Clinical Implementation of Second-generation Minimally Invasive Image-guided Cochlear Implantation Surgery.

OBJECTIVE: Minimally invasive, image-guided cochlear implantation (CI) surgery consists of drilling a precise tunnel from the surface of the mastoid cortex through the facial recess to target the scala tympani. In the first set of clinical trials of this technique, heat-induced facial nerve paresis (House-Brackmann II/VI) occurred on a patient on the last day of the initial trial which was scheduled to be halted secondary to a change in the regulatory requirements dictated by the 2012 the Food and Drug Administration Safety and Innovation Act requiring Investigational Device Exemption approval for previously exempted customized medical device testing. To address this adverse event, extensive changes were made to the drilling protocol; additionally, a custom insertion tool was developed. To address the Food and Drug Administration Safety and Innovation Act, an Investigational Device Exemption was submitted and, subsequently approved. Herein is described our first clinical implementation of the modified technique. PATIENT: Seventy-year-old with profound, postlingual sensorineural hearing loss who had previously undergone right CI via traditional approach in 2015. INTERVENTION: Minimally invasive image-guided left CI. MAIN OUTCOME MEASURE: Time of intervention, final location of CI electrode array within cochlea. RESULTS: Surgery took 155 minutes of which the largest components (in descending order) were soft tissue work, closure, and drilling. Full scala tympani insertion with angular insertion depth of 557 degrees of the electrode array was achieved. There were no complications, and the patient had an uneventful recovery and activation. CONCLUSIONS: Minimally invasive, image-guided CI surgery is achievable and reduces the mastoid depression associated with traditional CI surgery. CLINICALTRIALSGOV INFORMATION: Study NCT03101917, Microtable Microstereotactic Frame and Drill Press and Associated Method for Cochlear Implantation. LEVEL OF EVIDENCE: Case Report.

Workflow and simulation of image-to-physical registration of holes inside spongy bone.

PURPOSE: Mastoid cells as well as trabecula provide unique bone structures, which can serve as natural landmarks for registration. Preoperative imaging enables sufficient acquisition of these structures, but registration requires an intraoperative counterpart. Since versatile surgical interventions involve drilling into mastoid cells and trabecula, we propose a registration method based on endoscopy inside of these drill holes. METHODS: Recording of the surface of the inner drill hole yields bone-air patterns that provide intraoperative registration features. In this contribution, we discuss an approach that unrolls the drill hole surface into a two-dimensional image. Intraoperative endoscopic recordings are compared to simulated endoscopic views, which originate from preoperative data like computed tomography. Each simulated view corresponds to a different drill pose. The whole registration procedure and workflow is demonstrated, using high-resolution image data to simulate both preoperative and endoscopic image data. RESULTS: As the driving application is minimally invasive cochlear implantation, in which targets are close to the axis of the drill hole, Target Registration Error (TRE) was measured at points near the axis. TRE at increasing depths along the drill trajectory reveals increasing registration accuracy as more bone-air patterns become available as landmarks with the highest accuracy obtained at the center point. At the facial recess and the cochlea, TREs are ([Formula: see text]) mm and ([Formula: see text]) mm, respectively. CONCLUSION: This contribution demonstrates a new method for registration via endoscopic acquisition of small features like trabecula or mastoid cells for image-guided procedures. It has the potential to revolutionize bone registration because it requires only a preoperative dataset and intraoperative endoscopic exploration. Endoscopic recordings of at least 20 mm length and isotropic voxel sizes of 0.2 mm or smaller of the preoperative image data are recommended.

Safety margins in robotic bone milling: from registration uncertainty to statistically safe surgeries.

BACKGROUND: When robots mill bone near critical structures, safety margins are used to reduce the risk of accidental damage due to inaccurate registration. These margins are typically set heuristically with uniform thickness, which does not reflect the anisotropy and spatial variance of registration error. METHODS: A method is described to generate spatially varying safety margins around vital anatomy using statistical models of registration uncertainty. Numerical simulations are used to determine the margin geometry that matches a safety threshold specified by the surgeon. RESULTS: The algorithm was applied to CT scans of five temporal bones in the context of mastoidectomy, a common bone milling procedure in ear surgery that must approach vital nerves. Safety margins were generated that satisfied the specified safety levels in every case. CONCLUSIONS: Patient safety in image-guided surgery can be increased by incorporating statistical models of registration uncertainty in the generation of safety margins around vital anatomy.

Increasing Safety of a Robotic System for Inner Ear Surgery Using Probabilistic Error Modeling Near Vital Anatomy.

Safe and effective planning for robotic surgery that involves cutting or ablation of tissue must consider all potential sources of error when determining how close the tool may come to vital anatomy. A pre-operative plan that does not adequately consider potential deviations from ideal system behavior may lead to patient injury. Conversely, a plan that is overly conservative may result in ineffective or incomplete performance of the task. Thus, enforcing simple, uniform-thickness safety margins around vital anatomy is insufficient in the presence of spatially varying, anisotropic error. Prior work has used registration error to determine a variable-thickness safety margin around vital structures that must be approached during mastoidectomy but ultimately preserved. In this paper, these methods are extended to incorporate image distortion and physical robot errors, including kinematic errors and deflections of the robot. These additional sources of error are discussed and stochastic models for a bone-attached robot for otologic surgery are developed. An algorithm for generating appropriate safety margins based on a desired probability of preserving the underlying anatomical structure is presented. Simulations are performed on a CT scan of a cadaver head and safety margins are calculated around several critical structures for planning of a robotic mastoidectomy.

Incorporating Target Registration Error Into Robotic Bone Milling.

Robots have been shown to be useful in assisting surgeons in a variety of bone drilling and milling procedures. Examples include commercial systems for joint repair or replacement surgeries, with in vitro feasibility recently shown for mastoidectomy. Typically, the robot is guided along a path planned on a CT image that has been registered to the physical anatomy in the operating room, which is in turn registered to the robot. The registrations often take advantage of the high accuracy of fiducial registration, but, because no real-world registration is perfect, the drill guided by the robot will inevitably deviate from its planned path. The extent of the deviation can vary from point to point along the path because of the spatial variation of target registration error. The allowable deviation can also vary spatially based on the necessary safety margin between the drill tip and various nearby anatomical structures along the path. Knowledge of the expected spatial distribution of registration error can be obtained from theoretical models or experimental measurements and used to modify the planned path. The objective of such modifications is to achieve desired probabilities for sparing specified structures. This approach has previously been studied for drilling straight holes but has not yet been generalized to milling procedures, such as mastoidectomy, in which cavities of more general shapes must be created. In this work, we present a general method for altering any path to achieve specified probabilities for any spatial arrangement of structures to be protected. We validate the method via numerical simulations in the context of mastoidectomy.

A Compact, Bone-Attached Robot for Mastoidectomy.

Otologic surgery often involves a mastoidectomy, which is the removal of a portion of the mastoid region of the temporal bone, to safely access the middle and inner ear. The surgery is challenging because many critical structures are embedded within the bone, making them difficult to see and requiring a high level of accuracy with the surgical dissection instrument, a high-speed drill. We propose to automate the mastoidectomy portion of the surgery using a compact, bone-attached robot. The system described in this paper is a milling robot with four degrees-of-freedom (DOF) that is fixed to the patient during surgery using a rigid positioning frame screwed into the surface of the bone. The target volume to be removed is manually identified by the surgeon pre-operatively in a computed tomography (CT) scan and converted to a milling path for the robot. The surgeon attaches the robot to the patient in the operating room and monitors the procedure. Several design considerations are discussed in the paper as well as the proposed surgical workflow. The mean targeting error of the system in free space was measured to be 0.5 mm or less at vital structures. Four mastoidectomies were then performed in cadaveric temporal bones, and the error at the edges of the target volume was measured by registering a postoperative computed tomography (CT) to the pre-operative CT. The mean error along the border of the milled cavity was 0.38 mm, and all critical anatomical structures were preserved.

Preliminary Testing of a Compact, Bone-Attached Robot for Otologic Surgery.

Otologic surgery often involves a mastoidectomy procedure, in which part of the temporal bone is milled away in order to visualize critical structures embedded in the bone and safely access the middle and inner ear. We propose to automate this portion of the surgery using a compact, bone-attached milling robot. A high level of accuracy is required to avoid damage to vital anatomy along the surgical path, most notably the facial nerve, making this procedure well-suited for robotic intervention. In this study, several of the design considerations are discussed and a robot design and prototype are presented. The prototype is a 4 degrees-of-freedom robot similar to a four-axis milling machine that mounts to the patient's skull. A positioning frame, containing fiducial markers and attachment points for the robot, is rigidly attached to the skull of the patient, and a CT scan is acquired. The target bone volume is manually segmented in the CT by the surgeon and automatically converted to a milling path and robot trajectory. The robot is then attached to the positioning frame and is used to drill the desired volume. The accuracy of the entire system (image processing, planning, robot) was evaluated at several critical locations within or near the target bone volume with a mean free space accuracy result of 0.50 mm or less at all points. A milling test in a phantom material was then performed to evaluate the surgical workflow. The resulting milled volume did not violate any critical structures.

Clinical testing of an alternate method of inserting bone-implanted fiducial markers.

BACKGROUND: Deep brain stimulation (DBS) surgery utilizes image guidance via bone-implanted fiducial markers to achieve the desired submillimetric accuracy and to provide means for attaching microstereotactic frames. For maximal benefit, the markers must be inserted to the correct depth since over-insertion leads to stripping and under-insertion leads to instability. PURPOSE: The purpose of the study was to test clinically a depth-release drive system, the PosiSeat™, versus manual insertion (pilot hole followed by manual screwing until tactile determined correct seating) for implanting fiducial markers into the bone. METHODS: With institutional review board approval, the PosiSeat™ was used to implant markers in 15 DBS patients (57 fiducials). On post-insertion CT scans, the depth of the gap between the shoulder of the fiducial markers and the closest bone surface was measured. Similar depth measurements were performed on the CT scans of 64 DBS patients (250 fiducials), who underwent manual fiducial insertion. RESULTS: Median of shoulder-to-bone distance for PosiSeat™ and manual insertion group were 0.03 and 1.06 mm, respectively. Fifty percent of the fiducials had the shoulder-to-bone distances within 0.01-0.09 mm range for the PosiSeat group and 0.04-1.45 mm range for the manual insertion group. These differences were statistically significant. CONCLUSIONS: A depth-release drive system achieves more consistent placement of bone-implanted fiducial markers than manual insertion.

Minimally invasive image-guided cochlear implantation for pediatric patients: clinical feasibility study.

OBJECTIVE: Minimally invasive image-guided cochlear implantation (CI) involves accessing the cochlea via a linear path from the lateral skull to the cochlea avoiding vital structures including the facial nerve. Herein, we describe and demonstrate the feasibility of the technique for pediatric patients. STUDY DESIGN: Prospective. SETTING: Children's Hospital. SUBJECTS AND METHODS: Thirteen pediatric patients (1.5 to 8 years) undergoing traditional CI participated in this Institutional Review Board-approved study. Three fiducial markers were bone-implanted surrounding the ear, and a CT scan was acquired. The CT scan was processed to identify the marker locations and critical structures of the temporal bone. A safe linear path was determined to target the cochlea avoiding damage to vital structures. A custom microstereotactic frame was fabricated that would mount on the fiducial markers and constrain a tool to the desired trajectory. After traditional mastoidectomy and prior to cochleostomy, the custom microstereotactic frame was mounted on the bone-implanted markers to confirm that the achieved trajectory was safe and accurately accessed the cochlea. RESULTS: For all the 13 patients, it was possible to determine a safe trajectory to the cochlea. Custom microstereotactic frames were validated successfully on 9 patients. Two of these patients had inner ear malformations, and this technique helped the surgeon confirm ideal location for cochleostomy. For patients with normal anatomy, the mean and standard deviation of the closest distance of the trajectory to facial nerve and chorda tympani were 1.1 ± 0.3 mm and 1.2 ± 0.5 mm, respectively. CONCLUSION: Minimally invasive image-guided CI is feasible for pediatric patients.

Minimally invasive image-guided cochlear implantation surgery: first report of clinical implementation.

OBJECTIVES/HYPOTHESIS: Minimally invasive image-guided approach to cochlear implantation (CI) involves drilling a narrow, linear tunnel to the cochlea. Reported herein is the first clinical implementation of this approach. STUDY DESIGN: Prospective cohort study. METHODS: On preoperative computed tomography (CT), a safe linear trajectory through the facial recess targeting the scala tympani was planned. Intraoperatively, fiducial markers were bone-implanted, a second CT was acquired, and the trajectory was transferred from preoperative to intraoperative CT. A customized microstereotactic frame was rapidly designed and constructed to constrain a surgical drill along the desired trajectory. Following sterilization, the frame was employed to drill the tunnel to the middle ear. After lifting a tympanomeatal flap and performing a cochleostomy, the electrode array was threaded through the drilled tunnel and into the cochlea. RESULTS: Eight of nine patients were successfully implanted using the proposed approach with six insertions completely within the scala tympani. Traditional mastoidectomy was performed on one patient following difficulty threading the electrode array via the narrow tunnel. Other difficulties encountered included use of the backup implant when an electrode was dislodged during threading via the tunnel, tip fold-over, and facial nerve paresis (House-Brackmann II/VI at 12 months) secondary to heat during drilling. The average time of intervention was 182 ± 36 minutes. CONCLUSIONS: Minimally invasive image-guided CI is clinically achievable. Further clinical study is necessary to address technological difficulties during drilling and insertion, and to assess potential benefits including decreased time of intervention, standardization of surgical intervention, and decreased tissue dissection potentially leading to shorter recovery and earlier implant activation.

Validation of minimally invasive, image-guided cochlear implantation using Advanced Bionics, Cochlear, and Medel electrodes in a cadaver model.

PURPOSE: Validation of a novel minimally invasive, image-guided approach to implant electrodes from three FDA-approved manufacturers-Medel, Cochlear, and Advanced Bionics-in the cochlea via a linear tunnel from the lateral cranium through the facial recess to the cochlea. METHODS: Custom microstereotactic frames that mount on bone-implanted fiducial markers and constrain the drill along the desired path were utilized on seven cadaver specimens. A linear tunnel was drilled from the lateral skull to the cochlea followed by a marginal, round window cochleostomy and insertion of the electrode array into the cochlea through the drilled tunnel. Post-insertion CT scan and histological analysis were used to analyze the results. RESULTS: All specimens ([Formula: see text]) were successfully implanted without visible injury to the facial nerve. The Medel electrodes ([Formula: see text]) had minimal intracochlear trauma with 8, 8, and 10 (out of 12) electrodes intracochlear. The Cochlear lateral wall electrodes (straight research arrays) ([Formula: see text]) had minimal trauma with 20 and 21 of 22 electrodes intracochlear. The Advanced Bionics electrodes ([Formula: see text]) were inserted using their insertion tool; one had minimal insertion trauma and 14 of 16 electrodes intracochlear, while the other had violation of the basilar membrane just deep to the cochleostomy following which it remained in scala vestibuli with 13 of 16 electrodes intracochlear. CONCLUSIONS: Minimally invasive, image-guided cochlear implantation is possible using electrodes from the three FDA-approved manufacturers. Lateral wall electrodes were associated with less intracochlear trauma suggesting that they may be better suited for this surgical technique.

Evaluation of portable CT scanners for otologic image-guided surgery.

PURPOSE: Portable CT scanners are beneficial for diagnosis in the intensive care unit, emergency room, and operating room. Portable fixed-base versus translating-base CT systems were evaluated for otologic image-guided surgical (IGS) applications based on geometric accuracy and utility for percutaneous cochlear implantation. METHODS: Five cadaveric skulls were fitted with fiducial markers and scanned using both a translating-base, 8-slice CT scanner (CereTom(®)) and a fixed-base, flat-panel, volume CT (fpVCT) scanner (Xoran xCAT(®)). Images were analyzed for: (a) subjective quality (i.e., noise), (b) consistency of attenuation measurements (Hounsfield units) across similar tissue, and (c) geometric accuracy of fiducial marker positions. The utility of these scanners in clinical IGS cases was tested. RESULTS: Five cadaveric specimens were scanned using each of the scanners. The translating-base, 8-slice CT scanner had spatially consistent Hounsfield units, and the image quality was subjectively good. However, because of movement variations during scanning, the geometric accuracy of fiducial marker positions was low. The fixed-base, fpVCT system had high spatial resolution, but the images were noisy and had spatially inconsistent attenuation measurements, while the geometric representation of the fiducial markers was highly accurate. CONCLUSION: Two types of portable CT scanners were evaluated for otologic IGS. The translating-base, 8-slice CT scanner provided better image quality than a fixed-base, fpVCT scanner. However, the inherent error in three-dimensional spatial relationships by the translating-based system makes it suboptimal for otologic IGS use.

Convergent iterative closest-point algorithm to accomodate anisotropic and inhomogenous localization error.

Since its introduction in the early 1990s, the Iterative Closest Point (ICP) algorithm has become one of the most well-known methods for geometric alignment of 3D models. Given two roughly aligned shapes represented by two point sets, the algorithm iteratively establishes point correspondences given the current alignment of the data and computes a rigid transformation accordingly. From a statistical point of view, however, it implicitly assumes that the points are observed with isotropic Gaussian noise. In this paper, we show that this assumption may lead to errors and generalize the ICP such that it can account for anisotropic and inhomogenous localization errors. We 1) provide a formal description of the algorithm, 2) extend it to registration of partially overlapping surfaces, 3) prove its convergence, 4) derive the required covariance matrices for a set of selected applications, and 5) present means for optimizing the runtime. An evaluation on publicly available surface meshes as well as on a set of meshes extracted from medical imaging data shows a dramatic increase in accuracy compared to the original ICP, especially in the case of partial surface registration. As point-based surface registration is a central component in various applications, the potential impact of the proposed method is high.

Can otolaryngology compete with larger fields regarding impact factor?: is percentile-based impact factor a solution?

Impact factor (IF) consists of reporting the number of references an average article in a given journal receives over a 2-year period. Despite several valid criticisms, IF has become an important component of academic advancement. The authors sought to investigate the possible relationship between size of specialty field and IF. The top 10 journals of 13 specialty fields were selected based on IF as reported by Journal Citations Reports on the Web of Science. Specialty field population was obtained from the American Board of Medical Specialties. A highly positive correlation (r = 0.9) was noted with smaller fields (eg, otolaryngology) having lower IFs. To overcome this population bias, a percentile-based impact factor (PIF) may be used where the top journal within a field is given 100%, the worst 0%, and all other journals' IFs are proportionately scaled in between the 2 extremes. PIF acts to "level the playing field," allowing between-specialty field comparisons.

General approach to first-order error prediction in rigid point registration.

A general approach to the first-order analysis of error in rigid point registration is presented that accommodates fiducial localization error (FLE) that may be inhomogeneous (varying from point to point) and anisotropic (varying with direction) and also accommodates arbitrary weighting that may also be inhomogeneous and anisotropic. Covariances are derived for target registration error (TRE) and for weighted fiducial registration error (FRE) in terms of covariances of FLE, culminating in a simple implementation that encompasses all combinations of weightings and anisotropy. Furthermore, it is shown that for ideal weighting, in which the weighting matrix for each fiducial equals the inverse of the square root of the cross covariance of its two-space FLE, fluctuations of FRE and TRE are mutually independent. These results are validated by comparison with previously published expressions and by simulation. Furthermore, simulations for randomly generated fiducial positions and FLEs are presented that show that correlation is negligible (correlation coefficient < 0.1) in the exact case for both ideal and uniform weighting (i.e., no weighting), the latter of which is employed in commercial surgical guidance systems. From these results we conclude that for these weighting schemes, while valid expressions exist relating the covariance of FRE to the covariance of TRE, there are no measures of the goodness of fit of the fiducials for a given registration that give to first order any information about the fluctuation of TRE from its expected value and none that give useful information in the exact case. Therefore, as estimators of registration accuracy, such measures should be approached with extreme caution both by the purveyors of guidance systems and by the practitioners who use them.

Robotic mastoidectomy.

HYPOTHESIS: Using image-guided surgical techniques, we propose that an industrial robot can be programmed to safely, effectively, and efficiently perform a mastoidectomy. BACKGROUND: Whereas robotics is a mature field in many surgical applications, robots have yet to be clinically used in otologic surgery despite significant advantages including reliability and precision. METHODS: We designed a robotic system that incorporates custom software with an industrial robot to manipulate a surgical drill through a complex milling profile. The software controls the movements of the robot based on real-time feedback from a commercially available optical tracking system. The desired path of the drill to remove the desired volume of mastoid bone was planned using computed tomographic scans of cadaveric specimens and then implemented using the robotic system. Bone-implanted fiducial markers were used to provide accurate registration between computed tomographic and physical space. RESULTS: A mastoid cavity was milled on 3 cadaveric specimens with a 5-mm fluted ball bit. Postmilling computed tomographic scans showed that, for the 3 specimens, 97.70%, 99.99%, and 96.05% of the target region was ablated without violation of any critical feature. CONCLUSION: To the best of our knowledge, this is the first time that a robot has been used to perform a mastoidectomy. Although significant hurdles remain to translate this technology to clinical use, we have shown that it is feasible. The prospect of reducing surgical time and enhancing patient safety by replacing human hand-eye coordination with machine precision motivates future work toward translating this technique to clinical use.

Improved method for point-based tracking.

Image-guided surgery systems have a wide range of applications where the level of accuracy required for each application varies from millimeters to low sub-millimeter range. In systems that use optical tracking, it is typical to use point-based registration without any weighting schemes to determine the pose of the tracked tool with very good accuracy. However, recent advancements in methods to estimate the measurement uncertainty for each tracked marker and the development of an anisotropically weighted point-based registration algorithm have allowed for the optical tracking accuracy to be improved. In this article, we demonstrate a new tracking method that improves the tracking accuracy by 20-45% over the traditional tracking methodology.

Automatic determination of optimal linear drilling trajectories for cochlear access accounting for drill-positioning error.

BACKGROUND: Cochlear implantation is a surgical procedure in which an electrode array is permanently implanted into the cochlea to stimulate the auditory nerve and allow deaf people to hear. Percutaneous cochlear access, a new minimally invasive implantation approach, requires drilling a single linear channel from the skull surface to the cochlea. The focus of this paper addresses a major challenge with this approach, which is the ability to determine, in a pre-operative CT, a safe and effective drilling trajectory. METHODS: A measure of the safety and effectiveness of a given trajectory relative to sensitive structures is derived using a Monte Carlo approach. The drilling trajectory that maximizes this measure is found using an optimization algorithm. RESULTS: In tests on 13 ears, the technique was shown to find approximately twice as many acceptable trajectories as those found manually by an experienced surgeon. CONCLUSIONS: Using this method, safe trajectories can be automatically determined quickly and consistently.

Effect of MR distortion on targeting for deep-brain stimulation.

Deep-brain stimulation (DBS) surgery involves placing electrodes within specific deep-brain target nuclei. Surgeons employ MR imaging for preoperative selection of targets and computed tomography (CT) imaging for designing stereotactic frames used for intraoperative placement of electrodes at the targets. MR distortion may contribute to target-selection error in the MR scan and also to MR-CT registration error, each of which contributes to error in electrode placement. In this paper, we analyze the error contributed by the MR distortion to the total DBS targeting error. Distortion in conventional MR scans, both T1 and T2 weighted, were analyzed for six bilateral DBS patients in the typical areas of brain using typical scans on a 3-T clinical scanner. Mean targeting error due to MR distortion in T2 was found to be 0.07 +/- 0.025 mm with a maximum of 0.13 mm over 12 targets; error in the T1 images was smaller by 4%.

Percutaneous cochlear implant drilling via customized frames: an in vitro study.

OBJECTIVE: Percutaneous cochlear implantation (PCI) surgery uses patient-specific customized microstereotactic frames to achieve a single drill-pass from the lateral skull to the cochlea, avoiding vital anatomy. We demonstrate the use of a specific microstereotactic frame, called a "microtable," to perform PCI surgery on cadaveric temporal bone specimens. STUDY DESIGN: Feasibility study using cadaveric temporal bones. SUBJECTS AND METHODS: PCI drilling was performed on six cadaveric temporal bone specimens. The main steps involved were 1) placing three bone-implanted markers surrounding the ear, 2) obtaining a CT scan, 3) planning a safe surgical path to the cochlea avoiding vital anatomy, 4) constructing a microstereotactic frame to constrain the drill to the planned path, and 5) affixing the frame to the markers and using it to drill to the cochlea. The specimens were CT scanned after drilling to show the achieved path. Deviation of the drilled path from the desired path was computed, and the closest distance of the mid-axis of the drilled path from critical structures was measured. RESULTS: In all six specimens, we drilled successfully to the cochlea, preserving the facial nerve and ossicles. In four of six specimens, the chorda tympani was preserved, and in two of six specimens, it was sacrificed. The mean +/- standard deviation error at the target was found to be 0.31 +/- 0.10 mm. The closest distances of the mid-axis of the drilled path to structures were 1.28 +/- 0.17 mm to the facial nerve, 1.31 +/- 0.36 mm to the chorda tympani, and 1.59 +/- 0.43 mm to the ossicles. CONCLUSION: In a cadaveric model, PCI drilling is safe and effective.