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

47 Ankle and Foot 2217 47
Figure 47-13. Synovial cyst of
the anterior tibial tendon in a 23-
year-old. Axial (A) and sagittal (B)
T2-weighted images demonstrate
the cystic outpouching (white
arrow) of the synovial sheath
surrounding the anterior tibial
tendon (black arrow). The tendon
itself is normal.
A
B
proximal aspect of the calcaneus. The space contained
within this triangle is filled with fatty tissue, producing a
well-defined lucent triangle that can be seen on lateral
radiographs of the ankle (Fig. 47-14A). On rupture of the
Achilles tendon, this space becomes poorly demarcated,
and the normally lucent fatty tissue space becomes obscured
(see Fig. 47-21A).
The Achilles tendon is easily evaluated by physical
examination as well as by MRI or ultrasonography. 23 In the
sagittal plane, the Achilles tendon should appear uniformly
straight and black on T1-weighted images (Fig. 47-14B)
as well as on fluid-sensitive images (Fig. 47-14C). There
should be a sharp interface between the Achilles tendon
and Kager’s fat pad directly ventral to it. A normal retrocalcaneal
bursa may be present just in front of the Achilles
tendon (white arrowhead, Figs. 47-12D and 47-14C). The
normal retrocalcaneal bursa should measure less than
6 mm superior to inferior, 3 mm medial to lateral, and
2 mm anterior to posterior. 41 Any fluid behind the Achilles
tendon, in a retro-Achilles bursa, is abnormal. In the axial
plane, the Achilles tendon should appear flattened in the
anteroposterior direction. Distally, the ventral margin of
the tendon becomes concave, with upturned corners resembling
a smile (see Fig. 47-10D).
Injury
For practical purposes, the plantaris tendon is seldom clinically
relevant in the ankle. Tears of the plantaris tendon
tend to occur high in the calf, at the plantaris musculotendinous
junction, and have been called “tennis leg.” By
MRI, plantaris tears present as fluid tracking along the
length of the calf, between the underlying soleus and more
superficial gastrocnemius muscles (Fig. 47-15). Figure
47-16 illustrates a chronically swollen and scarred posterior
tibial tendon, with its cross-sectional area greater than
that of the normal anterior tibial tendon.
Ruptures of the Achilles tendon are usually diagnosed
clinically, often by the patients themselves. Patients can
often recall the exact instant the Achilles ruptured, describing
the sensation “as if someone kicked me.” The classic
Achilles tendon rupture occurs with forced dorsiflexion of
the planted foot, such as occurs in basketball or other
jumping sports. The classic patient is a middle-age
“weekend warrior” who leads a sedentary life and attempts
to participate in sports, perhaps with younger players,
without an adequate warm-up. Of all the tendons of the
foot and ankle, the Achilles is the only one for which disorders
have a male predominance. Complete ruptures of
the Achilles tendon typically occur at one of two locations.
One site is low, 3 to 5 cm just proximal to the calcaneal
insertion (Fig. 47-17). This is a relatively hypovascular
watershed region. The other site is relatively high, up at the
musculotendinous junction (Fig. 47-18). These more proximal
tears may require that the imaging coil be repositioned
around the lower calf rather than around the ankle
to visualize the torn and retracted proximal end (Fig.
47-19). When it is clinically apparent to all that the Achilles
tendon is completely ruptured, confirmation with MRI
is usually unnecessary. However, imaging with MRI or
ultrasonography is used to measure the tendinous gap
between the retracted ends of a complete tear.
Partial tears of the Achilles tendon are usually intrasubstance
tears, and edema-sensitive images reveal increased
signal in a swollen, abnormally rounded tendon (Fig.
47-20). Partial tears can also present as nearly complete
ruptures, with only a few remaining fibers intact (Fig.
47-21). In these cases, abnormal fluid can be seen surrounding
the intact fibers, within the distended paratenon
(see Fig. 47-21E). Imaging with MRI or ultrasonography is
used to assess the extent of partial tears.
An Achilles tendon that has undergone internal healing
and scar formation from a prior intrasubstance tear tends
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