Ankle and Foot 47 - Department of Radiology - University of ...
Ankle and Foot 47 - Department of Radiology - University of ... Ankle and Foot 47 - Department of Radiology - University of ...
2216 VII Imaging of the Musculoskeletal System A B C D Figure 47-12. The patient is a 45-year-old with pain over the dorsum of the midfoot, indicated by the marker (m). Axial proton-density– weighted (A) and T2-weighted (B) images well demonstrate normal anterior tibial (AT) and extensor digitorum longus (EDL) tendons. The extensor hallucis longus (EHL) tendon, which was well seen and normal on more proximal slices, is not seen on this slice, although it should be just below the marker. Could this be a rare EHL tear? The lack of edema in (B) argues against this diagnosis. The answer is revealed on the sagittal T1- weighted (C) and T2-weighted fat-suppressed (D) images: there is a navicular stress fracture (black arrow). The normal Achilles tendon (Ach) is uniform in thickness and dark signal in both sagittal sequences and has a sharp interface with the adjacent Kager’s fat pad. A portion of the normal AT tendon is seen, as well as a normal amount of fluid in the retrocalcaneal bursa (white arrowhead in D). and lateral ankle tendons, all of which are surrounded by synovial sheaths, the Achilles is surrounded by thin layers of filmy fibrous tissue with fine internal blood vessels, called the paratenon or paratendon. This paratenon is analogous to synovium in that it provides nutrients for the tendon, but because the Achilles tendon does not change its axis of motion, there is no need for the lubrication function of synovium. Thus, there should never be any fluid seen around a normal Achilles tendon. Directly anterior to the Achilles tendon is a triangular fat pad described radiographically by Kager in 1939. 26 Kager’s fat pad is located in the retromalleolar region and is defined anteriorly by the posterior aspect of the tibia and posteriorly by the Achilles tendon, with the base being the Ch047-A05375.indd 2216 9/9/2008 5:33:37 PM
47 Ankle and Foot 2217 47 Figure 47-13. Synovial cyst of the anterior tibial tendon in a 23- year-old. Axial (A) and sagittal (B) T2-weighted images demonstrate the cystic outpouching (white arrow) of the synovial sheath surrounding the anterior tibial tendon (black arrow). The tendon itself is normal. A B proximal aspect of the calcaneus. The space contained within this triangle is filled with fatty tissue, producing a well-defined lucent triangle that can be seen on lateral radiographs of the ankle (Fig. 47-14A). On rupture of the Achilles tendon, this space becomes poorly demarcated, and the normally lucent fatty tissue space becomes obscured (see Fig. 47-21A). The Achilles tendon is easily evaluated by physical examination as well as by MRI or ultrasonography. 23 In the sagittal plane, the Achilles tendon should appear uniformly straight and black on T1-weighted images (Fig. 47-14B) as well as on fluid-sensitive images (Fig. 47-14C). There should be a sharp interface between the Achilles tendon and Kager’s fat pad directly ventral to it. A normal retrocalcaneal bursa may be present just in front of the Achilles tendon (white arrowhead, Figs. 47-12D and 47-14C). The normal retrocalcaneal bursa should measure less than 6 mm superior to inferior, 3 mm medial to lateral, and 2 mm anterior to posterior. 41 Any fluid behind the Achilles tendon, in a retro-Achilles bursa, is abnormal. In the axial plane, the Achilles tendon should appear flattened in the anteroposterior direction. Distally, the ventral margin of the tendon becomes concave, with upturned corners resembling a smile (see Fig. 47-10D). Injury For practical purposes, the plantaris tendon is seldom clinically relevant in the ankle. Tears of the plantaris tendon tend to occur high in the calf, at the plantaris musculotendinous junction, and have been called “tennis leg.” By MRI, plantaris tears present as fluid tracking along the length of the calf, between the underlying soleus and more superficial gastrocnemius muscles (Fig. 47-15). Figure 47-16 illustrates a chronically swollen and scarred posterior tibial tendon, with its cross-sectional area greater than that of the normal anterior tibial tendon. Ruptures of the Achilles tendon are usually diagnosed clinically, often by the patients themselves. Patients can often recall the exact instant the Achilles ruptured, describing the sensation “as if someone kicked me.” The classic Achilles tendon rupture occurs with forced dorsiflexion of the planted foot, such as occurs in basketball or other jumping sports. The classic patient is a middle-age “weekend warrior” who leads a sedentary life and attempts to participate in sports, perhaps with younger players, without an adequate warm-up. Of all the tendons of the foot and ankle, the Achilles is the only one for which disorders have a male predominance. Complete ruptures of the Achilles tendon typically occur at one of two locations. One site is low, 3 to 5 cm just proximal to the calcaneal insertion (Fig. 47-17). This is a relatively hypovascular watershed region. The other site is relatively high, up at the musculotendinous junction (Fig. 47-18). These more proximal tears may require that the imaging coil be repositioned around the lower calf rather than around the ankle to visualize the torn and retracted proximal end (Fig. 47-19). When it is clinically apparent to all that the Achilles tendon is completely ruptured, confirmation with MRI is usually unnecessary. However, imaging with MRI or ultrasonography is used to measure the tendinous gap between the retracted ends of a complete tear. Partial tears of the Achilles tendon are usually intrasubstance tears, and edema-sensitive images reveal increased signal in a swollen, abnormally rounded tendon (Fig. 47-20). Partial tears can also present as nearly complete ruptures, with only a few remaining fibers intact (Fig. 47-21). In these cases, abnormal fluid can be seen surrounding the intact fibers, within the distended paratenon (see Fig. 47-21E). Imaging with MRI or ultrasonography is used to assess the extent of partial tears. An Achilles tendon that has undergone internal healing and scar formation from a prior intrasubstance tear tends Ch047-A05375.indd 2217 9/9/2008 5:33:38 PM
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2216 VII Imaging <strong>of</strong> the Musculoskeletal System<br />
A<br />
B<br />
C<br />
D<br />
Figure <strong>47</strong>-12. The patient is a 45-year-old with pain over the dorsum <strong>of</strong> the midfoot, indicated by the marker (m). Axial proton-density–<br />
weighted (A) <strong>and</strong> T2-weighted (B) images well demonstrate normal anterior tibial (AT) <strong>and</strong> extensor digitorum longus (EDL) tendons. The extensor<br />
hallucis longus (EHL) tendon, which was well seen <strong>and</strong> normal on more proximal slices, is not seen on this slice, although it should be just below<br />
the marker. Could this be a rare EHL tear? The lack <strong>of</strong> edema in (B) argues against this diagnosis. The answer is revealed on the sagittal T1-<br />
weighted (C) <strong>and</strong> T2-weighted fat-suppressed (D) images: there is a navicular stress fracture (black arrow). The normal Achilles tendon (Ach) is<br />
uniform in thickness <strong>and</strong> dark signal in both sagittal sequences <strong>and</strong> has a sharp interface with the adjacent Kager’s fat pad. A portion <strong>of</strong> the<br />
normal AT tendon is seen, as well as a normal amount <strong>of</strong> fluid in the retrocalcaneal bursa (white arrowhead in D).<br />
<strong>and</strong> lateral ankle tendons, all <strong>of</strong> which are surrounded by<br />
synovial sheaths, the Achilles is surrounded by thin layers<br />
<strong>of</strong> filmy fibrous tissue with fine internal blood vessels,<br />
called the paratenon or paratendon. This paratenon is analogous<br />
to synovium in that it provides nutrients for the<br />
tendon, but because the Achilles tendon does not change<br />
its axis <strong>of</strong> motion, there is no need for the lubrication function<br />
<strong>of</strong> synovium. Thus, there should never be any fluid<br />
seen around a normal Achilles tendon.<br />
Directly anterior to the Achilles tendon is a triangular<br />
fat pad described radiographically by Kager in 1939. 26<br />
Kager’s fat pad is located in the retromalleolar region <strong>and</strong><br />
is defined anteriorly by the posterior aspect <strong>of</strong> the tibia <strong>and</strong><br />
posteriorly by the Achilles tendon, with the base being the<br />
Ch0<strong>47</strong>-A05375.indd 2216<br />
9/9/2008 5:33:37 PM
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