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

predefined. Its successful application requires skill in order to balance the risk of headset slippage and inadvertent soft tissue erosion at the headset contact points. Recently, a less bulky and simplified head set has been demonstrated. The LandmarX headsets also limit access to the frontal region for open approaches.

The LandmarX’s optical tracking system requires that the overhead camera array recognize the signals produced by the DRFs in the operating field. Blocking the line of sight between the cameras and the DRFs will interfere with the system’s operation. Careful placement of instrumentation, which can be a significant problem in a crowded operating room, can minimize this problem.

Both the InstaTrak and LandmarX require specific CT scan protocols. In general, an axial CT scan at a slice thickness of 1 mm is performed for either system. As described above, the patient must wear a specialized headset if the scan will be used with the InstaTrak. Data must then be transferred to the computer workstation via a direct network connection or digital media. Data processing and transfer as well as software problems can lead to discrepancies in the reconstructed images and ultimately affect device accuracy. Of course, technical errors at each of the steps necessary for set-up of the system may result in the operating delays and may even prevent the use of either system.

11.6COST

The expense of new technology is often a deciding factor in its procurement and utilization. The average cost of a CAS system for sinus surgery today is approximately $150,000. Some of the cost could be deferred if other specialties share use of the system. On the other hand, supplementary equipment, such as modified surgical instruments, can add several thousand dollars to the package. An additional CT scan formatted for use by the specific CAS system adds to the total cost of the patient’s care. Other additional costs include added operating room time. Equipment problems can create delays that carry even additional expense.

Surgeon reimbursement for CAS in sinus surgery has been problematic. Although current definitions for procedural codes seem to support the use of Current Procedural Terminology (CPT) code 61795 [19], anecdotal evidence would suggest that third-party reimbursement of this code for sinus surgery applications has been minimal. In fact, the 2000 Health Care Financing Administration coding edit effectively prohibited the use of CPT code 61795 with other codes for endoscopic sinus surgery. Later, this coding edit was reversed, but implementation of this change has not yet been verified by common observations.

11.7THE FUTURE AND ALTERNATIVE CAS SYSTEMS

BrainLab (Munich, Germany) recently introduced a laser-based system for registration. In this approach, the computer correlates calculated surface contour data

dures while avoiding the necessity for external approaches. When used for selected cases, the benefits of CAS in FESS outweigh its disadvantages. Rhinologic surgeons are encouraged to use CAS as a guide for surgery, rather than as a substitute for surgical judgment, careful study, and cadaveric dissection experiences. Presently it is unknown if CAS can actually reduce FESS complications rates. Additional rapid developments in CAS technology may be anticipated. Surgeons should monitor these advances with significant interest, since they may offer advantages over currently accepted technology.

REFERENCES

1.Zinreich SJ, Dekel D, Leggett B. 3D CT interactive ‘‘surgical localizer’’ for endoscopic sinus surgery and neurosurgery. 76th Annual Meeting of the Radiological Society of North America, Chicago, 1990.

2.Zinreich SJ. Image-guided functional endoscopic sinus surgery. In: Kennedy DW, Bolger WE, Zinreich SJ, eds. Diseases of the Sinuses. Diagnosis and Endoscopic Management. Hamilton, Ontario: B.C. Decker Inc., 357–368, 2000.

3.Anon JB, Lipman SP, Oppenheim D, Halt RA. Computer-assisted endoscopic sinus surgery. Laryngoscope 104:901–905, 1994.

4.Fried MP, Kleefield J, Gopal H, Reardon E, Ho BT, Kuhn FA. Image-guided endoscopic surgery: results of accuracy and performance in a multicenter clinical study using an electromagnetic tracking system. Laryngoscope 107:594–601, 1997.

5.Bolger WE, Kennedy DW. Complications of surgery of the paranasal sinuses. In: Eisele DW, ed. Complications in Head and Neck Surgery. St. Louis: Mosby-Year Book, Inc., 1993, pp. 458–470.

6.Kennedy DW, Shaman P, Han W, Selman H, Deems DA, Lanza DC. Complications of ethmoidectomy: a survey of fellows of the American Academy of Otolaryn- gology-Head and Neck Surgery. Otolaryngol Head Neck Surg 111:589–599, 1994.

7.May M, Levine HL, Mester SJ, Schaitkin B. Complications of endoscopic sinus surgery: analysis of 2108 patients—incidence and prevention. Laryngoscope 104: 1080–1083, 1994.

8.Anon JB. Computer-aided endoscopic sinus surgery. Laryngoscope 108:949–961, 1998.

9.Roth M, Lanza DC, Zinreich J, Yousem D, Scanlan KA, Kennedy DW. Advantages and disadvantages of three-dimensional computed tomography intraoperative localization for functional endoscopic sinus surgery. Laryngoscope 105:1279–1286, 1995.

10.Watanabe E, Watanabe T, Manaka S, Mayanagi Y, Takakura K. Three-dimensional digitizer (Neuronavigator): new equipment for computed tomography-guided stereotaxic surgery. Surg Neurol 27:543–547, 1987.

11.Senior BA, Lanza DC, Kennedy DW, Weinstein GS. Computer-assisted resection of benign sinonasal tumors with skull base and orbital extension. Arch Otolaryngol Head Neck Surg 123:706–711, 1997.

12.Owens RG, Kuhn FA. Supraorbital ethmoid cell. Otolaryngol Head Neck Surg 116: 254–261, 1997.

13.Kennedy DW, Keogh B, Senior B, Lanza DC. Endoscopic approach to tumors of the anterior skull base and orbit. Oper Tech Otolaryngol Head Neck Surg 7:257– 263, 1996.

14.Kennedy DW. Functional endoscopic sinus surgery: Technique. Arch Otolaryngol 111:643–649, 1985.

15.Kennedy DW, Zinreich SJ, Hassab MH. The internal carotid artery as it relates to endonasal sphenoethmoidectomy. Am J Rhinol 4:7–12, 1990.

16.Anon JB, Klimek L, Mosges R, Zinreich SJ. Computer-assisted endoscopic sinus surgery. An international review. Otolaryngol Clin North Am 30:389–401, 1997.

17.Carrau RL, Snyderman CH, Curtin HB, Weissman JL. Computer-assisted frontal sinusotomy. Otolaryngol Head Neck Surg 111:727–732, 1994.

18.Mann W, Klimek L. Indications for computer-assisted surgery in otorhinolaryngology. Computer Aided Surg 3:202–204, 1998.

19.Current Procedural Terminology (CPT 2000). Chicago, IL: American Medical Association, 1999.

20.Fried MP, Topulos G, Hsu L, Jalahej H, Gopal H, Lauretano A, Morrison PR, Jolesz FA. Endoscopic sinus surgery with magnetic resonance imaging guidance: Initial patient experience. Otolaryngol Head Neck Surg 119:374–380, 1998.

It is important to remember that the impact of CAS on surgical techniques is very difficult to quantify. In fact, it is difficult even to describe that impact. Even those surgeons who have positive experiences with CAS probably will admit that conveying that experience to other physicians is a challenging task.

At this point, the need for further descriptions of the technology is minimal. Instead, reports should highlight the positive impact of CAS on surgical decision making and execution. This shift is critical, since CAS for otorhinolaryngology is at a critical point. The simple fact is that most sinus surgeons do not use CAS; these individuals will require convincing reports as justification for their adoption of CAS. The vendors and designers of CAS platforms also need additional guidance from surgeons; these engineers know how the systems work on a mechanistic level, but they do not know how surgeons use these systems or how these systems can be improved so that surgeons can use them more effectively. In addition, CAS is expensive technology, and hospital and surgery center administrators are requesting objective evidence of the benefits of CAS. Similarly, managed care companies and governmental regulatory agencies also have begun to ask appropriate questions about the cost-effectiveness of this technology. These demands are significant, and descriptions of an interesting technology will be insufficient for these audiences.

This chapter proposes an approach for the integration of CAS into functional endoscopic sinus surgery. Strategies for the adaptation of CAS for sinus surgery will be presented. Specific cases are presented, and the impact of CAS in these cases is demonstrated. In these discussions, the advantages of CAS should be self-evident. This chapter begins to answer the CAS critics who propose that CAS is an expensive novelty with little clinical use; this chapter also provides information for individuals who are independently trying to reach their own conclusions about CAS. Admittedly, this chapter will probably fall short of its lofty goals. At the very least it will provide a framework for further discussions, and it will engender additional debate in this area.

12.2RATIONALE FOR CAS IN ENDOSCOPIC SINUS SURGERY

Rhinologic surgeons have expressed interest in CAS technologies because of the unique challenges in rhinology. The paranasal sinuses are a complex, threedimensional space with close relationships with adjacent structures, including the optic nerve, the medial rectus muscle, the cavernous sinus, the carotid artery, etc. The patterns of paranasal sinus pneumatization can vary greatly among patients and even between sides in the same individual. The bony boundaries separating the intrasinus space from the adjacent structures may be congenitally absent. For instance, the bone of the lateral sphenoid wall may be dehiscent so that the sinus mucosa is draped directly across the optic nerve and/or carotid artery. For these reasons, minimal surgical imprecision can lead to catastrophic results.

The introduction of the rigid nasal telescopes and computed tomography (CT scan) has facilitated greater understanding of this anatomy. The patterns of anatomical relationships are better categorized, and, using nasal endoscopy and CT scans, experienced sinus surgeons can more readily recognize paranasal sinus anatomy in a specific patient. Together nasal endoscopy and CT scans have driven a fundamental shift in the strategy for the evaluation and treatment of inflammatory diseases of the paranasal sinuses; however, it must be remembered that even CT scans and nasal telescopes have inherent limitations.

Of course, CT scans offer tremendous advantages over standard plain films in the evaluation of the paranasal sinuses, since CT scans simply provide additional information in additional images, where as in plain films all of this information is superimposed in a single image. Yet standard CT scans, printed on film and reviewed on a traditional x-ray light box, provide only a finite amount of imaging information. During review of these images, the surgeon must interpret the images and mentally reconstruct ‘‘virtual image slices’’ for those areas that are between each pair of consecutive CT slices. Furthermore, the surgeon then must mentally fashion a three-dimensional model that is used for preoperative assessment and surgical planning. At the time of surgery, the surgeon then extrapolates from this model, which can only be seen in his or her personal ‘‘mind’s eye,’’ to the intraoperative anatomy. During each step of this process, inadvertent errors can compromise the ultimate precision of the process. Com- puter-enabled review of high-quality thin-cut CT images can minimize these issues.

The Hopkins rod telescopes provide brilliant illumination, and the resultant image quality is excellent. The slender profile of these instruments provides surgical access that would otherwise be impossible. It must be remembered that the endoscopic image is only a two-dimensional representation of three-dimensional anatomy. Furthermore, the endoscopic image also has a consistent amount of distortion that is intrinsic to the Hopkins rod lens systems; that is, looking through the telescope is simply not the same as looking through a window or through a standard movie camera viewfinder. The use of angled telescopes, which provide a field of view that is displaced 30o, 45o, 70o, or 120o from the main axis of the telescope, compounds this tendency for disorientation. The net result of these factors is that the views provided by the nasal telescopes are often difficult to interpret for even experienced sinus surgeons. CAS surgical navigation provides a potential solution for the problems of the nasal telescopes.

These issues also adversely effect physician education. Rhinologic surgery is fraught with potential catastrophic complications that reflect the position of the paranasal sinuses and adjacent structures as well as their variability. For this reason, physician education is of paramount importance, but learning the techniques of nasal endoscopy and CT scan review is not an easy task. Computerbased systems, to the extent that they simplify anatomical complexity, may facilitate physician education as well.

12.3IG-FESS PARADIGM

Messerklinger observed mucociliary clearance patterns directly through nasal and sinus endoscopy in live patients [10] as well as through time-lapse photography in fresh cadaveric specimens [11,12]. From these observations, it became apparent that relatively minimal swelling in the narrow mucosa-lined channels of the anterior ethmoid sinuses of the middle meatus can lead to secondary mucosal retention, which in turn leads to bacterial infection. From a clinical standpoint, the precise diagnosis of sinusitis depends on the accurate recognition of these often subtle abnormalities. Standard plain films and anterior rhinoscopy do not provide poor visualization of these critical areas; however, CT scans and nasal endoscopy permit reliable diagnosis of even mild mucosal changes. As rhinologists gained knowledge of normal mucosal mucociliary clearance and abnormal mucociliary dysfunction (i.e., the fundamental issue in the pathogenesis of sinusitis), the aim of sinus surgery shifted from the removal of ‘‘irreversibly diseased’’ tissue to the restoration of mucociliary clearance. Through the early work of Messerklinger [13] and others in Europe as well as rhinologists in Japan, this concept of functional endoscopic sinus surgery (FESS) became popular. In the mid-1980s, Kennedy introduced FESS to the United States [14,15].

The paradigm of image-guided functional endoscopic sinus surgery (IGFESS) attempts to integrate CAS technology into functional endoscopic sinus surgery [16]. Principles of this approach include:

The goal of sinus surgery that targets medically refractory chronic rhinosinusitis is the restoration of normal mucociliary clearance. The normal patterns of mucociliary clearance are well known, and nasal endoscopy provides direct visualization for diagnosis as well as surgical manipulations. The surgery is finesse-based, rather than destructive. IG-FESS extends these basic concepts of standard FESS.

CAS surgical navigation systems should provide more than simple localization; that is, they should not be used as mere ‘‘point-and-hunt’’ devices. Contemporary sinus surgery is derived from sophisticated principles, and the integration of new technology should be done in a sophisticated manner. Violation of this principle violates the fundamental core of functional endoscopic sinus surgery.

CAS computer workstations provide powerful platforms for the review of preoperative CT data. Through software-enabled review, surgeons can develop a more accurate understanding of three-dimensional anatomical relationships. Similarly surgical planning is facilitated by the use of the computer workstation.

Intraoperatively, the surgeon can relate the preoperative imaging to the intraoperative anatomy. In this way, the surgeon can directly implement his surgical plan.

The CAS surgical navigation system should be used throughout the entire procedure. In order to facilitate this, surgical instruments, including curved aspirators, pointers, forceps, and even microdebriders, should be modified so that their positions can be tracked through surgical navigation. The designs of these modified instruments should facilitate their use. Bulky or otherwise unworkable instruments not only serve to frustrate the operating surgeon; their poor design adversely effects the potential impact of CAS in FESS (Figure 12.1).

Through these principles, IG-FESS moves CAS from a means of confirmation to a critical method for the completion of the surgical procedure.

The term ‘‘image-guided functional endoscopic sinus surgery’’ obviously builds upon the widely accepted term ‘‘functional endoscopic sinus surgery.’’ One must not dismiss this as trivial wordplay. First, early reports on surgical navigation for sinus surgery in the United States and Japan, commonly talk about ‘‘image-guided surgery’’ (IGS); in rhinology, the IGS terminology still predomi-

FIGURE 12.1 Proper instrument design is critical for the successful integration of CAS instrumentation into routine endoscopic sinus surgery. Optically based surgical navigation requires line-of-sight between the instrument tracking platform and the overhead camera array. A simple post for the tracking platform is inadequate; instead, the post should include an offset (A) so that line of sight can be maintained during surgery (B).

nates, although the term ‘‘computer-aided surgery’’ has gained considerable international exposure through the actions of the International Society of ComputerAided Surgery (ISCAS) [17]. IG-FESS refers to this heritage of IGS. In addition, the IG-FESS term draws attention to the importance of images—which really guide the surgery. In IG-FESS, these images come from multiple sources. Clearly, the nasal telescopes provide a direct view of the operative field. The CAS system generates images that simulate a virtual world. Both the CAS-gener- ated images and the endoscopic images direct the surgery.

12.4COMPUTER-AIDED CT IMAGE REVIEW

IG-FESS incorporates not only intraoperative surgical navigation, but also the image review features of CAS systems. CAS software generally include a variety of tools:

Simultaneous view of the axial, coronal, and sagittal images in the three orthogonal planes through a given point

Coronal and sagittal image reconstruction 3D model reconstruction

3D model cut-view reconstruction Distance-measurement tools

CT window and level adjustment

These software tools provide technological means for the review of preoperative images in ways that the standard observation of the CT films on a radiology light box cannot. A standard sinus CT (namely 3 mm coronal slices, printed on regular x-ray film) provides a finite number of images. Anatomical structures between the scanned slices can only be inferred from the information captured on the surrounding slices. In contrast, CT scans for CAS are obtained at much

FIGURE 12.2 These series of images serve as example of the importance of CAS workstation review of preoperative CT scans in complex revision sinus surgery cases. (A) The standard coronal image depicts bilateral frontal sinus opacification as well as hazy new bone formation (star) and dehiscence of the right frontal sinus floor (arrow). (B) The midline frontal sinus seems well aerated; the reasons for the superior frontal sinus opacification are unclear. (C) The CT scan images of the three orthogonal planes through a point in the right frontal recess is shown. The sagittal reconstruction image demonstrates that complete dissection of the frontal recess has already been completed and that the inferior frontal sinus and frontal recess are both aerated and healthy; however, new bone formation separates the superior frontal sinus, which is opacified, from the inferior sinus and its normal outflow. (D) The findings on the left side are similar. The images in A and B are partial screen captures from the SAVANT (CBYON, Palo Alto, CA). C and D show screen captures from SAVANT (CBYON, Palo Alto, CA).