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6 Specific Reconstruction Techniques of Upper Cervical Spine and Craniovertebral Junction
Fig. 6.2 Extreme midline thickness of occipital bone in another
female patient. (a) Midsagittal CT reconstruction depicting
more than 20 mm of available bone. (b) Perfect monocortical
desire to fortify a multipoint construct involving the
condyle screws and/or failed previous occipitocervi-
screw anchorage in the same patient (“tooth-brush appearance”).
(c) Axial CT reconstruction in the same patient
anatomical works show the safety of using 3.5 mm
screws for transcondylar purchase [133, 229, 245].
cal fusion. Currently, the posterior approach using
either transcondylar or C1-0 transarticular screws is
most frequently discussed [81, 228, 229, 245].
Nevertheless, anterior transcondylar screw purchase
6.1.2.1 Posterior Transarticular
Atlantocondylar Screw (Fig. 6.3)
has also been described [52, 53]. As was depicted in
the anatomy chapter, the occipital condyle is normally twice as long as it is wide and is a medially
oriented structure. For practical purposes we can calculate the length as approximately 25 mm, the width
around 10 mm, and the height as 10 mm. Because of
its variability only CT scan can accurately show its
shape, orientation, mass, and relationship to neighboring structures [165]. Since it is a part of occipital
bone, cranial image guidance can be used not only to
model the ideal screw trajectory, but also to directly
guide the instruments. Most of the published
Dieter Grob from Zurich was the first who published
the posterior transarticular C1-0 screw fixation combined with a Y-shaped C2-occipital plate in a patient
with a failed previous wire fixation of AOD [81].
Gonzalez et al. [75] studied the feasibility of C1-0
transarticular screws on cadaveric model. They found
that for atlanto-occipital screw fixation the same stiffness as occipitocervical constructs in all directions,
with the exception of flexion-extension. Their recommendation was to supplement this technique with a
posterior buttress. Similarly to Magerl’s technique it
6.1 Occipital Bone as Anchoring Structure
Fig. 6.3 Schematic drawing of posterior atlantocondylar screw
fixation
can be strengthened by a Gallie type of graft [75]. The
same group of authors later published a report of a
patient with AOD treated using this method [60]
and also in two cases of posttraumatic instability where
they performed a combined double level transarticular
procedure (C1-0 and C2-1), simultaneously [76]. Yan
et al. in their works first defined the ideal entry point
for atlantocondylar fixation and then analyzed 20 dry
specimens and CT reconstructions of 30 healthy volunteers. They also conducted a simulation surgery on
another 12 fresh cadavers to establish the safety angle
ranges and length of screws [245, 246]. As a safe angle
of introduction, they established 53.3° (SD = 3.4°) in
sagittal plane and the medial inclination 20° (SD=2.6°)
in the axial plane. The appropriate length of transarticular C1-0 screw was between 24 and 34 mm.
However, always, one has to consider the individual
patient specific anatomy as well as the possible discrepancy in such values obtained in, generally, smaller
South-Eastern Chinese population.
69
anatomical works by Uribe et al. [229] and LaMarca
et al. [133].
The position of vertebral artery (VA) in the C1 arch
posterior groove does not usually affect the condyle
screw purchase as it is most frequently located more
caudally [133]. From an anatomical point of view, the
natural borders for screw placement are as follows: rostrally, the hypoglossal canal; rostrolaterally, the emissary vein; caudally, the occipitoatlantal joint; and
medially, the foramen magnum. The condylar emissary
vein can be of importance as a major drainage vein in
cases of jugular bulb occlusion (tumors) or in congenital anomalies but it can be sacrificed under normal conditions [13, 28]. LaMarca et al. [133] described the
ideal transcondylar screw trajectory after analyzing
thin sliced CT in a 3D navigational station. They found
that it was feasible to achieve safe screw purchase in all
the 12 cadaveric condyles studied with a safety rim of
bone surrounding the screw larger than 1.5 mm in all
cases. The average SAS on the posterior condylar wall
was 5–8 mm rostrocaudally and 5–9 mm mediolaterally. Uribe et al. [229] studying the feasibility of
transcondylar screw purchase on six silicone injected
cadaver heads determined the condylar entry point
(CEP) to be 4–5 mm laterally from posteromedial edge
of the condyle, anatomically defined as approximately
1–2 mm above the joint fissure. The base of condyle
(connection to occipital bone) was used. The pilot hole
was made by an awl and the trajectory of drill was 5°
6.1.2.2 Posterior Transcondylar Screw (Fig. 6.4)
The idea to use the transcondylar screw purchase as a
new point of fixation was described independently in
Fig. 6.4 Schematic drawing of posterior transcondylar screw
purchase
70
6 Specific Reconstruction Techniques of Upper Cervical Spine and Craniovertebral Junction
upward and tilted 17° (12°–22°), medially. They used
30–32 mm long screws where the unthreaded shaft
11–13 mm long was protruding above the bone to
enable the polyaxial screw head movement. They
attempted bicortical screw purchase and combined
anatomical guidance with fluoroscopy. All 12 operated
condyles were assessed by CT afterwards. Not one
screw violated the hypoglossal foramen or other important structures. The screw length inside the bone was,
on an average, 22 mm (20–24 mm) and the C1 arch
overhang (smooth shaft) 12 mm (11–13 mm).
This suggested technique of transcondylar screw
placement as a part of longer caudal construct was
later successfully used in patients with odontoid type
II fracture pseudoarthrosis and cranial settling [228].
Despite the fact that bicortical screw placement is
stronger than unicortical, we have to be aware of the
potential injury of structures located anteriorly to the
condyles. Most commonly, it is the pharyngeal wall but
variant carotid arteries can be anterior as well. There are
some advantages of posterior condyle screw purchase.
Using a polyaxial screw, the contoured rod connecting
the occipital plate is not necessary and thus the risk of
eventual stress rod fracture can be avoided. Also, the
construct connecting the occipital bone with the UCS or
subaxial spine is of low profile, and therefore, the muscular damage necessary for occipital plate placement
can be decreased as well as the risk of plate erosion.
6.1.2.3 Anterior Transarticular
Axial-Atlantocondylar
Screw (Fig. 6.5)
The original idea to fix the occipital condyle from an
anterior approach similar to the technique of anterior
atlantoaxial fixation came from Dvorak et al. [52, 53].
They suggested this method as a salvage procedure for
those unique situations where posterior fixation is not
possible, failed, or has to be fortified. Also, in some
very rare situations such as after total tumor removal or
in complex reconstruction due to congenital anomalies, these ideas can be utilized.
The first part of their work was an anatomical study
documented by a successfully treated patient with failed
posterior wire and graft fusion after repetitive trauma.
The second part was a biomechanical comparison of
anterior fixation with posterior methods. They confirmed the superior strength of posterior transarticular
Fig. 6.5 Artistic drawing of anterior C2-C1- occipital condyle
screw fixation
screw connected to suboccipital plate for all directions;
however, they found comparable stability of their anterior fixation in rotation and lateral bending. Both screw
methods were much more stable than posterior graft
and wiring alone. They have suggested approaching the
anterior surface of C2 the same way as for an odontoid
screw (high oblique anterolateral approach), to identify
the groove below the middle third of atlantoaxial joint
and introduce the K-wires tilted 25° posteriorly and 15°
laterally under biplanar fluoroscopical control. Then
cannulated self-tapping screws 24–30 mm long were
introduced along the wire. From the anatomical and CT
studies, they concluded that the angle of introduction
can vary substantially (posterior angle 15°–36°, lateral
tilt 10°–20°). As a major limitation, they cite the course
and volume of hypoglossal canal and the impossibility
to add graft material or to abrade the joint surface to
enable long-term bony fusion.
6.1.2.4 Our Preference
The occipital condyle has been confirmed as a solid
structure for screw anchorage. However, it has also been
found that the variability of VA course and location of
n.XII canal within the condyle can exclude the possibility of safe screw placement in as many as 17% of the
specimens studied [52, 53]. Also, in basilar impression
6.2 Atlas as an Anchoring Structure
ab
71
and other compound CVJ congenital anomalies, it could
be unsafe or even impossible to expose the occipital
condyle. Although case reports describing the successful use of either posterior transcondylar [228] and posterior atlanto-occipital transarticular screw fixation [76,
81] or anterior transarticular C1-0 screw placement [52]
were reported, no large series of patients treated with
condylar screw placement has been published. In our
opinion, the use of the condyle as a part of construct can
be more reasonable than transfixation of occipitoatlantal joint without support of bone grafting. Given the
technical and anatomical difficulties, although these
methods are feasible, they will likely have a limited role
mainly as salvage procedures.
6.1.3 Clivus
The upper part of the clivus belongs to sphenoid bone
whereas the lower part is a basilar portion of the occipital bone. These two parts are separated by spheno-occipital synchondrosis till the age 16.5 (13–18) in males and
14.4 (12–15) in females. This represents the growing
potential in correct formation of skull. In normal adults,
the length of the whole clivus is 4.5 cm (3.7–5.2) in the
sagittal plane and the basilar portion of occipital bone is
3.1 cm (SD = 0.3). In occipital hypoplasia, the basilar
portion may be only 1.7 cm long [134]. The thickest
portion is anterior and superior and contains the cancellous bone. The thinner part is formed only by compact
bone in the region of foramen magnum. Usually, the
outer cortex is more solid and thicker than the inner one.
For practical purposes, we can calculate that the
wedge-shaped clivus is only 4 mm thick and safely
reachable 10 mm on both sides from midline at the
level of FM. It gradually increasing in thickness to a
maximum of 22 mm at the level of pituitary fossa with
a safe strip of bone 10 mm from midline along its
entire course. The caudal half of the clivus can be
safely resected if necessary or can serve as a screw
anchorage or cage support (Fig. 6.6).
6.2 Atlas as an Anchoring Structure
C1 has no vertebral body, and therefore, its bone stock
has a limited volume for any screw anchorage; nevertheless, the lateral masses are frequently used for screw
Fig. 6.6 Two screws 12 mm long introduced in to the anterior lover clivus under image guidance. (a) Midsagittal CT reconstruction
showing appropriate screw length. (b) Coronal plane CT reconstruction
72
6 Specific Reconstruction Techniques of Upper Cervical Spine and Craniovertebral Junction
purchase, either from anterior or posterior approach.
Additionally, C1 lateral mass and laminar screws are
described in the literature. To reach desired lateral mass
posterior and/or anterior screw entry points, the arch is
the anatomical guiding structure. When performing
posterior approach, the dissection of C1 arch should
stay strictly subperiostally starting from the clearly distinguishable posterior tubercle. Ebraheim recommended [57] not to extend this dissection more than
12 mm lateral from midline and be aware 8 mm from
midline on superior arch. However, it is our experience
that following the lower edge of arch where we do not
expect any important neurovascular structures, safe
microdissection is possible even to the transverse process. The inferior half of the C1 lateral masses can be
exposed subperiostally under the thinnest part of posterior arch with care taken not to injure the C2 root, ganglion, and surrounding venous plexuses. Preoperative
evaluation of individual VA course is critical. Careful
observation of axial CT scans can show different C1
anomalies leading us eventually to further diagnostic
modalities (CTA). The anterior approach to the atlas is
limited to transoral exposure and/or high anterolateral
access and again for any anterior approach the most
important guiding point is the anterior tubercle of atlas.
It is most often visible on lateral fluoroscopy, it is not
surrounded by any danger and clearly localizes the
midline, and thus distances to vertebral arteries and
spinal cord. We have to be aware of possible slight
rotation, nevertheless, the safety defined by anterior
tubercle is always valid.
6.2.1 Posterior Lateral Massa Screw
a pilot hole in the middle of mass-arch base and thus
avoid bleeding from the venous plexuses surrounding
the C2 root. A straight or slightly convergent anteroposterior drilling trajectory parallel to the plane of the
C1 posterior arch was advised. The screws were introduced bicortically. Goel described the entry point as
the middle of the available bone area but introduced
the screws monocortically, initially. In his later works,
he conceded using bicortical purchase and an entry
point localized in the lower facet joint surface if not
enough bone is available in the posteroinferior C1 pillar, especially in children [70]. Other authors [148,
186] under influence of anatomical [225] and biome-
chanical works [148] suggested penetrating directly
through the surface of the posterior arch just above the
previously described entry point as another possibility
of how to avoid bleeding, not irritate the C2 root, and
prolong the screw pathway in order to increase the
pullout strength. Such a strong posterior lateral mass
screw anchor can also potentially limit previously necessary long occipitocervical constructs to occipitoatlantal or atlantocervical and thus avoid unnecessary
fusion of adjacent segments.
The originally suggested midline trajectory was
criticized by Blagg et al. [18] because of possible
injury of the VA in the transverse foramen. They recommend to start more medially, drill straight anteriorly
or in medial angle up to 20° and never tilt laterally during the procedure, but they concur with the original
concept of following the posterior arch attachment
angle in sagittal plane. Because of possible injury of
structures situated in front of atlas (n.XII and ICA)
another screw entry and trajectory was described by
Rocha et al. [191].
Fixation of the lateral mass of atlas with posteriorly
placed screws was first described by Atul Goel from
Mumbai, India in 1994 (first surgery 1988) as a part of
C1-2 plate construct where the inferior screws were
placed into the C2 pedicle [72]. This technique was
later popularized by Jürgen Harms from Karlsbad,
Germany who developed a more adaptable polyaxial
screw rod system allowing preservation of the C2
nerve root and enabling manipulation with atlas particularly in fractures and dislocations [94]. Both
authors used the middle posterior part of the C1 lateral
mass below the arch as the entry point. Harms recommended reaching this area subperiostally and to make
6.2.1.1 Anatomical Background
The space available for 3.5–4.5 mm lateral mass screw
(SAS) introduction had been repeatedly studied on anatomical specimen and CT images [30, 191, 233, 241].
The posterior inferior pillar of the C1 lateral mass is bordered superiorly by the arch and inferiorly by the joint
as measured by Blagg et al. [18] on 50 CT scans. They
established the height of available space to be 4.6 mm
(0–7 mm) and the width to be 14.9 mm (11–18 mm).
Rocha et al. measured the same parameters on 20 dried
atlantal specimens. They gauged electronically the mean
working space height as 4.5 mm (range 4.3–6.1 mm)
6.2 Atlas as an Anchoring Structure
73
and the mean width as 9.6 mm (range 7.7–12.8 mm). To
reach the aforementioned height they had to cut out the
posterior arch lip in 50% of the studied specimens and
they concluded that 93% of studied vertebras are able to
accept screws with 4.5 mm diameter [191]. Wang analyzed 74 dried cadaveric spines and found a mean height
of 3.9 mm and a width of 7.3 mm of the space for possible lateral mass screw entry [233]. All specimens
could accommodate 3.5 mm screws and 97% could
accept 4mm diameter screws. In 65% of cases, it was
necessary to remove at least a part of the posterior arch
overhang to be able to insert a 4mm screw. The possible
introductory angle for safe bicortical purchase varied
according to the position of entry point from 13° laterally to 45° medially. Cranial angulation without violation of C0/1 joint was acceptable until 19°.
Because this technique can be very advantageous in
fixation of the pediatric spine, Chamoun et al. [29] has
performed a CT morphometric analysis of 76 atlases in
children between 1.5–16 years old (mean 7.8 years)
and found that only in one case of a 19-month-old
infant one of the lateral masses was not of sufficient
size to accept 3.5 mm screw.
When performing the posterior approach to the
atlas surgeon can find different anomalies of the VA
course. Up to 15.6% of patients have a partial or total
covering (arcuate foramen) of the VA artery in the
groove of C1 by the “ponticulus posticus”. (Fig. 1.3,
Chap. 1) [31, 104]. Lee et al. in their anatomical work
analyzing 709 C1 cadaveric vertebrae found the
appearance of ponticuli significantly more frequent in
males (15.9%) vs. females (8.1%) [138]. Erroneous
evaluation of a “too broad” a lamina can lead to VA
injury [249] during arch subperiostal preparation or
translaminar screw introduction. There are also anomalies of the course of the horizontal (V3) segment of
the VA in 5.4% of the normal healthy population [104]
and in 13% of the group of patients selected for CVJ
surgery [244]. The most important and dangerous variant is the persistent first intersegmental artery. This
aberrant vessel partially or totally substitutes the VA
and courses below the posterior atlantal arch and thus
prohibits the subarcuate approach to posterior lateral
mass screw entry area. In a very large series of 1,013
patients with CT vertebral angiography, Hong found
persistent first intersegmental artery on one side in
3.8% and bilaterally in 0.8% [104].
Structures adjacent to the anterior surface of lateral
mass represent another risk during bicortical penetration
of drill, tap, or screw. The lumen of the internal carotid
artery (ICA) is located medially to FT in more than
80% of cases. The mean distance of the ICA medial
border measured from the medial border of FT is
2.78 mm on the left side and 3 mm on the right [37].
The average distance of ICA from anterior lateral mass
aspect is less than 3 mm on both sides (left, 2.88 mm;
right, 2.89 mm). Rotation of head due to positioning of
the patient probably has no effect on any change of
ICA position [38]. This close relationship can potentially be dangerous during placement of lateral mass
screws, C1/2 transarticular, and/or C2 transpedicular
fixation if bicortical purchase is chosen. Such a carotid
artery impingement has been described after transarticular fusion [38]. The risk of ICA injury during drilling or tapping or lateral injury due to the screw contact
and gradual ICA wall erosion was considered by
Currier et al. as high, if the artery was more than 4 mm
medial to the medial edge of FT and less than 2 mm
from anterior bone surface. In moderate risk are those
with the artery less than 2 mm from FT and within
2–4 mm from anterior C1 wall. They found that there
was no risk when the ICA is laterally from transverse
foramen and more than 6 mm away from the anterior
mass aspect [37]. Currier, in his series of 50 atlases
analyzed by CT with contrast, found that 12% of
patients were at high risk and another 46% at moderate
risk of ICA impingement at least on one side.
Additionally, the position of the hypoglossal nerve
in front of lateral C1 mass is of importance because of
possible injury during bicortical screw purchase as
described by Hong [103] and Jeanneret [116]. This
could be a cause of some reports of swallowing difficulties after purely posterior procedures [86, 151]. The
CN XII exits the skull via hypoglossal foramen at the
base of occipital condyle with a diameter of 2–3 mm.
It lies 2–3 mm laterally from the middle of the mass
and courses vertically to the C1/2 joint [56].
6.2.1.2 Surgical Technique
In a standard technique first described by Goel [72]
and later by Harms [94] the middle subarcuate portion of the posterior lateral mass was used as a screw
entry point. Goel, who performed his first procedure in
1988 in order to avoid venous bleeding from plexuses
accompanying the second nerve root, and to explore
widely the C1-2 joint, always cut out the C2 root with
74
6 Specific Reconstruction Techniques of Upper Cervical Spine and Craniovertebral Junction
its surroundings. This allowed to him to open the joint,
distract, or manipulate the joint in C1/2 dislocations
and also put bone graft or cages in between the roughened joint surfaces. Such a wide exposure enabled him
also to adapt a short plate directly onto the bone. The
price for this procedure was the denervation of C2
area which can be sometimes not well accepted by the
patients [73, 88]. In order to avoid this “non physiolog-
ical” dissection and to make the technique more versatile, Harms developed the polyaxial screw-rod modular
system and introduced the screws at the midline base
of posterior C1 arch. Later, the “transpedicular tech-
nique” was developed [186, 225] where the screw is
introduced through the posterior arch of atlas straight
forward in the lateral mass midplane. Unfortunately,
this anatomical term “C1 pedicle” or “C1 pseudopedicle” or “C1 pedicle analog” incorrectly naming the
posterior arch of atlas (as the atlas has no pedicles) has
gained wider acceptance [30, 31, 148].
Initially, the methods of C1 lateral mass fixation
[94, 152] were described with bicortical screw pur-
chase. Currently, there is discussion if that is necessary.
Cyr et al. described no statistically significant difference in pullout strength between bicortically or monocortically introduced C1/2 transarticular screw [40].
Eck et al. found that significantly much larger force is
necessary to pull out the lateral mass screws if bicortically introduced [58]. Advocates of so called “transpedicular” C1 screws argue from biomechanical work
of Ma [148] demonstrating that posterior arch monocortical screws present larger pullout force than bicortically placed screws through the lateral mass in standard
fashion. Nevertheless, in all the referenced studies, the
pullout strength for standard monocortical screws was
much larger than previously reported acceptable values
for subaxial spine [119, 126] thus giving rational for
surgeon’s preference to choose monocortical introduction whichever entry point is used.
Certainly, bone mineral density, presence of background disease (inflammatory bone destruction -RA,
pure bone quality in bone diseases or osteoporosis),
and surgeon’s feeling of bone solidity play important
roles in the decision if opposite cortex penetration can
increase the stability of the construct.
Some of the advocates of “transpedicular” method
accept that 5 mm height of the arch above lateral mass
and below VA is enough to accept 3.5 mm screw [30,
138]. Lee, in his series of 709 measured cadaveric
atlases, found the average thickness 3.95 mm at the side
of VA groove. He stated that only 6.9% of female and
17.4% male specimens can safely receive the 3, 5 mm
“transpedicular screw” and suggested as an alternative
the notching technique where the inferior part of arch is
drilled with 2 mm burr making a notch in which the
screw shaft can be placed. Nevertheless, even this technique was not possible in 26.7% of females and 8.3% of
males because the bony area of the atlas arch was less
than 3 mm thick [138]. Christensen et al. analyzing 240
lateral masses of cadaveric C1 vertebrae accepted the
smallest height of 4 mm and found that in19% of cases
there was not enough space available for 3.5 mm screw
placement [31]. In other studies [102, 148, 225], the
larger portion of studied C1 posterior arches was able to
accommodate screw placement and 4 mm arch thickness
was considered enough to accept 3.5 mm diameter
screws. This is debatable in our opinion because one can
hardly imagine this without breaching the cortex in practical application. Other complications can include arch
fracture and/or VA injury because of its atypical location
[14] or during taping of thin arch bone [8].
6.2.1.3 Our Preference
In our opinion, thin sliced CT, with 3D image reconstructions is essential for planning of C1 lateral mass
placement. As in other pathologies of UCS we always
procure MRI imaging as well. This can help to exclude
vascular anomalies and localize the position of VA in
relation to C1, as well as the ICA position anterior to
the lateral masses (Fig. 6.7). The surgeon could also
review if the ipsilateral FT is small or even absent on
standard axial scans. If any suspicion of anomalous
VA or ICA course arises from previous imaging, MRA,
or CTA should be performed to elucidate its course
(Fig. 6.8). With all images reviewed and analyzed the
longest safe bicortical screw trajectory is planned.
Based on anatomical studies, a quadratic area of 5 ×
5 mm should be available in the majority of cases but in
about 50% of patients the posterior arch lip overhang
has to be removed to reach this working space (Fig. 6.9).
Normally, the lip is resected with Kerrison rongeur or
high speed drill. If the bone overhang is not removed,
the subperiostal cleavage plane can be lost and the soft
tissue can be violated. The amount of bleeding from
injured venous plexuses surrounding the C2 root can
be very serious. The elevation of the operative field can
help but often we have to use haemostatic sealants and
6.2 Atlas as an Anchoring Structure
ab
ab
75
Fig. 6.7 Normal position of both carotid arteries shown on axial
CT and MRI. (a) Axial CT scan after contrast media application.
Notice the atypical loop of left V3 segment, VA hypoplasia on
the right side and the position of both carotid arteries.
(b) Transversal MRI in case of AA subluxation with clearly seen
position of both carotid arteries in front of atlas
Fig. 6.8 Course of arteries as shown on 3D CTA in two different patients. (a) Normal position of both carotid arteries, hypoplastic
left VA. (b) Abnormal coiling of right and kinking of left ACI just in front of C1 lateral mass
temporary cottonoid pressure. If these methods do not
help then we have to coagulate and/or ligate and
transect the whole C2 neurovascular bundle.
Once in the past we had a case of massive arterial
bleeding during subperiostal approach to the subarcuate
lateral mass on the left side. Compressive packing
together with a fibrin glue helped us to stop it and solve
a very difficult situation. In spite of no pathological vascularization visible on postoperative angiograms, with
our current knowledge we hypothesize that it could be
76
Fig. 6.9 Overhang of posterior arch impeding direct approach
to the posterior inferior lateral mass screw entry area
6 Specific Reconstruction Techniques of Upper Cervical Spine and Craniovertebral Junction
persistent first intersegmental artery. This experience
supports our previously mentioned complex preoperative investigation protocol.
Theoretically, the previously described difficulties
can be avoided if the screw is introduced directly
through the arch. However, from daily practice, we
know that the so called “transpedicular” drilling
through the posterior arch of atlas can be very difficult.
Even if an entry hole is prepared with a high speed
burr, the drill can easily slip up or down and injure the
VA or C2 root bundle. It is also very complicated to
hold the proper trajectory if the arch is less than 5 mm
wide. In general, we feel that this technique and its
modifications as a more risky option and we reserve the
notching technique for specific anatomical situations.
We prefer to locate the entry point in the middle of
the posterior lateral mass at the top of the arcuate surface, which is palpable by a blunt probe in the base of
posterior arch attachment or below it. To know the
exact position of medial and lateral mass pillar borders, the thin Penfield probe is used to palpate it
directly. Sometimes, the joint fissure can be visible but
mostly this is unnecessary. The joint cleft can be frequently visible on lateral fluoroscopy.
It is advantageous to make a small entry pilot hole
with a high speed drill or awl to avoid dislocation of
the drill in the beginning of pilot hole drilling. The C2
neurovascular bundle has to be covered and slightly
caudally dislocated during all the work. A special set
of instrument guides protecting the C2 bundle from
direct contact of drill, tap, screw, etc. can be used as
another option. If parallel to the posterior arch the
introductory angle of the entry point should be at least
3 mm cranially from C1/2 joint for a safe 3.5–4.0 mm
screw placement. Upward trajectory has to be used if
the available posterior vertical working distance is
smaller than normal. Using cranial trajectory inclination we have to take care not to encroach the C0-1 joint
(Fig. 6.10). Most often, we use a trajectory of 10°–15°
medial and 10°–15° cranial with active lateral fluoroscopy directing drilling (Fig. 6.11). Considering the
Fig. 6.10 (a) Parasagittal CT reconstructed image showing the vicinity of C1 lateral mass screw to the atlantooccipital joint,
(b) coronal plane reconstruction
6.2 Atlas as an Anchoring Structure
a
b
77
Fig. 6.12 Axial CT scan of correctly bicortically introduced C1
posterior lateral mass screw
Fig. 6.11 Schematic drawing of C1 lateral mass posterior screw
trajectory: The screw has partially smooth shank. (a) Screw is
introduced in medial angle between 10°–15°. (b) Trajectory is
tilted cranially in angle 10°–15°.
dominance of the left hemisphere in right-handed people, the author always prefers to put any hardware
jeopardizing the left side vessels, last.
The length of screw trajectory is defined by the position of the entry point and angle of introduction. Because
of ovoid shape of the lateral mass and its medial tilt in
transverse plane, (more pronounced in upper facets than
in the lower) we can expect that the screw thread length
will not correspond with the anatomical length of the
Fig. 6.13 Screw penetration of anterior C1 lateral mass shown
on 3D CT
lower facet pillar. Using the depth gauge helps us confirm or correct our measurements. The aim is to fully
accommodate the screw thread within the bone of lateral mass and to have a smooth contact with C2 root,
ganglion, and surrounding veins. Therefore, smooth
shank screws with different thread versus shank ratio
are used (often 60:40). The polyaxial screw head has to
be located freely behind the posterior atlantal arch to
enable its multidirectional movement and connection
rod attachment. Most frequently we use 4 mm smooth
shank screws 30–38 mm long. The final screw positioning is again always checked by lateral fluoroscopy.
We prefer bicortical screw purchase because the
pullout force is higher than in monocortical placement
(Figs. 6.12. and 6.13). However, in our opinion, not
only pullout force is important; especially, in longer
constructs, the vertical, horizontal, and rotational stabilities of the screw play an important role. As a simplified example, we will use a rod and brick analogy.
The rod is introduced in a predrilled hole of a slightly
larger diameter in a wall made from hollow bricks. The
resistance is weak if only placed into the hollow portion of the brick; however, its side load resistance can
be much improved when the opposite brick wall is
drilled and the rod is “bicortically introduced” even
though the effect on pullout effect may be negligible.
The bicortical screw tip should not overrun the
anterior mass surface more than one thread.