Minimally Invasive, Image-Guided, Facial-Recess Approach to the Middle Ear: Demonstration of the Concept of Percutaneous Cochlear Access In Vitro

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1 JOBNAME: ajo 26# PAGE: 1 OUTPUT: Fri July 1 9:30: Otology & Neurotology 26: Ó 2005, Otology & Neurotology, Inc. Minimally Invasive, Image-Guided, Facial-Recess Approach to the Middle Ear: Demonstration of the Concept of Percutaneous Cochlear Access In Vitro *Robert F. Labadie, *Pallavi Choudhury, Ebru Cetinkaya, *Ramya Balachandran, *David S. Haynes, Michael Fenlon, Steven Juscyzk, and J. Michael Fitzpatrick *Department of Otolaryngology Head and Neck Surgery, Vanderbilt University Medical Center, Nashville, Tennessee, U.S.A.; Department of Electrical Engineering and Computer Science, Vanderbilt University, Nashville, Tennessee, U.S.A.; Department of Prosthodontics, King s College, London, U.K. Hypothesis: Image-guided surgery will permit accurate access to the middle ear via the facial recess using a single drill hole from the lateral aspect of the mastoid cortex. Background: The widespread use of image-guided methods in otologic surgery has been limited by the need for a system that achieves the necessary level of accuracy with an easy-to-use, noninvasive fiducial marker system. We have developed and recently reported such a system (accuracy within the temporal bone = mm; n = 234 measurements). With this system, image-guided otologic surgery is feasible. Methods: Skulls (n = 2) were fitted with a dental bite-block affixed fiducial frame and scanned by computed tomography using standard temporal-bone algorithms. The frame was removed and replaced with an infrared emitter used to track the skull during dissection. Tracking was accomplished using an infrared tracker and commercially available software. Using this system in conjunction with a tracked otologic drill, the middle ear was approached via the facial recess using a single drill hole from the lateral aspect of the mastoid cortex. The path of the drill was verified by subsequently performing a traditional temporal bone dissection, preserving the tunnel of bone through which the drill pass had been made. Results: An accurate approach to the middle ear via the facial recess was achieved without violating the canal of the facial nerve, the horizontal semicircular canal, or the external auditory canal. Conclusions: Image-guided otologic surgery provides access to the cochlea via the facial recess in a minimally invasive, percutaneous fashion. While the present study was confined to in vitro demonstration, these exciting results warrant in vivo testing, which may lead to clinically applicable access. Key Words: Image-guided surgery Minimally invasive surgery Otologic surgery Fiducial markers Target registration error Cochlear implantation. Otol Neurotol 26: , BACKGROUND Image-guided surgery technology has been clinically available since the mid-1980s (1). Analogous to global positioning systems (GPS), image-guided surgery facilitates intraoperative surgical navigation by linking preoperative radiographs to intraoperative anatomy. Central to the image-guided surgery process is registration: the Address correspondence and reprint requests to Robert F. Labadie, M.D., Ph.D., 7209 Medical Center East, South Tower, Vanderbilt University Medical Center, Nashville, TN , U.S.A.; robert.labadie@vanderbilt.edu Presented at the Annual Meeting of the American Otologic Society, May 1 2, 2004, Phoenix, AZ. This work was supported by Vanderbilt University Medical Center (Discovery Grant, PI-RFL) and the National Institute of Biomedical Imaging and Bioengineering (R21 EB , PI-RFL). linking of the radiographic images to the patient. To achieve high accuracy, the registration is based on fiducial markers that are identified both in the radiographs and on the patient. A mathematical transformation matrix is created to optimize the alignment of the fiducials. This same transformation matrix is then applied to all information in the radiograph, allowing an overlay of the radiograph onto the patient s physical anatomy. This information is typically presented to the surgeon via a video monitor; a pointer placed within the surgical field is linked to a cursor on the monitor to show the corresponding radiographic position in axial, sagittal, and coronal sections. Image-guided surgery is widely used in neurosurgery, where the standard fiducial is a rigidly affixed N-frame. Screwed directly into the cranium, the N-frame is secured before imaging studies are obtained and remains in place throughout surgical intervention. Such stereotactic 557 Prod. #MAO200022

2 JOBNAME: ajo 26# PAGE: 2 OUTPUT: Fri July 1 9:30: R. F. LABADIE ET AL. frames are invasive and cumbersome. However, given a life-threatening disease such as a malignant brain tumor, they are tolerated by patients. Neurosurgical studies have shown that image-guided surgery decreases operative time (2) and allows more complete resection of pathologic tissue while minimizing collateral damage (3). As applied to otology and neurotology, image-guided surgery has found limited use. Isolated case reports describe their use in patients with unusual anatomy. Utilizing a modified neurosurgical unit, Sargent and Bucholz reported on image-guided surgery for middle cranial fossa approaches (4). Raine et al. (5) used an imageguided system for split-electrode cochlear implant placement in a patient with cochlear ossification. In perhaps the most widespread use, Caversaccio et al. (6) reported their series of aural atresia repair using image-guided surgery. The reasons that image-guided surgery technology has found limited clinical application in otology/neurotology remain unclear. Hypothetically, its use has been stalled by the need for noninvasive, yet accurate, fiducial systems. To achieve submillimetric image-guided surgery accuracy, necessary to prevent damage to vital structures within the temporal bone, bone-affixed fiducial systems have been necessary. At present, less invasive fiducial systems are less accurate; skin-affixed markers achieve accuracies in the range of 1.5 mm and laser skin contouring achieves accuracies in the range of 2.5 mm (7,8). To overcome these limitations and facilitate the use of image-guided surgery for otologic/neurotologic applications, our group previously reported on the development of a unique fiducial frame that achieves submillimetric accuracy in a noninvasive fashion. This fiducial frame, dubbed the EarMark system (Fig. 1), is described in detail elsewhere (9 11). Briefly, it consists of a dental bite block that affixes to a patient s maxillary dentition. Attached to the bite block is a lightweight yet rigid frame that extends to surround the external ears, placing fiducial markers in close proximity to the temporal bone. Using our fiducial frame with a commercially available image-guided surgery system, we have demonstrated submillimetric accuracy within the temporal bone (9 11). For more than 234 target registrations, mean target registration error (TRE) was 0.76 mm with a standard deviation of 0.23 mm. Using this system, we have now begun evaluating the versatility and power of image-guided surgery as applied to otologic/neurotologic surgery. For the round of experiments reported herein, we hypothesized that, given the system s accuracy, the middle ear can be safely accessed in vivo via the facial recess using a single drill hole from the lateral aspect of the mastoid cortex. The clinical correlate of this would be a percutaneous cochlear implant. MATERIALS AND METHODS Human skulls (n = 2) were fitted with a previously described dental bite block the locking dental acrylic splint (LADS) (12,13). The LADS resembles an athletic mouthguard but is made up of three pieces instead of one. It consists of a central piece, which engages the biting surfaces of the teeth, and right and left buccal pieces, which engage the lateral surfaces of the teeth. The three pieces are attached together with screws that lock the components around the crowns of the teeth, thereby fixing the mouthpiece reliably in place while allowing it to be removed and replaced in the same position and orientation. Affixed to the LADS was the fiducial system (13) with fiducial markers placed around the temporal bone (Fig. 1). This unit (skull + LADS + fiducial frame) was then CT scanned using clinically-applicable, temporal bone algorithms (slice thickness = 0.5 mm). Upon completion of the CT scanning, the LADS was removed from the skull as it would be clinically. The CT scan was loaded onto commercially available software (Voyager, Z-Kat Inc., Hollywood, FL, U.S.A.) which is designed to allow accurate identification of the centroids of the fiducial markers (14). The skull then was transported to our image guided laboratory and the IGS system was set up. The image-guided surgery system (Fig. 2) consists of a commercially available infrared tracking system with optical triangulation (Polaris R; Northern Digital Corp., Waterloo, Ontario, Canada) driven by an image analysis and display system (Voyager, Z-Kat, Inc.) running on a 1 GHz Intel PC system (Dell, Inc.) under Linux (Red Hat, Inc.). To allow navigation during operative intervention, the operative instrument, an otologic drill, was fitted with an infrared emitter. This device is registered to the system so that the tip of the drill is tracked in real time on the video monitor. The LADS and fiducial frame were reattached to the skull. In addition, to facilitate tracking of the skull, an IR emitter was affixed to the LADS. Using the drill as a localizing probe, the positions of FIG. 1. The fiducial frame system. (A) Reference frame worn during preoperative radiographic imaging. The small amber fiducial markers shown on the horizontal bar and vertical bar are arranged to surround the surgical field of interest: the temporal bone. The frame is affixed to the maxillary dentition via a customized mouthguard, the LADS. (B) The infrared emitter worn during operative intervention. It is attached to the LADS such that it is considered a rigid extension of the fiducial frame. This arrangement allows unimpeded access to the temporal bone during surgery. Fig. 1 live 4/C

3 JOBNAME: ajo 26# PAGE: 3 OUTPUT: Fri July 1 9:30: PERCUTANEOUS COCHLEAR IMPLANTATION 559 FIG. 2. The image-guided surgery set-up, including the central computer, infrared tracker, video monitor, skull with infrared emitter, and surgical tool with infrared tracking. Shown in the photographs are the skull with infrared emitter and the surgical tool with infrared tracking. the fiducial markers on the fiducial frame were determined. Rigid registration between physical space (the mock operating room) and radiographic space (the CT scan) was performed using a previously described algorithm (14,15). This algorithm creates a rigid transformation that minimizes the differences in position of the fiducials as identified on the CT scan with those identified in the mock operating room. This transformation was then applied to all data points in the CT scan to map the CT scan to the physical space the skull occupies in the mock operating room. IGS navigation was thus enabled with the drill serving as a localizer and the video monitor showing the corresponding position in the preoperative CT scan, which was actively updated in axial, coronal, and sagittal views. After registration was complete, the fiducial frame was removed from the LADS leaving the IR in place as a rigid extension of the frame. Removing it allowed unimpeded surgical access to the temporal bone (Fig. 1B). As both the drill and skull are being actively tracked, each can be moved independently of the other while continuously tracking (see lower inset of Fig. 2). Using this image-guided surgery system and tracked otologic drill fitted with a 2 mm cutting bit, a percutaneous approach to the middle ear via the facial recess was undertaken. The drill was advanced by watching the video monitors, which actively updated its position in the CT scan. Care was taken to avoid vital structures: the canal of the facial nerve, the horizontal semicircular canal, and the external auditory canal. With entry into the middle ear, the drill bit could be seen via the external auditory canal. Next, the mastoid was drilled in a conventional fashion, preserving the tunnel through which the percutaneous drill pass had been made. Photo documentation was performed to confirm that the track of the drill corresponded to that seen in the CT scan. RESULTS For each set-up, fiducial registration error was calculated to be below 0.8 mm and target registration error was calculated to be below 0.7 mm. Figure 3 shows an oblique magnified view with tracking of the drill. A wide line shows the path of the drill as it approaches the basal turn of the cochlea. The stylomastoid foramen is visible just below the path showing the distal, anteroinferior course of the facial nerve. Images from Skull #1 are shown in Figure 4. Panels A and B show images taken after minimally invasive, image-guided, facial-recess approach to the middle ear. The wire in Panel A extends down the drilled tunnel. Fig. 2 live 4/C

4 JOBNAME: ajo 26# PAGE: 4 OUTPUT: Fri July 1 9:30: R. F. LABADIE ET AL. FIG. 3. An optional view shows a magnified oblique image with the drill path shown as a wide line. This path can be seen approaching the basal turn of the cochlea. The stylomastoid foramen can be seen inferior to this. Panel B shows the view down the tunnel into the middle ear. Panels C and D are images taken after traditional mastoidectomy with preservation of the drill path. In Panels C and D, the vertical wire is placed in the stylomastoid foramen and the horizontal wire is placed through the tunnel. When turned anteriorly (panel D), the tunnel is noted to cross anterior to the facial nerve within the confines of the facial recess. Similar images from Skull #2 are shown in Figure 5. Panel A shows the minimally invasive, image-guided, facial-recess approach to the middle ear with a wire passing through the drilled tunnel. Panel B shows the result after mastoidectomy with exposure of vital structures. The vertical wire is located in the facial canal, the arched wires are in the semicircular canals, and bone over the central portion of the sigmoid sinus has been FIG. 4. Photographs of surgical dissection of Skull #1. (A and B) The path of the imageguided drill as it enters the middle ear via the facial recess. (C and D) The same skull after traditional mastoidectomy, preserving the path of the drill. In these panels, the vertical wire is located in the stylomastoid foramen and the horizontal wire passes through the drill path. Figs. 3,4 live 4/C

5 JOBNAME: ajo 26# PAGE: 5 OUTPUT: Fri July 1 9:30: PERCUTANEOUS COCHLEAR IMPLANTATION 561 FIG. 5. Photographs of surgical dissection of Skull #2. (A) Path of the imageguided drill as it enters the middle ear via the facial recess. A wire has been fed through this tunnel. (B) Post-mastoidectomy drilling with exposure of the semicircular canals (arched wire), sigmoid sinus, and facial canal (vertical wire). The drill path does not violate any of these structures. removed. As with Skull #1, no vital structures were mechanically damaged by the image-guided drilling. DISCUSSION The use of our fiducial system with a commerciallyavailable IGS system has been previously shown to achieve the necessary (submillimetric) accuracy for image-guided otologic/neurotologic surgery (9 11). Building on this, herein we have demonstrated the versatility of such a system by performing a minimallyinvasive, image-guided, facial-recess approach to the cochlea, in vitro. The clinical correlate we attempted to duplicate is that of a percutaneous cochlear implant. Cochlear implantation via mastoidectomy with extended facial recess is associated with a low incidence of complications and a high incidence of success (16). Alterations of this technique, such as the recently reported suprameatal approach, have met with controversy (17). Appropriate trepidation for new techniques is warranted. It is hoped that this trepidation will not impede the development of technology that may make procedures easier, safer, and more readily available to a wider breadth of surgeons. While our results suggest that percutaneous cochlear implantation is technically feasible in vitro, it remains to be seen whether there is clinical applicability. It is recognized that in actual surgical interventions, it is likely that there will be many more sources of error than those encountered in the temporal bone laboratory (e.g., placement of oral endotracheal tube in close proximity to the dental bite-block and surgical draping). Our initial efforts have been focused on in vitro demonstration of concepts. On-going efforts are aimed at in vivo validation, first with the accuracy studies cited above (9 11) and subsequently with clinical applications such as cochlear access. Our technique provides access to the cochlea but does not address the current need for surgical placement of the internal receiver, typically secured in a boney well in the parietal portion of the skull. Technological advances may obviate the need for such a large internal receiver. However, the need to access the cochlea will always be the central component of cochlear implantation inherent with the greatest surgical risks (e.g., facial nerve injury). As such, we focused our technique on this aspect. At a minimum, image-guided surgery seems ideally suited to provide an additional layer of safety for otologic/neurotologic procedures. The active tracking of an otologic drill will allow triggering of alarms or other safety mechanisms should a surgical border be approached. One proposed safety mechanism is shutting off a surgical drill to prevent damage to collateral tissue (18). Analogous to audible alarm the facial nerve monitor, such safety systems may allow more aggressive dissections while minimizing damage to vital structures. At a maximum, image-guided surgery may prompt reworking of the current paradigm of wide surgical exposure for otologic/neurotologic procedures. Approaches to the petrous apex may be accomplished under minimally invasive conditions. Retrofacial approach to the sinus tympani may be feasible during routine chronic middle ear surgery. This new paradigm may also include integration of other exciting technologies such as robotic surgery in the form of robotic mastoidectomy. This concept has already been suggested by others (19) and demonstrated by the drilling of cochlear implant, internal-receiver beds (20). Such ideals are already being embraced by other surgical specialties, including ophthalmology, where laser in situ keratomileusis (LASIK) surgery occurs under computer-control, and cardiothoracic surgery, where the DaVinci robot has received approval from the Food and Drug Administration. As a progressive field, we should remain on the forefront of such technological developments. REFERENCES 1. Roberts DW, Strohbehn JW, Hatch JF, et al. A frameless stereotaxic integration of computerized tomographic graphic imaging and the operating microscope. J Neurosurg 1986;65: Weinberg JS, Lang FF, Sawaya R. Surgical management of brain metastases. Curr Oncol Rep 2001;3: Wisoff JH, Boyett JM, Berger MS, et al. Current neurosurgical management and the impact of the extent of resection in the treatment of malignant gliomas of childhood: a report of the Children s Cancer Group trial no. CCG-945. J Neurosurg 1998;89: Sargent EW, Bucholz RD. Middle cranial fossa surgery with image-guided instrumentation. Otolaryngol Head Neck Surg 1997; 117: Fig. 5 live 4/C

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