Clinical History: A 17 year-old female injured her right ankle 4 days ago during a soccer game. MRI of the ankle was obtained with axial fast spin-echo T2-weighted (1a), coronal STIR (1b), and oblique coronal proton density-weighted (1c) images. What are the findings? What is your diagnosis?
Complete tear of the anterior inferior tibiofibular ligament at its fibular insertion, consistent with an anterior ankle syndesmotic injury (high ankle sprain).
Ankle sprains are the most common injury sustained by athletes1, and account for up to 10% of emergency room visits.2,3 The majority of ankle sprains involve the lateral ligament complex.2,3 A significant minority of patients with ankle trauma are diagnosed with injuries of the distal tibiofibular syndesmosis, ranging from 1% to 18% in the literature.4,5 Ankle injuries involving the syndesmotic ligaments are commonly called “high ankle sprains”, and are associated with a greater risk of residual ankle dysfunction and persistent pain.5 Athletes who sustain high ankle sprains generally require twice as long to return to their previous level of competition, compared to ankle sprains of similar severity which do not involve the syndesmosis.6
The distal tibiofibular syndesmosis is stabilized by the anterior inferior tibiofibular ligament (AITFL), posterior inferior tibiofibular ligament (PITFL), and transverse tibiofibular ligament (TrTFL) (also called the transverse interosseous ligament or inferior transverse tibiofibular ligament) (Figure 5a).7,8 The most inferior portion of the interosseous membrane (frequently called the interosseous ligament)8 and deep deltoid ligaments1 also contribute to syndesmotic stabilization.
On MRI, the normal ligaments are visualized as hypointense bandlike structures on all pulse sequences, consistent with their predominant collagen fiber content.9 The AITFL courses from the anterolateral tibia (Chaput’s tubercle) laterally and inferiorly to the anterior lateral malleolus (Wagstaffe’s tubercle). The PITFL courses from the posterior lateral tibial tubercle (also known as Volkmann’s tubercle or the posterior malleolus) to the posterior aspect of the lateral malleolus. The TrTFL is located just inferior and deep to the PITFL, extending from the posterior lateral malleolus to the posterior tibia and posterior surface of the medial malleolus.5,7,8 The interosseous membrane is located between the distal diaphysis of the fibula and tibia, usually terminating about 1 to 2 cm above the tibial plafond. A normal synovial recess extends superiorly from the tibiotalar joint into the interosseous region; the interosseous ligament forms the roof of this recess, and represents the lower margin of the interosseous membrane.10
Mechanism of injury and clinical presentation
A variety of mechanisms have been reported for syndesmotic injuries; the most common is thought to be forced external rotation with ankle dorsiflexion and pronation.1 The AITFL is the most commonly torn ligament, and will almost always be torn before the other syndesmotic ligaments.11 Patient symptomatology is similar to other ankle sprains, with the addition of supramalleolar pain and edema.12 Athletes may report pain predominantly in the push-off phase of their gait.5
Conventional radiographs are usually the initial imaging test obtained after ankle injury. Widening of the ankle mortise and lateral shift of the talus with respect to the medial malleolus are radiographic signs of syndesmotic diastasis. Fractures of the fibula and distal tibia are commonly associated with syndesmotic injury, and may also be diagnosed on radiographs.1,5 Stress radiographs in external rotation are advocated by some authors, but have been reported by others to demonstrate a high false-negative rate and low interobserver correlation.1,5,8 CT may be useful to detect small cortical fractures which are difficult to visualize on radiographs, and is more sensitive for mild syndesmotic injuries.13
MRI provides excellent soft tissue contrast and direct visualization of the syndesmotic ligaments.9,10,14 MRI criteria for syndesmotic ligament injury are similar to those used for other ankle ligaments. Absence or complete discontinuity of a ligament is consistent with complete tear. Wavy or irregular contour of remaining ligament fibers supports the diagnosis of tear, and increases specificity.9,10,12,14 Thickening or increased signal in the ligament with adjacent edema is compatible with acute sprain. Thickening of the ligament without acute edema may indicate chronic sprain, or healed injury with surrounding fibrosis. Reported sensitivity and specificity of MRI for acute syndesmotic ligament injury is very good in the literature, with sensitivity approaching 100% and specificity ranging from 70% to 94%.9,14 Sensitivity and specificity of noncontrast MRI was reported to be significantly lower in chronic syndesmotic injuries by Kim et al, with sensitivity ranging from 54.2 to 62.5% and specificity of 52.4% to 61.9%.10 Variable enhancement after intravenous contrast has been reported in ligament tears, with some authors reporting increased sensitivity and specificity by utilizing postcontrast imaging, particularly in chronic injuries.9,10
High ankle sprains are often associated with additional ankle injuries and fibular fractures. Brown et. al. found osteochondral lesions of the talar dome in 28%, bone contusions in 24%, and lateral ligament complex tears in 74% of 59 patients with syndesmotic injuries.15 Uys and Rijke observed an inverse correlation between severity of syndesmotic injury and lateral ankle sprains; more severe syndesmotic instability tends to be isolated and not associated with lateral ankle sprains. The authors hypothesized that syndesmotic injuries and lateral ankle sprains are usually caused by different mechanisms of injury.12
Conservative treatment is generally preferred for stable injuries of the syndesmosis, and initially includes rest, ice and immobilization. This is followed by limited weightbearing with application of joint protective devices such as a walker boot or functional brace. As the patient improves, strengthening and agility exercises may aid return to normal activities and athletic competition.1,5,17
In patients with unstable syndesmotic injury or frank diastasis of the syndesmosis, surgical reduction with placement of trans-syndesmotic screws may be indicated. This is usually followed by casting for 6 weeks and subsequent rehabilitation. Early surgical intervention ensures adequate reduction of the ankle mortise and positions the injured ligaments for optimal healing.1,5,16,18 Delayed surgery (due to initially missed diagnosis, or loss of reduction in a conservatively treated patient) is generally associated with less robust healing.5 Surgical treatment for chronic syndesmotic injuries has been reported to improve symptoms in the majority of patients, with residual pain and dysfunction in 14 to 20%.1,19,20
Ossification along the syndesmotic ligaments with synostosis may develop as a late complication 3 to 12 months after the initial injury.1,21 This can be associated with local pain, but up to 50% of patients with such ossification reported minimal or no symptoms.1,21,22 Surgical excision of painful synostoses has been reported, with relief of symptoms in most patients.1,21
Injury of the distal tibiofibular syndesmosis occurs in a significant minority of ankle sprains, and is associated with increased risk for chronic ankle dysfunction and persistent pain. Early diagnosis and prompt appropriate treatment has been advocated to decrease the likelihood of long-term sequelae. MRI provides excellent visualization of ankle anatomy, allowing accurate diagnosis of ligament abnormalities and any associated osteochondral or tendon injuries. MRI may be especially valuable for early detection of syndesmotic injury in patients with an unclear clinical history, and with equivocal or difficult clinical examinations.
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