Книги по МРТ КТ на английском языке / MRI for Orthopaedic Surgeons Khanna ed 2010

Fig. 9.13 This sagittal T1-weighted image shows tendon hypertrophy and heterogeneity of the substance (arrowhead), the characteristic MRI appearance of Achilles tendinosis.

is believed to be secondary to age-related degeneration and can also occur from repetitive strain or overuse, as is seen in running athletes.36 Achilles tendinosis can occur at the insertion on the posterior calcaneus or more proximally in the noninsertional tendon, approximately 3 to 6 cm above the insertion point.6,35 In Achilles tendinosis, there is a loss of the sharp tendon interface with the surrounding tissues, and the tendon may appear thickened and dysmorphic with loss of the normal concave appearance on axial images.6 Axial and sagittal T1-weighted images show tendon hypertrophy and heterogeneity of the substance (Fig. 9.13), whereas T2-weighted and fat-suppressed T2-weighted images show intratendinous edema and splitting.6 Insertional tendinosis may have an associated intratendinous spur on the calcaneus.

MRI is usually not necessary to diagnose acute rupture of the Achilles, which is often apparent clinically. The tear is typically located several centimeters above the insertion, but it may show complex patterns, including avulsion o the calcaneus or longitudinal splitting. MRI may be helpful for di erentiating partial and complete tears, or chronic ruptures that present in a delayed fashion.6 Sagittal and axial images show a gap between the torn ends with intervening fluid seen on T2-weighted images6 (Fig. 9.14). In chronic cases, fibrous tissue is present between the distal stump and the proximally retracted tendon.

Posterior tibial tendon dysfunction is the most common cause of adult-onset flatfoot deformity.30,37 It results from degenerative tendinosis, inflammatory disease, or, more rarely, trauma. The pathophysiology involves microscopic tears, collagen disorganization, and myxoid degenerative changes within the tendon. Clinically, patients present with flatfoot deformity, medial ankle pain, swelling, and weakness of the tendon.37

Conventional radiographs are useful in determining the severity of the deformity and the presence of secondary arthritis of the hindfoot and ankle. Although not necessary in all cases, MRI is a useful adjunct in assessing the posterior tibialis tendon and is superior to other modalities in detecting inflammation and tears within the tendon.37 Typically, the posterior tibial tendon is two to three times larger than the flexor digitorum longus tendon. It may have a small amount of synovial fluid around it, but usually no more than 1 to 2 mm.31 The tendon should have a homogeneous signal on both T1-weighted and T2-weighted images.31 Posterior tibial tendon tears are typically seen around the medial malleolus, or distally within 2 to 3 cm of its insertion on the navicu- lar.29–31 Posterior tibial tendon tears are best diagnosed on T2-weighted and fat-suppressed axial images, which show morphologic changes and intratendinous signal changes (Fig. 9.15). Sagittal images may be useful for assessing the distal insertion site on the navicular. Additionally, some

Fig. 9.14 A sagittal fat-suppressed T2-weighted image showing a tear of the Achilles tendon with overlapping ends and fluid and edema at the site of the tear (arrowhead).

Peroneal Tendon Disorders

Peroneal tendon disorders include tenosynovitis, tears, and subluxation. Pathology of the peroneal tendons can develop posterior to the lateral malleolus, on the lateral side of calcaneus inferior to the tip of the lateral malleolus, and distally

Fig. 9.16 An axial fat-suppressed T2-weighted image of the left foot shows marrow edema within an accessory navicular bone, superficial soft tissues, and the fibrous connection (synchondrosis) to the main navicular body (arrowhead). An accessory navicular bone is an occasional cause for posterior tibialis tendon dysfunction.

Fig. 9.18 In tenosynovitis of the peroneal longus tendon, the crosssectional area of the peritendinous fluid collection must be larger than the tendon itself (as seen on this axial STIR image of the left foot) (arrowhead) or complex.

appearance (Fig. 9.19); other suggestive findings include fluid within the sheath, a flat retrofibular groove, and spurring of the lateral edge of the fibula.32,39–41

Chronic peroneal tendon subluxation or instability has been reported with attenuation of the superior peroneal retinaculum and a shallow retrofibular groove. Both of these features can be well visualized on axial T1-weighted images39–41 (Fig. 9.20). The tendons may even be noted to be grossly dislocated on the lateral side of the lateral mal- leolus.39–41 Crowding of the retrofibular groove secondary to mass e ect, leading to peroneal instability, is another proposed mechanism. Such crowding can occur secondary to a low-lying peroneus brevis muscle belly, a ganglion cyst arising from the posterolateral ankle, or an accessory peroneus quartus muscle. A peroneus quartus muscle is present in 10% to 20% of individuals and occurs more commonly in men than in women.39,42 It is found posterior to the peroneus brevis and longus and inserts onto the calcaneus, cuboid, or peroneal tendons themselves.42 It is visualized on axial T1weighted images as a tendon separate from the brevis and longus tendons (Fig. 9.21).

Fig. 9.19 An axial T1-weighted image of the left foot showing a “split” peroneus brevis tendon with an irregular, bilobed shape (arrowhead).

Fig. 9.20 An axial T1-weighted image of the left ankle showing the peroneal tendon laterally subluxed over the lateral aspect of the retrofibular groove (arrowhead).

Distal peroneal pathology is less common; tenosynovitis or tear can occur inferior to the tip of the lateral malleolus or at the insertion sites of either tendon. Impingement of the peroneals can occur between the tip of the fibula and the peroneal tubercle on the lateral side of the calcaneus secondary to valgus hindfoot alignment or malunion of a previous calcaneal fracture. Distally, tenosynovitis can occur at the base of the fifth metatarsal at the insertion of the brevis tendon. Distal peroneus longus pathology can present as painful os peroneum syndrome (POPS).43 Patients with this entity present with pain and tenderness in the region of the cuboid inferolaterally secondary to tenosynovitis or tearing of the longus tendon. With complete rupture of the peroneus longus tendon, the associated os peroneum may retract proximal to the calcaneocuboid joint. Radiographically, a retracted or fragmented os peroneum may be seen. On MRI, there is fluid and edema in the sheath of the peroneus longus at the level of the cuboid, with edema and fragmentation of the os peroneum (Fig. 9.22).

Anterior Tibial Tendon Disorders

Disorders of the anterior tibialis tendon are relatively less common than those of the tendons listed above. Tendinitis can occur secondary to systemic inflammatory disease or overuse syndromes. Acute anterior tibialis tendon rupture can occur from trauma or laceration, and chronic attritional tear can present in elderly patients. Patients present with anterior ankle pain, localized soft-tissue swelling, and diminished dorsiflexion strength.33 Tendinitis appears as softtissue edema on axial and sagittal T2-weighted images33,44 (Fig. 9.23). As with tendinosis in other tendons, anterior tibial tendinosis appears on T1-weighted and T2-weighted images as hypertrophy, heterogeneous signal changes within the tendon substance, and even partial longitudinal splitting33,44 In complete rupture, sagittal T1-weighted images show discontinuity of the tendon, with proximal retraction and intervening fluid on T2-weighted images.33,44

Signal abnormality sometimes is seen on T1-weighted images distally at the insertion site because of the magic angle e ect. This phenomenon can occur when the foot is imaged in a neutral position.44 In addition, a longitudinal split of the tendon is occasionally seen at its insertion.

FHL and Posterior Impingement (Os Trigonum)

Tenosynovitis of the FHL tendon typically occurs at the posterior ankle at the level of the FHL tendon sheath posterior to the talus. Usually, patients present with deep ankle posteromedial pain, particularly with the ankle in plantarflexion. This condition is commonly seen in kicking athletes or ballet dancers performing in the en-pointe position.45 Clinically, tenosynovitis can overlap with posterior ankle impingement syndrome because of a prominent posterior process or

os trigonum. The os trigonum becomes impacted between the posterior tibia and calcaneus with ankle plantarflexion, which can cause osseous edema and localized inflammation; this impaction and inflammation can also lead to secondary FHL tenosynovitis and, potentially, a partial longitudinal tear. Passive plantarflexion on physical examination elicits posterior pain; passive dorsiflexion of the hallux stretches the FHL and may cause posterior ankle pain, also suggestive of FHL tendon pathology.

Conventional radiographs may show a large posterior talar process or an os trigonum. MRI shows fluid and softtissue signal intensity posteriorly around the region of the os trigonum, extending into the posterior portions of the ankle and the subtalar joints.6 The fibrous synchondrosis between the os trigonum and talus may show increased signal intensity on T2-weighted axial and sagittal images because of fluid traversing the articulation and forming a pseudarthrosis6 (Fig. 9.24). Edema also can be seen in the ossicle, posterior tibia, and dorsal calcaneus secondary to impaction. The FHL sheath appears enlarged and fluid-filled, and it may

Fig. 9.21 An axial T1-weighted image of the right foot showing an accessory peroneus quartus muscle posterior to the peroneus longus and brevis muscles (arrowhead).

Fig. 9.22 A sagittal T2-weighted image showing an edematous os peroneum with fluid and edema of the sheath of the distal peroneus longus tendon (arrowhead).

show loculation or a ganglion cyst; on rare occasions, the tendon may show intratendinous signal change consistent with a partial tear6 (Fig. 9.25).

Anterolateral Impingement

Impingement of the anterolateral aspect of the ankle joint is seen after a lateral ankle sprain in which hypertrophic scar tissue proliferates in the lateral gutter. This proliferation is sometimes called a meniscoid lesion because of its appearance as a meniscus-shaped focus of fibrous and synovial tissue that can lead to pain with dorsiflexion or eversion of the ankle.6,46 On physical examination, the patient exhibits tenderness at the anterolateral corner of the ankle that can be exacerbated by passive dorsiflexion. MRI findings in anterolateral impingement include the presence of inflamed soft tissue or increased fluid against the inner margin of the anteroinferior tibiofibular ligament and the anterior talofibular ligament instead of a small fat pad that sits anterior to the fibula on axial imaging6,46 (Fig. 9.26). MR arthrography with gadolinium contrast may help to outline this synovial tissue more accurately and confirm the diagnosis.46

Osteonecrosis of the Talus

Osteonecrosis (also termed avascular necrosis) of the talus typically occurs after talus fracture secondary to disruption

Fig. 9.23 Anterior tibialis tendon pathology. (A) An axial fat-sup- pressed T2-weighted image of the left ankle showing anterior tibial tendinitis with soft-tissue edema (arrowhead). (B) A sagittal fat-sup-

pressed T2-weighted image showing a complete rupture of the anterior tibialis tendon with discontinuity of the tendon, proximal retraction, and intervening fluid (arrowhead).

Fig. 9.24 A sagittal fat-suppressed T2-weighted image shows fluid signal within the fibrous synchondrosis between the os trigonum and talus (arrowhead), representing formation of a pseudarthrosis.

of the osseous blood supply.47 Atraumatic osteonecrosis can also occur in various medical conditions (such as vasculitis, sickle cell disease, or excessive alcohol use) or secondary to oral corticosteroid use.47 Patients complain of severe pain and di culty with ambulation. Conventional radiographs may show sclerosis, cystic changes, or bone collapse, although they are believed to have low sensitivity. MRI is a very sensitive test for making the diagnosis of osteonecrosis of the talus and has the added advantage of better anatomic resolution than bone scintigraphy.48 On T1-weighted images, low signal is seen with osteonecrosis secondary to marrow fat replacement, whereas T2-weighted images typically show edema in the avascular lesion (Fig. 9.27). Because there is a risk of osteonecrosis after talar neck fractures, titanium screws are recommended for surgical fixation to minimize the metallic artifact on postoperative MRI studies.47

reveals swelling, erythema, induration, tenderness, and warmth. Many infections in the foot and ankle result from a direct spread from contiguous ulceration, which occurs often in diabetic patients; penetrating trauma is another common cause.

Cellulitis and Abscess

An MRI study is often obtained in the setting of cellulitis to rule out deep-space abscess or associated osteomyelitis. Cellulitis typically shows low signal intensity in the subcutaneous fat on T1-weighted images and increased signal intensity on T2-weighted images. Fat-suppressed T2-weighted images are particularly sensitive in detecting subcutaneous edema; the underlying osseous structures appear normal. An abscess appears as a well-defined fluid collection that is low signal on T1-weighted and higher signal on T2-weighted and fat-suppressed images49 (Fig. 9.28). Administration of gadolinium contrast results in enhancement of the abscess rim.49 Cellulitis or abscess in the foot is best imaged on coronal and sagittal images; cellulitis at the ankle level is better visualized with sagittal and axial images. In cases of cellulitis or abscess secondary to penetrating trauma, a retained foreign

■ Infectious Processes

Infections in the foot and ankle are typically seen in the pediatric population, in patients with diabetes, and in immunocompromised individuals. Patients with infections present with pain, erythema, and swelling and can have fevers and other systemic responses. Physical examination

Fig. 9.25 An axial fat-suppressed T2-weighted image of the left ankle showing an enlarged and fluid-filled FHL with intratendinous signal change consistent with a partial tear (arrowhead).

Fig. 9.26 An axial fat-suppressed T2-weighted image of the left ankle showing a nidus of inflammatory soft tissue adjacent to the inner margin of the anteroinferior tibiofibular ligament (arrowhead), suggestive of anterolateral impingement.

Fig. 9.27 A sagittal fat-suppressed T2-weighted image showing osteonecrosis of the talus with patchy areas of bone marrow edema (arrowhead), some of which contain devitalized bone.

body may be di cult to identify but can present as a signal void in the midst of the edema pattern or fluid collection.

Fig. 9.28 A coronal fat-suppressed T2-weighted image of the left foot showing a well-defined fluid collection within the plantar forefoot (arrowhead), representing an abscess. Note that there is inflammation and edema of the surrounding tissues and probable osteomyelitis involving the first, second, and third metatarsals.

Septic Arthritis

Septic arthritis can occur secondary to direct penetration of a foreign body into the joint or secondary to hematogenous spread from a noncontiguous source. Occasionally, when the diagnosis is unclear, MRI may be helpful in identifying a joint e usion and may exclude a deep abscess or associated bone changes indicative of osteomyelitis. On MRI, septic arthritis shows a joint e usion with distention of the synovial recesses and intense contrast enhancement.50 Aspiration of the involved joint confirms the diagnosis and provides fluid for microscopic examination, culture, and sensitivity examination to help guide antibiotic management. The treatment involves drainage of the involved joint with repeated aspiration or open surgical drainage.

Osteomyelitis

Osteomyelitis, or bone infection, can be acute or chronic. In the foot and ankle, osteomyelitis is usually seen in the presence of a contiguous ulcer as in the diabetic or neuropathic patient. Radiographs in the early stages of osteomyelitis often are normal, and changes (including osteolysis, periosteal reaction, and osseous destruction) may not be seen until

Fig. 9.29 Osteomyelitis. (A) A sagittal T1-weighted image showing di use foci of low signal throughout the calcaneus and talus (arrowhead). (B) A sagittal fat-suppressed T2-weighted image of the same

patient showing di use foci of high signal in the same distribution (arrowhead). There is devitalization of a portion of the calcaneus.