The principle of computer-aided systems is to provide the surgeon with a direct interactive link with the pre-operative CT (or MRI) images (Fried and Morrison, 1998). This is achieved by reformatting patient specific CT (or MRI) images acquired preoperatively and displaying them on the screen in the coronal, sagittal, and
axial planes. During the operation, the system tracks the position of specialized surgical instruments and identifies these positions on the preoperative images (Fig. 10.31).
At present an intraoperative accuracy of 1-3 mm can be achieved (Fig. 10.32). Anatomically, computer-aided surgery is most useful in the sites that are associated with an increased risk of causing orbital or in-tracranial complications. Relevant procedures include opening the frontal sinus when normal landmarks have been altered, clearing sphenoethmoid cells, median drainage procedures, the excision of lesions of the skull base, and pituitary surgery. Other applications include optic nerve and orbital decompression (Anon et al„ 1997). C»VO 8
View the information provided critically in the light of your clinical judgment, as it is dangerous to rely on this information alone. Always remember that you are the "pilot" and that the image-guided system cannot "fly" on its own; it is just an instrument. Year by year the technology supporting these techniques is improving. The accuracy, the speed of setting up, and the cost are all getting better (Uddin et al., 2003).
• Navigational surgery helps the surgeon build up a 3D image.
• It is a valuable teaching tool for the trainee surgeon. Disadvantages:
• Computer-aided surgery is more expensive.
• Extra time is required in the set-up.
• The operator must not rely on navigational systems alone.
• The displayed image only shows the preoperative CT scan and does not take into account the tissue that has been removed.
• Added radiation results when finer CT cuts are required.
Computer-aided surgery adds a "third dimension" by allowing the surgeon to point to a specific structure in the surgical field and view its location on the preloaded CT images on the computer monitor (Olson and Citardi, 2000). However, these systems are no substitute for a thorough knowledge of paranasal sinus anatomy as, at present, the reformatted images only help the surgeon to confirm the position and they are not reliable enough to be used on their own (Simmen, 2000). It must be remembered that they do not account for tissue that is removed during surgery as they are based on the preoperative anatomy.
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