Ankle and Foot 47 - Department of Radiology - University of ...

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2230 VII Imaging of the Musculoskeletal System dental findings, in rare circumstances they are painful normal variants, conditions that can be difficult to diagnose and difficult to treat. Figure 47-31. The normal medial ankle ligaments on a T1-weighted image obtained using a high-resolution 512 acquisition matrix. This coronal image is located just behind the middle facet of the subtalar joint. (This image is seven slices anterior to Fig. 47-29D). The magnified dashed box to the right shows the superficial and deep components of the deltoid ligament. The broader deep fibers (black arrow) run from the medial malleolus (MM) to the medial process of the talus. The more slender superficial fibers (white arrow) run from the MM to the calcaneus at the sustentaculum tali (ST). Also shown is the flexor retinaculum (open arrowheads), forming the roof of the tarsal tunnel atop the three medial tendons (T, posterior tibial; D, flexor digitorum longus; H, flexor hallucis longus) and the posterior tibial neurovascular bundle (dotted oval). • Os Trigonum Syndrome The os trigonum is a common accessory ossicle located directly behind the talus, at the posterior end of the subtalar joint, adjacent to where the flexor hallucis longus wraps around the back of the talus. The os trigonum develops as a separate ossification center. During growth this fuses to the talus in most people, but in 5% to 15% of normal feet it remains nonunited and is variable in size and shape. There are no radiographic findings in a patient with symptomatic os trigonum syndrome, 44 although the diagnosis can be made with MRI by demonstrating marrow edema in the os trigonum and the adjacent talus (Fig. 47-33). • Accessory Navicular Syndrome The accessory navicular bone (os tibiale externum) is a common normal variant found adjacent to the medial pole of the navicular in approximately 10% of the population. As previously mentioned under “Medial Tendons,” the medial pole of the navicular is the primary insertion site of the posterior tibial tendon (see Figs. 47-10D and 47-11C). Small accessory navicular bones are called type 1 and are simply sesamoid bones in the substance of the posterior tibial tendon (Fig. 47-34). The posterior tibial tendon still inserts normally on the navicular, and the type 1 accessory navicular bones are of no clinical significance. Larger accessory navicular bones are called type 2 (Fig. 47-35), and with these the posterior tibial tendon inserts onto the accessory navicular, rather than on the navicular Figure 47-32. Deltoid ligament sprain in an 18-yearold. Mortise coronal T1-weighted (A) and T2-weighted fat-suppressed (B) images show abnormally increased signal in the deep deltoid (black arrow). There is bone marrow edema at its insertion site on the medial talus (arrowhead). The superficial deltoid (white arrow) is intact. A B Ch047-A05375.indd 2230 9/9/2008 5:34:01 PM
47 Ankle and Foot 2231 47 A B Figure 47-33. Os trigonum syndrome in an 11-yearold competitive Irish dancer. Lateral view of the symptomatic left ankle (A) shows a small os trigonum, a common normal variant. The asymptomatic right side (B) is shown for comparison. C, Midsagittal T1- weighted image shows the small os trigonum (arrow). D, Corresponding sagittal inversion recovery image shows bone marrow edema in the os trigonum (arrow) as well as in the adjacent talus (arrowhead). E, Straight axial proton-density–weighted image shows the irregular cleft (arrowhead) between the os trigonum and the talus. Well seen are the normal structures in the tarsal tunnel: the posterior tibial tendon (T), flexor digitorum longus tendon (D), neurovascular bundle (&), and flexor hallucis longus tendon (H). F, Corresponding axial T2-weighted fat-suppressed image shows bone marrow edema in the os trigonum (arrow) and in the adjacent talus (arrowhead). C D E F Ch047-A05375.indd 2231 9/9/2008 5:34:03 PM
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