(Radiology. 2000;217:359-360.)
© RSNA, 2000
The Fat C2 Sign1
David D. Pellei, MD
1 From the Medical College of Virginia of VCU, Richmond. Received March 3, 1998; revision requested April 16; revision received April 27, 1999; accepted June 15. Address correspondence to the author, Department of Emergency/Trauma Radiology, Charlotte Radiology, Carolinas Medical Center, PO Box 32861, Charlotte, NC 28232 (e-mail: RPellei@juno.com).
Index terms: Signs in Imaging • Spine, fractures, 311.412, 311.413 • Trauma
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APPEARANCE
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The so-called "fat" C2 sign refers to an apparent increase in the distance between the anterior and posterior margins of the C2 vertebra when compared with the distance between the two margins of the C3 vertebra (Fig 1) on a lateral conventional radiograph of the cervical spine in the setting of trauma.
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Figure 1. Lateral radiograph of cervical spine trauma shows that the anteroposterior diameter, measured as the distance (arrows) between the anterior and posterior margins (dotted lines), of C2 (A) is larger than that of C3 (B). Fractures are not readily apparent.
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EXPLANATION
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A fat C2 sign results from an obliquely oriented fracture, involving the body of C2, that causes displacement of either one or both of the anterior and posterior margins and creates an increased anteroposterior distance between the two surfaces. The degree of obliquity of the fracture plane with respect to the coronal plane will determine whether the actual fracture will be visualized on the lateral radiograph (1).
This injury may occur as a result of isolated hyperflexion or hyperextension forces or of combined hyperflexion and hyperextension. There may also be some component of vertical loading or distraction in these injuries (2). The fracture plane gains obliquity when there is some rotational component in the stress forces. The identification of a fat C2 sign implies that a potentially unstable fracture with fragment displacement is present and that further imaging evaluation is needed (Fig 2).
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Figure 2a. (a) Transverse and (b) sagittal reconstructed computed tomographic (CT) images obtained in the same patient as in Figure 1 demonstrate complex comminuted fractures (arrowheads) of C2.
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Figure 2b. (a) Transverse and (b) sagittal reconstructed computed tomographic (CT) images obtained in the same patient as in Figure 1 demonstrate complex comminuted fractures (arrowheads) of C2.
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DISCUSSION
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The fat C2 sign has been described as being indicative of a fracture that involves the body of C2. It may result from one of several types of injuries: a complex vertical fracture of the body of the C2 (1), a low (type III) dens fracture (3,4), or an atypical traumatic spondylolisthesis (3,5).
Complex vertical fractures of the body of C2 result from a combination of applied forces. Components of these forces may include hyperextension, hyperflexion, rotation, lateral bending, distraction, and vertical loading (1,2). These may be combined in any manner, with the resultant fractures reflecting the sum of the forces applied (2). These fractures are often obscured on conventional radiographs because the combined forces produce an oblique fracture that is not tangential to the x-ray beam.
However, the fracture fragments may be displaced by applied forces and result in the interruption of the anterior or posterior margins or both, with apparent enlargement of the body of C2 on the lateral radiograph. This creates the fat C2 sign. Primary hyperflexion injuries predominantly disrupt the anterior margin. If the posterior margin is disrupted, a primary hyperextension injury is suggested. Combined hyperextension-hyperflexion injuries may result in the interruption of both margins (1).
A low (type III) dens fracture may result from any combination of hyperflexion, hyperextension, and lateral bending mechanisms (3). It is actually a horizontal or oblique fracture of the body of C2, just inferior to the dens. It is considered unstable because it separates the upper portion of C2, the dens, the C1 vertebra, and the occiput from the remainder of the cervical spine (3). These fractures are often isolated injuries but are sometimes associated with mandibular fractures (5), extension teardrop fractures, Jefferson fractures, or atlanto-occipital dislocations (3).
In 1985, Levine and Edwards (6) modified the classification of traumatic spondylolisthesis (bilateral fractures through the neural arch of C2) and correlated each class with its mechanism of injury as follows: hyperextension-vertical loading (type I), hyperextension-vertical loading followed by flexion and compression (type II), flexion and distraction (type IIa), and flexion and compression (type III).
In 1993, Starr and Eismont (7) described an atypical traumatic spondylolisthesis in which the vertical component of the fracture line in a type I or type II injury passes through the posterior cortex of the vertebral body. In this variant, there is disruption of the posterior cortex of C2 (posterior margin), which forms the posterior portion of the ring of C2, and the anteroposterior diameter of C2 is subsequently widened (4,7). This injury has a greater tendency for mechanical instability, canal compromise, and neurologic sequelae than do the pure type I or type II injuries in which canal widening is typical (7).
The importance of recognizing the fat C2 sign and of identifying the causative fracture(s) is that these injuries are often accompanied by ligamentous disruption and are therefore considered potentially unstable (1). Additional CT and/or magnetic resonance imaging is indicated to evaluate the location of the fracture fragments and the condition of the spinal cord. Delay in detecting these injuries may be devastating.
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FOOTNOTES
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A trainee (resident or fellow) wishing to submit a manuscript for Signs in Imaging should first write to the Editor for approval of the sign to be prepared, to avoid duplicate preparation of the same sign.
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REFERENCES
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Smoker WRK, Dolan KD. The "fat" C2: a sign of fracture. AJNR Am J Neuroradiol 1987; 8:33-38.
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Benzel EC, Blaine LH, Perry AB, et al. Fractures of the C-2 vertebral body. J Neurosurg 1994; 81:206-212.[Medline]
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Harris JH, Mirvis SE. Injuries of diverse or poorly understood mechanisms In: The radiology of acute cervical spine trauma. 3rd ed. Baltimore, Md: Williams & Wilkins, 1996; 421-472.
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Harris JH, Burke JT, Ray RD, Nichols-Hostetter S, Lester RG. Low (type III) odontoid fracture: a new radiographic sign. Radiology 1984; 153:353-356.[Abstract/Free Full Text]
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Burke JT, Harris JH. Acute injuries of the axis vertebra. Skeletal Radiol 1989; 18:335-346.[Medline]
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Levine AM, Edwards CC. The management of traumatic spondylolisthesis of the axis. J Bone Joint Surg Am 1985; 67:217-226.[Abstract/Free Full Text]
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Starr JK, Eismont FJ. Atypical hangman’s fractures. Spine 1993; 18:1954-1957.[Medline]
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