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

2260 VII Imaging of the Musculoskeletal System
A
B
Figure 47-67. Acute talar dome fracture in
a 23-year-old who fell from a ladder. A and B,
Anteroposterior and mortise radiographs in which the
cortical fracture of the lateral corner of the talar dome
is so nondisplaced it is barely discernible (arrow). C
and D, Mortise coronal CT images obtained the same
day well demonstrate the cortical fragment (arrow) as
well as the full extent of the fracture (arrowheads).
C
D
Radiology and Computed Tomography
Although the development of a symptomatic OLT can
often be traced to a specific injury, radiographs are usually
read as normal early on. In part, this is because many of
these fractures are so nondisplaced that they can be difficult
to see radiographically (Fig. 47-67). But sometimes,
even in retrospect, the initial radiographs truly are negative,
and it may take months for the OLT to be radiographically
apparent (Fig. 47-68). At the UW we have a special
reformatting protocol just for such talar dome fractures
(see Fig. 47-47A) that includes 1-mm-thin slices reformatted
with no gaps in the mortise coronal and mortise sagittal
planes.
Magnetic Resonance Imaging and Staging
Although CT is good at showing a displaced fragment and
the size of the talar dome defect, MRI is better at showing
the integrity of the overlying articular hyaline cartilage
and the underlying bone marrow. Edema-sensitive MRI is
used to detect OLTs that are radiographically occult and
also is used to stage known OLTs to assess for healing
potential or need for surgery.
Several staging systems have been proposed. In 1959,
Berndt, 8 an orthopedic surgeon from the Cleveland Clinic,
working with Harty, an anatomist from the University of
Pennsylvania, analyzed 24 cases of what they called “transchondral
fractures of the talus.” In the process of tabulating
their data, “an arbitrary classification was developed to
aid understanding of the mechanism of the fracture and
to help in determining the appropriate treatment.” This
staging system was based solely on the radiographic appearance
of the fracture:
Stage I: A small compression fracture
Stage II: Incomplete avulsion fragment
Stage III: Complete avulsion without displacement
Stage IV: Avulsed fragment displaced within the joint
Thirty years later, Anderson and colleagues 2 from Australia
modified this staging system based on the MRI
appearance of the fracture. Anderson called stage I “subchondral
trabecular compression” and defined it as radiographically
negative, but with bone marrow edema on MRI
(Fig. 47-69). Anderson called stage II “incomplete separation
of the fragment,” requiring demonstration of an intact
attachment by either CT or MR (Fig. 47-70). Anderson
added a stage IIA, “formation of a subchondral cyst” (Fig.
47-71). Stage IIA cysts are thought to develop from stage I
injuries with post-traumatic necrosis of bone and subsequent
resorption of the necrotic trabeculae, leaving behind
a subchondral cyst. Anderson stage III, “unattached, undisplaced
fragment,” is the same as Berndt and Harty stage III.
Anderson noted, “In the T2 weighted image, the presence
of synovial fluid around a large fragment can help to differentiate
between stages II and III.” However, Anderson
went on to question the utility of MRI over CT in making
this determination (Fig. 47-72). Anderson stage IV, “displaced
fragment,” is the same as Berndt and Harty stage IV
(Fig. 47-73).
Around the same time as Anderson but half a world
away, De Smet and coworkers 19 from the University of
Wisconsin, Madison, were correlating surgical and MRI
findings and dividing OLTs into stable or unstable lesions.
Stable fragments were defined as being fixed firmly
with fibrous tissue or fibrocartilage, and these patients
were thought not to need surgery. Unstable fractures are
those that can be shown by MRI to be partially attached or
unattached, and these fractures were thought to require
more aggressive treatment with surgery or prolonged
immobilization. De Smet showed that the key factor in
distinguishing stability from instability by MRI is the
presence of bright signal on T2-weighted images at the
interface between the fragment and the donor site. In unattached
fragments this signal was as bright as fluid, and
surgery confirmed that these fragments were surrounded
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