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

47 Ankle and Foot 2297 47
Figure 47-102. Navicular fatigue (stress) fracture
in a 16-year-old who developed midfoot pain while
cross-country skiing. A, Anteroposterior radiograph of
the foot reveals a subtle nondisplaced fracture in the
middle third of the navicular (arrowhead in magnified
dashed box). MRI was obtained 4 days later. Axial
oblique T1- (B) and fat-suppressed T2-weighted (C)
images reveal a discrete fracture in the middle third of
the navicular (arrowhead) as well as diffuse bone
marrow edema.
Continued
A
B
C
developed a specific CT protocol that reformats the images
in thin, 1-mm slices using a small, 6-cm FOV centered on
the navicular (see Fig. 47-47D).
Whether seen by CT or MRI, navicular fatigue injuries
begin at the dorsal, central, proximal navicular where it
articulates with the head of the talus. This is illustrated by
the black arrows pointing to the dark regions of bone
marrow on the T1-weighted images in the stress reaction
in Figure 47-101. More fluid-sensitive fat-suppressed T2-
weighted or inversion recovery images show bone marrow
edema emanating from this dorsal/central/proximal site.
This is illustrated by the white arrows in Figure 47-101.
When stress reactions progress to stress fractures, the cortical
disruption starts at the dorsal/central/proximal site
on the navicular and propagates in a plantar direction
vertically in the sagittal plane (see Fig. 47-102) or in an
oblique sagittal plane (see Fig. 47-103). Because of the
primarily sagittal orientation of these fractures, they may
be difficult to appreciate on sagittal CT images and are
better seen on oblique coronal (see Fig. 47-102F) and
oblique axial (see Fig. 47-102G) images. Because they tend
to be nondisplaced incomplete fractures, they are best seen
on images that are reformatted into a small FOV with thin
slices. Because these patients may undergo serial CT scans
to follow the progress of fracture healing, it is useful to
have a standard protocol (as in Fig. 47-47D) to help retain
uniform reformatting parameters from one scan to the next
(see Fig. 47-103C to F).
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9/9/2008 5:35:56 PM