is an Associate Professor and Chief of, and
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.
. 2002;31(2):23-32. It is also available online at
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.
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
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.
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).
The presence of prevertebral soft-tissue swelling may prompt CT
scanning of the upper cervical spine, allowing detection of
otherwise radiographically occult fractures.
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
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
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.
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
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
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.
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