Учебники / Computer-Aided Otorhinolaryngology-Head and Neck Surgery Citardi 2002

endoscopic sinus surgery (FESS). The senior author has now been using these systems in selected surgical cases since 1988. He has participated in the evolution of these devices from tools that were very difficult to use routinely in the operating room settings. Early systems, which had mechanical arms and unfriendly software, were cumbersome. Today, newer systems are a very solid adjunct to surgery. The technology also supports the teaching of endoscopic techniques.

Sinus surgery is a particularly good application for CAS because of the anatomical ‘‘bony box’’ within which the surgery is performed. As long as the surgery is performed within these confines, anatomical relationships will not shift as tissue is removed, and thus preoperative radiographic studies remain valid for localization and resection margins. However, once the confines of this ‘‘box’’ have been violated, prolapse of tissue and other intraoperative shifting of soft tissue may occur. Thus, for accurate evaluation during intracranial surgery or surgery within the orbit, the value of computer-assisted techniques is much more limited. When working within the confines of the sinuses, CT and CAS allow the surgeon to see beyond the area visualized by the endoscope and provide valuable additional and potentially very accurate information.

Early image-guided systems were adapted from neurosurgery [1–4]. These systems used an attached sensing arm and rigid head frames and required the patient’s head to be fixed during the procedure. Because of these rather stringent, technologically based requirements, general anesthesia and frequently a Mayfield head frame were necessary. Additionally, this rigid field restricted patient exposure and surgical access. Furthermore, the localizing probes were unwieldy, and their maneuverability within the delicate confines of the nose and sinuses was problematic, since their attached rigid mechanical arms had only two or three joints. Current systems are significantly less cumbersome and more intuitive. They also allow surgery to be performed under general or local anesthesia. This continued development of CAS systems has facilitated their overall ease of use and accounts for wider appeal.

Major complications still occur during endoscopic sinus surgery, and the surgery remains a leading source of malpractice suits against physicians. In general, the term ‘‘major complication’’ refers to skull base injury with secondary cerebrospinal fluid leak, intracranial complications, intraorbital hematoma, visual loss, other orbital injury, and death [5]. The major complication rate of functional endoscopic sinus surgery is said range from 0.41 to 1.1% [6,7]. It should be remembered that these numbers may be misleading due to underreporting by surgeons. Computer-assisted surgery was designed in part to decrease the risk of such complications [8], although this has not been definitively proven to date.

Furthermore, the role of new technology in surgery must be kept in mind. Advancing technology is important in the diagnosis and treatment of sinonasal disease, but it is not a substitute for education, study of the anatomy through cadaver dissections, or experience.

This chapter will review the indications for CAS in functional endoscopic sinus surgery (FESS) and its advantages and disadvantages. We review the current systems in use at our institution along with their time-tested strengths and weaknesses. Future and alternative navigation systems are also briefly discussed.

11.2HISTORY

Systems for stereotaxis were first used in neuroanatomy in the early twentieth century and have been used in various surgical applications for much of the subsequent century. Reports of stereotaxis for use in surgery involving the human brain appeared in the middle of the twentieth century [2,9]. Extensive progress in intraoperative stereotactic navigation has occurred since the development of CT in the 1970s. These early CAS systems were used in neurosurgery. They relied upon modified stereotactic frames that required that the patient’s head remain rigidly fixed. In the mid-1980s, reports of frameless stereotactic systems began to appear in the surgical literature. In 1988, Watanabe et al. [10] reported on their development of a frameless, three-dimensional digitizer. The hardware of this system included a localizing probe that was attached to a jointed, motionsensing mechanical arm, which sent positional information in digital format to the system computer. External fiducial markers, which were placed on the patient’s head before a CT scan and then maintained through the surgical procedure, served as a system for the correlation of patient position (relative to the mechanical arm) with the corresponding points in the CT scan data set. Before intraoperative localization, each fiducial marker was touched with the localizing probe so that a correlation between points in the real world and virtual world could be established. Future localizations were then based upon extrapolation from this registration information.

Several other similar systems have been described in the neurosurgery literature. (For further information, please see Chapter 2.)

The first CAS system for sinus surgery was developed at Aachen University of Technology in Germany by Schlondorff in the mid-1980s [2]. This system also used a digitizing arm for intraoperative localization. Zinreich and colleagues reported on a similar but more versatile frameless CAS system, known as the Viewing Wand (ISG, Mississauga, Ontario, Canada), in 1989 [2]. Early work with this device during endoscopic sinus surgery led to the creation of other more versatile sensing systems that permitted head manipulation and thus facilitated their greater ease of use during FESS [2,8].

The first reports within otorhinolaryngology of an optical tracking system for use in CAS appeared in the early 1990s. Optical tracking systems incorporate infrared light-emitting diodes (LEDs) into surgical instruments and localizing probes. A camera array, which is situated at a distance from the operating table, can then monitor the relative positions of these instruments and send this digital

information to the computer. This positional information is correlated with the stored image data to provide the surgeon with intraoperative localization. These early systems, which were designed for industrial digitization, were large and hence cumbersome. They were also very accurate and very expensive.

In addition to optical tracking, electromagnetic tracking was introduced. These early electromagnetic systems were relatively inaccurate, and the magnetic field was easily distorted by any metallic object within even moderate distances of the electromagnetic sensing and emitting devices. In an electromagnetic tracking system, an electromagnetic emitting source is attached to the patient. Electromagnetic sensors, which are incorporated into the localizing probe and surgical instruments, provides positional information [2,8].

Early CAS systems were plagued by technological limitations, but progress in their development has been rapid. Over time, the problems associated with both early optical and electromagnetic systems have been largely overcome. As a result, the use of CAS in otorhinolaryngology is growing today.

11.3INDICATIONS

It is sometimes difficult to predict preoperatively which cases would benefit most from the use of CAS [9,11]. Therefore, some surgeons advocate the use of CAS for every case [4,8,9]. However, we find that there are specific situations in which such a system is especially useful. CAS is certainly beneficial in cases with unusual or distorted anatomy. During routine, primary FESS cases, standard landmarks, such as the medial orbital wall and skull base, may be identified with certainty, but massive disease can distort or destroy these and other less consistent anatomical landmarks. In these situations, CAS helps confirm location within the sinus cavities (Figure 11.1). CAS technology is also a guide in revision cases where the usual landmarks have previously been resected, as is frequently the case in patients with diffuse polyposis (Figure 11.2). CAS is also useful in the unexpected bloody field, which is often associated with significant mucosal inflammation. In this scenario, a suction attachment to the sensing probe is ideal. On the other hand, localizing probes without suction are much less helpful. Indeed, in a relatively bloodless field where location and anatomy is more straightforward, there is little call for probes attached to forceps.

CAS is particularly helpful during surgery in the difficult frontal recess. In 1997, Kuhn described the variable pneumatization pattern of the cells in the frontal recess and its effect on intraoperative identification of the frontal sinus [12]. An agger nasi or supraorbital ethmoid cell that extends into the frontal sinus can be close to the skull base and be mistaken for the frontal sinus by endoscopic examination alone. This circumstance, combined with the vulnerability of the adjacent areas, makes it an excellent situation for the use of CAS. The triplanar (axial, coronal, and sagittal) images provided by the computer system are also

(A)

(B)

FIGURE 11.1 (A) InstaTrak screen capture shows localization in the posterior ethmoid. The sagittal CT image shows skull base erosion, which was secondary to massive polyposis and allergic fungal sinusitis. (B) InstaTrak screen capture shows localization in the frontal recess. The sagittal CT image shows skull base erosion, which was secondary to massive polyposis and allergic fungal sinusitis.

FIGURE 11.2 This endoscopic image shows a 1 1.5 cm left ethmoid roof skull base defect due to skull base erosion from a mucocele. Exposed dura (arrows) surrounded by polyps is seen. Intraoperative surgical navigation can help define the bony margins of such defects.

particularly useful in planning the surgical approach to this area. The use of 45and 70-degree endoscopes, along with a localizing sensor attached to the microdebrider, aids in dissection of this narrow space. Even in FESS cases in which CAS is employed, frontoethmoid mucoceles are opened using the standard endoscopic ethmoidectomy and frontal sinusotomy as described by Kennedy et al. [13,14]. In these cases, CAS can aid in the identification of the usual anatomical landmarks as well as assist in avoiding violation of the orbit or skull base (Figure 11).

Posteriorly, the sphenoid sinus and its surrounding structures also present a difficult and sometimes unpredictable area in which to operate. The carotid canal may be clinically dehiscent in the lateral wall of the sphenoid sinus in up to 22% of specimens, and, significantly less commonly, dehiscences may occur also in the optic canal [15]. In addition to aiding surgical confidence during surgery in this area, CAS is also useful in orbital apex surgery or during optic nerve decompression [11,16]. Additionally, CAS has also been mentioned with respect to the drainage of orbital abscesses [16].

CAS has other potential applications. Surgical navigation can confirm the location of skull base defects for accurate surgery in meningoencephalocele re-

perspective gained by the sagittal view, coupled with the axial and coronal views, completes a ‘‘three-dimensional’’ picture and significantly helps with anatomical conceptualization for the surgeon [18]. By scrolling sequentially through these three views simultaneously, the surgeon may better comprehend the relevant surgical anatomy. Indeed, in some cases the capacity of CAS to foster the surgeon’s understanding of anatomical relationships may be the most important contribution of CAS. In this regard, computer-enabled CT review may be even more helpful than instrument localization.

The orientation afforded by intraoperative surgical navigation does enhance endoscopic sinus surgery and does facilitate a more complete procedure. Potentially, this may lead to improved treatment and reduce the need for revision surgery. Although the use of CAS is not a substitute for experience, it may also facilitate timely dissection for more hesitant and tentative surgeons. Although it is reasonable to believe that computer-guided information might allow for the surgery to be performed with increased safety, this remains to be proven. However, CAS is a perfect teaching tool for residents and those learning functional endoscopic sinus surgery [4,8,18]. CAS can enhance physician education both in the cadaver lab as well as in the operating room. Finally, the database capabilities of the systems also allow for comparison studies that can contribute to research.

11.4.2 Disadvantages

The most significant disadvantage of CAS is added cost. This includes the frequent necessity for a repeat CT scan, capital equipment expenses and disposable instrument costs, additional set-up time and labor, and, possibly, increased intraoperative time. Since fine-cut CTs are required for three-dimensional reconstruction, additional x-ray exposure also occurs to the lens, although this effect may be minimal. As is the case with all complex equipment, learning how to use and troubleshoot it can take considerable time and effort. CAS carries a rather steep learning curve, but this challenge is surmountable. Even surgeons who are well versed in CAS technology may find that troubleshooting these systems is problematic. For practical reasons, calling technical support in the middle of the procedure is not a good option. Alternatively, a CAS technician in the OR may be an ideal solution for this technical challenge, but most institutions do not have the resources for this position.

CAS system complexity can be overwhelming. Obviously, the probability of difficulties and delays related to the system varies directly with the number of steps necessary in the use of the system. Failure of the system can prolong or even lead to cancellation of the surgical procedure if the patient and surgeon were anticipating its use.

Another potential problem associated with CAS systems involves the performance of the preoperative CT scan and its transfer to the CAS workstation.

sized that small incremental changes in position are often not noticeable. If system accuracy degrades due to these problems, repeat system and/or tool calibration may be necessary, but this alternative disrupts the surgical procedure and increases the operative time. Since failure to recognize the need for recalibration can lead to complications, most CAS systems have built-in functions that require that the surgeon verify system accuracy against defined checkpoints and anatomical landmarks. In addition to these software precautions, it is important that the surgeon use the usual anatomical landmarks throughout the case for repeated comparisons for the confirmation of navigation accuracy.

The surgeon should keep in mind several other precautions with regard to use of CAS stereotactic navigation for endoscopic sinus surgery. There is a significant potential for a false dependence on CAS. CAS should only be used as a guide during surgery; it should not be the major means of orientation and localization. Again, the surgeon should depend on knowledge of paranasal sinus anatomy through study and cadaver dissections. The surgeon should constantly compare actual anatomy with the accuracy of system. A false sense of confidence could lead to more aggressive surgery with its attendant complications. Furthermore, it should be remembered that successful outcomes in sinus surgery do not come just from aggressive surgery; rather comprehensive perioperative medical management, meticulous, mucosal-sparing surgical techniques, and regular postoperative surveillance all contribute to the success of FESS procedures.

Finally, current CAS systems rely on imaging information that is obtained preoperatively. This does not allow for tissue shifts or changes that occur during the course of surgery [11].

11.5CURRENT SYSTEMS

We currently use both electromagnetic and infrared tracking systems. The current electromagnetic CAS in use at our institution is the InstaTrak System (Visualization Technology, Inc., Lawrence, MA). This system uses low-frequency magnetic fields to detect the relative positions of a probe with ferromagnetic sensors. The InstaTrak software then correlates this information with patient’s preoperative CT image data and displays the calculated localization (relative to the preoperative CT scan) on the computer monitor.

The Instrak requires a special preoperative CT scan. During this preoperative CT scan, the patient wears a special headset, which contains fiducial markers. The patient also must wear an identical headset, which is disposable, during the actual surgery. Since the headset can only fit on a patient in one way, the relationship between the fiducial markers and the anatomical region of interest is maintained during the preoperative CT scan and surgery. At the time of surgery, an electromagnetic transmitter is connected to the headset. For surgical navigation, a simple probe calibration is necessary. Registration, or the correlation of points

in the operative field and the CT scan volume, is automatic, since the relative position of the headset does not change. The InstaTrak software automatically recognizes the fiducial points in the CT scan. Since the headset remains stationary in relationship to the patient, the patient’s head can be moved during the procedure for optimal surgical access.

We find the system straightforward and user-friendly. Little teaching is required in its set-up and use. Use of the system results in few interruptions during the surgical procedure. There have been occasional technical difficulties relating to the transfer of the CT data to the computer, but tending to this task prior to surgery has minimized interruption of surgery.

The InstaTrak does impose restrictions on the surgical procedure. The headset extends across the forehead to the dorsum of the nose and is somewhat uncomfortable to wear for a prolonged periods. The headset also prevents the routine use of the device in open approaches to the frontal sinus. Because the localization is based upon an electromagnetic field, metallic instruments in the operative field may interfere with surgical navigation and localization. Additionally, because the electromagnetic source is connected to the anterior portion of the headset and the headset fiducial markers are all anterior, there is the potential for loss of accuracy in the more posterior anatomical structures.

The other system we use is LandmarX (Medtronic Xomed, Jacksonville, FL). This system uses an optical digitizer, which tracks the positions of LED arrays, known as dynamic reference frames (DRFs), on surgical instruments. In this system, anatomical fiducial landmarks, which must be manually identified in the preoperative CT data set, serve as guides for system calibration. During registration, each of these points is directly correlated to the corresponding point on the patient with the localizing probe. The patient wears a headset that contains a mount for a DRF that tracks patient position. Since this DRF is attached to the patient’s head, the head may be moved if necessary.

Before surgery, the LandmarX must reconstruct the sagittal and coronal images as well as a three-dimensional model. In order to minimize delays during the actual surgical procedure, this step may be performed in advance. In comparison with the automatic registration approach of the InstaTrak, LandmarX’s landmark-based registration process is more complicated. Registration based on anatomical landmarks requires practice. Adequate registration can be more timeconsuming if multiple iterations are necessary. Clinical impressions suggest that the LandmarX offers superior surgical navigation accuracy in the posterior ethmoid and sphenoid regions. This probably reflects the greater dispersion of the fiducial points in three-dimensional space in landmark-based registration. Unfortunately, LandmarX is less intuitive and more labor intensive during routine use.

The commonly used headset for the LandmarX system is rigid and somewhat bulky. As a result, the headset can be intrusive and can limit head positioning. Additionally, the correct degree of tension when applying the headset is not