Overview of Spinal Navigation

Mark R. McLaughlin, Juan Bartolomei Neuro-Group, PA, Lawrenceville, N.J., USA

An important advance in spinal surgery has been the development and application of image-guided techniques for spinal navigation and fixation. Image-guided technology includes both spinal stereotaxis as well as fluoroscopy-based image guidance systems. Both of these techniques offer significant advantages over commonly used plain radiography and fluoroscopy for complex spine procedures. Image-guided spine surgery has been utilized for cervical, thoracic, and lumbar fixation [1, 3-5, 7, 9-11, 13, 15-17, 19-22, 24, 28, 31, 34, 35]. This chapter will highlight the newest techniques in image-guided spine surgery and discuss their advantages and nuances compared to standard open techniques.

Unreliability of Intraoperative Radiography

Traditionally, intraoperative image guidance in spine surgery was directed primarily by plain radiography and fluoroscopy. Although these conventional imaging techniques offer surgeons better visualization compared with simple open exposure and recognition of the anatomy, they still have a limited accuracy [6, 7, 14, 23, 27, 29, 35]. Intraoperative plain films and fluoroscopy lack a three-dimensional (3-D) perspective of the anatomy. In addition, imaging in the thoracic region with these two techniques is problematic related to the rib cage and difficulty in localizing the appropriate level. Berlemann et al. [2] found that only 41% of 119 thoracolumbar pedicle screws were accurately placed with plain radiography at the time of surgery. Weinstein et al. [33] found a 21% failure rate in pedicle screw placement in a cadaver study. He found that success was independent of experience or approach and that 92% of the failures were cortical perforations within the spinal canal. Odgers et al. [23] reviewed a series of 72 patients undergoing placement of pedicle screws utilizing plain radiography for image guidance. Out of a total of 238 pedicle screw placements, 24 penetrated the pedicle wall and 2 resulted in neurological injury. Other studies have also documented a significant incidence of pedicle violation with standard techniques [6, 18, 29]. Anatomical variations have been described within the cervical spine that suggest image guidance may improve accuracy of screw placement [13, 17, 18, 26, 30, 34].

Numerous techniques have been described attempting to increase the accuracy of pedicle screw placement. These include open laminar techniques, evoked and spontaneous EMG potentials to record pedicle wall breakthrough, and intrapedicular or epidural endoscopy [8, 12]. Each of these techniques has significant limitations. Although intuitively open laminar techniques seem to the safest method of placing pedicle screws, there are various anatomical anomalies that sometimes can mislead a surgeon. In addition, optimal screw trajectory is difficult to predict based solely on anatomy. Evoked and spontaneous EMG potentials have been utilized to determine pedicle wall breakthrough; however, this technique presents information to the surgeon after potential neurological injury has occurred. Intrapedicular or epidural endoscopy is problematic primarily in the thoracic spine region given the small epidural space and presence of thoracic spine cord.

Image-Guided Technology

There are multiple advantages of image guidance in complex spine surgery. This technology can determine preoperatively the feasibility of performing certain difficult spinal instrumentation procedures and can assist the surgeon in navigating surgical instruments intraoperatively in real time. This technology can improve accuracy in the placement of spinal instrumentation minimizing the risk of neurological and vascular injury. In addition to reducing potential complications, there is an added advantage of achieving optimal bone purchase at each instrumented level. Once the surgeon and operating room team are familiar with the use of image-guided techniques, operating time can be reduced compared with other conventional technologies. Lastly, intraoperative exposure of patient and surgeon to ionizing radiation can be reduced or eliminated depending on the technique used [25].

Spinal Stereotaxis

The steps involved in utilizing spinal stereotaxis include both preopera-tive data acquisition and correlation of this data with the intraoperative field. First, a preoperative CT or MRI scan is obtained with 1-mm slices at a single

Fig. 1. The Stealthstation (Medtronic Sofamor Danek) includes a high-powered computer and high-resolution monitor. The display generates triplanar images and a 3-D model of the patient's spine. After registration, the digitized instruments are passed through the operative field and are displayed on the monitor.

gantry angle. Although the patient is in a prone position during surgery, a preoperative data set is obtained while the patient is in the supine position. Although there are slight differences in position, these are not quantifiably significant with the exception of cases of severe instability. Respiratory variation during scanning or surgery is not relevant to the navigation accuracy either [10, 11, 26]. Once the data set has been acquired, a 3-D model is built on the computer (fig. 1).

The patient is positioned in a standard fashion for surgery, adequate exposure is obtained and the reference arc is then firmly attached to a spinous

Fig. 2. The reference arc is attached firmly to one of the spinous processes exposed in the operative field. Theoretically the arc should be attached at each vertebral level being registered and instrumented. We have found for short segment fixation that only one level registration yields clinically relevant accuracy.

Fig. 2. The reference arc is attached firmly to one of the spinous processes exposed in the operative field. Theoretically the arc should be attached at each vertebral level being registered and instrumented. We have found for short segment fixation that only one level registration yields clinically relevant accuracy.

process (fig. 2). It has been our experience that the spinous process is the most secure and reliable landmark for anchoring the reference arch. The arc should be placed at each vertebral level being registered and instrumented. At L5, S1 it is useful to place the reference arc on the sacral spinous process with the angled arch toward the feet. This allows for free movement of the surgeon's hands and instruments without interference of the arc. For upper lumbar and thoracic instrumentation, the arc should be placed pointing cephalad.

Once the reference arc has been secured, it must be visible to the optical camera (fig. 3). The surgeon can then register analogous anatomical points on the computer and on the patient's spine. For registration in the lumbar and thoracic regions specific landmarks include the middle posterior lateral aspect of the transverse process tips bilaterally, the inferior and superior aspect of the spinous process tips, the mamillary tubercles as the base of the transverse processes (when present) (fig. 4).

These points are easily identifiable both on the computer model as well as the patient's spine. Once the reference arc is placed and registration is complete the computer will then verify the location of the patient's spinal anatomy in relation to the operating room with submillimetric precision. Accuracy is confirmed by correlating the patient's anatomy with the computer model. Digitized instruments then can be moved within the operative field and using both passive and active light-admitting diodes (LED). Frameless navigation can be performed. Standard lumbar instrumentation equipment is available and digitized

Fig. 3. The optical camera must be positioned approximately 6 ft away from the reference arc. It must have unobstructed views of the LEDs on the reference arc and the digitized instruments.

Fig. 4. Registration is carried out by locating homologous points of anatomy between the exposed osseous anatomy and the model generated on the computer workstation. Typically the spinous processes, transverse processes, and pars intra-articularis can be identified and correlated with the computer-generated model.

including a probe-all drill guide and tap that can contact the spine. With each movement, the surgeon can look on the computer screen and visualize the instruments in relation to the patient's spine. Images are available in the axial, sagittal and coronal planes as well as a 3-D image. This allows the surgeon to see the anatomy in any of the three necessary planes. As an instrument is passed

through the surgical field, its position is projected onto the computer screen with this triplanar view.

The advantages of frameless stereotaxis were discussed previously. Instrumentation can be placed more accurately decreasing the risk of vascular and neurological injury. Operating time can be decreased as a result of increased speed of hardware placement. The exposure of surgeon and patient to ionizing radiation is also decreased because there is less need for reliance on traditional imaging techniques such as fluoroscopy or plain films.

Limitations of frameless spinal stereotaxis include the additional cost of obtaining a preoperative CT or MRI scan. Also any significant unexpected movement related to instability at the time of positioning or surgery renders this type of spinal navigation inaccurate. This is because the anatomical relationships will differ from the preoperative data set. If the patient has had prior surgery, potential anatomical landmarks used for registration may not be available. Lastly, any disruption of the reference arc will require re-registration. Complicated multilevel fusions can pose a difficult problem in avoiding any unintended manipulation of the reference arc.

Virtual Fluoroscopy

The introduction of virtual fluoroscopy has offered surgeons several advantages over frameless spinal stereotaxis. Virtual fluoroscopy combines the optical tracking technology of spinal stereotaxis with fluoroscopic images. This technique does not require any preoperative data set acquisition and allows for instantaneous registration of the patient's anatomy. Virtual instruments are then superimposed on standard fluoroscopic images and data sets can be updated instantaneously by simply obtaining a new fluoroscopic image. Imaging studies utilized as anatomic data are obtained from lateral or AP fluoroscopic intraoperative views. Only one or two images are required to obtain a complete data set. The C-arm can then be removed from the field. The FluoroNav system (Medtronic Sofamor Danek, Memphis, Tenn., USA) allows for four images to be displayed simultaneously in navigated computer-generated representations of the digitized instruments (fig. 5). The advantages of utilizing virtual fluoroscopy include minimizing the surgeon's exposure to radiation and eliminating the need for protective aprons. Also, there is an ergonomic benefit being able to stand flush against the table when placing instrumentation rather than uncomfortably leaning against the C-arm. Equipment needed for virtual fluoroscopy includes a C-arm with LED attachments and fiducial array, a computer work station compatible with most stereotactic systems, an optical camera, a reference arc, and digitized instruments (fig. 6).

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