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Clin Sports Med 25 (2006) 763­780

CLINICS IN SPORTS MEDICINE

Imaging Sports Medicine Injuries of the Foot and Toes

Hilary R. Umans, MD

Albert Einstein College of Medicine, Division of Musculoskeletal Radiology, Jacobi Medical Center, Bronx, New York 10461, USA

T

he Lisfranc joint, aka the tarsal-metatarsal (TMT) joint, marks the transition between the more rigid midfoot and the relatively flexible forefoot. It provides critical stability in maintenance of both the transverse and longitudinal arch of the foot. That stability is derived from both its osseous geometry and complex capsuloligamentous architecture. The second metatarsophalangeal (MTP) joint is recessed with respect to the neighboring first and third MTP joints. Multiple facets at the second metatarsal base articulate with all three cuneiforms. The second metatarsal base is shaped like a keystone at the apex of the transverse arch of the foot. Intermetatarsal ligaments connect the second through fifth metatarsal bases, but there is no intermetatarsal ligament bridging the first and second. Instead, the Lisfranc ligament, the most substantial and strongest at the TMT joint, courses obliquely from the lateral surface of the medial cuneiform in a plantar and lateral direction to insert on the plantar medial base of the second metatarsal [1] (Fig. 1). Disruption or avulsion of the Lisfranc ligament, or fracture of the second metatarsal base, results in TMT instability. Left untreated, a Lisfranc injury can result in collapse of the longitudinal arch of the foot. Although the majority of Lisfranc fracture/dislocations result from highvelocity trauma or crushing injuries, sports-related Lisfranc injuries typically occur as a result of low-velocity indirect force. In athletes, the typical mechanism of injury is an axial load on a plantar flexed and slightly rotated foot [2]. These injuries are particularly common in but not unique to American Football, with offensive linemen most commonly affected [3]. Sports-related Lisfranc injuries are considered in a spectrum of midfoot sprains. Midfoot sprains may or may not include diastasis or fracture at the first intermetatarsal space or second metatarsal base, respectively, and therefore may elude conventional radiographic detection. Nunley and Vertullo [4] proposed a classification for midfoot sprains that differs from the standard classification systems used for high-velocity traumatic Lisfranc injury. Stage I injury is characterized by a dorsal capsular tear without

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Fig. 1. Axial T1 weighted MR image demonstrates the normal, intact Lisfranc ligament coursing between the lateral aspect of the medial cuneiform to its insertion onto the medial second metatarsal base (curved arrow).

elongation of the Lisfranc ligament; weight-bearing radiographs are normal. Stage II injury includes elongation or disruption of the Lisfranc ligament, with an intact plantar capsular ligament; weight-bearing radiographs demonstrate 2- to 5-mm diastasis at the first intermetatarsal space. Stage III injury includes disruption of the dorsal capsule as well as the Lisfranc ligament and the plantar capsuloligamentous structures; weight-bearing radiographs demonstrate greater than 5 mm diastasis at the first intermetatarsal space, loss of the longitudinal arch height, and, often, associated fracture. Even in the context of high-velocity traumatic Lisfranc injury, approximately 20% of cases are prospectively missed on conventional foot radiographs [5]. Although alignment may be assessed by evaluating cortical registration across each TMT joint, congruent alignment is most reliably evaluated at the medial cortex of the middle cuneiform and second metatarsal base on anteroposterior (AP) and oblique radiographs. Given a high index of suspicion based on mechanism of injury, midfoot tenderness/swelling, or TMT instability on examination, further imaging is indicated. Although some authors advocate stress views under fluoroscopy, weight-bearing radiographs more effectively stress the TMT joint and permit detection of subtle diastasis at the first intermetatarsal space [4,6] (Fig. 2). If pain precludes weight bearing, ankle block may facilitate the examination. Overlapping structures about the TMT joint often obscure midfoot fracture on conventional radiographs. Computed tomography (CT) permits improved fracture detection and, although it is a non­weight-bearing examination, may facilitate detection of subtle osseous malalignment [7]. An advantage of MRI over CT is that it can detect trabecular microfracture and bone bruise, and permits direct visualization of the Lisfranc ligament and the capsuloligamentous

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Fig. 2. AP weight-bearing radiographs of both feet. There is pathologic widening of the first intermetatarsal space with lateral subluxation of the second metatarsal with respect to the middle cuneiform (curved arrow); this is a grade II Lisfranc injury as described by Nunley and Vertullo [4]. Note the normal alignment in the comparison view of the right foot.

structures about the TMT joint [8,9]. It is important to realize that the Lisfranc ligament may appear intact on magnetic resonance imaging (MRI) in the context of mechanically significant injury (Fig. 3). Soft tissue edema on T2weighted imaging in and around the ligament should be considered suspicious for injury, as should associated bone bruise or fracture at the ligamentous origin and insertion at the medial cuneiform and second metatarsal base (Fig. 4).

Fig. 3. Axial STIR image through the mid and forefoot demonstrates an apparently intact Lisfranc ligament with surrounding soft tissue edema indicative of midfoot sprain.

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Fig. 4. Axial T1-weighted (A) and STIR (B) images of the mid and forefoot demonstrate an oblique intra-articular Lisfranc fracture (curved arrows) at the medial base of the second metatarsal. The STIR image demonstrates vague residual marrow edema.

FATIGUE FRACTURES OF THE MID AND FOREFOOT Stress fractures are characterized by bone pain and tenderness without a history of direct trauma. The fatigue type of stress fracture results from repetitive cyclical loading and prolonged muscular force on bone that has normal elastic resistance. Conventional radiographs are often unremarkable at the onset of symptoms. Fatigue fractures usually result from alteration of the duration, intensity, or manner in which a physical activity is performed. Stress fractures of the foot are relatively site specific based on the type of athletic activity. Recreational and competitive runners, basketball and football players, ice skaters, ballet dancers, and military recruits are particularly at risk. The most common midfoot stress fracture in athletes occurs in the tarsal navicular [10,11]. It is typically oriented in the midsagittal plane of the navicular (Fig. 5). The fracture may be partial, isolated to the dorsal cortex, or complete. Complete fractures may be complicated by delayed or nonunion or osteonecrosis of the lateral segment. Conventional radiographs are relatively insensitive for detection of navicular fracture. Historically, nuclear bone scintigraphy has been employed to detect clinically suspected, radiographically occult stress fractures. This has largely been supplanted by CT and MRI (Fig. 6). CT permits visualization of cortical defects, gapping at the fracture site and callus formation [12]. CT may reveal cortication or sclerosis at the fracture margins suggesting delayed or nonunion, whereas fragmentation, sclerosis, and cyst formation of the lateral fragment might suggest osteonecrosis (Fig. 7). MRI depicts the fracture as linear marrow signal abnormality, with surrounding marrow edema appearing as a penumbra of reticulated, ill-defined low T1 or bright T2 signal,

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Fig. 5. Close up radiograph demonstrates a vague linear lucency oriented in the sagittal plane in the central one third of the navicular, diagnostic of a stress fracture (curved arrow).

diminishing over time unless there is chronic instability and motion at the fracture site. Stress fracture of the ``lesser metatarsals'' most commonly occurs at the mid to distal shaft, typically affecting the second and third rays. Many factors can contribute to insufficiency of the first ray, shifting the stresses of weight bearing and ambulation from the first to the second and third rays. These include hallux valgus, metatarsus primus varus, previous corrective surgery of the first

Fig. 6. Axial CT image (A) and axial T1-weighted (B) and STIR (C) MR images through the midfoot demonstrate a navicular stress fracture. CT reveals surrounding sclerosis. STIR MRI reveals residual marrow edema.

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Fig. 7. CT images in the axial (A), coronal (B), and sagittal (C) planes demonstrate a chronic, ununited navicular stress fracture complicated by osteonecrosis and fragmentation.

ray, congenital shortening of the first ray, or a low-lying arch, all of which may predispose to stress fracture of the lesser metatarsals [13]. There are three different types of stress fracture of the proximal to midshaft fifth metatarsal. Fracture of the tip of the styloid process results from an inversion injury and results from avulsion either by the lateral cord of the plantar aponeurosis or by the peroneus brevis [14] (Fig. 8). A Jones fracture occurs approximately 1.5 to 2.0 cm distal to the tip of the tuberosity as a result dorsiflexion with the forefoot in supination [14,15] (Fig. 9); the distinction is important because of the tendency toward delayed healing or nonunion for these fractures at the junction of the metaphysis and proximal diaphysis. Midshaft fractures are related to chronic repetitive stress, and have been attributed in football players to fatigue resulting from insufficient diaphyseal support as a result of widely placed cleats [13]. Metatarsal stress fractures may be subtle or occult on conventional radiographs. Detection requires a discernible cortical defect, usually at the medial aspect of the mid to distal diaphysis. Cortical stress reaction or callus may obscure the lucent fracture line. MRI allows early visualization of stress-related marrow edema, which may be accompanied by parosteal soft tissue edema (Fig. 10). This marrow edema is nonspecific, but in the proper clinical context,

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Fig. 8. Oblique radiograph of the foot demonstrates a subtle avulsion fracture of the styloid process at the base of the fifth metatarsal (arrow).

may permit proper diagnosis and clinical intervention before progression to fracture [16]. On MRI a fracture appears as linear or band-like low signal on T1- or T2-weighted images contiguous with the cortex, with marrow edema most conspicuously demonstrated on fat-suppressed or STIR sequences (Fig. 11). Freiberg's infraction is characterized by subchondral collapse of the second or third metatarsal head with osteonecrosis and cartilaginous fissuring [17] (Fig. 12). It may result from acute or repetitive injury with vascular compromise to the subchondral bone. Radiographically, occult lesions may be visible by MRI as subchondral dark T1 and bright T2 signal. Over time, flattening and sclerosis of the metatarsal head will become radiographically evident, at which point MRI will demonstrate dark signal on both T1 and T2 weighting. Stress fractures of the phalanges are decidedly rare [18,19]. Case reports of stress fractures of the proximal phalanx of the great toe reveal a tendency toward the medial base, most commonly in the context of hallux valgus and a bipartite tibial hallucal sesamoid. Stress fracture of the proximal phalanx of the second toe is exceedingly rare, presenting with pain in the region of the metatarsal head. Most cases of phalangeal stress fractures occurred in young elite athletes engaged in basketball, volleyball, running, or ballet. SESAMOIDITIS Sesamoiditis is a clinical term used generically to refer to painful conditions in and around the region of the hallucal sesamoids. Some expand the term to refer

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Fig. 9. Close-up radiograph demonstrates a transverse fracture through the tuberosity of the fifth metatarsal; this is a Jones fracture.

to all painful conditions at the first MTP joint. Yet other authors have more specifically reserved the term to indicate chondromalacia of the sesamoids. Depending on its definition, this may account for up to 4% of overuse injuries of the foot [20] (Fig. 13). There is a general consensus that the condition results from overload at the plantar aspect of the first MTP joint. This may be related to acute injury or chronic repetitive trauma. Predisposing risk factors include wearing highheeled shoes, dancing, sports, and a cavus foot deformity with a rigidly plantar flexed first ray [21]. Patients may present with symptoms of sesamoiditis in the context of inflammatory arthritis, osteoarthritis, osteochondritis, or chondromalacia at the metatarso-sesamoid articulation. Alternatively, there may be stress fracture or osteonecrosis of the sesamoid [22] (Fig. 14). Imaging must include standard weight-bearing AP and lateral radiographs to assess congenital forefoot deformities and possibly identify arthritic changes. A sesamoid view is essentially an oblique coronally oriented radiograph, obtained tangential to the metatarso-sesamoid joint, which permits direct visualization of the joint space and articular surfaces, and eliminates osseous superimposition. Over time, radiographs may reveal fragmentation and sclerosis of the sesamoids. Nuclear bone scintigraphy is sensitive for demonstration of pathologic radiotracer uptake in the sesamoid region but does not effectively narrow the differential diagnosis. As compared with conventional radiography, CT affords

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Fig. 10. Sagittal STIR (A) and coronal T1-weighted (B) MR images demonstrate stress-related marrow edema in the midshaft of the fourth metatarsal without a discernible fracture line.

more sensitive and specific detection of fracture, and may permit visualization of periostitis, callus formation, articular irregularity, and pseudocyst formation, as well as subarticular or articular collapse of osteonecrosis. MRI may be reserved for cases in which CT is unrevealing, as in stress-related marrow edema, occult fracture, early osteonecrosis, or chondromalacia [23]. In addition to elucidating radiographically occult osseous changes, MRI delineates reactive soft tissue changes, including synovitis, tendonitis, and bursitis. TURF TOE The introduction of artifical sports surfaces in the late 1960s heralded a marked increase in injuries to the capsuloligamentous structures of the first MTP joint, presumably because of the higher friction coefficient of Astroturf as compared with grass. It is for this reason that the term ``turf toe'' was coined to describe this sports-related injury [24]. Turf toe is broadly defined by the Orthopedic Foot and Ankle Society as a ``plantar capsular ligament sprain'' of the first MTP joint. The mechanism of injury in the majority of cases is forced hyperextension. The injury occurs when the forefoot becomes fixed as a result of high friction and is positioned plantigrade with slight dorsiflexion and elevation of the heel off of the ground. Subsequently, an external force (another player) forces the first MTP joint into

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Fig. 11. Axial T1-weighted (A), axial STIR (B), sagittal STIR (C), and coronal T1-weighted (D) MR images demonstrate a midshaft second metatarsal stress fracture with a persistent linear fracture defect and exhuberant peripheral callus, with both marrow and parosteal soft tissue edema.

an even greater degree of dorsiflexion with a resultant tear of the capsular attachment at the level of the first metatarsal, which is its weakest point. The soft tissue injury may be complicated by cartilaginous or subchondral injury, as well as sesamoid fracture. American football cleats have evolved to include an increased numbers of cleats, with greater flexibility of the forefoot. Both of these adaptations have been associated with an increased incidence of turf toe [25]. Although it has not been proven, hardening of the artifical turf over time may have a small contributory role to the increased incidence of turf toe [26]. The diagnosis is often evident from the history. Clinically the patient presents with acute inflammation of the first MTP joint, which worsens over the first day. Painful guarding limits active range of motion. Nevertheless, passive ranging reveals a pathologically increased range of motion, often more than 100 (as compared with a normal of 65 dorsiflexion from a neutral position) reflecting plantar capsuloligamentous insufficiency. Pain is typically worst at the plantar surface of the first MTP joint and is potentiated with passive

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Fig. 12. Axial STIR (A) and sagittal T1-weighted and STIR (B,C) MR images demonstrate crescentic low-signal marrow changes (arrows) in the subarticular second metatarsal head with flattening of the subchondral cortex and associated marrow and soft tissue edema.

dorsiflexion. Turf toe may be complicated by associated dorsal dislocation of the great toe [27]. Conventional radiographs may be used in the differential diagnosis of possible fracture or dislocation about the first MTP joint. Alternatively, sesamoiditis, tendonitis, and bursitis may be considered; however, sesamoiditis may be

Fig. 13. Coronal T1-weighted (A) and STIR (B) images through the forefoot at the level of the first metatarsal head demonstrate sesamoiditis, manifest as uniform loss of fatty marrow signal localized to the tibial hallucal sesamoid (arrowhead). There is no contour defect or linear marrow signal alteration to suggest fracture or osteonecrosis.

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Fig. 14. Sagittal STIR image demonstrates marrow edema within the tibial hallucal sesamoid. The curved white arrow indicates a linear fracture line without displacement or diastasis.

differentiated clinically from turf toe by its more indolent onset and association with repetitive trauma rather than acute, traumatic hyperextension of the first MTP joint. The gold standard for diagnosis of turf toe is MRI, which permits direct visualization of a tear through the plantar capsule [28]. MRI also allows direct visualization of concomitant soft tissue injury including synovitis, plantar soft tissue swelling, and tendonitis of the flexor hallucis longus and adductor hallucis, as well as possible associated osseous or cartilaginous injury to the sesamoids or first metatarsal (Fig. 15). PLANTAR PLATE INJURY OF THE LESSER MTP JOINTS AND METATARSALGIA Metatarsalgia is a generic term applied to a spectrum of painful conditions in the region of the metatarsal heads resulting from chronic repetitive stress at the forefoot, most commonly affecting the second MTP joint. Differential diagnosis of metatarsalgia includes plantar plate injury, MTP joint synovitis, stress fracture, Freiberg's infraction (osteonecrosis of the metatarsal head), arthritis, interdigital (aka Morton's) neuroma, and synovial cyst formation. The plantar plate of the lesser MTP joints primarily differs from that of the first MTP joint by the absence of the hallucal sesamoids. That means that the plantar plate articulates directly with the plantar surface of the lesser metatarsal head and functions without the benefit of the sesamoids to provide critical articular stability and shock absorption. Whereas turf toe represents a sportsrelated acute traumatic rupture of the plantar plate of the first MTP joint, rupture of the plantar plate of the lesser MTP joints is typically a chronic acquired degenerative condition, developed over time as a result of increased loading [29]. The plantar plate is a firm, flexible fibrocartilaginous structure that has a mean length of 20 mm and average thickness of 2 mm at the second MTP joint [30]. Similar to the hallux, the plantar plate serves as the central attachment for ligamentous, capsular, and tendinous structures at the lesser MTP joint. It represents the distal insertion of the plantar fascia. The plantar third of the fibrocartilaginous plate blends with the deep transverse intermetatarsal

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Fig. 15. Coronal (A) and sagittal (B) STIR images through the forefoot demonstrate soft tissue edema plantar to the first metatarsal head in the region of the sesamoids and plantar plate. Straight arrows (A) indicate the sesamoids; an arrowhead indicates the flexor hallucis longus tendon. Curved arrows (A, B) demonstrate defects in the plantar plate in the intersesamoidal region and at the capsular attachment. Sagittal (C) and axial STIR (D) images demonstrate associated soft tissue edema in the adductor hallucis musculature.

ligament, whereas the dorsal surface has a smooth, articular-like surface, gliding deep to the metatarsal head during ambulation. Paired accessory collateral ligaments (ACL) course proximal-to-distal and dorsal-to-plantar originating at the dorsal tubercle of the lesser metatarsals to broadly insert on the medial and lateral margins of the plantar plate. Smaller, more obliquely oriented paired phalangeal collateral ligaments (PCL) also arise from the dorsal tubercle, but share a conjoint insertion along with the plantar plate at the medial and lateral base of the proximal phalanx [30]. The flexor tendon sheath is cradled within a central concavity at the deep surface of the plantar plate, anchored by a fibrous pulley [31]. The tendon sheath contains the flexor digitorum brevis (FDB) and the flexor digitorm longus (FDL) tendons. The FDB splits to straddle the FDL at the level of the proximal interphalangeal (PIP) joint to insert bilaterally onto the base of the middle phalanx, whereas the FDL inserts onto the plantar base of the distal phalanx. Dorsally, the extensor hood and sling represent a fibroaponeurotic expansion extending bilaterally from the borders of the extensor digitorum longus (EDL) tendon sheath, with direct insertions onto the plantar plate, the deep transverse intermetatarsal ligament, and base of the proximal phalanx [30].

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MTP joint synovitis most commonly results from chronic excessive loading of the MTP joint [32]. At the lesser MTP joints, compressive and tensile forces of weight bearing and ambulation are greatest at the second ray and are increased in the context of hallux valgus or developmental elongation of the second metatarsal. Shoe gear with elevated heels and a narrow toe box increases axial loading, to the greatest degree at the second MTP joint. Chronic synovitis often stretches the joint capsule and contributes to MTP joint instability [33]. Degeneration and attritional change of the plantar plate and collateral ligaments may ensue. MTP joint instability often accompanies plantar plate degeneration and rupture. Symptoms include pain and capsular and submetatarsal swelling. Pain is typically worst in the toe-off phase of ambulation, at which time the tensile forces across the degenerated plantar plate are maximal. Instability is detected and quantified by the Vertical Stress Test, which is simply performed by stabilizing the metatarsal head and forcibly displacing the proximal phalanx dorsally. A positive test not only reveals instability, but elicits pain at the dorsal base of the proximal phalanx. Plantar plate rupture most commonly occurs at the distal, lateral insertion onto the base of the proximal phalanx. High-resolution MRI of the forefoot is the gold standard for imaging of plantar plate rupture and differentiating it from other possible causes of metatarsalgia. Coronal (short axis) MR images through the forefoot demonstrate the plantar plate as a thick low signal band deep to the metatarsal head, thinnest centrally and thickest distally. A shallow groove at the central plantar surface accommodates the flexor tendon sheath (Fig. 16A). Collateral ligaments are seen as vertically oriented bands medially and laterally, inserting bilaterally onto the margins of the plantar plate and the base of the proximal phalanx (Fig. 16C,D). Oblique sagittal images are plotted off of an axial localizer along the axis of the second metatarsal shaft. In the normal, oblique sagittal imaging permits visualization of a distinct, narrow zone of high signal intensity representing hyaline cartilage undercutting the low signal fibrocartilage [34] near the distal insertion of the plantar plate, which should not measure more than 2.5 mm [29] (Fig. 16B). In plane visualization of the ACL and PCL is inconstant and fortuitous in the oblique sagittal plane. Whereas axial (long axis) imaging is not useful in detection of plantar plate or collateral ligament rupture, it permits qualitative evaluation of hallux valgus, second metatarsal protrusion, and identification of possible marrow signal abnormalities attendant to stress injury, osteonecrosis, and arthritis. In the context of plantar plate degeneration or rupture there is pathologic elongation and marginal indistinctness of the high signal intensity zone at the distal insertion of the plantar plate [29] (Fig. 17A). With capsular insufficiency and its attendant plantar plate and ligamentous degeneration, there is progressive hyperextension of the toe at the MTP joint. Degenerative thickening or thinning and signal distortion of the plantar plate and/or collateral ligaments is best demonstrated in the coronal plane. A rupture, seen as a high signal

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Fig. 16. Coronal 2D-Gradient Recalled Echo (GRE) image (A) at the level of the second metatarsal head demonstrates the intact plantar plate (arrowheads) with the subjacent flexor digitorum tendon (curved arrow). Sagittal 2D-GRE images demonstrate normal anatomy. (B) The plantar plate lies subjacent to the second metatarsal head (arrow); note the focal high signal zone representing undercutting of hyaline cartilage at the distal margin of the fibrocartilaginous plate (curved arrow). The arrowhead indicates the flexor digitorum tendon. (C) The phalangeal collateral ligament (arrow) is coursing obliquely from the dorsal tubercle of the second metatarsal to its conjoint insertion with the plantar plate at the base of the proximal phalanx (curved arrow). (D) The broader, more vertically oriented accessory collateral ligament (arrowhead).

defect on fluid-sensitive sequences, most commonly at the distal lateral conjoint insertion of the plantar plate and PCL at the base of the proximal phalanx (Fig. 17C), is often accompanied by medial displacement of the plantar plate with respect to the metatarsal head [29]. Partial tear may be associated with adjacent ganglion formation (Fig. 17D). Complete rupture may be associated with dorsal dislocation of the toe (Fig. 17B). Coronal fluid-sensitive sequences best demonstrate synovitis, submetatarsal soft tissue edema, and intermetatarsal bursitis, all of which are common in the setting of plantar plate degeneration. SUMMARY Imaging sports-related injuries of the mid and forefoot complements the physical examination and clinical history. Stress fractures may be radiographically occult, in which case CT may facilitate detection, or MRI may be necessary

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Fig. 17. Sagittal 2D-GRE image (A) demonstrates pathologic elongation of the high signal zone, indicative of degenerative tearing of the plantar plate; note hyperextension of the digit at the MTP joint. (B) Complete bilateral plantar plate rupture (arrow) with dorsal dislocation of the second toe. Coronal 2D-GRE image (C) demonstrates complete rupture at the lateral insertion of the plantar plate and phalangeal collateral ligament onto the base of the second proximal phalanx (black arrow). Coronal image (D) demonstrates a ganglion (white arrow) related to a partial tear at the lateral aspect of the plantar plate.

for identification of marrow signal changes in the absence of discernible cortical defects. Timely detection of stress-related marrow edema may permit early clinical intervention and prevent evolution to fracture, hastening the athlete's to return to training and competition. MRI has revolutionized the evaluation of soft tissue injury with or without associated occult osseous injury. As with footwear, however, MRI of the foot is not a one-size-fits-all proposition. In most individuals, it is not possible to image the foot from heel to toe without exceeding the limits of the surface coil or compromising the quality of the examination by field inhomogeneity or failure of fat suppression. It is important to tailor the MR examination of the foot to address the specific area of clinical concern. Ideally, imaging should be focused to the region of interest, be it the hindfoot, midfoot, or forefoot, so that protocols can be optimized to permit small field of view, high-resolution imaging. This is particularly crucial in imaging the forefoot in assessing small and subtle derangements of the capsuloligamentous, myotendinous, and osseous structures of the digits.

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