16 Surgical Navigation Systems
16.1 Introduction
Intraoperative imaging used in the placement of posterior pedicle screws is especially crucial in minimally invasive surgery where direct visualization of anatomic structures is limited. The conventional technique used by the majority of surgeons is fluoroscopy, which involves the introduction of a C-arm into the surgical field. However, conventional fluoroscopic imaging has been associated with variable screw placement accuracy and concerns regarding radiation exposure for the patient and operative staff.1,2,3,4,5,6 Inaccurate screw placement can lead to severe neurovascular injury with significant perioperative morbidity. Excessive radiation exposure has also been associated with increased risks of teratogenesis and carcinogenesis.7,8,9,10,11 As a result, image-guided navigation systems have been developed with the goals of improving screw placement accuracy and reducing operative radiation exposure.
Spinal navigation provides real-time, image-based guidance for insertion of pedicle screws and other spinal instrumentation. In this process, radiographic images of the anatomy of interest are uploaded into a computer workstation to produce a three-dimensional (3D) reconstruction of the relevant anatomy that can be viewed preoperatively or intraoperatively. This real-time display of the anatomy is also supplemented with specialized surgical tools whose positions and trajectories can be detected by the system and displayed to the surgeon.12 This real-time visualization of surgical anatomy and tools allows for insertion of instrumentation without the need to acquire multiple fluoroscopic images.
16.2 Components
16.2.1 Imaging Modalities
Navigation systems utilize multiple imaging modalities to acquire the necessary anatomical images. Fluoroscopy-based techniques can be adapted, using either two-dimensional (2D) or 3D modalities. Computed tomography (CT)-based techniques include 2D intraoperative CT (iCT), cone-beam 3D CT with a 190° scanning arc, or O-arm imaging with a 360° scanning arc. Additional techniques also include intraoperative magnetic resonance imaging (MRI) or ultrasonography. Imaging can be performed either preoperatively or intraoperatively, with subsequent uploading of the data into the navigation workstation.
16.2.2 Workstation
The navigation workstation contains multiple components essential for successful implementation of image guidance. The workstation houses a computer system containing software that integrates radiographic imaging studies, produces 3D anatomic reconstructions, and allows for positional tracking of specialized surgical instruments. The workstation also contains a camera, which is utilized to detect the position of reference frames and surgical instruments within the operative field. This camera can use a variety of different detection modalities, including optical imaging, electromagnetic detection, or acoustic detection.13 Workstations also contain high-resolution display screens which can be viewed from the operating table, allowing the surgical staff to analyze the patient’s anatomy and the instrument positions to determine the optimal trajectories for insertion of instrumentation.
16.2.3 Dynamic Reference Frames
Dynamic reference frames interact with the workstation’s camera and aid in positional tracking. These constructs are affixed onto anatomical landmarks within the surgical field, and act as a point of reference for the workstation’s camera system to determine the position of navigated instruments in relation to the patient’s anatomy. Most commonly, these reference frames are directly affixed to bony landmarks within the surgical field such as the spinous processes or the posterior superior iliac spine.14
16.2.4 Surgical Navigation Instruments
Navigation systems use specialized surgical instruments that are outfitted with tracking sensors or transmitters that are detected by the workstation’s camera.15 The navigation system then uses this information to plot the position and trajectory of these instruments in relation to dynamic reference frames and the patient’s anatomy. Instrument position is displayed in real time on the navigation screen, superimposed on the imaging studies or 3D anatomic reconstructions produced by the workstation. This capability allows for updatable, real-time guidance of instrument position to determine the optimal trajectory for insertion of spinal instrumentation.
16.3 Outcomes
The clinical efficacy of navigation systems is primarily measured against and compared to pedicle screw insertion accuracy rates seen in procedures done with conventional fluoroscopy. Multiple large meta-analyses have been performed that have demonstrated higher pedicle screw insertion accuracy in cases using spinal navigation.1,2,16,17 Mason et al, in a meta-analysis of 30 studies comparing conventional fluoroscopy to both 2D and 3D fluoroscopy-guided navigation, found significant differences in accuracy among the modalities.1 Navigation using either modality had a statistically higher rate of screw accuracy compared to conventional fluoroscopy, and this pattern was seen at all thoracic and lumbosacral spinal levels tested. Shin et al noted similar findings pertaining to accuracy, with navigated operations having an accuracy of 93.3% compared to 84.7% in nonnavigated procedures (p < 0.001).17 Furthermore, no neurologic complications were described in 719 navigation patients, while 2.3% of patients undergoing a procedure with conventional fluoroscopy experienced postoperative neurologic deficits. Shin et al further demonstrated the improved accuracy and complication profile of navigated procedures, while also determining that navigated and nonnavigated procedures had no statistical differences in operative time and intraoperative blood loss.
Spinal navigation systems demonstrate the advantage of intraoperative radiation exposure reduction. One manner by which navigation systems possibly reduce radiation dosage is by allowing surgical team members to exit the operating room during image acquisition and registration.18 This is especially true when preoperative images are used, or when CT-guided intraoperative imaging systems such as isocentric 3D C-arms and O-arms are used. Regarding measured intraoperative exposure, multiple studies have demonstrated significant, quantifiable reductions in radiation exposure to the surgeon and operative room staff when navigation is used compared to nonnavigated procedures.10,18,19,20,
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