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Файл:Ординатура / Хирургия / Библиотека им академика М.И. Перельмана / Книга_5225_Библиотеки_им_академика_М_И_Перельмана.pdf
6 Imaging-Based Navigation: Summary ofClinical Results
75
surgeons compared to those performing non-spinal musculoskeletal procedures.
The dose associated with increased risk of radiation-induced cancers is around
10,000 mrem [11]. Conventional projected images are subject to parallax, leading to
some degree of inaccuracy [12]. Lastly, only one plane of imaging can be controlled
for at a time.
Nolte etal. [4] initially described a system using computer-assisted surgical navigation (Fig.6.1). Critical invivo studies have demonstrated several advantages.
First, Rampersaud et al. demonstrated a clinically signicant pedicle breach of
>2mm at a rate of 1.4% [13]. As judged by postoperative CT imaging, which is the
standard tool used for measuring screw accuracy across many studies, pedicle cortical breaches of <2 mm can be attributed to metal artifact or screw-pedicle mismatch. Breaches of <2mm are clinically insignicant [14]. Fluoroscopic navigation
is more accurate in the placement of pedicle screws in the sagittal (cranial-caudal)
trajectory than in the axial (medial-lateral) trajectory [15]. The rate of accurate
screw placement with uoroscopic navigation is comparable to CT navigation [16].
In addition to accurate placement of implants, the trajectory imparts important biomechanical properties. Ideal pedicle screw trajectories are more often achieved
using uoroscopic navigation systems than with uoroscopy alone [17].
This technology allows for a signicant reduction in radiation exposure to the
surgeon [17–20]. The average radiation time per pedicle instrumented is between 3
and 4 s. This radiation exposure is still higher than that of CT-guided navigation.
Additionally, there is a signicant reduction in the amount of time it takes to instrument each level involved [17].
This technology is not without drawbacks. Operating room setup takes signicantly longer when using uoroscopic navigation as compared to standard uoroscopy. Despite having the benet of a faster time to instrumentation, the use of this
type of navigation does reect a longer overall operative time per level. This is
accounted for in the registration of navigated instruments [17, 18]. Moreover, the
use of this does not decrease the risk of inadvertent facet joint injury during screw
insertion [17]. Fluoroscopic imaging still relies on the surgeon’s ability to interpret
the quality of the imaging to verify its accuracy. The accepted clinical accuracy
range is ve vertebral segments for one reference array. In large multilevel constructs, this can require multiple image acquisition sessions; however, some studies
have demonstrated no change in pedicle screw placement accuracy when the array
is placed at T1 and the entire thoracic spine is instrumented [13].This technology is
also very sensitive to changes in spine position and alignment; however, the reliance
on intraoperative uoroscopic imaging allows these systems to have internal validation in real-time.
In addition to 2-dimensional (2D) uoroscopic navigation, 3-dimensional (3D)
uoroscopic navigation is commonly utilized in spinal surgery. 2D navigation relies
on standard intraoperative uoroscopy, while 3D navigation is based off of crosssectional imaging obtained via a CT scan or 3D uoroscopy. The CT scan can be
either obtained intraoperatively or preoperatively. If obtained preoperatively, certain
protocols must be followed in image acquisition. One of the largest studies to evaluate this technology (1100 screws) found 3D uoroscopic navigation to be as
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J. Whitaker and D. Ou-Yang
accurate as CT-guided navigation [21]. The choice of use between 2D and 3D navigation is often made by equipment availability and surgeon preference [22].
Gruetzner et al. compared 3D uoroscopic navigation with standard methods,
CT-guided navigation, and 2D navigation. 3D navigation outpaced all other methods in terms of screw accuracy and precision. 3D navigation also boasts the lowest
uoroscopic time as compared to the other groups. This resulted in a lower overall
radiation exposure [20]. Operative time, an area of mixed results for 2D navigation,
was found to be comparable among all four groups in this data [23].
Computerized Tomography-Guided Systems
CT-guided navigation systems are available from a wide variety of companies.
These systems can be broken down into two major categories. The rst type utilizes
information from CT imaging obtained prior to the surgical date. This is then crossreferenced intraoperatively by registering certain known anatomic landmarks (spinous process, facet, transverse process, etc.). This allows the computer system to
generate stereotactic navigation of instruments and implants. The alternative method
involves obtaining an intraoperative CT scan (or scans) and using this information
to calibrate all instrumentation facilitating stereotactic navigation.
Navigation based on preoperative CT imaging is the older of the two methods.
Thus a large body of literature exists to examine its efcacy. A signicant effort is
required to correctly set up the operating room to accommodate this technology.
There is also a well-documented learning curve that surgeons need to overcome
when integrating this into their practice [15, 19]. This learning curve is reported to
be around 6 months, after which surgeons typically experience reduced operative
time by over 30min and cortical perforation rate by 6% [24]. Other articles report a
learning curve for this technology around 50 cases, after which cortical perforation
rate drops by 5–6% [25].
This technology has been heavily compared against 2D and 3D uoroscopicguided navigation. Many studies available are, unfortunately, limited by their small
sample sizes and are likely underpowered. In 2009, Tian et al. [26] published a
meta-analysis evaluating accuracy of pedicle screw placement using uoroscopic
and CT-guided navigation (Fig.6.2). Some smaller invivo and invitro studies have
demonstrated no signicant differences in the accuracy rate of the three computerassisted navigation systems. There were trends noted that 2D uoroscopy was the
least accurate modality, then CT navigation, and nally 3D uoroscopy, which was
the most accurate. A meta-analysis noted that data for 3D uoroscopy was sparse,
and meaningful conclusions could not be drawn. There was a statistically signicant
difference in the accuracy of CT navigation over 2D uoroscopic navigation. These
results were most consistent when considering only thoracolumbar levels of instrumentation. The results also held true when examining only single-level instrumentation procedures (e.g. L4–L5). There was no signicant difference in overall operative
6 Imaging-Based Navigation: Summary ofClinical Results
77
Fig. 6.2 Evaluation of screw accuracy comparing 2D/3D uoroscopic navigation and CT navigation [26]
time, with CT navigation accounting for only 9min of additional operative time per
case [27].
Adjacent segment disease is a common complication that often necessitates reoperation in patients undergoing fusion surgeries. One of the key tenets in avoiding
this pathology is preservation of the cranial-level facet joint. Injury to this joint can
lead to accelerated degeneration once exposed to increased stress secondary to the
adjacent fusion. A unique advantage of CT-guided navigation systems is the clear
3D identication of this structure. When this has been compared against uoroscopic navigation modalities, CT-guided navigation systems have a signicantly
lower rate (4% vs. 26.5%) of cranial facet joint penetration during spinal instrumentation [28]. This study is limited in that it did not follow patients to determine if
there was a direct correlation between facet joint penetration and the development
of adjacent segment disease.
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J. Whitaker and D. Ou-Yang
Intraoperative CT imaging also demonstrates several benets over conventional
pedicle screw placement technique. Multiple studies have demonstrated that there is
a signicantly lower rate of pedicle re-instrumentation using this technology
[29–31]. This is a critically important factor to note as preserving pedicle screw
xation is crucial to achieving a stable fusion construct. Interestingly, these studies
did not note any signicant reduction in their rate of reoperation for symptomatic
screw misplacement.
There is mixed evidence correlating intraoperative CT navigation with surgical
efciency. Older studies indicate that there is no difference in case time [30], but
newer studies indicate that there is an increase in operative time using intraoperative
CT navigation [29]. This increase in operative time is, on average, 23min. The
increased operative time is balanced by the lower rates of intraoperative blood loss
and complication rates from screw misplacement [29].
Radiation exposure to the patient and the surgeon is another critical consideration when examining the role for CT-guided navigation in practice. Using intraoperative CT scans results in nearly a threefold increase in radiation exposure to
patients when compared to non-navigated cases [32]. This signicant increase in
patient exposure is reduced to insignicance when considering only long-construct
fusion. The total radiation exposure is, however, less than that of a conventional
lumbar CT (5.6 mSv vs. 7.2 mSv). In contrast, the surgical team sees a 2.5-fold
reduction in radiation exposure as compared to uoroscopy use. The radiation from
the CT scan to the patient is well below the accepted treatment threshold for patient
exposure (365 mGy vs. 2000 mGy).
Robotic Assisted Navigation Systems
Many surgical disciplines are seeing advances in the eld of robotic-assisted surgeries. This technology has widespread applications in areas including abdominal surgeries, gynecologic procedures, total joint arthroplasty, and spinal surgeries. The
goal of integrating robotics in to surgical practice is to improve accuracy, feasibility,
and efciency when performing certain portions of surgical procedures. In spine
surgery, an obvious application is placement of pedicle screws (Fig.6.3).
Since the earliest feasibility studies of robotic integration in the placement of
pedicle screw instrumentation, there have been clear advantages with its use.
Robotics has been shown to improve both the accuracy of templating planned screw
size/trajectories as well as screw placement [34]. Reportedly, screw placement is
within 1.02 mm ± 0.56 mm of planned placement. To put this into perspective,
screws with <2mm cortical breach have been considered safe. These early studies
focused on comparing robotic screw placement to conventional freehand technique.
More recently a meta-analysis compared robotics with uoroscopic (2D and 3D)
navigation and CT navigation. This study demonstrated that robotic placement of
pedicle screws had the highest rate of accurate placement [35]. Robotic placement
also minimized the risk of screw related complications. Some studies even cite that
6 Imaging-Based Navigation: Summary ofClinical Results
Fig. 6.3 Intraoperative display for robotic navigation [33]
79
as few as nine screws need to be placed with robotics to avoid one complication
related to screw misplacement [36].
As with other navigation technologies, most studies suggest that there is lower
overall radiation exposure to patients, surgeons, and operating room staff [36, 37].
Similarly, when comparing uoroscopic time of conventional pedicle screw placement with robotic pedicle screw placement, most literature suggest a clear benet to
robotics usage. However, in a 2017 systematic review by Joseph etal. [36] there
were studies that demonstrated similar overall intraoperative uoroscopic time
between robotic and conventional screw placement. When comparing this technology with standard navigation systems, robotic screw placement does tend to produce a higher dose of radiation exposure. Importantly, there is signicant dose
reduction when using a low-dose CT protocol for intraoperative imaging. This data
arises from several pediatric studies, which do not speak to screw accuracy using
this protocol.
A unique advantage to robotic screw placement is improved surgeon ergonomics
and efciency of movement when placing pedicle screws. Much like limiting radiation exposure is critical to preserving the occupational health of the surgeon and
their team, so too is improving ergonomics and efciency of repetitive OR practices
[36, 37].
When integrating innovative technologies into one’s practice, the surgeon and
team must understand that there exists a learning curve. This has been well documented in various surgical disciplines, and holds true with the incorporation of
robotic navigation. Surgical execution rates, which are dened as the beginning of
pedicle cannulation to the insertion of instrumentation, become signicantly more
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J. Whitaker and D. Ou-Yang
efcient as one becomes more familiar with the technology. Time per pedicle screw
placement also signicantly decreases by about 2 min per screw as the surgeon
becomes more procient. Fluoroscopic time (both total and per pedicle screw)
decreases by 50% during this time frame. Screw placement accuracy also improves
over time. The inection point that represents when a surgeon transitions from
slower than baseline to faster than baseline is not well dened in literature. Several
articles suggest that surgeons will see improvement from baseline within 15–30
cases using robotic systems [36].
There are multiple factors that inuence errors in robotic systems. The most
common factor cited is soft tissue pressure on the guide arm. If the robotic navigation system relies on preoperative CT imaging (obtained supine), there is a signicant increase in error rate compared to intraoperative CT acquisition while prone.
The surgeon must also factor in the shape of the facet. The drill guided can easily
slide off facets that are steeply sloped, which results in lateral and inferior deviation
of the screw compared to its intended path [36].
Despite showing excellent intraoperative outcomes, there remain several challenges to integrating robotic navigation into a surgeon’s practice. The cost of the
system ranges from $550,000 (USD) to over $1,000,000 (USD) for hardware and
installation. Maintenance costs are around 10% of the list price [38]. Many advocates of these systems say that one benet is the potential to drastically reduce direct
patient costs (by reducing re-operation rates) and indirect patient costs (avoiding
prolonged disability from postoperative complications). This claim does lack longterm, high-level evidence to support it. There are ongoing studies with promising
early-term results, but they are severely hampered by their small sample sizes and
short follow-up duration.
Navigation inMinimally Invasive Surgery (MIS)
Minimally invasive surgery is one area of spine surgery that has beneted from the
improvement and advancement of navigation technologies. Navigation has allowed
surgeons to conveniently operate in small spaces with limited visibility. Percutaneous
pedicle screw placement has been economized and rened with this technology.
Using navigation, the surgeon is now able to eliminate guidewire usage during percutaneous screw placement. This improves surgical efciency and removes risk
associated with guidewire placement.
Lateral interbody fusion is a technique employed often in minimally invasive
surgery. This style of interbody fusion is not typically done as a stand-alone procedure and is often accompanied by the placement of percutaneous pedicle screws.
Lateral interbody fusion was previously dependent on frequent uoroscopic imaging for intervertebral manipulation and instrumentation. With the incorporation of
navigation techniques, the repetitive use of uoroscopy is negated. This is especially benecial in cases of deformity or severe degenerative change. Navigation
allows for 3D orientation in the retroperitoneal and intervertebral disc space.
6 Imaging-Based Navigation: Summary ofClinical Results
81
Additionally, it allows for “single position” lateral interbody fusion which includes
percutaneous pedicle screw placement without repositioning. The technique has
been shown to decrease operative blood loss, shorten operative time, and lower cost
when compared to the standard procedure [37].
When performing minimally invasive surgeries, space available within the surgical eld is at a premium. Most commonly, the reference frame for the navigation
system is afxed to the patient via a clamp, pin, etc. An array is then attached for
detection by the navigation software. After image acquisition, if the frame is bumped
inadvertently this can lead to signicant inaccuracies in the system. In MIS procedures, the surgeon relies signicantly on image navigation. One solution to this
problem is to use a skin marker-based tracking system for the navigation software.
This system has been shown in several studies to provide reliable navigation. When
comparing standard uoroscopy and skin marker-based navigation for MIS transforaminal interbody fusion, the navigated procedure allowed for shorter overall operative time and less blood loss. For the majority of cases, set-up takes less than
25min [39].
Navigation inScoliosis/Spinal Deformity
Structural changes to the bony anatomy of the spine with scoliosis have been well
described. This includes abnormalities in the pedicle dimensions, congenital bony
abnormalities, and spatial changes due to the deformity itself. Freehand conventional screw placement is associated with a signicantly elevated risk of malposition. This is particularly common for screws placed in the convexity of a thoracic
curve. Screw malposition in this region can cause life-threatening great vessel
injury. Additionally, these levels are often critical xation points in long, multilevel
fusion constructs. Navigation offers surgeons the ability to visualize and identify the
bony anatomy for instrumentation, no matter what the orientation might be. Studies
that have specically evaluated the accuracy of screw placement with standard freehand techniques versus that of navigation have shown that there is an overall accuracy of greater than 98% when using navigation. The screw misplacement rate falls
from 4.9% to 0.6%, which represents a 90% improvement in accuracy [39].
Navigation does concern for elevated exposure to ionizing radiation. In the pediatric
scoliosis population, this is particularly relevant. Low-dose CT scan protocols have
been developed for this issue. These low-dose protocols have successfully lowered
the radiation exposure to patients without sacricing the screw accuracy rate.
Navigation is also very useful at the caudal end of long, multilevel fusion constructs. The S2-alar-iliac (S2AI) screw is a common pelvic xation option. Freehand
placement of this screw is technically challenging and can be dangerous.
Fluoroscopic assisted screw placement requires signicant exposure to radiation,
with multiple dedicated pelvic views. When there is a signicant change in the position of the uoroscope the contamination risk associated with its use dramatically
increases. Moreover, this imaging modality is quite time intensive. With all these
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J. Whitaker and D. Ou-Yang
factors optimized, the misplacement rate of this implant is around 6%. This subjects
intrapelvic vasculature and viscera to injury. Incorporating navigation technology
achieves a greater than 95% accuracy rate with this style of pelvic xation.
Navigation also allows the surgeon to measure the appropriate length and diameter
of the implant, as well as targeting ideal trajectories [37].
Navigation inCervical Spine Surgery
Most studies examining navigation pertain to instrumentation within the thoracic
and lumbosacral spine. The cervical spine can present interesting and unique challenges to appropriate instrumentation. The cervical spine is highly mobile, and its
positioning can be inadvertently changed with minimal motion. When conventional
methods are used, cervical lateral mass or pedicle screw placement have a misplacement rate as high as 30%. Misplacement of implants in the cervical spine can lead
to devastating injuries including spinal cord, nerve root, or vertebral artery injury.
Using intraoperative navigation, the rate of greater than 2mm misplacement drops
to 2.8%. This is still signicantly higher than instrumentation placed in any other
spinal segment using navigation technology. This technology has also improved
accuracy of C1 and C2 instrumentation (Fig.6.4), particularly when performing
MIS techniques [39].
Augmented Reality-Based Navigation Technology
The use of augmented reality in spine surgery is still in its nascent phase, especially
in comparison with the other types of navigational technology previously discussed
in this chapter. There is, however, some literature that demonstrates the benets of
this technology in spine surgery. This class of technology encompasses everything
ab
Fig. 6.4 Use of CT-guided navigation for placement of C2 pedicle screw [37]
6 Imaging-Based Navigation: Summary ofClinical Results
83
from a simple heads-up display that shows intraoperative imaging (uoroscopy or
CT) to superimposition of 3-dimensional overlays onto the surgical eld (Fig.6.5).
The accuracy of instrumentation placed with augmented reality has been shown
to be noninferior to standard navigation techniques and superior to standard freehand technique. This has best been demonstrated in the thoracic and lumbosacral
spine and applies to surgeons who have no familiarity with the system [40]. The
largest studies of screw placement accuracy have been cadaveric studies, which do
have some limitations in generalizability to invivo utilization. There are current,
ongoing studies to evaluate the accuracy of pedicle screw placement
intraoperatively.
This technology provides interesting advantages to the surgeonin that it provides
information to the user. Proponents of augmented reality argue that the technology
limits “attention shifts” and “line of site interruptions” by the surgeon during instrumentation of the spine. Attention shifts occur when the surgeon must look away
from the surgical eld to view a remote screen. Line of site interruptions implies
that an object blocks the standard navigation camera from seeing the navigation
arrays on the eld. Intraoperative adverse events signicantly increase with the
number of attention shifts that a surgeon has during a case [40]. Line of site interruptions limit real-time feedback from standard navigation systems. Augmented
reality technology is not subject to this as the information is relayed directly to the
surgeon’s visual eld via a headset, integrated microscope display, or other modality depending on which company is utilized.
There are several other advantages unique to augmented reality. The real-time
feedback for this technology allows for alerts when instrumentation is placed in
sub-optimal position or trajectory. No other navigation system can provide this
immediate feedback to surgeons. This is obviously limited in clinical applications
by the imaging input to the system [40]. Additionally, the technology allows the
surgeons to stand in a more ergonomic and natural position while operating. This
may prevent occupational injuries from repetitive tasks. Line of site interruptions
are very minimal, unlike with other navigation systems. The tracker is contained
within the device the surgeon is wearing, which allows for very minimal disruption
in the system’s understanding of where instruments are positioned. The use of
ab
Fig. 6.5 Use of intraoperative augmented reality navigation for instrumentation [5]
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J. Whitaker and D. Ou-Yang
augmented reality requires a minimal footprint in the operating room beyond what
is required for standard navigation equipment. The addition of this to standard navigation does not signicantly impact surgical workow.
As with many of the navigational technologies, there are logistic challenges to
implementing this system. There is a signicant capital investment required for procurement of specic equipment. The facility must also acquire intraoperative CT
imaging capabilities if it does not already have them. While the preliminary data is
very promising, there is a lack of high-level, long-term data for this technology.
Theoretical cost analysis suggests the technology will result in signicant cost savings in time, but currently, the body of available literature does not provide sufcient evidence to support this claim [40].
Light-Based Navigation
The newest spinal navigation systems are light-based, machine vision navigation.
Machine vision has been utilized for several years in numerous other industries. For
spinal navigation, a specic type of machine vision called structured light imaging
is utilized. This combines a light projector with two stereoscopic video cameras and
captures a precise and detailed three-dimensional image of the exposed surface
anatomy and co-registers it to a preoperatively or intraoperatively acquired image
(e.g., uoroscopy, CT) data set. Light-based navigation has the same rate of accurate screw placement as 3D navigation for the cervical, thoracic, and lumbar spine
[41]. Currently, the cost of this technology is 470,000 USD.The price compares
favorably to other navigation platforms in that it is approximately one third the price
of an intraoperative CT-based navigation system. The only disposables associated
with the system are the single use reective spheres that are attached to the reference array and navigation tools which come with the system [42].
The operating room footprint for this is small, particularly in comparison to other
navigation systems. A mobile workstation is positioned adjacent to the surgical
table and the arm adjusted to place the system head directly over the surgical eld.
The surgical lamp of the system head provides sufcient illumination of the eld
reducing the need for the standard ceiling mounted surgical lights. Navigation and
registration can begin after surgical exposure. The structured light projector in the
system head briey projects a linear light grid pattern onto the surgical eld and the
anatomy distorts the light path. The degree of this distortion is detected by the overhead stereoscopic video cameras. The specic distortion of the light pattern is then
used to calculate surface depths in order to reconstruct the three-dimensional topography of the exposed surgical surface anatomy and is registered to the previously
acquired images [42].
This system offers several unique features. Augmented reality can be incorporated into this navigation platform, which allows for a virtual “safe zone” for pedicle trajectory to be displayed for the surgeon. This feature is particularly useful
when placing screws and real-time feedback cannot be obtained due obstruction of