Over the past decade, image-guided surgical navigation has been welcomed as a technology in both primary and revision sinus surgery. The anatomic aid provided by navigation system is very important for the surgeon. It facilitates better intraoperative orientation and enables more thorough eradication of diseased tissue, especially in cases of extensive polyposis, revision surgery and neoplastic sinonasal disease. Due to the delicate neighboring structures in close proximity to the orbits, vessels and base of the skull, a variety of major surgical complications, including loss of vision, diplopia, injury to the internal carotid artery, cerebrospinal fluid leak and brain damage may occur when the surgeon is unaware of which anatomical areas are dangerous (5,30). The demand for safety can be particularly challenging in cases of anatomical complexity, mostly in patients with extensive sinus disease or requiring revision surgery. With the assist of this modern technology, surgeons may prevent accidental damage to vital structures in proximity to the surgical area by monitoring the real-time position of surgical
instruments, especially in cases in which there is anatomical complexity or intraoperative heavy bleeding (6,14,25,31).
The tracking modalities, registration methods and navigation locations all influence system accuracy. Hence, exact knowledge about the navigation systems under specific conditions is required for a confident use of CAS. Two main types of navigation modalities are in general use in current clinical practice. First modality, optical
navigation systems use active light-emitting diodes or passive reflecting spheres that are mounted on the navigational instruments and the reference headset to detect the
movement of the instruments by triangulation of an infrared camera system. Many studies have shown that optical navigation systems possess a high degree of accuracy for clinical use (11,18,23,26). However, a major drawback of the optic navigation system is that
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no obstacle can be allowed between the infrared camera system and the navigational instruments (23). Restated, continuous navigation may frequently be disturbed due to a blockage of the direct line of sight. Second modality, electromagnetic navigation systems do not need to keep the direct line of sight between all components because a different tracking technique is employed. While moving, the coil arrays embedded in the trackers can sense the change in the electromagnetic field that is constructed by an emitter. As a result, an electromagnetic tracking system can offer continuous
localization information on the monitor without interruption throughout the operation and is superior to an optic tracking system in a cluttered operating field (27,32). Currently, optical tracking systems are popular because of their availability and relative accuracy
(16,33). Numerous studies have shown that these systems achieve superior spatial
resolution and accuracy compared to electromagnetic tracking systems (1,26,34). However, electromagnetic tracking systems have improved the accuracy by means of advances in technology and have become more popular in clinical use (17). Nowadays, optical and electromagnetic tracking systems have both come to be accepted as suitable for CAS
(1,16,19).
Using the same electromagnetic navigation system (Medtronic S7®) as our study, Kral et al. (26) reported that the mean NE of electromagnetic tracking systems in human cadavers was 0.37 mm, which was more precise than the results in our study (0.8 - 2.1 mm). This may be explained by the use of externally fixed fiducial markers for
paired-point matching registration and immediate measurement following registration in Kral’s study. Furthermore, unlike cadavers, intraoperative stretching of the skin/soft tissue may occur easily and result in a displaced reference frame, which appears to have been the case in our study (16). The fixation of an endotracheal tube on the patient’s face with adhesive tape may also have contributed to the distortion of the facial contours and
caused surface registration discordance in the preoperative CT images. All of these factors influence the navigational accuracy during surgery. In this study, we measured the NE during the course of surgical procedures on patients diagnosed with severe sinus diseases. In contrast to the optimized conditions of laboratory settings, in clinical practice, the conditions tend to reduce system accuracy, especially when long periods have elapsed since surface registration. It may be appropriate to repeat surface
registration in consideration of intraoperative anatomical drift to overcome the increased NE in longer operations (7,32). However, we performed surface registration just once, at the beginning of surgery, and defined the NE value intraoperatively to evaluate whether it remained acceptable at the time the ESS was completed.
Although surface registration has been shown to be less accurate than the other registration methods in cadaveric heads, the errors are still within that which is typically deemed the acceptable ranges for navigation (25,35). To our knowledge, this is the first study to measure the NE of the surface registration obtained with an electromagnetic system in live surgery. The results show that the NEs in the 6 location groups have a maximal mean value of 2.1 mm; such variation may be important to surgeons if they, for example, want to assess the anatomical position in relation to the optic nerve or carotid artery. Although there is general agreement that the navigation system may be used for clinical purposes when the error is less than 3 mm (13,36), the acceptable range of the error may still depend on the location at which the operation is being carried out.
Regarding the additional time (mean 218 s) for equipment set-up and surface registration, only a small proportion (3.2%) of the total operation time was spent on system preparation. That is, the extra procedures of preparation for navigation surgery are not time-consuming.
Furthermore, our results show that the NE in the ML direction had a significantly
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higher accuracy than measurements in the AP and CC directions. A possible explanation is that the more dispersed registration points resulted in a more accurate anatomic localization. Because the extent of the divergence in tracing the paths is greater in the ML direction than in the AP and CC directions, the NE data consequently show the reading to be more precise on the ML axis. In addition, there is the possibility that depressed skin/soft tissue during the course of surface registration may shift the surface registration points and subsequently result in erroneous positional information, especially in the AP direction. Most importantly, before CAS may be used with confidence in sinus surgery, the surgeon has to keep in mind that the NEs in any of the axes may affect his or her judgment. As the dissection approaches an important anatomical structure, the users should be aware that the navigational instrument may have touched the critical site, in which case its location may not be correctly displayed on the monitor. Because the navigational accuracy is less reliable in the AP and CC directions than in the ML direction, dissections near the skull base are of greater risk than those closer to the lamina papyracea. In addition, sinus surgeons in training tend to have an excessive level of trust in the navigation system (37); such an excessive reliance on the navigation system can lead to a neglect of personal anatomical knowledge on the part of the surgeon and result in needless surgical complications. Although substantial but acceptable NE levels were confirmed in this study, it emphasized that CAS is only a useful adjunct to surgical experience and anatomical knowledge when it is applied properly (37).
In the second part of our study, the comparisons of the optic and electromagnetic navigation systems were conducted. In order to eliminate the need for the time-consuming application of the landmark and fiducial marker matching registration, a surface contour-based registration technique was proposed many years ago (22,23,25).
Although surface registration was shown to be less accurate as compared to the other registration methods (22,23,35), the NEs using the former registration for both the optic and electromagnetic navigation systems are still within the range that are typically deemed sufficient for clinical purposes (13,35,36). Because of small amount of registration time relative to the entire operation time (1.58% in the optically navigated surgery and 0.98%
in the electromagnetically navigated surgery), surface registration offers a great deal of practical convenience in navigation preparation. Although surface registration costs significantly less time in the electromagnetic navigation group than the optic one, the entire surgical times in both groups were similar.
To the best of our knowledge, this is the first report to measure NEs using both optic and electromagnetic navigation systems on the same patient in live surgery. The results show that the NEs in the 6 measured areas have maximal mean values of 1.7 mm in the optic navigation group and 1.4 mm in the electromagnetic navigation group. This finding can be taken as acceptable accuracy for clinical practice. Using the same navigation system as ours, Kral et al. (26) reported both the optic and electromagnetic navigation systems have excellent sub-millimetric accuracy in the case of anatomical specimens from an experimental setup. However, the optical tracking was reported to be significantly more precise than the electromagnetic tracking. In contrast, the NEs measured in our study demonstrated that the accuracy of electromagnetic tracking is comparable to that of optic tracking for live navigated endoscopic procedures. This may be because recent hardware and software advances in the electromagnetic navigation system have improved the system accuracy, which was initially highly influenced by ferromagnetic distortion (17,18). Furthermore, the advantage of the admirable accuracy of the optic tracking system when used under optimized laboratory conditions is somewhat offset in the course of live surgery, leading to a certain loss of precision. The surface
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registration method in this study also played a role in a decrease in accuracy, because either a displacement of the reference frame or distortion of the facial contour caused by intraoperative stretching of the skin/soft tissue may result in a discordance between the facial contour and the preoperative image (22,23,25). Consequently, our comparative study of these two systems used in the same patients provides objective evidence that both systems are similar in terms of accuracy, no matter which location of the NEs was measured. Together with previous reports in the existing literature (1,6,22-26,32), our results and experience provide a referential basis that should prove to be informative for a surgeon or institution in the selection of a navigation system.
Furthermore, in terms of both optic navigation and electromagnetic navigation, our results show that the NEs in the ML direction had a significantly higher precision than NEs in the CC and AP directions. The reason for the difference lies mostly in the fact that a more precise correlation between the pre-operative image and intra-operative anatomy depends on the collection of more widely dispersed registration points, regardless of the modality of the navigation system. Because the extent of the
divergence in tracing the points of the facial contour is greater for the ML axis than the CC and AP axes, it is more precise when analyzing the navigational accuracy in the ML direction. In addition, a shift of the patient’s skin/soft tissue, mostly in the AP direction, or changes in the tension in the muscles of expression during the course of surface registration may also play a role in an invalid data set correlation for CAS, especially in the AP direction. Since the measurement has even less precision on the CC and AP axes, the surgeon has to keep in mind that dissection near the skull base presents greater risks than when performed closer to the lamina papyracea. A thorough knowledge of the complex anatomy and surgical techniques, instead of just relying on the navigation system, remains essential for performing safe endoscopic sinus surgery (12,37), even
though this study has confirmed that the NEs in both systems are quite acceptable in terms of providing assistance during the surgery.
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