The value of computer-aided (image-guided) systems for endoscopic sinus surgery.

A number of image-guided systems for endoscopic sinus surgery have been developed in recent years. [1-3] In his authoritative paper in 1998, Anon compared four types of computer-aided endoscopic sinus surgical devices: the ISG Viewing Wand (ISG Technologies; Mississauga, Ont.), the ISG Infrared Optotrak, the IGT Flash Point 5000 (Image Guided Technologies; Boulder, Colo.), and the VTI InstaTrak (Visualization Technologies, Inc.; Woburn, Mass.) [1] Anon wrote that all of these units are relatively user-friendly and that they are all accurate to within 2 mm. The VTI InstaTrak image-guided system was first described by Fried et al in 1997. [2]

In another excellent paper published in 1998, Metson et al compared the InstaTrak system with the Stealth Station system (Sofamor Danek; Memphis, Tenn.). [3] The InstaTrak system employs an electromagnetic (radiofrequency) tracking device to monitor the real-time position of a hand-held suction cannula. The tip of the cannula is depicted by the crosshairs on the multiplanar preoperative scans on the viewing monitor. [2,3] The Stealth Station is an optical-based system for neurosurgical and orthopedic procedures that has recently been introduced for endoscopic sinus surgery. The Stealth Station employs an optical (infrared) tracking system to locate the real-time position of the surgical instruments, which include powered microdebriders, on preoperative CT scans. Metson et al reported that both the electromagnetic and optical systems provide anatomic localization to within 2 mm during surgery. They noted that with InstaTrak, the presence of metallic objects in the operative field interfered with the functioning of the electromagnetic system. They also reported that with the Stealth Station, the optical system required that a clear line of sight be maintained between the infrared camera and the surgical handpiece.

Advantages

Certainly, these devices are not necessary for all endoscopic sinus procedures. But our recent experience with the InstaTrak and the Stealth Station has convinced us that both systems are exceedingly valuable in detecting potentially risky anatomic sites and in helping prevent orbital and intracranial complications during surgery. Their greatest utility is in patients who have extreme polypoid disease, advanced tumors, a distorted anatomy (figure 1), or a lack of anatomic landmarks as a result of previous surgery.

Frontal sinus surgery. In frontal sinus surgery, computer-aided navigation is most valuable for locating the true opening of the frontal sinus (figure 1) and for performing intrasinus procedures safely. The technique is particularly helpful when the surgeon is performing the newer drill-out procedures or the modified Lothrop procedure.

Ethmoidectomy. Computer-aided navigation during ethmoidectomy is most useful in identifying the skull base, particularly medially in the area of the cribriform plate (figure 2). It also allows for the identification of the lamina papyracea laterally. Thus, computer assistance during endoscopic ethmoidectomy helps prevent injury to the skull base and cribriform plate area superiorly and the orbital contents laterally.

Sphenoidotomy. These systems are also valuable during a difficult endoscopic sphenoidotomy. They allow the surgeon to identify the anterior and posterior walls, the roof (Figure 3) and the lateral wall of the sphemoid sinus. The optic nerve and carotid artery are located in the lateral wall of the sphenoid sinus. Because it is sometimes difficult for the surgeon to determine if he is actually at the anterior wall of the sphenoid sinus or if he has already entered the sinus cavity, the computer-aided system is valuable in allowing the surgeon to locate and differentiate these important landmarks.

Maxillary Sinusotomy. Computer assistance in maxillary sinusotomy is helpful in difficult revision cases and in the presence of a large Haller's cell and a hypoplastic maxillary sinus. It can help prevent injury to the orbital floor and orbital contents.

Disadvantages

There are two main disadvantages to the newer image-guidance systems. First, they are costly, and because not all hospitals can afford them, they are not available to all endoscopic sinus surgeons. Second, they usually require two separate CT scans, which contributes to inconvenience as well as cost. Separate CT scans are required for diagnostic purposes (preoperative decision-making) and for use with the computer-aided surgical system.

Other disadvantages include 1) the need for additional preoperative setup time in order to position and calibrate the system, 2) the need for more space in the operating room, and 3) with the InstaTrak, the need for a headset, which can compromise an external approach. Some of these problems will certainly be corrected as future generations of image-guidance systems are developed.

In summary, computer-aided surgical systems allow the surgeon to see, in real time, the precise location of the surgical instrument during endoscopic sinus surgery, whether it is performed with a probe, forceps, or a microdebrider. They permit the surgeon to perform endoscopic surgery with greater confidence and safety, and they are an excellent teaching tool for surgeons learning endoscopic sinus surgical techniques.

It must be emphasized, however, that no amount technology can substitute for a thorough knowledge of the anatomy of the paranasal sinuses.

From the Section of Otolaryngology, Hospital of St. Raphael, New Haven, Conn.; Southern New England Ear, Nose, Throat and Facial Plastic Surgery Group; the Section of Otolaryngology, Yale University School of Medicine (Dr. Yanagisawa) and the University of South Florida College of Medicine, Tampa, Fla. (Dr. Christmas).

References

(1.) Anon JB. Computer-aided endoscopic sinus surgery. Laryngoscope 1998;108:949-61.

(2.) Fried MP, Kleefield J, Gopal H, et al. Image-guided endoscopic surgery: Results of accuracy and performance in a multicenter clinical study using an electromagnetic tracking system. Laryngoscope 1997;107:594-601.

(3.) Metson R, Gliklich RE, Cosenza M. A comparison of image guidance systems for sinus surgery. Laryngoscope 1998;108:1164-70.