(Radiographics. 2000;20:S153-S179.)
© RSNA, 2000
MR Imaging of the Ankle and Foot1
Zehava S. Rosenberg, MD,
Javier Beltran, MD and
Jenny T. Bencardino, MD
1 From the Department of Radiology, Hospital for Joint Diseases, NYU Medical Center, 305 E 17th St, New York, NY 10003 (Z.S.R.); the Department of Radiology, Maimonides Medical Center, Brooklyn, NY (J.B.); and the Department of Radiology, Massachusetts General Hospital, Harvard Medical School, Boston, Mass (J.T.B.). Presented as a refresher course at the 1999 RSNA scientific assembly. Received March 22, 2000; revision requested May 23 and received June 30; accepted July 5. Address correspondence to Z.S.R. (e-mail: zehava.rosenberg@usa.net).
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Abstract
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Magnetic resonance (MR) imaging has opened new horizons in the diagnosis and treatment of many musculoskeletal diseases of the ankle and foot. It demonstrates abnormalities in the bones and soft tissues before they become evident at other imaging modalities. The exquisite soft-tissue contrast resolution, noninvasive nature, and multiplanar capabilities of MR imaging make it especially valuable for the detection and assessment of a variety of soft-tissue disorders of the ligaments (eg, sprain), tendons (tendinosis, peritendinosis, tenosynovitis, entrapment, rupture, dislocation), and other soft-tissue structures (eg, anterolateral impingement syndrome, sinus tarsi syndrome, compressive neuropathies [eg, tarsal tunnel syndrome, Morton neuroma], synovial disorders). MR imaging has also been shown to be highly sensitive in the detection and staging of a number of musculoskeletal infections including cellulitis, soft-tissue abscesses, and osteomyelitis. In addition, MR imaging is excellent for the early detection and assessment of a number of osseous abnormalities such as bone contusions, stress and insufficiency fractures, osteochondral fractures, osteonecrosis, and transient bone marrow edema. MR imaging is increasingly being recognized as the modality of choice for assessment of pathologic conditions of the ankle and foot.
Index Terms: Ankle, anatomy, 46.92 • Ankle, fractures, 46.41 • Ankle, injuries, 46.41, 46.48 • Ankle, MR, 46.1214 • Foot, anatomy, 46.92 • Foot, fractures, 46.41 • Foot, injuries, 46.48 • Foot, MR, 46.1214
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Introduction
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The past 15 years have witnessed an explosion of information regarding the role of magnetic resonance (MR) imaging in assessing pathologic conditions of the ankle and foot. MR imaging has revitalized the study of musculoskeletal disease in this anatomic area due to its high soft-tissue contrast resolution and multiplanar capabilities. It provides a quick, noninvasive tool for the diagnosis of related injuries, which are often difficult to diagnose with alternative modalities. MR imaging is particularly advantageous for assessing soft-tissue structures around the ankle such as tendons, ligaments, nerves, and fascia and for detecting occult bone injuries. In this article, we review the normal MR imaging anatomy of the ankle and foot and discuss numerous related soft-tissue and osseous abnormalities that are seen with this modality.
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Imaging Technique
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Routine ankle MR imaging is performed in the axial, coronal, and sagittal planes parallel to the table top. The foot is imaged in the oblique axial plane (ie, parallel to the long axis of the metatarsal bones), oblique coronal plane (ie, perpendicular to the long axis of the metatarsals), and oblique sagittal plane (Fig 1). The patient is supine with the foot in about 20° of plantar flexion. Plantar flexion is useful for three reasons: it decreases the magic angle effect, it accentuates the fat plane between the peroneal tendons, and it allows better visualization of the calcaneofibular ligament. An extremity surface coil is used to enhance spatial resolution. A wrist coil or other small dedicated coils are often used to evaluate the distal foot. T1-weighted (repetition time msec/echo time msec = 600/20) and T2-weighted (2,000/20,80) MR images are obtained with a 12–16-cm field of view, a 256 x 192–512 acquisition matrix, 1–2 signals acquired, and a 3–5-mm section thickness with 1-mm intervals.
Marrow abnormalities are best evaluated with fat suppression techniques such as fat-suppressed proton-density–weighted imaging or with short-inversion-time inversion recovery (STIR) sequences (1,500/20; inversion time msec = 100–150). However, susceptibility to gradient inhomogeneity makes fat suppression techniques less optimal than STIR techniques in imaging the ankle and foot. Cartilage abnormalities can be visualized with two-dimensional or three-dimensional (3D) gradient-echo sequences.
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Normal Ligamentous Anatomy
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Three ligamentous groups support the ankle joint. The syndesmotic ligamentous complex is composed of the anterior and posterior tibiofibular and interosseous ligaments. The lateral collateral ligament is subdivided into the anterior talofibular, posterior talofibular, and calcaneofibular ligaments. The deltoid ligament has five bands: the anterior and posterior tibiotalar ligaments and the tibiospring, tibiocalcaneal, and tibionavicular ligaments.
The ligaments are readily identified as thin, linear, low-signal-intensity structures joining adjacent bones and are usually delineated by contiguous high-signal-intensity fat (1). Heterogeneity is typically seen due to the interposition of fat between the ligamentous fibers. This is particularly true for the anterior tibiofibular ligament, the tibiotalar components of the deltoid ligament, and the posterior talofibular ligament. Axial and coronal imaging with the foot in dorsiflexion and plantar flexion have been recommended to allow visualization of the ligaments in their entirety (2). The ligaments can also be studied with 3D Fourier transform reformatted images (3). In our experience, however, the ligaments can be reliably
evaluated on routine orthogonal ankle MR images as long as the section thickness is 3 mm or less.
The anterior and posterior tibiofibular ligaments are usually seen on two or more sequential axial and coronal MR images obtained at the level of the tibial plafond and talar dome (Fig 2). On axial images, these ligaments often appear striated and discontinuous owing to fat interposed between the fascicles of the ligaments and the downward, oblique course of the ligaments toward their insertion on the fibula.
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Figure 2a. Normal tibiofibular ligaments. (a) Axial T1-weighted MR image obtained at the joint level demonstrates the anterior (straight arrows) and posterior (curved arrow) tibiofibular ligaments. (b) Coronal T1-weighted MR image shows the posterior tibiofibular (straight arrow) and posterior talofibular (curved arrow) ligaments.
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Figure 2b. Normal tibiofibular ligaments. (a) Axial T1-weighted MR image obtained at the joint level demonstrates the anterior (straight arrows) and posterior (curved arrow) tibiofibular ligaments. (b) Coronal T1-weighted MR image shows the posterior tibiofibular (straight arrow) and posterior talofibular (curved arrow) ligaments.
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The anterior and posterior talofibular ligaments are usually seen on a single axial image obtained slightly distal to the tibiofibular ligaments. Fluid within the joint serves to highlight the anterior talofibular ligament on T2-weighted images. This ligament appears as a thin, straight, low-signal-intensity band extending from the talus to the fibular malleolus (Fig 3). The posterior talofibular ligament has a fan-shaped insertion on the distal fibula and may demonstrate marked heterogeneity and thickening, which should not be misinterpreted as a tear (Fig 3).
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Figure 3. Normal talofibular ligaments. Axial T1-weighted MR image depicts the anterior talofibular ligament (arrow). The posterior talofibular ligament normally demonstrates a striated pattern due to interspersed fat (*). Note the oblong shape of the talus as well as the medial indentation of the fibula (F), which represents the malleolar fossa. (Reprinted, with permission, from reference 4.)
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The morphologic features of the talus and the distal fibula can help distinguish the anterior and posterior tibiofibular ligaments from the anterior and posterior talofibular ligaments on axial MR images. The talar dome, where the tibiofibular ligaments are detected, is somewhat square. In addition, the ligaments insert onto the fibula above the malleolar fossa, where the cross-section of the fibula is round. Conversely, the talus is more oblong and the sinus tarsi is partially visualized at the insertion sites of the talofibular ligaments. The fibula demonstrates a medial indentation representing the malleolar fossa.
The calcaneofibular ligament is frequently seen as a band of low signal intensity parallel to the lateral calcaneal wall on routine axial MR images obtained with the foot in plantar flexion (Fig 4a). However, the ligament is more consistently visualized on coronal images, where it is depicted in cross-section as a round, homogeneous, low-signal-intensity structure that can be followed up on sequential T1-weighted images from its fibular origin to its calcaneal insertion site (Fig 4b). The various components of the deltoid ligament are well visualized on both axial and coronal images. The deep, tibiotalar component of the deltoid ligament normally demonstrates regular striations and thus has a heterogeneous appearance (Fig 5).
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Figure 4a. Normal calcaneofibular ligament. (a) Axial T1-weighted MR image shows the calcaneofibular ligament (straight arrows) immediately adjacent to the peroneal tendons (curved arrow). (b) Sequential coronal T1-weighted MR images display the calcaneofibular ligament as a round, hypointense structure (arrow) extending from the lateral malleolar tip (F) to the lateral wall of the calcaneus (C).
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Figure 4b. Normal calcaneofibular ligament. (a) Axial T1-weighted MR image shows the calcaneofibular ligament (straight arrows) immediately adjacent to the peroneal tendons (curved arrow). (b) Sequential coronal T1-weighted MR images display the calcaneofibular ligament as a round, hypointense structure (arrow) extending from the lateral malleolar tip (F) to the lateral wall of the calcaneus (C).
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Ligamentous Injuries
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Lateral ankle sprains represent 16%–21% of all sports-related traumatic lesions. The anterior talofibular ligament is the weakest ligament and therefore the most frequently torn. There is usually a predictable pattern of injury involving the anterior talofibular ligament followed by the calcaneofibular ligament and the posterior talofibular ligament. Anatomic classification of ankle sprains is based on the number of affected ligaments. First-degree sprain is characterized by a partial or complete tear of the anterior talofibular ligament. In second-degree sprain, both the anterior talofibular and calcaneofibular ligaments are either partially or completely torn. Third-degree sprain consists of injuries to the anterior talofibular, calcaneofibular, and posterior talofibular ligaments.
Because acute ankle ligamentous injuries are rarely treated surgically, the use of MR imaging is limited to the evaluation of athletes at advanced competitive levels in whom primary ligamentous surgical repair is contemplated and of patients with a history of chronic ankle instability (5). The MR imaging criteria for the diagnosis of acute rupture of the lateral collateral ligament include morphologic and signal intensity alterations within and around the ligament (2,6). Injuries of the anterior talofibular ligament are easily seen on routine axial ankle MR images. Discontinuity, detachment, thickening, thinning, or irregularity of the ligament may be encountered. Heterogeneity with increased intraligamentous signal intensity on fat-suppressed or T2-weighted images is indicative of intrasubstance edema or hemorrhage. Obliteration of the fat planes around the ligament, extravasation of joint fluid into the adjacent soft tissues, and talar contusions may also be seen. Chronic tear often manifests as thickening, thinning, elongation, and wavy or irregular contour of the ligament. There is usually no significant residual marrow or soft-tissue edema or hemorrhage (Fig 6). Decreased signal intensity in the fat abutting the ligaments with all pulse sequences is indicative of scarring or synovial proliferation.
Injuries of the calcaneofibular ligament may be detected on routine axial ankle MR images but are more consistently visualized on coronal T1-weighted images (Fig 7). On sequential coronal images, the normal calcaneofibular ligament is seen in cross-section as a low-signal-intensity, homogeneous, oval structure surrounded by fat. The injured ligament is frequently thickened (Fig 8) and heterogeneous, and the surrounding fat planes are often obliterated. Fluid within the peroneal tendon sheath can be a secondary sign of calcaneofibular ligament injury.
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Figure 7. Injury of the calcaneofibular ligament. Sequential coronal T1-weighted MR images demonstrate increased signal intensity and thickening of the calcaneofibular ligament (*) between the peroneal tendons (p) and the lateral wall of the calcaneus (c).
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MR imaging findings suggest that contusions of the deltoid ligament, particularly of its tibiotalar component, are frequently associated with inversion sprains (7). These contusions manifest as loss of the regular striations that are normally seen in the deltoid ligament (Fig 9). Thus, contrary to what one would expect, the ligament demonstrates homogeneous intermediate signal intensity, a finding that is consistent with injury. Reactive fluid within the tendon sheath of the posterior tibial tendon is also frequently noted.
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Figure 9a. Injury of the deltoid ligament. Coronal (a) and axial (b) T1-weighted MR images show indistinctness and swelling of the deltoid ligament as well as loss of the normal pattern of fatty striation (*), findings that are consistent with extensive partial tear. Some fibers of the tibionavicular ligament are still present (arrow in b).
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Figure 9b. Injury of the deltoid ligament. Coronal (a) and axial (b) T1-weighted MR images show indistinctness and swelling of the deltoid ligament as well as loss of the normal pattern of fatty striation (*), findings that are consistent with extensive partial tear. Some fibers of the tibionavicular ligament are still present (arrow in b).
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The accuracy of MR imaging in detecting injuries of the lateral collateral ligament has not yet been clearly established. The accuracy of 3D fast imaging with steady state precision in detecting acute tears of the anterior talofibular and calcaneofibular ligaments is reported to be 94.4% (3). MR arthrography has been shown to have an accuracy of 100% and 82% in detecting chronic anterior talofibular and calcaneofibular ligament tears, respectively, whereas conventional MR imaging has demonstrated an accuracy of 59% in diagnosing chronic lateral collateral ligament tears (8).
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Anterolateral Impingement Syndrome
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Anterolateral impingement syndrome is a common cause of chronic lateral ankle pain. Injuries to the anterior talofibular and tibiofibular ligaments and an accessory fascicle of the anterior talofibular ligament have been implicated as causes of anterolateral impingement syndrome. Repetitive synovial inflammation secondary to chronic lateral ankle instability produces a soft-tissue "mass" consisting of hypertrophic synovial tissue and fibrosis within the lateral gutter. Anteromedial or anterocentral bone impingement owing to osteophytes at the anterior ankle joint can exacerbate the condition (Fig 10). Arthroscopic debridement of soft-tissue impingement has produced excellent results, with 84% of patients returning to their previous sport activity (9).
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Figure 10. Anterolateral impingement syndrome. Sagittal STIR MR image shows a low-signal-intensity "meniscoid" mass (black arrow) related to redundant synovial tissue. Anterior tibial and talar "kissing" osteophytes are also noted (white arrows).
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MR imaging typically depicts a "meniscoid" mass within the lateral gutter of the ankle that demonstrates low signal intensity with all pulse sequences (Fig 10). This soft-tissue structure is best visualized on axial or coronal images when joint fluid is present within the lateral gutter. Accurate diagnosis necessitates distinguishing this mass from the adjacent anterior talofibular ligament. The accuracy of routine MR imaging in diagnosing soft-tissue anterolateral impingement syndrome has been questioned, particularly when no joint fluid is present (10–12). MR arthrography may be of value in equivocal cases.
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Sinus Tarsi Syndrome
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The sinus tarsi is a lateral space located between the talus and the calcaneus. It contains the cervical and interosseous talocalcaneal ligaments, the medial roots of the inferior extensor retinaculum, neurovascular structures, and fat. Sinus tarsi syndrome is caused by hemorrhage or inflammation of the synovial recesses of the sinus tarsi with or without tears of the associated ligaments. This disease entity commonly occurs following an inversion injury and is often associated with tears of the lateral collateral ligaments. It may also be related to rheumatologic disorders and abnormal biomechanics such as flat foot deformity secondary to posterior tibial tendon tear. Patients with sinus tarsi syndrome present with hindfoot instability and pain along the lateral aspect of the foot. Prior to the advent of MR imaging, arthrography of the subtalar joint and relief of pain following injection of a local anesthetic or steroid were the only techniques for diagnosing this syndrome. The MR imaging characteristics of sinus tarsi syndrome include the obliteration of fat in the sinus tarsi space. The space itself is replaced by either fluid or scar tissue, and the ligaments may be disrupted (Fig 11) (13,14). Osteoarthritis of the subtalar joint and subchondral cysts may be present in advanced cases. Normal recesses from the posterior subtalar joint may frequently extend into the sinus tarsi and should not be misinterpreted as pathologic conditions.
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Figure 11. Sinus tarsi syndrome in a patient with rheumatoid arthritis. Sagittal T1-weighted MR image shows obliteration of fat by an area of fluid-like signal intensity in the subtalar joint (*).
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Normal Tendon Anatomy
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The ankle tendons are well visualized as low-signal-intensity structures with all MR imaging sequences. In general, T1-weighted images provide good anatomic detail, whereas T2-weighted images are useful for assessing the abnormal increase in water that characterizes most pathologic conditions. Axial images are optimal for assessing morphologic features, longitudinal splits, tendon sheath fluid, and adjacent soft-tissue abnormalities. Sagittal images are most useful for depicting disease of the Achilles tendon. Coronal images are the least useful for assessing tendon disease. Occasionally, a minimal amount of fluid is noted within the tendon sheath, but this finding is clinically insignificant. Fluid within the sheath of the flexor hallucis longus tendon is common because of the normal communication between the sheath and the ankle joint.
The magic angle effect produces increased signal within normal tendons when they form an angle of about 55° with the main magnetic vector (15). This phenomenon is usually seen with echo times less than 20 msec (T1-weighted, proton-density–weighted, or gradient-echo sequences) and is quite common in the ankle tendons because they curve around the ankle joint. The posterior tibial tendon is particularly susceptible to the magic angle effect at its insertion on the navicular bone (Fig 12). Striation of the tendon at that site is also due to fat interposed between the several insertional fascicles of the tendon and should not be misinterpreted as a pathologic condition. Imaging the ankle in about 20° of plantar flexion decreases the angle between the tendons and the main magnetic vector and is therefore quite useful in decreasing the magic angle effect.
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Figure 12. Magic angle effect in an asymptomatic patient. Axial proton-density-weighted MR image demonstrates increased signal intensity within the posterior tibial (curved arrow), flexor digitorum longus (straight solid arrow), and flexor hallucis longus (open arrow) tendons. (Reprinted, with permission, from reference 16.)
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Tendon Injuries
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Tendon injuries can be grouped into six categories: tendinosis, peritendinosis, tenosynovitis, entrapment, rupture, and dislocation (17,18). These conditions often coexist, and overlap in their clinical, gross, and histologic manifestations can make them indistinguishable at MR imaging (6). The MR imaging characteristics of tendinosis include a fusiform shape and focal areas of increased tendon girth associated with increased signal intensity within the tendon on T1-weighted and proton-density–weighted images. T2 signal intensity alterations are noted when significant intrasubstance degeneration is present. Tenosynovitis and peritendinosis are caused by inflammation or mechanical irritation of the tendon sheath and peritenon, respectively. MR images reveal fluid accumulation, synovial proliferation, or scarring within the tendon sheath or adjacent soft tissues. Stenosing tenosynovitis occurs when synovial proliferation and fibrosis surround the tendon, causing entrapment and even rupture. It manifests as areas of intermediate to low signal intensity in the soft tissues around the tendon with all MR imaging sequences.
Partial rupture manifests on T1-weighted and proton-density–weighted images and occasionally on T2-weighted images as an area within the substance of the tendon having a signal intensity similar to that seen in advanced tendinosis. Complete rupture is depicted as complete disruption of the tendon fibers. MR imaging is useful in the detection of dislocation and subluxation of the peroneal and posterior tibial tendons and in the assessment of concomitant tendon disease.
Achilles Tendon Injuries
Achilles tendon injuries may be classified as noninsertional or insertional (17,18). The former group includes diffuse acute and chronic peritendinosis, tendinosis, and a rupture 2–6 cm above the insertion of the tendon on the calcaneus (19). The latter group includes insertional Achilles tendinosis, which may be associated with Haglund deformity of the calcaneus. Weinstabi et al (19) classified Achilles tendon lesions into four types on the basis of MR imaging findings. Type I represents inflammatory reaction; type II, degenerative changes; type III, partial rupture; and type IV, complete rupture.
The Achilles tendon lacks a tendon sheath. However, it has a peritenon whose vascular system extends both within and outside the tendon. Achilles peritendinosis manifests at MR imaging as linear or irregular areas of altered signal intensity in the pre–Achilles tendon fat pad, a finding that indicates the presence of edema (Fig 13), or scarring of the peritenon. The tendon itself is normal. Achilles tendinosis manifests on axial MR images as loss of the anterior concave or flat surface of the Achilles tendon and on sagittal images as fusiform thickening of the tendon (Fig 14) (20). Areas of increased signal intensity within the tendon are also noted.
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Figure 13. Acute Achilles peritendinosis. Sagittal T2-weighted MR image shows a reticular pattern of increased signal intensity in the pre-Achilles tendon fat (*), a finding that indicates the presence of edema.
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Figure 14. Chronic tendinosis of the Achilles tendon. Sagittal T1-weighted MR image shows fusiform thickening of the Achilles tendon without evidence of increased intrasubstance signal intensity (arrows).
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Preoperative MR imaging is useful for distinguishing partial from complete rupture and assessing the site and extent of the tear. Clinical misdiagnosis has been reported in up to 25% of patients with complete tears of the Achilles tendon due to swelling that obscures the tendon gap and retained weak plantar flexion (false-negative Thompson test). At MR imaging, partial Achilles tendon tears demonstrate heterogeneous signal intensity and thickening of the tendon without complete interruption (Fig 15). Differentiation between partial tear and severe chronic Achilles tendinosis may be difficult apart from clinical history. However, acute partial tears are often associated with subcutaneous edema, hemorrhage within the Kager fat pad, and intratendinous hemorrhage at MR imaging, whereas chronic tendinosis does not usually demonstrate increased subcutaneous or intratendinous signal intensity on T2-weighted images. Complete Achilles tendon rupture manifests as discontinuity with fraying and retraction of the torn edges of the tendon (Fig 16). In acute rupture, the tendon gap demonstrates intermediate signal intensity on T1-weighted images and high signal intensity on T2-weighted images, findings that are consistent with edema and hemorrhage, whereas in chronic ruptures, scar or fat may replace the tendon. Partial rerupture occurs in approximately 2% of surgically treated Achilles tendon ruptures (Fig 17). Postoperative MR imaging assessment includes evaluation of the extent of tendinous union and healing. On most follow-up MR imaging studies, intratendinous signal intensity will decrease as the tendon heals (21). However, the tendon may remain thickened, simulating chronic tendinosis, even after normal signal intensity has been regained.
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Figure 15. Partial tear of the Achilles tendon. Sagittal T2-weighted MR image demonstrates a thickened Achilles tendon containing irregular areas of high signal intensity and focal discontinuity of posterior fibers (arrow), findings that are consistent with partial tear.
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Figure 16. Complete tear of the Achilles tendon. Sagittal T2-weighted MR image depicts complete disruption and retraction of the torn edges of the Achilles tendon (arrows) with a fluid-filled gap (*).
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Figure 17. Partial retear of the Achilles tendon following surgical repair in a patient with recurrent symptoms. Sagittal STIR MR image demonstrates thickening and increased signal intensity at the surgical site (arrows) related to retear. Note the punctate and serpentine areas of signal void indicating the presence of surgical material.
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Insertional tendinosis is frequently associated with Haglund deformity of the calcaneus but may also be related to an ill-fitting shoe or to overuse. Focal pain at the insertion site of the Achilles tendon is usually present at clinical examination. Initial treatment of this condition is conservative. However, surgical treatment with removal of intrasubstance calcifications and bone osteophytes as well as resection of the Haglund deformity has proved highly successful. MR imaging findings include increased signal intensity and thickening at the insertion site of the Achilles tendon, intrasubstance calcifications, Haglund deformity, calcaneal marrow edema, and distended retrocalcaneal and Achilles bursitis (Fig 18).
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Figure 18. Insertional partial tear of the Achilles tendon. Sagittal STIR MR image shows increased signal intensity at the insertion site of the Achilles tendon (white arrowheads) associated with retrocalcaneal bursitis (black arrowhead). A prominent posterosuperior calcaneal tuberosity (Haglund deformity) and edematous bone marrow (*) are also noted.
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Fusiform thickening of the Achilles tendon associated with intrasubstance heterogeneity and stippling are consistent with the presence of xanthoma (Fig 19) (22). This entity is usually found in the presence of familial hypercholesterolemia and should be suspected in patients with bilateral Achilles tendon abnormalities. Obviously, differentiation from Achilles tendinosis is important.
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Figure 19. Xanthoma of the Achilles tendon in a patient with familial hypercholesterolemia. Sagittal T1-weighted MR image demonstrates massive enlargement and linear stranding of the Achilles tendon (arrows). Achilles bursitis is also seen (*). (Reprinted, with permission, from reference 4.)
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Posterior Tibial Tendon Dysfunction
Acute or chronic dysfunction of the posterior tibial tendon encompasses a spectrum of abnormalities ranging from tenosynovitis and tendinosis to partial or complete rupture of the tendon. Acute tenosynovitis is related to overuse and is usually encountered in young, athletic individuals. At MR imaging, fluid is seen within the tendon sheath (23). The tendon demonstrates normal signal intensity and morphologic characteristics, although nodular or diffuse thickening in chronic tenosynovitis and scarring of the peritenon may be encountered. Tendinosis manifests as mild to severe heterogeneity and thickening of the tendon.
Chronic posterior tibial tendon rupture typically develops in women during the 5th and 6th decades of life and is associated with progressive flat foot deformity. The tear is commonly noted behind the medial malleolus, where the tendon is subjected to a significant amount of friction. Acute partial or complete rupture of the posterior tibial tendon in young, athletic individuals is less common and is usually seen at the insertion of the tendon on the navicular bone (24).
Surgical and MR imaging classification of chronic posterior tibial tendon ruptures divides these injuries into three types (25). Type I partial tear consists of an incomplete tear with fusiform enlargement, intrasubstance degeneration, and longitudinal splits. On axial MR images, the diameter of the tendon may be five to 10 times that of the adjacent flexor digitorum longus tendon (Fig 20). High-signal-intensity foci representing longitudinal splits are noted within the substance of the tendon on T1-weighted and proton-density–weighted images (Fig 21). Thus, diagnostic overlap exists between severe tendinosis and partial type I tears because both demonstrate fusiform thickening of the tendon with intrasubstance signal intensity alteration (26). Further stretching and elongation of the tendon leads to a type II partial tear of the posterior tibial tendon. On axial images, a decrease in the diameter of the tendon, usually without signal intensity alterations, is diagnostic for this pathologic condition. The caliber of the tendon may now be equal to or less than that of the adjacent flexor digitorum longus tendon (Fig 22). Complete disruption of the tendon fibers is seen in type III posterior tibial tendon tears. These are quite rare and appear at MR imaging as tendon discontinuity. The gap may be filled with fluid or granulation tissue, depending on the chronicity of the injury (Fig 23).
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Figure 20. Type I tear of the posterior tibial tendon. On an axial T2-weighted MR image, the markedly thickened tendon (straight arrow) has a diameter 10 times that of the adjacent flexor digitorum longus tendon (curved arrow). Heterogeneous intrasubstance signal intensity representing longitudinal splits is also noted.
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Figure 21. Advanced type I tear of the posterior tibial tendon. Axial T1-weighted MR image shows marked tendon thickening as well as high-signal-intensity foci (arrow) representing longitudinal splits.
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Figure 22. Type II tear of the posterior tibial tendon. Axial proton-density-weighted MR image shows an attenuated posterior tibial tendon (open arrow) with a caliber equal to that of the adjacent flexor digitorum tendon (solid arrow).
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Figure 23. Type III tear of the posterior tibial tendon. Axial T2-weighted MR image demonstrates absence of the posterior tibial tendon. The tibial retromalleolar groove has been replaced by synovial fluid and debris (arrow). (Reprinted, with permission, from reference 27.)
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A number of soft-tissue and bone abnormalities are encountered at MR imaging in patients with posterior tibial tendon tears (28). These include fluid within the tendon sheath, fluid within the medial or lateral bursae, sinus tarsi syndrome, periostitis at the insertion of the flexor retinaculum on the tibia, hindfoot valgus, subtalar and talonavicular malalignment, and accessory navicular bone.
Dislocation of the posterior tibial tendon is a rare condition that is most commonly seen in young patients following a recognized injury to the ankle (29). The mechanism of injury is usually related to severe dorsiflexion associated with a torn flexor retinaculum, allowing the tendon to slide out of its groove. On axial MR images, subluxation or complete dislocation of the posterior tibial tendon is easily identified (Fig 24) (30). The tendon is seen medial or anterior to the medial malleolus. Additional findings include an avulsed or stripped flexor retinaculum, pressure erosion of the dislocated tendon on the medial malleolus, and a partially torn tendon.
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Figure 24. Dislocation of the posterior tibial tendon. Axial proton-density-weighted MR image shows the posterior tibial tendon anterior and medial to the tibial malleolus (black arrow). The flexor digitorum longus tendon is medially displaced within the retromalleolar groove (white arrow). (Reprinted, with permission, from reference 30.)
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Peroneal Tendon Injuries
Injuries to the peroneal tendons are frequently encountered and include peritendinosis, tenosynovitis, tendinosis, rupture, and dislocation (31). MR imaging characteristics of peritendinosis and tenosynovitis include scarring around the tendons and fluid within the common tendon sheath, respectively (Fig 25) (32). The morphologic features of the tendon are usually preserved. Care should be taken to differentiate tenosynovitis from fluid within the common peroneal sheath secondary to a tear of the calcaneofibular ligament.
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Figure 25. Peroneal tenosynovitis. Axial T2-weighted MR image shows a large amount of fluid within the common peroneal tendon sheath (arrow). The morphologic features of the tendons remain unchanged.
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Acute and chronic ruptures of the peroneal tendons occur in young, athletic individuals due to overuse or may be related to degenerative wear and tear in older, more sedentary patients. Calcaneal fractures typically predispose to partial tears, dislocation, and entrapment of the peroneal tendons. Chronic longitudinal tears of the peroneus brevis tendon often originate within the fibular groove, where the tendon is entrapped between the peroneus longus tendon and the lateral malleolus. They may be associated with either superior peroneal retinacular tear or laxity secondary to inversion injury. Longitudinal intrasubstance tears of the peroneus brevis tendon have a distinct appearance on axial MR images (33–35). The tendon assumes a C-shaped or boomerang configuration that partially envelops the peroneus longus tendon (Fig 26). Partial or full substance splits within the tendon and intrasubstance high-signal-intensity foci are noted on both T1- and T2-weighted images.
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Figure 26. Longitudinal tear of the peroneus brevis tendon. Axial proton-density-weighted MR image demonstrates a C-shaped split peroneus brevis tendon (white arrow) partially enveloping the peroneus longus tendon (black arrow).
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Acute or chronic tears of the peroneus longus tendon may be associated with peroneus brevis
tendon tears at the level of the medial malleolus (33). Isolated tears of the peroneus longus tendon are more frequently seen at the level of the peroneal tubercle or cuboid tunnel (36) (Figs 27, 28). MR imaging findings include morphologic and signal intensity abnormalities within the tendon representing partial or complete disruption. Marrow edema may be encountered within the lateral calcaneus and within a hypertrophic peroneal tubercle. Occasionally, proximal retraction of the os peroneum may be seen in patients with complete rupture of the peroneus longus tendon.
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Figure 27. Partial tear of the peroneus longus and brevis tendons. Axial proton-density-weighted MR image shows splits of the peroneus longus (open arrow) and peroneus brevis (solid white arrow) tendons. Marrow edema (arrowheads) is visible within a prominent peroneal tubercle (*). Debris, fluid, and scar (black arrows) are seen surrounding the tendons. (Reprinted, with permission, from reference 36.)
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Figure 28. Chronic partial tear of the peroneus longus tendon. Oblique coronal T1-weighted MR image reveals increased intrasubstance signal intensity in a thickened peroneus longus tendon (arrow) as it courses inferior to the cuboid bone (CU). (Reprinted, with permission, from reference 36.)
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Dislocation of the peroneal tendons is often clinically misdiagnosed as an ankle sprain. A flakelike fracture of the distal fibular metaphysis may be present on conventional radiographs, indicating an avulsed or stripped peroneal retinaculum. The mechanism of acute dislocation is a violent contraction of the peroneal muscles with secondary detachment of the superior peroneal retinaculum and lateral dislocation of the peroneal tendons out of the retromalleolar groove. Chronic ankle instability associated with superior peroneal retinacular laxity is considered a predisposing factor for chronic peroneal tendon dislocation.
MR imaging allows direct assessment of the position of the tendons relative to the fibular retromalleolar groove (32,34). Dislocation is best demonstrated on axial images, which show the tendons to be located anterior and lateral to the distal fibula. The tendons are often found within a "pouch" formed by a stripped-off superior peroneal retinaculum (Fig 29). Avulsion off the distal fibula and midsubstance tears of the superior peroneal retinaculum are less frequently encountered. Associated MR imaging findings include tenosynovitis or tears of the peroneal tendons, convex fibular groove, avulsion fracture of the distal fibula, and tear of the lateral collateral ligament.
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Figure 29. Dislocation of peroneal tendons. Axial proton-density-weighted MR image shows the dislocated peroneus brevis and longus tendons (arrowhead) within a "pouch" formed by the stripped-off superior peroneal retinaculum (arrows).
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Flexor Hallucis Longus Tendon Injuries
The flexor hallucis longus tendon is susceptible to injuries as it passes through the fibro-osseous tunnel between the lateral and medial talar tubercles. Repetitive friction at that site predisposes to chronic or stenosing tenosynovitis, tendinosis, and partial tear. Complete tendon rupture may also occur.
Injuries to the flexor hallucis longus tendon are best visualized on axial and sagittal MR images (Fig 30a) (37). Synovial fluid surrounding an otherwise intact tendon is characteristic of chronic tenosynovitis, particularly if only a small amount of fluid is noted within the ankle joint (Fig 30b). Tendon sheath fluid in the presence of a large ankle joint effusion most likely indicates a normal communication between the two structures and is usually of no clinical significance. Chronic and extensive inflammation of the peritenon leads to stenosing tenosynovitis, producing a functional hallux rigidus. Fusiform swelling and longitudinal splitting of the tendon associated with increased intrasubstance signal intensity is indicative of tendinosis and partial tear.
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Figure 30a. Os trigonum syndrome and flexor hallucis longus tenosynovitis. (a) Sagittal STIR MR image demonstrates abundant fluid (*) within the sheath of the flexor hallucis longus tendon (straight arrow). Edematous changes of the os trigonum, synchondrosis, and posterior talus (curved arrow) are also seen. (b) Axial T2-weighted MR image obtained in a different patient shows fluid and debris within the flexor hallucis longus tendon sheath (arrow), a finding that is consistent with tenosynovitis. Note the absence of joint fluid.
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Figure 30b. Os trigonum syndrome and flexor hallucis longus tenosynovitis. (a) Sagittal STIR MR image demonstrates abundant fluid (*) within the sheath of the flexor hallucis longus tendon (straight arrow). Edematous changes of the os trigonum, synchondrosis, and posterior talus (curved arrow) are also seen. (b) Axial T2-weighted MR image obtained in a different patient shows fluid and debris within the flexor hallucis longus tendon sheath (arrow), a finding that is consistent with tenosynovitis. Note the absence of joint fluid.
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Flexor hallucis longus tenosynovitis and tendinosis may also be seen in the region of the Henry knot and as the tendon passes between the sesamoid bones at the head of the first metatarsal. MR imaging helps distinguish flexor hallucis longus tendon abnormalities from other conditions with similar clinical characteristics (eg, sesamoiditis). Isolated distal rupture of the flexor hallucis longus tendon is a rare condition resulting from acute dorsiflexion or laceration injuries.
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Osseous Lesions
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Bone Contusion
During the acute stage, bone contusions (bone bruises) manifest at MR imaging as reticular areas of hypointensity on T1-weighted images and hyperintensity on T2-weighted and fat-suppressed images (Fig 31) (38,39). They are related to microfractures of the trabecular bone and edema or hemorrhage within the bone marrow. Bone contusions normally resolve within 8–12 weeks. In most cases, radiographic findings are negative. The clinical significance of bone contusions detected with MR imaging is unknown, but it is generally accepted that continued stress placed on a
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Abstract
Introduction
