Cervical Techniques with Image Guided Spinal Navigation

Iain H. Kalfas

Section of Spinal Surgery, Department of Neurosurgery, Cleveland Clinic Foundation, Cleveland, Ohio, USA

Image-guided spinal navigation is a computer-based surgical technology that was developed to improve intraoperative orientation to the unexposed anatomy during complex spinal procedures [12, 17]. It evolved from the principles of stereotaxy which have been used by neurosurgeons for several decades to help localize intracranial lesions. Stereotaxy is defined as the localization of a specific point in space using three-dimensional coordinates. The application of stereotaxy to intracranial surgery initially involved the use of an external frame attached to the patient's head. However, the evolution of computer-based technologies has eliminated the need for this frame and has allowed for the expansion of stereo-tactic technology into other surgical fields, in particular spinal surgery.

The management of complex spinal disorders has been greatly influenced by the increased acceptance and use of spinal instrumentation devices as well as the development of more complex operative exposures. Many of these techniques place a greater demand on the spinal surgeon by requiring a precise orientation to that part of the spinal anatomy that is not exposed in the surgical field. In particular, the various fixation techniques that require placing bone screws into the pedicles of the thoracic, lumbar and sacral spine, into the lateral masses of the cervical spine and across joint spaces in the upper cervical spine require 'visualization' of the unexposed spinal anatomy. Although conventional intraoperative imaging techniques, such as fluoroscopy, have proven useful, they are limited in that they provide only two-dimensional imaging of a complex three-dimensional structure. Consequently, the surgeon is required to extrapolate the third dimension based on an interpretation of the images and a knowledge of the pertinent anatomy. This so-called dead reckoning of the anatomy can result in varying degrees of inaccuracy when placing screws into the unexposed spinal column.

Several studies have shown the unreliability of routine radiography in assessing pedicle screw placement in the lumbosacral spine. The rate of penetration of the pedicle cortex by an inserted screw ranges from 21 to 31% in these studies [6, 9, 20]. The disadvantage of these conventional radiographic techniques in orienting the spinal surgeon to the unexposed spinal anatomy is that they display, at most, only two planar images. While the lateral view can be relatively easy to assess, the anteroposterior or oblique view can be difficult to interpret. For most screw fixation procedures, it is the position of the screw in the axial plane that is most important. This plane best demonstrates the position of the screw relative to the neural canal. Conventional intraoperative imaging cannot provide this view. To assess the potential advantage of axial imaging for screw placement, Steinmann et al. [19] used an image-based technique for pedicle screw placement that combined computed tomography (CT) axial images of cadaver spine specimens with fluoroscopy. This study demonstrated an improvement in pedicle screw insertion accuracy with an error rate of only 5.5%.

Image-guided spinal navigation minimizes much of the guesswork associated with complex spinal surgery. It allows for the intraoperative manipulation of multiplanar CT images that can be oriented to any selected point in the surgical field. Although it is not an intraoperative imaging device, it provides the spinal surgeon with superior image data compared to conventional intraoperative imaging technology (i.e. fluoroscopy). It improves the speed, accuracy and precision of complex spinal surgery while, in most cases, eliminates the need for cumbersome intraoperative fluoroscopy. This chapter will focus on its use in the cervical spine.

Principles of Image-Guided Spinal Navigation

The use of an image-guide navigational system for localizing intracranial lesions has been previously described [1, 2]. Image-guided navigation establishes a spatial relationship between a preoperative CT image data and its corresponding intraoperative anatomy. Both the CT image data and the anatomy can each be viewed as a three-dimensional coordinate system with each point in that system having a specific x, y and z Cartesian coordinate. Using defined mathematical algorithms, a specific point in the image data set can be matched to its corresponding point in the surgical field. This process is called registration and represents the critical step of image-guided navigation. A minimum of three points needs to be matched or registered, to allow for accurate navigation.

A variety of navigational systems have evolved over the past decade. The common components of most of these systems include an image-processing computer workstation interfaced with a two-camera optical localizer (fig. 1).

Localizers Cervical Spine
Fig. 1. Image-guided navigational workstation with infrared camera localizer system.

When positioned during surgery, the optical localizer emits infrared light towards the operative field. A hand-held navigational probe mounted with a fixed array of passive reflective spheres serves as the link between the surgeon and the computer workstation (fig. 2). Alternatively, passive reflectors may be attached to standard surgical instruments. The spacing and positioning of the passive reflectors on each navigational probe or customized trackable surgical instrument are known by the computer workstation. The infrared light that is transmitted towards the operative field is reflected back to the optical localizer by the passive reflectors. This information is relayed to the computer workstation which can then calculate the precise location of the instrument tip in the surgical field as well as the location of the anatomic point on which the instrument tip is resting.

The initial application of navigational principles to spinal surgery was not intuitive. Early navigational technology applied to intracranial surgery used an external frame mounted to the patient's head to provide a point of reference to link preoperative image data to intracranial anatomy. This was not practical for spinal surgery. The current generation of intracranial navigational technology

Fig. 3. Navigational workstation screen demonstrating a paired point registration plan for the insertion of C2 pedicle screws. Three discreet bony landmarks are selected at the C2 level. In this case the lateral margins of the two C2-3 facets and the spinous process tip of C2 have been selected.

uses reference markers or fiducials that are glued to the patient's scalp prior to imaging. However, the use of these surface-mounted fiducials for spinal navigation is not practical because of accuracy issues related to a greater degree of skin movement over the spinal column [4, 5]. This is less of a problem with intracranial applications because of the relatively fixed position of the overlying scalp to the attached fiducials.

The application of navigational technology to spinal surgery involves using the rigid spinal anatomy itself as a frame of reference. Bone landmarks on the exposed surface of the spinal column provide the points of reference necessary for image-guided navigation. Specifically, any anatomic landmark that can be identified intraoperatively as well as in the preoperative image data set can be used as a reference point. The tip of a spinous or transverse process, a facet joint or a prominent osteophyte can all serve as potential reference points (fig. 3). Since each vertebra is a fixed, rigid body, the spatial relationship of the selected registration points to the vertebral anatomy at a single spinal level is not affected by changes in body position.

Two different registration techniques can be used for spinal navigation, paired point registration and surface matching. Paired point registration involves selecting a series of corresponding points in a CT or magnetic resonance imaging data set and in the exposed spinal anatomy. The registration process is performed immediately after surgical exposure and prior to any planned decompressive procedure. This allows for the use of the spinous processes as registration points.

A specific registration point in the CT image data set is selected by highlighting it with the computer cursor. The tip of the probe is then placed on the corresponding point in the surgical field and the reflective spheres on the probe handle are aimed towards the camera. Infrared light from the camera is reflected back allowing the spatial position of the probe's tip to be identified. This initial step of the registration process effectively links the point selected in the image data with the point selected in the surgical field. When a minimum of three such points are registered, the probe can be placed on any other point in the surgical field and the corresponding point in the image data set will be identified on the computer workstation.

Alternatively, a second registration technique called surface matching can be used. This technique involves selecting multiple nondiscreet points only on the exposed and debrided surface of the spine in the surgical field. This technique does not require the preselection of points in the image set although several discreet points in both the image data set and in the surgical field are frequently required to improve the accuracy of surface mapping. The positional information of these points is transferred to the workstation and a topographic map of the selected anatomy is created and matched to the patient's image set [18].

Typically, paired point registration can be done more quickly than surface mapping. The average time needed for paired point registration is 10-15 s. The time needed for surface mapping is much longer with difficult cases requiring as much as 10-15 min. With the need to perform several registration processes during each surgery, this time difference can significantly impact the length of the navigational procedure and the surgery itself [16].

The purpose of the registration process is to establish a precise spatial relationship between the image space of the data with the physical space of the patient's corresponding surgical anatomy. If the patient is moved after registration, this spatial relationship is distorted making the navigational information inaccurate. This problem can be minimized by the optional use of a spinal tracking device which consists of a separate set of passive reflectors mounted on an instrument that can be attached to the exposed spinal anatomy (fig. 4). The position of the reference frame can be tracked by the camera system. Movement of the frame alerts the navigational system to any inadvertent movement of the spine. The system can then make correctional steps to keep the registration process accurate and eliminate the need to repeat the registration process. The disadvantage of using a tracking device is the added time needed

Fig. 4. Reference frame attached to a spinous process in the surgical field. The reference frame monitors inadvertent movement of the spinal anatomy that may affect navigational accuracy.

for its attachment to the spine, the need to maintain a line of sight between it and the camera and the inconvenience of having to perform the procedure with the device placed in the surgical field. It is particularly cumbersome when image-guided navigation is used during cervical procedures.

Alternatively, image-guided spinal navigation can be performed without a tracking device [12, 16]. This involves acknowledging the effect of patient movement on the accuracy of image-guided navigation and maintaining a reasonably stable patient position during the relatively short amount of time needed (i.e. 10-20 s) for the selection of each appropriate screw trajectory. Patient movement can potentially occur with respiration, from the surgical team leaning on the table or from a change of table position. Movement associated with patient respiration is negligible and does not require any tracking even in the thoracic spine. Although movement associated with leaning on the table or repositioning the table or the patient will affect registration accuracy, it can be easily avoided during the short navigational procedure. If inadvertent patient movement does occur, the registration process can be repeated. Repeating the registration process is easiest when using the shorter paired point technique as opposed to the more time-consuming surface mapping technique.

When the registration process has been completed, the probe can be positioned on any surface point in the surgical field and three separate reformatted CT images centered on the corresponding point in the image data set are immediately presented on the workstation monitor. Each reformatted image is referenced to the long axis of the probe. If the probe is placed on the spinal anatomy directly perpendicular to its long axis, the three images will be in the sagittal, coronal and axial planes. A trajectory line representing the orientation of the

long axis of the probe will overlay the sagittal and axial planes. A cursor representing a cross section through the selected trajectory will overlay the coronal plane. The insertional depth of the trajectory can be adjusted to correspond to selected screw lengths. As the depth is adjusted, the specific coronal plane will also adjust accordingly with the position of the cursor demonstrating the final position of the tip of a screw placed at that depth along the selected trajectory. As the probe is moved to another point in the surgical field, the reformatted images as well as the position of the cursor and trajectory line will also change. The planar orientation of the three reformatted images will also change as the probe's angle relative to the spinal axis changes. When the probe's orientation is not perpendicular to the long axis of the spine, the images displayed will be in oblique or orthogonal planes. Regardless of the probe's orientation, the navigational workstation will provide the surgeon with a greater degree of anatomic information than can be provided by any intraoperative imaging technique.

The application of image-guided navigation to spinal surgery is directed by the complexity of the procedure and, specifically, by the need to 'visualize' the unexposed spinal anatomy. Image-guided navigation can be used with or without standard intraoperative imaging techniques (i.e. fluoroscopy). In either case, image-guided navigation provides the surgeon with an improved orientation to the pertinent spinal anatomy, which subsequently facilitates the accuracy and effectiveness of the procedure.

Clinical Applications

Prior to using image-guided navigation for spinal surgery, testing of the system was carried out in cadaver spine specimens. Image guidance was used to direct screws into the thoracic and lumbosacral pedicles of these specimens. The accuracy of screw insertion was assessed with plain film radiography and thin section CT imaging of the instrumented levels. All inserted pedicle screws were determined to be satisfactorily positioned [17].

The initial clinical application of image-guided spinal navigation was for lumbosacral pedicle fixation [8, 10, 12]. It proved to be an efficient and effective replacement for intraoperative fluoroscopy and was gradually applied to other spinal procedures such as thoracic pedicle fixation and anterior thora-columbar decompression and screw fixation. The use of image-guided technology in the cervical spine has also evolved. It is now used for such procedures as C1-2 transarticular screw fixation, lateral mass screws at C1, pedicle screws at C2 and C7 and anterior procedures such as transoral surgery and cervical corpectomy [3, 16, 21].

Fig. 5. Positioning of the image-guided system for a C1-2 transarticular screw fixation procedure. The camera is positioned at the head of the table in a vertical orientation. This minimizes any potential visual obstruction between the camera and the surgical field. The workstation is positioned across the table from the surgeon (left arrow). The fluoro-scopic monitor sits next to the navigational workstation (right arrow).

Fig. 5. Positioning of the image-guided system for a C1-2 transarticular screw fixation procedure. The camera is positioned at the head of the table in a vertical orientation. This minimizes any potential visual obstruction between the camera and the surgical field. The workstation is positioned across the table from the surgeon (left arrow). The fluoro-scopic monitor sits next to the navigational workstation (right arrow).

Navigational Technique

Image-guided navigation requires the acquisition of a preoperative CT scan through the appropriate spinal segments to be instrumented. The image data is then transferred to the computer workstation via optical disc or a high-speed data link. If paired point registration is to be used, three to five reference points for each spinal segment to be instrumented are selected and stored in the image data set. For most cervical procedures, the camera can be positioned at the head of the table with the navigation workstation positioned across from the surgeon. If fluoroscopy is also used, it can be positioned next to the workstation (fig. 5).

Following a standard surgical exposure, either the paired point or surface matching registration technique is performed. When the registration process has been completed, most navigational workstations will calculate a registration error (expressed in millimeters) that is directly dependent on the surgeon's registration technique. The error presented does not represent a linear error but rather a volumetric calculation comparing the spacing of registration points in the surgical field to the spacing of the corresponding points in the image data set. This figure is, at best, a relative indicator of accuracy.

A more practical method of assuring registration accuracy is the verification step. This step is typically performed immediately after completing either registration process. The surgeon places the navigational probe on a discreet landmark in the surgical field. With the navigational system now tracking the movement and position of the probe, the trajectory line and cursor on the workstation screen will, if accurate registration has been achieved, move to the corresponding point in the image data set (fig. 6a). If registration accuracy has not been achieved, the cursor and trajectory line may rest on something other than the point selected in the surgical field (fig. 6b). If this occurs to a significant degree, the registration process needs to be repeated. This step is more of an absolute indicator of registration accuracy and is a necessary step to perform prior to proceeding with navigation.

C1-2 Transarticular Screw Fixation

This procedure involves the passage of a screw through the pars interartic-ularis of C2, across the facet joint and into the lateral mass of C1. The risks of screw insertion include injury to the vertebral artery if the screw is placed too laterally or ventrally, injury to the spinal cord if the screw is placed too medially, and failure to engage the lateral mass of C1 if the screw trajectory is too ventral. The insertion of a screw on either side may be contraindicated if the pars interarticularis of C2 is too narrow. The procedure is typically performed bilaterally using fluoroscopic guidance.

The selection of the appropriate screw entry site and trajectory requires a thorough understanding of the atlantoaxial anatomy. Although fluoroscopy provides real-time imaging of the relevant spinal anatomy, the views generated represent only two-dimensional images of a complex three-dimensional anatomic region. Manipulation of the fluoroscopic unit can reduce this problem but these maneuvers can be cumbersome and time-consuming. Other disadvantages include the radiation exposure and the need to wear lead aprons during the procedure. Fluoroscopy cannot provide a view of the spinal anatomy in the axial plane. It is this axial view provided by image-guided navigation that makes it superior to fluoroscopy for spinal screw fixation procedures. The application of image-guided navigation to this procedure adds a significant layer of accuracy for proper screw placement.

The technique for applying image-guided navigation to posterior C1-2 screw fixation involves acquiring a preoperative CT scan that extends from the lower occipital region to C3. The image data is transferred to the computer workstation and can be used to create a preoperative screw trajectory plan. A proposed entry point and target can be selected at the C2 and C1 levels, respectively. The image data set can then be manipulated in multiple planes between these two points to demonstrate the position of a screw placed along the selected trajectory. In addition to a sagittal image that demonstrates the same information provided by lateral fluoroscopy, two other images are

presented. One of the images lies perpendicular to the sagittal image along the selected trajectory. It represents an orthogonal view that lies approximately midway between the coronal and axial planes through the spine. It demonstrates a second view of the selected trajectory.

An additional view demonstrates an image oriented perpendicular to the long axis of the probe and, therefore, the selected trajectory. A cursor superimposed on this image can show the position of the screw tip along the selected trajectory at millimetric increments. By scrolling through this image, the proposed position of the screw along the selected trajectory can be assessed along its entire path. While this planning technique does not assure safe screw placement intraoperatively, it can preoperatively alert the surgeon to avoid screw placement in patients with insufficient anatomy and to select an alternate approach.

Intraoperatively, the patient is positioned and the posterior C1-2 complex is exposed. A wire (cable) and bone graft stabilization procedure at the C1-2 level is performed prior to navigation and screw insertion. Performing this step first minimizes any independent motion between C1 and C2 during navigation and makes tap and screw insertion easier. If a reference frame is used, it is typically attached to the spinous process of C2.

Following placement of the graft and cable, three to five registration points are selected at the C2 level. It is not necessary to include registration points at C1. Although the spatial relationship of C1 and C2 may change between the preoperative scanned position and the intraoperative position, the ability of image-guided navigation to facilitate accurate screw placement is not significantly affected. The technical difficulty of this procedure is the accurate passage of the screw through the narrow pars interarticularis of C2. The lateral mass of C1 is a relatively large target that can be easily reached provided there is a reasonably acceptable realignment of C1 and C2 as well as an optimal positioning of the screw within the appropriate C2 anatomy. While the relative position of C1 and C2 in both the preoperative image set and in the surgical field is important, it is not critical enough to interfere with the process of image-guided navigation.

Fig. 6. a Navigational workstation screen demonstrating satisfactory verification of registration accuracy. While the navigational probe is positioned on the C2 spinous process in the surgical field, the workstation screen should show the cursor and trajectory line in a correlative position in the CT image set. b Navigational workstation screen demonstrating an unsatisfactory verification of registration accuracy. If the navigational probe is positioned on the C2 spinous process in the surgical field but the workstation screen shows the cursor and trajectory line in a noncorrelative position (i.e. not on the C2 spinous process) accurate registration has not been achieved and the registration process needs to be repeated before proceeding with navigation.

Voqager

Spine Navigation Registration Target

Track

Action Verify ♦ Trajectory

Screw Length (mm) 36

Fig. 7. Workstation screen demonstrating a trajectory for insertion of a C1-2 transarticular screw. The lower right screen shows the trajectory in the sagittal plane. The lower left screen represents an orthogonal plane lying between the axial and coronal planes. It conveys the medial-lateral trajectory. The upper left screen represents a plane that is perpendicular to the two other images. It demonstrates the location of the screw tip inserted along the selected trajectory at the indicated depth.

Two separate stab incisions are made on either side of the midline at the C7-T1 level. A drill guide is placed through one of the stab incisions, passed through the paravertebral musculature and into the operative field. A small divot is drilled at the proposed entry site in order to provide for secure placement of the drill guide. The registration process is performed at the C2 level and its accuracy confirmed using the verification step. The probe is passed through the drill guide and, as its position is adjusted in the surgical field, the images on the workstation screen will adjust accordingly to show the corresponding trajectory in two separate planes and the projected location of the screw tip in the third plane. Orientation to the correct screw position can be assessed rapidly and accurately (fig. 7). Any errors in trajectory or entry point selection can be determined and corrected by adjusting the position of the probe and the drill guide through which it passes. When the correct screw insertion parameters have been selected, the probe is removed from the drill guide and a drill inserted. A hole is drilled along the selected trajectory, tapped and the appropriate length screw inserted. The process is repeated on the opposite side.

Spine Navigation Registration Target

Track

Action Verify ♦ Trajectory

Screw Length (mm) 36

Fig. 7. Workstation screen demonstrating a trajectory for insertion of a C1-2 transarticular screw. The lower right screen shows the trajectory in the sagittal plane. The lower left screen represents an orthogonal plane lying between the axial and coronal planes. It conveys the medial-lateral trajectory. The upper left screen represents a plane that is perpendicular to the two other images. It demonstrates the location of the screw tip inserted along the selected trajectory at the indicated depth.

The purpose of the drill guide is to preserve the physical trajectory and entry point information just acquired through the navigation of that pedicle. If a drill guide is not used, it may be difficult to precisely position a drill or pedicle probe on the same point and with the same trajectory previously conveyed by the navigational probe after probe removal.

While image-guided navigation does not guarantee accurate screw placement, it does provide the surgeon with a greater degree of anatomical information than fluoroscopy alone. The addition of fluoroscopy to this navigational technique provides the greatest degree of precision to the procedure. In this case however, navigational technology significantly reduces the time of intraoperative fluoroscopic usage as it is typically used only to help position the patient preop-eratively and as a final check of the selected trajectory in the sagittal plane immediately following the navigational step.

Segmental Cl-2 Screw Fixation

As an alternative to transarticular screw fixation, segmental fixation of C1-2 can be used for managing atlantoaxial instability [11]. The procedure involves placing a screw into each of the two lateral masses of C1 and two screws down the pedicles of C2. The polyaxial screw heads on each side are then connected with rods. Although this approach potentially reduces the risk of injury to the vertebral artery during screw insertion, it does not eliminate the risk altogether. As with the transarticular technique, precise anatomic orientation is required to avoid arterial or neural injury. Image guidance can supplement intraoperative fluoroscopy in order to provide the necessary orientation for accurate screw insertion.

As with the transarticular screw fixation technique, a preoperative CT is obtained. The posterior C1-2 spine is exposed and a wire and cable fixation procedure is carried out. Registration is first performed at C1 for placement of the C1 lateral mass screws. The three registration points typically used at C1 include the midline posterior tubercle and the bilateral points marked by the junction of the small pedicle of C1 with its lateral mass (immediately above the two exiting C2 nerve roots). Once registered, the correct trajectory into the lateral mass can be displayed on the workstation screen and the screws inserted (fig. 8).

To use image guidance for inserting C2 pedicle screws, the same registration points are used at C2 as those used for transarticular fixation (the C2 spinous process and the two lateral margins of the C2-3 facet). The entry point for the screw is more lateral and the trajectory more medially oriented than for a transarticular screw. The navigation probe is placed through a drill guide onto this entry point and the selected trajectory is displayed on the workstation screen. When the correct entry point and trajectory have been selected, the probe is removed, a drill is inserted and the pilot hole is drilled (fig. 9).

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