malleolus fracture has occurred. If there is pain or swelling on the inner side of the ankle, a bimalleolar ankle fracture may be present. 

Special x-rays, called stress x-rays, can be performed to look for signs of instability of the ankle joint.

Trimalleolar Ankle Fractures

Trimalleolar ankle fractures are another variant of this type of injury. The typical bimalleolar fracture involves bone injury to the inner and outer side of the ankle. People who sustain a trimalleolar ankle fracture also have a bone injury at the back of the tibia (posterior malleolus fracture) near the ankle joint.

Often, this does not change the treatment from that of a bimalleolar ankle fracture. However, if bone injury in the back of the tibia, called the posterior malleolus, causes instability of the ankle joint, it may need to also be repaired at the time of surgery.

Treatment of Complex Ankle Fractures

Treatment of all of these injuries is similar, and almost always requires surgery. The surgical procedure is performed to repair the bones, most often with metal plates and screws. These implants repair the bones to restore the stability of the ankle joint. It is very important to repair the bones with proper alignment. If not lined up perfectly there is a higher chance of developing early ankle arthritis.

One of the concerns of these complex ankle fractures is that they are usually accompanied by significant ankle swelling. Often this swelling can be serious, and may even cause blisters (called fracture blisters) to form on the skin. Surgery is frequently delayed days or weeks if there is significant swelling. Severe swelling not only makes the surgery more difficult to perform but can dramatically increase the risk of infection and healing problems after surgery.

As mentioned, infection and wound complications are the most worrisome concerns associated with ankle fracture surgery. Other common complications include stiffness and long-term swelling. Many patients notice swelling after ankle fracture surgery for six months, and may always have some increased swelling. Another concern is that because the bone is directly under the skin, metal plates and screws are sometimes bothersome and require removal. Lastly, even with appropriate care, surgical repair, and proper rehabilitation, ankle arthritis can occur.

A Word From Verywell

Bimalleolar and trimalleolar ankle fractures are typically unstable injuries that require surgical intervention in order to adequately restore alignment and stability to the ankle joint. Surgical treatment can be complicated by a tenuous soft tissue envelope surrounding the ankle joint causing the potential for complications such as infection and healing problems. Because of this, surgeons often proceed cautiously, allowing soft tissue to dictate the timing of surgical intervention. While most people fully recover, long-term mobility in the strength of the ankle joint can be an issue after these injuries