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Depth = Length Drill Depth (mm) 1$

Screw Length (mm) 20

Screw Width (mm) 3.0

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Fig. 8. Workstation screen demonstrating navigational information for placement of a screw into the lateral mass of C1.

Fig. 9. Workstation screen demonstrating navigational information for placement of a screw into the pedicle of C2.

The process is repeated for the other side. The heads of the screws are then connected with two short rods.

Transoral Surgery

Transoral decompression of the upper cervical spine typically requires intraoperative fluoroscopy to help maintain proper anatomic orientation during the procedure. Although orientation in the sagittal plane is easy to obtain with fluoroscopy, depth and medial-lateral orientation are more difficult to assess. Image-guided technology can be used to orient the surgeon in multiple planes during transoral surgery [16, 21].

Unlike other spinal applications of image guidance, discreet registration points are not readily available during transoral surgery. In this setting, surface-mounted markers (fiducials) are applied to the patient prior to obtaining the preoperative CT. Typically, two fiducials are applied to the mastoid processes and two are applied to the lateral orbital margins or to both malar eminences. The nasal septum can also be used as an inherent registration point.

The patient is positioned in a three-point head holder. The registration process is performed prior to draping the patient using the surface-mounted fiducials. Because the registration points will not be accessible during the procedure, a reference frame is used for transoral navigation. This allows for changes in patient positioning during surgery without the need to re-register. The reference frame can be attached to the three-point head holder.

During the procedure, the probe can be placed into the site of the decompression. Reformatted sagittal, axial and coronal CT images are immediately generated providing the surgeon with a precise orientation to the pertinent surgical anatomy. In particular, orientation in the axial plane minimizes the risk of lateral deviation towards the vertebral artery during the decompression (fig. 10). If a posterior fixation is indicated following transoral decompression, the same CT image data set can be used for C1-2 screw placement.

Other Cervical Applications

There are several other applications for image-guided navigation in the cervical spine. Any procedure in which intraoperative imaging is required to improve a surgeon's orientation to nonexposed spinal anatomy can benefit from image guidance. The other cervical procedures to which image guidance has been applied include navigation during the removal of cervical neoplasms (fig. 11), the placement of anterior fixation screws for the management of nondisplaced odontoid fractures, lateral mass screw fixation in the subaxial

Fig. 10. Workstation screen demonstrating navigational information during transoral decompression (probe tip location and trajectory highlighted by arrows).
Fig. 11. Workstation screen demonstrating the use of image guidance to help localize an osteoid osteoma within the lamina and articular pillar of C7.

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Spine Navigation Registration

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Screw Length (mm) 30

Screw Width (mm)

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