The Laryngoscope Lippincott Williams & Wilkins, Inc., Philadelphia 0 1999 The American Laryngological, Rhinological and Otological Society, Inc.

Computer-Aided Surgery in the Petrous Bone

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Andreas R. Gunkel, MD; Michael Vogele; Arno Martin; Reto J. Bale, MD; Walter F. Thumfart, MD; Wolfgang Freysinger, PhD

Objectives: We demonstrate that computer- assisted frameless stereotactic navigation with the ISGiELEKTA Viewing Wand system in the petrous bone i s routinely possible with sufficient application accuracy. Methods: High-resolution computed tomog- raphy imaging is done with a dedicated structure at- tached to the mouthpiece of the Vogele-Bale-Hohner (VBH) head holder, an integral part of our intraoper- ative patient fixation. The patient image registration can be reliably performed before surgery in an un- sterile environment with the registration structure of the mouthpiece. For intraoperative navigation either the position-sensitive articulated arm or the optical three-dimensional digitizer of the ISGELEKTA sys- tem is used. Results: In the operations of the petrous bone performed so far, i.e., mastoidectomy, cholestea- toma surgery, and lateral skull base revision surgery, the clinical value of three-dimensional navigation was clearly demonstrated with an application accu- racy, constant throughout surgery, mostly limited only by the resolution of the computed tomography. Key Words: Petrous bone, middle ear surgery, cho- lesteatoma, ISG Viewing Wand, computer, assisted surgery, three-dimensional, navigation, frameless stereotaxy.

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INTRODUCTION Frameless stereotaxy -three-dimensional (3D) nav-

igation1,2 -has found widespread use and has signifi- cantly improved the performance of many surgical proce- dures’-“ We have used the Viewing Wand navigation system (ISG Technologies, Mississauga, Ontario, Canada) extensively7 with the VBH head holders in mostly video endoscopic endonasal procedures. The system is well es- tablished for surgery in the anterior skull baseg and the paranasal sinuses,1° for 3D-navigable instruments,11J2 and for interstitial brachytherapy.13

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From the ENT Clinic (A.R.G., M.v., A.M. , w F.T., w.F.), Clinic of Radiol- ogy I (R.J .R. ) , University of Innsbruck, Innsbruck, Austria.

Editor’s Note: This Manuscript was accepted for publication July 14, 1999.

Send Reprint Requests to Univ. Prof. Dr. Wolfgang Freysinger, PhD, ENT Department, University Innsbruck, Anichstrasse 35, A-6020 Inns- bruck, Austria. E-mail: wolfgang.freysinge&ibk.ac.at.

Laryngoscope 109: November 1999

Computer-assisted surgery provides orientation in difficult cases involving anatomical variations, excessive pathology, bleeding, or combinations thereof.14 Positional intraoperative information is crucial for obtaining biopsies close to vital anatomical structures, such as at the poste- rior and lateral wall of the sphenoid sinus. Microsurgery of the petrous bone is complicated by pathological changes and anatomic variations and requires augmented intraop- erative orientation.

This bony anatomical region seems to be ideally suited for 3D navigation, but the use of high-resolution computed tomography (CT) imaging eliminates external referencing landmarks. The necessary accuracy of less than 1 mm is a challenge for 3D navigation. Theoretically, with accuracy of 0.25 mm and 0.35 mm for the mechanical arm and the optical digitizer, respectively, the ISG/ELE- KTA Viewing Wand system meets this challenge. Practi- cally, however, application accuracies of up to 5 mrnl5J6 are reported. Therefore, frameless stereotaxy has not been used clinically in the petrous bone and only preliminary work exi~ts.17-~9 We report what is to our knowledge the first successful clinical “routine” application of 3D-image- guided surgery in the petrous bone.

MATERIALS AND METHODS

Three-Dimensional Navigation Equipment Detailed descriptions of the ISGELEKTA Viewing Wand

intraoperative navigation system, the VBH head holder, and the VBH mouthpiece can be found elsewhere8J6.20 In brief, the View- ing Wand shows the actual position of a probe in the patient’s axial, coronal, sagittal, and multiplanar reformatted CT/mag- netic resonance (MR) images; the spatial position of the probe is detected by an articulated position-sensing arm (FAR0 Technol- ogies, Orlando, FL) (Fig. l), or the Polaris infrared digitizer (NDI, Waterloo, Ontario, Canada). The patient image registration is done by correlating at least four well-defined anatomical points from the patient with corresponding positions on the imagery stored in the navigation system. In general, anatomic features, fiducial skin markers, implantable markers, and external inva- sive and noninvasive reference frames serve as matching points. For reliable results, stable patient immobilization is mandatory: we achieve this with the VBH head holder.8 Unlike head immo- bilization for paranasal sinus surgery, the patient’s head is se- cured by unilaterally positioning two hydraulic arms contralat- erally to the side where the surgery will take place.

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Fig. 1. Setup with the mechanical arm. The anaesthetized patient is already secured in the VBH head holder; the patient image regis- tration is now to be performed with the position-sensitive articulated arm mounted on the cranial end of the operating table. The patient wears the mouthpiece with the first-generation registration element secured to it. The staff is positioned as for a standard ear surgery; the arm is placed so that it does not interfere with the manipulations of the surgeon and the nurse, respectively. The Viewing Wand’s cabinet is placed so that the surgeon can conveniently view the monitor with a slight turn of the head, even when using the operating microscope

Preoperative Steps To use the VBH head holder, before surgery a mouthpiece

was fabricated and secured to the supporting rod.8 Unilaterally, at the side of surgery, a dedicated registration device (Fig. 2) was securely mounted to the carbon-fiber mouthpiece. It was essen- tially a Plexiglas cube (3X3X3 cm) and a carbon rod (6-mm diameter) to which radiopaque fiducials were attached. For high- resolution CT imaging the patient was immobilized with the VBH head holder on the CT couch with the registration device in place, close to the outer ear structures and possibly entering the outer ear canal. To assure exact repositioning of the mouthpiece to the patient’s upper jaw, an underpressure of 0.8 atm was applied.20

We used Siemens Somatom Plus (Siemens, Erlangen, Ger- many) and GE Hispeed Advantage (General Electric, Milwaukee, WI) CT scanners for helical (spiral) and axial scanning. Images of the petrous bone were reconstructed in l-mm axial slices, 512 X 512 pixels; the smallest field of view ( < l o cm) was chosen to include the anatomy and the registration device. After scanning, the mouthpiece registration device assembly was removed from the patient’s mouth and stored until surgery. Careful handling of the whole assembly is indicated so as not to destroy future patient image registrations relative to the mouthpiece. Creation of a 3D reconstruction on the Allegro workstation (ISG Technologies, Mississauga, Ontario, Canada) and transfer of the data to the navigation system in the operating room complete the preopera- tive steps.

Intraoperative Procedure After induction of general anesthesia, the assembly mouth-

piece registration device was repositioned and secured in the patient’s mouth by underpressure. Repositioning was considered optimal if the pressure approached the pressure registered dur- ing fabrication. The patient was positioned according to the sur- gical needs and rigidly immobilized with the VBH head holder, as shown in Figure 1. For intraoperative use, either the position- sensitive articulated arm (Fig. 1) or the reference frame of the optical Polaris digitizer (Fig. 4) was mounted to the base plate of the head holder. In either case the Plexiglas structure of the

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registration element of the mouthpiece (Figs. 2 and 3) was refer- enced to the 3D CT data set. An initial registration accuracy check was done by comparing the actual and calculated position of the probe on the registration cube. Thereafter the accessible anatomical structures of the scanned region were used for the next intraoperative registration control according to our proto- ~01.21 If a satisfactory patient image registration was deterrnincd, the registration element was removed. If not, the registration was repeated. Surgery commenced after sterile washing, cleaning, and draping of the surgical site. During surgery bony landmarks and available unique structures were gently touched with the pointer of the navigation system, and these positions were com- pared with the calculated ones (Fig. 4). The anatomical structures used for these quality checks were, depending on surgery and availability, stapes and stapes footplate, promontory, facial nerve genu, tragus, external ear canal, and (with precaution) parts of the cartilaginous external structures. If accessible, the temporo- petrosal fissure was also used.

Patients The patients, who all gave informed consent, underwent

operations of the petrous bone and are detailed in Table I. Al- though the upper dentition was used as part of our strategy for intraoperative patient fixation, we did not use completeness of the upper dentition as a selection criterion for inclusion in this investigation. We are, however, studying the reproducibility of the mouthpiece’s repositioning accuracy in edentulous patients. In only one patient were multiple operations necessary.

In three patients 3D-assisted navigation was used during middle ear revision. Two patients were treated for recurrent

Fig. 2. Close-up of the patient-to-computed tomography referenc- ing element. The carbon structure carries a plastic tray to hold the two-component self-hardening dental imprint mass (Impregum F, Espe, Seefeld, Germany). The lateral arm of the mouthpiece holds an assembly of Plexiglas cubes and carbon rods. These compo- nents are labeled with radiopaque markers (x-like structures), allow- ing a precise intraoperative registration. The whole construction provides maximum flexibility in all directions to fit all patients, is radiolucent, stable, and rigid, and provides exact spatial reference points by means of the markers.

Gunkel et al.: Computer-Aided Surgery in the Petrous Bone

Fig. 3. A. lntraoperative unsterile patient image registration. The mouthpiece registration element assembly is secured to the upper jaw with underpressure and carries the registration element with randomly placed radiopaque markers on its surface. With sufficient registration accuracy, the operation site is cleaned and draped. 6. lntraoperative screenshot of the ISG/ELEKTA Viewing Wand monitor showing a three-dimensional (3D) reconstruction of the referencing element with the radiopaque markers (bright dots), a window to control the transparency of the selected structures, and a 3D reconstruction of the patient’s skin, skull, and brain.

cholesteatoma. Another patient, a 33-year-old man suffering from cholesteatoma, underwent surgery of the right ear (retro- auricular approach), and three consecutive surgeries of the left ear (two retro-auricular approaches and transtemporal ap- proach), each time under 3D guidance. Large masses of cholestea- toma were present in both middle ears and mastoid bones; in the left petrous bone the facial nerve canal was eroded (Fig. 5 ) , the labyrinthine block and the large vessels were packed by the cholesteatoma matrix. The patient showed a slight binaural hear- ing loss and no vestibular disorders. He was referred to our clinic from an outside hospital. A 69-year-old woman was referred for continuing vertigo under slight physical activity. She underwent implantation of a middle ear ossicular alio loco. In our clinic a vestibular fistula of the left vestibular organ was diagnosed; for localization of the fistula a 3D-navigated transtympanic approach was chosen. One female patient was referred to us for continuing middle ear infections. The patient had undergone an earlier emergency neurosurgery for multiple fractures of the skull; parts of the superior aspect of the right petrous bone were recon- structed with bone wax, a large defect of the lateral skull was closed with bone cement. Given this history, a radiological exam- ination suggested a prolapse of the brain into the middle ear region; a second-look operation by ENT surgeons was suggested. It was found that the bone wax used to remodel the roof of the pyramid was completely dissolved and a large, soft mass, with inflammatory symptoms was present instead. The mass was his- tologically identified as granulomatous tissue and removed and the defect closed with transplanted body fat.

None of the patients complained of postoperative effects from being mechanically fixated throughout the surgery, nor was any effect of the inserted mouthpiece felt. Immediate postopera- tive inspection of the hard palate and the dentition did not show any effect on the gingival or buccal mucosa. The intraoperative patient positioning with the head specially positioned and immo- bilized was well tolerated by all patients. In cases when we used CT immobilization of the awake patient, no discomfort of any type was reported.

RESULTS In all cases the patients had an almost complete

upper dentition so that the results of Martin et a1.20 could be applied directly. Insertion of the VBH mouthpiece into the anesthetized patient’s mouth was easy and uncompli- cated. When carefully positioned, the immobilization de- vice did not interfere with radiologic imaging and did not produce any artifacts. Finally, when used for image acqui- sition, rigid patient immobilization resulted in optimal, artifact-free CT data sets.

In the 10 operations performed in our clinic to date we used 3D computed navigation for assistance during cholesteatoma surgery, middle-ear revision surgery, tym- panoplasty, closure of a vestibular fistula, and revision of the lateral skull base after a posttraumatic reconstruc- tion. In all cases a satisfactory intraoperative patient im-

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TABLE I. Patient Information.

indication for Surgery Surgical Access Cases (n) Application Accuracy Digitizer

Cholesteatoma Transtympanic 3 1 mm Far0

C holesteatoma Retro-auricular 2 1 mm Polaris C holesteatoma Transtemporal 1 See text Faro, Polaris Middle ear revision Retro-auricular 3 <1 mm Faro, Polaris

Lateral skull base revision Transtemporal 1 <1 mm Polaris Vestibular fistula Retro-auricular 1 1 1 mm Far0

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age registration was achieved, making reliable navigation The mathematical error determined by the system, possible. We were able to achieve in essence two- frequently cited as the root mean square (RMS), was ex- dimensional sub-millimetric, pixel-limited application ac- tremely low in all cases: < 0.6 mm. This was because the curacy, as demonstrated by the localization of a vestibular reference element with its precisely defined (cubic) spatial fistula (Fig. 6) . In three dimensions the resolution of the referencing locations was used, Clinical accuracy, how- CT scanner has shown to be the limiting factor for intra- ever, is determined by intraoperative measurements of operative registration error. the difference between exact anatomical and calculated

location. With our setup the repositioning accuracy of the

Fig. 4. A. lntraoperative measurement with the infrared digitizing system during a transtemporal cholesteatoma removal. Note that simulta- neous working with the navigation system and the microscope is possible if a large enough focal length of the microscope is chosen. This surgery was done with the first generation of ISG’s optically navigable probes. The only potentially complicating equipment in the surgical field is the starburst, which is draped with a sterile transparent self-adhesive foil. The starburst remains visible to the infrared-cameras. B. Video print of the microscopic view corresponding to Figure 5A. 3D measurement to assess the actual position when entering the roof of the petrous bone showing the probe coming in from the lower left, sliding on top of the brain retractor, lower center, and parts of a retractor, lower right. C. 3D position as determined by the ISGIELEKTA navigation system. The four panels show (clockwise from lower left) axial, coronal, sagittal, and a trajectory view. The trajectory 0 view can be compared with ultrasound B images. The calculated position correlates exactly to the actual one.

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1796 Gunkel et al.: Computer-Aided Surgery in the Petrous Bone

Fig. 5. A. lntraoperative screen-shot of the navigation system’s monitor during transtemporal cholesteatoma surgery. The surgeon used the Wand to determine the actual position. The canal of the facial nerve was eroded by the cholesteatoma. The CT images are shown as they were acquired. (Slice orientation is as in Fig. 4C.). 6. lntraoperative video print of a position measurement with the navigation system, in this case employing the position-sensitive articulated arm. The probe’s tip is placed on the exposed facial nerve. To obtain exact orientation in the surgical field see Figure 5A.

mouthpiece is the central limiting factor. It is known from the literature20 that the 3D error introduced by the repo- sitioning of the mouthpiece is smaller than 0.6 mm, which simultaneously forms the lower limit for application accu- racy. Possible deviations in excess of this value are due to either registration errors or malpositioned mouthpieces. Registration errors can be avoided by re-registration of the patient. Mouthpiece malpositioning can be avoided by repositioning the mouthpiece exactly and by monitoring the applied underpressure, which should be close to 0.8 atm.

Using available exterior anatomical structures (such as the tragus) or bony landmarks (such as the emissary foramina, stapes footplate, facial nerve genu, round win-

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dow, ampulla of lateral semicircular canal, ampulla of superior semicircular canal, or canal of the carotid artery), the surgeon determined the intraoperative application ac- curacy to be approximately 1 to 2 mm 5 0.5 mm.zl

DISCUSSION The close relationship of the minute anatomical

structures in the petrous bone demands a high degree of application accuracy of computer-assisted 3D navigation. Thus, for imaging, settings have to be used for radiologic imaging such that “near-diagnostic” CT images can be used for navigation. Therefore, only limited external fea- tures of the patient’s anatomy will be available for regis- tration. Patient image registrations based on anatomical features and externally placed radiopaque fiducials will hardly be possible in this anatomical region, where only

A

B Fig. 6. A. Screenshot of the navigation system’s monitor during a revision ear surgery. The four views show the axial, coronal, and sagittal views and a multiplanar reformatted (MPR) view of the 30 CT data set (clockwise). The MPR view shows the trajectory of the probe attached to the position-sensitive articulated arm that is currently placed on the vestibular fistula. 6. Video print of the intraoperative situation encountered while determining the position of the suspected vestibular fistula. The probe of the navigation system, attached to the position-sensitive mechanical arm, is placed on the lateral semicircular canal directly.

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parts of the nonrigid outer ear structures without reliable anatomical landmarks are available for providing spatial points as input to the patient image registration.

In an experimental study Sipos et a1.16 evaluated the in vitro accuracy of an optical 3D digitizer and a motion tracker mounted on phantoms, finding application accu- racies of 0.87 mm and 2.64 mm, respectively. They con- cluded that intraoperative patient fixation is indispens- able for assuring proper and satisfactory application of their 3D-image-guided navigation system. Vrionis et a1.lS report localization errors in the range of 0.91 to 2.44 mm and 0.71 to 1.52 mm for dry and fresh petrous bones, respectively. However, the transfer of these results to a clinical setting is severely limited by the availability of suitable usable structures and the navigational skills of the surgeon who performs the registration. At almost the same time as the Vrionis study, the possibility of using the ISG Viewing Wand in the petrous bone was demonstrated on a cadaver.17 Various “patient” image registration tech- niques were optimized on the model, resulting in the de- velopment of the current clinical setup. Our procedure extends and applies the results of Sipos et a1.,16 suggesting that a broad distribution of reference points around the area of interest, instead of a collinear, coplanar spatial arrangement, enhances patient image registration re- sults. One major advantage of the presented approach is that registration can be performed quickly, without the need for sterilization, and spatially separated from sur- gery, so that intraoperative patient image registration is reduced to a quick check thereof. The average error in- duced by this technique is < 0.7 mm.

Our intraoperative patient image registration for paranasal sinus and anterior skull base surgery is be- tween 1 and 2 mm. Because of the rigid structures of the skull, the small field of view, and the stable anatomical relations, the achievable intraoperative application accu- racy for petrous bone surgery is remarkably better. How- ever, it is a rather difficult task to exactly determine a possible difference between the position in the patient and the corresponding calculated one. This is the reason why we do not give precise figures for x, y, and z deviations. Application of a slightly modified measurement protoco121 was possible and yielded reliable values. Joint use of the registration device and elaborated registration procedures allow a reliable intraoperative use. In view of the values of the achieved clinical application accuracy (uide the local- ization of minute lesions of the vestibular organ), it is justified to describe our patient image registration as z- limited: assuming a homogenous 3D registration, l-mm- thick CT slices limit the achievable intraoperative accu- racy. In the two other dimensions, much smaller values can intrinsically be reached.

The presented approach of 3D navigation in the pe- trous bone has been successful. One drawback, however, is that with the removal of the registration element, a sec- ond intraoperative registration procedure becomes impos- sible, hence the term “lost registration.” The Viewing Wand software does provide the possibility of realigning an intraoperative registration with landmarks that should then be re-registered. So far we have not encoun- tered any intraoperative movements of the patients

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caused by the employment of the VBH head holder. We have had, however, three operations (not discussed in this paper) that were started as 3D surgeries but had to be completed in the classic manner because of changes of the spatial position of the Far0 arm and the infrared-emitting “starburst” (i.e., the origin of the Polaris digitizer’s coor- dinate system) after the registration element was removed and the patient image registration could not be repeated any more. Because of this experience, we now carefully retain the same position of these components in space throughout the complete surgery. In conclusion, we found that the surgeon acquired valuable information about his actual intraoperative position inside the patient. The value of intraoperative visualization could be much higher if the handling of the 3D setup were more adapted to applications in microsurgery, Specifically, the simulta- neous handling of the microscopic view and the navigation information on the workstation is cumbersome. Addition- ally, the stable and reliable placement of both the arm’s probe and the freehand probe on a certain delicate ana- tomical structure is a difficult task, especially if the sur- geon must look away from the surgical field to the navi- gation system’s monitor. An integration of navigational information into the (bin)ocular view of the surgeon would be desirable.

CONCLUSION We have successfully used a mechanical and an op-

tically based navigation system for petrous bone surgery. The patient was rigidly fixed with the VBH head holder throughout the surgery; for registration a modified VBH mouthpiece was used. The navigation system proved to be a valuable instrument for the surgeon, providing valuable spatial information, especially in cases in which orienta- tion is difficult because of tumor invasion. This system is not, however, a substitute for a knowledge of anatomy.

Even though we use 3D navigation routinely in our department, it is still far from a “routine” surgical proce- dure. The tools are not ideally suited for microsurgery: instruments must still undergo an evolution in size and functionality to really fit into surgical routine of the pe- trous bone and to allow a practical coexistence with the operating microscope.

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