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

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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.
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