Previously, arthrography and tenography were the primary means of imaging ankle ligament injuries. They had the limitations of being invasive, providing only an indirect depiction of ankle ligament disruption, and yielding potentially false- negative results. MRI provides a noninvasive means of directly imaging all the ligaments in the vicinity of the ankle as well as all the other osseous and soft tissues.
Axial MR images provide good visualization of the tibiofibular ligaments of the tibiofibular mortise. All the lateral collateral ligaments of the ankle have an oblique orientation, and to image these ligaments in full length, either an oblique imaging plane that parallels their length must be chosen or the foot must be placed in sufficient dorsiflexion or plantar flexion to bring the ligaments into one of the standard imaging planes. With the imaging plane parallel to the anterior talofibular ligament, it is displayed as a low–signal-intensity band extending anteromedially from the lateral malleolus to gain attachment to the talus just anterior to its fibular articular surface (Fig. 6-33A) (50). The calcaneofibular ligament is visualized as a low–signal-intensity structure extending from the lateral malleolus to the calcaneus, with the peroneus longus and brevis tendons situated superficial to its fibular end (Fig. 6-33B). The posterior talofibular ligament is visualized as a wide low–signal- intensity structure extending from the deep surface of the lateral malleolus to a broad attachment on the talus from its fibular articular surface to its posterior process (Fig. 6-33C).
FIGURE 6-33. T1-weighted images of the normal lateral collateral ligaments of the ankle. A: The anterior talofibular ligament (ATAF) extends from the fibular malleolus to the neck of the talus. B: The calcaneofibular ligament (CFL) attaches to the calcaneus and is deep to the peroneus tendons. C: The strong talofibular ligament (between arrowheads).
MRI of ankle ligament injuries offers promise for the noninvasive evaluation of the site and severity of both acute ankle ligament injuries and chronic ankle instability (51).
The mechanism of injury of the lateral collateral ligaments typically involves plantar flexion and inversion, and they are usually injured in a predictable sequence from anterior to posterior. The anterior talofibular ligament is the most commonly injured, followed in sequence by injury to the calcaneofibular and posterior talofibular ligaments. The major MRI finding in a complete rupture (i.e., grade III sprain) of the anterior talofibular ligament is a complete discontinuity of the ligament visualized at all imaging levels (Fig. 6-34A,B). This is accompanied by periarticular edema or hemorrhage and joint effusion because this ligament is a thickening of the ankle joint capsule. The edema and effusion are visualized with moderate signal intensity on T1-weighted MR images and hyperintensity on T2-weighted images. A partial tear (i.e., grade II sprain) of the anterior talofibular ligament is visualized on MRI as a discontinuity of the upper part of the ligament, with the lower portion remaining intact. Again, there is periarticular edema, hemorrhage, and joint effusion. Grade II sprains of the calcaneofibular ligament may appear as a longitudinal splitting In contrast to the three discrete lateral collateral ligaments, or waviness of the ligament with fluid accumulation within the the medial collateral or deltoid ligament is a continuous tendon sheath of the overlying peroneal tendons (Fig. 6-35). ligamentous sheet with an apical attachment to the tibial malleolus, and a broad base attaching below to the navicular, talar neck, spring ligament, sustentaculum tali of the calcaneus, and posterior talus. The posterior tibiotalar part of the deltoid ligament is its thickest and strongest (52). The deltoid ligament can be visualized by either axial or coronal MRI. Axial images allow simultaneous visualization of all parts of the deltoid ligament, the overlying flexor retinaculum, and the walls and contents of the tarsal tunnel (Fig. 6-36A). The contents of the four compartments under the flexor retinaculum include, from anterior to posterior, the tibialis posterior tendon, flexor digitorum longus tendon, posterior tibial artery, tibial nerve, and flexor hallucis longus tendon. Coronal MR images through the deltoid ligament display the proximal and distal attachments of each part of the deltoid ligament (Fig. 6-36B).
MRI has the potential to visualize even grade I sprains, which are microtears confined to the interior of the ligament. The minute foci of edema and hemorrhage accompanying such tears become hyperintense on T2-weighted images. Findings compatible with such grade I tears have been identified in the posterior tibiotalar portion of the deltoid ligament. They are frequently accompanied by fluid within the tendon sheath of the overlying tibialis posterior.
In chronic ankle instability, MR images show thinned, lengthened, wavy ligaments in some locations and thickened, scarred ligaments in others.
FIGURE 6-34. Complete rupture of the anterior talofibular ligament. Axial T1-weighted (A) and T2-weighted fat suppressed sequences (B). There is discontinuity to the ligament fibers (arrows) and associated soft tissue swelling (arrowhead in B).
FIGURE 6-35. Partial tear to the calcaneofibular ligament. There is thickening and increased signal within the ligament fibers (long arrow) and edema within the soft tissues between the ligament and the calcaneus.