Musculoskeletal ultrasound of the ankle and hindfoot

INTRODUCTION — The structures of the ankle and hindfoot are superficial and ideally suited for sonographic evaluation. In the authors' experience, ultrasound is the imaging modality of choice when evaluating the ankle tendons, ligaments, and peripheral nerves. The dynamic nature of the sonographic examination allows for detection of instability of both tendon and ligament, which may not be apparent on static imaging. In addition, ultrasound can display the small peripheral nerves of the ankle and foot that are prone to entrapment and insult from trauma and surgery.

This topic will describe a systematic approach to complete fundamental sonographic evaluation of each ankle quadrant as well as limited structures of the plantar hindfoot. The conditions that cause ankle and foot pain are reviewed separately. (See "Ankle sprain in adults: Evaluation and diagnosis" and "Ankle fractures in adults" and "Non-Achilles ankle tendinopathy" and "Achilles tendinopathy and tendon rupture" and "Calf injuries not involving the Achilles tendon" and "Overview of foot anatomy and biomechanics and assessment of foot pain in adults".)

USES, ADVANTAGES, AND LIMITATIONS OF ANKLE AND FOOT ULTRASOUND — A discussion of the general advantages and limitations of musculoskeletal ultrasound is provided separately. (See "Musculoskeletal ultrasound of the shoulder".)

In brief, ultrasound offers many advantages for imaging the ankle and foot. Its superior spatial resolution (ie, greater ability to differentiate between objects) allows for detailed evaluation of tendon, ligament and nerve disorders that may not be apparent on other imaging modalities. Complaints of instability can be rapidly assessed in real time with dynamic ultrasound examination, and the location of pain can be correlated with precise anatomic structures by palpation with the ultrasound transducer (ie, sonopalpation). Another advantage of ultrasound is the lack of artifact associated with orthopedic hardware, which enables the clinician to examine structures surrounding the hardware for impingement or signs of inflammation.

Although many structures of the foot and ankle are easily accessible for sonographic evaluation, intra-articular structures are not. This is an inherent limitation of ultrasound technology, and other imaging modalities may be required for complete evaluation of a joint.

ANKLE AND FOOT ANATOMY FOR ULTRASOUND EVALUATION — For the purposes of sonographic evaluation, the ankle is divided into anatomic quadrants: anterior, lateral, medial, and posterior. These quadrants are divided by the long axis of the tibia. In addition to the ankle proper, pertinent structures of the plantar aspect of the hindfoot may be included in the ultrasound examination of the ankle and some of these are listed below. The clinical anatomy of these regions is reviewed in detail separately. (See "Ankle fractures in adults", section on 'Clinical anatomy' and "Non-Achilles ankle tendinopathy" and "Overview of foot anatomy and biomechanics and assessment of foot pain in adults", section on 'Basic foot anatomy'.)

●Anterior ankle – Structures evaluated in the anterior ankle quadrant include (figure 1 and picture 1): tibialis anterior muscle and tendon (picture 2); extensor hallucis longus muscle and tendon (picture 3); extensor digitorum longus muscle and tendon (picture 4); fibularis (peroneus) tertius muscle and tendon (picture 5); deep fibular nerve; dorsalis pedis artery (figure 2 and figure 3); anterior joint recess; anterior inferior tibiofibular ligament (figure 4); superficial fibular nerve.

●Lateral ankle – Structures evaluated in the lateral ankle quadrant include (figure 5 and picture 6): fibularis (peroneus) longus and brevis muscles and tendons (picture 7 and picture 8); superior fibular retinaculum; anterior talofibular ligament (figure 5); lateral tibiotalar joint; calcaneofibular ligament; posterior subtalar joint (figure 6); sural nerve (figure 7); fibular trochlea of calcaneus (peroneal tubercle).

●Medial ankle – Structures evaluated in the medial ankle quadrant include (figure 8 and figure 9): tibialis posterior muscle and tendon (picture 9); flexor digitorum longus muscle and tendon (picture 10); tibial nerve; medial calcaneal nerve; medial and lateral plantar nerves; inferior calcaneal nerve; tibial artery and vein (figure 10); flexor hallucis longus muscle and tendon (picture 11); deltoid ligament (figure 11); medial tibiotalar joint.

●Posterior ankle – Structures evaluated in the posterior ankle quadrant include: Achilles tendon (figure 12); retrocalcaneal bursa; retro-Achilles bursa; plantaris tendon; posterior tibiotalar and subtalar joints.

●Plantar hindfoot – Structures evaluated in the plantar hindfoot include (figure 13): plantar fascia (figure 14); plantar heel pad.

ULTRASOUND EXAMINATION OF THE ANKLE AND FOOT

Guidelines, structures to image, and positioning — The sonographic examination of the ankle is organized by quadrants: anterior, lateral, medial, and posterior. Additional examination of portions of the plantar hindfoot and forefoot are typically limited in scope. It is often appropriate to focus the sonographic examination on the region of the patient's complaints, but other structures may need to be evaluated as clinically indicated, and examinations may involve more than one quadrant. A list of structures to be evaluated for each quadrant is provided above. These lists are drawn from the American Medical Society for Sports Medicine (AMSSM) Recommended Sports Ultrasound Curriculum for Sports Medicine Fellowships, which in turn was adopted from the American Institute of Ultrasound in Medicine (AIUM) Guidelines for Performance of the MSK US Examination [1,2]. The approach described is consistent with that of the European Society of Musculoskeletal Radiology [3,4]. (See 'Ankle and foot anatomy for ultrasound evaluation' above.)

A high frequency (eg, 12 to 5 MHz) linear array transducer is preferred for the diagnostic evaluation of the ankle and foot. If available, a high frequency (eg, 15 to 7 MHz) linear array transducer with a short footprint (ie, "hockey stick") is ideal (picture 12). All structures should be imaged in both the short and long axis, and any possible pathology confirmed with orthogonal imaging. Doppler evaluation may be needed to evaluate possible synovitis, tenosynovitis, paratenonitis, and intratendinous neovascularization, as is commonly seen in chronic tendinosis.

Anterior ankle region — To examine the anterior ankle, the patient lies supine with their hips flexed approximately 45 degrees, knees flexed approximately 90 degrees, and ankle plantar-flexed with the plantar surface supported by the examination table.

The transducer should be placed in the sagittal plane (ie, long-axis view) at the junction of the foot and lower leg (picture 13). In this plane, the hyperechoic lines of the tibia, caudal hyperechoic curvilinear line of the talus, and the hyperechoic triangular structure residing between both bones (anterior fat pad) should be seen (image 1). The talus can be distinguished from the tibia by the superficial layer of articular cartilage. With the transducer centered over the proximal convex curve of the talus, the transducer is rotated 90 degrees (picture 14). In this transverse plane (or short-axis view), the talus is identified as a concave hyperechoic line (talar dome), with overlying articular cartilage. (See "Talus fractures".)

At the medial edge of the talus the largest tendon in the anterior ankle, the tibialis anterior tendon, is recognized by its fibrillar structure in short axis. Its location can be accentuated by tilting the transducer to appreciate its relative anisotropy compared to the surrounding subcutaneous fat (image 2). In this short-axis view, the extensor hallucis longus tendon resides immediately lateral to the tibialis anterior tendon, and can be differentiated from the tibialis anterior tendon not only by its location, but by its size (half the size), and the distal continuation of its muscle belly. The extensor digitorum longus tendon (short axis) resides immediately lateral, and appears relatively flatter than the extensor hallucis longus tendon (image 2). (See "Non-Achilles ankle tendinopathy".)

Each tendon should be followed in short axis down to its insertion, and then reimaged in long axis at the talus, and followed down again to its insertion (image 3 and image 4). A hyperechoic line (extensor retinaculum) can be noted superficial to each tendon in the short-axis view at the talar dome. Of note, when scanning the extensor digitorum longus tendon in short axis and imaging the divisions to the middle three phalanges (numbers 2-4), the lateral most portion of the fibrillar structure is the fibularis (peroneus) tertius tendon, which should be identified and evaluated to its insertion onto the dorsal surface of the fifth metatarsal (image 5).

To image the neurovascular structures of the anterior ankle, the transducer is returned to the transverse plane over the talar dome. Deep to the flexor digitorum longus tendon an artery, the dorsalis pedis artery, is noted (image 6). The deep fibular nerve is lateral to the dorsalis pedis artery at this level. As the transducer is moved caudally, the deep fibular nerve is seen crossing superficial to the dorsalis pedis artery and to the medial side of the artery (image 7). The artery and nerve can also be imaged in the long axis (ie, sagittal plane).

Lateral ankle region — To examine the lateral ankle, the patient is positioned on their side with the contralateral lower extremity on the examination table and flexed at the hip, and the ipsilateral leg supported by a pillow or bolster at the medial aspect, placing the ankle in slight eversion (picture 15).

One can choose to begin the examination with the lateral ligaments or fibular tendons depending on the clinical presentation. We begin here with the lateral ligaments. The cranial edge of the transducer is positioned on the lateral malleolus, while the caudal edge is placed on the talus with the transducer relatively parallel to the foot and examination table (picture 16). In this plane, the fibula and talus are visualized, with a hypoechoic structure connecting both bones (anterior talofibular ligament, ATFL). The ATFL has an oblique superficial border relative to the surface of the transducer (image 8). To minimize anisotropy, align the transducer with the ligament by applying pressure to its caudal end (heel-toe maneuver). The integrity of the ATFL can be assessed dynamically by performing an anterior drawer test while the transducer remains in position. (See "Ankle sprain in adults: Evaluation and diagnosis".)

From this position, the anterior inferior tibiofibular ligament can be assessed by rotating the caudal edge of the transducer (picture 17) until it rests on the tibia and a fibrillar structure (anterior inferior tibiofibular ligament) (image 9) is seen connecting both bones. This ligament can also be assessed dynamically, by performing a squeeze test or an external rotation stress test while the transducer remains in position.

To image the interosseous membrane, both bones are kept in view as the transducer is moved cranially to the mid lower leg, where a hyperechoic line connecting both bones (interosseous membrane) is seen (image 10). If the transducer is moved inferiorly in the same plane used to image the anterior inferior tibiofibular ligament, translating the transducer posteriorly so that the cranial edge of the transducer is posterior to the lateral malleolus (picture 18) reveals a hyperechoic fibrillar structure originating from the lateral malleolus and extending over the calcaneus (calcaneofibular ligament) (image 11). In this image, two tendons (fibularis longus and brevis) are seen in short axis. The calcaneofibular ligament can be assessed dynamically, by having the patient dorsiflex the ankle, which causes the ligament to become taut and elevates the fibular tendons. The ligament can be assessed in short axis by rotating the transducer 90 degrees, which also brings into view the fibular tendons in long axis. (See "Syndesmotic ankle injury (high ankle sprain)".)

To image the fibular tendons in short axis, place the transducer in the same position used to image the calcaneofibular ligament in its long axis, and move cranial to the lateral malleolus (picture 19). A superficial tendon in short axis (fibularis longus) is seen overlying another tendon in short axis (fibularis brevis) accompanied by its muscle belly (image 12). As the transducer is moved caudally over the lateral malleolus, the muscle belly of the fibularis brevis begins to disappear, and a hyperechoic fibrocartilaginous plate is seen between the superficial edge of the fibularis longus tendon and the lateral malleolus (image 13). This structure is the anterior support for the superior fibular retinaculum, which attaches to the posterolateral border of the lateral malleolus and supports the fibular tendons at the lateral malleolus. The stability of the fibular tendons can be assessed by having the patient dorsiflex their ankle while looking for dislocation or subluxation of the tendons anterior to the lateral malleolus.

The fibular tendons can be traced caudally while rotating the transducer to ensure the tendons remain in short axis (picture 20). As the transducer is moved caudally over the calcaneus a hyperechoic ridge (fibular trochlea or peroneal tubercle) is seen separating the fibularis brevis and longus tendons, with the former dorsal to the trochlea and the latter plantar (image 14). As the two tendons diverge, relative anisotropy appears, and careful scanning technique of each tendon is required to ensure that this artifact is not confused with pathology. Such anisotropy can be appreciated in the following image with the fibularis longus appearing hypoechoic (image 14).

From this point, the fibularis brevis can be traced distally to its insertion onto the base of the fifth metatarsal. At this position, the transducer can be rotated 90 degrees (picture 21) to visualize the fibularis brevis insertion in long axis (image 15). With the transducer in the same orientation, it is moved plantar and the lateral band of the plantar fascia is appreciated inserting onto the base of the fifth metatarsal in long axis (image 16).The transducer can then follow the fibularis longus tendon distal to the fibular trochlea in short axis to the cuboid tunnel, and trace it proximally in long axis by rotating the transducer 90 degrees. A round or oval-shaped sesamoid bone may be noted within the fibularis longus tendon near the level of the calcaneocuboid joint, which is referred to as the os peroneum. This sesamoid is present in approximately 10 percent of patients (picture 22) and should not be confused with an avulsion fracture or calcific tendinopathy [5]. (See "Sesamoid fractures of the foot".)

The sural nerve can be imaged by placing the transducer proximal to the lateral malleolus where both fibular tendons were imaged in short axis and moving the transducer posteriorly to bring the Achilles tendon into view (picture 23). From this position, the sural nerve is seen in short axis between the fibularis muscles and the Achilles, accompanied by the lesser saphenous vein (image 17).

Medial ankle region — To examine the medial ankle, the patient is positioned on their side with the lower extremity to be examined on the examination table and the hip and knee of that extremity extended (picture 24), while the hip of the opposite lower extremity is flexed about 45 degrees [6].

The transducer is placed in the transverse plane, posterior to the medial malleolus with the anterior edge of the transducer fixed on the tibia (picture 25). At this position, looking from anterior to posterior, the tibialis posterior, flexor digitorum longus (FDL), and flexor hallucis longus (FHL) tendons are seen in short axis (image 18). Between the FDL and FHL, lies the neurovascular bundle, which includes the tibial vein (branches), tibial artery, and tibial nerve. The flexor retinaculum overlies the tendons and neurovascular bundle, which is collectively referred to as the tarsal tunnel. All three tendons can be followed caudally in the short axis to the level of the sustentaculum tali of the calcaneus (picture 26), where the tibialis posterior tendon resides dorsal to this bony structure, the FDL tendon superficial, and FHL tendon plantar (image 19). (See "Non-Achilles ankle tendinopathy" and "Ankle sprain in adults: Evaluation and diagnosis".)

From the sustentaculum tali, the tibialis posterior tendon can be followed to its insertion onto the navicular bone. At this point, the caudal end of the transducer can be fixated on the navicular, and the cranial end rotated 90 degrees (picture 27) to image the tendon in long axis (image 20). Note that the insertions of the tibialis posterior tendon onto the navicular and onto the cuneiforms, cuboid, and metatarsals are variable. This broad attachment introduces anisotropy, which should not be confused for tendinosis or tear. In addition, the presence of an accessory navicular (ie, os tibiale) has been reported in up to 21 percent of individuals and it should not be mistaken for an avulsion fracture or calcific tendinopathy [5]. The accessory navicular is classified as type-I (sesamoid bone within distal tibialis posterior), type-II (persistent accessory ossification center with synchondrosis to navicular bone), and type-III (incorporation to the navicular tuberosity). While typically an incidental finding, the accessory navicular has been associated with a variety of potential pain syndromes. (See "Evaluation, diagnosis, and select management of common causes of midfoot pain in adults", section on 'Medial arch (navicular) injury'.)

The transducer can be moved back to the sustentaculum tali to image the FDL tendon (short axis), which sits superficial to this bony structure. The FDL can be followed caudally from here as it courses laterally over the FHL (at the Knot of Henry) (picture 28 and image 21). The transducer is then moved cranially to a position over the tarsal tunnel, with the FHL (short axis) centered in the image. From this position, the transducer is moved caudally, as the flexor hallucis tendon continues to be seen in short axis. Over the talus the tendon is seen in a concave groove, bordered on either side by the medial and lateral tubercles of the talus (image 22). The FHL tendon can be followed caudally to the Knot of Henry, and further caudally to its insertion onto the first distal phalanx. If clinically warranted, the FHL tendon can also be imaged in long axis by rotating the transducer 90 degrees at the location where its insertion is seen in the short axis, and fixing the caudal edge of the transducer on the distal phalanx (picture 29 and image 23). Caution must be used when assessing fluid in the hindfoot, and in particular the tibialis posterior tendon, as up to 4 mm is considered normal [1].

The transducer is repositioned over the tarsal tunnel, centering the transducer over the tibial nerve (short axis) (image 24). From this position, the transducer is moved caudally, while rotating it, to maintain a short-axis view of the nerve (picture 30). The first branch off the nerve that is seen is the calcaneal sensory branch (medial calcaneal nerve), which is followed further caudally by the two major branches, the medial and lateral plantar nerves (image 25). There is variability in the origin of the medial calcaneal nerve, which can also arise from the lateral plantar nerve or less commonly the medial plantar nerve. The medial calcaneal nerve is followed as it courses superficially through the flexor retinaculum and becomes subcutaneous.

The inferior calcaneal nerve (ie, Baxter’s nerve) is typically the first branch off the lateral plantar nerve and is often referred to as such [7]. However, this nomenclature can be confusing if the medial calcaneal nerve arises from the lateral plantar nerve. The inferior calcaneal nerve courses between the abductor hallucis and quadratus plantae muscles (image 26), where it is at risk of entrapment. It then travels between the flexor digitorum brevis and quadratus plantae muscles in close proximity to the anterior aspect of the medial calcaneal tuberosity.

To image the different components of the deltoid ligament, the transducer is placed in the coronal plane with the cranial edge on the medial malleolus and the caudal edge over the calcaneus (picture 31). The fibrillar structure connecting both bones is the tibiocalcaneal component of the deltoid ligament (image 27). If the caudal edge of the transducer is rotated and fixed on the navicular bone (picture 32), several structures are seen, including the fibrillar structure connecting the tibia and navicular, which is the tibionavicular component of the deltoid ligament, and deep to it the anterior tibiotalar component (image 28). The distal edge of the transducer is now rotated posteriorly (picture 33) to visualize the posterior tibiotalar component of the deltoid ligament (image 29).

Posterior ankle region — To examine the posterior ankle, the patient lies prone with their foot hanging free over the edge of the examination table.

The transducer is placed in the sagittal plane over the posterior ankle, with the caudal edge of the transducer fixed on the distal calcaneus (picture 34). The Achilles tendon (long axis) can be appreciated as the fibrillar structure inserting onto the posterior aspect of the calcaneus (image 30). One should appreciate that a portion of the superficial fibers of the Achilles tendon extend distal to the calcaneus and are continuous with the plantar fascia [2]. The potential space for the retro-Achilles bursa is immediately posterior to the distal Achilles tendon as it courses over the calcaneus. The retrocalcaneal bursa can be appreciated as the potential space between the Achilles tendon and cranial edge of the calcaneus. It is not uncommon to see a small amount of physiologic fluid within the retrocalcaneal bursa. (See "Achilles tendinopathy and tendon rupture" and "Calcaneus fractures".)

Kager's fat pad resides deep to the Achilles tendon between the calcaneus and muscle belly of the soleus (image 30). The tibiotalar and subtalar joints can be appreciated deep to Kager's fat pad by increasing the depth of the image. An accessory ossicle, the os trigonum, may be seen at this level posterior to the lateral tubercle of the posterior process of the talus. The os trigonum is the largest and most common of the accessory ossicles in the foot and ankle and identified in up to 25 percent of patients [5]. If bony fusion has occurred then this is referred to as the Stieda process. Both the os trigonum and Stieda process can be involved in posterior impingement syndrome of the ankle. The os trigonum should not be confused with a fracture or loose body.

If the transducer is moved cranially in the sagittal plane, the Achilles tendon can be appreciated, with the soleus muscle appearing deep to the tendon. The above structures (Achilles tendon, Kager's fat pad, retro-Achilles bursa, retrocalcaneal bursa, and soleus muscle) can also be viewed in short axis by rotating the transducer 90 degrees.

When following the Achilles tendon in short axis to its insertion, one can appreciate that the tendon rotates 90 degrees before inserting on the calcaneus, and the soleus tendon component lies medial while the gastrocnemius tendon component lies lateral (image 31). When viewing the Achilles tendon in short axis, a small tendon may be seen usually on the medial side, which is the plantaris tendon (image 32).

Plantar hindfoot region — Patient positioning to examine the plantar hindfoot is identical to that used to assess the posterior ankle.

To image the plantar fascia in long axis, the transducer is oriented in the sagittal plane (picture 35) with the proximal edge fixed on the medial calcaneal tuberosity (medial to midline of foot) (image 33). The origin of the plantar fascia should be imaged in the short axis to ensure that the entire width of the fascia, including the origin of both the central and lateral bands, has been evaluated.

The lateral band of the plantar fascia originates at the lateral aspect of the medial calcaneal tuberosity and often blends with the origin of the central band of the plantar fascia [8,9]. The lateral band is best identified by moving the transducer slightly distal to the calcaneus where the lateral band becomes more distinct as it courses towards the base of the fifth metatarsal (image 34). Plantar fascia thickness should generally be less than 4 mm (image 35), but asymptomatic thickening has been reported [10,11]. The plantar heel pad is seen in both the short-and long-axis views as a pattern of uniform heterogeneity in which circular or ovoid hypoechoic fat pockets are separated by hyperechoic fibrous septa (image 36) [11]. (See "Plantar fasciitis".)

ADDITIONAL ULTRASOUND RESOURCES — Instructional videos demonstrating proper performance of the ultrasound examination of the ankle, hindfoot, and related pathology can be found at the website of the American Medical Society for Sports Medicine: posterior ankle US examination, anterior ankle US examination, medial ankle US examination, lateral ankle US examination, plantar foot US examination, sports US ankle-foot pathology, US guided interventional procedures of the ankle and foot. Registration must be completed to access these videos, but no fee is required.

SOCIETY GUIDELINE LINKS — Links to society and government-sponsored guidelines from selected countries and regions around the world are provided separately. (See "Society guideline links: Musculoskeletal ultrasound".)

SUMMARY AND RECOMMENDATIONS

●The ankle is susceptible to a variety of traumatic and nontraumatic injuries. Due to the relatively superficial location of the ankle and its surrounding soft tissues, many of these pathologic conditions can be diagnosed or assessed using ultrasound (US). The advantages of US include portability, absence of radiation, and dynamic imaging capability. Incomplete evaluation of intra-articular structures is an inherent limitation of US, and other advanced imaging modalities may be required for complete evaluation. (See 'Uses, advantages, and limitations of ankle and foot ultrasound' above.)

●An understanding of the anatomy of the ankle and surrounding structures is essential for interpreting US images. A brief outline of the structures assessed as part of the fundamental ankle examination is provided above; detailed descriptions of ankle and foot anatomy are provided separately. (See 'Ankle and foot anatomy for ultrasound evaluation' above and "Foot and ankle pain in the active child or skeletally immature adolescent: Evaluation", section on 'Anatomy' and "Overview of foot anatomy and biomechanics and assessment of foot pain in adults", section on 'Anatomy and biomechanics'.)

●A high frequency linear array transducer is required for adequate evaluation of the superficial structures of the ankle and foot. If available, a short footprint linear array transducer (ie, "hockey stick") is ideal for smaller structures. (See 'Guidelines, structures to image, and positioning' above.)

●Sonographic evaluation of the ankle is organized into a quadrant system as described in the text. The major quadrants and structures examined include the following:

•Anterior ankle – Structures evaluated in the anterior ankle quadrant include (figure 1 and picture 1): tibialis anterior muscle and tendon (picture 2); extensor hallucis longus muscle and tendon (picture 3); extensor digitorum longus muscle and tendon (picture 4); fibularis (peroneus) tertius muscle and tendon (picture 5); deep fibular nerve; dorsalis pedis artery (figure 2 and figure 3); anterior joint recess; anterior inferior tibiofibular ligament (figure 4); superficial fibular nerve. (See 'Anterior ankle region' above.)

•Lateral ankle – Structures evaluated in the lateral ankle quadrant include (figure 5 and picture 6): fibularis (peroneus) longus and brevis muscles and tendons (picture 7 and picture 8); superior fibular retinaculum; anterior talofibular ligament (figure 5); lateral tibiotalar joint; calcaneofibular ligament; posterior subtalar joint (figure 6); sural nerve (figure 7); fibular trochlea of calcaneus (peroneal tubercle). (See 'Lateral ankle region' above.)

•Medial ankle – Structures evaluated in the medial ankle quadrant include (figure 8 and figure 9): tibialis posterior muscle and tendon (picture 9); flexor digitorum longus muscle and tendon (picture 10); tibial nerve; medial calcaneal nerve; medial and lateral plantar nerves; inferior calcaneal nerve; tibial artery and vein (figure 10); flexor hallucis longus muscle and tendon (picture 11); deltoid ligament (figure 11); medial tibiotalar joint. (See 'Medial ankle region' above.)

•Posterior ankle – Structures evaluated in the posterior ankle quadrant include: Achilles tendon (figure 12); retrocalcaneal bursa; retro-Achilles bursa; plantaris tendon; posterior tibiotalar and subtalar joints. (See 'Posterior ankle region' above.)

In addition to these four major quadrants, structures of the plantar hindfoot are often included as part of the US examination. (See 'Plantar hindfoot region' above.)