There have been considerable advances in spine surgery over the past number of decades.1 Image-guided navigation and robotic-assisted surgery are technologies that have progressed considerably and have potential to reduce complications and improve outcomes. These technologies can make a major impact on all areas of spinal surgery, such as minimally invasive spinal surgery (MISS), deformity correction, tumor resection, and revision surgery. These technologies typically require considerable financial investment and in the early adoption phase may be associated with potential disastrous operative complications. For these reasons, it is imperative that they are proven superior and safe before their mainstream use is adopted.
Robotic assistance offers the potential to mitigate fatigue and may facilitate superior precision of fine movement, irrespective of the length of the surgical procedure.2 This is desirable in a field such as spinal surgery, where procedures are often prolonged, physically taxing and demand delicate fine motor control throughout. Furthermore, the radiopaque and rigid nature of the adult axial skeleton offers great potential for the utilization of image-guided navigation and robotic assistance.3 The ever-increasing interest in achieving such precision and consistency is evident from the growing body of literature in the field of spinal robotics and the number of platforms coming to market.4 This review will give an overview of the most popular platforms available in both image-guided navigation and robotic assistance and provide an insight into the emerging technologies likely to impact spine surgery in the near future.
Navigation systems currently determine an instrument’s location in space and in relation to their surroundings. Modern surgical navigation systems use a stereoscopic camera and reference markers attached to the patient in combination with marked instruments to determine instrument location and trajectories.5 This information is relayed to a monitor in the operating theater allowing real-time feedback. The following include some of the more commonly utilized navigation systems currently available:
Airo Mobile Intraoperative Computed Tomography (CT)
Airo, by Brainlab, is a circular scanner, attached to an operating room table, providing a full 360-degree CT scan (Fig. 1). Instruments have 3 attached reference points recognized by a scanning stereotactic camera. The main advantages of this system are the slim gantry (30.5 cm×38 cm) with a large gantry opening (107 cm), and the mobility of the scanner and table. For navigation purposes, instruments may be calibrated during anesthesia, preparation and draping or exposure. An anatomic reference clamp is attached to a spinous process, allowing registration of the CT imaging. For minimally invasive procedures, 2 reference pins are placed in the iliac crests. A full 32-slice CT scan is obtained and the imaging is registered to Brainlab software. A real-time 3D map is then registered with precalibrated instruments, allowing stereotactic guidance of instrumentation.
O-Arm and StealthStation
The O-Arm and StealthStation S8 navigation system, by Medtronic involves a 360-degree CT-like scanner that can function as a 2D fluoroscopy device or collect 3D volumetric imaging data. It relies on a scanning camera for instrument registration and allows for real-time instrument navigation and screw projection. Reference Clamps are placed on spinal processes and must not be touched, as it can alter the registration process thus leading to errors in stereotaxis. This causes the “map” generated to be shifted relative to instruments in space. If this occurs the patient will need to be rescanned.
Ziehm Vision FD Vario 3D
The Ziehm Vision FD Vario 3D, from Ziehm Imaging, is a C-arm based technology with the ability for 2D and 3D reconstructions. The 3D CT-like reconstructions can be combined for use with other navigation systems (such as Brainlab, Stryker, Medtronic). It also relies on a scanning camera for image registration and real-time navigation.
Stryker SpineMask Tracker
Stryker’s SpineMask Tracker, attempts to negate the problem of reference point translation and allows the surgeon to work free of obstacles. It uses a flat rectangle of trackers, applied directly onto the skin of the patient’s back. The system’s camera connects with up to 31 tracking LEDs. It must maintain an unobstructed view of a minimum of 5 LEDs and furthermore, the surgeon’s arm or body must not obstruct more than half of the reference points of the rectangular tracker to allow accuracy of navigation.
This system is limited by the size of the operative field, which cannot be extended beyond the boundary of the rectangular trackers, which is defined at the start of the procedure. Retraction or other movement of the skin may shift the trackers in relation to the axial skeleton, thus altering the navigation accuracy. This system is potentially better suited to minimally invasive procedures rather than more open procedures which demand more vigorous and dynamic retraction.
Accuracy of Navigation Systems
Accurate placement of pedicle screws is essential to achieving a solid purchase for any spinal construct, as well as avoiding damage to structures in close proximity to the spinal column. Schwarzenbach et al6 were the first group to examine the effect of computer-assisted navigation systems on pedicle screw placement. They demonstrated a breach rate of 2.7% in lumbar pedicle screws with the use of a navigation system. Numerous meta-analyses and systematic reviews have been published in recent years examining the safety and accuracy of navigation-assisted pedicle screw placement versus free-hand placement.7 The primary endpoints utilized are typically accuracy of pedicle screw placement, radiation dose used, and operative time.
Overall, current evidence points towards increased accuracy in terms of screw placement with image-guided navigation systems.8 Kotani et al9 demonstrated a lower breach rate and a corresponding improved safety profile in cervical spine pedicle screw placement when computer assistance was used. Verma et al10 performed the first meta-analysis on this subject, published in 2010, reporting an improved accuracy with navigation-assisted pedicle screw placement. Although they found no neurological complication in those utilizing navigation (0/327 patients) and a neurological complication rate of 2.3% (13/569) in those undergoing free-hand placement, this failed to reach significance.10 Therefore, conclusions were unable to be drawn on whether this improved accuracy of pedicle screw placement results in improved outcomes for patients.
A number of reasons may explain why this accuracy did not readily translate into improved patient outcomes. First, free-hand pedicle screw insertion has an established high success rate.11 Aoude et al12 in their systematic review found 91.4% of free-hand pedicle screws to be placed safely. Second, there exists the assumption that increased accuracy of pedicle screw placement will result in improved patient outcomes. However, studies published when pedicle screw placement was in its infancy in the 1990s reported free-hand pedicle screw misplacement rates as high as 20%–40%, yet this did not translate into increased complication rates.13,14 Gertzbein and Robbins13 reported an accuracy of 81% of free-hand pedicle screws placed but without significant complications, a study that included only thoracolumbar pedicle screws. Although there is a relative paucity of studies reporting patient outcomes, recent studies failed to show decreased complication rates in those undergoing navigation assistance.7
One difficulty when evaluating this topic is the lack of consensus on grading systems for pedicle screw accuracy.12 Many use 2-mm increments as the unit of breach severity, however, there remains no standardized grading system which takes into account degree of breach, side of breach (lateral or medial cortex), location within the spinal column and correlation with clinical symptoms.12
Intraoperative fluoroscopy is an integral part of many spinal procedures, commonly utilized for localization and guidance of instrumentation. Unfortunately, consequently, the ionizing radiation exposure is considerably higher in spinal surgery than in other subspecialties for all operating room staff.15 Furthermore, the increasing use of minimally invasive surgery, such as percutaneous pedicle screw placement and microdiscectomy, results in increased radiation exposure to the spinal surgeon.16,17 Rampersaud et al18 examined the ionizing radiation exposure to chest, thyroid, and hands during cadaveric lumbosacral pedicle screw placement. They found that spinal surgeons experience as much as 12 times greater exposure than nonspinal surgeons and are at a significantly increased risk of exceeding recommended annual dose limits. Bindal et al19 found that performing an unprotected minimally invasive transforaminal lumbar interbody fusion would reach annual occupational exposure limit after 194 cases, when averaging 1.38 levels per case.
The importance of lead protection cannot be overemphasized during these fluoroscopically assisted cases. Mroz et al17 found that the use of lead protection allowed the spinal surgeon to place up to 6396 screws before extremity exposure limits were met. Taher et al20 echoed these findings in their study suggesting that 2700 lateral lumbar interbody fusion procedures can be performed per year before occupational exposure limits are exceeded if radiation safety guidelines are adhered to.
Computer-assisted navigation has the potential to further lower occupational exposure to ionizing radiation. Kim et al21 demonstrated a significant decrease in radiation exposure to both patient and surgeon while using a navigation-assisted technique versus traditional fluoroscopy, without a significant increase in operative time. Kraus et al22 found that radiation dose for fluoroscopy in posterior lumbar fusion far exceeded the dose for CT computer-assisted navigation registration, with navigation significantly reducing the radiation dose administered to the patient. Yu et al23 in their systematic review echoed these results noting radiation exposure can be significantly reduced by the use of radiation protection and potentially with navigation-assistance technologies.
Advancement in technology has led to the refinement of navigation-assisted techniques, however, limitations still exist and accuracy is dependent on several variables as result of the interactions between the navigation platform, the operating environment and surgeon. Robotic-assisted technologies use the same navigation systems but have the advantages of avoiding the surgeon interfering with the tracking systems and decreasing the risk of human error through fatigue.
Robotic assistance can be compartmentalized into 3 categories based on the level of assistance provided. First, there is the telesurgical systems, which have remote command stations whereby the surgeon controls every movement of the machine. Second, there is the “shared-control” model where both the surgeon and robot control motions. Lastly, there is the supervised controlled systems whereby the robot performs actions autonomously under close supervision of the surgeon.24
SpineAssist/Renaissance/ Mazor X (Mazor Robotics)
Mazor have been the pioneers in spinal robotics and their products are the most studied in the field.25 Their system operates under a shared-control model, with the pedicle drilling operation being performed by the surgeon. Preoperative or intraoperative surgical planning is possible with the assisted navigation system, which allows production of a template for screw entry point, size, and trajectory. The robot is mounted to the patient, at the spinal process in the open procedure or with a frame triangulated with K-wires for MISS. This eliminates relative motion between the patient and the robot. A short verification procedure using tracked K-wires in the mounted robot and the virtual template ensures accuracy of the procedure (within 1.5 mm).
The accuracy of this platform was confirmed in cadaveric studies, reporting accuracies within 1 mm of the preoperative template.26,27 Kantelhardt et al28 demonstrated a 95% accuracy of robotic-assisted pedicle screws versus 92% for fluoroscopically guided screws in their retrospective study of 112 patients. Hu et al reported on 102 patients and 1085 planned robotic-guided pedicle screws.29 In 95 patients, 960 screws were implanted with robotic assistance, with 98.9% accuracy. In the remaining patients, pedicle screws were manually placed due to technical problems, with poor registration and trajectory problems as the main reasons for conversion to traditional methods.
Thus far, the only study to demonstrate inferiority of robotic assistance is a randomized control trial published by Ringel et al.30 This study with 60 patients and 298 lumbosacral pedicle screws demonstrated a 93% accuracy with the conventional free-hand placement versus 85% accuracy of the robot. The authors suggest that robotic assistance was a potential source of inaccuracy, with instability of the K-wire attached to the spinous process leading to malposition of drill sleeves and drill cannula. They therefore, suggested superior fixation of the mounted robot.
The ROSA robot, by Zimmer Biomet, is a freestanding robotic surgical assistant with a floor-fixable base and a rigid robotic arm, which mitigates concerns of fixation strength to the patient. The robotic arm moves with the patient using tracking camera monitoring and percutaneously placed tracking pins to the patients’ bony anatomy. Lonjon et al31 demonstrated an improved accuracy of this technique (97.3% for robot-assisted pedicle screws versus 92% for freehand placed) in a study of 20 patients and 40 screws. It is suggested that this robotic platform would be best suited for MISS and percutaneous procedures due to the improved robotic arm fixation.
Da Vinci Surgical System
The Da Vinci Surgical System, by Intuitive, is a telerobotic platform which has gained widespread use across various specialties including general surgery, urology, and gynecology.32 It consists of 2 units: the surgical console and the robotic arms. The surgical console/booth is equipped with 3D vision screens and portals for the surgeon to control the surgical instruments. It has the advantage of having high definition stereoscopic vision (magnification ×10), tremor filtering, limitless wrist range of motion and improved surgeon ergonomics.
In terms of application to spinal surgery, this platform has been used in laparoscopic anterior lumbar interbody fusion (ALIF). However, despite early promise of laparoscopic ALIF, no advantage has been shown over conventional open ALIF in terms of length of stay (LOS), blood loss, or complication rates.33–36 This platform has a steep learning curve leading to an increased operative time and the system is not in common use by spinal surgeons. Currently, it is not FDA approved for spinal instrumentation.
The Excelsius GPS by Globus Medical is a new system launched in the USA in 2017. It is a system which offers both navigation assistance and robotic guidance capabilities. This system offers integrated instrumentation, involves minimal set-up time and has the potential to lead to a more optimal spine workflow. In a cadaveric study, Vaccaro et al37 demonstrated improved accuracy and reduced surgical time with 0 radiation exposure in this robotic model compared with conventional minimally invasive techniques.
Cirq is BrainLab’s new Robotic Arm, awaiting FDA approval. It is a slender, lightweight robotic device which attaches onto the operating table side-rail. This allows for minimal set-up time and interference with the surrounding operating room equipment and staff. Grip sensors allow for adjustment of individual segments providing significant flexibility. Currently this device is compatible with Kick and Curve navigation platforms and intraoperative imaging platforms such as Airo Mobile CT and Ziehm 3D C-arm. Attachable modules allow for operation-specific assistance.
There remains a paucity of literature pertaining to cost-effectiveness of robotic assistance in spinal surgery, without which there will remain a barrier to investment for both navigation and robotic technologies in spinal surgery centers. Robotic systems, irrespective of surgical specialty, have high fixed costs. These costs include the initial purchasing of the system, maintenance and instrument costs. Annual service contracts of certain robotic systems are in excess of US$100,000. Furthermore, training costs and the learning curve associated with these technologies are significant. In order to stimulate and maintain investment, these systems must confer advantages in both surgical outcomes and hospital/operating room efficiency. Several factors such as operative room time, length of hospital stay and surgical revision rates are measurable outcomes which influence cost that may be offset by use of these technologies.24
Operative room time is considerate proportionate to operating room costs. A cost-effective analysis in 2010 by Watkins and Gupta estimated that operative room time was ~$93 per minute.38 Therefore, it stands to reason that decreased operative length will translate to cost savings. Vaccaro et al37 reported significant time savings from a cadaveric study using the Excelsius robot comparing robot versus conventional MIS techniques. Length of hospital stay is also considered a surrogate marker for overall hospital costs. Hyun et al39 in their randomized control trial comparing MIS robot assistance versus fluoroscopy-guided open lumbar fusions reported significantly less hospital LOS. Decreasing LOS not only reduces the hospital cost per admission, but also allows for increased patient turnover thereby improving overall hospital efficiency. Revision surgery is a significant expense as a result of a repeated hospital admission and an additional surgical operation which has potentially increased operative length and complexity compared with the index procedure. Schroder and Staartjes40 in their review noted significantly less revision surgical rates in those undergoing robotic-assisted MIS—posterior and transverse lumbar interbody fusion versus navigated and freehand procedures.
Therefore, although sparse, current evidence suggests that robotic-assisted procedures result in indirect cost savings through decreased operative time, LOS, and revision surgical rates. Further studies are required to compare if the investment cost of the robot technology is offset or improved by these savings.
The future for navigation and robotic-assisted spine surgery is full of promise. Current areas for development lie in enhanced preoperative planning, improved navigation to include navigation for rods and interbody devices, disc space preparation and decompression procedures, as well as integration of rigid retraction systems. Further technological advances are on the horizon. Philips’ in 2017 announced their “augmented reality” (AR) operating suite or “hybrid OR” which aims to improve on existing navigation systems by combining images of patients’ anatomy with 3-dimensional reconstructed images. Xvision-spine AR surgical navigation, by Augmedics, is a system under development, funded by the AO foundation. It utilizes a headpiece worn by the surgeon which incorporates the patients preoperative CT imaging to allow real-time navigation without having to look away from the patient. Reference points on both the headpiece and on the patient allow for both patient and surgeon movement during navigation.
Cost-effectiveness must be proven to ensure widespread uptake of these technologies. This may inevitably occur with time as more systems become available ensuring competitive pricing, as well as with expanded applications such as in tumor and deformity surgery.
AUTHORS’ PREFERRED TREATMENT
Robotic systems further advance the assistance provided by navigation platforms, offering the potential for greater mitigation of fatigue to the surgeon. They allow for accurate and efficient pedicle screw placement. At present, these platforms are demonstrating the greatest potential to improve spinal surgery outcomes. However, time will tell whether the AR platforms will prove dominant over the robotics-assistance systems.
In conclusion, spinal navigation systems and robotic assistance offer huge potential for improving modern spinal surgery. This developing technology has been demonstrated to be safe and efficacious for pedicle screw instrumentation. It offers the advantages of improved dexterity, tremor elimination, indefatigability, and image magnification. This field is rapidly growing, typified by the proposed AR systems, which are currently under development. It is important that this evolving technology demonstrates superiority when compared with traditional techniques before its acceptance and widespread use.
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