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

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
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