Distal fibula fracture (basic)

This is a basic article for medical students and other non-radiologists

Distal fibula fractures are the most common type at the ankle and are usually the result of an inversion injury with or without rotation. They are the extension of a lateral collateral ligament injury.

Background

Pathophysiology

Most ankle injuries occur because of an inversion injury. A pure inversion injury will result in tension being applied to the supporting soft tissues of the lateral ankle, particularly the lateral collateral ligament. This results in either a pure ligamentous injury (complete or partial tear) or avulsion of the tip of the fibula (the lateral malleolus). Avulsion injuries do not involve the syndesmosis and the ankle remains stable.

In some cases, inversion coupled with rotation leads to a more complex injury. They tend to causes fractures that are higher up the fibula and the rotational component of the injury may cause syndesmosis tears.

Relevant anatomy

The ankle is a pseudo-ball-and-socket joint; the talus is the ball and the distal tibia and fibula act as the socket. This socket is only functional because the tibia (medial and posterior malleolus) and fibula (lateral malleolus) are held together tightly by the syndesmosis. The syndesmosis is a strong ligament that pulls the tibia and fibula together just above the distal tibiofibular joint.

Etiology

Ankle fractures may be the result of a vast array of injuries that range from an inversion injury to a complex high trauma sporting injury. Cigarette smoking and high BMI are both risk factors for ankle fractures.

Epidemiology

Ankle injuries, like many fractures have a bimodal distribution. Young patients present following injuries in relatively high-energy trauma (e.g. motor vehicle accident, sporting injury), while older patients present following minor trauma (e.g. a simple fall).

Clinical features

Presentation

Most patients present following an episode of trauma with ankle pain, tenderness and an inability to weightbear.

Diagnosis

The Ottawa ankle rules allow evidence-based decision making regarding the need for plain films in patients with ankle injury.

Patients should have a plain film, if there is malleolar tenderness and:

tenderness along the posterior surface of the distal fibula
OR tenderness along the posterior surface of the distal tibia
OR inability to weightbear after the trauma and when being assessed

An ankle x-ray (AP and lateral views) are usually all that is needed to make a diagnosis.

Treatment

Treatment depends on the type of distal fibula fracture which is a reflection of the severity of the fracture and the surrounding ligamentous structures.

The majority of injuries are relatively simple avulsion injuries from the fibular pole and only require immobilization with a cast. However, more severe injuries with ligamentous injury and ankle instability may require operative reduction and internal fixation.

Radiographic features

In most cases an ankle x-ray is all that is required for diagnosis and follow up. It is worth noting that fractures may be invisible on one projection.

Classification

Classification of distal fibula fractures attempts to split fractures into groups by severity. The commonest classification is the Weber classification that uses the position of the fracture relative to the syndesmosis to group fractures:

Weber A: below the syndesmosis (stable)
Weber B: at the syndesmosis (may be unstable)
Weber C: above the syndesmosis (unstable)

In Weber B and C fractures the syndesmosis may have been torn (partially or completely). This results in widening of the distal tibiofibular joint and loss of integrity of the socket.

X-ray features

The AP and lateral views from an ankle x-ray will almost always allow detection of a lateral malleolar fracture. If there is a lot of soft tissue swelling over the lateral malleolus, but no fracture, then there has been a ligamentous injury. Remember that avulsion injuries may be small, and just involve the tip, or the internal surface of the malleolus.

Once you have seen the fracture, remember to describe:

which bone is involved (fibula)
where the fracture is in the bone (relative to syndesmosis)
what type of fracture (transverse, oblique, spiral, comminuted)
whether there is displacement (translocation, angulation, rotation)
whether there is another fracture (medial malleolus, talus)

The joint spaces around the talus should be the same all the way around. If they are not and the talar dome is not parallel to the tibial plafond, the syndesmosis has been torn. This is called talar shift and the ankle joint is unstable.