Anterior ankle

Evaluation of the ankle may be performed with the patient supine on the table with the knee slightly flexed and the foot free to allow manipulation during scanning. Place the transducer in the sagittal plane to visualize the anterior recess of the tibiotalar joint distal to the tibia. The anterior fat pad lies anterior to the talar neck, between the talar head and dome. Scan medial to lateral to examine the full extent of the talar dome. The anterior aspect of the ankle joint is assessed in the sagittal plane for synovitis or joint effusion. The hyperechoic joint capsule is seen superficial to the anterior distal tibia and hypoechoic cartilage of the talar dome. The anterior fat pad lies superficial to the joint capsule and can be displaced in the presence of increased joint fluid. In normal people, up to 3 mm of anechoic fluid may be seen. It is important to scan laterally to identify smaller joint effusions. Plantar flexion may shift fluid out of the anterior recess to further assist in visualization of small effusions. Increased echogenicity of the fluid may indicate complex fluid from infection, gout, or hemorrhage. Compressibility, redistribution with ankle joint motion, and the absence of flow on power Doppler suggests the presence of fluid rather than synovitis. Synovitis can be seen with infection, and with inflammatory (eg, rheumatoid arthritis) or noninflammatory arthridities. The detection of cartilaginous or bony intra-articular loose bodies is assisted by the presence of a joint effusion, or by the injection of sterile normal saline into a joint without an effusion. Successful injection and aspiration of the tibiotalar joint using ultrasound guidance has been demonstrated with an accuracy approaching 100%.

For evaluation of the tibialis anterior tendons, start with the transducer placed in the axial plane over the dorsal and medial aspect of the ankle from the musculotendinous junction proximally to the insertion distally on the first cuneiform. Most tibialis anterior tendon tears occur within 3.5 cm of the insertion. Hypoechoic changes and swelling of the tendon are seen with tendonopathy. A muscle hernia can present as a defect in the fascia at the site of a perforating vessel confirmed with power Doppler. Active dorsiflexion with dynamic imaging at the site of maximal tenderness may be the only means to visualize the hernia because contraction pushes the muscle through the fascia.

Scanning laterally, one next finds the extensor hallucis longus tendon, tibialis anterior artery, and deep peroneal nerve. Moving further laterally find the multiple tendons of extensor digitorum longus. The superior extensor retinaculum lies superficial to these tendons. Tenosynovitis, tendon tears, and avulsion fractures rarely occur in this region.

Ligaments may demonstrate hypoechoic thickening if partially torn, or a hypoechoic gap and hemorrhage if acutely and completely torn. Osseous avulsions appear as hyperechoic fragments with acoustic shadows attached to ligament tissue. The anterior talofibular ligament can be imaged by inverting and plantar flexing the ankle slightly. With the probe in the transverse plane place the anterior edge of the transducer over the talus and posterior edge over the distal lateral malleolus. Gentle anterior traction on the forefoot while imaging allows the examiner to identify potential defects in this commonly injured ligament. To visualize the anteroinferior tibiofibular ligament rotate the anterior edge of the transducer proximally to the distal tibia, while maintaining the posterior edge on the lateral malleolus. It passes a region of talar cartilage before positioning over the ligament. Injury to this ligament may be associated with a tear of the interosseous membrane between the tibia and fibula, or a high fibular Maisonneuve fracture. Interosseous membrane injury also can be identified on ultrasound imaging. The calcaneofibular ligament can be identified in the dorsiflexed ankle by rotating the anterior edge of the transducer distally toward the calcaneus, while keeping the other edge on the inferior and slightly posterior aspect of the lateral malleolus. The peroneus longus and peroneus brevis tendons are seen superficial to the calcaneofibular ligament.

Lateral ankle

The peroneal tendons are best visualized in the transverse plane superior to the lateral malleolus in the retromalleolar groove. The peroneus longus tendon lies posterior to the peroneus brevis tendon and muscle. Scanning inferiorly the peroneus brevis muscle tapers to a tendon at the distal tip of the fibula unless a low-lying muscle belly is present (a normal variation). Follow the tendons to the point where they diverge at the peroneal tubercle, a bony prominence on the lateral calcaneus. At this point it is difficult to follow both tendons because of anisotropy. Follow the peroneus longus tendon under the lateral foot to its insertion on the medial cuneiform and first metatarsal base. A sesamoid, the os peroneum, may appear as an echogenic mass within the tendon. Peroneus longus tendon tears occur more distally (at the cuboid notch or os peroneum insertion) and are rare. The peroneus brevis can be followed to its insertion on the base of the fifth metatarsal. Transverse imaging is beneficial to identify hypoechoic clefts or longitudinal split tears, which may occur in association with an inversion sprain because the peroneus longus tendon is tensioned against the peroneus brevis tendon. Hypoechoic tenosynovitis, tendon sheath thickening may also be encountered.

Superficial to the tendons, the hyperechoic superior and inferior peroneal retinacula are seen. Dynamic imaging transverse to the tendons during active dorsiflexion and eversion of the ankle has been demonstrated to reveal subluxation of the tendons anterior to the fibula with 100% positive predictive value. Subluxation is often associated with disruption of the superior peroneal retinaculum and Oden’s classification system is useful for grading the severity of injury. The most common, type I injuries, include elevation of the superior peroneal retinaculum off the periosteal attachment at the level of the retromalleolar groove. A pouch forms allowing the peroneal tendons to dislocate anteriorly. In the much less common type II injuries, the superior peroneal retinaculum tears at its attachment to the distal fibula. Type III injuries consist of an avulsion fracture of the superior peroneal retinaculum at its distal fibular attachment and are the second most common form of injury. Type IV injuries involve a tear of the superior peroneal retinaculum at its posterior attachment. Intrasheath subluxation of the peroneal tendons can occur without disruption of the superior peroneal retinaculum and can usually be identified with ultrasound imaging.

Lateral ankle

The peroneal tendons are best visualized in the transverse plane superior to the lateral malleolus in the retromalleolar groove. The peroneus longus tendon lies posterior to the peroneus brevis tendon and muscle. Scanning inferiorly the peroneus brevis muscle tapers to a tendon at the distal tip of the fibula unless a low-lying muscle belly is present (a normal variation). Follow the tendons to the point where they diverge at the peroneal tubercle, a bony prominence on the lateral calcaneus. At this point it is difficult to follow both tendons because of anisotropy. Follow the peroneus longus tendon under the lateral foot to its insertion on the medial cuneiform and first metatarsal base. A sesamoid, the os peroneum, may appear as an echogenic mass within the tendon. Peroneus longus tendon tears occur more distally (at the cuboid notch or os peroneum insertion) and are rare. The peroneus brevis can be followed to its insertion on the base of the fifth metatarsal. Transverse imaging is beneficial to identify hypoechoic clefts or longitudinal split tears, which may occur in association with an inversion sprain because the peroneus longus tendon is tensioned against the peroneus brevis tendon. Hypoechoic tenosynovitis, tendon sheath thickening may also be encountered.

Superficial to the tendons, the hyperechoic superior and inferior peroneal retinacula are seen. Dynamic imaging transverse to the tendons during active dorsiflexion and eversion of the ankle has been demonstrated to reveal subluxation of the tendons anterior to the fibula with 100% positive predictive value. Subluxation is often associated with disruption of the superior peroneal retinaculum and Oden’s classification system is useful for grading the severity of injury. The most common, type I injuries, include elevation of the superior peroneal retinaculum off the periosteal attachment at the level of the retromalleolar groove. A pouch forms allowing the peroneal tendons to dislocate anteriorly. In the much less common type II injuries, the superior peroneal retinaculum tears at its attachment to the distal fibula. Type III injuries consist of an avulsion fracture of the superior peroneal retinaculum at its distal fibular attachment and are the second most common form of injury. Type IV injuries involve a tear of the superior peroneal retinaculum at its posterior attachment. Intrasheath subluxation of the peroneal tendons can occur without disruption of the superior peroneal retinaculum and can usually be identified with ultrasound imaging.

Medial ankle

The tibialis posterior tendon is the most commonly abnormal tendon in the medial ankle. The tendon may be visualized by scanning transversely proximal and posterior to the medial malleolus. It is typically larger than the other tendons of the medial ankle. Adjust the probe to minimize anisotropy and distinguish the tendon from adjacent structures. Up to 4 mm of fluid distention may be seen in the distal tibialis posterior tendon sheath. Hypoechoic enlargement of the tendon is suggestive of tendinopathy. Tenosynovitis caused by mechanical stress, systemic arthritis, or infection may demonstrate increased blood flow on power Doppler imaging. Partial-thickness tears may progress to a longitudinal split tear, commonly near the medial malleolus, and are associated with tendon dislocation or subluxation. Dynamic imaging is helpful in evaluation of abnormal tendon function. Full-thickness tears may demonstrate tendon stump retraction, hemorrhage, or synovitis, and complete fiber disruption.

In the longitudinal axis, scan the tendon to its distal insertions. The main portion inserts on the plantar aspect of the navicular tuberosity. Accessory navicular bones are the most common accessory ossicle in the foot. A type 1 accessory navicular bone (os tibiale externum) is a normal variant in up to 11.7% of the population and is seen within the tibialis posterior tendon. Types 2 and 3 (cornuate navicular) accessory bones are more commonly symptomatic and associated with soft tissue edema. A tendon sheath is absent at this location and fluid should not be present. The plantar portion of the posterior tibialis tendon inserts on the first, second, and third cuneiforms and second, third, and fourth metatarsals. The recurrent portion inserts on the sustentaculum tali, a bony prominence of the medial calcaneus that forms the middle facet of the anterior subtalar joint. The main portion of the tibialis posterior tendon lies dorsal and superficial to the sustentaculum tali. The flexor digitorum longus tendon lies superficial to the sustentaculum tali. The flexor hallucis longus tendon lies plantar to the sustentaculum tali in a groove.

Scanning posterior to the tibialis posterior tendon, next find the flexor digitorum longus tendon, posterior tibial artery and veins, tibial nerve, and flexor hallucis longus tendon. Scan longitudinal to the distal aspects of the flexor digitorum longus and flexor hallucis longus tendons to assess for tendinopathy, tendon rupture, longitudinal splits, stenosing tenosynovitis, or dislocation. Injury to the flexor hallucis longus tendon generally occurs with forced, repetitive, or prolonged great toe extension or ankle plantar flexion. Three common areas of injury include (1) the fibro-osseous tunnel along the posteromedial ankle; (2) under the base of the first metatarsal where the flexor digitorum longus tendon crosses over the flexor hallucis longus tendon (known as the “knot of Henry”); or (3) where the flexor hallucis longus tendon passes between the great toe sesamoids under the metatarsal head.

Entrapment of the tibial nerve in the tarsal tunnel may be caused by ganglion cysts or talocalcaneal coalition or varicose veins. The medial calcaneal branch, which supplies sensation to the medial and plantar skin of the heel, can be seen superficially before the tibial nerve splits into the medial and lateral plantar branches of the tibial nerve. Superficial to the tendons, nerves, and artery lays the hyperechoic flexor retinaculum. Deep to the tendons lays the deltoid ligament, which consists of four components: (1) the anterior tibiotalar, (2) tibionavicular, (3) tibiocalcaneal, and (4) posterior tibiotalar ligaments. The ligaments are best visualized scanning longitudinal to the hyperechoic fibrillar fibers.

Each component can be visualized by maintaining the proximal aspect of the ultrasound probe on the medial malleolus while placing the distal aspect on the neck of the talus (anterior tibiotalar ligament); navicular bone (tibionavicular ligament); sustentaculum tali (tibiocalcaneal ligament); or posterior process of the talus (posterior tibiotalar ligament deep to the tibialis posterior tendon). Ultrasound imaging has been shown to accurately diagnose deltoid ligament rupture with a sensitivity of 85.6% and a specificity of 83.3%.