An image-guided robot system for direct cochlear access Brett Bell 1, Tom Williamson 1, Nicolas Gerber1 , Kate Gavaghan1, Wilhelm Wimmer 1, Martin Kompis2, Stefan Weber 1, Marco Caversaccio 2 1
ARTORG Center for Biomedical Engineering, University of Bern, Bern, Switzerland, 2Department of ENT Surgery, University Hospital Bern, Bern, Switzerland The aim of direct cochlear access (DCA) is to replace the standard mastoidectomy with a small diameter tunnel from the lateral bone surface to the cochlea for electrode array insertion. In contrast to previous attempts, the approach described in this work not only achieves an unprecedented high accuracy, but also contains several safety sub-systems. This paper provides a brief description of the system components, and summarizes accuracy results using the system in a cadaver model over the past two years. Keywords: Robot, minimally invasive, direct cochlear access, image guidance
Background Since the first clinical use of the cochlear implant in the mid-eighties, the surgical technology and methods implemented to gain access to the middle ear for electrode insertion have remained largely unchanged. Alternative surgical techniques such as the ‘Veria’ (Kiratzidis et al., 2002), ‘Suprameatal’ (Kronenberg et al., 2004), and ‘Pericanal’ (Häusler, 2002) have not been widely adopted. In 2004, Schipper et al. (2004) proposed a new use of this stereotaxy to provide a minimally invasive access to the cochlea, which we call direct cochlear access (DCA). Similar to the manual techniques mentioned above, the procedure eliminates the need for a mastoidectomy. In essence, a small tunnel originating at the lateral mastoid, passing through the facial recess, and terminating in the middle ear on the basal turn of the cochlea is drilled with guidance from the navigation system. Schipper quickly realized, however, that the navigation accuracy available with traditional hand-held instruments was not sufficient for safe traversal of the facial recess which has a width of 1.8–4.1 mm in 95% of the population (Bielamowicz et al., 1988). Thus, alternative approaches using patient-specific templates (Labadie et al., 2010) or robotic drilling assistance (Bell et al., 2012) have also been developed. This report summarizes our work in the field of minimally invasive cochlear implantation using robot-assisted stereotaxy. Specifically, we have Correspondence to: Nicolas Gerber, ARTORG Center for Biomedical Engineering, Research University of Bern, Murtenstrasse 50, 3010 Bern, Switzerland. Email: [email protected]
© W. S. Maney & Son Ltd 2014 DOI 10.1179/1467010014Z.000000000192
focused on the overall accuracy of the system including high-accuracy patient–image registration, and safety mechanisms including integrated neuromonitoring of the facial nerve and redundant drill position estimation based on process measurements.
Description of the surgical robot system The surgical robot system was designed and constructed specifically for surgery on the lateral cranial base with a focus on DCA. The robot arm has five rotational axes connected in series and weighs ∼5.5 kg. This construction allows the robot to be easily mounted directly to the rails of the operating table and the large workspace of the serial arm allows the end effector to efficiently move in and out of the surgical workspace as necessary during the intervention. The robot system also utilizes an optical position measurement system (Cambar B1, Axios 3D Gmbh, Oldenburg, Germany), tracking both the robot’s end effector (tool position) and also the patient’s head by means of rigidly fixed optical references. Thus, corrections of patient movement and robot positioning inaccuracies can be made by passing measurements from the optical tracking system to the robot controller. This so-called visual servoing technique effectively enables the position of the tool to be controlled with an accuracy of 0.05 mm (measurement accuracy of the optical system). This is a significant improvement over the absolute robot positioning accuracy (through its internal encoders), which was verified through metrological measurements to be 0.3 mm in a 200 mm cubic workspace.
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Bell et al.
An image-guided robot system for DCA
Figure 1 Surgical robot mounted on operating table with head fixed in non-invasive clamp. The stereo camera tracks the position of the drill relative to the patient and can compensate for movement of the head.
In addition to the high accuracy provided through the visual servoing technique, the system also utilizes an accurate patient-to-image registration process based on bone-anchored fiducial screws. Screws are located in the image space using a surface model registration technique, and screw locations are digitized on the patient with the same accuracy as the camera system. Together, this results in a patient-to-image registration error (target registration error) of 0.1 ± 0.04 mm (8) (Fig. 1).
Accuracy assessment in a cadaver model Materials and methods The accuracy of the robot-assisted stereotaxy system was evaluated using a cadaver model under cliniclike conditions. Each case begins with the placement of bone-anchored titanium screws (M-5220.03, Medartis, Basel, Switzerland, Planmeca, Helsinki, Finland) in the mastoid surrounding the external auditory canal. Next, preoperative imaging is performed using high-resolution cone beam computed tomography (CBCT) (ProMax 3D Max, Planmeca, Finland). These data are then transferred to a custom planning software. During this stage, the structures of the
Figure 2 Postoperative CBCT scan showing the electrode inserted through the DCA tunnel into the round window and its progression into the cochlea.
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facial recess (facial nerve, chorda tympani, external auditory canal, and ossicles) are segmented along with a target location on the cochlea. Finally, a drill trajectory passing safely through the facial recess is defined. In the operating room (OR), the robotic arm is attached to the side rails of a standard operating table using a custom mounting platform. The arm is draped with a sterile covering, and is connected to the navigation platform containing the graphical user interface. The cadaver head was rigidly fixed to the OR table using a pin-type head clamp. The surgical site was then prepared with a retroauricular incision after which a patient tracking reference was fixed rigidly to the skull. Following these preparations, the patient was registered using the bone-anchored fiducial screws using the methods described in Gerber et al. (2013). Using this information, the robot system automatically calculates the position where the tunnel should be drilled in three phases: centering hole, intermediate drill, and final depth. The centering hole is created with a short stiff drilling bur to ensure that later steps do not deviate from the planned path. Likewise, the intermediate step is performed with a medium length bur to ensure maximal tool stiffness, which reduces lateral drilling error. Finally, the long bur is used to drill to the middle ear cavity. Accuracy of the DCA tunnel was evaluated using postoperative CBCT scans of the temporal bone region with a titanium wire inserted into the tunnel for increased contrast. Postoperative data were registered to the preoperative plan using a mutual information matching algorithm (Amira, FEI Visualization Sciences Group, Düsseldorf, Germany, MED-EL Worldwide Headquarters, Innsbruck, Austria). The wire was segmented using thresholding and a surface created. A line was subsequently fitted to the resulting 3D surface points. This line was then compared to the planned location and the deviation of the actual tunnel from the plan was evaluated at the lateral bone surface and the target point at the round window. In a separate stage, the insertion feasibility was assessed. A tympanomeatal flap was prepared to provide visual control of the electrode insertion process and manually remove the bony overhang of the round window. The electrodes (4 Standard 12 channel 24 contact electrodes and 4 Flex28 12 channel 19 contact electrodes, MED-EL GmbH, Austria) were fed through the DCA tunnel by hand. Once the electrode tip entered the middle ear space, it was guided to the round window as necessary using a micropick. After the tip of the electrode was introduced into the round window, the insertion was primarily performed with the fingers from the lateral bone surface. Occasionally, additional tools such as
Bell et al.
the micropick or forceps were used to manipulate the electrode in the middle ear space.
Results The accuracy of the drilling attempts measured at the lateral surface was 0.08 ± 0.05 mm with a range of 0.01–0.17 mm. More importantly, the accuracy at the target (round window) was measured at 0.15 ± 0.08 mm with a range of 0.02–0.26 mm for the eight cases drilled. The facial nerve was preserved in every case. On an average, the robotic procedure including robot setup and patient marker attachment required 21.5 minutes to perform. Electrode insertion was easily performed, and the drilled tunnel aligned perfectly with the round window in every case. Full insertion was obtained in seven of the eight cases. In one case, a single electrode pair remained extra-cochlear (Fig. 2).
Discussion In this study, we were able to show that a minimally invasive DCA is feasible and safe using the described robot system. The demonstrated high accuracy of the system means that a large portion of the patient population can be treated with the proposed approach. Also, a highly accurate tunnel increases the likelihood of a successful insertion due to the ideal alignment of the tunnel with the intended target at the round window. DCA can be achieved in ∼20 minutes. The additional steps leading up to electrode insertion are estimated to take an additional 15 minutes. This means that a time savings in the OR is likely to be achieved in most cases. At present, a tympanomeatal flap was necessary to control the electrode insertion. Future investigations will address this aspect of the surgical workflow in more detail with the aim of a complete insertion through the DCA tunnel without the necessity of opening the tympanic membrane further increasing the efficiency of the operation. Although the method is intended to be compatible with any electrode type, only straight electrodes were utilized in this study. We also intend to investigate the use of precurved electrodes. A natural extension of the robotically drilled tunnel is an automated electrode insertion process which could reduce intra-cochlear damage. The safety of the intervention is the primary concern with a priority of protecting the facial nerve. With the safety of this structure in mind, we have also
An image-guided robot system for DCA
investigated alternative and redundant safety measures. We have developed an estimator of the drill position which is independent of the optical tracking system. This estimator correlates drilling forces with bone density information available from preoperative image data (Williamson et al., 2012). In addition to this, we are also investigating the integration of neuromonitoring systems which will alert the surgeon should the drill come too close to the facial nerve. We believe that a safe and effective tunnel procedure can be achieved through the combined effects of high accuracy, drill position estimation redundancy, and facial nerve monitoring. This work was financially supported by the Swiss National Science Foundation (NanoTera Initiative project title HearRestore) and from the European Commission FP7 (Project title HearEU). Cochlear electrode arrays were furnished by the Med-El Worldwide Headquarters, Innsbruck, Austria. B. Bell, T. Williamson, and S. Weber hold a patent on the drill position detection method, which could have future financial implications.
References Bell B., Stieger C., Gerber N., et al. 2012. A self-developed and constructed robot for minimally invasive cochlear implantation. Acta Oto-Laryngologica, 132(4): 355–360. Bielamowicz S.A., Coker N.J., Jenkins H.A., Igarashi M. 1988. Surgical dimensions of the facial recess in adults and children. Archives of Otolaryngology – Head & Neck Surgery, 114(5): 534–537. Gerber N., Gavaghan K., Bell B., et al. 2013. High accuracy patientto-image registration for the facilitation of image guided robotic microsurgery on the head. IEEE Transactions on Biomedical Engineering, 60(4): 960–968. Häusler R. 2002. Cochlear implantation without mastoidectomy: the pericanal electrode insertion technique. Acta OtoLaryngologica, 122(7): 715–719. Kiratzidis T., Arnold W., Iliades T. 2002. Veria operation updated. I. The trans-canal wall cochlear implantation. ORL; Journal for Oto-Rhino-Laryngology and its Related Specialties, 64(6): 406–412. Kronenberg J., Baumgartner W., Migirov L., Dagan T., Hildesheimer M. 2004. The suprameatal approach: an alternative surgical approach to cochlear implantation. Otology & Neurotology, 25(1): 41–44; discussion 44–45. Labadie R.F., Balachandran R., Mitchell J.E., et al. 2010. Clinical validation study of percutaneous cochlear access using patient-customized microstereotactic frames. Otology & Neurotology, 31(1): 94–99. Schipper J., Klenzner T., Aschendorff A., Arapakis I., Ridder G.J., Laszig R. 2004. Navigation-controlled cochleostomy. Is an improvement in the quality of results for cochlear implant surgery possible? HNO, 52(4): 329–335. Williamson T., Bell B., Gerber N., Salas L., Zysset P., Caversaccio M., et al. 2013. Estimation of tool pose based on forcedensity correlation during robotic drilling. IEEE Trans. Biomed. Eng., 60(4): 969–76.
Cochlear Implants International
2014
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ARTORG Center for Biomedical Engineering, University of Bern, Bern, Switzerland, 2Department of ENT Surgery, University Hospital Bern, Bern, Switzerland The aim of direct cochlear access (DCA) is to replace the standard mastoidectomy with a small diameter tunnel from the lateral bone surface to the cochlea for electrode array insertion. In contrast to previous attempts, the approach described in this work not only achieves an unprecedented high accuracy, but also contains several safety sub-systems. This paper provides a brief description of the system components, and summarizes accuracy results using the system in a cadaver model over the past two years. Keywords: Robot, minimally invasive, direct cochlear access, image guidance
Background Since the first clinical use of the cochlear implant in the mid-eighties, the surgical technology and methods implemented to gain access to the middle ear for electrode insertion have remained largely unchanged. Alternative surgical techniques such as the ‘Veria’ (Kiratzidis et al., 2002), ‘Suprameatal’ (Kronenberg et al., 2004), and ‘Pericanal’ (Häusler, 2002) have not been widely adopted. In 2004, Schipper et al. (2004) proposed a new use of this stereotaxy to provide a minimally invasive access to the cochlea, which we call direct cochlear access (DCA). Similar to the manual techniques mentioned above, the procedure eliminates the need for a mastoidectomy. In essence, a small tunnel originating at the lateral mastoid, passing through the facial recess, and terminating in the middle ear on the basal turn of the cochlea is drilled with guidance from the navigation system. Schipper quickly realized, however, that the navigation accuracy available with traditional hand-held instruments was not sufficient for safe traversal of the facial recess which has a width of 1.8–4.1 mm in 95% of the population (Bielamowicz et al., 1988). Thus, alternative approaches using patient-specific templates (Labadie et al., 2010) or robotic drilling assistance (Bell et al., 2012) have also been developed. This report summarizes our work in the field of minimally invasive cochlear implantation using robot-assisted stereotaxy. Specifically, we have Correspondence to: Nicolas Gerber, ARTORG Center for Biomedical Engineering, Research University of Bern, Murtenstrasse 50, 3010 Bern, Switzerland. Email: [email protected]
© W. S. Maney & Son Ltd 2014 DOI 10.1179/1467010014Z.000000000192
focused on the overall accuracy of the system including high-accuracy patient–image registration, and safety mechanisms including integrated neuromonitoring of the facial nerve and redundant drill position estimation based on process measurements.
Description of the surgical robot system The surgical robot system was designed and constructed specifically for surgery on the lateral cranial base with a focus on DCA. The robot arm has five rotational axes connected in series and weighs ∼5.5 kg. This construction allows the robot to be easily mounted directly to the rails of the operating table and the large workspace of the serial arm allows the end effector to efficiently move in and out of the surgical workspace as necessary during the intervention. The robot system also utilizes an optical position measurement system (Cambar B1, Axios 3D Gmbh, Oldenburg, Germany), tracking both the robot’s end effector (tool position) and also the patient’s head by means of rigidly fixed optical references. Thus, corrections of patient movement and robot positioning inaccuracies can be made by passing measurements from the optical tracking system to the robot controller. This so-called visual servoing technique effectively enables the position of the tool to be controlled with an accuracy of 0.05 mm (measurement accuracy of the optical system). This is a significant improvement over the absolute robot positioning accuracy (through its internal encoders), which was verified through metrological measurements to be 0.3 mm in a 200 mm cubic workspace.
Cochlear Implants International
2014
VOL.
15
NO.
S1
S11
Bell et al.
An image-guided robot system for DCA
Figure 1 Surgical robot mounted on operating table with head fixed in non-invasive clamp. The stereo camera tracks the position of the drill relative to the patient and can compensate for movement of the head.
In addition to the high accuracy provided through the visual servoing technique, the system also utilizes an accurate patient-to-image registration process based on bone-anchored fiducial screws. Screws are located in the image space using a surface model registration technique, and screw locations are digitized on the patient with the same accuracy as the camera system. Together, this results in a patient-to-image registration error (target registration error) of 0.1 ± 0.04 mm (8) (Fig. 1).
Accuracy assessment in a cadaver model Materials and methods The accuracy of the robot-assisted stereotaxy system was evaluated using a cadaver model under cliniclike conditions. Each case begins with the placement of bone-anchored titanium screws (M-5220.03, Medartis, Basel, Switzerland, Planmeca, Helsinki, Finland) in the mastoid surrounding the external auditory canal. Next, preoperative imaging is performed using high-resolution cone beam computed tomography (CBCT) (ProMax 3D Max, Planmeca, Finland). These data are then transferred to a custom planning software. During this stage, the structures of the
Figure 2 Postoperative CBCT scan showing the electrode inserted through the DCA tunnel into the round window and its progression into the cochlea.
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NO.
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facial recess (facial nerve, chorda tympani, external auditory canal, and ossicles) are segmented along with a target location on the cochlea. Finally, a drill trajectory passing safely through the facial recess is defined. In the operating room (OR), the robotic arm is attached to the side rails of a standard operating table using a custom mounting platform. The arm is draped with a sterile covering, and is connected to the navigation platform containing the graphical user interface. The cadaver head was rigidly fixed to the OR table using a pin-type head clamp. The surgical site was then prepared with a retroauricular incision after which a patient tracking reference was fixed rigidly to the skull. Following these preparations, the patient was registered using the bone-anchored fiducial screws using the methods described in Gerber et al. (2013). Using this information, the robot system automatically calculates the position where the tunnel should be drilled in three phases: centering hole, intermediate drill, and final depth. The centering hole is created with a short stiff drilling bur to ensure that later steps do not deviate from the planned path. Likewise, the intermediate step is performed with a medium length bur to ensure maximal tool stiffness, which reduces lateral drilling error. Finally, the long bur is used to drill to the middle ear cavity. Accuracy of the DCA tunnel was evaluated using postoperative CBCT scans of the temporal bone region with a titanium wire inserted into the tunnel for increased contrast. Postoperative data were registered to the preoperative plan using a mutual information matching algorithm (Amira, FEI Visualization Sciences Group, Düsseldorf, Germany, MED-EL Worldwide Headquarters, Innsbruck, Austria). The wire was segmented using thresholding and a surface created. A line was subsequently fitted to the resulting 3D surface points. This line was then compared to the planned location and the deviation of the actual tunnel from the plan was evaluated at the lateral bone surface and the target point at the round window. In a separate stage, the insertion feasibility was assessed. A tympanomeatal flap was prepared to provide visual control of the electrode insertion process and manually remove the bony overhang of the round window. The electrodes (4 Standard 12 channel 24 contact electrodes and 4 Flex28 12 channel 19 contact electrodes, MED-EL GmbH, Austria) were fed through the DCA tunnel by hand. Once the electrode tip entered the middle ear space, it was guided to the round window as necessary using a micropick. After the tip of the electrode was introduced into the round window, the insertion was primarily performed with the fingers from the lateral bone surface. Occasionally, additional tools such as
Bell et al.
the micropick or forceps were used to manipulate the electrode in the middle ear space.
Results The accuracy of the drilling attempts measured at the lateral surface was 0.08 ± 0.05 mm with a range of 0.01–0.17 mm. More importantly, the accuracy at the target (round window) was measured at 0.15 ± 0.08 mm with a range of 0.02–0.26 mm for the eight cases drilled. The facial nerve was preserved in every case. On an average, the robotic procedure including robot setup and patient marker attachment required 21.5 minutes to perform. Electrode insertion was easily performed, and the drilled tunnel aligned perfectly with the round window in every case. Full insertion was obtained in seven of the eight cases. In one case, a single electrode pair remained extra-cochlear (Fig. 2).
Discussion In this study, we were able to show that a minimally invasive DCA is feasible and safe using the described robot system. The demonstrated high accuracy of the system means that a large portion of the patient population can be treated with the proposed approach. Also, a highly accurate tunnel increases the likelihood of a successful insertion due to the ideal alignment of the tunnel with the intended target at the round window. DCA can be achieved in ∼20 minutes. The additional steps leading up to electrode insertion are estimated to take an additional 15 minutes. This means that a time savings in the OR is likely to be achieved in most cases. At present, a tympanomeatal flap was necessary to control the electrode insertion. Future investigations will address this aspect of the surgical workflow in more detail with the aim of a complete insertion through the DCA tunnel without the necessity of opening the tympanic membrane further increasing the efficiency of the operation. Although the method is intended to be compatible with any electrode type, only straight electrodes were utilized in this study. We also intend to investigate the use of precurved electrodes. A natural extension of the robotically drilled tunnel is an automated electrode insertion process which could reduce intra-cochlear damage. The safety of the intervention is the primary concern with a priority of protecting the facial nerve. With the safety of this structure in mind, we have also
An image-guided robot system for DCA
investigated alternative and redundant safety measures. We have developed an estimator of the drill position which is independent of the optical tracking system. This estimator correlates drilling forces with bone density information available from preoperative image data (Williamson et al., 2012). In addition to this, we are also investigating the integration of neuromonitoring systems which will alert the surgeon should the drill come too close to the facial nerve. We believe that a safe and effective tunnel procedure can be achieved through the combined effects of high accuracy, drill position estimation redundancy, and facial nerve monitoring. This work was financially supported by the Swiss National Science Foundation (NanoTera Initiative project title HearRestore) and from the European Commission FP7 (Project title HearEU). Cochlear electrode arrays were furnished by the Med-El Worldwide Headquarters, Innsbruck, Austria. B. Bell, T. Williamson, and S. Weber hold a patent on the drill position detection method, which could have future financial implications.
References Bell B., Stieger C., Gerber N., et al. 2012. A self-developed and constructed robot for minimally invasive cochlear implantation. Acta Oto-Laryngologica, 132(4): 355–360. Bielamowicz S.A., Coker N.J., Jenkins H.A., Igarashi M. 1988. Surgical dimensions of the facial recess in adults and children. Archives of Otolaryngology – Head & Neck Surgery, 114(5): 534–537. Gerber N., Gavaghan K., Bell B., et al. 2013. High accuracy patientto-image registration for the facilitation of image guided robotic microsurgery on the head. IEEE Transactions on Biomedical Engineering, 60(4): 960–968. Häusler R. 2002. Cochlear implantation without mastoidectomy: the pericanal electrode insertion technique. Acta OtoLaryngologica, 122(7): 715–719. Kiratzidis T., Arnold W., Iliades T. 2002. Veria operation updated. I. The trans-canal wall cochlear implantation. ORL; Journal for Oto-Rhino-Laryngology and its Related Specialties, 64(6): 406–412. Kronenberg J., Baumgartner W., Migirov L., Dagan T., Hildesheimer M. 2004. The suprameatal approach: an alternative surgical approach to cochlear implantation. Otology & Neurotology, 25(1): 41–44; discussion 44–45. Labadie R.F., Balachandran R., Mitchell J.E., et al. 2010. Clinical validation study of percutaneous cochlear access using patient-customized microstereotactic frames. Otology & Neurotology, 31(1): 94–99. Schipper J., Klenzner T., Aschendorff A., Arapakis I., Ridder G.J., Laszig R. 2004. Navigation-controlled cochleostomy. Is an improvement in the quality of results for cochlear implant surgery possible? HNO, 52(4): 329–335. Williamson T., Bell B., Gerber N., Salas L., Zysset P., Caversaccio M., et al. 2013. Estimation of tool pose based on forcedensity correlation during robotic drilling. IEEE Trans. Biomed. Eng., 60(4): 969–76.
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![[Direct acoustic cochlear stimulation for therapy of severe to profound mixed hearing loss: Codacs™ Direct Acoustic Cochlear Implant System].](/assets/img/hold.png)
