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58
Fig. 5.3 Lag screw
principle – fully threaded
screw with the need of
proximal overdrilling
5 Basic Principles of Reconstruction Techniques
Fig. 5.4 Lag screw
principle – partially thread
screw without necessity of
proximal overdrilling
Other unique posterior screw constructs include
atlantoaxial transarticular screws as initially described
by Magerl, in 1979 [28]. The pronounced rotator and
translational instability present with Type II odontoid
fractures or TAL injury results in decreased fusion
rates when the initial posterior wiring techniques alone
were used. Therefore, any form of rigid fixation at
this level was going to enhance fusion success rates
(Chap. 6). The transarticular screw resulted in immediate fixation that allowed for posterior fusion of any
kind (Gallie, Sonntag, Brooks) to take place. This took
away the new, abnormal axis and amount of rotation
5.2 Construct Design
Fig. 5.5 For
adequate stability,
atlantoaxial screws
need to be placed
sufficiently deep
within C1 lateral
mass (tricortical) or
even through the far
cortex of C1
(quadricortical).
Increased risk of
neurovascular
injury with long
screws exists
59
and translation seen after isolated posterior wire/cable
constructs [13], offered significant shear shielding and
decreased pseudoarthrosis rate [18]. The atlantoaxial
screws can offer a neutralizing and compressive effect
on the AA complex. Although, theoretically, they do
not need to offer any kind of compression across the
joint as the point of fusion is distant from the joint (i.e.,
posterior if C1 arch remains intact), the strength of the
transarticular screw can be enhanced by quadricortical rather than tricortical screw purchase (Fig. 5.5).
Quadricortical screw purchase obviously carries a risk
of neurovascular injury anterior to the C1 as described
in Chap. 6.
Quadricortical screw purchase can be intentional
for screw pull out strength and toggle avoidance as
mentioned above, or can serve as a rescue option.
C2 pedicle screws are a good example. The technique
(Chap. 6) and anatomical reasoning (Chap. 1) are
discussed elsewhere; however, a small C2 pedicle can
make placement of a 3.5 mm screw impossible.
Exceptionally, as an alternative, a more medial trajectory through the lateral spinal canal can provide a
robust screw anchor through four cortices (Fig. 5.6).
Fig. 5.6 C2 quadricortical pedicle screws
5.2.2 Anterior Structural Constructs
The anterior column reconstruction techniques at the
CVJ, just as elsewhere in the spine, must result in restoration of a stable load-bearing column, maintenance
of appropriate height, and sagittal alignment for a long
enough period, so that bony integration and fusion can
take place. The eventually biologically integrated
construct will be replaced by living bone and become
obsolete. The available constructs include tricortical
autografts or allografts and synthetic cages. The lack
of vertebral body at the C1 level and the unique biomechanical profile of the UCS make structural anterior constructs much less common. Unlike in the
subaxial spine, axial loading forces are transmitted
from the head to C2 via the occipital condyles, C1
lateral masses, atlantoaxial joints in the middle column, and then C2/3 disk space. Therefore, from a biomechanical viewpoint, an isolated anterior column
construct at the CVJ (except C2/3 interbody cages/
60
5 Basic Principles of Reconstruction Techniques
grafts) does not make sense and most likely would fail
without posterior support. Nonetheless, they can be
used for stability restoration in defects created by
treatment of neoplastic, inflammatory, or infectious
lesions in combination with posterior techniques. The
presence of a robust anterior weight-bearing column
with a posterior tension band (occipitocervical fusion)
represents a tension band principle that allows dynamic
compression of the anterior column and thereby
encourages fusion [4].
The difficulty of anterior column reconstruction at
the CVJ is the relative lack of sufficient anchors at the
superior end of the construct (i.e., clivus or C1 attachment) and the forces applied by the head. Some authors
have attempted to replace anterior elements of C2
vertebra with a specific C2 prosthesis [24] that utilizes
a load-bearing interbody device with buttress-platelike attachment principles. The authors gradually
developed an implant that respects the loading force
distribution of the head from a two-column system of
the C0-C1-C2 segment to the three-column one present
in the subaxial spine. When C2 corpectomy or vertebrectomy is undertaken, a clear reconstructive plan
must be present. In our opinion, two main strategies
exist: (1) reconstruction is going to include both anterior and posterior instrumentation from the occiput/
clivus to subaxial spine as shown in Fig. 5.1 where
anterior cage was anchored into clivus and middle column support was created through inter-facet cages
between C1 and C3 or (2) decreasing the forces transmitted to the construct from the large lever arm of the
head by a structural attachment to C1 both anteriorly
and posteriorly (Fig. 5.7) and thus excluding the C0-C1
segment from the construct and allowing for a shorter
period of postoperative immobilization [33].
Without anterior column reconstruction at the C2
level, a bridge fixation principle needs to be applied
[4] with posterior constructs. An increased stress
transfer and thus minimized fatigue failure can be
achieved by creation of a posterior construct with
multiple points of fixation. At the CVJ, that means
extension of the construct to the lower cervical spine
[16] over segments not involved in the pathological
process. And, without the prospect of any anterior
support over time, it also means a likely fatigue failure
of the construct, which would need to be able to
endure three million loading cycles to survive one
year after insertion [4]. Long term, a posterior fusion
Fig. 5.7 Combined atlanto-cervical construct utilizing an
interbody cage anteriorly and bicortically anchored screws posteriorly used to reconstruct a C2 spondylectomy defect.
Compare to Fig. 5.1. This construct is off-loaded by exclusion
of the normal C0-C1 segment. (a) Sagittal CT reconstruction.
(b) Axial images (courtesy of R. Bohinski, MD, Mayfield
Clinic, Cincinnati, OH)
5.3 Fracture Healing/Bone Fusion
61
with instrumentation cannot compensate for a complete defect in the anterior column.
Irrespective of the construct created, the basic
principle of sufficiently rigid immobilization of the
involved segment must be achieved in order for a bone
fusion to take place. Ideally, the construct design
allows for both anterior and posterior (or lateral) with
sufficient mediation of bone growth, as solid, stable
bony fusion in an anatomically aligned and balanced
CVJ is the ultimate goal of any construct created.
5.3 Fracture Healing/Bone Fusion
Although, historically, spinal instrumentation [19]
preceded attempts at fusion [3, 23], it is clear that
the two need to go hand in hand. Frequently, a spinal
reconstruction cannot be considered successful without
the presence of bone fusion. Fractures heal by means
of an indirect bone healing [32] that involves sequential steps of tissue differentiation, resorption of surfaces of the fracture, uniting of the fracture fragments
by callus, and internal remodeling [31]. Therefore,
with any fracture, there is an attempt to reduce it,
align it, and maintain it in reduced position until bone
healing is complete (i.e., biological fixation has taken
place). On the other hand, internal fixation produces a
stable, rigid construct until bone healing occurs (direct
bone healing). Direct healing occurs under compressive conditions and skips the intermediate steps of the
indirect process and proceeds directly (not necessarily
faster) to internal remodeling of the Haversian system.
Therefore, direct healing does not result in callus formation. If this is visible after an internal fixation, it is
understood that the stability did not reach the intended
levels [32]. However, a completely rigid fixation of a
fracture gap results in lack of mechanical induction of
callus formation (strain theory) [11]. These basic orthopedic concepts, derived from fractures of long bones,
hold value when assessing healing of fractures at the
CVJ that were treated by means of an internal fixation
with direct osteosynthesis (e.g., odontoid screw, C2
compressive pedicle screws). Vascular supply to the
fracture site also plays a role in healing [34].
Unlike fractures, fusion techniques at the CVJ
(Brooks, Gallie, Sonntag, anterior cage construct)
require deposition of a new bone in intersegmental
locations that are not biologically structured for bone
formation. Although instrumentation significantly
improves fusion rates as demonstrated by addition of
atlantoaxial screws to posterior fusion techniques in
atlantoaxial instability treatment [18], failures of
long-term stability occur. Fusion rates are dependent
on multiple local, host’s, technical, and environmental
factors. The internal fixation factors affecting healing
are discussed above. Graft properties (osteo-induction,
-conduction and –genecity) and type (autograft,
allograft, xenograft), mechanical stability, and graft
site preparation are among some of the local factors
determining fusion occurrence [5]. Nicotine, osteoporosis, hormonal imbalance, and certain pharmacotherapy are well-known interferers of bone fusion. An
organized effort has been made to supplement bone
graft materials with fusion enhancers, such as growth
factors or electrical stimulation [5]. Discussion of
those factors is, however, beyond the scope of this
chapter.
5.3.1 Our Preference
Constructs at the CVJ are complex and basic biomechanical as well as biological principles need to be
applied to each individual scenario. During anterior
decompressive procedures at the UCS, we favor the
use of perimesh cage reconstruction fashioned in such
a manner as to be utilized as an anterior plate also. This
then fulfills the role of a buttress plate, interbody loadbearing construct with packed autograft to facilitate
anterior column stability, minimize subsidence, and
allow anterior fusion.
We believe bicortical screw purchase is important
in design of a stable construct and we aim for all hardware to be well anchored. We do not use a cannulated
odontoid lag screw construct over a K-wire with the
understanding that it could be inadvertently advanced
with catastrophic results (Fig. 5.8).
Economic restraints only allow selective use of
bone morphogenic enhancers of fusion in our practice
but all patients are screened for any potential factors
that would influence adequate healing.
62
5 Basic Principles of Reconstruction Techniques
Fig. 5.8 Lag screw
introduction along the
K-wire. Note that if odontoid
apex is drilled through the
wire become free (not fixed)
presenting a danger of its
cranial dislocation during
screw purchase
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37. Tuite, G.F., Papadopoulos, S.M., Sonntag, V.K.: Caspar plate
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38. Wellman, B.J., Follett, K.A., Traynelis, V.C.: Complications
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Specific Reconstruction Techniques
of Upper Cervical Spine
and Craniovertebral Junction
P. Suchomel and O. Choutka
6
A thorough knowledge of anatomy is necessary when
instrumenting the upper cervical spine. Because of the
frequency of anomalies of the bone and neurovascular
structures in this region, multiple diagnostic studies
are usually required for a comprehensive evaluation
and surgical planning. The use of simple axial CT
imaging is insufficient. In the majority of cases, using
CT reconstructions and MRI is essential. At times,
other imaging modalities such as CT angiography or
CT myelogram may be required. Generally, for a safe
3.5 mm screw purchase, the diameter of available bone
surrounding the screw should exceed 5 mm if visual
and/or fluoroscopic control is used [153, 232]. When
using image-guided navigation, the diameter of available bone should be at least 4 mm [19]. Some authors
primarily prefer the use of 4 mm screws [9, 234], and
thus the available bone amount in the plane perpendicular to the axis of screw trajectory should be adapted
by adding a minimum of 1 mm to the previously
mentioned dimensions. If, in exceptional cases, direct
real-time visualization is used (prioperative CT or isofluoroscopy), the outer diameter of the bone can be the
same diameter as the screw. Some experienced surgeons will accept the core diameter of screw being
P. Suchomel
Department of Neurosurgery, Neurocenter,
Regional Hospital Liberec,
Husova St. 10, 46063 Liberec, Czech Republic
O. Choutka
Department of Neurosurgery,
University of Cincinnati College of Medicine,
231 Albert Sabin Way,
Cincinnati, OH 45267-0515, USA
slightly smaller than the diameter of pedicle, arch, or
isthmus in the belief that the slight cortex “blow-out”
is not dangerous. In our opinion, this philosophy can
be accepted only if there are no other options and if the
target structure has an appropriate “guiding tunnel” of
cancellous bone surrounded by cortex. The other
extreme possibility is to intentionally go out of the
bone (e.g., out of the pedicle) when the standard purchase can endanger vitally important structures. In
such a situation, one can select tri- or quadri-cortical
purchase involving the spinal canal and/or extravertebral space.
Despite numerous techniques of fixation described
in the literature using different types of very sophisticated constructs manufactured from state-of-the-art
materials, these technical developments are only supportive tools facilitating the correct environment for
bony fusion and healing. The preparation of fusion
surface and the use of osteoinductive and osteoconductive biomaterials are of paramount importance. Only a
stringent, independent evaluation of fusion result can
confirm the validity of one’s own work. Certainly, in
some situations (e.g., the elderly), radiographic and
functional stability without evident bony fusion can be
enough. Strictly, the term “fusion” should not apply
to a situation when there is lack of movement on
dynamic radiographs. There should also be evidence
of bony mass bridging the fused segment (best documented on CT) without any radiolucency surrounding
the hardware.
First, we shall describe occiput, atlas, and axis one
by one as anchoring structures and then the potential
fusion constructs of craniovertebral junction(CVJ)
and upper cervical spine (UCS).
P. Suchomel and O. Choutka, Reconstruction of Upper Cervical Spine and Craniovertebral Junction,
DOI: 10.1007/978-3-642-13158-5_6, © Springer-Verlag Berlin Heidelberg 2011
65
66
a b
6 Specific Reconstruction Techniques of Upper Cervical Spine and Craniovertebral Junction
6.1 Occipital Bone as Anchoring Structure
6.1.1 Occipital Squama
In any type of CVJ instability, the head has to be included
in the stabilization construct. The occipital bone therefore is always involved and most frequently the occipital
squama is used as a cranial anchor. Historically, pure
onlay bone grafting was used [169] and then various
wiring techniques were utilized to fix either bone strut
grafts [78, 235] or polymethacrylate inlays [160]
between occiput and UCS and thus stabilize the CVJ.
Despite mandatory use of external supports (halo or
Minerva), previous techniques often fail in the long
term. The application of contoured rods, loops, and
frames which are fixed to intact posterior spinal elements and doubled holes in the occiput started the semirigid era of craniovertebral fixation [183]. To increase
the solidity and reliability of CVJ fixation, screws
connected to plates, both locked and unlocked, were
used in the early 1990s [84, 143, 199, 207]. These constructs demonstrated much higher rigidity and substantially increased the fusion rate. This decreased the
necessity of rigid external fixation; however, the fixed
design of the plates often dictated the position of screw
to suboptimal locations. Additionally, a straight line
concordant with UCS fixating points and the plated was
essential. This was disadvantageous, especially when
the lateral placement of screws in the thin part of occipital bone often leads to loosening or breakage. Currently,
modular screw – rod fixating systems are available [1,
115, 178]. They provide flexibility to place occipital
screws in the area of thickest bone independently of the
positions of the spine screws. The majority of occipital
plates have movable and multiaxial U-shaped fixating
heads that enable variable positioning of the contoured
rods to the cervical polyaxial screws. Such variable rigid
constructs allow shorter segments of fixation preserving
more motion segments. When instrumenting the occipital squama, a thorough knowledge of anatomy and any
patient variation is necessary. Evaluating the bone thickness in the planned screw locations and the intracranial
position of venous sinuses is paramount.
6.1.1.1 Anatomical Background
The occipital squama bone thickness is the greatest at
the external occipital protuberance (EOP) and decreases
in a radial distribution (Fig. 1.1, Chap. 1) [55, 250] The
superior nuchal line does not reflect the internal position
of transverse sinus accurately; the relation of the confluence of sinuses to EOP is more consistent [189]; therefore, our screw position should be approximately 1 cm
below it and not more than 2 cm, laterally. As was
described in the prior chapter on anatomy, we can expect
the bone thickness in the EOP to be 15 mm in males and
12 mm in females on an average (Fig. 6.1a). The other
“safe area” is relatively thin strips of bone that extends
caudally from the EOP and is a reflection of the internal
occipital crest. The thinnest bone is directly above the
cerebellar hemispheres inferior to INL (Fig. 6.1b).
Fig. 6.1 Normal thickness of occipital bone visible on CT sagittal reconstructions documented in a male patient. (a) Midsagittal
scan with bone thickness between 10–15 mm. (b) Less than 4 mm thin bone over cerebellar hemisphere in the same patient
6.1 Occipital Bone as Anchoring Structure
67
6.1.1.2 Surgical Technique
Usually, the posterior midline skin incision starts 1 cm
above the inion and continues splitting the nuchal ligament inferior to the desired level of cervical spine.
Subperiostal dissection with or without sparing the EOP
muscular attachments exposes the external anatomical
landmarks, the SNL, INL, and the edge of foramen
magnum. Palpating the C2 spinous process and critically, the C1 posterior tubercle helps to identify the posterior FM rim. Depending on technique chosen, the
extent of occipital bone exposure is defined. In the case
of in line lateral plates, which are in continuity with lateral masses of spine, more lateral dissection is required,
whereas midline fixation only needs limited exposure.
Reviewing the anatomical landmarks, preoperative
radiographs, and CT, the holes corresponding to plate
are drilled. On an average, 12 mm screw purchase in
midline is safe. If lateral screw location is chosen then
the necessary bone thickness for screw purchase should
be at least 6 mm but preferably, 8 mm. Bicortical screw
purchase is probably not necessary in occipital area.
This is supported by Zipnick’s paper, which demonstrates that the outer cortex contributes 45% of total
occipital bone thickness whereas the inner one is providing only 10% [250]. However, Haher et al. [90] found
that bicortical pullout strength was 50% greater than
unicortical. Because the strength of screw fixation is
proportional to thickness of the bone, the EOP (inion)
and caudal midline are ideal for screw placement [177,
189]. Whichever type of screw introduction is selected,
the holes should be tapped as the bone can be very hard
and the screw may break during tightening. Four to six
millimeters diameter occipital screws are used to fix the
plates. Although the length of screws is often established preoperatively, lateral fluoroscopy is recommended to double-check the desired length, to achieve
correct perpendicular screw angle and to verify the full
contact between the plate and the bone. Final tightening
is done in controlled fashion with the torque wrench.
Comparing these landmarks with preoperative CT and
plain X-ray, we select the appropriate position of the
occipital plate on the occipital squama inferiorly to
SNL. The final position of the occipital plate is also
influenced by the availability of skin coverage above
it. Sometimes, we have to localize the plate more caudally to avoid the potential of erosion over the plate.
Holding the plate in the planned location, we mark the
planned drill entry points with high speed burr or awl.
Then with a chisel or high speed reamer we prepare the
surface of the bone to accept the plate (eventually contoured) without any air gaps in the interface. The plate
is held in the final position and the final holes are
drilled with the safety stop drill guide.
We always start with the deepest screw to firmly
attach the plate for further drilling. If bicortical screw
placement is attempted, then one has to be aware of
dural and/or sinus injury. Potential sinus injury has to
be treated properly. The head must not be above the
level of right cardiac atrium to avoid air embolism. It is
much better to err on the side of venous bleeding than
a dry field that is sucking air. In such cases, the “sucking” hole must be plugged with cottonoid and flooded
with water. The surgical position of the patient has to
be changed immediately (Trendelenburg) and a shorter
screw, away from the sinus is placed. Violation of the
sinuses can cause not only bleeding but also much
more dangerous venous sinus thrombosis. Simple
dural penetration is frequently seen with CSF leak
plugged with the screw introduction. However, such a
“minor complication” can injure surface vessels of the
cerebellum causing subdural or epidural hematoma.
In conclusion, we can say that the best and the safest anchorage can be achieved in the midline below the
EOTP and that bone thickness less than 6 mm is not
sufficient for firm screw purchase. We believe that the
safest approach to this procedure is ensured by precise
preoperative evaluation of occipital bone by thin cut
CT (Fig. 6.2).
6.1.1.3 Our Preference
Radiological evaluation prior to any occipital bone
screw introduction is essential to achieve firm, bicortical screw placement while avoiding the venous sinuses.
We prefer to dissect the occipital bone subperiostally
without any midline muscular attachment left in place
and then to define the external anatomical landmarks.
6.1.2 Occipital Condyles
In the case of thin or missing (after craniectomy)
occipital bone, the occipital condyles can be used as
an anchoring structure. Also, a monosegmental transarticular atlantooccipital (atlantocondylar) screw
fixation can be used to fix AOD, especially if it is
mild and reduced. Other indications include the