Ankle Fracture Imaging

The ankle is one of the most frequently injured areas of the skeleton and the site of the most common intra-articular fracture of a weight-bearing joint. Ankle injuries are responsible for over 5 million emergency department visits each year. ^[1]  Although many of these injuries are ligament sprains, the radiologist plays a key role in the thorough evaluation of complex injuries and the detection of subtle fractures (see the images below). ^{[2, 3, 4, 5, 6, 7]}  When evaluating for ankle fractures, also consider conditions such as ankle impingement syndrome, ankle sprain, metatarsalgia, metatarsal fractures (eg, fifth metatarsal fractures), talar dome osteochondral injuries, and injuries to other surrounding ligaments and/or tendons. ^[8]

Anatomy

The shapes of the ankle bones and the supporting ligamentous structures are important anatomic features of the ankle area. The distal tibia has a large, flat articular surface (the plafond), a prominent medial malleolus, and a less prominent posterior malleolus. The talar dome is wedge-shaped, wider anteriorly than posteriorly. ^[8]

The distal fibula or lateral malleolus is bound to the distal tibia by the anterior and posterior inferior tibiofibular ligaments, an inferior transverse ligament, and a syndesmotic ligament. The fibula is also bound to the talus by the anterior and posterior talofibular ligaments and to the calcaneus by the calcaneofibular ligament. The medial malleolus is bound to the talus, calcaneus, and navicular by the superficial and deep portions of the deltoid ligament.

Imaging modalities

Brandser et al emphasized the necessity of obtaining 3 conventional radiographs in anteroposterior (AP), internal oblique (mortise), and lateral projections. ^[9] Other imaging studies, such as arthrography, ultrasonography, computed tomography (CT) scanning, magnetic resonance imaging (MRI), and nuclear medicine, are rarely used. Radiographic stress views may be done, although they can be difficult to obtain. Park et al reported that stress views with dorsiflexion and external rotation of the ankle best show tears of the deltoid ligament by resultant widening of the medial clear space when measured at 5 mm or more. ^[10]

Despite the use of the standard 3-view conventional radiographic survey, some ankle fractures cannot be seen at the time of initial evaluation. The presence of a large ankle-joint effusion on the initial lateral radiograph suggests an occult fracture.

One third of patients with an effusion measuring 13 mm or more had occult fractures in a series reported by Clark et al. ^[11]  Many of these occult fractures involve the talar dome. The radiographic appearance often suggests the presence of associated ligamentous injuries, but in a series of 59 patients, Gardner et al showed that MRI is much more specific for ligamentous injuries. ^[12] Additionally, although radiographic widening of the syndesmotic space of greater than 5 mm is reported to be abnormal, in an MRI series of 70 patients, Nielson et al found no association between the MRI findings of syndesmotic injury and the radiographic measurements. ^[13] In a prospective series of 51 patients with ankle fractures, Hermans et al confirmed that radiographic measurements of the syndesmotic space, amount of tibiofibular overlap, and width of the medial clear space did not correlate with ligamentous injuries that were shown on concurrent MRI studies. ^[14]

Van Gerven et al found that routine follow-up radiographs rarely affect the treatment strategy for ankle fractures. Of 936 routine radiographs taken during the follow-up period, only 11 (1.2 %) resulted in changes to treatment strategy. ^[15]

MRI is not needed for the evaluation of most ankle fractures. This imaging modality can show additional injuries in children with Salter-Harris fractures and also may be used to check for occult injuries, especially injuries of the talar dome, or soft-tissue injuries, such as surrounding ligament or tendon abnormalities. ^[16]  If MRI is performed specifically for evaluation of the distal tibiofibular ligaments, an oblique axial imaging plane should be included. ^[14]

Imaging Guidelines

The Ottawa Ankle Rules (OAR) have been developed to predict the necessity of radiographs for acute ankle injuries, with the goal of protecting patients from unnecessary radiation exposure. These rules provide practical guidelines for selecting patients for radiographic studies. According to the OAR, indications for ankle radiographs for patients with acute ankle pain include pain in the ankle region plus one of the following ^{[17, 18]} :

• Bony tenderness at the distal 6 cm of the posterior edge of the medial malleolus
• Bony tenderness at the distal 6 cm of the posterior edge of the lateral malleolus
• Inability to bear weight both immediately and in the ED (defined as 4 steps)

In a prospective study of 403 acute nonpenetrating ankle injuries, the OAR had high sensitivity (95-100%) and negative predictive value (100%) but low specificity (40-51%) and positive predictive value (24-28%). ^[19]

Diagnostic guidelines for suspected ankle fracture are from the American College of Radiology. For acute trauma to the ankle, according to the ACR, radiographs are usually appropriate for initial imaging when the OAR rules are met, or when exclusionary criteria such as a neurologic disorder or neuropathy are present but OAR is not met.  For secondary imaging, MRI without contrast and CT without contrast are equivalent alternatives and usually appropriate. ^[20]

 

As some ankle fractures are initially occult, patients with significant injury should be treated symptomatically and asked to return for additional radiographs in 7-10 days if symptoms persist. The physician should pay special attention to certain target areas, such as the medial and lateral edges of the talar dome, the anterior process of the calcaneus, and the base of the fifth metatarsal, in order to check for subtle fractures.

Lauge-Hansen Classification

Many ankle fractures occur in well-known, predictable patterns. ^{[21, 22, 17, 18]} Three similar classification schemes are frequently used to describe the findings: the Lauge-Hansen, the Danis-Weber, and the AO-Muller/Orthopaedic Trauma Association (AO/OTA) classification systems. ^{[23, 24, 25, 26]} These classifications are nearly identical, but they have different emphases for the radiologist and orthopedic surgeon, respectively. ^[27] Because the Lauge-Hansen scheme is designed for radiologists, it will be emphasized here.

The Lauge-Hansen classification scheme has 4 injury patterns: supination-adduction (SA) (or Weber A in the Danis-Weber scheme), supination external (SE) rotation (or Weber B), pronation-abduction (PA) (or Weber C1), and pronation external (PE) rotation (or Weber C2). ^{[25, 26]} The names of the Lauge-Hansen injury patterns can be thought of as indicating the initial position of the foot and hindfoot (supination or pronation) and the direction of the injuring force acting through the talus (adduction, abduction, external rotation). The location and type of fibula fracture are key to understanding the classification (see the image below).

Supination adduction (Weber A)

In an SA injury, the foot is supinated (inverted), and an adducting force is exerted on the talus, resulting in 2 sequential injuries. First, tension on the lateral ligaments (the calcaneofibular ligament, primarily) leads to a transverse fracture of the lateral malleolus below or up to the level of the tibiotalar joint line, or a ligament tear occurs. Second, the talus adducts, impacts the medial malleolus, and causes an oblique medial malleolar fracture (see the image below).

Supination external rotation (Weber B)

The SE rotation is the most common mechanism for a "twisted ankle" injury. The foot is supinated, and an external rotation force acts on the talus, resulting in up to 4 sequential injuries, as described in the following:

• First, the anteroinferiortibiofibular ligament tears.

• Second, a short oblique fracture of the fibula occurs (which is best seen on lateral radiographs). The direction of this fracture line is typically from posterosuperior to anteroinferior. (See the following images.)

  Anteroposterior radiograph from a 31-year-old woman with a supination external rotation stage 2 ankle injury. This image only shows lateral soft-tissue swelling. See also the next image.

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  Lateral radiograph from a 31-year-old woman with a supination external rotation stage 2 ankle injury. This image shows a short, oblique fracture of the distal fibula that extends to the level of the tibiotalar joint line (supination external rotation stage 2 injury). Note that there is no fracture of the posterior malleolus (stage 3) or medial malleolus (stage 4).

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• Third, fracture of the posterior malleolus is observed.

• Fourth, transverse fracture of the medial malleolus or tear of the deltoid ligament occurs. (Sorrento and Mlodzienski also reported lesions of the lateral aspect of the talar dome in 38% of patients with SE stage 4 injuries. ^[28] )

Pronation abduction (Weber C1)

With a PA injury, the foot is in a pronated position (everted), and an abducting force is exerted on the talus, resulting in up to 3 sequential injuries, as follows:

• First, the deep portion of the deltoid ligament becomes tense, and a transverse fracture of the medial malleolus occurs (75% of cases) or the deltoid ligament tears (25% of cases).

• Second, the talus abducts and stresses the ligaments of the tibiofibula syndesmosis, resulting in a tear of the anteroinferior tibiofibula ligament.

• Third, further abduction of the talus results in oblique fracture of the distal fibula (see the image below). This fibula fracture ends above the level of the joint line and is best seen on anteroposterior (AP) or mortise views. It may not be visible on lateral radiographs. Injury of the syndesmosis should be suspected when the distance between the lateral edge of the tibia and the medial edge of the fibula measures more than 5 mm on either the AP or mortise views, as reported by Sclafani. ^[29]

  Anteroposterior radiograph from a 22-year-old man with a posteroanterior stage 3 ankle injury. This image shows medial soft-tissue swelling, indicating ligamentous injury (stage 1) and an oblique fracture of the fibula just above the level of the tibiofibular syndesmosis (stage 3 injury). Syndesmosis injury (stage 2) is not evident in this patient.

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Pronation external rotation (Weber C2)

In a PE rotation injury, the foot is in a pronated position (everted), and an external rotation force acts through the talus, resulting in up to 4 sequential injuries, as follows:

• The first 2 injuries are the same as in the PA mechanism (medial malleolar fracture and syndesmosis injury) (see the images below).

  Anteroposterior radiograph from a 27-year-old woman with a pronation external rotation–type ankle injury. This image shows fracture of the medial malleolus (stage 1), widening of the tibiofibular syndesmosis (indicating ligamentous tear, stage 2), and a high fibula fracture (stage 3). See also the next image.

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  Lateral radiograph from a 27-year-old woman with a pronation external rotation–type ankle injury. This image shows additional fracture of the posterior malleolus, making this a pronation external rotation stage 4 injury.

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• For the third injury, the external rotation force results in a different fibula fracture. It is a short spiral or oblique fracture well above the level of the syndesmosis (usually 6-8 cm above the syndesmosis, but the fracture may be as high as the midshaft level). The direction of this fracture line is often opposite the SE fracture line; that is, it extends from anterosuperior to posteroinferior.

• The fourth injury is a fracture of the posterior malleolus.

Maisonneuve fracture (Weber C3)

Maisonneuve fracture typically involves deltoid ligament rupture, tibiofibular ligament disruption, and a spiral fracture of the proximal fibula. ^[30] The exact mechanism leading to a Maisonneuve fracture is not clear. The injury sequence as described by Pankovich clearly differs from those above. ^[31]

• First, a tear of the anteroinferior tibiofibular ligament and the interosseous membrane occurs.

• Second, fracture of the posterior malleolus or a posterior ligament tear is observed.

• Third, anteromedial capsular injury is present.

• Fourth, fracture of the proximal fibula occurs (usually at the neck).

• Fifth, fracture of the medial malleolus or a deltoid ligament tear is observed (see the images below). (The timing of the last injury in this mechanism distinguishes it from the usual pronation injuries, where the medial malleolar fracture is the first injury in the sequence.)

  Maisonneuve injury. This mortise view shows transverse fracture of the medial malleolus and widening of the tibiofibular syndesmosis without a fracture of the fibula. This injury is suggestive of a proximal fibula fracture (Maisonneuve fracture). See also the next image.

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  Lateral radiograph in a patient with Maisonneuve injury. This image, taken after a short leg cast was applied and the patient reported pain, reveals Maisonneuve fracture of the proximal fibula.

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Pilon (pylon) fracture

Some of the fracture patterns listed above include fractures of the medial malleolus or posterior malleolus, but the horizontal articular surface of the tibia, the plafond, is uninvolved. Pilon (pylon) fractures are comminuted fractures involving the plafond. Many other associated fractures may exist, including any or all of the malleoli. The key feature is comminution of the distal tibia articular surface (see the radiograph below and the CT scans from the same patient).

Most authorities now include the old Lauge-Hansen type pronation dorsiflexion injury as a pilon-type fracture. ^[32]

Salter-Harris fractures

All types of Salter-Harris injury may involve the distal tibia or fibula. Most simple Salter-Harris fractures of the distal tibia are type 2 (they have a metaphyseal component). Special types of Salter-Harris injury in the ankle region include the triplane and juvenile Tillaux fractures.

Triplane fracture

Triplane ankle fractures are complex traumatic Salter-Harris IV fractures. As the name implies, fractures are seen in 3 different axes (planes) with triplane fracture. ^[33] These 3 axes (planes) are an axial or horizontal injury through the distal tibia physis, a sagittal component through the distal tibia epiphysis, and a coronal component posteriorly through the distal tibia metaphysis (see the images below).

Juvenile Tillaux/Tillaux

In children, a Tillaux fracture is basically a Salter-Harris type 3 fracture of the distal tibia epiphysis that occurs, by definition, at the lateral edge of the epiphysis from tensile avulsion by the syndesmotic ligaments (see the radiograph and CT scan from the same patient below). Its adult counterpart is simply a Tillaux fracture, and the fibula avulsion counterpart is known as a Wagstaff-LeFort fracture.

 

CT scanning is not needed for the evaluation of most ankle fractures. It may be used to better define pilon fractures or triplane fractures. Thin overlapping sections should be taken in case coronal and sagittal reconstructions are needed, or newer multislice isotropic techniques should be used. ^{[24, 34]}

(The following are radiographs and CT scans of ankle fractures from the same patients.)

Ultrasonography is not usually used for the evaluation of patients with ankle fractures. However, this technique can depict fractures and associated soft-tissue injuries, especially injuries of the peroneal tendons. ^[1] In addition, Hsu et al found ultrasonography to be useful for identifying ligament injuries in patients with inversion ankle sprains. ^[35] Mei-Dan et al, using dynamic ultrasound, reported normal values for the width of the syndesmosis in a study of 110 healthy subjects: in neutral, 3.78 mm; with internal rotation stress, 3.64 mm; and with external rotation stress, 4.08 mm. ^[36]

The combination of Ottawa Foot and Ankle Rules (OFAR) and bedside ultrasound was found, in one study, to have a high sensitivity and specificity for detecting foot and/or ankle fractures in the ED, which could decrease the number of x-rays and improve the efficiency and costs associated with evaluating these injuries in the ED. The sensitivity of US in detecting foot and/or ankle fractures was 100%, and the specificity of OFAR increased from 50% to 100% with the addition of US. The negative predictive value and the positive predictive value were both 100%. ^[37]