The correlation of preoperative imaging studies to a patient’s anatomy during spinal surgery is essential to good outcomes in spinal surgery. Using Intraoperative plain radiographs have been used for years to properly relate the correct level, side, and location of pathology to the patient’s anatomy during an operative procedure. In recent years, three-dimensional imaging technology, such as computed tomography (CT) has been obtained in the operating room and linked with real-time tracking of surgical instruments to provide precise intraoperative mapping of instruments. First developed for cranial procedures, intraoperative image-guided surgical navigation has established itself as a useful tool for cranial-based surgery, especially tumors. There are, however, several differences between cranial and spinal surgery that make the use of image guidance in spinal surgery more difficult. In image-guided cranial surgery, the patient’s skull is rigidly affixed to the operating table, and the relationship between the skull and the reference arc or array is immobile. By contrast, most spinal surgery is done with the patient in the prone position resting on a mobile abdomen and thereby allowing significant movement of the spine during the operation. In addition, image-guidance accuracy during spinal surgery can also be compromised by movement between vertebrae during the operation or by surgical manipulations of the vertebrae during the operative procedure.
57.2 Patient Selection
Image-guided surgical navigation can help facilitate safer and more effective spinal surgery. The capability of three-dimensional multiplanar image manipulation inherent in most modern systems may be especially useful in identifying pertinent anatomy in cases in which the anatomy has been disrupted from a previous operation ( ▶ Fig. 57.1), distorted by deformity, or destroyed by tumor. Intraoperative image guidance is not a replacement for detailed knowledge of normal anatomy or the patient’s pathology.
Fig. 57.1 Operating room shown during image acquisition. Patient is draped in center of gantry. Camera array is at patient’s head, above computer screen.
57.3 Preoperative Preparation
In addition to identification and characterization of pathology, image-guidance techniques may be useful for preoperative planning. Preoperatively, a screw trajectory can be mapped to determine if the anatomy is favorable for screw placement (i.e., the anatomy of the C2 pars interarticularis). The optimum screw length may also be determined. Careful review of the preoperative studies may assist in determining whether there are any contraindications for screw placement, such as an aberrant vertebral artery. 1 The technology may be particularly useful in this setting because it avoids some of the problems inherent to live fluoroscopy, such as obstruction of the relevant anatomy by surgical equipment or the patient’s skull, rib cage, or scapulae.
57.4 Operative Procedure
57.4.1 Thoracic and Lumbar Spine
The pedicles of the thoracic spine are smaller and more variable in dimensions compared with the lumbar spine. Moreover, obtaining fluoroscopic and X-ray views of this region of the spine is somewhat difficult because of scapular hindrance, the costotransverse elements, and the presence and degree of deformity. Thus, in certain cases, the precise insertion of thoracic pedicle screws can be challenging. The application of lumbar pedicle screws, although commonplace, is frequently accomplished in the face of deformity or surgically altered anatomy.
The initial laboratory and clinical studies into the efficacy of spinal image guidance focused on the lumbar spine. This was in part stimulated by research which revealed that the actual lumbar pedicle screw misplacement rates using conventional techniques were higher than previously thought. 1
A typical system relies upon preoperative CT imaging, specialized surgical instruments, a dynamic reference array (DRA), an electro-optical camera array, and a computer workstation (primary system interface). Light-emitting diodes (LEDs, also known as active arrays) or reflective spheres (otherwise known as passive arrays) are attached to the surgical instruments as well as the DRA and are monitored or tracked by the electro-optical camera array. The three-dimensional locations of the arrays are measured by the optical tracking digitizer, and the information is transferred to the computer workstation to manufacture a spatial orientation of the surgical and image anatomy ( ▶ Fig. 57.1). The technique of re-creating the patient’s anatomy is a straightforward process but one that demands familiarity ( ▶ Table 57.1).
Stage | Process |
1 | Preoperative computed tomography (CT) scan is performed of the relevant spine level |
2 | Transfer of the digitized CT scan to the workstation; |
3 | Computer workstation reformats images to various spinal anatomical views |
4 | Patient registration; at each involved surgical level, three to four anatomical sites are selected on the reformatted CT images |
5 | Further registration points are selected following the surgical exposure |
6 | The dynamic reference array (DRA) is attached and fixed to the spine |
7 | Paired-point matching; registration probe touches surgical field points corresponding to the workstation monitor; to reduce registration error, surface matching can be pursued where random points on the exposed surface of the vertebral level of interest are selected |
8 | The DRA facilitates the electro-optical camera to track the vertebral position; this process allows proper registration accuracy and reduces error attributed to camera or patient movement |
9 | Registration errors are calculated by the computer workstation |
10 | Verification of system accuracy is conducted by matching virtual and real probes to the corresponding CT image and anatomical area, respectively |
11 | Three-dimensional anatomical views are generated on the workstation monitor and various trajectories and points of spinal location are illustrated |
To simplify the setup and ease of use and to avoid potential line of sight tracking issues of existing operational modes of optical array navigation systems, an electromagnetic tracking technology was introduced shortly after optical tracking systems as a different method for referencing and navigating rigid anatomical structures, such as the vertebrae, on CT, magnetic resonance imaging (MRI), or X-ray medical images.
A fiducial transmitter emitting radio frequency signals establishes three orthogonal electromagnetic fields (EMFs) surrounding the surgical volume. This transmitter is rigidly attached to the bony anatomy. Surgical instruments and the C-arm unit each have a receiver that communicates with the EMF to calculate their relative positions and orientations to each other, and are then superimposed over the X-ray anatomy in near-real-time updates on the navigation screen. These screens can be sterile draped and controlled via a touch screen in the surgical field or remotely by a surgical assistant. The C-arm unit can be removed from the surgical field.
Low-ferrous instruments and distance orientations between the sensors in the electromagnetic (EM) volume measuring up to a 42-cm radius manage potential physical barriers of EM distortion.
57.4.2 Cervical Spine
Image-guided navigation has been applied successfully for treating conditions affecting the cervical spine. The reference array can be easily applied outside of the surgical field because the head can be secured rigidly to the operating table using a standard three-pin head holder. Image guidance in the cervical spine has been shown to provide optimal trajectory and entry points to minimize patient injury that via traditional approaches would otherwise be problematic because of the small bony anatomy, variations in bone morphology, and intimate proximity—and in some cases—anomalous course of the vertebral artery to the cervical spine.
Several clinical and cadaveric studies have shown the utility of image-guidance for C1–2 transarticular screw placement and found it useful as a preoperative planning tool and intraoperative navigational device. 2, 3, 4 The technology is effective in reducing, but not eliminating, the risk of screw misplacement. New navigation algorithms are now capable of expanding the usefulness of preoperative registration of CT data sets by intraoperatively registering them via a set of X-ray fluoroscopy views for quicker alignment between patient anatomy and the image data sets. Additionally, these newer methods allow the high-resolution CT images to be correlated and updated with periodic two-dimensional X-ray fluoroscopic images to monitor changes in the anatomy or registration process. These additions allow surgeons to obviate the need for slower surface-based registration methods to the dorsal percutaneous instrumentation placement.
57.4.3 Fluoroscopy and Virtual Fluoroscopy
Fluoroscopy is currently the most frequently used radiographic imaging modality for performing minimally invasive surgery (MIS). Ease of use and instant dynamic visualization of pertinent anatomy, in large part, contribute to the popularity of fluoroscopy. As such, fluoroscopy is commonly used for surgical localization and guiding implant placement during MIS procedures; however, the fluoroscopic views generated represent only two-dimensional images of a complex three-dimensional structure. With repositioning, frequently from the AP to lateral views, a better appreciation of the surgical anatomy can be obtained. Nevertheless, these maneuvers are cumbersome and time consuming. Biplanar fluoroscopy could be used to diminish C-arm repositioning time; however, such use and equipment setup could be spatially a hindrance in the operating room and with respect to the surgical field. Other disadvantages to using fluoroscopy include significant radiation exposure, poor image resolution in certain cases, and the surgeon’s restricted access to the surgical field.
Because of the shortcomings of traditional fluoroscopy, virtual fluoroscopy was developed ( ▶ Fig. 57.2). A reference array is mounted directly to the fluoroscopic unit, which allows accuracy that is similar to three-dimensional image-guided surgery systems. Fluoroscopically based surgical navigation obviates the need for CT surface registration between the bony anatomy and the X-ray image through a process of automatic registration. It also minimizes the need of repetitive C-arm positioning. Multiple X-ray views are acquired up front, saved, and used later for real-time multiplanar spinal navigation of virtual instrument trajectories, with the ability to update the dataset to compensate for shifts in the bony spine.
Fig. 57.2 (a, b) Intraoperative images showing the use of the electromagnetic computer image to guide a Jamsheedi-like needle, which enables percutaneous pedicle access using image guidance instead of fluoroscopy.
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