Imaging of upper cervical spine injuries—Part II: The dens

O. Clark West, MD; Ronald M. Bilow, MD; Amanda Mignon Jarolimek, MD

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Abstract:  his pictorial essay is the second in a series of articles on spinal injuries. To avoid diagnostic error, radiologists must pay careful attention to the cortical margins of the dens, the spinolaminar line in the craniocervical region, the C2 rings, and the prevertebral soft tissues. Each of these signs is illustrated in this article.

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Dr. West is an Associate Professor and Chief of, and Dr. Bilow and Dr. Jarolimek are Instructors in, the Emergency and Trauma Radiology Section, Department of Radiology at the University of Texas Medical School, Houston, Houston, TX.

[Editor’s note: The first article in this series was: West OC. Imaging upper cervical spine injuries—Part I: CO­C1. Appl Radiol . 2002;31(2):23-32. It is also available online at www.appliedradiology.com.]

This is the second in a series of articles that emphasizes the radiography and computed tomography (CT) of spine injuries. It is hoped that the subject will be covered with a depth and breadth rarely possible in print media. To achieve this goal, this article will focus exclusively on dens fractures. Throughout this series, our motto will be Few words, many pictures.

Dens fractures are common, accounting for 7% to 17% of all cervical spine fractures. While displaced dens fractures are readily apparent, less displaced fractures are easily overlooked. To avoid diagnostic error, the radiologist must pay careful attention to the cortical margins of the dens, the spinolaminar line in the craniocervical region, the C2 rings, and the prevertebral soft tissues. Each of these signs will be illustrated in this pictorial essay.

Dens fractures of C2 are classified into three types. 1 A type I fracture represents an avulsion at the insertion of the alar ligament and appears as an obliquely oriented fracture at the rostral end of the dens (Figure 1). An isolated type I fracture is rare—we have never encountered one in our practice--and is considered to be stable. We do occasionally find type I fractures in association with an unstable craniocervical dislocation or subluxation.

A type II fracture is the most common type. This horizontally oriented fracture occurs at the base of the dens, an area of diminished bone density compared with the rest of C2. 2 Type II fractures are the least stable dens fracture and are most likely to develop nonunion when treated nonoperatively.

Minimally displaced type II dens fractures may be difficult or impossible to see on lateral radiographs (Figure 2). The open-mouth odontoid view may depict a fracture that is not otherwise visible, either as a fracture line (Figure 3) or a tilted dens (Figure 4). 3 The presence of prevertebral soft-tissue swelling may prompt CT scanning of the upper cervical spine, allowing detection of otherwise radiographically occult fractures. 4 However, not all dens fractures have abnormal prevertebral soft tissues (Figure 2). The existence of radiographically occult dens fractures in the absence of soft-tissue swelling is a compelling reason to perform screening CT in high-risk patients.

Most type II dens fractures are horizontally oriented and occur at or below the level of the inferior margin of the anterior arch of C1 (Figures 2 to 8). On occasion, the fracture line is oriented obliquely, from rostral anteriorly to caudal posteriorly (Figure 9).

Using thick CT sections (3 to 5 mm), type II fractures may be difficult to detect on axial CT. Our experience using 2.5-mm axial images made with the bone algorithm is that a type II dens fracture is almost always visible on axial CT images as a cortical break at the base of the dens. We routinely make a second set of 1.25-mm axial images with the standard algorithm for high-quality sagittal and coronal reformatted images. Rarely, a minimally displaced horizontal type II dens fracture is aligned perfectly with the plane of axial scanning, making it difficult to detect on axial images. Fortunately, the fracture is very easy to detect on high-quality multiplanar reformatted images (Figure 5). Displaced type II dens fractures are recognized easily on radiographs or CT scans (Figures 6 through 8).

Nonunion of a type II dens fracture occurs with sufficient frequency to present a diagnostic challenge. Well-corticated fracture margins and a history of previous neck injury, or, better still, a history of previous fracture, helps establish the diagnosis. However, the crucial issues are whether or not a quasi-stable fibrous union has formed and whether or not acute re-injury has resulted in instability. The presence of pain, pathologic motion on flexion/ extension lateral radiographs, edema on MRI, and increased radiopharmaceutical uptake on bone scan provides evidence of acute re-injury and instability (Figure 8).

A type III fracture is an oblique fracture at the base of the dens, extending into the cancellous bone in the body of C2. Type III fractures are relatively more stable and less prone to nonunion when compared with type II fractures. Minimally displaced type III fractures may be difficult to detect. Such fractures are usually not apparent in the anteroposterior (AP) projection. Careful analysis of the lateral radiograph with attention to the C2 rings may indicate fracture through the C2 body. 5,6 The C2 rings are a pair of composite shadows forming two inverted horseshoes within the C2 body caudal to the base of the dens (Figures 9F and 9G). The C2 rings are superimposed on a perfectly lateral radiograph but are visible as separate structures if the patient is rotated or the X-ray beam is oblique to C2. The anterior margin of this cortical ring is formed by the oblique groove between the body and articular mass. The top is formed by both the notch between the base of the dens and the superior articular facet of C2, and by the superior margin of the superior articular facet itself. The posterior cortex of the C2 body forms the posterior margin. Minimal breaks in the C2 ring are a sensitive means of detecting small fractures (Figures 10 and 11). Displaced type III fractures typically cause interruption of the C2 rings (Figures 12 and 13). Fracture lines often extend into one or both superior articular surfaces.

Conclusion

After studying the cases of dens fracture in this pictorial essay, several important facts should be clear. First, fractures of the dens are often, but not always, detectable by radiographs. Second, prevertebral soft-tissue swelling is helpful when present, but the absence of swelling does not exclude a dens fracture. Third, because radiographs are not completely sensitive for the detection of potential unstable injuries in the craniocervical region, screening high-risk trauma patients for injury using CT is suggested. 7,8 Fourth, routine use of sagittal and coronal reformatted images is important for detecting minimally displaced type II dens fractures and for characterizing all dens fractures. Fifth, type II dens fractures are commonly encountered in clinical practice—type III fractures are less common; type I fractures are part of more complex craniocervical junction injuries and rarely, if ever, occur as isolated fractures. Finally, the C2 ring sign is a valuable tool in detecting type III dens fractures.

References

  1. Anderson LD, D’Alonzo RT. Fractures of the odontoid process of the axis. J Bone Joint Surg Am. 1974;56:1663-1674
  2. Amling M, Hahn M, Wening VJ, et al. The microarchitecture of the axis as the predisposing factor for fracture of the base of the odontoid process. A histomorphometric analysis of twenty-two autopsy specimens. J Bone Joint Surg Am. 1994;76:1840-1846.
  3. West OC, Anbari MM, Pilgram TK, Wilson AJ. Acute cervical spine trauma: Diagnostic performance of single-view versus three-view radiographic screening. Radiology. 1997;204:819-823.
  4. Harris J, Jr. The cervicocranium: Its radiographic assessment. Radiology. 2001;218:337-351.
  5. Harris JH, Jr., Burke JT, Ray RD, et al. Low (type III) odontoid fracture: A new radiographic sign. Radiology. 1984;153:353-356.
  6. Mortelmans LJ, Geusens EA, Sabbe MB, Delooz HH. Harris or axis ring: An aid in diagnosing low (type 3) odontoid fractures. Eur J Surg. 1999;165:1138-1141.
  7. Link TM, Schuierer G, Hufendiek A, et al. Substantial head trauma: Value of routine CT examination of the cervicocranium. Radiology. 1995;196:741-745.
  8. Blackmore CC, Mann FA, Wilson AJ. Helical CT in the primary trauma evaluation of the cervical spine: An evidence-based approach. Skeletal Radiol. 2000;29:632-639.

Tables & Figures

  • Figure 1A. A 13-year-old with multiple craniocervical injuries, including a type I dens fracture. (A) Intraoperative lateral radiograph (initial radiographs are no longer available) shows widening of C1–C2 articulation (white doubleheaded arrows), widening of the rostral portion of the atlantal-dental articulation (black doubleheaded arrow), and anterior tilting of C1. No sign of a dens fracture is present. (B) A 2.5-mm axial CT image shows empty fossae of C2 [F], a sign of occipito-atlantal (C0–C1) dissociation. (C) The next slice shows that the rostral aspect of the atlantal-dental interval (C1–C2) is widened at 5.5 mm (double-headed arrow). Avulsion of the attachment of the alar ligament—the type I dens fracture—is a minor finding (large arrow). (D) Six slices caudally, widening of C1–C2 facet joints is difficult to perceive on axial images (double-headed arrows). (E) Coronal reformatting of 1.25-mm axial data readily depicts C0–C1 and C1–C2 widening. Type I dens fracture is easier to detect (arrow).
    Figure 1A.
  • Figure 1B.
    Figure 1B.
  • Figure 1C.
    Figure 1C.
  • Figure 1D.
    Figure 1D.
  • Figure 1E.
    Figure 1E.
  • Figure 2A. A 19-year-old man with a type II dens fracture. (A) The initial lateral radiograph shows minimal findings. Prevertebral soft tissues are not thickened and have a normal contour. No fracture is visible. In retrospect, density posterior to the expected location of the dens must represent the posteriorly displaced dens (arrow). (B) A 2.5-mm axial CT image shows a posteriorly displaced dens fracture. (C) Sagittal reformatted image better depicts the typical location of a type II fracture at the inferior margin of the anterior arch of C1. (D) Lateral radiograph obtained after halo placement. Dens fracture is slightly more posteriorly displaced but is much more apparent, primarily because of better radiographic technique.
    Figure 2A.
  • Figure 2B.
    Figure 2B.
  • Figure 2C.
    Figure 2C.
  • Figure 2D.
    Figure 2D.
  • Figure 3A. A 36-year-old woman with a type II dens fracture. (A) Fracture at the base of the dens is 1 to 2 mm wide. It is invisible to many readers because it is directly superimposed over the normal lucency created by the C1–C2 facet joints (double-headed arrows). (B) Openmouth odontoid view more readily depicts the fracture as an irregular lucency across the base of the dens. (C) and (D) Two adjacent 3-mm axial CT images through the base of the dens show the minimally displaced, mildly distracted fracture.
    Figure 3A.
  • Figure 3B.
    Figure 3B.
  • Figure 3C.
    Figure 3C.
  • Figure 3D.
    Figure 3D.
  • Figure 4A. A 71-year-old man with a type II dens fracture that demonstrates a tilted dens sign. (A) Lateral radiograph is similar to Figure 3 in showing a fracture line superimposed over the C1–C2 facet joints. (B) Open-mouth odontoid view shows the dens tilted 14° to left, although the fracture line is not apparent. Normally, the dens is perpendicular to the horizontal axis of C2.
    Figure 4A.
  • Figure 4B.
    Figure 4B.
  • Figure 5A. An 82-year-old woman with a type II dens fracture that illustrates the more common horizontal fracture plane and importance of routine use of multiplanar reformatted images. (A) Lateral radiograph shows abnormal convexity of prevertebral soft tissues with only mild soft-tissue thickening. A horizontal fracture is visible as breaks in the anterior and posterior cortices (arrows). (B) A 2.5-mm axial CT image through the base of the dens shows relatively inconspicuous cortical breaks (arrows) with no other sign of a dens fracture. This fracture might be overlooked if only axial images were reviewed. (C) Sagittal and (D) coronal reformatted images of 1.25-mm axial data readily depict the horizontal fracture in the plane of axial scanning.
    Figure 5A.
  • Figure 5B.
    Figure 5B.
  • Figure 5C.
    Figure 5C.
  • Figure 5D.
    Figure 5D.
  • Figure 6A. A 67-year-old man with a posteriorly displaced type II dens fracture. (A) Lateral radiograph shows a horizontally oriented fracture at the base of the dens displaced posteriorly one-half width of the dens. Note the thickening and the abnormally convex contour of prevertebral soft tissues. A fracture of the posterior arch of C1 is also present. (B) A 2.5-mm axial CT image shows a horizontal fracture at the base of the dens. Hairline fractures of the posterior arch of C1 are also visible. (C) Two slices lower, a tag of the posterior cortex projects into the spinal canal. Subluxation of the left C1–C2 facet joint is expected from posterior displacement of the dens fracture (double-headed arrow). (D) Sagittal and (E) coronal reformatted images illustrate the severity of the posterior displacement.
    Figure 6A.
  • Figure 6B.
    Figure 6B.
  • Figure 6C.
    Figure 6C.
  • Figure 6D.
    Figure 6D.
  • Figure 6E.
    Figure 6E.
  • Figure 7A. A 23-year-old man with an anteriorly tilted type II dens fracture imaged several weeks following blunt trauma. (A) Lateral radiograph shows a horizontal dens fracture with 26° anterior angulation. (B) Sagittal and (C) coronal reformatted images illustrate the severity of the anterior displacement.
    Figure 7A.
  • Figure 7B.
    Figure 7B.
  • Figure 7C.
    Figure 7C.
  • Figure 8A. A 35-year-old woman with an unstable nonunion of type II dens fracture presented with neck pain after a recent car crash. She gave a history of a cervical spine injury after a car crash several years previously. Lateral radiographs in extension (A) and flexion (B) show gross instability with anterior displacement of the dens in flexion. Note narrowing of the spinal canal that results from anterior motion of C1. (C) Open-mouth odontoid radiograph shows nonunion of the base of the dens with well-corticated margins on either side. A bone scan with single-photon emission computed tomography showed increased uptake of radiopharmaceutical at the site of nonunion.
    Figure 8A.
  • Figure 8B.
    Figure 8B.
  • Figure 8C.
    Figure 8C.
  • Figure 9A. A 25-year-old man with a type II dens fracture illustrating soft-tissue swelling, oblique fracture line, and intact C2 rings. (A) Lateral radiograph shows thickening and straightening of the normally concave prevertebral soft-tissue contour anterior to C2. (B), (C), and (D) A series of 3 contiguous 2.5-mm axial CT images shows a steeply oblique fracture line from anterior cortex rostrally to posterior cortex caudally. (E) Sagittal reformatted image shows an oblique hairline fracture through the right lateral cortex of the dens (arrows). Oblique fracture lines are less common than horizontal lines. (F) A series of white arrows outlines one of two C2 rings. (G) A series of black arrows outlines the anterior portion of the second C2 ring. In this case, two rings merge into a combined cortical line over the top of the C2 body and continue postero-inferiorly as a single combined line. Intact C2 rings indicate that this type II dens fracture does not extend into the C2 body.
    Figure 9A.
  • Figure 9B.
    Figure 9B.
  • Figure 9C.
    Figure 9C.
  • Figure 9D.
    Figure 9D.
  • Figure 9E.
    Figure 9E.
  • Figure 9F.
    Figure 9F.
  • Figure 9G.
    Figure 9G.
  • Figure 10A. A 26-year-old man with a minimal type III dens fracture. (A) Lateral radiograph shows a focal cortical break in one of the C2 rings (arrow). Note the thickened, abnormally convex prevertebral soft tissues. (B) Axial CT image at the base of the dens shows a minute fracture (arrows) along the anterior aspect of the C2 body, immediately lateral to the dens. This case illustrates the sensitivity of the C2 ring sign for the detection of fractures through this region.
    Figure 10A.
  • Figure 10B.
    Figure 10B.
  • Figure 11. A 38-year-old man with a type III dens fracture. Lateral radiograph shows a focal cortical break in one of the C2 rings anteriorly (white arrow) similar to Figure 10. Displaced fracture at the base of the dens posteriorly (black arrow) makes radiographic detection easy. Prevertebral soft tissues are only minimally thickened with a nearly normal contour.
    Figure 11.
  • Figure 12A. A 76-year-old woman with a displaced type III dens fracture. (A) Initial lateral radiograph shows fractures through the C2 body involving the anterior and posterior portions of the C2 ring (arrows). Note marked prevertebral soft-tissue thickening with abnormal convex contour. The white line illustrates normal spinolaminar alignment from C1–C4. (B) Lateral radiograph obtained later the same day shows several millimeters of anterior displacement of the dens and illustrates fracture instability. Note the marked anterior motion of C1 compared with spinolaminar line. The skull, C1, and the dens have moved anteriorly as a unit. (C) through (E) A series of 3 contiguous 2.5-mm axial CT images shows the fracture involving the vertebral body caudal to the base of the dens. (F) Right parasagittal reformatted image shows breaks in the narrow portion of the C2 body, correlating with a broken C2 ring (arrows). (G) Midsagittal reformatted image illustrates anterior displacement and anterior impaction. (H) Left parasagittal reformatted image also shows breaks in the C2 ring (arrows).
    Figure 12A.
  • Figure 12B.
    Figure 12B.
  • Figure 12C.
    Figure 12C.
  • Figure 12D.
    Figure 12D.
  • Figure 12E.
    Figure 12E.
  • Figure 12F.
    Figure 12F.
  • Figure 12G.
    Figure 12G.
  • Figure 12H.
    Figure 12H.
  • Figure 13A. A 23-year-old woman with a displaced type III dens fracture involving articular facets. (A) Sagittal RaySum 3D reformatted image simulates a lateral radiograph (GE Medical Systems, Waukesha, WI). Note the anterior displacement of the dens with anterior and posterior breaks in the C2 ring. (B) Coronal RaySum 3D reformatted image simulates an open-mouth odontoid radiograph. The fracture line extends across the C2 body involving both superior articular facets (arrows). (C) through (E) A series of 3 contiguous 2.5-mm axial CT images shows a 2-mm wide fracture line across the body of C2 immediately caudal to the base of the dens. The fracture involves constriction in the body of C2 immediately lateral to the dens, correlating with disruption of both C2 rings (arrows). (F) Right parasagittal reformatted image shows anterior and posterior disruption in a C2 ring (arrows). (G) Midsagittal reformatted image shows a slightly distracted and anteriorly displaced fracture. (H) through (J) A series of 3 coronal reformatted images from anterior to posterior through the dens shows a fracture line extending across the medial aspects of both C2 superior articular facets. These coronal images graphically illustrate the difference between type II and type III fractures—type III fractures involve a substantial portion of the C2 vertebral body caudal to the dens.
    Figure 13A.
  • Figure 13B.
    Figure 13B.