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TECHNICAL CASE REPORT
Navigation and Image Injection for Control of Bone
Removal and Osteotomy Planes in Spine Surgery
Michael Kosterhon, MD
BACKGROUND AND IMPORTANCE: In contrast to cranial interventions, neuronavigation
in spinal surgery is used in few applications, not tapping into its full technological potential.
We have developed a method to preoperatively create virtual resection planes and
volumes for spinal osteotomies and export 3-D operation plans to a navigation system
controlling intraoperative visualization using a surgical microscope’s head-up display.
The method was developed using a Sawbone model of the lumbar spine, demonstrating feasibility with high precision. Computer tomographic and magnetic resonance
image data were imported into Amira , a 3-D visualization software. Resection planes
were positioned, and resection volumes representing intraoperative bone removal were
defined. Fused to the original Digital Imaging and Communications in Medicine data, the
osteotomy planes were exported to the cranial version of a Brainlab navigation system.
A navigated surgical microscope with video connection to the navigation system allowed
intraoperative image injection to visualize the preplanned resection planes.
CLINICAL PRESENTATION: The workflow was applied to a patient presenting with a
congenital hemivertebra of the thoracolumbar spine. Dorsal instrumentation with pedicle
screws and rods was followed by resection of the deformed vertebra guided by the inview image injection of the preplanned resection planes into the optical path of a surgical
microscope. Postoperatively, the patient showed no neurological deficits, and the spine
was found to be restored in near physiological posture.
CONCLUSION: The intraoperative visualization of resection planes in a microscope’s headup display was found to assist the surgeon during the resection of a complex-shaped bone
wedge and may help to further increase accuracy and patient safety.
Angelika Gutenberg, MD
Sven Rainer Kantelhardt, MD
Elefterios Archavlis, MD
Alf Giese, MD
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Department of Neurosurgery, University
Medical Centre, Johannes GutenbergUniversity Mainz, Mainz, Germany
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Parts of this technical case report
(methods and results) have been
presented previously as a poster at the
66th Annual Meeting of the German
Society of Neurosurgery (DGNC) on
June 8, 2015 in Karlsruhe, Germany.
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Correspondence:
Alf Giese, MD,
Department of Neurosurgery,
University Medical Centre,
Johannes Gutenberg-University Mainz,
Langenbeckstrasse 1,
D-55131 Mainz, Germany.
E-mail: alf.giese1@gmail.com
Received, September 17, 2015.
Accepted, October 31, 2016.
Published Online, January 2, 2017.
KEY WORDS: Augmented reality, Osteotomy, PSO, Spinal deformity correction, Spinal navigation
Operative Neurosurgery 13:297–304, 2017
DOI: 10.1093/ons/opw017
C 2017 by the
Copyright Congress of Neurological Surgeons
F
irst introduced in spinal surgery in 1995
by Nolte et al,1,2 neuronavigation has
become an indispensable tool, increasing
patients’ safety and minimizing invasiveness.
Nevertheless, navigation in spinal procedures
today is only used in limited applications.
Foremost is the positioning of pedicle screws,3
a task that greatly benefits from image guidance
regarding pedicle perforation rates.4
On the other hand, neuronavigation in cranial
surgery is a highly developed technique offering
many advanced features, such as the implementation of surgical microscopes or augmented
reality applications.
To transfer these benefits to spinal surgery,
we have used the cranial version of an optical
navigation system (Kolibri 2.0, Brainlab ,
Feldkirchen, Germany) that offers the possibility
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OPERATIVE NEUROSURGERY
to implement a surgical microscope and visualize
preplanned objects in a head-up display (HUD;
Pentero, Zeiss , Oberkochen, Germany).
Due to the fact that the spinal navigation
system’s planning software is not intended to
handle complex spinal osteotomies or other
experimental applications, we have used a 3D software for editing and visualization of
tomographic images (Amira , FEI Visualization
Sciences Group, version 5.4.2, Mérignac Cedex,
France) to design and plan complex objects and
surgical procedures.
We have recently demonstrated that 3-D
surgical planning in Amira offers the possibility of performing virtual tissue or bone resections and to position implants or any other
object directly within a virtual model of the
patient’s anatomy. Furthermore, it is possible
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VOLUME 13 | NUMBER 2 | APRIL 2017 | 297
KOSTERHON ET AL
to transfer these virtual objects back to any navigation system by
converting them into the Digital Imaging and Communications
in Medicine (DICOM) format.5 Here, we use a similar method
to create virtual resection planes with the Amira software and
export those to a navigation system coupled with a surgical microscope.
Pedicle subtraction osteotomy (PSO) and other osteotomies
allow multiplanar correction of deformities but remain technically challenging and are associated with high complication
rates.6 The presented technique, in combination with precise 3D planning, provides the option to perform appropriate-sized
osteotomies and, thus, might help increase surgical accuracy.
Initially tested in an experimental setup performing a “PSO” in a
spine model, the method was found to be highly accurate. Using
this technique, we performed a fully 3-D, preplanned, and intraoperatively navigated osteotomy in a clinical case of a patient
presenting with a congenital spinal deformity.
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CLINICAL PRESENTATION
We present a clinical case of a 56-yr-old female, presenting with
a congenital wedge-shaped hemivertebra between T12 and L1,
causing a complex kyphotic deformity, which necessitated surgical
correction due to severe back pain. During surgery, only certified
medical devices were applied. The patient gave her informed
written consent to perform navigated corrective osteotomy and
to publish these data.
Method
Three-dimensional Reconstruction and Surgical Planning in
Amira
Computer tomographic (CT) images of patients or models
were acquired by an Aquilion RXL multislice scanner (Toshiba,
Zoetermeer, the Netherlands) with a slice thickness of 0.5 mm.
Subsequently, the DICOM images were imported into the
Amira software, and 3-D reconstructions were displayed
using volume rendering. Additionally acquired datasets, such
as magnetic resonance imaging scans and CT-scanned surgical
implants or instruments, may be added and used within the
3-D environment.5
Using the open-source CAD-software blender (Stichting
Blender Foundation, Amsterdam, the Netherlands), virtual
resection planes were created and exported to Amira in the
surface tessellation language (.stl) format. To visualize these
objects on 0.5-mm sliced DICOM data, it was necessary to assign
them a minimum thickness of 1.0 mm. The planes were designed
to have a trapezoid shape, making it possible to detect their spatial
orientation when displayed in the microscope’s HUD during
surgery.
In Amira , the planes were positioned in the 3-D volume
renderings of the patient’s spine according to the desired
osteotomy, and enclosed bony structures to be resected were
defined (Figure 1A).
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298 | VOLUME 13 | NUMBER 2 | APRIL 2017
Conversion of Resection Volumes and Osteotomy Planes into
Navigable Objects
Next, the resection plane objects were transformed from surface
to volume data by converting each into a binary image sequence,
consisting of white and black pixels only. Thus, areas inside or
outside the resection planes were defined and used to raise density
values of each voxel of the original CT lying in these planes.
The majority of density values of the original CT varied from
−1000 to +3000 Hounsfield units (HU). To ensure identification of the 2 resection planes and the resection volume, their
density values were raised to levels over 10 000 HU (Figure 1B).
Subsequently, this virtual planning CT with integrated
resection planes was exported in DICOM format and uploaded
to the navigation system (Kolibri 2.0, Brainlab , Feldkirchen,
Germany). During interventions on the patient, certified
navigation software (iPlan cranial 2.1, Brainlab, Feldkrichen,
Germany) was applied exclusively. Due to the assigned high HU,
it was possible to automatically detect the resection planes using
the navigation platform’s planning software and to define them
as separate objects to be displayed in the HUD of the surgical
microscope (Figure 1C).
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Registration, Navigation, and Augmented Reality
Landmarks for intraoperative registration were defined in
the navigation software. Compared to its cranial application,
intraoperative acquisition of spinal anatomic landmarks is not
possible until surgical exposure of the spine has been performed.
Therefore, preoperative analysis of landmarks to be clearly visible
during surgery is important. The inherent spinal flexibility necessitates the registration of each single vertebra that is included in
the osteotomy. We have chosen the upper and lower curvature of
the spinal process, as well as the middle of the left and right facet
joint, as anatomic landmarks for each vertebra. However, in the
present case, partial interbody fusion between T11 and L1 has
eliminated spinal flexibility in these segments, and, thus, registering the T11-L1 segment as a whole was sufficient to maintain
navigation accuracy.
In the operating room, the spine was exposed by a dorsal
midline approach, and the reference base with infrared reflectivereflective markers was clamped to the spinous process of L1. The
predefined landmarks were localized with the navigation pointer
to undergo intraoperative registration. Additionally, a diamond
drill was used to mark intraoperative unique landmarks, allowing
the restoration of the registration in case of loss of the original
registration. The high-speed drill was registered with the system
as well as the surgical microscope, which additionally was linked
to the navigation system by means of a data exchange connection
(Figures 2A and 2B). This allowed the transfer of images as seen
from the perspective of the microscope, data on the focal plane,
and the zoom ratio to display resection planes correctly in the
HUD of the microscope (Figures 2B and 2C).
The microscope’s position was adjusted until the resection
plane displayed in the HUD appeared as a single line (Figure 2B),
www.operativeneurosurgery-online.com
AUGMENTED REALITY IN SPINE SURGERY
FIGURE 1. Planning procedure and export to the navigation system. A, Three-dimensional planning of a PSO in a lumbar
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spine model in Amira
. The resection planes (red and blue) are positioned to enclose the volume of the vertebral body to be
resected (yellow). B, Upper panel: DICOM image showing the resection planes integrated with high-density values into the
original CT scan data of the spine model; lower panel: histogram of Hounsfield units showing the densities of the main CT data
(left peak) and the densities of the 2 resection planes (intermediate and right peak) to be clearly separated from each other. C,
Screenshot of the final plan transferred to the navigation system’s software with density-based, autodetected resection plane and
registration points set.
OPERATIVE NEUROSURGERY
VOLUME 13 | NUMBER 2 | APRIL 2017 | 299
KOSTERHON ET AL
FIGURE 2. Navigation environment, setup, and in-line visualization. A, Tracking unit. B, Navigation system’s processing unit connected
to the tracker and displaying in-line perspective. C, Surgical microscope with integrated HUD receiving data from the navigation
processing unit. D, Visualization of the microscope’s line of view set exactly in line with the first resection plane. E, Cutting procedure
seen through the HUD’s visualization of the first resection plane that appears as a single line superimposed to the real image. F, Cutting
procedure seen by external observer.
indicating the axis of view to be exactly parallel to the osteotomy
plane (Figure 2D). Subsequently, the resection procedure could be
controlled visually (Figures 2E and 2F) or by using the navigated
drill.
Results of Simulation of PSO and Experimental
Navigation-Guided Osteotomy in a Spine Model
To demonstrate its feasibility, the method was used to perform
a “pedicle subtraction-like osteotomy of L3” in a rigid foam
model of a human lumbar spine (Sawbone , Malmö, Sweden).
The radiopaque model underwent CT scanning, and the images
obtained were loaded into the Amira software. In the lateral
view, the wedge to be resected was defined, and a resection angle
of 27◦ was measured. Subsequently, the resection planes were
placed (Figure 3A). The objects were integrated into the original
CT scan and exported to the navigation system in DICOM
format, as described above, and were subsequently automatically segmented. With anatomic landmarks set for registration,
the reference base was mounted to the spinal process of the
model’s fourth lumbar vertebra and registered to the system. With
the microscope connected to the navigation system, the axis of
view was adjusted to be in line to the first osteotomy plane
(Figure 3A), and the experimental resection could be performed
by optical guidance of the integrated HUD (Figure 3B). After the
“osteotomy,” the model underwent a CT scan, and the resulting
data were registered to the preoperative images showing the
resected bone volume to meet the planning by measuring an
actual resection angle of 27◦ (Figure 3C).
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300 | VOLUME 13 | NUMBER 2 | APRIL 2017
Patient Results: Surgical 3-D Planning and Definition
of Osteotomy Planes in a Case of Spinal Deformity and
Intraoperative Image Guidance by Augmented Reality
The patient suffered severe back pain due to the complex
kyphotic deformity caused by a congenital wedge-shaped
hemivertebra between T12 and L1 (Figure 4A). CT images were
imported into Amira , allowing analysis of the vertebral malformation in a 3-D virtual reality environment. Based on a 3-D
simulation of the desired posture, the asymmetrical angles of a
wedge osteotomy were determined. Subsequently, the resection
planes were aligned to the adjacent vertebras’ endplates. Thus, the
intermediary wedge-shaped vertebra to be removed was defined
(Figure 4B). The planes and the resection volume were converted
into DICOM images and were subsequently exported to the
navigation system.
In the operating room, the spine was exposed via a dorsal
midline approach and registered using anatomic landmarks.
Additionally, the surgical microscope and a high-speed drill were
also registered with the navigation system. The virtual overlay
of the resection volume in the microscope’s HUD was found to
match the anatomic situation with good accuracy (Figure 4C).
Subsequently, intraoperative anatomic landmarks were defined
to facilitate re-registration in the case of registration loss or
failure.
The surgical microscope was positioned to visualize the
first resection plane in-line. Guided by these virtual images
(Figure 4D) and with the aid of the navigated drill (Figure 4E),
it was possible to perform the whole vertebral body subtraction
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www.operativeneurosurgery-online.com
AUGMENTED REALITY IN SPINE SURGERY
FIGURE 3. Concept of augmented reality in guiding a PSO resection. A, Depending on the point of view at which
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the surgical microscope is positioned, the red or blue resection plane appears as a single line. In Amira
, a resection
angle of 27◦ was defined. B, Visualization seen through the HUD. C, Original CT data with preplanned resection
wedge of the spine model and superimposed CT scan of the model after PSO (blue) showing high accuracy.
corpectomy of the hemivertebra T12 as a triangular resection at
the targeted vertebral body via a posterior approach. Reduction
of the osteotomy was achieved using a rod-centered system to
correct the lordosis placed over the pedicle screws 3 levels above
and below the osteotomy T9-10-11-L1-2-3. The entire procedure
was performed under motor-evoked potential and sensory-evoked
potential neurophysiological monitoring.
Postoperatively, no neurological deficits appeared, and postoperative imaging showed a near physiological sagittal and coronal
profile (Figure 4F) as well as sufficient interbody fusion
(Figure 4G).
OPERATIVE NEUROSURGERY
Pre- and Postoperative Pain Assessment and Spinal Parameters
The preoperative clinical examination confirmed a sagittal
imbalance with insufficient compensating mechanisms, so that
the patient was not able to stand upright and walk. The clinical
assessment showed a lumbar and radicular pain index on the visual
analogic scale (VAS) each measuring 8/10.7
The patient was clinically assessed postoperatively 12 mo
following the surgery and presented a lumbar VAS of 3/10 and
radicular VAS of 2/10. The preoperative and 12-mo follow-up
radiographic pelvic (compare Figure 4) and spinal parameters are
summarized in Table.
VOLUME 13 | NUMBER 2 | APRIL 2017 | 301
KOSTERHON ET AL
FIGURE 4. Corrective osteotomy in a clinical case of severe spinal deformity. A, Three-dimensional reconstruction of a 56-yr-old female
patient presenting with a congenital wedge-shaped hemivertebra T12 and L1. B, Preoperative planning of resection planes (red and blue)
with enclosed resection volume (yellow). C, Intraoperative view seen through the microscope’s HUD demonstrating an accurate match of
real and virtually augmented elements. D, Sagittal view in the navigation system showing the microscope’s direction of view (blue line)
in relation to the wedge to be resected (yellow). E, Outline of the resection volume seen through the HUD helps to visually control the
navigated high-speed drill. F, Postoperative radiograph showing restored sagittal and coronal profile. G, Twelve-month follow-up CT
showing fusion of T12 and L1, and a maintained sagittal profile.
302 | VOLUME 13 | NUMBER 2 | APRIL 2017
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AUGMENTED REALITY IN SPINE SURGERY
TABLE. Preoperative and latest radiographic pelvic and
spinal parameters
Segmental kyphosis T11-12
Index curve
SSA
LL
TK
PI
SVA
Preop
Postop
45◦
43◦
114◦
20◦
25◦
40◦
Not ambulatory
5◦
4◦
128◦
34◦
37◦
40◦
11 mm
SSA: spinosacral angle; LL: lumbar lordosis (L1S1); TK: thoracic kyphosis; PI: pelvic
incidence; SVA: sagittal vertical axis.
The index curve (the curve containing the hemivertebra) was
43◦ , 5◦ postoperatively, and 4◦ at the most recent follow-up
evaluation. A measurable improvement in sagittal balance was
achieved and maintained in the last follow-up, and the patient
was fully mobile and without pain medication. No complications
and no neurological deficits were observed in the postoperative
course.
DISCUSSION
Intraoperative navigation in general is a tool that has been
heavily utilized in cranial neurosurgery for numerous years.
Augmented reality applications, such as surgical microscopes with
built-in HUDs, have been established in cranial procedures for
many years as well, but are not employed to the same extent
in spinal surgery. We have applied the cranial version of our
navigation system to spinal procedures, offering in-view visualization of virtual objects through the surgical microscope demonstrating a new approach that may help to increase accuracy and
patient safety.
Navigation in spinal surgery is more complicated than in its
cranial application, particularly with regard to the registration
process. This is primarily due to spinal flexibility and to the fact
that preoperative CT imaging is acquired in a supine position,
while surgery is performed in a prone position. To overcome
these limitations, intraoperative CT imaging8 might be used, but
ongoing surgery does not allow for complex planning as used
in the presented case. Therefore, paired point matching based
on anatomic landmarks was used, although this method necessitates registering the system to every treated level separately and
to reposition the reference base. It has been demonstrated that
registration accuracy depends on the distance of the instrumented
spine level to the reference base as well as the operation time.9 In
case of osteotomies spanning more than 1 vertebra, such as in
expanded PSO, it might be necessary to re-register the system
for each osteotomy plane. This can be done by clamping the
reference base to the adjacent vertebra cranial to the first vertebra,
followed by registration for this vertebra. For the second plane,
OPERATIVE NEUROSURGERY
the reference base is placed to the adjacent vertebra caudal of
the last vertebra, and subsequent registration is performed. This
process might need approximately 5 min for each osteotomy
plane. However, in the presented case, interbody fusion between
T11 and L1 (Figure 4A) made it possible to register all 3 levels as
a whole, and, thus, no re-registration was necessary to maintain
navigation accuracy.
PSO is challenging because of the complex deformities that
need to be managed and the high incidence of complications of over 45%.10 Achieving adequate bone resection is an
important factor in managing reduction, requiring extensive
experience in spine surgery, and must be performed in specialized
centers equipped with reliable intensive care units.11 Numerous
studies demonstrated that the aim of the targeted LL correction
was close to the ideal LL which, according to Schwab,12
is given by the following formula: ideal LL = PI +/− 9,
or an SVA\5 cm or an FBI < 10◦ .13 These corrections,
therefore, had to be checked preoperatively by radiographic
measurements or alignment verification using a 3-D navigation
system.
Anatomic restoration of spinal alignment is a difficult task and
necessitates complex preoperative planning. Therefore, there is a
genuine need for improving the accuracy of resection margins and
angles of the osteotomized vertebra. In recent years, there have
been some reports on the use of navigation in assisting resections
of bone.14-16 With real-time instant visual feedback, intraoperative navigation can assist surgeons in locating osteotomy planes
more precisely.
We suggest that this technique may be helpful in achieving safe
osteotomy particularly in patients with hemivertebrae undergoing
PSO.
Our patient needed a high degree of correction; 40◦ , according
to the segmental kyphosis angle achievable only by a PSO.
However, according to some authors, the average correction
of a PSO can reach up to 25◦ or 30◦ .17,18 Otherwise, the
correction must be augmented by Smith-Petersen osteotomies or
a second PSO. This could increase the invasiveness of the surgery
and the incidence of complications. Although the majority of
spinal osteotomies today are performed without navigation, we
postulate that 3-D simulation, navigation, and image injection
of preplanned osteotomy planes may be an additional option
to improve bone resection accuracy by avoiding inadequate
osteotomies that would not achieve the preplanned correction
angle. With the presented technique, the resection of the adjacent
posterior arch and lamina can be precisely planned and performed
to avoid excessive resection, which could lead to the absence
of contact between the posterior elements and, thus, nonunion.
However, this has to be considered carefully, because in extensive
correction, posterior decompression of adjacent segments may be
required to avoid neurological injury. Radiological and clinical
follow-up of our case confirmed the sufficient correction of the
segmental and global imbalance and demonstrated good posterior
fusion.
VOLUME 13 | NUMBER 2 | APRIL 2017 | 303
KOSTERHON ET AL
CONCLUSION
This technical note demonstrates that the intraoperative
visualization of resection planes via a microscope’s HUD was
found to be very helpful during the resection of a complex
shaped bone wedge. The presented method offers means of
improving resection accuracy and, thus, may assist spine surgeons
in performing appropriate-sized osteotomies, particularly in
complex spinal procedures.
Disclosure
The authors have no personal, financial, or institutional interest in any of the
drugs, materials, or devices described in this article.
REFERENCES
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on the use of navigation in spine surgery. World Neurosurg. 2013;79(1):162-172.
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systematic review and meta-analysis of perforation risk for computer-navigated
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MA. Intraoperative image-guided spinal navigation: technical pitfalls and their
avoidance. Neurosurgical Focus. 2014;36(3):E3.
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subtraction osteotomies for fixed sagittal imbalance. Spine. 2003;28(18):20932101.
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subtraction osteotomy in the thoracic spine and thoracolumbar junction: a retrospective series of 28 cases. Eur Spine J. 2015;24(suppl 1):S42-48.
12. Schwab FJ, Blondel B, Bess S, et al. Radiographical spinopelvic parameters and
disability in the setting of adult spinal deformity: a prospective multicenter analysis.
Spine. 2013;38(13):E803-812.
13. Le Huec JC, Leijssen P, Duarte M, Aunoble S. Thoracolumbar imbalance
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17. Bridwell KH, Lewis SJ, Rinella A, Lenke LG, Baldus C, Blanke K. Pedicle
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COMMENTS
T
he authors have described an interesting application of neuronavigation to surgery for spinal deformity. Although in theory it is
perfectly applicable, there are a number of variables that may affect spinal
stability and thus navigation accuracy during the case. It is rare to find
a perfectly fused pathology in adult deformity such as the authors have
described, and inherent movement or additional osteotomies may result
in severe errors if the surgeon is relying solely on navigation. The authors’
routine of registering each vertebra individually minimizes these variables
but is time consuming. As in any application of neuronavigation, it
should never be relied upon blindly, and anatomy is always the final
reference for the osteotomy planes and its closure. It is probably a useful
tool for surgeons who are planning their first 3-column osteotomies and
the authors should be commended for describing and applying it.
Ricardo Braganca de Vasconcellos Fontes
Chicago, Illinois
T
his is an interesting paper highlighting a clinically useful topic in
modern spine surgery, although based on a single case. The application of navigation to complex procedures, like osteotomies, could
certainly improve surgical precision and, most likely, the overall outcome
by executing preoperative surgical plans.
However, the concept that high technology may overcome surgical
experience may be misleading. Complex spinal surgery requires extensive
experience, as the authors correctly state, and the use of high-tech aids,
like computer-based navigation, cannot replace such experience. Spinal
osteotomies are currently performed without navigation in the majority
of spine centers, and I believe many surgeons will continue to perform
them well even without navigation.
Giuseppe Barbagallo
Catania, Italy
I
n this technical case report, the feasibility of a navigated PSO using
currently available technology and software is confirmed. In contrast
to cranial navigation, the applications for spinal navigation have been
limited mainly to pedicle screw insertion, although there has been
recent use of navigation for interbody cage placement. As navigation
technology advances expanding applications such as these will likely
become common given the potential for navigation to increase accuracy,
efficiency, and safety.
Paul Park
Ann Arbor, Michigan
www.operativeneurosurgery-online.com
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