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J Spine Surg 2020;6(1):136-144 | http://dx.doi.org/10.21037/jss.2019.11.13© Journal of Spine Surgery. All rights reserved.
Introduction
The intimate relationship between the cervical vertebra and
its surrounding neurovascular structures creates inherent
surgical challenges when operating on the cervical spine.
Accurate placement of spinal implants is critically important
to avoid iatrogenic complications and costly returns to
the operative suite. Screw breach places nearby structures
at risk, which has spurred critical appraisal of screw
placement using volumetric imaging (1). In an effort to
improve accuracy, there has been a surge of image-guided
technology. As a result, the use of stereotactic navigation has
become increasingly more mainstream. This is especially
the case when usual anatomic landmarks cannot reliably
orient the surgeon intraoperatively, as is the case with
significant trauma, severe degeneration, or developmental
malformations.
The first iterations of the technology relied on
preoperatively acquired computed tomography (CT) scans
to map the patient anatomy (2-4). Intraoperatively, the
CT images were loaded to a navigation station and the
digital three-dimensional (3D) model was referenced to the
patient’s anatomy using a guided probe touching specific
anatomic landmarks (e.g., spinous processes) or externally
applied reference markers. This referencing method was
termed the “point merge technique”. The protocol worked
well for cranial surgery when the software needed only
register an immobile skull fixed in a rigid external frame.
However, as one would expect, alterations in spine position
on the operating table created considerable registration
Review of Techniques on Advanced Techniques in Complex Cervical Spine Surgery
Computer-assisted navigation in complex cervical spine surgery:
tips and tricks
Nicholas Wallace1, Nathaniel E. Schaffer1, Brett A. Freedman2, Ahmad Nassr2, Bradford L. Currier2,
Rakesh Patel1, Ilyas S. Aleem1
1Department of Orthopedic Surgery, Division of Spine Surgery, University of Michigan, Ann Arbor, MI, USA; 2Department of Orthopedic Surgery,
Mayo Clinic, Rochester, MN, USA
Contributions: (I) Conception and design: IS Aleem, R Patel, BL Currier, A Nassr, BA Freedman; (II) Administrative support: All authors; (III)
Provision of study materials or patients: IS Aleem, R Patel; (IV) Collection and assembly of data: N Wallace; (V) Data analysis and interpretation:
N Wallace, NE Schaffer, IS Aleem, R Patel; (VI) Manuscript writing: All authors; (VII) Final approval of manuscript: All authors.
Correspondence to: Ilyas S. Aleem, MD, MSc. Department of Orthopedic Surgery, Division of Spine Surgery, University of Michigan, 1500 East
Medical Center Drive, 2912 Taubman Center, SPC 5328, Ann Arbor, MI 48109, USA. Email: ialeem@med.umich.edu.
Abstract: Stereotactic navigation is quickly establishing itself as the gold standard for accurate placement
of spinal instrumentation and providing real-time anatomic referencing. There have been substantial
improvements in computer-aided navigation over the last decade producing improved accuracy with
intraoperative scanning while shortening registration time. The newest iterations of modeling software
create robust maps of the anatomy while tracking software localizes instruments in multiple display modes.
As a result, stereotactic navigation has become an effective adjunct to spine surgery, particularly improving
instrumentation accuracy in the setting of atypical anatomy. This article provides an overview of stereotactic
navigation applied to complex cervical spine surgery, details the means for registration and direct referencing,
and shares our preferred methods to implement this promising technology.
Keywords: Stereotaxic techniques; surgery; computer-assisted/instrumentation; cervical vertebrae/surgery; spine/
surgery; orthopedic procedures/instrumentation
Submitted Oct 30, 2019. Accepted for publication Nov 22, 2019.
doi: 10.21037/jss.2019.11.13
View this article at: http://dx.doi.org/10.21037/jss.2019.11.13
144
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errors and instrument inaccuracy when this technology was
applied to spine surgery (5). To compensate for changes
in vertebrae position, each level of interest would need to
be registered individually, adding substantial time to the
navigation set up. The advent of intraoperatively acquired
imaging dramatically improved accuracy and registration
times, transforming the technology from novelty to
standard practice. Now navigation systems use frameless,
integrated registration processes, which have reduced the
time to place an image-guided pedicle screw in half (5).
The literature describes several freehand and
fluoroscopically-guided methods of screw placement
(6-8), with reported rates of pedicle wall violation ranging
from 5.2–54.7% (9-11). Though errantly placed screws are
rarely clinically relevant (<5%), in the cervical spine the
screws place vital neurovascular structures at risk (9,12-16).
Computer-navigation is shown to decrease rates of pedicle
wall violation, lower operative times, and decrease the rate
of revision procedures (11,17-24). Despite the improved
accuracy, it is important to note that no form of navigation
has proven to decrease neurologic or vascular complications,
increase fusion rates, or improve pain or health outcome
scores (10,18,25).
This article describes the practicalities of stereotactic
navigation, details our methods for registration and
direct referencing, and shares tips on best practices for
this burgeoning technology, all with a focus on complex
cervical spine surgery using the Medtronic O-arm and
StealthStation (Medtronic, Minneapolis, MN, USA).
Several other systems are available including Iso-C
C-arm (Siremobil Iso-C 3D; Siemens Medical Solutions,
Erlangen, Germany) and NaviVision (VectorVision,
BrainLab, Germany), and our discussion should be
generally applicable as, to date, the systems have shown no
differences in pedicle screw placement accuracy (22,26-28).
Additionally, many instrumentation systems have been FDA
cleared for use with Medtronic’s Navlock Tracker system
on its StealthStation including Alphatec Spine Inc., Globus
Medical Inc., Orthofix Inc., among others. Each system
has its pros and cons, and every surgeon has his or her
individual preferences, but the principles remain constant.
The O-arm and SteathStation are only highlighted due to
our familiarity with the systems rather than any superiority
over other commercially available systems.
How it works
Stereotactic navigation systems will use CT or pulsed
fluoroscopic images (obtained either preoperatively or
intraoperatively) and an image processing software to
generate a volumetric model of the patient’s anatomy. Both
two-dimensional (2D) and 3D projections are available as
the surgeon and case requires. For 3D modelling, a series of
pulsed X-ray exposures are collected by an image intensier
that spins 360 degrees around the patient, and from these
images a reconstruction algorithm generates a 3D model.
The resolution of the voxel rendered image depends on
several factors including the image frequency and intensity
and rotational speed of the X-ray source.
Prior to image acquisition, a reference frame is rigidly
positioned with respect to the patient’s anatomy to permit the
navigation station to correlate the 3D image to points with
the patient’s position in space. Optical or electromagnetic
(EM) localization is used to detect the frame. With optical
tracking, a camera detects infrared light from optical markers
(either reflective spheres or light-emitting diodes attached
to the instruments and reference frame). Once infrared
light is emitted by the camera and reected off the spheres
(or emitted directly by LEDs), the system uses two camera
lenses to geometrically triangulate the spatial coordinates of
each optical marker and transmits the data to the navigation
software for computation. EM tracking works similarly but
uses an emitter, which emits a low-energy magnetic field
with unique field properties at every coordinate within the
eld. The instruments contain EM sensors which allow the
navigation software to identify the instrument’s location
within the field. Spine cases will normally employ optical
tracking rather than EM, as the navigation field for optical
tracking is much larger than that for EM tracking.
After receiving the localization data, the navigation
station processes the sensor data in real-time to compute the
position and angle of the surgical instruments in relation to
the registered model. For the software to correctly display
the instrument’s spatial location, the software must create
a map between points on the patient and points in the
images. This process is called registration. After registration
is complete, the computer uses the created map to identify
corresponding points between the image and patient. The
navigation station can then display the data in several forms,
including simultaneous axial, coronal, and sagittal images;
3D models; or 2D projections analogous to uoroscopy.
Method for employing stereotactic navigation
Room set up
The operating table must be selected to be compatible with
138 Wallace et al. Navigated cervical spine surgery
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the image acquisition system. In this case, a radiolucent
table with table supports at the head and foot are used
which allows the O-arm gantry to pass around a patient and
close the telescoping door (i.e., Jackson or Allen table). For
prone positioning, the patient’s arms are secured to their
sides to provide additional room at the head of the bed.
Whereas securing a patient’s arms at their sides typically
diminishes resolution of 2D radiography, 3D acquired
images are protected from this image degradation. If there
is concern about having sufcient space at the head of the
bed, the O-arm should be tested before prepping to conrm
that the appropriate placement is possible.
Next, attention should be turned toward room layout.
The O-arm, navigation station, and StealthStation take up
considerably more room than a standard uoroscopy unit,
and therefore, image-guided cases are ideally performed in
larger operative suites. For cervical cases, the StealthStation
is preferably placed at the head of the bed to ensure the
reference frame and instruments fall within the ideal
navigation eld (between 0.95 and 2.4 meters). However, if
the angle of approach for instrumentation favors the sensors
at the foot of the bed, this would supersede convention.
Ideally, the passive reference frame is transxed on the side
of the wound nearest the sensor. This prevents obstructing
the line-of-sight between the camera and reference frame
when using instruments within the navigation field. The
O-arm should remain on the side closest to the door, so it
may be removed when not in use. It need only be present
for a brief time during image acquisition, and therefore, a
single O-arm can support several simultaneous stereotactic
navigation cases if separate navigation stations are available.
Tracked instruments and the reference frame
With any image-guided system, special instruments are
needed, and costs scale with the number used due to the
disposable tracking spheres that must be attached. We
generally use the following tracked instruments for pedicle
screw placement: drill, tap, ball-tip probe, and screw driver.
For optical tracking, each instrument has a unique array
where reflective spheres are secured, which the infrared
camera then uses to track. These balls must be rmly set in
place (conrmed by a click). If not fully seated, the tracking
software will be unable to register or track the array. If the
spheres become dirtied, the infrared light will no longer
reflect and the tracking will fail. If blood covers a sphere,
wipe it with a moist sponge followed by a dry one to restore
its reective surface.
Selecting the method of transxing the passive reference
array is critically important to ensure accurate registration
of patient anatomy without obstructing the surgical field.
For cases involving the upper cervical spine, we prefer
to use a Mayfield attachment (Figure 1) when possible as
this provides a rigid position in relation to the spine while
remaining out of the surgical eld. The non-sterile post is
covered with a sterile clear plastic drape then the remainder
B
C
A
Figure 1 Setup for the reference array attached to the Mayeld. (A) A non-sterile arm connects to the back side of the Mayeld to provide
a mount point for the reference array; (B) the mounting arm is covered with a sterile clear plastic drape then brought through a hole in the
standard surgical drape. The hole in the surgical drape is closed off with a rubber band; (C) the reference array is inserted into the mounting
bracket by poking through the sterile clear drape.
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of the draping is performed as usual. The outer drapes are
cut over the sterilely-draped post with scissors that are
passed off the eld. The blue drape is secured to the post
with rubber bands and the frame is inserted through the
clear plastic drape into the post. If direct fixation to the
skeleton is required, the frame can be placed on the spinous
process of C2 for higher accuracy at the cost of crowding
the field. Alternatively, a spinous process clamp on T1
or T2 can be used for lower cervical levels. The spinous
process clamp minimizes the distance of the reference
frame to instrumented levels which optimizes accuracy, yet
the proximity of the frame to the working eld creates new
challenges. Instruments will have to circumvent the frame
while working in the wound, and if the frame is bumped,
the accuracy may degrade.
Before securing the reference frame to the patient,
register the instruments with the unsecured passive frame.
Registering the instruments after image acquisition
introduces risk of inaccuracy if the reference frame or
patient anatomy is inadvertently displaced by the process.
Because instrument registration confirms a known spatial
relationship between the frame and instrument arrays, the
frame does not need to be fixed to the patient. Thus, the
surgical scrub can register instruments at any time after the
StealthStation is set up.
Wound management
The entire exposure should be performed before securing
the passive frame. The surgeon should conrm the infrared
lenses can visualize the frame and instruments at their
desired levels and trajectories. If the frame and instruments
obstruct each other or the lenses cannot identify them
individually, the frame must be adjusted or moved to a
different spinous process.
The deep retractors can remain in the wound throughout
the case, including image acquisition. Leaving the retractors
in the wound limits the risk of inadvertently bumping the
reference frame while replacing them, saves some time,
and avoids potential motion within the registered anatomy.
However, this constant retraction risks potential tissue
necrosis and the retained retractors will blemish images
with metal artifact. When placing or removing retractors,
avoid contacting the passive frame to preserve the fidelity
of the system’s map. Special care should be taken when the
passive frame post is located at the apex of the skin incision.
The retracted tissues can tension the apex which will
shorten the length of the wound and distort the position of
the spinous process clamp after image acquisition.
Direct referencing
For cases involving multiple levels or spinal instability, we
use a “direct referencing” technique in order to repeatedly
verify accuracy of the map anatomy. To do so, we place
1.8 mm cranial plate xation screws strategically on lamina
across the planned surgical levels. These act as fiducial
markers. The locations are recorded and the screws
removed before decortication or wound closure. While
navigating, the screws create easily identiable and reliable
reference points. The fiducial markers should be used
to verify proper orientation of the images and accuracy
of the instruments (Figure 2). The system uses dynamic
referencing and will constantly recompute instrument
location using the reference frame. Throughout the case,
and particularly before key portions of instrumentation,
verify the accuracy and responsiveness of the tracking
system by using the probe to touch bony landmarks or
ducial markers at various points in the eld. Conrm these
points correspond to the correct position on the imaged
model. Should the accuracy degrade, the surgeon should
pause and re-register the system, abandon the process,
or use additional confirmatory imaging with fluoroscopy.
Occasionally, moving the reference frame closer to the
vertebrae of interest may improve accuracy.
Image acquisition
There are several techniques to prepare the surgical field
to ensure sterility and improve registration accuracy.
Commercial drapes are available for the O-arm to maintain
sterility during gantry positioning, but they incur additional
cost. We employ an alternative method to protect the
sterile eld. First, the wound is lled with sterile saline to
prevent tissue desiccation and minimize air-tissue contrast
within the image. Two three-quarter drapes are placed over
the patient, slightly overlapping and connected together
at midline with staples or clips. The reference array is
excluded from this draping and sticks out from between
the two drapes. A third drape or towel covers the reference
frame while the gantry positions itself and telescoping door
closes around the patient. This should be removed, and
gloves changed, to reveal the reference frame prior to image
acquisition. After all images are acquired and the O-arm
removed, the protective drapes are separated and discarded,
taking caution not to contaminate the field. Again, gloves
140 Wallace et al. Navigated cervical spine surgery
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should be changed after touching these protective drapes.
This setup preserves sterile environment below the drape.
With the O-arm in position, 2D scout images are
obtained to confirm the gantry is properly positioned. If
the levels of interest cannot be captured in a single spin,
then multiple spins can be performed without changing
the draping or frame. The occiput to T2 levels typically
requires no more than two spines. Spins should marginally
overlap to guarantee that all pedicles and lateral masses are
captured. The O-arm has two settings for image quality:
high-denition (HD) and standard. We use HD modes with
larger patients, when metallic implants are already present,
when retractors remain in place during image acquisition,
or when working at the occipitocervical or cervicothoracic
junctions. In standard mode, the rotor spins the X-ray
source at 30 degrees per second, acquiring images at
30 frames per second. In HD mode, the rotor spins at
15 degrees per second, effectively doubling the exposure
dose. If the recommended HD 3D dosing is selected
for a large patient (120 kVp, 240 mAs) for the smallest
field of view (20 cm), this will result in an exposure of
approximately 38 mGy. However, most other protocols
range from 10 to 20 mGy depending on the field and
dosing. If radiation exposure is a concern, a low dose mode
is available that will decrease the dosing by 35% from
the standard protocol. Once the image quality is selected,
anesthesia should hold respirations during image acquisition
(usually 14–28 seconds). After image acquisition, the O-arm
can be removed from the suite.
Image-guided instrumentation
After constructing the navigated image, instrumentation
can proceed. Decompression is deferred until after
instrumentation (or at least after preparing the screw holes)
because decompression initiates new bony bleeding, exposes
vulnerable neural structures, and risks decreased navigation
accuracy from displacing the spine from its imaged position.
Using anatomic landmarks, select a desired start point
and use the navigated ball-tip probe to conrm the proper
trajectory and depth to pass the drill through the pedicle and
into the vertebral body (seen on sagittal and axial planes)
(Figure 3). A projection from the tip of the instrument
assists visualization of the path the instrument will take
during advancement. Images are displayed on overhead
monitors or directly on the navigation station within clear
view of the surgeon while using the instruments.
The StealthStation screen can display up to four separate
images simultaneously (axial, sagittal, probe’s eye, 2D
uoroscopy, or 3D model). We routinely use the axial and
sagittal views (Figure 4). The axial view will confirm the
appropriate start point, guide midline angulation, and help
estimate depth. The sagittal view will assist in centering the
tool within the pedicle. The probe’s eye view is the least
helpful, but it shows a composite view in the coronal plane
along the axis of the instrument. Finally, the 2D and 3D
Figure 2 A small screw placed prior to the O-arm spin can act as a ducial marker to verify accuracy of the navigation system. The ball-
tipped probe is placed in the head of the screw, and the projection is conrmed to reect placement at this landmark.
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model views provide a more global assessment.
After an appropriate start point is identified with the
ball-tip probe, the dorsal cortex is burred to create a pilot
hole. The drill tip is positioned in the pilot hole. As the drill
advances, it is tracked on the navigated image in real-time
so minor adjustments can be made to center the drill within
the pedicle. The mapping software can modify the image to
overlay a simulated pedicle screw, which conrms the length
and diameter of the screw. On the navigation system, the
length of the prepared screw track is measured by placing
one cursor at the start point and another at the preferred
depth of insertion producing a display of the linear distance
between them. If the images differ from the preoperative
plan or intraoperative landmarks, the preoperative
measurements are preferentially used or position and depth
are veried with uoroscopic imaging.
We use a standard ball-tip probe to conrm the ve walls
were not breeched by the drill (lateral, medial, inferior,
superior, and the screw hole floor). The navigated ball-
tip probe can conrm drill hole depth is appropriate. The
hole is then tapped using a navigated tap, one millimeter
undersized from the intended screw. The probe again
confirms intact walls. The screw is then placed using a
navigated driver (Figure 5). Both handheld and powered
drivers are available. Powered insertion provides a steadier
navigated image, however current drivers are off-the-shelf
systems that have been adapted to attach a navigation array.
As such, these modified drivers tend to be awkward and
unwieldy.
While inserting the screw, attention is given to the
capture between the driver and the screw head because it
can loosen during screw insertion and produce an inaccurate
display of the screw position. The connection between the
screw and the shaft of the driver is routinely retightened
during insertion. After completion of instrumentation,
Figure 3 The O-arm is brought in on the side of the patient
closest to the door with the eld covered by two drapes that leave
only the reference array exposed, and the arm is closed around the
patient without touching the drape.
Figure 4 The ball-tipped probe may be used to assess the anatomy and nd a safe screw trajectory. A projection from the tip of the probe
demonstrates the path of the planned screw.
142 Wallace et al. Navigated cervical spine surgery
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standard 2D fluoroscopic images confirm proper screw
position so any hardware complications can be identified
and addressed before closure. Typically, an O-arm spin after
instrumentation is not required unless a screw is close to a
critical structure or the native anatomy is so distorted that
uoroscopic imaging is ineffective.
Other indications
Beyond screw insertion, computer-assisted navigation can
be utilized in a number of other ways to aide complicated
spine procedures. The ball-tip probe is commonly used to
verify adequate decompression and to localize and measure
anatomic structures. Stereotactic navigation has found a role
in both short and long segment instrumentation. In some
studies, it has been shown to shorten implantation times
and decrease blood loss (17,29). Additionally, stereotactic
navigation clearly improves implantation accuracy. This
is most notable within the thoracic spine, where pedicle
breeches are reported as high as 47% (11,13,18,22,30).
These benets are offset by increased radiation exposure to
the patient and higher capital costs compared to standard
fluoroscopic or freehand techniques. Though the surgical
team benets from lessened radiation exposure, the patient
on average is subject to an effective radiation dose of 6 mSv.
Fortunately, this is a low dose exposure and should not pose
a specific carcinogenic risk (31,32). However, a standard
abdominal CT is ~8 mSv, and, by epidemiologic data, is
correlated with a small cancer risk. Therefore, the surgeon
should include an honest discussion of radiation risks in the
informed consent if the O-arm will be utilized.
ary and case example
A 25-year-old patient with Klippel-Feil Syndrome
presented to our clinic with signs and symptoms of
severe progressive cervical myelopathy. Imaging showed
numerous formation and segmentation abnormalities, as
well as anomalous vertebral artery anatomy. We undertook
a circumferential cervical decompression and fusion
consisting of C5–6 anterior cervical discectomy and fusion
surgery (ACDF) and posterior C1–T2 decompression and
fusion. We summarize below our steps with regards to use
of stereotactic navigation in this complex case:
(I) reference frame secured opposite to the Mayfield
clamp (Figure 1);
(II) after exposure, O-arm brought in for image
acquisition (Figure 3);
(III) identication of ducial markers to verify accuracy
(Figure 2);
(IV) navigated drill used to confirm screw start point,
followed by projected drill track and screw
placement (Figure 5);
(V) intraoperative uoroscopic imaging showed screw
placement along projected track.
Conclusions
Stereotactic navigation is a burgeoning technology that
has a proven benefit in certain situations. Provided is a
historical, theoretical, and methodological background to
permit an informed decision about using image-guidance
in practice. Navigated surgery requires constant vigilance.
Figure 5 The screw is inserted on a navigated driver so that a projection from the tip of the driver may be tracked as it advances into the
bone.
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Surgeons should not fall into a state of complacency when
using navigated instruments. Repeated accuracy checks with
direct referencing screws should be employed to verify the
navigation mapping has not degraded. If images deviate
from the preoperative plan or intraoperative landmarks,
the system must be re-registered or abandoned. Employed
correctly, stereotactic navigation is a powerful tool in
complex cervical cases, as described here, where traditional
techniques fall short.
Acknowledgments
Funding: None.
Footnote
Provenance and Peer Review: This article was commissioned
by the Guest Editors (Lee A. Tan and Ilyas S. Aleem) for
the series “Advanced Techniques in Complex Cervical Spine
Surgery” published in Journal of Spine Surgery. The article
was sent for external peer review organized by the Guest
Editors and the editorial ofce.
Conflicts of Interest: The series “Advanced Techniques in
Complex Cervical Spine Surgery” was commissioned by
the editorial ofce without any funding or sponsorship. ISA
served as the unpaid Guest Editors of the series “Advanced
Techniques in Complex Cervical Spine Surgery” published
in Journal of Spine Surgery. The other authors have no
conicts of interest to declare.
Ethical Statement: The authors are accountable for all
aspects of the work in ensuring that questions related
to the accuracy or integrity of any part of the work are
appropriately investigated and resolved.
Open Access Statement: This is an Open Access article
distributed in accordance with the Creative Commons
Attribution-NonCommercial-NoDerivs 4.0 International
License (CC BY-NC-ND 4.0), which permits the non-
commercial replication and distribution of the article with
the strict proviso that no changes or edits are made and
the original work is properly cited (including links to both
the formal publication through the relevant DOI and the
license). See: https://creativecommons.org/licenses/by-nc-
nd/4.0/.
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Cite this article as: Wallace N, Schaffer NE, Freedman BA,
Nassr A, Currier BL, Patel R, Aleem IS. Computer-assisted
navigation in complex cervical spine surgery: tips and tricks. J
Spine Surg 2020;6(1):136-144. doi: 10.21037/jss.2019.11.13
