Tendons

Posterior

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.

Anterior

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

Figure 7.14 • (A) Axial T1-weighted MR image demonstrating the anterior tendon structures. Using the lateral malleolus and Achilles tendon to orientate to the ankle, note that there are a number of low signal intensity structures lying anterior to the tibia and fibula. From medial to lateral, these represent the tibialis posterior (1), the extensor hallucis longus (2), the extensor digitorum longus (3) and the position of the peroneus tertius (4). (B) Axial T2-weighted sequence demonstrating laterally the anterior and posterior tibiofibular ligaments (white arrows). Notice the bright signal in the anteromedial aspect of the distal tibia due to recent trauma, sustained during a rugby match.

Ligaments

The ankle and foot ligaments may also be well visualized using MR imaging; this modality is particularly useful in evaluation of complex or refractory ankle sprains or in those patients who solicit their ankles regularly.¹⁵ However, for routine ankle sprains, clinical examination alone, augmented if necessary by diagnostic ultrasound, is usually sufficient. The ligaments most frequently involved in clinical syndromes are the medial ligament complex (deltoid ligament) and lateral ligament complex. These ligaments are best evaluated using axial MR images, although the sagittal and coronal planes complement these sequences. Generally, ligaments are thin, low signal intensity structures; their identification is relatively straightforward if their anatomical relations are familiar to the examiner.

The ankle ligaments can be divided in to three regions: the lateral ligamentous complex, which is the most prone to injury; the medial group, consisting of the deltoid complex; and the central syndesmosis, or tibiofibular complex, which is important for retaining the stability of the ankle and which may be injured in high-grade ankle traumas, resulting in instability of the ankle.¹⁶ Often, there is a predominant group of ligaments involved with a secondary or tertiary group affected by means of the ‘contre coup’ mechanism, so, as the patient rolls over the ankle into inversion, often, a rapid eversion will occur following this, thereby resulting in lateral and medial involvement. The physical examination and diagnostic imaging should take this into consideration.

Osseous variants and paediatric considerations

A number of common variations in the osseous and soft tissue structures may be encountered about the foot and ankle. It is important for the clinician to be familiar with these and their clinical significance.

A frequent cause of foot and ankle pain is the development of accessory ossicles. In the ankle, the two most common accessory ossicles are the os trigonum and the accessory navicular (or os tibiale externum) bone (Figure 7.15). An os trigonum will be found immediately posterior to the talus and is not usually palpable; therefore, its existence can only be determined by diagnostic imaging. It arises from the failure to fuse of an accessory ossification centre located just posterior to the lateral talar tubercle ossification centre. Normally, this will occur at around 13 years of age; however, in about 10% of the population it remains separated and forms a separate bone (Figure 7.15). In some patients, a pseudoarticulation with the talus may occur.

Figure 7.15 • MR imaging of the ankle in the axial plane in T1-weighted (A) and STIR (B) sequences, and in the coronal plane in STIR (C), in a 33-year-old female with pain along the medial aspect of her midfoot, which had begun after wearing new shoes. The ovals enclose the region of abnormality, demonstrating a large navicular with an accessory navicular just medial and posterior to the navicular bone. In (B), a homogeneous region of increased signal is noted in the navicular and accessory navicular, which is also seen in (C). This is due to oedema created by the altered mechanics in this region, and the irritation created by the presence of the accessory navicular bone. By comparison, (D) and (E) represent sagittal slices using T1-weighted sequences and show a separate, marrow-filled structure posterior to the talus, which represents an os trigonum (white circle).

Although many patients with an os trigonum will be asymptomatic, some will complain of pain in the posterior aspect of the ankle that is worsened by plantar flexion, thus those who dance, run downhill or play football are particularly affected, although even activities of general daily living may be enough to provoke the development of pain. Pain may develop with onset of the condition or remain dormant until later life when altered regional biomechanics create stress between the bony structure and the surrounding soft tissues, most significantly the adjacent tendon of the flexor hallucis longus muscle; this is termed os trigonum syndrome or posterior impingement syndrome.^17,18

Posterior impingement syndrome may also develop as a result of a large talar process (also known as the Stieda process), located on the posterolateral surface of the talus. Both an os trigonum and a long talar process may be detected on routine radiography, particularly on the lateral view. The inflammatory process itself may also be suggested by the obliteration of the posterior margin of Kager’s fat pad; however, MR imaging is easily superior in the depiction of the precise location of the bony structure and its effect on the surrounding structures. This allows for the specific grading of any lesion located in the flexor hallucis longus and, with this, the prognosis of the patient. Although MR imaging in any of the three main planes will assist in the precise anatomic location of the bony structure, the sagittal sequence is particularly helpful as it allows for determination of the relative positions of both the osseous structures and the flexor hallucis longus tendon.

In the case of an accessory navicular, the patient may complain of discomfort and pain on the medial side of the midfoot and exhibit swelling in the region. With larger accessory navicular bones, the patient may even remark on the presence of a hard mass on the inside of the midfoot, which may cause pressure on the inside of their shoes. On radiographs, a prominent or accessory navicular may be demonstrated just medial to the parent navicular bone. Although an accessory navicular is considered by some to be a post-traumatic lesion – an avulsive fracture of the cartilaginous growth plate – the fact that it is so frequently found bilaterally suggests otherwise.¹⁹

With MR imaging, the relationship of the accessory bone with the surrounding soft tissue structures can again be appreciated to a fuller extent; in particular, the posterior tibialis tendon is particularly vulnerable to tears in the presence of an accessory navicular. Other pathologies that may arise include the development of a painful bursa, or the development of bone marrow oedema in either the accessory navicular or parent navicular bone.²⁰

Although diagnostic ultrasound can also be used to detect the dynamic relationship of the soft tissue structures, the detail that is achieved with MR imaging provides a higher degree of accuracy and a better overview of the region in question; musculoskeletal ultrasound is limited in the depth to which it can penetrate and cannot readily demonstrate conditions such as bone marrow oedema that can add considerably to the clinician’s clinical thinking.

Muscular variations

The most common and clinically significant muscular variant is the presence of a fourth peroneus muscle, peroneus quartus. Although small, this supplementary muscle can have clinical consequences (Figure 7.16).

Figure 7.16 • Axial T1-weighted MR sequence of a 57-year-old male who complained of discomfort in the posterior aspect of his ankle; the region outlined by the white oval contains three low signal intensity structures. The lateral two, indicated by the white arrows, represent the tendons of peroneus longus (left arrow) and brevis; however, there is a third region of low signal intensity noted, marked by a black arrow. This represents the peroneus quartus, which may on occasion be associated with peroneus brevis tears, owing to the increased pressure on the peroneal retinaculum, which is stretched around the tendons.

Osseous conditions

Trauma

Trauma to the ankle and foot constitutes a major proportion of injuries to the musculoskeletal system as a whole; which of us has not, at some time, turned our ankle or stubbed our toe, even if the consequences are usually no more than a few days’ discomfort? Although radiography is useful in identifying the basic injury in many cases, further imaging may be necessary if symptoms persist or complications develop or, in the case of an athlete, to determine exactly the degree of injury sustained.¹⁴ MR imaging is ideal to assess such injuries, being able to determine the osseous and soft tissue relationships, aiding in the development of clinical management strategies and in establishing the patient’s prognosis.

Although a slightly simplistic approach, it is helpful to divide foot and ankle trauma into acute and chronic presentations (some patients will of course present with an acute clinical syndrome following recent trauma on top of a pre-existing chronic injury, as in the case of a new ankle sprain in a patient who has had a previous clinical history of multiple sprains).

Acute trauma

Acute trauma represents a common presenting complaint, such as that relating to a sporting activity whereby an acute twisting injury results in impaction between the osseous structures (particularly of the ankle) and tears of the soft tissue structures surrounding the region. In such cases, regions of high signal intensity within the osseous structures may be detected soon after the injury; this represents bone marrow oedema. The pattern of this oedema may assist in determining the direction of injury and therefore help to identify the associated soft tissue structures that may be implicated.¹⁴

A common complication of such an injury is fragmentation of a piece of articular cartilage, which may be attached to the underlying bone, forming a chondral or osteochondral defect respectively. The talar dome is a particularly common site for such injuries to occur. In addition to a history of recent ankle trauma, the patient may also report a sensation of the articulation being ‘blocked’ during normal activity or, at times, sudden immobility or ‘locking’ following simple movement. The patient may also notice that the joint becomes intermittently swollen; the physical examination will often confirm this through visual observation or a related reduction in the joint’s range of motion.^21,22

Whilst routine radiography is often used in the first instance to assess the osseous and articular aspects of the ankle, avascular necrosis in its early stages is usually negative on these studies. MR imaging allows identification of such injuries at a much earlier stage and can give much more detail regarding the level of the articular cartilage damage, the extent of displacement of the fragment and the viability of the donor site. Osteochondral defects are particularly well demonstrated using MR imaging and intra-articular contrast is not normally required (Figure 7.17).¹⁵

Figure 7.17 • MR images of a 57-year-old male suffering with ankle pain and a sensation of the joint catching on occasion. T1-weighted (A) and STIR (B) sequences in the sagittal plane demonstrate an abnormality of the dome of the talus. In (A), a region of low signal is noted involving the majority of the articular surface of the talus. This is well visualized as a region of high signal intensity on the STIR sequence, where, in addition, irregularity of the articular surface of the talus is noted.

This makes the acquisition of images non-invasive, as compared with computed tomography (CT), wherein iodinated contrast is normally necessary to evaluate the articular cartilage, carrying a risk of complications and potential discomfort for a patient who is already in pain. MR imaging is also superior in providing an overall view of the articulation, identifying other factors that may affect the clinical management of the patient, such as bone marrow oedema or injury to surrounding soft tissue structures.²³

If there is less than 10 mm of fragment displacement and no rotation of the fragment with respect to the donor site, treatment may simply involve partial or complete immobilization; however, surgery may be required to return the fragment to the donor site if there is significant displacement or an awkward positioning of the fragment. If there is an associated chondral injury (for example fibrillation), grafting, autologous or otherwise, may be considered.^24,25

On MR imaging, the view that is particularly useful in determining the presence of an intra-articular fragment and assessing cartilage damage is the STIR or T2-weighted coronal images; the sagittal and axial images will provide the dimensions of the donor site and, therefore, the extent of the lesion. MR imaging can also provide further information regarding associated injuries. These lesions are often associated with ankle sprains; consequently, a careful evaluation of the ligaments about the ankle is an important part of the evaluation routine.^17,26

Chronic trauma

Chronic trauma may also be well visualized on MR imaging and may result from repeated injuries such as those sustained in consecutive ankle sprains, or be related to abnormal biomechanics. This may be intrinsic (congenital, developmental or acquired) or extrinsic, for example from ill-fitting footwear (Figure 7.18).

Figure 7.18 • A 33-year-old female presenting with pain in the ankle and foot following surgery for ligamentous tears about the ankle (the low signal intensity regions in the distal tibia are bony screws). MR imaging of the ankle and midfoot was performed in the coronal plane (A) and sagittal plane (B, C); these demonstrate that, on the T2-weighted images of (A) and (B), regions of diffuse heterogeneous increased signal intensity are noted in the talus, navicular and cuneiformis. This represents bone marrow oedema, which, on a T1-weighted image (C), is visible as a heterogeneous decrease in signal intensity. The patient is thought to have developed the regions of bone marrow oedema secondary to the altered mechanical stress placed on the ankle and foot following surgery. No associated soft tissue swelling was noted.

An example of a clinical syndrome related to inappropriate shoes is Haglund’s syndrome (or deformity), whereby wearing high heels or poorly fitting ‘basketball shoes’ causes chronic irritation to the posterior aspect of the ankle. The patient, often a woman, will complain of pain and swelling around the posterosuperior aspect of the calcaneus and demonstrate a noticeable lump in the same region. The condition is often complicated by retrocalcaneal bursitis, retro-Achilles bursitis and thickening of the distal Achilles tendon (Figure 7.19).^27,28

Figure 7.19 • A 40-year-old female presented with discomfort about the posterior aspect of her ankle, which was worsened when using high heels. Sagittal STIR imaging of the ankle demonstrates a retrocalcaneal (oval) and retro-Achilles (arrow) bursitis. The Achilles tendon is thickened at its most distal aspect due to chronic irritation. This imaging triad of bursitis in the retrocalcaneal and retro-Achilles bursa and thickening of the Achilles tendon has been referred to as Haglund’s deformity.

Although a number of treatment options are available, first and foremost to correct the precipitating cause, surgery is often required to modify the surrounding bony structures and to reduce the thickening of the tendon. Similar effects may occur in the other bones of the ankle and of the foot, either due to poor-fitting footwear or in those who place a high demand on their feet, such as long-distance runners.^29,30

Such patients are also at risk of developing stress fractures, which usually present with diffuse pain that is difficult to localize. Eventually, particularly if the precipitating activity is not curtailed, the pain will focus over the affected region, most commonly the diaphysis of the second or third metatarsal. Although routine radiography may demonstrate a local periosteal response and a bone scan may demonstrate focal increased uptake corresponding to a local increase in bone metabolic activity, the MR images will be able to grade the stress fracture from a simple local periostitis to a complete, though often radiographically subtle, fracture.³

The T2-weighted images will demonstrate oedema in the periosteum, perhaps in the medullary cavity and in the surrounding periosteal tissues, whilst the T1-weighted sequences can be used to determine the degree of cortical thickening due to the periostitis and the actual fracture line. Clinical management includes reducing or eliminating the causative activity, modifying footwear and, if necessary, the use of an air cast. Further evaluation of underlying bone density issues is important to eliminate osteopenia, which can cause an insufficiency fracture.

Ligaments

The most commonly injured ligaments in the region are the lateral ligaments of the ankle; often, the patient will have a history of repetitively turning over on their ankle with the development of pain, swelling and perhaps discoloration about the anterolateral aspect of the ankle. Although ankle sprains may be the result of the application of altered stress to the ankle, conditions such as tarsal coalition may predispose the patient to repetitive sprains. Coalition should be evaluated during the clinical examination, where a stiff midfoot will be detected, perhaps associated with a pes planus deformity.

Radiographs are useful in identifying coalition, which tends to occur between the calcaneus and navicular or calcaneus and talus. Although MR imaging is not necessarily systematically used in those patients with coalition, if the patient has a clinical history of repeated ankle sprains and pain or limitation of activity, MR imaging can be useful to determine the presence of a fibrous synchondrosis, and associated bone marrow oedema that may accompany this, as well as for the evaluation of the effects of the coalition, in determining the most appropriate management of the patient.^34,35

In cases of a partial or complete rupture, this ligament is most easily assessed using the axial images. By placing the patient’s foot in slight dorsiflexion, the whole ligament will be included in a single slice, making the interpretation easier. If this is not possible, it is important to trace the ligament in its entirety as it courses obliquely from its origin on the medial aspect of the lateral malleolus to its insertion on the lateral aspect of the talus. If this ligament is torn, either partial or complete disruption of the fibres will be noted on MR images; in addition, on the fluid-sensitive sequences, the high-signal synovial fluid will be seen tracking out anterior to the normal position of the ligament, indicating that the synovial capsule has been broached.^34,35

In patients with a history of repeated sprains, the ligament may appear irregular and thickened with small foci of increased signal within it on the fluid-sensitive sequences; this is due to chronic inflammatory changes. The ATAF ligament may be injured as an isolated phenomenon, or be a component of more complex sprains. Depending on anatomical factors and the position of the ankle during the injury event, the calcaneofibular and, to a lesser extent, the PTAF may also be involved.

The calcaneofibular ligament is also often involved clinically; again, fluid may be noted surrounding the ligament in cases of partial or complete rupture, which helps to confirm the presence and severity of injury.

When injured, the posterior tibiotalar ligament may be associated with a small amount of fluid, which extends to surround the descending tibialis posterior fibres, suggesting a tendinopathy of this tendon, when in fact it is just the result of the inflammation associated with the damaged tibiotalar ligament.

Although the PTIF ligament is rarely injured in isolation, when damaged, as may be the case following a severe trauma to the ankle, which may be accompanied by fractures affecting the ankle complex, loss of the normally well-defined low signal intensity linear ligament as well as noting surrounding high signal owing to the leakage of the synovial fluid is useful to determine injury.^10,36

Both the Lisfranc and Chopart ligaments may be involved in disruption following a direct or indirect trauma and may be associated with fracture/dislocations of the articulations contributing to the region in question. This is the case particularly with patients affected by diabetes where the Lisfranc fracture/dislocation injury may occur, causing either an adduction or abduction injury as well as dorsal subluxation of the base of the second metatarsal.^37,38

Tendons

Tears of the tendons about the ankle and foot may also be evaluated well with MR imaging, especially when associated bony injuries and additional soft tissue structures need to be evaluated. If small foci of high signal are noted within tendons on T2-weighted images, a diagnosis of tendinosis should be considered. This appearance is caused by small micro-tears in the tendon, arising from degenerative changes or overuse. Partial or complete ruptures of the tendon may also occur; however, owing to the relative ease of clinical differentiation and the importance of rapid surgical intervention, MR imaging is not routinely performed. In cases where the diagnosis is unclear, where the presence of a partial, high-grade rupture is being considered, MR imaging will demonstrate the tendon as being irregular and frayed, with small or moderate regions of high signal on the T2-weighted sequence.⁵

In the Achilles tendon, which is particularly prone to rupture, these regions are usually found 4–6 cm proximal to the insertion of the tendon into the calcaneus, thought to be a less vascular region, thus rendering the tendon more susceptible to injury (Figure 7.20). They need to be differentiated from the small regions of slightly brighter signal arising from the internal architecture of the tendons, which are seen on both the T1- and T2-weighted sequences rather than the fluid-sensitive sequences alone.

Figure 7.20 • A 51-year-old male with sudden pain in the back of his calf and ankle underwent T1-weighted imaging in the sagittal plane (A) and T2-weighted imaging in both the sagittal (B) and axial (C) planes. Compared to normal, when the Achilles tendon is noted as a low signal intensity structure that gradually blends proximally to become the muscle belly, in these sequences there is discontinuity of the tendon [oval in (A)] and a moderate region of increased signal intensity (B, C) in the location of the normal tendon due to a complete Achilles tendon tear. The retracted fibres of the Achilles tendon can also be well seen particularly in (B) (oval) and (C).

The clinical presentation of a patient with an Achilles tendinosis will differ from that of a complete rupture of the tendon. With a patient who has tendinosis, there is, in general, local pain that worsens with activity such as running or an activity that will solicit the use of the tendon. In a patient who has a complete rupture of the tendon, the predominant complaint of the patient is a total inability to move the ankle, due to the loss of the normal function of the tendon. As a result of this, the patient will not necessarily have pain as the major symptom. In a complete rupture, MR imaging is not necessarily the technique of choice to confirm the diagnosis, the clinical history and examination usually suffice for this purpose, whereas in a tendinosis, which may be associated with micro- or small macro-tears, further imaging may be useful to determine the extent of the tear. In this case, ultrasound may be useful but not as informative as MR imaging to determine the local effect of the problem.

The tibialis posterior tendon is also prone to rupture, though not nearly as frequently as that of the Achilles tendon. During its course, proximal to its passage past the medial malleolus, the tendon can become irritated with a tendinopathy being created. This can be observed on the fluid-sensitive sequences as bright or high signal in the region of the tendon, which is normally found positioned deeply to its companion tendons, flexor digitorum longus and flexor hallucis longus.

As the descending tendon passes the navicular, normal variants may occur; these may, on occasion, cause irritation and tendinopathy. Such normal variants can include a large accessory navicular bone, located medial to the parent navicular. In certain populations, such as dancers and athletes, such large accessory navicular bones can produce a friction effect with the posterior tibialis tendon and also create irritation, and an inflammatory reaction with the parent bone via the partially formed synchondrosis.^39,40

The posterior tibial artery, nerve and vein are located between flexor digitorum longus and flexor hallucis longus (Figure 7.12). Occasionally, vascular lesions such as varices can be detected on MR imaging, usually after the clinical examination. MR imaging would not be the imaging modality of choice to detect the varicosities, being detectable following the clinical examination with ultrasound; however, on occasion, large tortuous varicosities may irritate surrounding soft tissue structures such as the tendons and ligaments about the ankle, leading to the development of symptoms of pain, swelling and limitation of movement about the ankle joint.

The peronei can also develop tendinoses: the peroneus longus tendon is more prone to tendinopathy; the peroneus brevis tendon is more prone to acute tears, either partial or complete. Such tears may be associated with bony lesions, such as the Dancer’s or Jones’ fracture, an avulsion fracture located at the styloid process of the base of the fifth metatarsal. With such an injury, the patient will normally have focal pain, swelling and a sense of lack of stability about the lateral aspect of the midfoot (it is important to determine whether this sensation is from the ankle or, as in this case, more distally located).^31,41

Infection

In predisposed populations, such as those with diabetes mellitus, infection of the distal lower extremity is relatively common and carries with it high morbidity and mortality; therefore, astute diagnosis and management of the patient is essential. Despite other techniques being available for the detection of infection, MR imaging remains the most cost-effective and accurate examination modality in differentiating osteomyelitis from soft tissue infection and visualizing associated complications such as abscess formation (Figure 7.21).⁵

Figure 7.21 • A 79-year-old female presented with pain and swelling in her first ray. Sagittal T1-weighted (A) and STIR (B) images demonstrate a region of decreased signal intensity within the bone in (A), with an associated mass in the soft tissue surrounding the bone both superior and inferiorly. On the STIR sequence (B), the lesion is well identified in the proximal phalanx, with increased signal intensity throughout, destruction of the cortices and extension in to the surrounding soft tissues; this is an example of osteomyelitis, in this case secondary to known diabetes mellitus.

Whilst the patient may not be fully aware of the presence or extent of infection, examination may reveal a region of focal swelling or redness, accompanied by an alteration in the general health status of the patient. Depending on the degree of sensory loss, pain and tenderness will be variable features. Close inspection of the plantar surface is particularly important, to search for portals of entry for infection; in some cases, evidence of the sinus tract will also be identifiable. Extra care is needed when palpating the foot of a diabetic, owing to the increased friability of the skin.

On routine imaging, osteomyelitis may not become visible for up to 10 days post-implantation, the radiographic latent period. However, it is clearly important to make the diagnosis as early as possible to instigate the appropriate management protocols, namely antibiotics and control of associated comorbidity; therefore, where clinical suspicion exists, further imaging is appropriate. This may involve a bone scan but, to best determine the presence of osteomyelitis and the effect that it has on the surrounding osseous and soft tissue architecture, MR imaging is the modality of choice, using a normal series with, if possible, the addition of gadolinium to detect subtle foci of active infection that may otherwise remain occult.⁴²

Within the bone marrow, osteomyelitis will provoke a high signal intensity on the fluid-sensitive sequences; by contrast, the signal on the T1-weighted sequences will appear as hypointense compared to the normal marrow. Regions of osseous destruction will be well defined on these images. Any involvement of the surrounding soft tissue can also be demonstrated well with MR imaging: the development of a periperiosteal inflammatory response, abscess formation and eventually sinus tract development can all be evaluated, as can the response to medical management. Where surgical debridement has been employed, the results can be monitored non-invasively to confirm resolution of the osteomyelitis and the development of possible complications. This is particularly useful for patients such as diabetics, who may need to be evaluated over many years, and allows comparisons to be made with previous studies, which helps to determine subtle changes suggestive of new infective foci.^43,44

Morton’s neuroma

Most commonly occurring in between the third and fourth rays, Morton’s neuroma is an enlargement of a nerve, usually at the point where the lateral plantar nerve combines with part of the medial plantar nerve. The nerve lies in subcutaneous tissue, just above the fat pad of the foot, close to an artery and vein and superficial to the deep transverse metatarsal ligament.^9,16 In patients predisposed by pes planus or by unsuitable footwear, the nerve is compressed between the contact point of a shoe and the strong, relatively inflexible ligament.^42,45

The condition, which is not a true neuroma and is more accurately termed Morton’s metatarsalgia, needs to be differentiated from intermetatarsal bursitis (Figure 7.22). It can be extremely disabling, not just causing local, sometimes exquisite, pain but also altering lower limb biomechanics, limiting the choice of footwear and activities of daily living, such as shopping. Often, the patients are middle-aged women who have progressively developed pain in the second and third metatarsal spaces, either unilaterally or, at times, bilaterally.⁴⁶

Figure 7.22 • Axial T2-weighted (A) and T1-weighted with contrast (B) MR images of the foot, demonstrating a region of high signal intensity about the inferior aspect of the intermetatarsal space between the second and third metatarsal. This region exhibits a moderate degree of enhancement with the addition of contrast (B), due to the presence of an intermetatarsal bursitis, which, clinically, can easily be confused with Morton’s neuroma.

Although observation of the region does not usually reveal a great deal, palpation will often recreate the exquisite pain associated with the conditions. The standard orthopaedic test is the squeeze test, whereby pressure is applied in a superficial to deep direction from the first and fifth metatarsals, thereby compressing the inter-metatarsal structures and recreating pain in patients with either pathology.⁴⁷

Routine radiography will normally provide no useful information. Ultrasonography, in the hands of an experienced operator, can be extremely helpful both in diagnosis and for guiding hydrocortisone injection into the site; however, despite the requirement in this instance for contrast, MR imaging is still widely used for evaluation of this region, partly because it is easier to interpret but also because it offers better scope for differentiating Morton’s neuroma from soft tissue tumours. The lesion will be intermediate signal on T1-weighted imaging and low signal on T2-weighted imaging. If contrast is used, the lesion is well demonstrated with intense uptake; contrast can also help with patients who redevelop symptoms after surgical resection in differentiating the development of a new neuroma from scar tissue.^42,45

Plantar fasciitis

A potentially disabling condition, plantar fasciitis is commonest in the middle-aged patient, who typically reports the development of progressive discomfort and then pain along the proximal aspect of the plantar surface of their hind- to midfoot. On observation, there is not usually evidence of swelling, but on direct palpation over the origin of the short plantar muscles as well as of the fascia, exquisite pain may be elicited.

Whilst MR imaging is not warranted for simple cases of plantar fasciitis, for those patients with persistent pain, or where surgery is being considered, MR imaging may provide additional clinical information to determine the origin of the pain and identify which anatomical structures are involved (Figure 7.23).⁴⁸ The plantar fascia and involved muscles are well visualized on the sagittal, fluid-sensitive images, which will demonstrate regions of increased signal in and about the tendinous insertions into the calcaneus as well as thickening of the plantar fascia to more than 5 mm in thickness. MR imaging is important in those patients who are resistant to conservative management, to determine the extent of inflammation, identify the muscle origins that are the cause of the pain, provide a visual guide to any intervention for pain relief, and indicate the presence and extent of the commonly associated plantar enthesophyte, which may require surgical excision. It can also differentiate the condition from osteomyelitis in the underlying calcaneus, which can mimic the symptoms.⁴⁹

Figure 7.23 • A 54-year-old female presented with severe pain in the base of the ankle. MR imaging of the ankle has been performed in the sagittal plane using T1-weighted (A) and T2-weighted (B) sequences. On the T2-weighted image, a focal region of increased signal intensity is noted in the calcaneus at the insertion of the plantar fascia (oval). On the T1-weighted image, the same region is noted, though now with low signal intensity, with thickening of the plantar fascia noted (oval); this is an example of moderate plantar fasciitis.