This article is the first in a series of pictorial essay illustrating the radiography and CT of spine injuries. Each article will focus on a small group of such injuries. This installment demonstrates injuries to the upper cervical spine, including the skull base, the atlas vertebra, and the associated joints. The unique anatomy of this region requires careful inspection, since several injuries are suitable on radiographs or CT.
Dr. West is an Assistant Professor and the Chief of
Emergency and Trauma Radiology, Department of Radiology, The
University of Texas Health Science Center at Houston, TX.
Pictorial essays often have strict limits on the number of cases
and the number of pictures in each case. Subtleties of diagnosis
may be lost if insufficient axial computed tomography (CT) images
are presented because of editorial limits. Variations from
patient-to-patient in the appearance of injuries may be lost due to
fiscal necessities of the journal. In failing to present cases in
sufficient number or in sufficient detail, the author may create
the false impression that all cases are "classic" or "typical."
This article and the series that will follow in subsequent issues
will not suffer from a lack of sufficient illustration. The theme
of these articles will be "Few words, many pictures."
This series will emphasize the radiography and CT of spine
injuries with the depth and breadth rarely possible in print media.
To achieve this goal, each article will focus on a small group of
spine injuries. The series will be organized using a cranial-caudal
approach, beginning with the upper cervical spine. Where
appropriate, injuries in a given anatomic region will be
subcategorized into pathomechanical families.
The upper cervical spine includes the skull base (C0), the atlas
vertebra (C1), the axis vertebra (C2), and the associated joints.
In contrast to the remainder of the spine, the upper cervical
region does not have a repetitive, segmental pattern. Tracing a
series of lines through the upper cervical region is of little
value. Instead, the radiologist must look for each of the major
injuries and exclude their presence. The unique anatomy of this
region requires careful inspection, since several injuries are
subtle on radiographs or CT.
The terminology and classification scheme used in this article
is based on that advocated by the Cervical Spine Research Society.
Occipital condyle fractures are not frequently visualized on
radiographs, but are readily visible on CT of the head or upper
cervical spine performed for suspected head or spine injury. The
type I fracture is a comminuted impaction fracture of the occipital
condyle that results from a direct blow to the head and is a stable
injury (figure 1).
The type II fracture represents extension of an occipital bone
fracture into the occipital condyle. Like type I, the type II
fracture is the result of a blow to the head and is stable unless
the entire condyle is separated from the skull base (figure 2). The
type III fracture is a wedge-shaped avulsion fracture of the
transverse alar ligaments on the medial aspect of the occipital
condyles and is frequently associated with occipitocervical
instability (figure 3). Thin-section (1 to 1.5 mm) CT with sagittal
and coronal reformatted images is required to assess for
Occipitocervical subluxation or dislocation is occasionally
survivable, particularly in children.
On radiographs, occipitocervical malalignment may be recognized by
identification of uncovered occipital condyles (figure 4).
Alignment may be assessed qualitatively by comparing the position
of the anterior margin of the foramen magnum (the basion) and the
posterior margin of the foramen magnum (the opisthion) to the
subjacent cervical vertebrae. In a normal individual, the basion
should be positioned over the dens and the opisthion should fall
along the curved spinolaminar line drawn upward along the
spinolaminar junctions of the upper cervical vertebrae.
Quantitatively, a diagonal line drawn from the basion to the tip of
the dens, the basion-dens interval (BDI) should not exceed 12 mm.
On thin-section CT, the occipital condyles should reside within the
concavities (fossae) of the C2 lateral masses. The joint space
should be no more than 2 mm wide and the articular surfaces should
The relationship between the basion and the dens is well seen on
sagittal reformatted views; again, the basion-dens interval should
not exceed 12 mm.
In occipitocervical subluxation or dislocation, the
occipito-atlantal are usually both anteriorly translated and
distracted. The injury is usually readily apparent on radiographs.
Less readily apparent cases have less displacement, either in
anterior translation (figure 5) or distraction (figure 6).
The C1 posterior arch fracture is typically bilateral and
results from hyperextension of the upper cervical spine.
Posterior arch fractures are usually visible on lateral radiographs
(figure 7). However, when the fracture is located far laterally at
the junction with the lateral mass, the overlying dens or lateral
mass often obscures the fracture line (refer to figure 11 for a
similar example). Computed tomography detection may be difficult if
the scanning plane passes obliquely through the ring of C1. Even if
the posterior arch is seen segmentally, fractures may be detected
by looking for sharp cortical margins. If
1- to 1.5-mm source axial images are available, axial reformatted
images may be made to correct for obliquity, so that the C1 ring
may be seen in its entirety on a few contiguous images. These "true
axial" reformatted images make evaluation of the C1 ring much
The isolated anterior C1 arch fracture is uncommon (figure 8).
It may be distinguished from the much more common congenital
midline cleft by recognition of sharp cortical margins indicating
The burst fracture of C1 (Jefferson) is the result of an axial
Two, 3 or 4 fractures are present in the C1 ring, allowing the
lateral masses to slide laterally if the fractures are displaced.
Radiographic signs include lateral subluxation of one or both of
the lateral masses on the open-mouth odontoid view (figure 9). As
in posterior arch of C1 fracture, a fracture line through the
posterior arch is usually, but not always, visible on the lateral
radiograph (figures 10, 11, and 12). Fractures through the anterior
and posterior arches of C1 are seen easily on axial CT images.
While the 4-part Jefferson bursting fracture with 2 fractures
through the anterior arch and 2 fractures through the posterior is
considered "classic" or "typical" (figure 10), axial CT frequently
reveals variations from the classic pattern (figures 11 and 12).
Lateral subluxation of the lateral masses of C1 is readily visible
on coronal reformatted images, but may be difficult to recognize on
axial images. Subluxation of the lateral masses by more than 7 mm
from their expected anatomic position implies disruption of the
transverse atlantal ligament.
Fracture of the C1 lateral mass is characterized by ipsilateral
fractures of the anterior and posterior C1 arch (figure 13). Less
commonly, the articular surface of the lateral mass is fractured
Most of the CT images presented in this article were obtained on
a General Electric LightSpeed QX/i CT scanner (GE Medical Systems,
Milwaukee, WI) that has a 4-channel detector capable of making 4
simultaneous source axial images 1.25 to 5 mm thick. At The
University of Texas, primary diagnostic images are scanned in the 4
* 1.25 mm mode, and 2.5 mm thick axial images using the bone
algorithm are created. Images made in the HS mode (beam pitch 1.5)
are of excellent quality, but many of the images presented in this
article were made using the HQ mode (beam pitch 0.75). Sagittal and
coronal reformatted images have been made using various techniques,
with modifications made based on additional experience with
multislice scanning. Our department's current technique, which
produces the best reformatted images, involves creating a set of
1.25-mm axial images spaced every 1.0 mm using the standard
algorithm--the secondary raw data--which is made from the same raw
data that is used to make our 2.5-mm primary diagnostic images. The
secondary raw data is reformatted into 0.3- to 0.6-mm sagittal and
coronal sections spaced every 2 to 3 mm. A less resource-intensive
technique that also produces excellent reformatted images is to use
1.25-mm images spaced every 1.25 mm made with the bone algorithm.
These are reformatted into 2-mm sagittal and coronal images spaced
every 2 to 3 mm. The thicker reformatted images reduce image noise,
which is a problem when bone algorithm is used. The least
aesthetically pleasing images in this article were made using an
older technique wherein the 2.5-mm primary diagnostic images were
reformatted into 0.3- to 0.6-mm sagittal and coronal sections
spaced every 3 to 4 mm. Images made with this older technique have
less detail, more image noise, and suffer from noticeable stairstep
artifact. Nevertheless, the images are adequate for diagnosis and
are of sufficient educational value to merit their inclusion in
this pictorial essay.
After studying these cases of C0C1 upper cervical spine injury,
there are several important facts to remember: 1) fractures of the
C0C1 region are often, but not always, detectable
radiographically; 2) prevertebral soft-tissue swelling is helpful
when present, but the absence of swelling does not exclude an upper
cervical spine injury; 3) because radiographs are not completely
sensitive for detection of potential unstable injuries in the C0C1
region, screening high-risk trauma patients for injury using CT is
a logical approach
; and 4) because C0C1 injuries may be difficult to detect or
evaluate completely using axial CT alone, high-quality radiographs
or high-quality sagittal and coronal reformatted images should be
part of the routine evaluation of the C0C1 region.