Ankle, Foot, and Lower Leg Ultrasound

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Chapter 8 Ankle, Foot, and Lower Leg Ultrasound

imageAdditional videos for this topic are available online at www.expertconsult.com.

Ankle and Foot Anatomy

Osseous Anatomy

The ankle joint is a hinged synovial articulation between the talus and the distal tibia and the fibula (Fig. 8-1). Inferiorly, the talus articulates with the calcaneus through three facets, joined by the cervical and interosseous talocalcaneal ligaments located in a cone-shaped region termed the sinus tarsi, which opens laterally.1 The Chopart joint represents the articulations between the talus and navicular and the calcaneus and cuboid bones. The navicular, in turn, articulates with the medial, middle, and lateral cuneiforms, which then articulate with the first through third metatarsals. The fourth and fifth metatarsals articulate directly with the cuboid bone, and the tarsometatarsal articulations collectively are called the Lisfranc joint. Phalangeal bones extend beyond the metatarsals.

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FIGURE 8-1 Leg, ankle, and foot anatomy.

A, Anterior view of left leg. B, Medial view of left leg. C, Medial view of left ankle. D, Lateral view of left ankle. E, Posterior view of calf. F, Medial ankle and midfoot ligaments. G, Lateral ankle and midfoot ligaments. H, Plantar aspect of left foot.

(A and E, From Schaefer, EA, Symington J, Bryce TH [eds]: Quain’s anatomy, 11th ed, London, 1915, Longmans, Green, with permission from Pearson Education; B, C, D, and H, from Standring S: Gray’s anatomy: the anatomical basis of clinical practice, 39th ed, Edinburgh, 2005, Churchill Livingstone; F and G, drawn from a specimen in the Museum of the Royal College of Surgeons of England, with permission from the Council.)

Muscle and Tendon Anatomy

Anteriorly, from medial to lateral, are the tibialis anterior (origin: proximal tibia and interosseous membrane; insertion: base of first metatarsal and medial cuneiform), the extensor hallucis longus (origin: fibula and interosseous membrane; insertion: distal phalanx of the first digit), and the extensor digitorum longus tendons (origin: tibia, fibula, and interosseous membrane; insertion: phalanges of the second through fifth digits) (see Fig. 8-1A to C). The peroneus tertius extends from the fibula and interosseous membrane to the base of the fifth metatarsal. The anterior tendons are held in place by the superior and inferior extensor retinacula. The anterior tibial artery courses beneath the superior extensor retinaculum and becomes the dorsalis pedis artery, located between the extensor hallucis and extensor digitorum longus tendons. The deep peroneal nerve follows the anterior tibial artery and dorsal pedis and bifurcates as medial and lateral branches anterior to the ankle.

Medially, from anterior to posterior, are the tibialis posterior (origin: tibia, fibula, and interosseous membrane; insertion: navicular, cuneiforms, and second through fourth metatarsals), the flexor digitorum longus (origin: tibia; insertion: distal phalanges of second through fifth digits), and the flexor hallucis longus tendons (origin: fibula; insertion: base of distal phalanx of first digit) (see Fig. 8-1A to D). Between the flexor digitorum and flexor hallucis longus tendons at the posterior ankle are the tibial nerve and posterior tibial artery and veins. The order of structures from anterior to posterior from the medial malleolus can be remembered with the phrase “Tom, Dick, And Very Nervous Harry” (T, Tibialis posterior tendon; D, flexor Digitorum longus tendon; A, tibial Artery; V, tibial Veins; N, tibial Nerve; and H, flexor Hallucis longus tendon). The flexor retinaculum extends from the medial malleolus to the calcaneus superficial to the medial tendons and tibial nerve, which forms the roof of the tarsal tunnel. The tibial nerve divides into medial and lateral plantar nerves and a smaller medial calcaneal nerve.2 The inferior calcaneal nerve usually originates from the medial plantar branch and courses between the abductor hallucis and quadratus plantae muscles and then adjacent to the calcaneus.3 The medial and lateral plantar nerves continue toward the digits as the common plantar digital nerves and then as the proper plantar digital nerves. More distally under the mid-foot, the flexor digitorum and flexor hallucis longus tendons cross each other, a configuration termed the knot of Henry. The flexor digitorum and flexor hallucis brevis muscles are located in the plantar aspect of the foot.

Laterally, the peroneus brevis (origin: distal fibula; insertion: fifth metatarsal base) and peroneus longus tendons (origin: proximal fibula and tibial condyle; insertion: first metatarsal base and medial cuneiform) are found posterior to the fibula (see Fig. 8-1A, B, D, and E). The musculotendinous junction of the peroneus longus is more superior to that of the peroneus brevis; at the level of the distal fibula, the peroneus brevis muscle and tendon are found medial and anterior to the tendon of the peroneus longus. More distally, the peroneus brevis tendon is typically in contact with the posterior fibular or retromalleolar groove. The normal peroneus muscle belly should taper so that only tendon is present at the fibula tip.4 The peroneal tendons are held in place by the superior and inferior peroneal retinacula.5 The peroneal tendons then course anteriorly on each side of the peroneal tubercle of the calcaneus and extend to their insertions. As a normal variant, an accessory tendon called the peroneus quartus may be found posterior to the fibula; this tendon most commonly originates from the peroneus brevis and inserts on the lateral aspect of the calcaneus at the retrotrochlear eminence.6 Over the lateral aspect of the calcaneus, the extensor digitorum brevis muscle originates from the calcaneus and extensor retinaculum and inserts distally on the second through fifth phalanges.

Posteriorly in the calf, the medial and lateral heads of the gastrocnemius muscle converge with the soleus to form the Achilles tendon (termed the triceps surae), which inserts onto the calcaneus (see Fig. 8-1B and E).7 The plantaris muscle originates from the lateral femur, courses obliquely through the popliteal region, continues as a thin tendon between the muscle bellies of the medial head of the gastrocnemius and soleus muscles, courses distally at the medial aspect of the Achilles tendon, and then inserts onto the calcaneus. At the plantar aspect of the calcaneus, the plantar aponeurosis originates from the medial calcaneus and extends distally as medial, central, and lateral cords (see Fig. 8-1H). The central cord envelops the flexor digitorum brevis muscle.

Ligamentous Anatomy

The stabilizing structures of the lateral ankle include the anterior talofibular ligament, which extends from the fibular to the talus in the transverse plane; the calcaneofibular ligament, which extends from the fibula inferiorly and posteriorly to the calcaneus deep to the peroneal tendons; and the posterior talofibular ligament, which extends from the fibula to the posterior aspect of the tibia in the transverse plane (see Fig. 8-1G).8 In addition, the anterior and posterior tibiofibular ligaments extend laterally and inferiorly in an oblique fashion from the tibia to the fibula. An accessory anterior tibiofibular ligament may be present, also called Bassett ligament.9 At the medial aspect of the ankle, the deltoid ligament is found, consisting of deep (anterior tibiotalar and posterior tibiotalar) and superficial (tibiocalcaneal and tibionavicular) components (see Fig. 8-1F).8 The spring ligament complex consists of superomedial, medioplantar, and inferoplantar calcaneonavicular ligaments.10 In addition to other small ligaments that connect the various tarsal bones and are named by their osseous attachments, the Lisfranc ligament proper is a strong ligament that connects obliquely from the medial cuneiform to the base of the second metatarsal bone.11 The bifurcate ligament extends from the calcaneus to the navicular and cuboid bones at the lateral aspect of the mid-foot.

Ultrasound Examination Technique

Table 8-1 is a checklist for ankle, calf, and forefoot ultrasound examination. Examples of diagnostic ankle ultrasound reports are available online at www.expertconsult.com (see eBox 8-1 and 8-2).

TABLE 8-1 Ankle, Calf, and Forefoot Ultrasound Examination Checklist

Location Structures of Interest
Ankle: anterior

Ankle: medial Ankle: lateral Ankle: posterior Calf Forefoot

General Comments

Ultrasound examination of the ankle and foot is comfortably completed with the patient supine and the foot and ankle on the examination table. Although limited examination of the distal Achilles and plantar aponeurosis may be completed in supine position with external rotation of the leg to gain access to these structures, a more thorough examination is best accomplished with the patient prone. This is essential when the clinical indication is to assess for Achilles tendon or calf abnormalities. A high-frequency transducer of at least 10 MHz is typically used because most of the structures are superficial. In general, the ankle tendons are first evaluated in short axis (with Achilles being the exception) to identify each structure and for orientation. Following this, evaluation of each tendon in long axis is completed for diagnosis of tendon tear or tendinosis. Evaluation of the calf, ankle, and foot may be initially focused over the area that is clinically symptomatic or that is relevant to the patient’s history. Regardless, a complete examination of all areas should always be considered and is suggested for one to become familiar with normal anatomy and normal variants, to develop a quick and efficient sonographic technique, and to appreciate subtle or early pathologic changes. In addition to use of an ultrasound imaging protocol, it is essential that evaluation include any area of focal symptoms as directed by the patient. This is often a clue to locate pathologic processes and may include areas and structures not routinely assessed. This approach is quite important in the foot, where there are many structures closely associated that may produce symptoms. One example is identification of an osseous stress fracture.

Anterior Ankle Evaluation

The primary structures evaluated from the anterior approach are the anterior ankle joint recess, the tibialis anterior, the extensor hallucis longus, the dorsalis pedis artery and superficial peroneal nerve, and the extensor digitorum longus. The transducer is first placed in the sagittal plane at the level of the tibiotalar joint with the foot in mild plantar flexion (Fig. 8-2A). The hyperechoic bone landmarks of the distal tibia and proximal talus are used for orientation, and the anterior ankle joint region is evaluated for joint abnormality (see Fig. 8-2B). It is important to evaluate not only the anterior joint recess in the sagittal plane but also the parasagittal plane laterally near the anterior talofibular ligament because small amounts of joint fluid may be present only at this site. Next, to evaluate the anterior tendons, the transducer is placed transversely at the level of the ankle joint (Fig. 8-3A). It is important to begin in the transverse plane, short axis to the tendons so that each of the tendons can be accounted for and differentiated from each other as they may appear similar in long axis. The tibialis anterior tendon is the largest, located most medially, with the typical hyperechoic and fibrillar echotexture (see Fig. 8-3B). One may toggle the transducer (see Fig. 1-5B) to assist in identification of the tendons in short axis. This maneuver causes the normally hyperechoic tendon to appear artifactually hypoechoic from anisotropy, which will make the tendon more conspicuous surrounded by the hyperechoic fat (see Fig. 8-3C). Lateral to the tibialis anterior is the extensor hallucis longus (see Fig. 8-3D). The muscle belly of this structure extends more inferiorly compared with the other anterior tendons, and this hypoechoic muscle tissue should not be mistaken for tenosynovitis. The adjacent anterior tibial artery is seen as it crosses from medial to lateral deep to the extensor hallucis longus, which continues as the dorsal pedis artery once beyond the superior extensor retinaculum. The next lateral structure is the extensor digitorum longus with its multiple tendons that extend distally to the digits (see Fig. 8-3D and E). Lateral to this, the peroneus tertius extends to the fifth metatarsal base. Each of these structures should then be evaluated in long axis from proximal to the ankle joint to at least the mid-foot region, the extent of which can be guided by physical examination findings or patient history (Fig. 8-4). For identification of the deep peroneal nerve, the anterior tibial artery is an ideal landmark (Fig. 8-5); when moving the transducer from proximal to distal over the anterior tibial artery in short axis, the deep peroneal nerve is identified as it crosses from medial to lateral over the anterior tibial artery.

Medial Ankle Evaluation

For medial evaluation, the supine patient externally rotates at the hip or rolls partially onto the ipsilateral side to gain access to the medial aspect of the ankle. Ultrasound examination begins in the transverse plane superior to the medial malleolus (Fig. 8-6A). The hyperechoic and shadowing surface of the tibia is seen, and the transducer is moved posteriorly. The first tendon identified is the tibialis posterior tendon in short axis (see Fig. 8-6B). One may toggle the transducer (see Fig. 1-3B) to assist in identification of the tendons in short axis, which causes the tendon to appear hypoechoic from anisotropy and improves conspicuity compared with the adjacent hyperechoic fat (see Fig. 8-6C). The transducer is then moved posteriorly to identify the flexor digitorum longus tendon, the posterior tibial artery and veins, the tibial nerve, and then the flexor hallucis longus tendon in order from anterior to posterior (see Fig. 8-6D). The tibialis posterior tendon is typically twice the size of the adjacent flexor digitorum longus tendon. The thin and hyperechoic flexor retinaculum can also be identified superficial to the tendons, and it attaches to the tibia.

Evaluation is continued distally with the transducer short axis to each tendon; the transducer is rotated to the coronal plane as each tendon is followed distally (Fig. 8-7A). Anisotropy is again used to help delineate each tendon in short axis (see Fig. 8-7B and C). At the medial aspect of the calcaneus, a bony protuberance called the sustentaculum tali protrudes medially to articulate with the talus as the middle facet of the anterior subtalar joint. The medial tendons have characteristic locations relative to the sustentaculum tali (see Fig. 8-7B). The tibialis posterior tendon is dorsal and superficial, the flexor digitorum longus lies immediately superficial, and the flexor hallucis longus tendon lies plantar to the sustentaculum tali in a bony groove of the calcaneus.

In the supramalleolar region, the tibial nerve is located between the flexor digitorum longus and flexor hallucis longus tendons. In cross section, the individual hypoechoic nerve fascicles surrounded by hyperechoic connective tissue take on a honeycomb appearance (see Fig. 8-6D), whereas in long axis a fascicular pattern is appreciated that, in contrast to adjacent tendons, is coarser in echotexture. In the supramalleolar region, a small medial calcaneal nerve arising from the tibial nerve can be identified; this branch courses directly inferior, medial to the calcaneus (Fig. 8-8). The tibial nerve then divides into medial and lateral plantar branches, which continue under the mid-foot to give off the common plantar digital nerves and then the proper plantar digital branches.

To assess for medial tendon abnormality in long axis, the transducer is then moved back to the level of the distal tibia over the tibialis posterior tendon and is turned 90 degrees (Fig. 8-9A and B). As the transducer follows the course of the tibialis posterior tendon in long axis, the transducer moves from a coronal plane relative to the body to the axial plane (see Fig. 8-9C to F). At the navicular bone, it is common to visualize mild thickening and decreased echogenicity of the distal tibialis posterior tendon, related to its insertion on the talus and anisotropy from several of the tibialis posterior tendon fibers that course plantar to the navicular to insert at the cuneiforms and the second through fourth metatarsals (see Fig. 8-9F). It is also common to see a small amount of fluid within the tendon sheath of the tibialis posterior tendon just beyond the medial malleolus, usually seen only along one side of the tendon; asymptomatic fluid should not be present at the navicular where a tendon sheath is absent.12 An accessory navicular bone may be seen within the distal tibialis posterior tendon near the navicular bone (see Fig. 8-79). To assess the flexor digitorum longus tendon, examination again begins transversely superior and posterior to the medial malleolus, followed by assessment in long axis and distally (Fig. 8-10A). Similarly, the flexor hallucis longus tendon can be assessed first in short axis and then in long axis (see Fig. 8-10B). As the flexor digitorum longus and flexor hallucis longus tendons are followed distally beneath the mid-foot, the two tendons cross, called the knot of Henry (see Fig. 8-10C).

After assessment of the medial tendons, the components of the deltoid ligament are evaluated. The transducer is initially placed in the coronal plane at the medial malleolus (Fig. 8-11A). At this location, a superficial hyperechoic and fibrillar tibiocalcaneal component of the deltoid ligament is identified, extending from the tibia to the calcaneus (see Fig. 8-11B). With rotation of the distal aspect of the transducer anteriorly with the proximal aspect fixed to the medial malleolus, the more superficial tibionavicular and deeper anterior tibiotalar components are identified (see Fig. 8-11C). The distal aspect of the transducer is then rotated posteriorly while the proximal aspect remains fixed to the medial malleolus with the foot in dorsiflexion. In this position, the thick hyperechoic and fibrillar posterior tibiotalar component of the deltoid ligament is identified deep to the tibialis posterior tendon (see Fig. 8-11D).

The spring ligament complex consists of superomedial, medioplantar, and inferoplantar calcaneonavicular ligaments.10 To visualize each component, the transducer is initially placed in the transverse plane inferior to the medial malleolus and over the sustentaculum tali. By moving the transducer anteriorly and angling superior toward the talar head, the superomedial calcaneonavicular ligament is identified in long axis between the tibialis posterior tendon and the talus (Fig. 8-12).13

Lateral Ankle Evaluation

Structures of interest include the peroneal tendons and the lateral ligamentous structures of the ankle. Examination begins in the supramalleolar region in the transverse plane, directly posterior to the fibula in the retromalleolar groove or sulcus (Fig. 8-13A). At this location, the muscle belly and tendon of the peroneus brevis are identified in short axis (see Fig. 8-13B). An adjacent tendon, the peroneus longus is also seen, characterized by lack of a muscle belly at this level. With movement of the transducer from superior to inferior, the normal peroneus brevis muscle belly will taper; only the peroneus brevis and longus tendons should be visible at the extreme fibula tip (see Fig. 8-13C). If the peroneus brevis muscle is present beyond the fibular tip, this normal variation is termed a low-lying muscle belly of the peroneus brevis and may be associated with tendon tear (see Fig. 8-92).4 Although variable, the peroneus brevis is usually directly against the posterior cortex of the fibula, with the adjacent peroneus longus tendon more posterior. The thin and hyperechoic superior peroneal retinaculum can be seen extending over the tendons to insert on the posterolateral margin of the fibula.

Assessment is continued short axis to the peroneal tendons. Toggling the transducer is a helpful maneuver to identify the tendons in short axis, which causes the tendon to appear hypoechoic from anisotropy and improves conspicuity compared with the adjacent hyperechoic fat (see Fig. 1-12). As the transducer crosses the oblique plane between the tip of the fibular and the posterior aspect of the heel, the normal calcaneofibular ligament can be seen deep to the peroneal tendons (Fig. 8-14). As the peroneal tendons are followed in short axis, the transducer becomes positioned in the coronal plane (Fig. 8-15A). At the lateral aspect of the calcaneus, a bony prominence of variable size called the peroneal tubercle is present (see Fig. 8-15B). At this site, the peroneus brevis and longus tendons diverge into different directions. Because of their different respective orientations at the peroneal tubercle, it is difficult to image both tendons in short axis without one tendon appearing artifactually hypoechoic from anisotropy (see Fig. 8-15B). With minimal clockwise and counterclockwise transducer rotation and toggling, anisotropy of each tendon can be eliminated (see Fig. 8-15C). The peroneus brevis can be followed distally to its insertion on the fifth metatarsal base, and the peroneus longus similarly can be imaged under the mid-foot and forefoot to its insertion on the medial cuneiform and first metatarsal base.

Imaging in short axis is important in evaluation of the peroneal tendons because this is the optimal plane to visualize the common longitudinal split tears. It is also important to use dynamic maneuvers in evaluation of the peroneal tendons, to assess for subluxation or dislocation lateral and anterior to the fibula. This is accomplished with placement of the transducer in the transverse plane posterior to the distal fibula, and the patient is asked either to reproduce symptoms or to actively move the ankle into dorsiflexion and eversion. It is important to place only minimal transducer pressure throughout the dynamic examination so as not to inhibit abnormal movement of a peroneal tendon. The peroneal tendons should remain posterior to the fibula with an intact superior peroneal retinaculum.

For assessment of the peroneal tendons in long axis, one again returns to the supramalleolar region and places the transducer over the retromalleolar groove with the transducer in the oblique-sagittal plane toward the posterior aspect of the fibula (Fig. 8-16A). This approach allows visualization of the peroneus brevis and longus tendons in one imaging plane (see Fig. 8-16B). As the transducer is moved distally, the tendons begin to diverge distal to the fibula (see Fig. 8-16C and D). At this point, the peroneus longus and brevis are followed individually (see Fig. 8-16E). The peroneus longus courses deep toward the cuboid, where it commonly demonstrates anisotropy (see Fig. 8-16F). An echogenic os peroneum may be seen within the peroneus longus tendon.14 More distal assessment of the peroneus longus may be completed if symptoms warrant. The peroneus brevis tendon can be followed distally from the fibula to its insertion on the base of the fifth metatarsal (see Fig. 8-16G).

The first lateral ankle ligament to be assessed is the anterior talofibular ligament. For localization, the transducer is first placed directly over the lateral aspect of the distal fibula. The transducer is then moved inferiorly. Once the extreme distal fibula tip is reached, the transducer is moved slightly superiorly and anteriorly to visualize the talus (Fig. 8-17A). In this position, the anterior talofibular ligament appears as a homogeneously hypoechoic structure from anisotropy resulting from the oblique course of the ligament toward the talus (see Fig. 8-17B). The transducer is then angled (heel-toe maneuver) so that the ligament fibers are perpendicular to the sound beam, to eliminate anisotropy, and the normal anterior talofibular ligament is seen as a continuous compact fibrillar structure that extends from the fibula to the talus in long axis (see Fig. 8-17C) (Video 8-1)image. Anisotropy is used to one’s advantage in this application because initial identification of the anterior talofibular ligament is enhanced; the hypoechoic ligament is more conspicuous adjacent to the hyperechoic fat. Once the ligament is identified, it is important to eliminate anisotropy to exclude ligament abnormality.

To evaluate the calcaneofibular ligament in long axis, the transducer is placed in an oblique-coronal plane between the fibular tip and the posterior aspect of the heel where the calcaneofibular ligament is identified between the peroneal tendons and calcaneus (Figs. 8-18A and B). The calcaneofibular ligament is often incidentally seen during evaluation of the peroneal tendons in (see Fig. 8-14B). In short axis, the normal calcaneofibular ligament may appear hypoechoic from anisotropy and simulate a complex ganglion cyst associated with the peroneal tendons (see Fig. 8-18C and D).

To evaluate the anterior inferior tibiofibular ligament, the transducer is initially placed in the axial plane over the distal tibia and fibula. As the transducer is moved inferiorly, the cortex of the tibia disappears from view, and the talus appears, a finding that indicates the level of the ankle joint. The transducer is moved superiorly again to identify the most distal aspect of the tibia, and then the lateral aspect of the transducer is rotated inferiorly to visualize the hyperechoic and compact fibrillar anterior inferior tibiofibular ligament, which courses inferiorly from the tibia to the fibula (Fig. 8-19A and B). Another manner in identifying the anterior inferior tibiofibular ligament is to begin at the anterior talofibular ligament; fix the transducer over the fibula, and rotate the transducer so that the medial aspect moves superiorly from the talus to the tibia in an oblique plane. An accessory anterior inferior tibiofibular ligament (Bassett ligament) may also be identified as a discrete ligament bundle inferior to the anterior inferior tibiofibular ligament, slightly more horizontal and spanning a greater distance between tibia and fibula (see Fig. 8-19C).9 Variability exists in the number of bundles or fascicles in the anterior inferior tibiofibular ligament (see Fig. 8-19D).15,16

It is very important to evaluate the interosseous membrane between the tibia and the fibula in the setting of an anterior tibiofibular ligament tear. At ultrasound, the interosseous membrane appears as a thin and hyperechoic often bilaminar structure extending from the tibia to the fibula and best evaluated in the transverse plane perpendicular to the sound beam (Fig. 8-20).17 The interosseous membrane extends inferiorly and becomes thickened as the interosseous ligament superior to the tibiotalar joint. The combination of the interosseous ligament, the anterior and posterior inferior tibiofibular ligaments, and the posteriorly located inferior transverse ligament stabilizes the ankle syndesmosis or articulation.15 Although visible, the posterior talofibular is not routinely evaluated (Fig. 8-21; see Fig. 8-13C). The posterior inferior tibiofibular ligament may also be assessed; however, the posterior ligamentous structures are more difficult to evaluate, given their depth.

Posterior Ankle and Heel Evaluation

If the patient has no symptoms posteriorly and one wants simply to screen the distal Achilles tendon and plantar aponeurosis for abnormalities, the patient can externally rotate the leg while supine to gain limited access to the posterior ankle. However, for a thorough examination, the patient should lie prone for complete access to the calf and posterior ankle. Dorsiflexion of the ankle elongates the Achilles tendon and reduces anisotropy. The Achilles tendon is easily evaluated because the transducer is placed in the sagittal plane long axis to the tendon fibers from a posterior approach (Fig. 8-22A). In long axis, the Achilles tendon should be fairly uniform in thickness (see Fig. 8-22B and C). The transducer is moved superiorly from the distal calf to the calcaneus, and the transducer is turned 90 degrees for evaluation in short axis; in this plane, the anterior margin of the Achilles tendon is predominantly flat or concave and should not be diffusely convex posterior (see Fig. 8-22D). When imaged from superior to inferior in short axis, the Achilles tendon fibers rotate 90 degrees, with the gastrocnemius component lateral and the soleus medial. A thin tendon, the plantaris, can be seen directly medial to the Achilles tendon (see Fig. 8-22D) but is often best appreciated in the setting of an Achilles tendon tear. The plantaris tendon may be absent in up to 20% of individuals.7 Anterior to the Achilles tendon is a somewhat heterogeneous fat pad called Kager fat pad. Distally, a small amount of anechoic fluid (up to 2.5 mm anteroposterior) can be seen in the retrocalcaneal bursa.12 In evaluation of the retro-Achilles bursa, located superficial to the distal Achilles tendon, it is important to float the transducer on a layer of thick gel so as not to efface the bursa and displace fluid out of the field of view.

The transducer is then moved over the plantar aspect of the heel to evaluate the plantar aponeurosis (Fig. 8-23A). The transducer is placed in the sagittal plane over the plantar and medial aspect of the heel long axis to the plantar aponeurosis, which appears hyperechoic, uniform, and 4 mm or less in thickness at the calcaneal attachment (see Fig. 8-23B).18 Any identified disorder is also assessed in short axis. More distal assessment of the plantar aponeurosis can be carried out if symptoms or history warrants such evaluation.

Evaluation of the Calf

Structures of interest in the posterior calf include the soleus, the medial and lateral heads of the gastrocnemius, and the plantaris. Evaluation begins in the transverse plane over the posterior mid-calf (Fig. 8-24A). At this location, the medial and lateral heads of the gastrocnemius muscle are identified superficial to the larger soleus muscle (see Fig. 8-24B). At this point, the transducer is centered over the medial head of the gastrocnemius and then is moved distally until the muscle tapers. The transducer is then turned 90 degrees to visualize the normal tapering appearance of the medial gastrocnemius head over the soleus in long axis, a very common site of injury (see Fig. 8-24C and D). The lateral head of the gastrocnemius can be evaluated in a similar manner. It is also important to evaluate the entire calf for pathologic processes, although the patient often indicates a site of symptoms to focus evaluation. The thin, hyperechoic plantaris tendon, when present, can be seen in the posterior calf deep to the gastrocnemius muscle.7 Initially, the plantaris crosses midline posterior to the knee joint and then moves medial directly between the muscle bellies of the medial head of the gastrocnemius and soleus muscles. Distally, the medial and lateral heads of the gastrocnemius combine with the soleus to form the Achilles tendon. The plantaris tendon courses along the medial aspect of the Achilles tendon to insert on the calcaneus.

Evaluation of the Forefoot

Evaluation of the distal aspect of the foot is largely guided by the patient’s symptoms or history. Tendons around the digits, joint processes, soft tissue fluid collections, and masses can be assessed with ultrasound. If indicated, the forefoot can be assessed for Morton neuroma.19 This is accomplished by placement of the transducer in the coronal plane on the body or short axis to the metatarsals, over the metatarsal heads from a plantar approach (Fig. 8-25A). The examiner’s finger from the other hand is placed at the dorsal aspect of the forefoot over the web space to be evaluated (see Fig. 8-25B). This maneuver assists evaluation because the distal metatarsals are separated and the intermetatarsal space is widened, and it also reproduces the patient’s symptoms when a neuroma is present. Evaluation for Morton neuroma also continues in long axis in the sagittal plane (see Fig. 8-25C). A similar long axis image can be obtained with the transducer over the dorsal foot and manual palpation over the plantar aspect. Resolution is often improved given the thinner dorsal soft tissues compared with the plantar aspect. Returning to the plantar short axis approach, dynamic assessment for Morton neuroma can be completed by manually squeezing the metatarsals together from side to side and imaging from a plantar approach. This maneuver (called the sonographic Mulder sign) will cause plantar displacement of a neuroma also producing symptoms.20 When screening for inflammatory arthritis, in addition to evaluation of a symptomatic region, the fifth metatarsal head and medial first metatarsal head should be routinely imaged to assess for rheumatoid arthritis and gout, respectively.

Joint and Bursal Abnormalities

Joint Effusion and Synovial Hypertrophy

Evaluation for joint pathology should focus on key joint recesses for effusion and synovial hypertrophy. For the ankle or tibiotalar joint, the anterior recess with the foot in slight plantar flexion is the most sensitive position and location to identify joint effusion.21 Simple fluid distention of a joint is typically anechoic. Joint distention is seen in the sagittal plane or in the lateral aspect of the anterior joint recess (Fig. 8-26). A small amount of fluid may be found in the anterior ankle joint recess in normal volunteers; this fluid may measure up to 1.8 mm anteroposterior.12 It is important not to mistake the 1 to 2 mm of hypoechoic hyaline cartilage that covers the talar dome to the talar neck for joint effusion (see Fig. 8-2B). For the foot, dorsal recesses of the tarsal, metatarsophalangeal, and interphalangeal joints are targeted. With regard to the metatarsophalangeal joints, each dorsal joint recess distends proximally over the metatarsal (Fig. 8-27A) as well as over the proximal phalanx when large (see Fig. 8-27B). Causes for anechoic joint effusion are many and include infection (Fig. 8-28), trauma, osteoarthritis (Fig. 8-29), and other arthritides (discussed later). Although commonly seen and asymptomatic, joint fluid within the first metatarsophalangeal joint often relates to early degenerative joint disease because this joint is a common site for osteoarthritis. Intra-articular bodies from degenerative arthritis and trauma appear hyperechoic with possible shadowing within a joint recess (Fig. 8-30). Intra-articular bodies may also migrate to the medial ankle tendon sheaths (see Fig. 8-68B) because communication with the ankle joint is common.

Increased echogenicity of joint fluid can be the result of complex fluid, as seen in infection (Figs. 8-31 and 8-32) and hemorrhage (Fig. 8-33). Echogenic joint fluid may resemble synovial hypertrophy (Figs. 8-34). To assist in this differentiation, compressibility and internal echo movement with transducer pressure, redistribution with joint movement, and lack of flow on color and power Doppler imaging suggest complex fluid rather than synovitis (Videos 8-2 and 8-3)image. Echogenicity and vascularity do not predict infection, and ultrasound-guided aspiration should be considered if there is concern for infection. In the setting of synovial hypertrophy, adjacent cortical irregularity may be from erosions, which can be seen in inflammatory (see below for inflammatory arthritis and Chapter 2 for infection) and noninflammatory conditions, which include pigmented villonodular synovitis22 (Fig. 8-35) and synovial (osteo)chondromatosis (Fig. 8-36). In the latter condition, hyperechoic and possibly shadowing foci may be identified.23 Synovial hypertrophy may also be found in the ankle joint deep to the anterior talofibular ligament in anterolateral impingement syndrome (Fig. 8-37), where echogenic synovial hypertrophy greater than 10 mm is associated with symptoms and adjacent ligament abnormality.24 Nonspecific mild synovial thickening, usually with little or no flow on color or power Doppler imaging, can be seen with osteoarthritis and may not correlate with patient symptoms (Fig. 8-38).25,26

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