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Similar to the use of frameless stereotaxis, the patient is positioned in a standard fashion and the relevant anatomy is exposed. Again, the reference arc is attached to the spine, and lateral, AT or oblique fluoroscopic images can be obtained depending on the surgeon's preference. The initial image is demonstrated with an overlaid array of spherical fiducials, which are called the calibration target. The anatomy is then instantaneously registered and the computer combines the known geometry of the calibration target with the fluoroscopic image. Signals are sent from LEDs on the reference arc, instruments, and C-arm to the optical camera, which then transmits this data to the computer. The computer calculates the anatomy of the spine relative to the C-arm. No point for point registration is required.

Fig. 7. Fluoro to CT registration. Through computer deformation algorithms, the software merges the selected vertebral body with the fluoroscopic images. This minimizes extensive tissue dissection and it becomes convenient when there are no bony anatomical landmarks (i.e. postlaminectomy) for registration.

Fig. 7. Fluoro to CT registration. Through computer deformation algorithms, the software merges the selected vertebral body with the fluoroscopic images. This minimizes extensive tissue dissection and it becomes convenient when there are no bony anatomical landmarks (i.e. postlaminectomy) for registration.

The surgeon can choose any instrument needed for pedicle screw placement and a single universal instrument handle is attached. The software system projects to the computer screen a virtual instrument in relation to fluoroscopic image of the spine.

Various instruments can be projected onto the screen. As the instruments move in real time with surgeon manipulation, the changes are replicated instantaneously on the computer screen. The surgeon is thus aware of the depth and specific location of the instrument at all times. Distances such as pedicle length can be calculated and screw angulation for optimal screw trajectory can be determined with simple movements of the hand.

Because virtual fluoroscopy relies on fluoroscopic images, it is most useful in navigating bony anatomy. Tumor resections and soft tissue abnormalities are not suited to virtual fluoroscopy but are best suited for standard frameless stereotaxis. Patients with 3-D or coronal deformities are also unsuitable candidates for virtual fluoroscopy and would be better suited for frameless stereo-tactic techniques as well.

Another spinal navigational system has recently gained popularity with the advancement in fluoro to CT registration technology in which the two fluoroscopic images are used to noninvasively merge into the preoperative CT image data set (Vector Vision, Brainlab, Germany) (fig. 7). A preoperative CT scan with the standard image-guided protocol is necessary prior to using the fluoro to CT registration technology. This protocol consists of 1-mm axial cuts incorporating one or two levels above and below the area of interest or the surgical field where instrumentation is going to be placed. The images are then transported via ether net or zip drive into the working station where a 3-D image is created. With the 3-D image operative planning can be performed such as identification of entry and target points for pedicle, transarticular, and lateral mass screw insertion or biopsies.

Fig. 8. Computer screen displaying (clockwise) 3-D image, lateral fluoro image, axial CT, and Auto Pilot view while inserting a pedicle screw.

Once the images are transformed into a 3-D rendition, surgical exposure is obtained in a standard fashion and the reference array is clamped to the spinous process. A new device is also available that allows the reference array to be placed anteriorly into the vertebral body if an anterior surgical approach is desired. This has been described previously with other systems [32]. By adjusting the flexible joints, the reference clamp can be positioned without obstructing the surgical field. Once the array is secured AP and lateral fluoroscopic images over the area to be instrumented are obtained. There are three methods for registration. Like other systems there is the paired point registration in which anatomical landmarks are identified and registered on both the spine and on the CT images, and there is the surface matching algorithm in which ten or more points are rapidly collected on the surface of the spine and registered to the CT images. As mentioned above, unique to the Brain Lab system is the fluoro to CT registration.

Fluoro to CT registration is performed by matching on the computer screen the vertebral body of interest in a spine model to the AP and lateral films obtained. Once the vertebral bodies are identified and matched, it takes approximately 5min for the software to register and have a navigational 3-D image. Once registered, the software allows the surgeon to navigate in 3-D, CT, and fluoro images or a combination of all by adjusting the computer screen (fig. 8).

When targeting a specific end point such as a lesion biopsy or placement of a pedicle screw, the Brain Lab provides a software called Auto Pilot view that

Fig. 9. Auto Pilot view. Concentric circles form a tunnel which provides immediate directional feedback for pedicle screw insertion or biopsy guidance.

enables safe navigation and instant feedback. The tool consists of a series of aligned concentric circles forming a tunnel on a direct path to the target. The first concentric circle is the entry point and the target point is the smallest black circle at the end of the tunnel. If during navigation there are any deviations from the trajectory, the tunnel will curve as the last and first circle become noncongruent. The software provides immediate feedback providing distance to end point or any compensatory changes in direction (i.e. angle) necessary to safely reach the desired target. This tool can be used on difficult cases were the anatomy might be distorted such as in scoliotic patients or when performing percutaneous pedicle screw insertion where visible landmarks are not available (fig. 9).

The Brain Lab system allows the surgeon to adapt any desired instrument into the image-guided field. This is performed by clamping a passive marker array to any instrument (i.e. penfield, pedicle finder, tap, or awl) and inserting the tip of the instrument into the calibration matrix box. This allows any instrument to be utilized during the surgery.

Conclusions

Image-guided spine surgery enhances a surgeon's ability to navigate instruments and spinal instrumentation throughout the entire spine. These techniques not only minimize morbidity related to instrumentation, but also optimize screw purchase, decrease operating time, and minimize patient and surgeon exposure to ionizing radiation. Frameless stereotactic techniques offer certain advantages in the 3-D navigation of the spine. This technology is particularly helpful in the placement of C1,2 transarticular screw fixation and in patients with tumors infiltrating the spinal column. Virtual fluoroscopy, which is a combination of computer-assisted stereotaxis with C-arm fluoroscopic guidance, allows a surgeon to manipulate instruments within a surgical field while viewing a virtual image of the instrument in real time on a computer screen. Both of these techniques represent significant advances in spinal stabilization and will most definitely advance future minimally invasive percutaneous techniques. One remaining challenge in spinal navigation is improving registration accuracy when the spinal alignment is deformed during surgery. Also, severe spinal deformities tend to pose significant obstacles when trying to register the images using fluoroscopy. Innovative advances using fluoro to CT merge registration technology and the 3-D fluoroscopy units appear to provide new horizons for the improvement of real time registration and safe navigation.

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Mark R. McLaughlin, MD Neuro-Group, PA, 123 Franklin Corner Road Lawrenceville, NJ 08648 (USA)

Tel. +1 609 895 8898, Fax +1 609 895 8330, E-Mail [email protected]

Haid RW Jr, Subach BR, Rodts GE Jr (eds): Advances in Spinal Stabilization. Prog Neurol Surg. Basel, Karger, 2003, vol 16, pp 84-95

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