7 The ankle and foot

Introduction

Non-invasive imaging of the ankle and foot can comprise a variety of approaches including plain film radiography, ultrasound, computed tomography (CT) and magnetic resonance (MR) imaging, depending on the clinician’s suspicions and the available resources. Whilst the basic imaging techniques may be useful in many circumstances, an increasing number of patients undergo MR imaging owing to the unsurpassed detail that it provides, particularly of the ligamentous complexes around the ankle.

In cases of ankle trauma, MR imaging also gives useful information regarding the bone marrow and allows assessment of the effect of the injury on joint biomechanics – altered weight-bearing can result in areas of bone marrow oedema in other areas of the mid- and forefoot that may contribute to the patient’s clinical syndrome.

For most primary contact healthcare providers, it would be a rare month that did not see at least a couple of patients attend having ‘turned’ their ankle; it is therefore important to have a good knowledge of the basic anatomical appearance of the ankle and foot on MR images and to be aware of the arrangement of the ligamentous complexes about the ankle, the intricate path of muscle tendons through the ankle and foot, and the normal and pathological appearance of the tibiofibular syndesmosis, which can help considerably with the classification of an ankle injury and its resulting prognosis and management.

It is also necessary to appreciate the importance of accurate diagnosis and prompt treatment of conditions in this area; disorders affecting this region, such as Morton’s neuroma or intermetatarsal bursitis, can have a far-reaching effect on the patient due to the disruption of lower kinematic chain mechanics.^1,² Although orthopaedic testing can provide some information as to the range of diagnostic possibilities, further imaging, particularly MR imaging, can often prove key in identifying the pain-producing structure. Although ultrasonography is being increasingly utilized, this modality can be very operator-dependent, which makes it less reliable and prone to producing false negative results.^3,⁴

History and examination

Much of what holds true for the knee also applies to the ankle and foot; this should not be surprising as they are directly connected elements in what, in standing posture, is a closed-loop kinematic chain.² Because of this, the knee, ankle and foot often need to be assessed as a unit and the reader should direct themselves to Chapter 6 in order to review this material.

Obviously, the orthopaedic tests specific to the knee are not appropriate for those of the ankle and foot; however, apart from range of motion and digital palpatory pressure, there are far fewer such tests for this region. Those that there are will be dealt with on a condition-by-condition basis later in the chapter.

Differential diagnosis

The foot is susceptible to many conditions; these can mimic each other and exist as comorbidities. Although a thorough history will help to direct the clinician’s thinking, more than any other joint, the ‘V-I-N-D-I-C-A-T-E’ mnemonic will help to avoid the missed diagnosis (Table 7.01).

Table 7.01 Differential diagnostic considerations for the foot and ankle

Vascular	Diabetes mellitus, thrombophlebitis, Raynaud’s syndrome
Infection/Inflammation	Osteomyeltis/gout, Reiter’s syndrome/ plantar fasciitis, bursitis, tendinitis, cellulitis
Neoplasm	Primary or secondary cancers
Dermatological	Complex regional pain syndrome (Sudek’s atrophy)
Iatrogenic	Drug interactions, postsurgical syndromes
Congenital	Morton’s toe, talipes
Anatomical	Accessory muscles, Morton’s neuroma, calcaneal spur
Trauma	Acute or chronic
Endocrine/Environmental	Syndromes related to footwear

Clinical indications for imaging

The clinical indications for MR imaging of the ankle and foot are varied and may also depend on the clinical protocols established by the imaging centre and the availability of resources. Based on current evidence, MR imaging of the ankle and foot is considered the imaging modality of choice for conditions such as internal derangements (for example, impingement syndromes or tarsal tunnel syndrome) that commonly affect the distal lower extremity or for postoperative evaluation.⁵^–⁷ MR imaging gives the anatomical detail needed to identify both the pathology and the effect on the surrounding structures; this is important to evaluate since it will affect both the clinical management of the patient and their prognosis.

Although more basic imaging studies can imply a disorder and, indeed, demonstrate the dynamic effects of the condition, it is the soft tissue detail that MR imaging affords that sets it apart. Not only is it possible to have 3-mm slices in the sagittal, axial and coronal planes but also it is relatively simple to angle these planes obliquely in order to analyse the course of tendons or to position the foot with different angulations to optimize the visualization of specific structures; this ability gives MR imaging unsurpassed flexibility. As with other areas of the body, the correlation of the clinical presentation and examination findings to the MR images is the skill that leads to effective diagnosis and management. Proper evaluation of the imaging sequences can also identify other areas of clinical interest: contributory causative factors, secondary pathologies or incidental findings, all of which enhance the clinician’s understanding of their patient.

An example of the benefits offered by MR imaging can be easily seen if you consider a common clinical entity such as os trigonum syndrome in which the flexor hallucis longus tendon may be compromised by the aberrant ossicle, leading to a posterior impingement syndrome. Although the os trigonum itself is demonstrated well on plain film images, the tendinous involvement can only be inferred from the partial or complete obliteration of Kager’s fat pad, a triangular region of decreased density noted posterior to the tibia and fibula and superior to the calcaneus on the lateral view in most well-exposed digital radiographic images. Just anterior to this fat pad lie various structures, which, if irritated (as is the case in os trigonum syndrome), can cause partial or complete obliteration of the anterior margin of Kager’s fat pad. This, however, is an inference; by comparison, MR images will show clearly the precise location and severity of the soft tissue involvement.

Likewise, in cases of trauma, bony and associated soft tissue lesions can be easily identified, and monitored throughout the patient’s clinical management. This is particularly so when surgery is needed, when MR imaging can be used to determine the postsurgical response and evaluate intermediate- to long-term complications, including avascular necrosis and osteochondral injuries, which may remain undetected with other imaging modalities.

Procedure and sequences

MR imaging of the ankle requires that the patient lies supine with the ankle perpendicular to the lower leg; however, if the course of a particular tendon requires analysis, the position of the ankle may need to be modified in order to avoid technical artifacts such as the magic angle effect (explained in detail in Chapter 8). A support may be used to immobilize the patient and maintain the position during the imaging study.

A surface coil is usually used to increase the signal capture; a skin marker may be added to signal to the person reading the image the region of clinical interest (Figure 7.01). The sequences usually consist of all three planes: sagittal (Figures 7.02–7.04), axial (Figures 7.05–7.07) and coronal (Figures 7.08–7.10), using a combination of T1- and T2-weighted (or STIR) sequences, depending on the clinical scenario.

Figure 7.01 • Axial T1-weighted MR image of the ankle of a 33-year-old female with pain on the medial aspect of the midfoot. A region of high signal intensity is noted on the medial aspect of the skin, just medial to the navicular. This is a skin marker to indicate the epicentre of pain or swelling noted by the patient and is useful to note when interpreting the final image.

Figure 7.02 • Sagittal T1-weighted MR image of the ankle of a 33-year-old female with hind- and midfoot pain demonstrating the fibula with the tendons of the peronei wrapping around the distal aspect. When interpreting the sagittal plane, by identifying the fibula, the laterality of the slice is more easily determined.

Figure 7.03 • Sagittal T1-weighted MR image of the ankle of a 33-year-old female with hind- and midfoot pain. This image is in the midsagittal plane and demonstrates well the bone and soft tissue detail.

Figure 7.04 • This sagittal T1-weighted MR image of the ankle of a 33-year-old female with hind- and midfoot pain is one of the most medial slices that was captured during this study; notice the position of the medial malleolus (arrow).

Figure 7.05 • Axial T1-weighted MR image of the left ankle of a 33-year-old female with hind- and midfoot pain. This slice is taken in the region of the distal tibia and fibula. Orientation to this image can be made by using the Achilles tendon (arrow) to determine the posterior, and then, opposing this, the anterior aspects. The fibula, with the peronei tendons wrapping around it (dashed arrow), help determine the lateral aspect of the ankle and, therefore, by elimination, the medial aspect.

Figure 7.06 • MR imaging of the ankle of a 33-year-old female with hind- and midfoot pain. This axial T1-weighted image is slightly more inferior than in the previous figure. It is useful to start from superior and to work inferiorly to retain an orientation to the image. Note the most distal tip of the lateral malleolus (dashed arrow) and the sickle-shaped Achilles tendon (arrow).

Figure 7.07 • Axial T1-weighted MR image of the ankle of a 33-year-old female with hind- and midfoot pain. This is one of the more distal slices; note the insertion of the Achilles tendon on the calcaneus.

Figure 7.08 • MR imaging of the ankle of a 33-year-old female with hind- and midfoot pain. This is one of the most posterior slices in the coronal plane. On this T1-weighted image, one’s orientation is based around the fibula, a lateral structure (arrow); once found, the remaining structures are more easily identified.

Figure 7.09 • MR imaging of the ankle of a 33-year-old female with hind- and midfoot pain. This image is also in the coronal plane, slightly more anterior than the previous slice. Note the changing form of the bony structures, and the appearance of the sustentaculum tali.

Figure 7.10 • MR imaging of the ankle of a 33-year-old female with hind- and midfoot pain. This is the most anterior slice of a coronal T1-weighted series. It is useful to start with the most posterior slice and then work forward to remain orientated to the image. This slice is performed in the posterior aspect of the midfoot.

Contrast is used where a mass or infection is suspected, in order to determine the character and behaviour of the lesion and therefore aid the diagnosis; if this is the case, a T1-weighted, fat-suppressed image is used with the addition of gadolinium. Contrast can be given to the patient either via intravenous injection or directly into the articulation. Intra-articular injection is used to better visualize the state of the articular surface of the tibiotalar joint and to determine more completely the presence of any intra-articular body, which may be difficult to confirm on regular MR studies. Each contrasted slice is 4 mm in thickness, usually with an overlap of 0.5 mm.

MR imaging of the foot is ordinarily performed with the patient in the prone position. Although this position may be difficult for some patients, it allows for the toes to be maintained in the anatomically neutral position, and for the patient to be as immobile as possible during the imaging study, therefore limiting motion artifact, which can degrade the final image quality and therefore reduce the diagnostic quality. The sequences are the same as in the ankle; however, the sagittal image is performed along the long axis of the metatarsals, and may be referred to as the long axis; the axial slice perpendicular to the second or third metatarsal long axis and which may be referred to as the short axis; and the coronal image in a plane connecting the second and fifth metatarsals. These slight variations are useful to know in order to better orientate the anatomical structures when interpreting the final MR image.

Normal imaging appearance

Although there are a variety of methods to evaluate imaging of the ankle and foot, it is useful to approach the interpretation by identifying individual anatomic structures; from the first, it is important to be aware that there can be a wide variation in appearances, further complicated by the number of potential anomalies. These can be divided into those comprising the soft tissues, including tendons, ligaments and synovial capsules, and fascia; neural; and osseous structures. Our emphasis will be placed primarily on those structures that are commonly implicated in the development of clinical syndromes.

In order to help identify the different structures, the ankle and foot may be divided anatomically into four sections – anterior, posterior, medial and lateral; this helps with both orientation and systematic evaluation.

The posterior tendons include the Achilles (calcaneal) and plantaris, both of which have associated bursae (Figure 7.11). The Achilles tendon has a broad insertion along the posterior aspect of the calcaneus, which is often irregular in this region due to the development of degenerative changes associated with soliciting the tendon during the activities of daily living. The tendon is best seen on the sagittal and axial cuts. Normally, on the sagittal imaging, it is noted as a long, relatively thick, low signal intensity structure that courses from the intermediate signal of the gastrocnemius muscle to its insertion on the calcaneus. It thickens somewhat just above this insertion; this should not be interpreted as a focal tendinosis. The tendon should be evaluated on both the T1- and T2-weighted images; on both sequences, it should be seen as a low signal intensity structure.

Figure 7.11 • Axial T1-weighted MR image of a patient with ankle discomfort. The posterior tendon structures can be identified (circle) with the most posterior structure representing the Achilles tendon with its low signal intensity. The tendon may demonstrate a small focus of increased signal in its centre, though this is not visible in this patient. Just medial to the Achilles tendon, located anteriorly, is a region of grey, intermediate signal; this represents the soleus muscle.

On axial images, the normal Achilles tendon may contain small regions of slightly brighter signal within it. These are seen on both the T1- and T2-weighted sequences and are due to the internal architecture of the tendons, representing areas where the fibres are gathered together in small pockets. This appearance is normal and should not be confused with the small foci of high signal arising from small micro-tears in the tendon representing degenerative changes or overuse. These are only seen on the T2-weighted images, will be visible on all planes, and suggest a diagnosis of tendinosis. This makes it important to verify the tendon in at least two planes in order to differentiate the normal appearance of the tendon from pathology; correlation with clinical presentation can also help the clinician considerably in this aspect.

Medial

Medially, the flexor tendons descend under the flexor retinaculum; these comprise the tibialis posterior, flexor digitorum longus and flexor hallucis longus (mnemonically referred to as ‘Tom, Dick and Harry’). The tibialis posterior tendon is prone to rupture, though not as frequently as that of the Achilles tendon.⁸

As the tendon descends to its insertion on the navicular and metatarsals, the shape of the tendon changes on the axial slices from an oval structure to a flatter configuration as it courses past the medial aspect of the talus. As the tendon continues to descend, it divides into smaller bundles as it approaches the navicular and metatarsals.⁹ The appearance of this normal fibrous division may be confused with a distal tear; however, tears in the insertions of the tendon are rare and the normal appearance can be confirmed by the lack of high-signal oedema on the fluid-sensitive sequences (Figure 7.12).

Figure 7.12 • Axial T1-weighted MR image demonstrating the medial soft tissue structures. The hatched oval represents the tibialis posterior tendon located anteriorly, with the flexor digitorum longus tendon lying more posteriorly (hint – use the Achilles tendon to orientate to the image). The arrow points to a small, intermediate signal intensity structure, which represents the posterior tibial vein. Directly posterior to this is an oval within which are located two small round structures, the most medial of these is the posterior tibial artery; the tibial nerve lies laterally. The small circle surrounding the low signal intensity structure represents the flexor hallucis longus.

The flexor digitorum longus tendon runs alongside and superficial to that of the tibialis posterior. It is rarely injured in isolation and is more commonly associated with a generalized regional tendinopathy. The flexor hallucis longus tendon has a rather unusual course that can render it susceptible to injury. As it passes behind the talus, in those patients who have an os trigonum, friction may be created between the tendon and the accessory ossicle; this can lead to symptoms of posterior impingement, including discomfort and pain on plantar flexion and perhaps focal swelling palpable behind the ankle joint. In the same region, the tendon changes its course from being a predominantly vertical to a more horizontal orientation; during this transition, it can be more prone to injury. The tendon of the flexor hallucis longus is well depicted in its proximal course in both the sagittal and axial planes; as it turns to become more horizontal, it is initially well visualized on the axial and then the sagittal/long axis images as it approaches its insertion.¹⁰

Lateral

The peroneus longus and brevis tendons are located laterally (Figure 7.13). Both these muscles share the action of eversion and dorsiflexion of the foot.¹¹ As both tendons descend, they are located posterolaterally to the lateral malleolus. As the tendons pass this region, they are stabilized in part by the superior peroneal retinaculum, which, depending on the imaging quality, can be identified. At this level, the tendon of the peroneus longus is seen as a rounded structure in the axial plane; this helps distinguish it from the flatter appearance of the peroneus brevis tendon. As it passes further distally, the peroneus brevis is located anterior to the peroneal tubercle of the calcaneus. Both tendons may be surrounded by a thin layer of fluid on the fluid-sensitive sequences, which is normal. Although there is no formal measurement for the quantification of fluid, more than just a thin layer may be considered as tenosynovitis.

Figure 7.13 • Axial T1-weighted MR sequence of the ankle demonstrating two small low signal intensity structures enclosed within a white oval. These structures represent the peronei tendons. The most lateral structure (hint – note the lateral malleolus) represents the peroneus longus tendon and the more medial structure, the peroneus brevis tendon. Note their intimate association with the posterior aspect of the lateral malleolus.

The tendons in this region comprise the tibialis anterior, which lies most medially, then the extensor hallucis longus, extensor digitorum longus and peroneus tertius (Figure 7.14).¹² The tendons pass inferiorly to the superior and inferior divisions of the extensor retinaculum, which may be identifiable on MR imaging. These structures are only occasionally affected by pathology; of the anterior tendons, the tibialis anterior tendon is most frequently involved. In particular, the tibialis anterior tendon course is straight with limited opportunity to be impinged by surrounding structures, unlike the posterior compartment structures. The appearance of the tendon on axial slices is that of a round or oval structure, which, as it approaches the insertion points on the first metatarsal and medial cuneiform, becomes flatter. The normal thickness of the tendon within 3 cm of these insertion points is that of no more than 5 mm.¹³ Injuries are usually either of a direct nature, which may occur at any age and which may be associated with fractures of soft tissue lesions, or as a result of degenerative processes, in which case the patient is often 60 to 70 years old with chronic symptoms.¹⁴