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

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2268 VII Imaging of the Musculoskeletal System A D B E C Figure 47-79. Fracture of lateral process of the talus (LPT) in a 45- year-old who was the driver in a front-end automobile collision. A, Lateral radiograph does not clearly demonstrate the LPT fracture. There is a small ossicle (gray arrow) just behind the subtalar joint that could be mistaken for an os trigonum but is in fact a small fragment off the posterior corner of the talus. B, Mortise radiograph shows a tiny ossicle (white arrow) between the LPT and lateral malleolus, too small to characterize. C, Anteroposterior radiograph reveals the large LPT fragment (white arrow) as well as a medial fragment (open arrow). D and E, CT scans obtained the same day as the radiographs, reformatted in the oblique coronal plane, perpendicular to the subtalar joint. D, Image through the middle facet of the subtalar joint (M-STJ) shows the large LPT fragment (arrow). E, A more posterior image demonstrates the LPT fracture extending into the posterior facet (arrowhead), as well as the separate fragment off the medial talus (arrow). Surgery was performed 1 week later, after the soft tissue swelling had diminished. Lateral (F) and mortise (G) radiographs were obtained after the LPT fracture was repaired with two screws. (There is also a Mitek suture anchor in the lateral malleolus.) F G Ch047-A05375.indd 2268 9/9/2008 5:35:11 PM
• Calcaneal Fractures 6,18,33 47 Ankle and Foot 2269 47 The calcaneus is the most commonly fractured tarsal bone, with fractures typically occurring as the result of traumatic axial loading, such as can occur with a front-end automotive collision or falling from a height and striking the ground feet first. Here in Wisconsin, we have hunters falling from their tree-mounted deer stands. As a clinical aside, patients who present to the emergency department with bilateral calcaneal fractures should also be evaluated for lumbar spine fractures at the time of the initial trauma workup. The same traumatic axial loading that drives the talus into the calcaneus also drives the lumbar vertebrae together, and the risk of a lumbar burst fracture with fragments retropulsed into the vertebral canal is high. An example of the workup of such a patient with nondisplaced lumbar fractures is outlined in Figure 47-80. Calcaneal fractures can usually be recognized on the lateral radiograph by the presence of lucent fracture lines and displaced fragments (see Fig. 47-80A) or by a compression deformity of the calcaneus with flattening of “Böhler’s* angle” (see Fig. 47-80B). 9 When calcaneal fractures are identified, it is important to evaluate for related injuries. Lumbar compression fractures related to axial loading forces are particularly associated with calcaneal fractures. At the UW it is typical for severely traumatized patients to receive a contrast-enhanced CT scan of the abdomen and pelvis as part of the initial trauma workup (see Fig. 47-80C). Although this scan is designed to reconstruct the raw data into large FOV images that are relatively thick (5 mm) to assess for soft tissue organ injury, the same raw data can also be reconstructed into images centered on the spine with a smaller FOV and thinner (1 mm), overlapping slices (see Fig. 47-80D). These thin, overlapping source images can then be reformatted into sagittal (see Fig. 47- 80E) and other planes. Although CT images of the spine are well suited to demonstrate the presence or absence of cortical fragments displaced into the vertebral canal as well as the overall alignment of the spine, MRI is used to visualize epidural hematomas and other possible soft tissue causes for neural compromise. T1-weighted (see Fig. 47- 80F) and proton-density–weighted (see Fig. 47-80G) images are less sensitive to bone marrow edema than are fat-suppressed T2-weighted (see Fig. 47-80H and I) or inversion recovery images. With traumatic axial loading, the wedge-shaped LPT is driven into the calcaneus at the angle of Gissane, fracturing and depressing the calcaneus (see Fig. 47-80J; Fig. 47-81B). This fracture invariably involves the calcaneal articular surface of the posterior facet of the subtalar joint (see Figs. 47-80K and 47-81C). The fracture then propagates inferiorly and medially (see Fig. 47-81C), involving the sustentaculum tali and the middle facet to varying degrees (see Figs. 47-80L and 47-81D). Assessment of the integrity of the middle facet of the subtalar joint is an important part of preoperative surgical planning. Surgeons prefer to operate on the calcaneus from the lateral side, meaning that they will not directly visualize the middle facet and sustentaculum tali. Thus, they require the preoperative CT scan to show them these structures. For this reason, the oblique coronal plane, angled perpendicular to the subtalar joint, is the primary imaging plane in the assessment of calcaneal fractures. Our CT hindfoot/midfoot reformatting protocol (see Fig. 47-47B) also includes straight sagittal and straight and oblique axial images to assess for extension into the calcaneocuboid joint (see Fig. 47-81E). One additional clinical point regarding calcaneal fractures: they tend not to be surgical emergencies. Surgeons typically wait for several days after the initial trauma for the soft tissue swelling to decrease before operating. Therefore, the preoperative CT scan of the calcaneus does not need to be performed emergently when there may be other, more serious injuries that need to be addressed. • Anterior Process of the Calcaneus The APC is the upper outer corner of the calcaneus where it articulates with the cuboid, indicated by the orange box on gross Figure 47-4C and the red arrow on sagittal CT Figure 47-7A. Like LPT fractures, APC fractures can be easily overlooked, and this structure should be carefully scrutinized on all lateral radiographs of the ankle and foot (Fig. 47-82). APC fractures are more common in women and are the result of an inversion injury while the foot is in plantar flexion, such as when wearing high-heeled shoes. Even when APC fractures are only minimally displaced they have a tendency for nonunion despite prolonged immobilization (Fig. 47-83). CT is useful for both detecting these fractures and following their progress. One potential pitfall in the diagnosis of an APC fracture is the os calcaneus secondarius, an occasionally seen normal variant that resides between the APC and the lateral pole of the navicular (see Fig. 47-39). The os calcaneus secondarius can be thought of as a forme fruste of tarsal coalition, and it should not articulate with the cuboid as the APC does. CT can be used to distinguish an acute APC fracture from the normal-variant accessory ossicle (Fig. 47-84). • Lisfranc Dislocation Dislocations along the tarsometatarsal joint are not uncommon. These can be the result of severe acute trauma, but the Lisfranc joint is also a common site for dislocation in diabetic patients with peripheral neuropathy. As mentioned in a footnote earlier in this chapter, Jacques Lisfranc was a very aggressive surgeon in Napoleon’s army, and although he did not describe the dislocation that now *Lorenz Böhler (1885-1973) is most notable as the creator of modern accident surgery. He was the head of the AUVA-Hospital in Vienna, Austria, that was later named for him. This hospital was an international model during his time as the leading surgeon there. Text continued on p. 2277 Ch047-A05375.indd 2269 9/9/2008 5:35:11 PM
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