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

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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 Ch047-A05375.indd 2260 9/9/2008 5:34:57 PM
47 Ankle and Foot 2261 47 A C B D Figure 47-68. Osteochondral lesion (OLT) in a 9- year-old with right ankle pain, without any specific trauma. A, Mortise radiograph, even in retrospect, is negative for a lesion in the medial talar dome. B, Nine months later, the same mortise view reveals a subtle lesion in the medial talar dome (arrows). C and D, CT scans obtained 1 month after the radiograph in part B well demonstrate the medial osteochondral lesion of the talus (open and black arrows). The difference between C and D is the way the images were reformatted: C was reformatted using our hindfoot protocol (3 × 3 mm in the oblique coronal plane), whereas D was reformatted using our specialized OLT protocol (1 × 1 mm in the mortise coronal plane). The thinner, 1-mm reformatted images yield edges with sharper margins. E, Mortise coronal reformatted CT well shows the extent of the OLT (black arrows). F, Axial source images through both ankles demonstrate not only the symptomatic OLT in the posterior medial corner of the right talar dome (black arrows), but an asymptomatic OLT in the posterior medial corner of the left talar dome (white arrows). The patient was treated conservatively and was asymptomatic bilaterally. LM, lateral malleolus; MM, medial malleolus; Ta, talus. E F Ch047-A05375.indd 2261 9/9/2008 5:34:59 PM
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Ken L. Schreibman Richard Bruce Ank
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