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

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2254 VII Imaging of the Musculoskeletal System surgical emergencies. The patients with significantly displaced fractures may go to the operating room the day of the injury for traction reduction and external fixation to restore relative alignment to the mortise, and then wait several days for the swelling of the surrounding soft tissues to reduce before returning to the operating room for the more anatomic ORIF of the pilon fracture. This means that these patients typically receive their CT scans during this interim period, after the external fixator is in place. However, as illustrated in Figure 47-62, such external fixation hardware is no impediment to obtaining the CT images the surgeon requires. To maintain alignment between the hindfoot and leg, the surgeon will percutaneously drill thick metal pins through the calcaneus (white arrows in Fig. 47- 62A, B, and D) and through the tibia proximal to the fracture (this pin is not seen in Fig. 47-62). These pins are rigidly attached by metal clamps (white arrowheads in Fig. 47-62A and B) to nonmetallic connecting bars (gray arrows in Fig. 47-62A to C). It is these nonmetallic bars that span the length of the fracture and maintain the tibia length. Because these nonmetallic bars are made of materials (usually carbon fiber) that block very few x-rays from reaching the detectors, they are nearly radiolucent and cause no CT streak artifacts (see Fig. 47-62C). The metal pin-bar clamps block many x-rays from reaching detectors and thus will cause some CT streak artifacts. However, because the clamps are always placed proximal and distal to the pilon fracture, they never cause any CT streak artifacts across the reformatted fracture margins (see Fig. 47-62B and D). Using our standard bone CT scanning protocol of thin/overlapping slices, metallic streak artifacts are often not appreciable. Notice the good visualization of the calcaneus cortex in Figure 47-62B and D, which is only minimally affected by streaking caused by the metal pin-bar clamps. Juvenile Tillaux Fracture Juvenile Tillaux fractures are Salter-Harris type 3 fractures.* These fractures have a characteristic appearance, particularly on CT. The fracture is the result of an external rotation force pulling on the anterior tibiofibular ligament, causing avulsion of the anterolateral corner of the distal tibial epiphysis (Fig. 47-63A). These fractures always occur laterally because the distal tibial physis fuses from medial to lateral as a child matures (Fig. 47-63B). As such, juvenile Tillaux fractures occur exclusively in adolescents in whom the lateral growth plates have not yet fused, usually between the ages of 12 and 15 years. Coronal and sagittal images are useful to demonstrate the degree of displacement particularly at the articular surface (Fig. 47-63B and C, white arrow). While minimally displaced juvenile Tillaux fractures are usually treated nonoperatively, fractures displaced more than 2 mm should have orthopedic consultation and surgery to restore the congruity of the joint surface. Triplane Fracture Triplane fractures are Salter-Harris type 4 fractures. Like the juvenile Tillaux fracture, triplane fractures occur in adolescents in whom the lateral growth plates have not yet fused. When minimally displaced, triplane fractures can be difficult to see radiographically, and frontal and lateral views are needed to appreciate their multiplanar nature (Fig. 47-64A to C): the epiphysis fracture running vertically in a sagittal orientation (plane 1), the physeal fracture running horizontally in the axial plane (plane 2), and the metaphyseal fracture running obliquely vertically in a coronal orientation (plane 3). Multiplanar CT scans are ideally suited to visualize these fractures in all planes (Fig. 47-64D to F) and often reveal more deformity of the articular surface than would be anticipated from radiographs alone. • Talar Fractures 3,47 Talar fractures can be thought of as either traumatic or insidious. Traumatic fractures are considered surgical emergencies because of the high risk of avascular necrosis, and patients usually go straight from the emergency department to the operating room without stopping at CT (although CT scans of displaced fractures of the body of the talus can be dramatic; Fig. 47-65). Even with anatomic internal fixation, avascular necrosis sometimes occurs, and CT can be useful to confirm the presence of abnormal medullary sclerosis suspected radiographically (Fig. 47-66). • Osteochondral Lesions of the Talus Fractures of the talar dome are insidious. They typically occur at the medial edge or posterolateral corners of the talar dome and are thought to be the result of an impaction of the talar dome on the tibial plafond during an inversion or eversion twisting injury. Refer to the illustrations of Weber injuries (see Fig. 47-60). Because they involve the cortical bone and the overlying articular hyaline cartilage, they are referred to as osteochondral fractures. A gross example is indicated by the green arrow in Figure 47-2. Osteochondral fractures notoriously occur on convex articular surfaces, including the femoral condyles of the knee and capitellum of the elbow. Generically, these fractures have been referred to by many names, including osteochondral defect, osteochondral lesion, and osteochondritis dissecans. The last term is the oldest and perhaps the most misleading, for although the suffix “itis” by definition implies inflammation, histologically these lesions have not been shown to be inflammatory. We prefer the term osteochondral lesions of the talus, to distinguish them from osteochondral lesions at other sites. *The Salter-Harris system is applied to fractures that involve the growth plate (physis) at the ends of skeletally immature bones. Type 1 refers to simple transverse fractures that involve the physis only. Type 2, the most common, refers to fractures that involve the physis and the adjacent metaphysis. Type 3 fractures extend from the physis through the epiphysis at the end of the bone, typically disrupting the articular surface at a joint. Type 4 fractures involve the epiphysis, the physis, and the metaphysis. Type 5 fractures are rare and are crush injuries to the growth plate. Text continued on p. 2260 Ch047-A05375.indd 2254 9/9/2008 5:34:47 PM
47 Ankle and Foot 2255 47 A B C D Figure 47-62. Pilon fracture. A, Anteroposterior radiograph showing an external fixation device. The radiopaque hardware that could potentially cause streak artifacts on CT—the thick metal pin through the calcaneus (white arrows), the distal pin-bar clamps (white arrowheads), and the proximal pin-bar clamps (black arrowheads)—are below and above the pilon fracture and thus will not be in the axial CT scanning plane through the fracture. The longitudinal carbon fiber connecting bars are barely radiopaque, and as such they are barely discernible on this radiograph (gray arrows). These will cause no CT streak artifacts. B, Coronal plane CT scan. The carbon fiber connecting bars (gray arrows) cause no CT streak artifacts across the fractures. The CT streak artifacts from the metal percutaneous pin (white arrows) and pin-bar clamps (white arrowheads) are all distal to the pilon fracture and only minimally effect visualization of the calcaneus cortex. C, Axial plane CT scan through the level of the fractured plafond. The carbon fiber connecting bars (gray arrows) cause no CT streak artifacts across the fractures. D, Sagittal plane showing the talar dome impacted into a large cortical gap in the plafond. This is the type of visual information the surgeon needs to plan the open reduction and internal fixation. The white arrow shows the percutaneous pin passing through the calcaneus. Ch047-A05375.indd 2255 9/9/2008 5:34:49 PM
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Ankle and Foot 47 - Department of Radiology - University of ...

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