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A. S. Farooqi et al.
Remaining Steps Are Similar Between Both Systems
Navigation accuracy is tested by placing the probe over a spinous process and correlating the imaging seen on the screen with the anatomy. We have found that it is
also helpful to “paint” the lamina by sliding the probe tip down both sides of the
spinous process from supercial to deep while observing the navigation screen.
This process allows the user to determine if the probe tip on screen correlates with
the anatomic location on the patient to conrm accuracy of (the CT scan and) navigation registration (Fig.4.3). Once verication is completed, we recommend performing segmental instrumentation of each level performing the facetectomy, if
desired, followed by the normal sequence of steps for placement of the screw(s).
The inferior articular facetectomy is performed, and an acorn burr is used to decorticate the starting point for instrumentation using the usual anatomic landmarks of
the transverse process and superior articular process. Then, a navigated awl is used
to cannulate the pedicle while visualizing trajectory on the image-based software.
Alternatively, a navigated drill with a calibrated drill sleeve set to a predetermined
depth can be used to drill the pedicle tract instead of manual dilation with an awl.
The navigated pointed probe may be subsequently used to verify the trajectory of
the pedicle screw tract. A navigated tap is used for tapping, and a ball-tipped feeler
probe is utilized to evaluate for breaches. Finally, a navigated screwdriver is used to
insert the pedicle screw, and the nal placement is checked with the navigated
pointed probe. The benet to this technique is that real-time assessment of the awl,
tap, and pedicle screw can be used to easily guide trajectory and allows for immediate correction during any step of instrumentation. The decision to use powered
instruments or manual instruments for navigation-guided instrumentation is left to
surgeon preference, as prior studies have demonstrated comparable accuracy with
both techniques, although the use of powered instruments may be associated with
faster pedicle screw placement, less physical exertion, and lower risk of occupational injury [14–16]. Emerging evidence on the safety prole for powered instrumentation techniques could lead young surgeons to pair it with navigation given the
recent surgeon preservation data.
Another technique that the authors frequently employ selectively uses imagebased navigation tools to conrm pedicle screw starting point and axial trajectory
while completing the other steps of instrumentation in a freehand fashion which we
refer to as “navigation-assisted freehand.” After decorticating the starting point
using typical anatomic landmarks, a navigated pointed probe is then used at the
starting point to visualize the trajectory for the pedicle screw tract (Figs.4.4 and
4.5). At this point, the image-guided on-screen projection can be used to project
different sized pedicle screws to help determine appropriate pedicle screw dimensions, including length and width. The remaining steps of instrumentation are then
completed in a freehand fashion, relying on the navigated pointed probe solely to
conrm axial trajectory. The pedicle is cannulated using a straight or curved awl, or
“gearshift,” and the navigated pointed probe again is used to conrm tract trajectory.
4 Image-Based Navigation: Instrumentation
35
Fig. 4.3 7D Flash Navigation screen showing the number of points on the intraoperative image that correlate with the preoperative CT data (more green rep-
resents more points of data). “Painting” the lamina with the navigated probe to conrm accuracy of the registration before moving forward with the instrumen-
tation portion
36
a
A. S. Farooqi et al.
b
Fig. 4.4 (a) Planned starting point and screw trajectory for lumbar pedicle screw using navigation
guidance. (b) Cannulated lumbar pedicle with diameter and length overlay projected by the navigation software
The pedicle tract is then tapped, and a ball-tipped feeler probe is used to evaluate for
any breaches. The appropriately sized pedicle screw is inserted. The navigated
pointed probe can again be used to conrm the nal placement of the pedicle screw,
which is aided with the use of cannulated screws as the probe can be placed into the
cannulation.
There are many important differences when comparing the fully navigationbased and navigation-assisted freehand technique. Although fully navigation-based
4 Image-Based Navigation: Instrumentation
37
Fig. 4.5 Planned starting point and screw trajectory for thoracic pedicle screw using navigation
guidance
instrumentation allows for real-time assessment of screw trajectory, the added array
to these instruments can make them cumbersome. Often surgeons may also nd
themselves more reliant on the on-screen trajectories rather than the tactile feedback
or “feel” of the pedicle. Furthermore, there can be an increased amount of “toggle”
with the on-screen trajectories during each step which can lead to indecision.
Periodically removing one’s hands from the instrument to assess the true trajectory
and accuracy of pedicle screw placement is benecial, and sometimes necessary as
the surgeon’s hand can block the array.
Minimally Invasive Instrumentation Technique
Minimally invasive, navigation-guided instrumentation may be accomplished with a
variety of techniques, with or without the use of a guidewire. In the technique using
a guidewire, uoroscopy is used to localize the operative vertebral levels and the
dynamic reference array is percutaneously attached to either the iliac spine for
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A. S. Farooqi et al.
lumbar surgery or the spinous process of the most cephalad or caudad vertebral level.
An intraoperative CT scan is obtained and registered with the navigation software.
The navigated pointed probe is used over the skin to visualize the trajectory for dissection down to the vertebra. Using the visualized trajectory, small, paraspinal incisions are made followed by a transfascial stab incision and sequential dilation to
expose the vertebra. One must be conscious to make the incision large enough to
pass through the appropriate instruments without compromising the soft tissue.
Navigation accuracy is veried using the navigated pointed probe at the tip of the
transverse process. A navigated Jamshidi needle or Pak needle may be used to engage
the vertebra with a few gentle taps at the appropriate starting point. The inner stylet
is removed, and a guidewire is gently tapped into the starting point. Alternatively, a
guidewire can be tapped into position at the starting point without the prior use of a
Jamshidi or Pak needle. Following initial insertion of the guidewire, a navigated drill
is used to visualize the pedicle tract trajectory and advance the guidewire. Image
projection is also used at this time to identify the appropriately sized pedicle screw
for the vertebra. A navigated, cannulated tap is used over the guidewire, and a long
ball-tipped feeler probe is used to evaluate for breaches. A navigated screwdriver is
used to insert the pedicle screw with nal placement conrmed using the navigated
pointed probe [17].
Minimally invasive, navigation-guided instrumentation can also be performed
without the use of a guidewire [18]. In this technique, the dynamic reference array
is placed in the iliac crest, proximal spinous process, or distal spinous process
using percutaneous pins. An intraoperative CT scan is obtained and registered with
the corresponding image navigation software. The navigated pointed probe is used
to visualize the trajectory and guide the small, paraspinal incisions. A transfascial
stab incision is made, and a navigated guide tube is used to determine pedicle tract
trajectory, which is then impacted to engage the vertebra. A navigated hand drill
locked at 35mm is used to cannulate the pedicle and then to subsequently tap the
pedicle tract. A long ball-tipped feeler probe is used to evaluate for breaches.
Then, a navigated screwdriver is used to place the pedicle screw, with nal placement checked using the navigated pointed probe. Other minimally invasive techniques have also been described using a navigated awl or awl-tipped tap instead of
a navigated drill, with or without use of a guidewire, with comparable accuracy
[19, 20].
A one-step minimally invasive navigation-guided instrumentation technique
with acceptable pedicle screw accuracy has also been introduced [21]. In this
technique, the pedicle screw is placed without a pedicle probe or tapping. A
guidewire integrated with a cannulated, navigated screwdriver and self-tapping
pedicle screw is docked into the pedicle at the appropriate starting point. Then,
the guidewire is removed and the pedicle screw is inserted under navigation
guidance.
4 Image-Based Navigation: Instrumentation
39
Navigation-Guided Cervicothoracic Instrumentation Techniques
Navigation guidance has improved the safety of cervical instrumentation, especially
in the upper cervical spine. For open, navigation-guided C1-C2 fusion, the patient
is placed in cervical traction with a Mayeld frame. A midline, posterior approach
is used for subperiosteal exposure of C1 and C2. To minimize bleeding, the C1/C2
complex and dorsal arch of the atlas are only exposed up to the medial part of the
facet joints without visualizing the complete facet joint capsule. The dynamic reference array is attached to the caudal most spinous process, followed by intraoperative CT scan and registration with navigation software as described previously. For
other navigation systems with point-to-point registration, a minimum of 3 registration markers should be inserted before intraoperative CT.A navigated pedicle awl
is used to determine appropriate screw trajectory and project the appropriate screw
size. The entry point for C1 lateral mass screw is at the junction of the lamina and
superior-posterior aspect of the lateral mass, with a trajectory parallel to the C1 ring
in the sagittal plane. The entry point is marked with a burr and cannulated with a
navigated manual drill. A ball-tipped feeler probe is used to evaluate for breaches.
The appropriately sized self-tapping polyaxial screw is then placed. The C2 screws
are inserted similarly using the Harms technique at the pars, lateral to the superior
margin of the C2 lamina [22].
The technique for minimally invasive, navigation-guided cervical pedicle screw
placement is similar to minimally invasive, navigation-guided lumbar screw placement. Cervical traction is applied followed by percutaneous placement of the
dynamic reference array at the spinous process of the lowest instrumented vertebra
level. After intraoperative CT is acquired and registered by navigation software,
the navigated pointed probe is used to determine skin entry point for the small,
paraspinal incisions. The navigated pointed probe is used to establish the pedicle
screw entry point, and a navigated drill is advanced into the midpoint of the pedicle. A navigated tap is used followed by use of a ball-tipped feeler probe or navigated pointed probe to evaluate for breaches. Image projection is used to determine
appropriate size of pedicle screw which is then placed using a navigated screwdriver. Instrumentation is performed cranially to caudally due to the concern of
decreasing accuracy with increasing distance from the reference array and possibility of introducing additional navigation inaccuracy from spine micromotion [23].
Navigation-Guided Spinopelvic Fixation Techniques
Image-based navigation has improved spinopelvic xation with iliac screws and
S2-alar-iliac (S2AI) screws. For iliac or S2AI screws, the dynamic reference array
may be attached to the sacral spinous process, a proximal lumbar spinous process,
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A. S. Farooqi et al.
or superolateral aspect of the posterior superior iliac spine (PSIS). For iliac
screws, after intraoperative CT scan is obtained and registered with navigation
software, the PSIS is exposed through a second fascial incision with subperiosteal
exposure. Using a traditional approach, a rongeur is used to remove a bone notch
to create a lower prole for the tulip head, whereas for the anatomic approach,
there is a more medial exposure without removal of the bone notch. A navigated
awl is used to establish the appropriate starting point and trajectory down the
ilium. Image projection is used to determine appropriate screw length and diameter followed by navigated iliac screw insertion. This technique can be performed
percutaneously without direct notch palpation or extensive soft-tissue exposure
[24, 25]. For S2AI screws, following intraoperative CT and registration with the
navigation software, the entry point of the lateral border of the midpoint of the S1
and S2 foramina is marked with the burr [26–28]. Kebaish etal. reported the entry
point to be 25mm caudal to the superior endplate of S1 and 22mm lateral to
midline (one-third of the way caudal to the dorsal S1 foramen or approximately
25mm lateral to midline) [29]. The starting point is veried using a navigated
pointed probe and conrms the screw trajectory which is usually 45° from horizontal and 25° caudal, aimed toward the greater trochanter and superior to the
sciatic notch (Fig.4.6). Image projection determines appropriate screw diameter
and length. A freehand gearshift is used to cross the SI joint, and a longer custom
probe is used to traverse the ilium. Trajectory is again conrmed with the navigated pointed probe, followed by tapping and placement of S2AI screw [30, 31].
Alternatively, S2AI screws may be placed using all navigation- guided instruments. In this technique, a navigated drill guide can be used to determine appropriate screw trajectory and cannulate the sacrum and ilium. After using a
ball-tipped feeler probe to check for breaches, a nitinol guidewire is inserted and
tapped over with a navigated cannulated tap. A navigated screwdriver is used to
place the S2AI screw and the guidewire is removed [25].
Other navigation-guided techniques include S2AI xation with concomitant SI
fusion in adult spinal deformity surgery using triangular titanium rods (TTRs) [32].
In this technique, a starting point 2–3mm proximal to the S2AI screw is marked
with a burr. A navigated drill guide is used to insert a guidewire proximal to the
S2AI screw and into the distal lateral iliac cortex. A cannulated drill is used to make
the pilot hole into the SI joint, followed by use of a navigated broach. Image projection is used to determine the size of the implant, followed by placement of the triangular titanium rod and removal of the guidewire [32].
a
4 Image-Based Navigation: Instrumentation
41
Fig. 4.6 (a) Intraoperative picture with guidewire in the entry point for S2AI screw. Navigation
sagittal and coronal plane imaging for screw diameter and length. (b) Planned trajectory for S2AI
screw placement using the 7D Flash Navigation system with simulated pelvis images
42
b
Fig. 4.6 (continued)
A. S. Farooqi et al.
Learning Curve forNavigation-Guided Instrumentation
Image-based navigation for spinal instrumentation is associated with a notable
learning curve, and numerous studies have demonstrated that navigation-guided
instrumentation efciency and accuracy improve with experience. For example,
Khanna etal. found that there was a signicant decrease in operative length over
time for image-guided single-level lumbar fusion as compared to freehand singlelevel lumbar fusion [33]. In contrast, other studies have found that navigationguided instrumentation time per screw at the thoracic or lumbar spine rapidly
decreases from 34 to 10min after just the rst ten patients [34]. Another study of
navigation-guided instrumentation at the thoracic or lumbar spine demonstrated a
signicantly higher breach rate of 13% for the rst 30 patients, although breach rate
decreased to 5.6% in the subsequent 30 patients after technique modications such
as adjusting the reference array and instrumenting prior to decompression [35].
Ryang etal. evaluated the implementation of a navigation-guided protocol between
the rst quarter and last quarter of a 1.5-year study period and found that initial use
of navigation guidance at the thoracic or lumbar spine was associated with greater
intraoperative navigation scan time (15.4 vs. 8.4min), greater pedicle screw insertion time (5.3 vs. 3.2min), and decreased pedicle screw accuracy (83.1% vs. 92.4%),
although signicant improvements in pedicle screw time and accuracy were also
noted in the second quarter of the study period [36]. Minimally invasive, navigationguided pedicle screws have also been shown to be associated with a notable learning
curve for surgeons. Wood et al. reported that the malposition rate of navigationguided, percutaneous lumbar pedicle screw placement decreased from 5.1% to
4 Image-Based Navigation: Instrumentation
43
2.0% when comparing the rst and last 50 patients over a 4-year study period [37].
The learning curve for navigation-guided sacral pedicle screws may be greater than
the learning curve for thoracic or lumbar pedicle screws. Sargut etal. demonstrated
that the accuracy of navigation-guided tricortical S1 screws drastically improved
from 39% to 91% during the rst and last periods of a 3-year study [38]. The authors
prefer to use navigation on all cases to allow formation of an efcient workow
which also eases troubleshooting when using navigation in complex three-column
osteotomy cases.
Conclusion
Across all areas of spine surgery, including deformity, degenerative, trauma, whether
it be pediatric or adult, the last 10years have seen a signicant improvement in navigation technology to improve the spine surgery safety prole (precision and accuracy), with many exciting innovations in the decade ahead (robotics, radiation free
navigation, etc.).
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