Forefoot: a basic integrated imaging perspective for radiologists.

Clinical Imaging xxx (2014) xxx–xxx

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Clinical Imaging journal homepage: http://www.clinicalimaging.org

Forefoot: a basic integrated imaging perspective for radiologists Mohamed R. Nouh a, b,⁎, Ahmed A. Khalil b a b

Department of Radiology and Clinical Imaging, Faculty of Medicine, Alexandria University, Egypt Department of Radiology and Clinical Imaging, El-Razi Hospital, Ministry of Health, Kuwait

a r t i c l e

i n f o

Article history: Received 21 October 2013 Received in revised form 29 January 2014 Accepted 20 February 2014 Available online xxxx Keywords: Metatarsalgia Forefoot MR Imaging Sesamoid Plantar plate

a b s t r a c t Imaging of the forefoot is increasingly requested for patients with metatarsalgia. Awareness with specific anatomic arrangements exclusive for the forefoot and widely variable pathologic entities associated with metatarsalgia helps the radiologist to tailor a cost-effective imaging approach. This will enable reaching a specific diagnosis as much as possible with subsequent proper patient management. This pictorial review aims to provide basic understanding for the different imaging modalities used in studying the forefoot. After that, certain anatomic arrangements exclusive for the forefoot are discussed. The final section of this review describes the imaging findings of some common forefoot problems.

Forefoot pain is a fairly common clinical problem. The source of pain is most often the metatarsals and adjacent soft tissues and is often termed metatarsalgia clinically [1–3]. Forefoot disorders are more common in females, prevalence related to high-heeled footwear [4]. A wide spectrum of traumatic and non-traumatic conditions are associated with pain of the forefoot including stress fractures of the metatarsals, Freiberg infraction, sesamoid disorders, Morton neuroma, as well as tendinopathies, arthropathies, and infection [1–3]. Detailed history as well as physical examination usually narrow the differential diagnosis but may not yield a specific diagnosis. Hence, imaging is often utilized to aid in narrowing these differentials [1,3]. The aim of this pictorial review is to provide a basic understanding for the different imaging modalities used in studying the forefoot along with the limitations of each modality. This is followed by illustrations of disorders that are specific to the forefoot. The final section of this review describes the imaging findings of some common forefoot problems. 1. Imaging modalities used in forefoot imaging 1.1. X-ray Initial assessment of forefoot pain usually starts with plain radiography in three planes: the dorso-plantar (AP), the medial oblique, ⁎ Corresponding author. Department of Radiology and Clinical Imaging, El-Razi Hospital, Gamal Abd El-Nasser Street, Sulibakhat 13001, Kuwait City, Kuwait. Tel.: +965 65099562; fax: +965 24825508. E-mail address: [email protected] (M.R. Nouh).

© 2014 Elsevier Inc. All rights reserved.

and the lateral views [5]. The medial oblique view is most useful for midfoot joints [5]. In most non-trauma patients, these films should be done weight bearing. Tangential axial projections may be used to assess hallux sesamoid as well as their articulations with the first metatarsophalangeal (MTP) joint and the transverse arch of the foot [5,6]. This view is basically an ankle view centered slightly different. Radiographic evaluation of the forefoot can address osseous structures and their interrelationships but is unable to visualize soft tissue structures and their derangements. 1.2. Nuclear medicine Bone scanning is a valuable tool for evaluating the entire skeleton in one field view. Radionuclide imaging studies are of high sensitivity and are advantageous in inferring functional data about bone turnover rather than the mere anatomic data supplied by other imaging modalities [7]. However, they are limited by their lower spatial resolution and specificity, warranting the correlation of their results with those of other imaging modalities [7–11]. Scintigraphic evaluation of the forefoot can be used to diagnose stress fractures, avascular necrosis, arthritis, reflex sympathetic dystrophy (RSD), and infection [8–11]. The latter two often require a triple phase study including flow images. 18F-FDG PET/CT and hybrid single-photon emission computed tomography/computed tomography (SPECT/CT) using 99mTc-labeled leukocytes may differentiate between osteomyelitis and soft tissue infection in diabetic foot [12,13]. However, many centers currently use magnetic resonance (MR) for this purpose.

http://dx.doi.org/10.1016/j.clinimag.2014.02.011 0899-7071/© 2014 Elsevier Inc. All rights reserved.

Please cite this article as: Nouh MR, Khalil AA, Forefoot: a basic integrated imaging perspective for radiologists, Clin Imaging (2014), http://dx. doi.org/10.1016/j.clinimag.2014.02.011

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M.R. Nouh, A.A. Khalil / Clinical Imaging xxx (2014) xxx–xxx

2. Computed tomography (CT) CT superiority in assessing bony details and depicting cortical and trabecular pathologies is well established [14]. The introduction of multi-detector CT (MDCT) with its fast imaging, volume acquisition, and high spatial as well as contrast resolution helped the recognition of subtle fractures and improved the definition of fracture patterns [15]. CT is mainly advantageous in detailing complex osseous injuries and follow-up of fracture healing and post-operative patients [14,15]. It can study soft tissue calcifications as in depositional diseases like gout and bone forming matrix lesions. Dual-energy CT has proved to be beneficial for crystal characterization in joints and periarticular soft tissues of gouty patients [16]. 3. Ultrasound (US) US has been proved a widely used tool for focused screening of forefoot problems via both dorsal and plantar approaches [17]. It can categorize lesions as cystic or solid. Applying color and pulsed Doppler in soft tissue masses adds more value about the nature of the lesion. Although US is widely available, its main drawback is the operator dependency that overshadows the test results [17,18]. Additionally, musculoskeletal ultrasonography needs special training and has a long learning curve. US is widely used for the diagnosis of Morton's neuroma (fibroma) and its differential diagnostic considerations such as intermetatarsal bursitis [17]. It can be used to check the integrity of the long tendons of the forefoot and the plantar plate [19]. Its rule in localization and extraction of foreign bodies is well established [20,21]. Recently, US has been proved to detect metatarsal fractures in proper clinical settings with negative radiographs [22,23]. Moreover, it can guide biopsy of soft tissue masses and alcohol ablation strategy for Morton's neuroma [24]. 4. Magnetic resonance imaging (MRI) MRI allows superior soft tissue contrast, multiplanar imaging capability, and detailed visualization of interlacing small anatomic structures as well as absent ionizing radiation [25,26]. Optimum forefoot MR image acquisition requires compatible coil size and small fields of view [25,26]. The standard protocol includes a combination of anatomic sequences (T1W) and water-sensitive sequences (fat-saturated T2, PD, and/or STIR) in the sagittal, short-axis (Coronal) and long-axis (Axial) planes of the foot [26]. The main limitations for the use of MRI in forefoot studies are those general contraindications for MRI including claustrophobia, patients with pacemakers and cochlear implants and metallic hardware insertions within the region of interest, degrading the image quality. A detailed description of the forefoot MR anatomy is beyond the scope of the current review but the reader may be referred to previous manuscripts dedicated to the topic [25–27]. In the following section, we will highlight some applied anatomic arrangements specific to the forefoot that will help the reader to understand their relevant imaging pathologic findings. 5. Specific anatomic arrangements of the forefoot Anatomically, the forefoot describes that part of the foot distal to the Lisfranc (tarsometatarsal) joints. It is formed of five bony rays originating from the tarsus, numbered one through five from medial to lateral. Each ray is formed by the articulation of a metatarsal (M) bone with a phalangeal column, forming the toe skeleton, through an MTP joint. The toe's skeleton includes two phalanges for the first (medial) toe and three phalanges for the second through the fifth toes.

These phalanges are inter-articulating via interphalangeal (IP) joints. All MTP and IP joints are synovial joints [28]. 5.1. Hallucal sesamoid complex Sesamoid bones are small oval ossicles embedded, partially or totally, in the substance of a corresponding tendon crossing a joint. In the forefoot, it nests within the tendons of the flexor hallucis brevis under the first MTP joint [29]. The hallucial sesamoid complex includes two separate sesamoids, the tibial (medial, larger, and oval) and fibular (lateral, smaller, and rounded) sesamoids. They are separated by the tendon of the flexor hallucis longus muscle. Their dorsal surfaces are covered by hyaline cartilage and articulate with the opposing articular facets on the plantar aspect of the first metatarsal (M1) head to form the metatarsosesamoidal joints. Thus, they cushion the M1 head, disperse body weight on walk, and protect plantar tendons of the foot [29]. Disturbances in their ossification may results in bipartite or multipartite configurations, especially for the tibial one [30]. Rarely, hallux sesamoids may be absent or resorped following infections and/or altered biomechanics around their joint [29]. 5.2. Accessory ossicles Accessory ossicles are frequent incidental findings on forefoot imaging. They are regular in shape, smooth, well corticated, and hosted within their expected anatomic location [31,32]. However, the radiologist has to be aware of the more frequent ones, as they may be the subject of degenerative changes and painful overuse syndromes. More importantly, they may be confused with bone fractures [31,32]. The os intermetatarseum is an accessory ossicle of the forefoot located dorsally between the medial cuneiform and the base of the first and second metatarsals in 1–10% of humans [33]. It has the potential to be misinterpreted as a fracture fragment in the settings of Lisfranc injury-dislocation spectrum. Similarly, small accessory ossicles may be seen just close to the superior surfaces of the navicular or talar bones named os supranavicular and os supratalar, respectively. These are likely the sequelae of a prior injury but have the potential to be symptomatic [32,34]. Os vesalianum is an accessory bone hosted within the peroneus brevis tendon adjacent to the fifth metatarsal base in 0.1–0.4% of subjects. It can be a single or a bipartite bone and should not be confused with a fifth metatarsal avulsion fracture [35]. 6. MTP joint stabilizing structures 6.1. The plantar plate The plantar plate is a small but thick fibrocartilaginous structure on the plantar aspect of MTP joints, underneath the flexor digitorum tendon of each toe. It is formed by the blending of the distal plantar aponeurosis slips with the MTP joint capsule. At the first MTP articulation, it extends to include each sesamoid, the terms of metatarso-sesamoidal ligament and sesamoid-phalangeal ligament are applied for its proximal and distal portions, respectively. The plantar plate portion intervening between the two sesamoids, horizontally, is named the inter-sesamoid ligament [36,37]. Distally at the plantar aspect of the proximal phalanx base, it is firmly attached to the periosteum and spans proximally to attach to the metatarsal head neck junction in a sagging fashion so that sometimes a small recess of fluid is physiologically seen [36,37]. The plantar plates function to cushion the metatarsal heads and MTP joint during foot take-off phase of walking and running preventing phalangeal hyperextension. The flexor tendon sheath (containing both flexor digitorum longus and brevis tendons) sits within a shallow central groove on the overlying surface of the plantar plate [36,37].



6.2. The extensor hood and sling

7. Imaging of common forefoot disorders

A compound hood-like aponeurotic expansion originates from the fascial planes around the extensor hallucis longus, extensor digitorum longus, and lumbrical and interosseous tendons named the extensor aponeurosis of the toe. It holds these structures in place as well as aligning them to the dorsum of MTP joints. Additionally, from adjacent dorsal tubercle of each metatarsal head, fibrous slips join the aponeurosis and encircle each MTP articulation and descend volarly to insert with the plantar plate at the base of the proximal phalanx, named the collateral ligament. It is reinforced by an accessory ligament adjacent to its origin and their distal fibers blend with the plantar plate also, the accessory collateral ligament [36,38,39]. These structures collectively form the extensor hood of the MTP joint and act as a pulley to the extensors during dorsiflexion of the toes on walking and running [36,38,39].

7.1. MTP joint instabilities

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7.1.1. Plantar plate injury (turf toe) During walking, it has been postulated that repeated hyperextension poses forces on the plantar plates during toes push off and predispose them to attenuation or rupture [37]. Plantar plate degeneration is often seen in patients with a narrow shoebox and high-heeled shoes hence their predominance in young and middle-aged females [29]. It is also seen in patients with inflammatory MTP joint diseases as rheumatoid arthritis (RhA) [41]. A more common form usually encountered in more acute to subacute settings and involves the first or second MTP joint is termed the “turf toe”. It commonly occurs in athletes playing on artificial surfaces and wearing flexible shoes [42]. Hyperextension of the MTP results in stretching or tearing of the opposing plantar plate at its weak point of attachment at the metatarsal head neck junction [42].

6.3. Normal sonographic and MR appearances On US, the normal plantar plate is a slightly echogenic, homogeneous structure curving over the metatarsal head to insert into the proximal phalanx in a longitudinal plane. The flexor tendon sheath is seen beneath it in the short-axis plane [19,40]. On MRI, the normal plantar plate appears, in the sagittal plane, as a smooth, low-signal structure abutting the plantar aspect of the metatarsal head, attaching at the proximal phalangeal base adjacent to the joint surface in all pulse sequences. It may be barely identified from the adjacent flexor tendon. However, in the gradient echo sequences, the plantar plate may be easily distinguished from the adjacent flexor tendons as it appears slightly higher in signal intensity than them [19,40]. Coronal short-axis images (Fig. 1) demonstrate the plantar plate as a thick low-signal band centered subjacent to the lesser metatarsal heads, with a smooth central indent housing the flexor tendon sheath on its plantar surface [19,40]. The accessory collateral ligament (Fig. 1) is seen as vertically oriented low-signal bands on either side of the metatarsal head descending to fuse with the plantar plate and becomes thicker distally as it becomes joined by its accessory collateral counterpart, proximal to the MTP joint. At the other end, near the base of proximal phalanx, thin low-signal extensions of this complex encircle the flexor tendon [19,40].

Fig. 1. Short-axis view of the forefoot at the level of the lesser MTP joints showing the different stabilizing structures around these joint. Open arrow heads point to the long extensor tendons, short arrows point to the long flexor–plantar plate complexes, curved arrows point to collateral ligaments, and elbow arrow connector points to the accessory collateral ligaments. Note the interdigital neurovascular bundle within the dotted oval shape showing the dorsal interdigital nerve deep and interdigital artery and vein superficially.

7.1.2. Imaging Initial imaging usually begins with X-ray that may rarely shows edema of the soft tissues around the affected MTP, dorsal dislocation of it, or very rarely in first MTP affections (turf toe) proximal migration of the sesamoid. Correlation with previous images and comparison with other side may be beneficial. Radiographs can evaluate the associated arthropathy, fracture, inflammation, and infection. On US, plantar plate tears appear as a plantar plate discontinuity with a focal hypoechoic defect in the region of tear [19,40]. Associated

Fig. 2. (a) Sagittal fat-saturated PD and (b) short-axis fat-saturated PD images of the first MTP joint showing high signal intensity of the plantar plate [arrow in (a)] as well as intermetatarsal ligament [arrow in (b)] inferring plantar plate tear. This is associated with edema of the tibial sesamoid [white star in (a) and (b)] and mild MTP joint effusions.


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osteophyte formation seen as highly echoic foci similar in echogenicity to bone could be seen in chronic cases. MTP effusions, synovial thickening, and entrapment of the flexor tendon sheath in the plantar plate tear may be seen, additionally [19,40]. On MRI, plantar plate tears are best recognized on sagittal and shortaxis images. A tear of the plantar plate appears as a hyperintense focus replacing the normally hypointense insertion at the phalangeal base on water-sensitive sequences (Fig. 2). Edema of the surrounding soft tissues may be seen. In chronic cases, variable degrees of dorsal hyperextension may be seen along with synovial thickening of the MTP joint. Variable degrees of osteochondral injuries of the metatarsal heads may also be seen [43]. 7.1.3. Skimboard toe Another related hyperextension injury associated with MTP instability is called “skimboard toe”, described by Donnelly et al. [44]. It has to be differentiated from turf toe. It is characterized by edema of the soft tissues on the toe dorsum and failed visualization of intact extensor hood around the affected MTP. In pure skimboard toe, the corresponding plantar plate is preserved. Combination with turf toe may be seen in traumatic instabilities of the MTP joint (Fig. 3). 7.1.4. Sand toe This a special type of MTP joint instability with similar clinical presentation to that of turf toes. It has been described in beach volleyball players referred to as “sand toe”. It is caused by vigorous hyperflexion of MTP associated with dorsal capsular rupture with subsequent disabilities [45]. 7.2. Stress-related disorders 7.2.1. Stress response/fracture spectrum Bone responses to stress include a spectral continuum ranging from normal osseous remodeling, through adaptive remodeling (stress response) with early fatigue failure, to frank stress fracture [8]. Stress reaction/response occurs when microfractures are healing and a complete fracture has not yet developed. A fatigue fracture occurs when normal bones are exposed to repeated abnormal stresses as classically described in military recruits and runners. On the other hand, insufficiency fractures occur when normal or physiological stresses are applied to weakened bones, such as bones with osteoporosis or Paget disease [8,43]. The second, third, and fourth metatarsals are the most common site of stress fractures in the forefoot. The first and fifth metatarsals are rarely involved. It typically occurs in the proximal metaphysis of the first metatarsal and the middle or distal segments of the other four metatarsal bones [46,47]. The sesamoid bones of the hallux, the synchondrosis between the portions of a bipartite sesamoid bone, accessory ossicles, and the base of the proximal phalanx of the great toe are other potential sites for stress injuries in the forefoot [3,46,47]. On imaging, plain radiography may be entirely normal in an early phase, but with time, a small cortical break progressing to a periosteal reaction and finally forming a linear area of sclerosis perpendicular to the direction of stress appears. It can take up to 6 weeks for changes to occur or the fracture line to be appreciated in the plain film [8]. MRI is the next step in imaging of suspected stress fractures, if radiography is negative. It is highly sensitive for the bone marrow edema that may be present with stress reaction before the fracture is appreciated [8,43]. MRI shows stress fractures as very low signal intensity linear lesions on T1 images, representing the fracture line, surrounded by bone marrow edema, which is low signal on T1 weighting and bright on water-sensitive images (Fig. 4). There may be para-osteal soft tissue edema or a small sympathetic effusion in the adjacent joint. Periosteal reaction if present appears as low-signal

Fig. 3. Combined turf toe and skimboard injuries in amateur fisher who obtained trauma to his feet during landing from fishing boat to the dock. Sagittal fat-saturated PD (a) and T1W SE (b) images and fat-saturated PD images in long-axis (c) and short-axis planes from MR series acquired 1 week following trauma showed focal edema surrounding the dorsal capsule of second MTP joint [white star in (a) and (b)], joint effusion [images (a) through (d)] and high signal within the barely visualized collateral ligament of MTP joint [white chevrons in (c)]. Also note high signal intensity of the plantar plate (arrow) and posttraumatic marrow edema of M2 head [images (a), (b), and (d)].

stripe paralleling the cortical bone, occasionally separated from it by high-signal edema on fluid-sensitive sequences [8,43]. Before the advent of MRI, bone scan was a valuable tool for early detection of clinically suspected stress fractures with negative



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Fig. 4. MR of the forefoot in long-axis plane in (a) STIR and (b) T1W sequences revealing third metatarsal mid-shaft low-signal fracture line, florid marrow edema on STIR image, and surface low-signal exuberant callus formation representing a missed stress fracture.

radiographs, thanks to its inherent ability for detecting early changes in bone metabolic activity. However, it is limited by lower sensitivity and specificity, radiation exposure, and low spatial resolution [8,46,47]. Although CT would be more sensitive than the plain film in detecting early stress fracture, it can also be negative. CT is particularly useful in defining an abnormality discovered with scintigraphy or MRI. It is especially valuable in differentiating a stress fracture from an osteoid osteoma in pediatric age groups [8,48]. A point worth mentioning here is that insufficiency fractures, commonly seen in elderly population, may coincide with osteoarthritic changes of the forefoot joints. However, typical exclusive affection of articular surfaces on both sides of a joint with subchondral cystic changes and sclerosis, joint space narrowing, as well as the presence of marginal osteophytes can discern osteoarthrosis from insufficiency fractures. On the other hand, presence of low-signal fracture line on T1W images, bone marrow edema, periosteal reactions, and para-osteal soft tissue edema will favor the diagnosis of insufficiency fracture [8,49]. Recent reports [22,23] emphasized an emerging role for ultrasonography in early diagnosis of suspected stress fractures of the metatarsal. US features aiding the diagnosis of a metatarsal stress fracture include periosteal reaction appearing as a hyperechoic band along the cortex, with an underlying hypoechoic band representing periosteal hemorrhage, and step off the underlying bony cortex. Color Doppler imaging shows perilesional increased flow. Also, localized tenderness is a confirmatory positive finding on US examination [22,23,50].

7.2.2. Sesamoid disorders Patients with sesamoid disorders usually present with limited and painful dorsiflexion of the first MTP joint [29]. Clinically there may be swelling at the plantar aspect of the joint with tenderness when the examiner pushes the pathological sesamoid distally [29]. Radiography can play an important role in the initial diagnosis of sesamoid disorders. A special sesamoid (oblique coronal) view (Fig. 5) can be performed if a sesamoid disorder is highly suspected. The beam is

directed tangential to the metatarso-sesamoidal articulation allowing direct visualization of the joint without osseous overlap [5,6]. Radiography may be unremarkable or reveal fracture lines and separate fragments in case of fractures, marginal osteophytes in osteoarthrosis, and periosteal reaction in cases of old stress fractures and avascular necrosis [6].

Fig. 5. Radiograph of the forefoot (sesamoid projection) showing areas of mixed lysis and sclerosis of the fibular sesamoid (white arrows) in the course of evolving sesamoiditis.


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CT is an excellent modality in assessing disorders of the sesamoid. It can adequately reveal fractures, differentiate it from bipartite sesamoids and delineate the articular surfaces of the metatarso-sesamoidal articulation, and reveal subarticular cysts and marginal osteophytic formation [51,52]. Recently, Sharma et al. [53] highlighted the utility of hybrid SPECT/CT imaging in exact localization of sesamoid disorders in workup for metatarsalgia. MRI is the most sensitive modality in the evaluation of sesamoid disorders. It can early demonstrate diffuse bone marrow edema, which is usually present at the time of the initial symptoms (Fig. 6). These are common MR findings shared by most sesamoid pathologies however [29,51,52]. With persistent offending factors, a fracture line commences and MRI will reveal a low-signal-intensity fracture line in the affected sesamoid. Later on, avascular necrosis ensues where progressive flattening and fragmentation can be seen [29,51]. Moreover, MRI is advantageous in depiction of coexistent reactive soft tissue changes, as synovitis, tendonitis, and bursitis [29,51]. On radiography and CT, a bipartite sesamoid is usually larger than a non-partite sesamoid and its fragments are neatly rounded with smooth sclerotic margins. On the contrary, a fracture fragment will often show an irregular non-sclerotic margin often with some degree

Fig. 6. Sesamoiditis and callus formation in a 32 Y handyman with habitual tip toeing to catch above head objects complaining of forefoot pain localized to the first MTP joint since 6 months. Sagittal T1W (a) and short-axis fat-saturated PD (b) as well as T1W (c) images shows extensive edema of both tibial and fibular sesamoids (white arrows) with focal replacement of the plantar metatarsal fat beneath it with low to intermediate signal intensity focus representing callus (star).

of fragments separation. MR is more beneficial in depicting marrow edema of the fractured sesamoid. 7.2.3. Freiberg disease Freiberg disease is a self-limited condition, postulated to be osteonecrosis resulting from repetitive microtrauma to the metatarsal head. It commonly targets the second metatarsal followed by the third metatarsal [54]. The disease is common in adolescent females indulged in high-heeled shoes footwear, with a female-to-male ratio of 3–5:1 [3,43]. The patient may be asymptomatic until degenerative arthrosis develops. Symptomatic subjects usually present by pain, tenderness, swelling, and limping in pediatrics [3,54]. Pathologically, there is subchondral trabecular collapse of the involved metatarsal head with osteonecrosis and cartilaginous fissuring [3,54]. Radiography shows cystic changes of the metatarsal head, sclerosis, and widening of the joint space in early disease. Later on, there may be fragmentation and collapse of articular surfaces. If the disease progresses, flattening of the targeted metatarsal head, secondary osteoarthritis, and loose bodies may appear [3,54]. MRI can detect these changes early in the course of the disease as areas of marrow edema and subchondral cystic changes. Associated effusion and periarticular edema could be seen [55,56]. However, stress response has to be considered and imaging data are cautiously interpreted in view of clinical setting [55,56]. Early, scintigraphy was used to detect the disease [9] yet its role had been overtaken by the MR with its lack of ionizing radiation. 7.2.4. Interdigital fibroma (Morton's neuroma) Interdigital fibroma or Morton's neuroma is a common cause of pain and paraesthesia of the forefoot [1–4]. The entity is more common in women than men, with a high prevalence in the fifth and sixth decades of life and a predilection to arise in the third and less commonly the second intermetatarsal space [1–4]. Histologically, it has been proved to represent a non-neoplastic, fibrous proliferation involving a plantar digital nerve likely sequel to its compression against the intermetatarsal ligament [57]. This is supported by its prevalence in people engaged in high sports activities, women wearing tight high-heeled shoes, obese females, and association with intermetatarsal bursitis [1–4,43,58]. Morton neuroma diagnosis is commonly accomplished on the basis of clinical history and physical examination. However, multiplicity and other diagnostic considerations may warrant the use of clinical imaging to confirm the diagnosis. Lesions with a transverse diameter (b 5 mm) are often asymptomatic and may be an incidental finding on imaging of healthy volunteers [59]. In general, imaging diagnosis of Morton's fibroma relies on localizing a small intermetatarsal space mass lesion usually oriented parallel to the long axis of the metatarsals, deep to the interosseous muscles, distal and plantar to the intermetatarsal ligament [43]. This could be achieved by using US and/or MRI. MRI is a reproducible modality with high sensitivity and specificity in diagnosing Morton's fibroma. Imaging in short-axis plane is the gold standard for lesion depiction [58]. Occasionally, Morton's fibroma is identified on MRI (Fig. 7) as a small lesion with intermediate signal intensity on T1-weighted images and variable signal intensities on T2weighted images allocated in one or more of the intermetatarsal spaces [3,43,58]. Use of contrast is controversial as the lesion may exhibit variable enhancement depending on their maturity and associated inflammation [3,43,58]. Weishaupt et al. [60] stressed on increased conspicuity of small lesions if imaged in prone and upright weight-bearing positions. US is an excellent handy modality, to diagnose suspected interdigital fibroma, when MRI is unavailable, and/or cost-effectiveness is a prime consideration. Morton neuromas have a variable sonographic appearance. It may appear as a rounded or oval lesion proximal to the metatarsal head, hypoechoic when new, and hyperechoic as it gets older [43]. Mulder's sonographic sign refers to a dynamic US maneuver described to increase conspicuity and diagnostic perception of the



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on T1W1 and high signal intensity on fluid-sensitive images. Subtle peripheral enhancement usually follows intravenous administration of gadolinium [43].

Fig. 7. Surgically proven Morton's fibroma (neuroma). MR images of the forefoot in short axis showing small heterogeneous oval soft tissue mass lesion (black dotted oval) on the plantar aspect of second intermetatarsal space on PD image (a). The lesion showed mild enhancement on post-gadolinium T1W images (b).

7.2.6. Chronic regional pain syndrome (CRPS) CRPS was described using different terms as RSD and Sudek's atrophy. CRPS is a pain dysfunction disease entity characterized by intense pain and swelling involving a limb and resulting in severe disuse osteoporosis [10]. This condition has been reported in association with prior trauma, surgery or infectious states, as well as vasculitis, calcific tendonitis, degenerative disc disease, and cardiovascular disorders [10,64,65]. The clinical presentation is divided into three stages. Initially, an inflammatory stage characterized by a painful stiff and swollen extremity. A dystrophic stage with activity-related pain and hyperesthesia then follows. Lastly, a final stage characterized by skin and muscle atrophy ensues [64]. The imaging characteristics of RSD depend on the stage of the disease. In early stages, radiographs are usually normal. Three phase bone scan is considered the most sensitive imaging modality at early stage and usually shows increased blood flow on the early and blood pool images and increased bone uptake on the delayed scintigraphs due to subperiosteal resorption [64]. In this stage, MRI is useful in detecting soft tissue and bone marrow edema and joint effusions that are not depicted or well appreciated with other imaging modalities [65,66]. In later stages, imaging studies will demonstrate periarticular osteopenia, endosteal and subperiosteal bone resorption along with demineralization, especially on CT (Fig. 8). MRI will depict skin and muscular atrophic changes [65].

lesion [61]. The examined forefoot is held in one hand exerting gentle lateral compression over the metatarsals and applying the transducer to the plantar aspect of the intermetatarsal regions by the other hand. This displaces small lesions for more reliable measurements. It also helps visualization of occult lesions [61]. Moreover, US can aid non-surgical interventions in Morton's neuroma. US-guided intra-lesional injections of alcohol or corticosteroids have been used for variable short- and medium-term symptom relief in those patients [24,62]. Chuter et al. [63] recently described the use of US to guide radiofrequency ablations of Morton's neuromas as an outpatient procedure with 85% success in alleviating patients' symptoms. 7.2.5. Intermetatarsal bursitis The intermetatarsal bursa is a naturally occurring bursa in the first through fourth intermetatarsal spaces between the metatarsal heads just dorsal to the deep transverse metatarsal ligament [18,28,43]. They may contain a small amount of lubricating fluid that may be recognized as a physiological finding (b3 mm in transverse diameter) on MRI studies [18,28,43]. However, its visualization in the last intermetatarsal space is often associated with clinical symptoms [18]. Intermetatarsal bursitis is usually associated with chronic mechanical compression and irritation between the metatarsal heads as with tight shoes and repetitive microtrauma; hence, it is commonly associated with Morton's fibroma [1–4,43,58]. It is also common with infection, RhA, and gout [18]. On sonography, intermetatarsal bursitis is typically depicted as hypoechoic or anechoic fluid collections in-between and to some extent wrapping around the adjacent metatarsal head. If infected, variable echogenicity is perceived with significant hyperemia on Doppler interrogation [43]. On MRI, it typically appears as a well-defined fluid collection in its typical location between the metatarsal heads with low signal intensity

Fig. 8. Reconstructed MDCT image of both feet in long-axis plane showing evident subcortical cysts involving the proximal phalanx and M1 head as well as midfoot bones in a patient with chronic forefoot pain following internal fixation for a tibial fracture. These features were concordant with the suspected clinical diagnosis of RSD.


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8. Degenerative and inflammatory conditions targeting the forefoot The small joints of the forefoot are potential targets for degenerative, inflammatory, and metabolic osteoarthrosis presenting with metatarsalgia. 8.1. Osteoarthritis Primary osteoarthritis is a common finding in the first MTP joint and is most often produced by repetitive loading injury [1–3]. Radiographic findings include joint space narrowing, subchondral sclerosis, osteophytes formations, and subchondral cysts or geodes formation [1–3]. MRI can demonstrate subchondral marrow edema, in early disease. In the more advanced disease, subchondral cyst formation and sclerosis ensue [1–3]. 8.2. RhA RhA commonly affects the MTP joints of the forefoot even before the affection of the hands and the wrists. Symmetric bilateral involvement is commonly seen in the feet. Asymmetric or unilateral involvement is sometimes found [67,68]. Plain films usually are the initial imaging modality for diagnosing articular changes in RhA. It can demonstrate soft tissue swelling and juxtaarticular osteopenia in early disease. Additionally, the medial and lateral aspects of the fifth MTP joint are common locations for osseous erosions in early RhA. In more advanced disease marginal erosions, joint space narrowing and malalignment appear. However, radiographic changes always appear late in the disease course [67,68]. US and MRI can reliably address early RhA changes, in clinically suspect subjects. This is empirical for initiating proper treatment to preserve joint function and delay joint destruction [67–70]. US is increasingly used in the evaluation of early disease [18]. It easily detects joint effusion, synovitis, cartilage loss, and subarticular erosions of the MTP joints [70]. It can be used for evaluation of soft tissue changes as tenosynovitis, tendinopathies, and rheumatoid nodules [61,67,68,70]. US demonstrates nodules as mixed echo texture masses within the subcutaneous tissues. More commonly, the nodules are hypoechoic and often contain fluid components with poor internal vascularity [67]. Moreover, recent reports suggest a future role for US elastography techniques in differentiating inflammatory, depositional, and infective arthritides [71,72]. MRI has been shown to be a sensitive modality that can assess both inflammatory and structural lesions in rheumatoid patients. It can show joint effusions, synovial thickening, bursitis, and bone erosions. Moreover, MRI surpasses other imaging modalities for its unique ability to demonstrate bone marrow edema, a marker of active inflammation and predictor for developing erosions in early RhA [67,69]. 8.3. Gout Gout is a metabolic disorder characterized by intra- as well as extraarticular deposition of monosodium urate crystals. Gout typically targets the MTP and IP joints of the great toe with characteristic juxtaarticular punched out erosions and hanging out margins. An extensive form characterized by multiple, soft tissue masses with destruction of multiple bones has been described in the foot [73]. In the acute settings, radiography may show soft tissue swelling, yet in the majority of cases, radiographs are normal. On US, effusions within the affected joint and/or tendon synovial sheath may be seen with hyperemia on color Doppler. These effusions can visualize the crystals as hyperechoic foci in up to one third of patients [74,75]. In the chronic disease, when tophi develop, US shows it as hyperechoic soft tissue masses within the subcutaneous tissues and

Fig. 9. Long-axis T1W image of the forefoot showing the characteristic juxtaarticular punched out erosion of a capsular gouty tophus with hanging out edges (thick white arrow). The tophus signal intensity is iso-intense to that of adjacent skeletal muscles.

around joints. It usually has a central hypoechoic zone in about 50% of patient [76]. On CT, tophi appear as soft tissue masses with 160–170 HU and may show calcifications [77]. Dual-energy CT can disclose the nature of a periarticular soft tissue mass to be a gouty tophus by detecting monosodium urate deposits within it [16]. On MR, erosive changes are seen. When gouty tophi are present, it exhibits iso-intense signal to muscle on T1-weighted sequences with variable enhancement following gadolinium administration (Fig. 9). However, it exhibits variable signal intensity on fluid-sensitive sequences [74,78]. 8.4. Diabetic foot infection and Charcot's arthropathy The forefoot of diabetic patients can show a lot of pathologic changes including callus formation (Fig. 6), adventitious bursitis (Fig. 10), skin ulcers, neuropathy, diabetic myopathy (Fig. 10), and diabetic osteoneuropathy and osteomyelitis. The forefoot bones and joints are a common site of predilection for osteomyelitis and Charcot's arthropathy in ambulatory patients, underneath the first and fifth metatarsal heads. They often coexist and are associated with high patient's morbidity [79–83]. Detailed discussion of both entities is beyond the scope of this review and the reader may be refereed to good articles dealing with these topics in imaging literature [79–83]. Early detection of either disease is crucial to initiate appropriate treatment and abort further disease progression. So, the radiologist is the key personal in the workup of these patients saving them the hazards of unnecessary amputations. Both processes share common imaging findings including soft tissue edema, periosteal reaction and subchondral sclerosis on radiographic and CT studies, increased focal perfusion on nuclear medicine studies, and bone marrow edema on MRI. Hence, differentiation of both entities is a challenging task for radiologists as it is clinically [79–83].



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9. Forefoot mass lesions A swelling arising in the forefoot could be categorized as a bony tumor, a soft tissue tumor, or a pseudo-soft tissue tumor. 9.1. Forefoot osseous neoplastic lesions By far, osseous tumors and tumor-like conditions of the foot are rare [84]. The small bones of the forefoot can be the site of any benign or malignant neoplastic processes that may arise elsewhere in the skeletal system. Some lesions, however, have a predilection for the foot. In a series by Murari et al. [85], the metatarsal bones were the most commonly affected. In the subset of benign osseous lesions, osteoid osteoma (Fig. 11), giant cell tumor, and enchondroma are the major types encountered. On the other hand, Ewing's sarcoma, chondrosarcoma, and osteosarcoma are the commonest lesions of the malignant skeletal neoplasia targeting the forefoot [86]. Radiographs are the initial imaging used in patients with forefoot pain. It usually detects a suspected bony tumor and provides valuable information predicting its matrix and biologic behavior. It remains the cornerstone in framing the most likely differential diagnosis. Further staging of forefoot bone tumors requires further crosssectional imaging using MDCT and MR to map the compartmental extensions of the lesion. 9.2. Pseudotumoral soft tissue lesions

Fig. 10. Short-axis fat-suppressed PD (a) and T1W SE (b) images of type I diabetic patient showing edema of the foot dorsal subcutaneous tissues (chevron) and fatty atrophy of the intrinsic foot muscles (dotted oval) and evolving adventitial bursitis underneath the fifth metatarsal head (small arrows).

According to the American College of Radiology recommendations, MRI with or without contrast is the examination of choice to detect presence of osteomyelitis while other imaging modalities are complementary to it in different clinical scenarios [80]. There are multiple points that can aid differentiating both disease entities on MRI, summarized in Table 1. Nevertheless, the key fact in differentiating both entities is that osteomyelitis almost exclusively develops by contiguous spread of infection from skin ulceration at predictable sites whereas neurosteoarthropathy is primarily epicentered to an articular surface with multiple joints involvement. In general, replacement of soft tissue fat, fluid collection, and sinus tracts over a pressure point leading to an area of extensive marrow edema are MRI features consistent with infection [79–83].

In general, pseudotumoral soft tissue lesions form the most common forefoot masses [58]. A useful clinical tool is its separation into cystic and non-cystic lesions. The cystic pseudo-masses targeting the forefoot can include synovial cysts and/or ganglia, adventitial bursae, and abscesses. The non-cystic pseudo-masses include calluses, rheumatoid nodules, and Morton's neuroma. Forefoot granuloma can belong to the non-cystic group if the inflammatory reaction was predominantly fibrotic. On the other hand, if infection ensues around the inciting agent, an abscess will develop. We will stress on the most common lesions in this section. Other entities as Morton's neuroma and rheumatoid nodules have been described in earlier sections of this review. 9.3. Foreign body granuloma Foreign body granuloma is a tissue reaction in response to foreign objects penetrating the soft tissue. The forefoot is commonly involved especially in the pediatric age and in labor workers [87]. Plain radiographs can show soft tissue fullness (Fig. 12a). If the foreign body is radiopaque and sizable, it may be apparent. On the other hand, non-opaque foreign bodies as wooden splinters won't be visible [88]. US can reveal the foreign body as an echogenic structure with a posterior acoustic shadow. The surrounding granuloma appears as a non-specific soft tissue mass. The key to the correct diagnosis is to identify the foreign body and clarify the previous trauma [89].

Table 1 Key MRI features to differentiate osteomyelitis from Charcot's arthropathy [79–83] Difference

Osteomyelitis

Charcot's arthropathy

Disease topography Deformity

Focal involvement of the affected bone and adjacent soft tissues Usually no deformity unless there is an underlying neuropathic joint Localized to a single bone and exhibits high signal on water-sensitive sequences and low on T1 images with enhancement following IV gadolinium Always present with overlying ulcer, abscess, or sinus tract

Regional distribution including multiple bones and joints Deformity is common along with bony debris

Bone marrow abnormality

Soft tissue changes

Epicentered to joints with periarticular and subarticular distribution. Acute form mimics osteomyelitis. Chronic: normal marrow signal or low on T1 and T2 Overlying skin is usually intact but may be edematous


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Fig. 11. An adolescent male presented with forefoot pain. Oblique AP radiograph showed marked periosteal reaction (black arrows) of M4 with an area of distal metaphyseal lucency (white arrow). Short-axis CT in bone window image showing a calcific nidus of an osteoid osteoma within the M4 cortex (curved white arrow).

Fig. 12. (a) X-ray AP projection of the forefoot shows divergence of the second and third metatarsal rays with soft tissue fullness (white dotted circle). (b) Short-axis post-contrast T1W image of the forefoot showing dorsal bi-locular abscess formation (white arrows) with wall enhancement and septic intermetatarsal bursitis (black dotted oval) of second intermetatarsal space around a small signal void wooden splinter (black chevron).

MRI can show the foreign body as low intensity in both T1 and T2 surrounded by a reactive edema (low signal on T1 and high signal on T2) giving a characteristic target appearance when the lesion is imaged perpendicular to its main axis [90]. When superadded infections coincide, an abscess formation may follow (Fig. 12b).

ment, if simple cysts or echogenic with edge enhancement, if complicated with infection and/or chronic inflammation. A connection with adjacent joint or tendon can be traced in most cases [3,58]. On MRI images, they exhibit typical characteristics of fluid (low T1W or slightly higher in case of ganglia due to mucin content and high T2W) (Fig. 13) unless they are complicated with hemorrhage or infection, a higher signal on T1W images indicates a high proteinaceous content. Sometimes, a communication with adjacent joint or tendon may not be elicited on both US and MRI [3,58]. Some researchers recommend the use of CT arthrography to demonstrate this relationship if surgery is contemplated [91].

9.4. Synovial cysts and ganglia Ganglion and synovial cysts are the most common soft tissue lesions in the foot region, most frequently located at the dorsum of the forefoot [3,58]. Synovial cysts are true cysts lined with synovial cells while ganglia are pseudo-cysts with non-continuous flattened pseudo-synovial cells developing in close vicinity to diseased joints, tendons, and ligaments [58]. They are commonly suspected clinically and imaging may be just requested to confirm their cystic nature and communication with adjacent joint and/or tendon and rule out other causes of swelling. On US, ganglion and synovial cysts appear as round to oval, monoor multi-loculated anechoic lesions with posterior acoustic enhance-

9.5. Forefoot adventitial bursitis Bursae are synovial-lined, fluid-filled structures located at areas of friction to ease the gliding of a movable tendon over an adjacent bony surface and minimize its friction [92]. Bursae may be (a) native (naturally occurring), in certain anatomic locations, e.g., intermetatarsal bursae, or (b) adventitious bursae, developing when abnormal frictions occur between opposing rigid structures [92].



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A detailed description of these entities is beyond the scope of this review. We will only stress on plantar fibromatosis as it may be considered in the differential diagnosis of the more common entity of foreign body granuloma, if history is not clear. 9.7. Plantar fibromatosis

Fig. 13. Short-axis PD image showing two small dorsal ganglia (black arrows) on the dorso-lateral aspects of proximal fourth and fifth metatarsals in an adult female with a long history of high-heel shoe wearing.

Plantar fibromatosis is a nodular fibrous proliferation arising from the plantar aponeurosis, often on the medial aspect of the plantar arch. It can extend to the skin or deep structures of the foot [96]. The disease is more often common in men and bilateral in 20–50% of cases. Diabetes mellitus, epilepsy, keloids, and alcoholism with liver disease were reported as co-morbidity factors [96]. On US, plantar fibromatosis appears as a fusiform, hypoechoic, or heterogeneous mass epicentered to the plantar fascia. However, in large lesions, US may not be able to confirm with certainty the anatomic localization. Cystic components and intra-tumoral neovascularity may be seen [97]. On MR (Fig. 14), lesions have iso-intense to hypointense signal intensity to the skeletal muscles on T1- and PD-weighted images and usually demonstrate heterogeneous signal intensity on T2-weighted images. Contrast enhancement is variable and linear tails of extensions (fascial tail sign) along the aponeurosis are frequent post-contrast finding [96].

Adventitious bursae usually develop in the subcutaneous tissues at sites vulnerable to weight bearing, high pressure, and repetitive frictions [28,92]. In the forefoot, they commonly develop under metatarsal heads and medial to the hallux in patients with valgus deformity. Repetitive stress leads small fluid collections within the loose mesenchyma of the subcutaneous tissues to coalesce into a well-defined fluid-filled cavity lined by synovium-like columnar cells the adventitious bursa. In difference with the native bursae, they lack the mesothelial lining [93]. On sonography, in early stages, it may appear as broad areas of illdefined compressible anechoic or heterogeneous collections in the subcutaneous tissues. Later on, a well-formed adventitious bursa will be depicted as a focal well-defined fluid collection [43]. On MR, they appear as an ill-defined lesion in the subcutaneous fat that is of low signal intensity on T1W1 and high signal intensity on fluid-sensitive sequences (Fig. 10) and with rim enhancement if intravenous gadolinium was used, consistent with the enclosed fluid. Usually, there may be associated stranding of the adjacent soft tissues [43,93]. Enlarged inflamed bursae can also be seen in the course of inflammatory connective tissue diseases, e.g., RhA.

9.6. Forefoot non-cystic soft tissue masses True soft tissue neoplasms targeting the foot are rare. Previous large oncopathologic series [94,95] figured out their common distributions (Table 2).

Table 2 Most common foot soft tissue neoplasms [94,95] Age group/neoplasm

Pediatrics

Adulthood

Benign

Granuloma annular Fibromatosis Rhabdomyosarcoma Synovial sarcoma Dermatofibrosarcoma protuberance

Plantar fibromatosis GCT of the tendon sheath Synovial sarcoma Malignant fibrous histiocytoma (undifferentiated pleomorphic sarcoma) leiomyosarcoma

Malignant

Fig. 14. Surgically proven plantar fibroma of the forefoot. (a) Long-axis PD image of the forefoot revealing an oval soft tissue lesion of to high signal intensity on PD image. The lesion showed marked enhancement following IV gadolinium administration on shortaxis post-gadolinium T1W image (b).


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