The anterior inferior tibiofibular ligament is a strong multifascicular fan-shaped ligament that lies between the anterior aspect of the distal tibia (Tillaux-Chaput tubercle) and the adjacent fibula (Fig. 8.1). It runs slightly obliquely upward and medially from the anteromedial aspect of the distal fibula at the upper margin of the ankle joint and is widest at the tibial insertion (Fig. 8.1). Moving the transducer proximally in a transverse plane from the ankle joint shows the anterior tibiofibular ligament (Fig. 8.2). Ligament conspicuity can be improved by slight angulation of the transducer or by rotating the tibial end of the transducer slightly more cranially.
The ligament is <2-mm wide at its mid-portion and provides major support to the distal tibiofibular syndesmosis.¹ The distal interosseous membrane is seen as a thin, hyperechoic (almost isoechoic to bone) line between the tibia and the fibula² proximal to the anterior tibiofibular ligament.

The anterior ankle tendons comprise the tibialis anterior, extensor hallucis longus (EHL), and extensor digitorum longus (EDL) from medial to lateral (Fig. 8.3). They can be memorized by “Tom Has a Date Perhaps” from medial to lateral (Tom = Tibialis anterior, Has = extensor Hallucis longus, A = Anterior tibial artery/vein and deep peroneal nerve, Date = extensor Digitorum longus, and Perhaps = Peroneus tertius). They are covered by retinaculum. The superior extensor retinaculum extends from the lateral malleolus to the medial malleolus and is continuous with the flexor retinaculum medially and superior peroneal retinaculum laterally. The inferior extensor retinaculum is Y-shaped. The stem of the Y lies on the lateral aspect of the retinaculum and divides into superomedial and inferomedial limbs, which attach to the medial malleolus and navicular-cuneiform area, respectively.

The tibialis anterior tendon is the strongest, largest, and most superficial of the anterior tendons. It begins at the junction of the lower and middle-thirds of the tibia. At the level of the ankle joint, it lies medial to the midline
and then runs toward the medial border of the foot, where it attaches to the superomedial aspect of the medial cuneiform and the first metatarsal³,⁴ (Fig. 8.3). The tendon is ovoid proximally and becomes flatter distally, measuring <5 mm in width at 3 cm from its insertion.⁵

The EHL tendon lies just lateral to the tibialis anterior tendon and inserts dorsally on the base of the distal phalanx of the hallux. The anterior tibial artery, anterior tibial vein, and deep peroneal nerve lie just lateral to EHL. In the distal leg, the deep peroneal nerve lies on the medial side of the anterior tibial artery. At the ankle, the nerve crosses over the artery to lie on its lateral side (Fig. 8.3). This arrangement is important to note during aspiration of an ankle joint effusion using an anterior approach.

The EDL tendon is the most lateral of the anterior tendons, and branches at the anterior ankle joint line into separate tendons to the second to fifth toes (Fig. 8.3). The peroneus tertius tendon crosses the ankle joint lateral to the EDL tendon to insert into the dorsal surface of the fifth metatarsal base. It is absent in about 10% of individuals.

The deep and superficial peroneal nerves arise from the common peroneal nerve at the level of the fibular neck and run distally deep to the peroneus longus muscle. At the ankle, the deep peroneal nerve runs deep to the extensor retinaculum initially medial to and then lateral to the anterior tibial artery. Just distal to the retinaculum it divides into a medial sensory branch, which supplies the first digital interspace, and a lateral motor branch, which supplies the extensor digitorum brevis muscle. It can become trapped between the extensor digitorum brevis muscle and the head of the talus.⁶ The superficial peroneal nerve separates from the deep peroneal nerve 10 to 15 cm above the ankle. It pierces the investing fascia about 9 cm proximal to the lateral malleolus, where it is prone to injury,⁷ then runs anterolateral to the lateral malleolus, superficial to the superior and inferior extensor retinacula, before dividing into medial and intermediate dorsal cutaneous nerves. It provides sensory innervation to the skin on the dorsum of the foot and most of the toes.

The anterior ankle joint is best assessed in the longitudinal plane with the ankle in slight plantar flexion. The anterior joint capsule is attached along the distal margin of the tibia and fibula approximately 10 mm from the ankle joint and extends to the neck of the talus.⁸ High-resolution imaging is needed to distinguish the capsule from the triangular-shaped fat pad that lies slightly deeper and divides the anterior recess of the ankle into smaller superior and larger inferior recesses (Fig. 8.4). A small amount of joint fluid is normal and best appreciated in the medial and lateral recesses. Larger quantities of fluid distend the anterior and posterior recesses. Increased fluid in the anterior recess displaces the fat pad anteriorly. A thin (approximately 2 mm), hypoechoic layer of articular cartilage covers the anterior aspect of the talar dome when the ankle is plantar flexed and should not be confused with joint fluid. Articular cartilage in the central and posterior ankle joint cannot be seen with ultrasound.

The anterolateral recess or gutter is bounded laterally by the fibula, medially by the talus and tibia, anteriorly by the tibiotalar joint capsule, the anterior talofibular and tibiofibular ligaments, anteroinferiorly by the calcaneofibular (CFL) ligament, and superiorly by the tibial plafond and distal tibiofibular syndesmosis.⁹ The anterolateral and anteromedial recesses should be routinely assessed on ultrasound of the ankle joint as fluid, crystal aggregates, synovial proliferation, and other pathologies may be visible, although not apparent in the anterior recess.

The lateral ankle ligaments, or lateral collateral ligaments, comprise the anterior talofibular, CFL, and posterior talofibular ligaments (Fig. 8.1).

The tendons posterior to the lateral malleolus are the peroneus brevis and peroneus longus tendons (Fig. 8.7). The peroneus longus tendon starts about 4 cm above the level of the ankle joint and peroneus brevis more distally. Both tendons run posterior and then inferior to the lateral malleolus where they share a common supramalleolar and retromalleolar synovial sheath. They lie within the fibro-osseous retromalleolar groove of the fibula covered by the superior peroneal retinaculum. The groove is lined by fibrocartilage and varies in width and depth. The superior peroneal retinaculum is about 1mm thick, 1- to 2 cm wide; extends from the posterolateral edge of the distal fibula to the lateral wall of the calcaneum, although variations of this insertion exist¹³; and is the primary restraint to peroneal tendon subluxation. The peroneus longus tendon lies posterolateral to the brevis tendon in the retromalleolar groove and takes the longer route around the inferior aspect of the malleolus (remembered by the peroneus longus takes the longer route round the malleolus or peroneus brevis lies closer to the bone). The peroneus longus is marginally (10% to 15%) larger in cross-sectional area than the peroneus brevis tendon.¹⁰

The common peroneal tendon sheath divides just proximal to the peroneal tubercle of the calcaneus, and the tendons run anteriorly in their respective tendon sheaths. Peroneus brevis runs dorsal and peroneus longus plantar to the peroneal tubercle, deep to the inferior peroneal retinaculum. The tubercle varies in size from a clearly palpable and distinct tubercle to a barely discernible one. As peroneus longus moves around the peroneal tubercle, it may become irritated when the tubercle is hypertrophied. Small amounts of fluid in the peroneal tendon sheaths are normal, particularly in the dependent inframalleolar region. The peroneus brevis tendon extends horizontally from the peroneal tubercle to insert on the base of the fifth metatarsal.

Peroneus longus passes into the sole of the foot running in a groove on the cuboid (the cuboid tunnel) before turning sharply and running obliquely across the midfoot to insert onto the undersurface of the medial cuneiform and the base of the first metatarsal. The os peroneum is a sesamoid bone located in the peroneus longus tendon just proximal to the cuboid tunnel in up to 30% of adults, bilateral in 60% of cases, bipartite in about 10%,¹⁴ and seen as a small (5 to 15 mm) hyperechoic body with posterior acoustic shadowing. The primary function of the peroneus longus tendon is plantar flexion of the medial forefoot. Along with the peroneus brevis, it also everts and stabilizes the ankle joint.

An accessory peroneus quartus muscle, present in about 10% of subjects, lies medial and posterior to the peroneus brevis and longus. Its tendon runs in the common peroneal tendon sheath and usually inserts into the retrotrochlear eminence of the calcaneus, which is often hypertrophied, just posterior to the peroneal tubercle.¹⁵ Other insertions include the peroneus longus and brevis tendons, the base of the fifth metatarsal, and the cuboid.¹⁶ Occasionally, the muscle belly inserts directly into the retrotrochlear eminence.

The sural nerve runs initially between the Achilles tendon and the peroneal tendons accompanied by the small saphenous vein, then inferior to the peroneal tendons. It bifurcates into terminal lateral and medial branches at the base of the fifth metatarsal and provides sensory supply to the lateral ankle, foot, and the small toe. It anastomoses with branches of the superficial peroneal nerve.

The spring (plantar calcaneonavicular) ligament complex is located between the calcaneus and navicular on the medial and inferoplantar aspects of the foot. It helps to support the head of the talus and, with the tibialis posterior tendon, forms a crucial support for the medial longitudinal arch¹⁹ (Figs. 8.8A, B). It has superomedial, oblique medioplantar, and inferoplantar components (also known respectively as the medial, intermediate, and lateral components). The superomedial fibers are functionally the most important, and extend from the sustentaculum tali, over the surface of talar head, to
insert onto the superomedial aspect of the navicular. There is a “gliding layer” of fibrocartilage between these fibers and the overlying posterior tibial tendon, though this gliding layer is not seen as a discrete separate structure on ultrasound imaging. There are also some connecting collagenous fibers between the posterior tibial tendon and the superomedial fibers of the spring ligament. The superomedial fibers are seen as an echogenic band just deep to the posterior tibial tendon,²⁰ best demonstrated by placing the probe with one end over the sustentaculum tali and the other over the talar head and medial aspect of the navicular bone.²⁰ The superomedial component of the spring ligament can also be seen deep to the posterior tibial tendon just proximal to the tendon insertion when the tendon is scanned transversely. Proximally, the normal thickness of the superomedial component of the spring ligament is 4 mm overlying the talar head.²¹ The medioplantar and inferoplantar components of the spring ligament are functionally less important and not consistently seen on ultrasound (Fig. 8.8B).

The tendons that run posterior to the medial malleolus are from anterior to posterior (or medial to lateral), the posterior tibial (tibialis posterior), flexor digitorum longus (FDL), and flexor hallucis longus (FHL) tendons. The neurovascular bundle, comprising the posterior tibial vessels and tibial nerve, lies between the FDL and FHL tendons (Fig. 8.10). The arrangement can be memorized by “Tom, Dick, And Harry” from anterior to posterior (Tom = Tibialis posterior, Dick = FDL, And = posterior tibial Artery, concomitant veins and tibial nerve, and Harry = FHL). In the retromalleolar region, they are covered by the flexor retinaculum to form a fibro-osseous tarsal tunnel.²² The retinaculum is a thin (<1 mm) echogenic band extending from the medial malleolus to the posterosuperior aspect of the calcaneus. The proximal tunnel is covered by a deep aponeurosis of the leg and has an osseous floor formed by the tibia and the talus. The more distal tunnel is covered by the flexor retinaculum and the fascial covering of the abductor hallucis muscle and has an osseous floor formed by the calcaneus, sustentaculum tali, and inferomedial navicular.

The tibial nerve trifurcates about 1.5 cm proximal to the tip of the medial malleolus.²³ Its first branch is the sensory medial calcaneal nerve, which arises proximal to the flexor retinaculum in 40% of subjects. When it does so, sensory deficit due to compression of the tibial nerve in the tarsal tunnel tends to spare the heel.²⁴ The tibial nerve then divides into the medial and lateral plantar nerves, deep to the flexor retinaculum and separated by the interfascicular septum. The medial plantar nerve runs with the posterior tibial tendon along the medial aspect of the foot to innervate the small muscles (abductor hallucis, flexor hallucis brevis, flexor digitorum brevis, and the first lumbrical muscles). The lateral plantar nerve descends under the midfoot to innervate the small muscles on the lateral side of the foot (abductor digiti minimi, abductor hallucis, quadratus plantae, interosseous muscles, second to fifth lumbricals).

The posterior tibial tendon is held against the posteromedial aspect of the medial malleolus by the flexor retinaculum. It courses around the inferior tip of the medial malleolus. The inframalleolar segment of the tendon runs over the deltoid ligament and the body of the calcaneum before inserting on the plantar aspects of the medial pole of the navicular, the cuneiforms, and the second to fourth metatarsal bases. Its tendon sheath extends from just distal to the musculotendinous junction to 1 to 2 cm proximal to the navicular insertion. The tendon sheath is usually dry, but a trace of fluid (<2 mm in thickness and not circumferential) is normal. The posterior tibial tendon is best examined in short axis in the supra-, retro-, and inframalleolar regions. Distally, it is best examined in long axis as this allows easier evaluation of the insertion. The posterior tibial tendon normally fans out near its insertion, leading to thickening and hypoechogenicity that may mimic a tendon tear or tendinosis. It may insert into an elongated (cornuate) medial pole
of the navicular or an accessory navicular. The normal cross-sectional area/mean width of the posterior tibial tendon are: retromalleolar <0.16 cm²/<2 mm; inframalleolar <0.18 cm²/<2.5 mm; and pre-insertional <0.27 cm²/<3.5 mm. The retromalleolar and inframalleolar segments of the posterior tibial tendon are about twice the size of the adjacent FDL tendons. The largest variability in size of the posterior tibial tendon is the most distal portion where it flares out to insert onto the undersurface of the navicular medially, the three cuneiforms, and the medial metatarsal bases.²⁵

The FDL tendon lies immediately posterolateral to the posterior tibial tendon behind the medial malleolus. It is important on transverse scanning to appreciate whether both tendons are present, as FDL can slip forward if tibialis posterior is ruptured and simulate a normal tibialis posterior tendon. In the inframalleolar region, FDL separates from the posterior tibial tendon and runs superficial to the sustentaculum tali. The normal caliber of the FDL is approximately half that of the tibialis posterior. Usually, there is no detectable fluid within the FDL tendon sheath on ultrasound.²⁶

The FHL tendon is the most posterior and deepest ankle tendon in the medial ankle. The fleshy muscle belly extends to the level of the ankle joint. The tendon originates behind the ankle joint and lies in a fibro-osseous inframalleolar groove between the medial and lateral talar tubercle. The tendon is covered by the retinaculum and may become entrapped by reactive changes affecting the posterior talar process. Distally, the tendon courses inferior to the sustentaculum tali before inserting on the base of the distal phalanx of the hallux. Due to its deep location, it is less easily appreciated than the other medial ankle tendons on ultrasound. The retromalleolar portion is best seen on sagittal scanning medial to the Achilles tendon. Passive flexion and extension of the big toe during scanning help to confirm continuity of the FHL. In the hindfoot, FHL crosses the tendon of FDL to which it is connected by a thin fibrous slip. In 20% of normal individuals, the FHL tendon sheath communicates with the ankle joint, and an ankle effusion may result in considerable fluid distension of the FHL tendon sheath.

The posterior inferior tibiofibular and posterior intermalleolar ligaments are deep, not entirely accessible by ultrasound and better assessed by MRI. The posterior tibiofibular ligament is a short ligament between the posterior tibia and fibula. The posterior intermalleolar ligament is present in half of normal subjects and extends obliquely from the superior margin of the medial malleolus to the superior margin of the fibular malleolar fossa where it attaches just proximal to the posterior talofibular ligament.²⁷

The Achilles tendon, the largest tendon in the human body, is a merger of the tendons of the triceps surae (i.e., gastrocnemius and soleus muscles) (Fig. 8.11). The gastrocnemius muscle ends in the mid-calf, soleus more distally. The Achilles tendon inserts via a broad attachment on the posterosuperior aspect of the calcaneus²⁸ (Fig. 8.12). There is a narrow band of fibrocartilage binding the tendon to the bone. The tendon is moderately echogenic with fine parallel echogenic lines on longitudinal scanning. It is generally oval-shaped with a flat or concave anterior border on transverse scanning and is covered by an incomplete tendon sheath or paratenon that envelops the dorsum and sides of the tendon. The paratenon is a thin echogenic band that outlines the moderately echogenic tendon. Anterior to the distal tendon is an echogenic fat pad known as
Kager’s fat. There are two bursae at the insertion. The retrocalcaneal bursa is a comma-shaped hypoechoic structure (2 to 3 mm in depth) between the tendon and the calcaneus. A subcutaneous bursa lies between the tendon and the skin. It is usually not distended or visible. Focal tendon calcification, fluid in the retrocalcaneal bursa, and bony contour abnormalities at the calcaneal insertion are seen in 2%, 35%, and 63% of asymptomatic subjects.²⁸ The normal Achilles tendon is 4 to 6 mm (mean 5.3 mm) wide and 5 to 6 cm long, although size is greater in taller and heavier subjects, and in men.²⁴,²⁵ Achilles tendon measurements are best made on transverse scanning as measurements made on longitudinal scanning tend to be overestimations due to tendon obliquity. Vascularity on Doppler ultrasound is normally minimal or absent.²⁸ The middle third of the tendon is almost avascular,²⁸ while the proximal and distal portions are better vascularized from the soleus muscle belly and calcaneal periosteum, respectively. The Achilles tendon receives its sensory nerve supply from the nerves innervating the adjacent muscles and from cutaneous nerves, such as the sural nerve.²⁹

The plantaris muscle, which is present in about 90% of the population, has a short (7 to 10 cm) muscle belly arising from the lateral supracondylar region of the femur superficial to the origin of the lateral head of gastrocnemius.³⁰ The long slender plantaris tendon runs inferomedially, deep to the medial belly of gastrocnemius, to the medial side of the Achilles tendon, and either fuses with the Achilles tendon or inserts into the flexor retinaculum or the calcaneus. The myotendinous junction of plantaris is in the upper calf region.³¹ The tendon is a thin ribbon-like structure between the medial head of gastrocnemius and the soleus muscle proximally, medial to the Achilles tendon distally (Fig. 8.13). It is best identified just proximal to the Achilles myotendinous junction, deep to the medial belly of gastrocnemius, as a small oval hyperechoic structure with an internal fibrillar appearance (Fig. 8.13). The whole course of the tendon may be traced, although it is more difficult to trace proximally than distally. Plantaris should be distinguished from an accessory soleus muscle and tendon. The accessory soleus muscle is a larger more fleshy structure, usually located at the anteromedial aspect of the Achilles tendon, and its tendon inserts into the medial side of the calcaneum.

Fluid distension of the posterior recess of the ankle joint is best appreciated with the patient prone and can lead to unusual configurations such as an inverted “figure of 3” shape due to fluid bulging between the posterior tibiofibular and tibiotalar ligaments.

The posterior talocalcaneal articulation is continuous with the ankle joint in about 20% of normal subjects. The posterior talar process originates from a secondary ossification center that mineralizes between 7 and 14 years and fuses with the talar body within a year to form the lateral talar tubercle. If elongated, it is called a trigonal (Stieda) process. Failure of fusion occurs in approximately 25% of subjects, and the resulting ossicle, or os-trigonum, articulates via a synchondrosis with the talus. Ultrasound cannot usually distinguish between an os-trigonum and a Stieda process as acoustic shadowing precludes assessment of any synchondrosis.

The plantar fascia is examined with the patient prone or supine. The plantar fascia is not strictly fascial but can be regarded as a common tendon aponeurosis for the superficial layer of the intrinsic plantar foot muscles. It is a thin structure that arises from the medial and lateral inferior calcaneal tubercles and attaches to the plantar plates of the metatarsophalangeal joints and the proximal phalangeal bases. It is a homogeneous echogenic band with internal linear interfaces on longitudinal scans, normally 2- to 3.6-mm thick near its calcaneal origin (Fig. 8.14), becoming thinner (1 to 2 mm) and more superficial distally. It comprises a thicker central cord and smaller medial and lateral cords that are not distinct entities but merge indistinguishably with each other. The plantar fascia is the main supporting structure for the longitudinal arches of the foot, connecting the posteroinferior calcaneus and the proximal phalanges.

The intertarsal joints, tarsometatarsal joints, metatarsophalangeal and interphalangeal joints, and their ligaments are assessed individually. The Lisfranc ligament extends obliquely between the lateral aspect of the medial cuneiform and the medial aspect of the second metatarsal base and is an important stabilizer of the tarsometatarsal joints, although it is not assessable by ultrasound. The normal joint space between the medial cuneiform and second metatarsal base is <1.0 mm.³²

The sinus tarsi is an area bounded by the talus, calcaneum, talonavicular joint, and the posterior compartment of the subtalar joint. It contains the cervical and talocalcaneal interosseous ligaments, inferior extensor retinaculum, fat, and nerve endings of the deep peroneal nerve. The interosseous ligaments cannot be seen on ultrasound as they are deep, partially obscured by bone, and surrounded by adjacent echogenic sinus fat. The sinus tarsi region can be assessed by placing the probe anteroinferior to the tip of the lateral malleolus and angling toward the anterior subtalar joint.

Small plantar digital nerves and their accompanying vessels run between the metatarsal bones. The diameter of the normal plantar digital nerve is 1 to 2 mm at the level of the intermetatarsal heads, and the nerve can be seen on high-resolution (>10 MHz) ultrasound. The intermetatarsal bursa is normally present in each interspace, dorsal to the deep transverse intermetatarsal ligament and neurovascular bundle, and just proximal to the metatarsal head.³³ The bursa may reach ≤3 mm in asymptomatic subjects.³⁴

The capsulo-ligamentous-sesamoid complex on the plantar aspect of the first metatarsophalangeal joint includes the joint capsule, collateral ligaments, fibrous plantar plate, and the flexor hallucis brevis, abductor hallucis, and adductor hallucis tendons. Flexor hallucis brevis originates on the cuboid and lateral cuneiform and splits beneath the first metatarsal into medial and lateral tendons that blend with the medial and lateral sesamoid bones just proximal to inserting into the proximal phalanx. The sesamoids are paired oval hyperechoic structures with posterior acoustic shadowing. The sesamoids are embedded in the medial and lateral tendon slips of the flexor hallucis brevis muscle and in the tendon of abductor hallucis muscle. Between the sesamoid bones runs the FHL tendon and deep to this, the thick interconnecting intersesamoid ligament. The sesamoids are bipartite in about 10% of subjects. In this situation, the summated width of the two bipartite ossicles is more than the width of the normal adjacent sesamoid allowing the distinction of bipartite sesamoid to be made.

The plantar plate is a triangular thick biconcave hyperechoic fibrocartilaginous structure on the plantar aspect of the metatarsophalangeal joints. It is firmly attached to the proximal phalanx and extends posteriorly to cover the articular cartilage on the plantar aspects of the metatarsal heads. The plantar plates are connected at the sides to the collateral ligaments of the metatarsophalangeal joints. The long and short toe flexors, invested in a synovial sheath, run just superficial to the plantar plates within a common fibrous sheath. In full-thickness tears of the plantar plate, the tendon sheath may communicate with the metatarsophalangeal joint.

The tibialis anterior, EHL, and the four slips of the EDL tendons can be identified on the dorsal aspect of the midfoot (Fig. 8.3). The most medial tendon is the tibialis anterior tendon, which tapers distally to insert on the anteromedial aspect of the first cuneiform and the base of the first metatarsal bone. The distal portion of this tendon is more medial than dorsal and may normally show a mild longitudinal separation just prior to insertion. This
should not be confused with a longitudinal split tear. The EHL tendon is thin and more easily appreciated by passive flexion and extension of the big toe. The four diverging slips of EDL insert onto the middle and distal phalanges of the toes. Peroneus tertius may be seen as an accessory fifth lateral slip of the EDL extending toward the base of the fifth metatarsal bone. The dorsal pedis artery and medial branch of the deep peroneal nerve both run between EHL and EDL distal to the ankle joint. The medial branch of the deep peroneal nerve lies just lateral to the dorsalis pedis artery.

The intertarsal, tarsometatarsal, metatarsophalangeal, and interphalangeal joints and corresponding ligaments are assessed individually. A small amount of fluid in the dorsal recess of the metatarsophalangeal or interphalangeal joints is normal. Extensor digitorum brevis and extensor hallucis brevis are the only muscles on the dorsal foot. They extend from the superolateral aspect of the calcaneus (just lateral to sinus tarsi) to the toes, the flat muscle belly lying deep to the EDL tendons.³⁴ The extensor hallucis brevis muscle is on the medial aspect of the extensor digitorum brevis muscle belly and may be indistinguishable from it. The extensor digitorum tendons insert into the lateral aspects of the EDL tendons.

The anterior tendons are the least prone of all the ankle tendons to tendinosis or tenosynovitis, and of these the tibialis anterior is the most commonly affected (Fig. 8.15). It is the strongest dorsiflexor of the ankle and also assists foot inversion. As it runs a relatively straight course within a fibrous tunnel, the mechanical demands on the tendon are low. Complete tears are rare but can result from acute injury or spontaneous rupture of a chronically degenerate tendon.³ Acute rupture usually presents in younger patients and acute-on-chronic spontaneous rupture in elderly patients, predominantly males.³⁵,³⁶ Predisposing factors include diabetes mellitus, inflammatory arthropathy, gout, and local steroid injections. The distal tendon is relatively avascular, and this, together with friction against the retinaculum, may explain why most tears occur distally. Ruptures occur at the medial cuneiform, beneath the superomedial limb of the inferior extensor retinaculum, or just distal to the superior extensor retinaculum. Ultrasound shows complete fiber discontinuity. The proximal end of the tendon usually retracts, appears moderately enlarged and hypoechoic, and is typically located deep to the superomedial limb of the inferior retinaculum. It may manifest clinically as a soft tissue mass anterior to the ankle joint. The distal end of the tendon tends not to swell or retract to the same degree and is not as well seen.

The next most common disorder of the tibialis anterior tendon is tendinosis of the distal tendon, which results in a swollen, hypoechoic tendon.⁵ Partial tears can occur on a background of tendinosis, usually located near the superomedial limb of the inferior retinaculum.³,³⁷ The tendon is focally swollen to a moderate degree and hypoechoic. Surface fraying or a discrete partial tear may be visible. Small partial tendon tears may be too small to detect, although swelling and hypoechogenicity are apparent.

Tenosynovitis, tears precipitated by large dorsal osteophytes, and cortical avulsion at the distal insertion are occasionally encountered in EHL and EDL.

The peroneus longus and brevis tendons primarily evert the foot and stabilize the ankle. After the Achilles and posterior tibial tendons, disorders of the peroneal tendons are the next most common.