Ultrasound of the distal tibiofibular syndesmosis

The AiTFL is the most frequently visualized and commonly injured ligament⁽³⁾. It runs obliquely at an angle of 35° from the anterolateral tibial tubercle (Chaput tubercle) to the anterior fibular tubercle (Wagstaffe tubercle). The AiTFL consists of 2–5 bands, depending on individual anatomy (Fig. 1). Its oblique orientation resists external rotation and lateral translation of the fibula^(6,7).

Fig. 1.

AiTFL – anterior inferior tibiofibular ligament; ATFL – anterior talofibular ligament; IoL – interosseous ligament

Fig. 2.

AiTFL – anterior inferior tibiofibular ligament; PiTFL – posterior inferior tibiofibular ligament; IoL – interosseous ligament; ATFL – anterior talofibular ligament; PTFL – posterior talofibular ligament; CFL – calcaneofibular ligament

The PiTFL is the strongest syndesmotic component, extending from the posterior tibial tubercle (Volkmann tubercle) to the posterior distal fibula (Fig. 2). It prevents posterior displacement of the fibula and contributes significantly to ankle mortise stability^(6,7).

The interosseous ligament represents the distal thickening of the interosseous membrane (Fig. 2) and acts as a central stabilizer, transmitting load between the tibia and fibula, and maintaining their alignment during weight bearing^(6,7).

The coordinated function of these structures allows minimal physiologic motion, typically less than 2 mm of separation and 1–2° of rotation between the distal tibia and fibula during ankle movement⁽⁷⁾.

A high-frequency linear transducer (10–18 MHz) provides optimal visualization. Use of a small-footprint “hockey-stick” probe can improve access in smaller ankles^(9,10,11,12).

The patient is examined supine with the ankle slightly plantarflexed. To visualize the PiTFL, the patient may be positioned prone or on the contralateral side of the injured ankle. Comparison with the contra-lateral side is useful to establish normal baseline measurements^(9,10,11,12).

• AiTFL: Visualized in an oblique axial plane extending from the tibial to the fibular tubercle. Appears as a thin, well-defined, fibrillar, echogenic band crossing the syndesmotic interval (Fig. 3).

• PiTFL: Best seen posteriorly, slightly deeper and thicker, extending between the posterior tibia and fibula (Fig. 4).

• Dynamic assessment: Performed during gentle external rotation and dorsiflexion stress. The tibiofibular interval should not widen by more than 2 mm in healthy individuals^(9,10,11,12).

The ligaments appear as continuous, fibrillar, hyperechoic bands with sharply defined margins. There should be no fluid or hypoechoic cleft between the tibia and fibula. The adjacent cortex is smooth, and no motion asymmetry is observed during stress maneuvers^(9,10,11,12).

Fig. 3.

US image of the superior (A) and (B) bands of the AiTFL. The ligament appears as continuous, fibrillar, hyperechoic bands with defined margins (arrows), extending from the anterior aspect of the lateral malleolus to the anterolateral aspect of the distal tibia. The US probe is oriented obliquely at 35° from the anterior aspect of distal tibia to the lateral malleolus (C)

Fig. 4.

US image of the PiTFL obtained with a 20 MHz matrix linear probe. Ultrasound beam-steering was applied to orient the beam as perpendicular as possible to the PiTFL to reduce anisotropy artifacts (A). The ligament appears as continuous, fibrillar, hyperechoic band with defined margins (arrows), extending from the posterior aspect of the lateral malleolus to the posterior aspect of the tibia. The US linear probe is oriented obliquely at 35–45° from the posterior aspect of distal tibia to the lateral malleolus (B). CFL – calcaneofibular ligament; IoL – interosseous ligament

• Ligament thickening, loss of normal fibrillar pattern, or fiber discontinuity.

• Hypoechoic or anechoic gap representing tear or hematoma.

• Presence of small, avulsed bone fragments.

• Increased tibiofibular distance during dynamic external rotation.

• Adjacent soft tissue edema and hyperemia on color Doppler⁽¹³⁾.

Fig. 5.

US images of a 46-year-old male who sustained a twisting ankle injury while snowboarding. Initial US (A) shows loss of the normal fibrillar pattern of the AiTFL, fiber discontinuity, and a small hypoechoic hematoma (arrows). Follow-up US after 4 years (B) of adequate conservative treatment, shows continuity of the ligament with mild thickening of the AiTFL but no hematoma (arrows), consistent with adequate ligament healing

Fig. 6.

US and radiographic evaluation of a 22-year-old male soccer player with ankle twisting injury. Lateral view plain radiograph (A) shows a small avulsion fracture of the posterior aspect of the distal tibia (arrow). US long-section image (B) confirms the presence of the small avulsed bone fragment in the posterior aspect of the tibia. Axial oblique US image (C) shows thickened PiTFL (arrowheads) with discontinuity of the tibial attachment and the small, avulsed bone fragment (*). MRI sagittal proton density fat-saturated sequence (D) image shows mild bone marrow contusion in posterior aspect of the distal tibia. Axial proton density fat-saturated sequence (E) and T1W (F) show injury to both the AiTFL (large circle) and the PiTFL (small circle)

Ligament thinning, irregularity, or heterogeneous scarring.

• Hypoechoic fibrotic changes or calcification at insertion sites.

• Cortical irregularity or periosteal reaction at the fibular or tibial attachment.

• Persistent instability on dynamic testing⁽¹³⁾.

• Grade I: Mild strain or partial tear of AiTFL; stability intact.

• Grade II: AiTFL disruption with interosseous ligament involvement; mild diastasis.

• Grade III: Complete AiTFL and IoL disruption with (Fig. 6) or without PiTFL injury; significant instability.

Ultrasound is more sensitive in demonstrating small avulsion fragments (Fig. 6, Fig. 7) or calcifications. In addition, dynamic assessment may help differentiate Grade II from Grade III injuries, with the amount of syndesmotic widening determining surgical versus conservative management⁽¹³⁾.

• Anisotropy: The oblique course of the AiTFL makes it prone to apparent hypoechogenicity if the US beam is not perpendicular. Proper probe angulation is essential.

• Overlying edema or hematoma may obscure ligament definition. Contralateral comparison helps confirm pathology.

• PiTFLs may be challenging to visualize due to their deeper location and overlying tendons.

Operator experience is crucial for accurate interpretation, especially during dynamic assessment.

MRI provides comprehensive visualization of all syndesmotic structures, including deep interosseous components, bone marrow edema, and associated intraarticular lesions^(2,5,16). However, ultrasound offers unique advantages. Several studies have shown that ultrasound demonstrates strong agreement with MRI in identifying AiTFL injuries, particularly in acute trauma settings. The modality’s dynamic capability allows assessment of functional stability, which static MRI cannot replicate. MRI remains indispensable when evaluating deeper structures or associated injuries (e.g., IoL, cartilage, bone marrow, deltoid complex)^(15,16).

Early and accurate identification of syndesmotic injury influences both treatment and prognosis.

• Grade I–II injuries: Managed conservatively with functional rehabilitation and gradual return to sport.

• Grade III injuries: Typically require surgical fixation using screws or suture-button devices.

Ultrasound can also assist in patient follow-up after treatment (Fig. 5), detecting complications such as persistent diastasis, screw loosening, or heterotopic ossification, and guiding targeted interventions for chronic pain^(15,16).