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3 Special Radiology
by the atlas (88%) but the odontoid tip in only 34%
of patients in a study of 131 rheumatoid patients [41].
The authors went on to determine the sensitivity and
specificity of the above-mentioned criteria in diagnosing basilar invagination and found that individually,
none of them reached sensitivity or negative predictive value (NPV) greater than 90%, not even those that
did not require odontoid visualization (i.e., RedlundJohnell and Ranawat). However, when combining
Clark station, the Redlund-Johnell, and the Ranawat
criteria to plain film assessment, the combined sensitivity and negative predictive value reached 94% and
91%, respectively. Practically, they recommended further imaging if basilar invagination was suspected by
the combined criteria in rheumatoid patients. Using
the combined criteria, 55% patients could be spared
of unnecessary MRI evaluation in their opinion. The
Klaus height index measures the perpendicular height
from the tip of the odontoid process to the line connecting tuberculum sellae and the internal occipital protuberance [24]. This measurement can be more accurate
if obtained from an MRI as the tentorium is clearly visible. If the posterior fossa height is less than 30 mm, it
suggests platybasia and basilar invagination.
All criteria/lines mentioned above utilized some
sort of relationship between the skull and the cervical
spine. However, Clark station of the atlas uses the vertical atlantoaxial relationship to predict presence of basilar invagination [7]. It divides the axis into three equal
parts on sagittal film, and if the anterior ring of atlas
crosses the middle or caudal thirds, basilar invagination exists. In fact, this method had 83% sensitivity and
85% NPV in diagnosing the condition in the abovementioned study by Riew et al. The vertical atlantoaxial index also describes the relationship between the
first two cervical vertebrae and was actually developed
with the understanding that a subgroup of patients with
basilar invagination may, indeed, have a problem of the
atlantoaxial joints rather than atlanto-occipital ones
[26]. Their subgroup of patients was treated with a posterior atlantoaxial joint release, distraction, graft, or
cage placement and fixation and thus avoiding transoral surgery. In anteroposterior view, the atlantooccipital joint axis angle formed by lines parallel to
those joints is useful in detecting occipital condyle
hypoplasia, with normal values ranging 124°–127° and
increasing to even over 180° with the condition [45].
The majority of the above-mentioned parameters
apply to non-traumatic situations and mostly to
rheumatoid patients with CVJ involvement. The
pathophysiology of the process results in cranial settling/basilar invagination. However, in the setting of
trauma, highly sensitive and specific criteria are essential to detect atlanto-occipital and atlantoaxial dislocations and instabilities that are commonly missed. These
injuries result in distraction, translation, or rotation
between the individual components of the CVJ, making the above criteria obsolete for their use in most
traumatic scenarios.
However, multiple indices and ratios have been
described to alert physicians to usually subtle and often
missed signs of injury at the CVJ. Starting with survivors of atlanto-occipital dissociations, the most commonly used craniometric parameters include Harris’
measurements, Powers ratio, Lee’s lines, and occipital
condyle displacement.
Harris et al. derived the so called “Rule of Twelve” in
a normal adult spine (Fig. 3.7). He used the basion-dental
interval (BDI) previously described by Wholey et al.
[57] and basion-axial (BAI) interval (distance between
basion and posterior C2 tangent) to describe normal values in adults and children [17]. It was found, that in
adults, the CVJ is intact if BDI and BAI are less than
12 mm. The BDI cannot be applied to children in whom
the dens ossification has not been completed. Harris
et al. then applied those rules retrospectively to analyze
plain radiographs of 37 patients previously diagnosed
with AOD on the basis of Lee X-lines, Powers ratio, and
BDI/BAI [18]. In all 23 patients with frank AOD, the
BDI and BAI were both greater than 12 mm. Neither
Powers ratio nor Lee X-lines could be applied to 46% of
patients due to anatomical variants. In the remaining 20
patients, Powers ratio detected the correct type of atlantooccipital subluxation in only 60% and Lee X-lines in
only 20% of cases. The types of atlanto-occipital subluxations identified were purely anterior, purely distracted, combined anterior and distracted, and purely
posterior. The in specificity inferior Powers ratio is calculated as a distance between basion and posterior C1
arch divided by the distance between anterior C1 arch
and opisthion [35]. It is considered normal when the value
is less than 1 (Fig. 3.7). However, it will remain normal
in purely vertical distraction and posterior types of AOD,
thus resulting in delay in diagnosis. Lee X-lines [27]
were applied to 12 cases of AOD as well as 100 normal
patients and compared with Powers ratio, Wholey’s
BDI, the Dublin method, and direct atlanto-occipital
joint measurement. The Lee X-line method was the
most accurate on plain radiograph assessment and correct in 75% of cases; the other methods were accurate
3.1 Radiographic Data Analysis
BAI – basion-posterior axial line interval
BDI – basion-dental interval
Posterior arch. of C1
Opisthion
Anterior arch. of C1
ADI – anterior atlanto-dental interval
PADI – posterior atlanto-dental interval
O
B
C
A
Basion
Fig. 3.7 Different measure-
ments of CVJ and UCS
alignment
29
in 50% or less. Ultimately, all cases were confirmed by
2 mm cut CT with reconstructions. The X-line method
is considered abnormal if the basion-axis spinolaminar
junction line does not intersect C2 and if the opisthionaxis line does not cross C1. According to Wholey et al.,
the direct measurement of the width of the atlantooccipital joints (condyle-C1 interval – CCI, atlantooccipital interval – AOI) should be less than 2 mm in
adults and 5 mm in children [57].
3.1.3 Atlanto-Axial Parameters
The atlanto-axial relationship can be described by means
of an anterior atlantodental interval (AADI, predental
space) on plain radiographs [20]. Closely related is the
posterior atlantodental interval (PADI) (Fig. 3.7). Both
have been defined validated on dynamic plain films of
patients with rheumatoid arthritis, developmental anomalies, and/or chronic posttraumatic instabilities but traumatic atlantoaxial instability is relatively uncommon
and thus no such validation exists in the setting of acute
trauma [2]. The AADI should be less than 3 mm in
adults and less than 5 mm in children on plain dynamic
films. Posterior ADI of 14 mm or less in combination
with abnormal AADI warrants an MRI evaluation, even
in the absence of neurological signs and symptoms.
The craniocervical relationships were thus restudied
in the era of multi-slice CT and the previously mentioned
plain radiograph parameters challenged. Rojas et al. studied 200 patients with a multidetector row CT (MDCT)
and re-examined Harris’ measurements, Powers ratio, as
well as atlantodental and atlanto-occipital intervals [42].
They found significant variances from the previously
accepted normal values. Firstly, they found no differences
when comparing men and women. Secondly, the Rule of
Twelve described above was valid but on CT those values
were much tighter for BDI (<8.5 mm) but unreliable for
BAI. Powers ratio was less than 0.9 when measured on
MDCT, thus not significantly different. Pang et al. found
the normal CCI to be less than 2 mm in 89 children [34]
and recommended surgical treatment in patients with
greater than 4 mm widening of CCI. The differences in
craniometric values on CT and plain radiographs were
probably a result of magnification, landmark visualization difficulties, and patient positioning. Similarly, when
craniometric parameters for basilar invagination were
validated on MRI, there were significant differences
when compared to plain radiographic criteria [48].
Historically, one of the most commonly used indicators of atlantoaxial instability secondary to transverse
ligament (TAL) injury was the rule of Spence [46], which
states that if the combined overhang of the C1 lateral
masses over C2 adds up to 7 mm or more, the TAL is
likely disrupted (Fig. 2, Chap. 10). One has to be aware
of the fact that the original study was done on cadaveric
specimens and therefore, the number is actually greater
than 8.1 mm when assessed on plain films [19]. Spence’s
actual number of 6.9 mm can be resurrected in the era
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3 Special Radiology
of CT with direct measurement of the displacement on
coronal reconstructions, although a direct assessment of
TAL integrity can always be confirmed on MRI [9].
3.2 Dynamic Imaging
Several mobile CVJ and UCS abnormalities require
testing under loading, i.e., in dynamic conditions of
flexion and extension, or rotation. These include, for
example, reducible cranial settling, AA instability, os
odontoideum, or fracture malunions. Just as for static
films, several radiographic indices have been developed
to describe dynamic relationships of structures of the
CVJ. Abe et al. proposed an atlantoaxial instability
index, defined as a change of rate of space available for
the spinal cord between flexion and extension [1]. They
recommended surgical intervention for all patients
with neurological deficit, or instability index greater
than 20% or maximum space for cord of less than
14 mm. This is useful for sagittal instability but, for
example, in patients with os odontoideum, the instability is multidirectional. Therefore, Watanabe attempted
to define a sagittal plane rotation angle as well as the
above-mentioned indices in 37 patients with AAI on
dynamic tomograms and concluded that not only the
instability index of more than 40% but also rotation
angle of more than 20° is predictive of cord signs and
symptoms [56]. The extent of instability as measure by
any dynamic means in an awake patient can be an
underestimate of that seen during anesthesia.
The previously mentioned vertical atlantoaxial
instability index (VAAII) as described by Kulkarni
and Goel is a good indicator of vertical instability.
Goel et al. successfully used the VAAII to predict the
feasibility of a pure vertical instability reduction and
fixation without transoral decompression in cases of
vertically mobile instability [14].
Although the majority of atlantoaxial pathologies
mentioned so far affect the complex in vertical or anteroposterior or coronal planes, the primary movement
within the AA complex is that of rotation. Atlantoaxial
rotatory fixation (AARF) represents a pathological phenomenon that gained a standardized definition based on
dynamic imaging of the CVJ only in recent years. Pang
et al. were exemplary in defining the norm of atlantoaxial
rotation in normal subjects using dynamic CT scans in
rotation [34]. They defined the dynamic relationship of
C1 on C2 throughout the turn and found very little variance from the mean. When C1 rotates on C2, it crosses it
at 0°, moves alone until 23° then moves faster than C2 up
to 65° and then they move in unison beyond that angle
with a maximal separation angle of 43°. They used a
three-position CT to generate a physiological motion
curve (Fig. 3.8) according to which they classified AARF
into three subtypes based on a diagnostic paradigm [33].
Type I AARF represented a fixed deformity whereas
Fig. 3.8 Atlantoaxial rotatory fixation (AARF) is defined on the basis of a normal three position CT evaluation that generates a
physiological curve of the atlantoaxial relationship change in rotation
3.3 Vascular Imaging
Fig. 3.9 Dynamic MRI. Neural compression that would otherwise not be obvious can be visualized on MRI in flexion and
extension (a–d)
31
Type III was the least severe type with C1 crossing C2
axis with rotation to contralateral side.
More recently, MRI evaluation of the CVJ was done
in flexion and extension to further visualize soft tissue
compression or changes in instabilities of the region.
Epstein et al. was the first to use the term “dynamic”
MRI [12] when she documented cord compression in
flexion/extension MRI although the dynamic plain
films failed to show osseous instability (Fig. 3.9).
Gupta et al. studied 25 patients with suspected CVJ
abnormalities due to various reasons with MRI and CT
performed in neutral, flexion, and extension positions.
They found that dynamic MRI detected every cord
compression under loading that was not apparent in
neutral position [15]. Other authors have also utilized
dynamic MRI to assess patients with RA and suspected
atlantoaxial instability [22] and correlated it to
findings on plain radiographs. Krodel et al. even argued
that dynamic MRI plays a role in surgical planning
when treating patients with rheumatoid-related upper
cervical instabilities [25]. Three out of his 11 patients
did not demonstrate reduction of cord compression on
dynamic MRI and thus required removal of the synovial proliferative tissue around the dens.
3.3 Vascular Imaging
Conventional cerebral angiography, CTA, and MRA
have been used to assess vasculature at the CVJ. CTA
and MRA, in particular, have revolutionized preoperative evaluation of the vascular tree at the CVJ and obviated the need for invasive angiography (Fig. 3.10). With
Fig. 3.10 Vascular evaluation of CVJ vessels. (a) CTA is particularly useful in delineating the relationship between VA and bone.
(b) MRA
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3 Special Radiology
respect to surgical reconstruction techniques, one is
particularly interested in anatomical variants of the vertebral arteries and their relationship to bony anatomy.
The details of VA anatomy, its variants and relationships are discussed in detail in Chap. 1. Nonetheless,
we would like to mention dynamic angiography that
is performed in rare cases of dynamic vertebral artery
compression at the CVJ due to bony spurs, AAI or cranial settling. Janeway et al. described positional VA
occlusion with head turning in a patient with basilar
impression diagnosed on dynamic angiography already
in 1966 [21]. Our case (Fig. 3.11) demonstrates an
example of V3 segment injury due to bone spur compression of the VA in rotation in a 16-year-old boy with
recurrent posterior circulation strokes. Dynamic angiogram demonstrated vessel occlusion with head rotation.
He was successfully treated by C1 partial laminectomy.
Similar cases are described in the literature such as that
Fig. 3.11 Sixteen year old boy with recurrent posterior fossa
strokes as seen on MRI (a). Complete VA occlusion on left
head turn (b). CTA demonstrating posterior C1 arch bone spur
(c). Dynamic intraoperative angiogram with complete release of
compression in rotation (d) (courtesy of T. Abruzzo MD, Mayfield
Clinic, Cincinnati)
3.4 Our Preference
33
of a 34-year-old man with recurrent embolic strokes
and vertebrobasilar insufficiency secondary to dynamic
VA compression during head rotation [49]. He was
also successfully treated by osseous decompression.
Dynamic angiogram is a rare investigation required for
only specific problems at the CVJ; however, it may be
essential to diagnosis and management in those rare
cases. Dumas et al. further assessed the use of dynamic
MRA and CTA to evaluate VA compression during
head rotation in healthy volunteers as well as an actual
symptomatic patient [11]. However, these non-invasive,
dynamic vascular imaging techniques have not been
validated further.
3.4 Our Preference
Radiographic evaluation of disorders of CVJ is not
complex but requires knowledge of patient’s clinical
details to guide an appropriate selection of modalities
for a particular clinical scenario. Rather than using all
imaging modalities for everyone, we like to divide the
patients in traumatic, degenerative/inflammatory, and
neoplastic scenarios.
3.4.1 Developmental/Degenerative/
Inflammatory
The majority of rheumatoid patients belong to this
group. Most of them are referred based on clinical history of the disease with associated neck pain and/or
neurological symptoms and have had plain radiographs
obtained prior to the referral. We proceed with obtaining standard neutral, non-contrast MRI images, particularly in patients with neurological compromise. If bony
relationships remain questionable on plain radiographs,
we obtain a thin cut CT with sagittal, coronal, and threedimensional images. This is mandated prior to any surgical intervention at the CVJ. These images are also
supplemented by dynamic flexion-extension lateral
plain radiographs. The CT images as well as plain
radiographs allow for direct electronic measurement of
the earlier mentioned craniometric parameters for disease severity assessment as well as surgical planning.
It is clear that no single parameter should be used in
isolation for evaluation of the alignment of CVJ structures
due to individual anatomical variations and differences
in clinical situations. A combination of multiple parameters relevant to the clinical scenario should always be
applied which can lead to further, more detailed evaluation if abnormal. In non-traumatic cases with disease
processes that result in cranial settling, basilar impression/invagination, or platybasia, we find the following
parameters the most useful: Wackenheim basal clival
line, cervicomedullary and basal angles, McRae and
Chamberlain lines, and the BDI. We select a combination of multiple parameters in questionable cases and
sometimes we are forced to use a particular parameter
due to limited visualization of structures at the CVJ. For
all of them, mid-sagittal CT reconstructions have certainly made the process significantly easier.
3.4.2 Traumatic Cases
For patients with traumatic situations, basilar impression parameters are not very useful as our biggest concerns are: (1) distraction injuries of the occipito-cervical
junction, (2) atlantoaxial instability secondary to bony
or ligamentous injuries, (3) coronal alignment in injuries of atlas, and (4) sagittal alignment of axis/dens. At
our institution, all victims of traumatic injuries are transported with spinal precautions and with a hard collar,
until radiographically and clinically cleared. In majority
of emergently admitted trauma victims with suspicion
of spinal injury (particularly if polytraumatized), we
perform directly helical CT with appropriate reconstructions. Those referred from other hospitals or patients
with isolated UCS trauma without neurological deficit
are standardly assessed by plain radiography first.
An MRI evaluation is often indicated in the acute setting unless partial spinal cord injury exists or an injury
remains questionable based on mechanism of injury,
clinical syndrome, and CT evaluation (Fig. 3.12).
3.4.3 Neoplastic Conditions
In patients with suspected neoplastic disease of the CVJ,
a contrast enhanced MRI is added to the above modalities. As tumors (metastatic or primary) commonly result
in bony erosion and instability, relevant indices described
above still apply and thus require a careful evaluation
of bony structures by CT, stability by dynamic films,
and neural compression by MRI. Occasionally, with
34
3 Special Radiology
Fig. 3.12 MRI depicting posttraumatic extra medullary
intraspinal hematomas. (a, b) clinically silent posttraumatic
subdural hematoma of CVJ. (c, d) initially clinically silent but
invasive tumors, vascular invasion/occlusion needs to
be evaluated by CTA/MRA or DSA.
In conclusion, in the modern era of continuous
improvement of imaging modalities, many craniometric parameters have become obsolete. However, their
knowledge is important for the CVJ surgeon as they
define “normal” relationships of the complex joints of
the CVJ and UCS.
later decompensated epidural midcervical posttraumatic hematoma in another patient
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Section
Principles of Reconstruction Techniques
II