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

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FIGURE 12.10 (A) A dehiscent medial orbital wall is seen on the coronal CT, but the endoscopic view suggests residual uncinate process. Intraoperative localization on the apparent ‘‘uncinate process’’ directly illustrates that the area of the dehiscence. (B) A dehiscent medial orbital wall is seen on the coronal CT, but the endoscopic view suggests residual uncinate process. Dissection around the dehiscence to the posterior ethmoid has been completed. In this CAS screen capture, localization to a posterior ethmoid cell is shown. The endoscopic image is a view into this posterior ethmoid cell. (LandmarX 2.6.4, Medtronic Xomed, Jacksonville, FL.) (From Ref. 16.)

Since the roof of the ethmoid defines the anatomical extent of superior dissection during endoscopic ethmoidectomy, the identification of the skull base is obviously important. Although this is a critical boundary from a surgical perspective, the ethmoid roof often does not have any distinguishing features. Previous surgery can further obscure landmarks, and the inflammatory changes, such as osteitic new bone formation, can further distort the anticipated anatomical relationship. In these instances, CAS surgical navigation can supply critical localization information.

Case 6: Orbital Dehiscence

L.M., a 60-year-old man with a history of chronic rhinosinusitis, sinonasal polyposis, and asthma, described episodic congestion, rhinorrhea, and midfacial pressure. He reported that he had received sinus surgery several years earlier; according to his recollection, the surgery was uneventful. His symptom severity seemed to be worsening, although he received aggressive medical treatment. He then underwent bilateral IG-FESS. Computer-enabled review of the preoperative CT scan revealed that the right medial orbital wall was partially dehiscent. Since he did not have a history of significant trauma, this finding was ascribed to his previous sinus surgery. Endoscopic examination of the right lateral nasal wall suggested residual uncinate process; in reality, the orbital dehiscence was mimicking the appearance of an uncinate process. During surgery, CAS surgical navigation was used to define the limits of the orbital wall dehiscence and to guide dissection around the dehiscence for access to the posterior ethmoid cells (Figure 12.10).

It is important to remember that the endoscopic view can be misleading. In this case, the orbital dehiscence was the critical finding, but the endoscopic view suggested residual uncinate process! CAS defined the extent of the orbital dehiscence and intraoperatively, the boundaries of dehiscence, was mapped by successive localizations. This case also demonstrates another critical point. Previous surgery can disrupt standard anatomy in ways that are difficult to predict. CAS can minimize this complexity if revision surgery proves necessary.

12.7INDICATIONS FOR CAS IN SINUS SURGERY

Discussions about the indications for CAS in sinus surgery have engendered considerable debate and controversy. Most surgeons will at least admit that CAS has the greatest role in the more complex rhinologic surgeries. Of course, procedural complexity reflects anatomical complexity; as a result, CAS is most useful for surgery of the frontal recess, the sphenoethmoid recess, and the posterior ethmoid region. In addition, certain conditions can obscure anatomical landmarks—mak- ing procedures, which would otherwise be simple and straightforward, especially

complex and challenging. Therefore, sinonasal polyposis, pansinusitis, and previous surgery can all be considered indications for CAS in sinus surgery.

It is important to consider those instances in which CAS is probably inappropriate. Simple, limited procedures do not require CAS. For instance, endoscopic maxillary antrostomy for isolated chronic maxillary sinusitis in a patient who has failed medical treatment but has not already undergone sinus surgery would not routinely require CAS.

The indications for CAS are relative, not absolute indications. The decision to use CAS is really at the discretion of the rhinologic surgeon, since CAS is merely a tool that the operating surgeon may select for a specific application. CAS is not a substitute for surgical expertise.

12.8LIMITATIONS

Since IG-FESS depends upon the robustness of CAS technology, surgeons must be cognizant of the limitations of CAS. With such knowledge, the surgeon may recognize CAS technical failures early and thereby avoid potentially catastrophic complications. In addition, the knowledgeable surgeon can better integrate CAS into his surgical decision-making and execution.

The limitations of CAS for sinus surgery can be divided into three catego-

ries:

1.Since current CAS platforms rely upon preoperative CT images, intraoperative surgical navigation cannot reflect the anatomical changes caused by the actual procedures. For soft tissue surgery, where intraoperative tissue deformation can be significant, this limitation may be a major problem; however, standard sinus surgery is performed within a bony box, which is the ideal environment for surgical navigation that depends upon preoperative imaging. Intraoperative MR and CT scanners may permit real-time updates for surgical navigation, but they have not been fully integrated with CAS surgical navigation. Furthermore, intraoperative scanners are cumbersome, time-consuming, and expensive. Also, the quality of the images from current intraoperative scanners is also less than the quality of comparable preoperative images, since the intraoperative scanners use faster scan sequences and thicker scan slices. The poorer resolution of these images limits the maximal surgical navigation accuracy that they can support.

2.Accurate surgical navigation depends upon precise registration. If the registration tightly maps imaging data set volume to the surgical field of the real world, then surgical navigation will be optimal. It should be emphasized that all registration protocols are less than perfect. Even registration based upon bone-anchored fiducial markers may be com-

promised by minor deviations in instrument tracking precision. Bent instruments can also degrade registration. Registration protocols also contain adaptations so that the CAS system is more useful in the clinical setting. Bone-anchored fiducial markers clearly provide the best registration accuracy, but they are unacceptable for ambulatory procedures (such as sinus surgery). As a result, alternative approaches for registration—including anatomical fiducial points, surface contour mapping, and DRF headset frames—have been developed. Although these alternatives increase the usability of the system, they all degrade registration accuracy to a certain degree. In general, this trade-off has been considered an acceptable compromise. Since registration is the critical step in intraoperative surgical navigation, all surgeons who use CAS must be able to recognize registration failures and troubleshoot registration problems.

3.Hardware failure and software bugs can compromise all CAS systems. Although these problems are becoming less common, they can occur in all systems. Most CAS platforms include diagnostics, which can help determine the source of a potential system failure. For instance, optically based systems often can display aiming information that depicts the relationship between the overhead camera array and the DRF. Software engineers and designers have modified the CAS software interfaces so that these software diagnostic tools are more helpful. Surgeons should be familiar with these aspects of the software.

Although CAS is an impressive tool for sinus surgery, CAS is not a substitute for surgical expertise. CAS merely provides additional information that can simplify complex procedures. As a result, CAS is an enabling technology. It does not radically change the nature of the surgery; rather CAS facilitates the completion of procedures according to currently accepted surgical principles. CAS does not change the details of the surgery.

12.9CONCLUSION

The image-guided functional endoscopic sinus surgery paradigm incorporates CAS technology into sinus surgery. CAS provides intraoperative surgical navigation as well as a series of software tools that support computer-enabled review of preoperative CT images. Because of the anatomical complexity of paranasal sinuses, rhinologic surgeons have begun to embrace CAS as a means of improving surgical outcomes and decreasing surgical morbidity. Clinical experiences have shown that CAS is most helpful in the frontal recess, in the sphenoethmoid recess, in the posterior ethmoid and at the roof of the ethmoid. In addition, CAS offers significant advantages in specific situations, including previous sinus sur-

gery, extensive sinonasal polyposis, and other scenarios that are characterized by a loss of surgical landmarks. CAS (namely preoperative computer-based CT review and intraoperative surgical navigation) should be routinely utilized for these more complex cases. This IG-FESS paradigm should serve to decrease surgical morbidity and increase surgical effectiveness.

REFERENCES

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5.MP Fried, J Kleefield, R Taylor. New armless image-guidance system for endoscopic sinus surgery. Otolaryngol Head Neck Surg 119:528–532, 1998.

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7.R Metson. Intraoperative image-guidance technology. Arch Otolaryngol Head Neck Surg 125:1278–1279, 1999.

8.R Metson, M Cosenza, RE Gliklich, et al. The role of image-guidance systems for head and neck surgery. Arch Otolaryngol Head Neck Surg 125:1100–1104, 1999.

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sinus surgery. Laryngoscope 108:1164–1170, 1998.

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trolled allergic disease, underlying immunological disorder, ciliary dyskinesia, and osteitis. Incomplete surgery and/or scar tissue formation may interfere with paranasal sinus ventilation and drainage and thus lead to revision surgery. The initial evaluation of postoperative patients with persistent/recurrent sinusitis should focus on potential nonsurgical etiologies. If such an etiology is confirmed, then appropriate treatment can be instituted. If systems persist despite such treatment or the search for such an etiology is inconclusive, then careful comprehensive nasal endoscopy, in conjunction with repeat CT scans, will often identify a disease focus that is amenable to revision surgery [8]. Since the normal landmarks have been altered by the previous surgery and/or persistent inflammatory disease, intraoperative recognition of critical structures by endoscopic criteria alone can be very challenging. Computer-aided surgery (CAS), which incorporates soft- ware-enabled review of preoperative images as well as intraoperative surgical navigation and localization, can provide critical supplemental information and thereby improve surgical outcomes.

Functional endoscopic sinus surgery has a reported major complication rate of 0–8% [3,12–18]. Major complications include orbital injuries with significant sequelae such as restricted gaze, altered vision, or even blindness. Penetration of the ethmoid roof may result in cerebrospinal fluid fistula, meningitis, or brain abscess. Death may result from catastrophic injury to the cavernous portion of the internal carotid artery or severe brain injury. Minor complications, including bleeding requiring packing, symptomatic synechia formation, periorbital ecchymosis and/or emphysema, and temporary epiphora occur in up to 21% of patients [3,12–18]. Several authors have reported their results with revision FESS: there does not appear to be a higher incidence of major or minor complications with these procedures (compared with primary FESS cases) [1,19,20]. To the extent that CAS facilitates the surgeon’s comprehension of intraoperative anatomy, it may be anticipated that risk of both minor and major complications should decrease.

13.2PATTERNS OF SURGICAL FAILURES

When performing revision FESS, the most important features to recognize are the alterations of normal anatomical surgical landmarks. Most commonly one finds scar tissue between the middle turbinate and the lateral nasal wall [1–4,19]. This scar tissue will contract with time and result in middle turbinate lateralization (Figure 13.1). Scar tissue may also obscure the maxillary sinus natural ostium as well as make identification of the lamina papyracea and anterior skull base difficult [1,4,6,19,21]. When there has been prior partial or complete middle turbinate resection, the orientation for the revision surgeon is significantly altered (Figure 13.2). The inferior free edge of a shortened middle turbinate remnant will be closer to the ethmoid roof when compared to an intact middle turbinate.

Failure to recognize this may lead to an otherwise avoidable injury of the ethmoid roof. Also, the position of the maxillary sinus ostium will appear ‘‘lower’’ in comparison with an intact middle turbinate. Penetration of the lamina papyracea may result if the inferior border of a shortened middle turbinate is used as a landmark during a revision maxillary antrostomy in the presence of a scarred lateral nasal wall

13.3REPORTS OF CAS IN FESS

The primary goals of surgical navigation are the reduction of complications and the improvement in outcomes as a result of early anatomical landmark recognition and increased surgical precision. With CAS, important anatomical surgical landmarks such as the lamina papyracea, middle turbinate basal lamella, sphenoid face, frontal recess, and posterior skull base can be more easily identified by correlating endoscopic and multiplanar CT images. Although it appears that CAS will allow for increased surgical precision and decreased complications, this has not been proven definitively [22–26].

In a review of the first 107 consecutive patients undergoing endoscopic sinus surgery of the paranasal sinuses and anterior skull base with CAS, Neumann et al. reported no major complications [20]. Minor complications, including persistent symptomatic synechiae (4/107) and postoperative bleeding requiring placement of packing (1/107), occurred in 4.7% of patients. Overall 81 of 107 patients were diagnosed with chronic rhinosinusitis, 78% of whom had undergone prior sinus surgery. Revision surgery was required in 4.7% of patients (5/107). CAS was utilized with an estimated accuracy of better than 3 mm in 97.2% of cases (104/107). In three patients the estimated accuracy was worse than 3 mm. In these cases, CAS was therefore not used. Since this paper was a description of an early experience utilizing new technology, long-term outcomes were not reported.

Caversaccio et al. reported their experience in 25 patients undergoing revision surgery using CAS [26]. Results were compared to a control group of 10 patients undergoing revision FESS without CAS. The reported surgical navigation accuracy was 0.5–2.0 mm. There were no major or minor complications in either group. The author emphasized the improved ability to localize important surgical landmarks with CAS and a subsequent decrease in operator anxiety when compared to patients operated without surgical navigation.

Metson et al. reported 34 surgeons’ experience with CAS in 754 patients undergoing sinonasal surgery [27]. Chronic rhinosinusitis was the most common diagnosis. There were no major or minor complications, and outcomes were not reported. In a survey of participating surgeons, 71% responded that they would utilize CAS for revision sinus surgery, while 11% would use surgical navigation in all cases.