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

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2292 VII Imaging of the Musculoskeletal System A B C D E F Figure 47-95. Tarsal coalition in a 29-year-old radiology resident with a long history of bilateral foot pain. The pain is worse on the right, is aggravated by athletics, and was improved with custom orthoses. A, Lateral radiograph of the more symptomatic right ankle reveals a talar beak (arrow). B, Lateral radiograph of the less symptomatic left ankle shows no enthesophyte arising from the dorsal head of the talus. Oblique radiographs of the right (C) and left (D) feet reveal no coalition between the calcaneus and navicular (bidirectional arrows). E, Sagittal CT scan of the right foot reveals the talar beak (arrow) as well as a portion of the solid osseous coalition across the subtalar joint (arrowhead). F, CT scan in the coronal oblique plane through the middle facets of both subtalar joints demonstrates solid osseous coalition across the right middle facet (white arrowheads). There is also a nonosseous coalition of the left middle facet (black arrowheads), as manifest by a joint that is abnormally broad with irregular, noncongruent articular surfaces. Ch047-A05375.indd 2292 9/9/2008 5:35:49 PM
A C B D 47 Ankle and Foot 2293 47 Figure 47-96. Calcaneonavicular coalition seen radiographically in this 21-year-old who has been complaining of left ankle pain for at least 7 years. On physical examination, the subtalar range of motion of the left foot is half that of the asymptomatic right. A, Oblique radiograph of the asymptomatic right midfoot shows the normal relationship between the calcaneus (Ca) and navicular (N), with no contact between them. B, Oblique radiograph of the symptomatic left foot shows the abnormal joint (arrowheads) between the calcaneus and navicular. This is a nonosseous coalition. Lateral radiographs of the right (C) and left (D) ankles reveal elongated anterior processes of the calcaneus bilaterally (arrows). These bilateral “ant-eater” signs suggest that the patient has an asymptomatic calcaneonavicular coalition on the right that was not radiographically apparent on the oblique view (A). Talar beaks occur less frequently with nonosseous coalition because some subtalar motion remains. • Calcaneonavicular Coalition Of the two common locations for tarsal coalitions, calcaneonavicular coalitions can often be seen radiographically on the oblique view (Fig. 47-96). Normally, the calcaneus and navicular do not touch (see Fig. 47-96A); it is abnormal any time they get close enough to each other to form a joint (see Fig. 47-96B), let alone a solid bony bridge. Another radiographic indication of a calcaneonavicular coalition is the presence of an elongated APC, sometimes called an ant-eater sign (Fig. 47-96C and D). An elongated APC and an asymptomatic calcaneonavicular coalition may be seen as incidental findings on radiographs and CTs obtained for other reasons. And although an elongated APC may not cause a symptomatic coalition, it may be at increased risk of fracture (Fig. 47-97). • Talocalcaneal Coalition Talocalcaneal coalitions, which occur across the middle facet of the subtalar joint, are difficult to demonstrate with conventional radiographs because these radiographs do not well profile the middle facet. For this reason, coronal CT images obliqued to be perpendicular to the subtalar joint are the key imaging plane (Fig. 47-95F). With osseous coalitions, solid bony ankylosis is present across the middle facet (see Fig. 47-95F, white arrowheads). Nonosseous coalitions across the middle facet are not difficult to recognize by CT because they do not look like the flat, uniform middle facet we typically see on coronal oblique CT. In nonosseous coalitions, the articular surfaces are not smooth or congruous and tend to have an overgrown appearance (see Fig. 47-95F, black arrowheads). An additional example of nonosseous middle facet coalition is shown in Figure 47-98. • Tarsal Tunnel Syndrome The tarsal tunnel in the ankle is analogous to the carpal tunnel in the wrist. Both are spaces confined by the underlying bones and overlying fibrous ligaments through which pass tendons, blood vessels, and nerves. The roof of the tarsal tunnel is the flexor retinaculum, a broad, fibrous band extending between the medial malleolus and the medial tubercle of the calcaneus (Fig. 47-99A). These retinacular fibers help prevent the medial tendons from becoming dislocated and can be identified on highresolution images (Fig. 47-99B; see Fig. 47-31). Because the tarsal tunnel is a relatively tight space, otherwise innocuous volume-occupying lesions, such as synovial cysts (Fig. Ch047-A05375.indd 2293 9/9/2008 5:35:50 PM
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