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

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47 Ankle and Foot 2287 47
Figure 47-92, cont’d C, Shortaxis
(coronal) CT scan confirms the
erosion of the left lateral sesamoid
(ellipse) as well as an erosion in
the adjacent metatarsal head
(arrow). D, Axial CT scan proximal
to B reveals the marginal erosion
seen radiographically in the left
medial first metatarsal head (white
arrow) and an erosion in the right
second cuneiform (black arrow).
Both erosions have well-defined,
slightly sclerotic margins with
sharp overhanging edges,
characteristic of gout. Aspiration
of the patient’s left great toe
metatarsophalangeal joint yielded
uric acid crystals.
the source images consist of thin, overlapping slices
(Fig. 47-93).
• Tarsal Coalitions 17,32
The term coalition comes from the verb “coalesce,” which
means “to grow together and form a union.” These abnormal
unions are either osseous, in which there is a solid cortical
bridge between the bones, or nonosseous, in which there
is a fibrous or cartilaginous union between the bones.
Although abnormal bone coalitions have been reported
throughout the body, certain locations predominate. In the
wrist, for example, carpal coalitions usually occur between
the lunate and triquetrum. In the hindfoot, tarsal coalitions
most commonly occur across the middle facet of the
subtalar joint, and between the APC and the lateral pole
of the navicular. 46 An example of the latter was already seen
as an incidental finding in Figure 47-81B.
The subtalar joint complex consists of the subtalar
joint itself and the talonavicular and calcaneocuboid joints.
These joints function in unison during the gait cycle, and
limitation of motion of any one of these joints limits the
motion of the other joints. 37 The clinical syndrome of tarsal
coalition consists of pain and reduced or absent subtalar
motion, as well as pes planus (flat-foot) and peroneal
muscle spasm (clonus on inversion stress). 4,43 The exact
cause of the peroneal spasm is uncertain; however, peroneal
muscle tightness is the result of tarsal coalition, not
the cause. Symptoms usually manifest between 12 and 16
years of age and worsen with increasing age. Conservative
treatment options include anti-inflammatory medication
and a trial of reduced activity, cast immobilization, and
molded orthoses. If conservative treatment fails, surgical
options include resection of the coalition and arthrodesis
if secondary osteoarthritis has developed.
According to the literature, tarsal coalitions are bilateral
in 50% to 60% of cases. However, in searching our
database for examples for this chapter, we found that bilaterality
was the rule. Perhaps because our CT protocol
entails scanning both feet, we are apt to find asymptomatic
coalitions and other incidental variants, such as the os
calcaneus secondarius (a forme fruste of calcaneonavicular
coalition), in the contralateral foot (Fig. 47-94).
A talar beak is an indirect sign of a tarsal coalition. Seen
best radiographically on the lateral view (Fig. 47-95A) or
on a sagittal CT (Fig. 47-95C), the talar beak is not part of
the coalition but a result of it. The altered biomechanics
across the talonavicular joint can result in a traction spur
(enthesophyte) arising from the dorsal head of the talus.
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