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128
Fig. 7.3 Comparison of virtual plan with real drill position vis-
ible on navigational computer screen
the procedure follows a preoperative virtual plan and
can be checked directly on the screen (Fig. 7.3).
Images for navigation systems are currently
obtained prior to surgery or, more frequently, after
7 Virtual and Real TimeNavigational Techniques
positioning of the patient or even after spine exposure but prior to placement of any hardware. One has
to be aware of possible movement or intersegmental
relationship change after data acquisition and appropriately re-register or use automatic updates. This is
more of a potential problem with systems that rely on
preoperative rather than intraoperative scan.
A comprehensive data acquisition and computer
analysis allows essentially for any intervention to be
virtually planned (Fig. 7.4). In terms of UCS and CVJ,
this usually applies to determination whether a certain
screw is anatomically feasible, the location and course
of VA, the best tumor approach, and clival anatomy.
Virtual planning can determine whether a complex
procedure is feasible and can even be utilized in procedures where further intraoperative navigation is not
possible or necessary [26, 33, 34].
7.1.1.1 Preoperative Imaging Based vIGS
CT-based preoperative vIGS is the most accurate
method of spinal bone navigation, especially when it
comes to deformity and anatomically difficult regions.
With accurate registration (1.5 mm accuracy), this guidance can be utilized for any screw placement in the UCS
and CVJ. Its most useful application is the placement of
transarticular C1/2 screw [1, 5, 15, 17, 56] due to the
lack of direct visualization of anatomical structures and
its accuracy is certainly better than with traditional
methods [12, 27, 32, 53]. The downside of preoperative
CT-based navigation is the need for a specific scanning
Fig. 7.4 Virtual plan for transarticular screw introduction. (a) Left side. (b) Right side (notice high riding VA)
7.1 Technique Description
129
protocol, which obviously increases the cost in cases
where a CT has already been obtained but is incompatible with the navigation system. Registration process is
lengthy, has a steep learning curve and each individual
vertebra requires separate registration. Usually, only C2
vertebra is registered/guided and C1-reduced position is
confirmed by lateral fluoroscopy.
7.1.1.2 Intraoperative Imaging Based vIGS
Intraoperative use of CT scanner has been well
described. Navigation system registers images obtained
after patient positioning and spinal exposure and thus
minimizing intersegmental movement that can occur
with preoperatively obtained data [20]. It also allows for
immediate repeat intraoperative scan to check hardware
position. However, the cost and common small gentry’s
window of a mobile scanner are the major downsides of
this technique. Other drawbacks include the need for a
special table, draping technique and the fact that, for
example, mobile fracture fragments cannot be seen in a
changed position without new registration.
Virtual two-dimensional fluoroscopy is perhaps bet-
ter known to most spine surgeons as it combines the
use of familiar fluoroscopy and image guidance techniques [4, 20, 37]. Although its accuracy and virtual
guidance is only improved in one plane at a time,
application to UCS and CVJ surgery has not been
described. Nevertheless, when compared to classic
fluoroscopy, the radiation exposure is decreased. This
type of navigation depends on regular fluoroscope
image acquisition and is, therefore, problematic in
obese or osteopenic patients and in poorly visualized
regions of the spine. UCS anatomy is not always visible, in particular, in patients with deformity and certainly lacks the necessary detail.
Therefore, surgeons have adopted three-dimen-
sional fluoroscopy with the use of isocentric C-arm
that automatically rotates around the patient, obtaining
fluoroscopic images in the surgical position with the
spine centered [21, 22]. The iso-C then generates axial,
sagittal, and coronal images of the anatomy that are
close to CT quality. The iso-C arm can be connected to
a navigation station and images are acquired after the
patient is positioned on the table prior to or after the
exposure. Spinal exposure prior to registration is not
necessary thus obviating the surgeon-driven registration process completely. This fact is important for
minimally invasive or even percutaneous spinal
procedures [12, 21]. The accuracy in UCS and CVJ
has been established in the initial series using iso-C
navigational techniques and is frequently good enough
[22, 23, 41, 42]. Axial images can be reconstructed
three-dimensionally and sent to the navigation system.
A real-time position of spinal elements and implants
can be transmitted to the computer and any subsequent
navigation is actualized and more accurate. Also, the
final position of hardware can be monitored at the end
of the procedure [22]. The radiation dose is reduced to
57–77% [22] in comparison with standard CT protocols. Images obtained with iso-C arm are obviously of
lesser quality than CT and the volume of the scan is
limited to 12 cm3, which means visualization of only
four cervical or three lumbar vertebrae.
Intraoperative spine image acquisition with MRI
became possible with open design of the scanner [44].
This has found application mostly in intracranial surgery of intra-axial lesions to define extent of resection.
Similarly, resection of intramedullary spine tumors
can be controlled with MRI-based navigation techniques; however, bony structures are poorly defined by
this modality [28]. The most limiting nature of an
intraoperative MRI is its cost, size, and need for nonmagnetic equipment.
7.1.2 “Real Time” Image Guided
Surgery (rIGS)
Ideally, “real time” image-guided surgery would allow
for a continuous check of the extent of an intervention.
Although a real-time feedback in its true sense of a
word does not currently exist, the real-time techniques
available allow for an immediate check of each step of
the procedure. It is safer than any virtual navigation.
Surgical strategy can be adapted based on changes during the procedure or target movement. Any dynamic
process in surgery (e.g., resection of tumor, extent of
decompression, deformity reduction, and/or fracture
fragment reposition) will be automatically reflected on
subsequent intraoperative scan. The drill/screw angle
or length can be modified and visualized. Subsequent
steps of surgery minimize the risk to vascular or neural
structures as long as frequent updated scans take place.
The longer the time interval between “real time”
updates is, the higher the risk of inadvertent event.
Minimally invasive or even percutaneous spine surgery
is clearly safer due to real-time imaging guidance.
130
7 Virtual and Real TimeNavigational Techniques
However, the price for improved safety and accuracy is
paid in time necessary for repeated scans, need for a
good radiographer or radiologist, higher radiation
exposure, need for special table hardware connection
for CT or MRI, and surgeons’ discomfort caused by
ergonomic problems.
The most ergonomic real-time navigational tool is
probably three-dimensional isofluoroscopy [22, 23].
The machine is smaller and thus offers some flexibility
in the management of operating room and the procedure. It fares favorably against CT or MRI in terms of
image acquisition times [24, 40]. Bony structures are
well visualized on iso-C arm and it is, therefore, a good
tool for delicate procedures of the UCS and CVJ (e.g.,
transpedicular or transarticular C2 screw placement,
odontoid-compressive osteosynthesis or C1–0 screw)
[22, 23]. Real-time navigation can also be combined
with a virtual one whenever necessary. A good example is the placement of an odontoid-compressive screw
that can be guided Iso-C arm and also monitored by
lateral fluoroscopy instead of using biplanar fluoroscopy [22]. Three-dimensional isofluoroscopy does
account for motion of bone fragments or deformity
corrections in real time, which is one of its main advantages. Image quality in obese patients as well as
increased radiation exposure remains an issue.
The use of direct spinal CT guidance in stationary
scanners in radiology suites is well known from percutaneous interventions [3, 43]. Alternatively, it was limited to procedures performed in stationary scanners in
radiology departments and suites adapted for use of
general anesthesia and surgery [10, 54]. Mobile CT
scanners with radiolucent surgical tables then allowed
for real-time navigational surgery to move to real operating rooms for larger, open procedures [10, 25]. The
superior image quality of CT when compared to isofluoroscopy resulted in broader range of intraoperative
applications of this modality and also enabled percutaneous procedures at the UCS and CVJ. As with any
intraoperative scanner, the major problems are of ergonomic nature due to the gantry size and need for a special operating table.
Likewise, intraoperative MRI (iMRI) surgical systems have been extensively described in the literature
[28, 60]. Soft tissue pathologies (extra/intradural
tumors, intramedullary tumors, or CVJ anomalies
compressing the neural tissue) can be imaged very well
with iMRI. Again, the advantages are clear and drawbacks were already mentioned above. The widespread
use of iMRI in spine surgery is prohibited by the low
yield of high start-up costs, poor bone visualization,
significant metal artifact, and low MRI image quality.
7.2 Our Preference
We advocate the use of computer guidance in the surgery of UCS and CVJ, in particular in situations when
safety and accuracy can be significantly enhanced. This
is especially important when navigating invisible areas.
Neural compression can be caused by developmental or acquired deformity, trauma, inflammation or
tumor. The use of virtual or real-time CT or MRI to
localize or monitor the extent of decompression is well
described [55, 57, 58]. Whenever pure bony compression exists, the Iso-C arm may be sufficient.
Reconstructive techniques during transoral procedures may require the use of navigational techniques
for planning and guiding anchorage of construct to the
clivus (Fig. 6.6, Chap. 6; Fig. 19.22, Chap. 19). The
position of hypoglossal canal can also be monitored in
cases of atlanto-occipital instrumentation when screws
pass through the condyle [23].
Although, the anatomy of atlas is relatively simple
and rarely requires navigational techniques, this technology facilitated percutaneous fixation of an unusual C1
fracture in our department. The fracture comprised a
unilateral sagittal split of the lateral mass with an intact
transverse ligament (Figs. 7.5–7.8). The reason for surgical treatment in our situation was the intraarticular
nature of the fracture and we believe such fractures result
in subsequent pain syndrome, deformity, and poor healing, despite the advocated use of conservative treatment
only. Indeed, Bransford et al. [7] later reported on three
out of six patients with similar injuries that were initially
treated with external orthosis developing a late cockrobin deformity with significant pain and rotatory restriction. All three patients were eventually successfully
treated with an occipitocervical fusion that might have
been avoided with an initial aggressive treatment.
Image guidance can be applied to just about any type
of a C2 screw. Anterior approaches are complicated by
the lack of a stable DRA attachment to the navigated
vertebra but IGS is not impossible as demonstrated in
placement of odontoid screws with iso-C navigation [23,
52]. Deformity correction and fracture dislocations can
make posterior vIGS difficult. Any mobile part of spine
7.2 Our Preference
Fig. 7.5 Unstable two part C1 ring fracture with intact TAL. (a) Axial CT image showing intraarticular extend of fracture. (b) CT
reconstruction in coronal plane. (c) MRI confirming the intact TAL
Fig. 7.6 First step of “real
time” percutaneous CT
guided osteosynthesis of
distracted C1 lateral mass
fracture (the same patient
from Fig. 7.5). (a) Planning
of introductory angle. (b–d)
Consecutive “step by step”
introduction of K-wire
through the fracture. Each
step controlled by repeated
CT image
131
or fracture fragment cannot be registered preoperatively;
perioperative data update is time-consuming and it only
depicts one actual position of target in space. This
becomes obvious when reducing hangman’s fracture
posteriorly. This issue was overcome by real-time
CT-based guidance [54]. Judet’s transpedicular screw
compressive osteosynthesis [2, 29, 41, 42] can effectively be performed in patients with some Effendi type II
fractures without disk bulge or, more appropriately, in
Effendi type I injuries, where the fracture gap is larger
than 3 mm on the CT scan (see Chap. 12). The UCS is
approached through a standard midline approach and
132
ab
cd
Fig. 7.7 Percutaneous
drilling and cannulated screw
purchase along the K-wire.
(a) Cannulated drill
introduced along the K-wire.
(b) Cannulated screw driver
in position. (c, d) Consecutive
cannulated lag screw passage
7 Virtual and Real TimeNavigational Techniques
Fig. 7.8 Final tightening of
the lag screw compressing the
fracture (result after 1.5 year
on Fig. 10.13, Chap. 10). (a)
Before tightening. (b) Final
fracture compression
entry points are planned according to the navigation
computer optimal trajectory. The gantry of a scanner is
accordingly to maximize screw visualization as it passes
through the axis. Repeated CT scans monitor the stepby-step gradual screw introduction. The fracture is
finally compressed by tightening of the lag screw; the
length of which is chosen based on navigation images
(Fig. 7.9; Fig. 12.16, Chap. 12). We believe that, soon
many of those procedures will be achievable percutaneously thanks to the development of mobile scanners,
Iso-C arms, and better computer software.
Currently, we use two main surgical techniques for
C1–2 fixation: atlantoaxial screw fixation as described
by Magerl in 1987 [35] and the Harms [16] modification
7.2 Our Preference
a
Fig. 7.9 Open surgical
compression of hangman
type I fracture with the use of
“real time” CT navigation.
(a) Before tightening of the
lag screw. (b) After final
tightening
133
of Goel’s [14] method of screw and rod construct
between C1 lateral mass and the C2 pedicles/isthmi (see
Chap. 6). Because both techniques involve a passage of
the screws through the axis and may potentially result in
inadvertent injuries of neural or vascular structures (i.e.,
spinal cord and vertebral artery), it would appear very
reasonable to use surgical navigation in such cases.
We use a CT-based vIGS where images are obtained
by a CT scanner according to a specific protocol and
transferred to the workstation. We focus the scanner
on the C2 vertebra mainly in order to create its large
and precise 3D model. On the preoperative virtual plan,
we determine whether the isthmus is large enough to
accommodate a 3.5 mm screw. Once the feasibility is
confirmed virtually, we proceed to define clearly visible
anatomical feducial points for registration purposes.
The patient is then positioned prone, standard posterior exposure carried out, and a DRA frame is firmly
attached to the C2 spinous process as not to hinder the
procedure (Fig. 7.2b). Registration then takes place
and instruments are tracked. The virtual picture should
always be checked against the visible anatomy in order
Fig. 7.10 Navigational plan
(a) and postoperative axial CT scan
(b) confirming correct position of
crosslaminary (with help of vIGS)
introduced screws in thin C2
laminas
134
b
7 Virtual and Real TimeNavigational Techniques
real-time data acquisition and thus enable the growth
of minimally invasive or even percutaneous [6, 12, 21,
50] and robotic [38] surgeries of the UCS and CVJ.
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Section
Indications for Surgery and Examples
of Reconstruction
III