Otolaryngology–Head and Neck Surgery (2010) 142, 421-426

ORIGINAL RESEARCH–OTOLOGY AND NEUROTOLOGY

Percutaneous cochlear implant drilling via

customized frames: An in vitro study

Ramya Balachandran, PhD, Jason E. Mitchell, MS, Grégoire Blachon,Jack H. Noble, MS, Benoit M. Dawant, PhD, J. Michael Fitzpatrick, PhD,

and Robert F. Labadie, MD, PhD, Nashville, TN

Sponsorships or competing interests that may be relevant to con-tent are disclosed at the end of this article.

ABSTRACT

OBJECTIVE: Percutaneous cochlear implantation (PCI) surgeryuses patient-specific customized microstereotactic frames toachieve a single drill-pass from the lateral skull to the cochlea,avoiding vital anatomy. We demonstrate the use of a specificmicrostereotactic frame, called a “microtable,” to perform PCIsurgery on cadaveric temporal bone specimens.STUDY DESIGN: Feasibility study using cadaveric temporalbones.SUBJECTS AND METHODS: PCI drilling was performed onsix cadaveric temporal bone specimens. The main steps involvedwere 1) placing three bone-implanted markers surrounding the ear,2) obtaining a CT scan, 3) planning a safe surgical path to thecochlea avoiding vital anatomy, 4) constructing a microstereotacticframe to constrain the drill to the planned path, and 5) affixing theframe to the markers and using it to drill to the cochlea. Thespecimens were CT scanned after drilling to show the achievedpath. Deviation of the drilled path from the desired path wascomputed, and the closest distance of the mid-axis of the drilledpath from critical structures was measured.RESULTS: In all six specimens, we drilled successfully to thecochlea, preserving the facial nerve and ossicles. In four of sixspecimens, the chorda tympani was preserved, and in two of sixspecimens, it was sacrificed. The mean � standard deviation errorat the target was found to be 0.31 � 0.10 mm. The closestdistances of the mid-axis of the drilled path to structures were1.28 � 0.17 mm to the facial nerve, 1.31 � 0.36 mm to the chordatympani, and 1.59 � 0.43 mm to the ossicles.CONCLUSION: In a cadaveric model, PCI drilling is safe andeffective.

© 2010 American Academy of Otolaryngology–Head and NeckSurgery Foundation. All rights reserved.

Percutaneous cochlear implantation (PCI) involves dril-ling a linear path from the lateral skull to the cochlea

through the facial recess. PCI can be performed with acustomized microstereotactic frame that constrains the drillto the desired path. We have shown in our previous in

Received September 14, 2009; revised November 5, 2009; accepted Novemb

0194-5998/$36.00 © 2010 American Academy of Otolaryngology–Head and Necdoi:10.1016/j.otohns.2009.11.029

vitro1,2 and in vivo3,4 validation studies that it is possible tochoose a safe linear drill path for PCI and to construct acustomized microstereotactic frame that guides the drill tothe cochlea without causing risk to the facial nerve.

The main steps in PCI involve bone implanting at leastthree fiducial markers surrounding the mastoid, acquiring aCT scan, planning a safe trajectory in the CT scan relativeto the location of the fiducial markers, designing and con-structing a customized microstereotactic frame that mountson the fiducial markers and achieves the specified trajectory,and using the frame to drill to the cochlea. Our prior studiesinvolved validation of the PCI technique to make sure thata drill passing through the frame will target the cochleawithout risk to the facial nerve.1-4

For this application, we tested two microstereotacticframes (Fig 1)—the commercially available STarFix micro-Targeting Platform (FHC, Inc., Bowdoin, ME) and the re-cently developed microstereotactic table (which we refer toas a “microtable”)—both of which achieve the submillimet-ric accuracy necessary for PCI. The STarFix platform is amicrostereotactic frame approved by the Food and DrugAdministration for deep brain stimulation surgeries. It ismanufactured by using rapid-prototyping technology at acentralized facility. As a result, there is a two-day delay ingetting the microstereotactic frame ready for the surgeryafter obtaining the CT scan. This delay necessitates implant-ing markers on patients in the clinic at least two days priorto the surgery, which is inconvenient for both patients andsurgeons.

To avoid the manufacturing delay, we developed themicrotable as an alternative microstereotactic frame that canbe constructed onsite within a few minutes.2 The microtableconsists of a tabletop supported by three legs that mount onthe bone-implanted markers such that the tabletop is per-pendicular to the desired trajectory (Fig 2). The tabletop hasfour holes, three of which are used to connect to three legsand a fourth that serves as a targeting hole that determinesthe trajectory. The three holes for the legs are countersunkon the basis of the anatomy of the patient and the desired

er 19, 2009.

k Surgery Foundation. All rights reserved.

422 Otolaryngology–Head and Neck Surgery, Vol 142, No 3, March 2010

trajectory. The targeting hole is always designed such thatthe top of the countersink is 75 mm from the cochlea. Thetabletop can be manufactured in less than four minutes by astandard computer-numeric-control (CNC) machine. The invitro targeting accuracy of the microtable has been shown tobe 0.37 � 0.18 mm2, which is similar to the accuracy of theSTarFix platform (0.45 � 0.15 mm).5

We previously reported on the use of the microtable foran application similar to PCI, namely percutaneous accessto the petrous apex.6 The drilling mechanism we used in thatrecent report has been improved to achieve better accuracy.In this report, we present the results of PCI drilling incadaveric temporal bone specimens to target the cochlea.

Figure 1 Clinical validation of the percutaneous cochlear im-plantation technique using a customized microstereotactic frame.

A, STarFix platform. B, Microtable.

We follow the same procedure we propose to use for theclinical implementation of PCI.

Methods

Cadaveric temporal bone specimens were used. Therefore,institutional review board approval was not necessary forthis study.

We designed a drill press for this study to advance thedrill linearly along the path constrained by the microtable.On the basis of our previous drilling experience6 and guide-lines from the Machinery’s Handbook,7 we designed a newdrill press (Fig 3) consisting of a linear guide system thatcontrols the straight travel of the drill to a specific distanceand a drill bushing adaptor that is used to restrict themovement of the drill bit during drilling. We chose to use atraditional “twist” drill bit for our application because oto-logic drill bits, which are spherical in shape with side-

Figure 2 Microtable design. The tabletop has three holes forlegs that mount on markers and a targeting hole for the trajectory.

Figure 3 Drill press system. The linear guide system controlsthe advancement of the drill, and the drill bushing adaptor controls

the movement of the drill bit.

423Balachandran et al Percutaneous cochlear implant drilling via customized . . .

cutting flutes, are not ideal for forward drilling. For eachspecimen, we begin drilling with a 6-mm diameter drill bitand then switch to a 1.9-mm diameter drill bit for the last 20mm to the target, during which the facial recess is traversed.The linear guide system of the drill and drill-tip bushingadaptors are designed such that we drill the exact desireddistance and cannot physically override the system to drillfarther down the path. The desired distances are accuratelypreset by a FARO Gage Plus coordinate measuring machine(Faro Technologies Inc., Lake Mary, FL). The 6-mm be-ginning hole can be used to provide irrigation during thedeeper drilling. The 1.9-mm hole size to the cochlea willprovide sufficient room for electrode implantation with acustomized insertion tool (Schurzig D, Labadie RF, Hus-song A, et al. A second generation automated cochlearimplant insertion tool with integrated force sensing. In sub-mission.).8

For the current study, we followed the same procedurethat we would potentially follow for clinical implementa-tion. The procedure involves the following steps. (Note thatexcept for steps 1 and 2, which are done prior to the surgery,all the steps are performed in the operating room.)

Step 1: Acquisition of Preoperative CT ScanA clinically applicable temporal bone CT scan is acquired.For this study, we used a Philips Mx8000 IDT 16 (16-sliceacquisition) CT scanner that can provide a pixel size of0.25-by-0.25 mm and a slice thickness of 0.8 mm with 0.4mm overlap.

Figure 4 Trajectory planning. Significant structures are auto-matically segmented, and a drill trajectory that targets the scala

tympani and avoids the facial nerve is determined.

Step 2: Preoperative Trajectory PlanningA safe drill trajectory is automatically planned on the basisof the preoperative CT scan. To achieve this automaticplanning, we segmented the significant structures aroundthe target—namely, the facial nerve, the chorda tympani,the cochlea, the labyrinth, the ossicles, and the externalauditory canal—which are automatically segmented usingmethods described previously by our group.9,10 A drill tra-jectory is then automatically determined that targets thescala tympani avoiding the facial nerve11 (Fig 4). Segmen-tation followed by trajectory determination is completed inapproximately three minutes with an Intel Xeon 2.4 GHzdual quad-core 64-bit machine with 10 GB random accessmemory. The segmentation results and the trajectory areverified by the surgeon.

Step 3: Implantation of Fiducial MarkersMarkers are bone-implanted at three locations surroundingthe target region—typically in the mastoid tip, the supra-helical region, and the region posterior to the sigmoid sinus(Fig 5). Each marker consists of an anchor that screws intothe bone and an extension, at the tip of which is a sphericalportion that serves as a fiducial (Fig 2). Using these spheresas fiducials, instead of the anchors themselves, provideshigher accuracy.2

Step 4: Acquisition of Intraoperative CT ScanAfter the spherical fiducial markers are attached, an xCATENT mobile CT scanner (Xoran Technologies, Ann Arbor,MI) is used to acquire a CT scan with a voxel size of 0.3 mm

Figure 5 Locations of the three spherical fiducial markers.

in all directions.

424 Otolaryngology–Head and Neck Surgery, Vol 142, No 3, March 2010

Step 5: Intraoperative PlanningThe preoperative and intraoperative CT scans of the patient areregistered by using a standard mutual information method,12

and the trajectory is transformed from the preoperative scanto the intraoperative scan. Also, the centers of the sphericalfiducial markers are automatically localized. This step takesapproximately four minutes.

Step 6: Design and Fabrication of CustomizedMicrostereotactic FrameThe marker locations and the desired trajectory are used todesign a customized microtable. The design involves deter-mining the location and depths of four holes in the tabletopof the microtable and the lengths of the three legs thatconnect the tabletop to the markers. A custom Matlab (TheMathworks, Natick, MA) program automatically determinesthese values, creates a virtual model of the microtable, andgenerates the commands (in G-code) necessary to run theCNC milling machine for fabricating the tabletop. Thisprogram takes two or three seconds to run. The tabletop isthen fabricated in less than four minutes from a slab ofUltem (Quadrant Engineering Plastic Products, Reading,PA) by a CNC milling machine (Ameritech CNC, Brous-sard Enterprises, Inc., Santa Fe Springs, CA). Legs oflengths specified by the program are fitted into the three legholes of the microtable, and a drill-press adaptor is attachedto the targeting hole (Fig 2). A quality assurance check isperformed on the parts of the microtable. Attachment ofparts to the tabletop and quality assurance takes approxi-mately two minutes.

Step 7: Attachment of Frame and DrillingThe microtable is attached to the markers by tightening thegrippers, which are attached to the spherical fiducial mark-ers by thumbscrews (Fig 6). The drill bushing adaptor forthe 6-mm drill bit is inserted into the drill press, and then the

Figure 6 Microtable and drill press assembly attached to acadaveric temporal bone specimen for percutaneous cochlear im-

plantation drilling.

assembly is inserted into the targeting hole of the mi-crotable. Thumbnuts are tightened to hold the assembly inplace. Drilling is then performed along the constrained pathuntil the stop is reached. For the initial 6-mm drill bit, thestop is set at 20 mm from the desired target. The drill pressand the drill bushing adaptor are then removed from the drillpress adaptor. The 6-mm diameter drill bit is replaced witha 1.9-mm diameter drill bit. The drill bushing adaptor forthe 1.9-mm drill bit is inserted into the drill press; then theassembly is attached to the drill press adaptor as before.Drilling is again performed until the drill press stops at75 mm—the depth of the cochlea.

Step 8: Acquisition of Postoperative CT Scan forValidationTo analyze the quality of the drilling, we acquired a post-operative CT scan using the xCAT CT scanner with the1.9-mm drill bit in the drilled trajectory. The sphericalfiducial markers are present in this scan and are used toregister it to the intraoperative scan by means of a rigidbody point-based registration algorithm.13 The planned tra-jectory in the intraoperative scan is transformed to thepostoperative scan, and the deviation of the drilled trajec-tory from the planned trajectory is determined. The postop-erative scan is also registered, by using standard mutualinformation registration technique,12 to the preoperativescan, in which the critical structures have been automati-cally segmented (see Step 2). Applying this registrationtransformation, we transform the axis of the drilled path tothe preoperative scan and measure the closest distance ofthe drilled path to the critical structures.

Results

Six (n � 6) cadaveric temporal bone specimens were usedfor this study, and the procedure described in the previoussection was followed for each specimen. Both 6-mm and1.9-mm diameter holes were drilled as specified. It took lessthan five minutes for drilling including both the 6-mm and1.9-mm path. In all the specimens, the drill reached its target(i.e., the cochlea). Figure 7 shows the postoperative scanacquired with the 1.9-mm drill bit in the drilled hole.

To analyze the quality of the drilling, we localized themid-axis of the drilled path in the postoperative CT scan andtransformed it to the intraoperative CT scan using the reg-istration based on the spherical fiducial markers. The errorof the system, defined as the deviation of the drilled pathfrom the planned path, was computed at the target point andat a point 30 mm lateral from the target. The lateral pointwas chosen to assess any wandering of the drill tip shortlyafter impact with the lateral skull. Table 1 reports these errorvalues for each specimen. The mean � SD error was 0.31 �0.10 mm at the target and 0.15 � 0.09 mm at the laterallocation.

There was no contact between the drill and the facial

nerve in any of the specimens. The shortest distance from

425Balachandran et al Percutaneous cochlear implant drilling via customized . . .

the mid-axis of the drill to the facial nerve, chorda tympani,and ossicles were computed. Table 2 reports these distancesfor each specimen. The mean � SD of the closest distancewas 1.28 � 0.17 mm to the facial nerve, 1.31 � 0.36 mmto the chorda tympani, and 1.59 � 0.43 mm to the ossicles.Margin distances between the surface of the 1.9-mm drilland the surface of a critical structure can be determined bysubtracting the radius of the drill, which is 0.95 mm. Re-sulting negative numbers (specimens 1 and 2, chorda tym-pani) indicate that the structure was sacrificed. The meanmargin distance was 0.33 mm to the facial nerve, 0.36 mmto the chorda tympani, and 0.64 mm to the ossicles.

Discussion

In a cadaveric model, we have demonstrated drilling for aPCI procedure as guided by customized microstereotactic

Figure 7 Postoperative CT scan with the drill bit in the drilledhole. The dotted line shows the desired trajectory.

Table 1

Deviation of the drilled trajectory from the planned

trajectory

Specimen Ear

Deviation (mm)

At target At lateral point

1 Left 0.22 0.112 Right 0.22 0.123 Left 0.31 0.054 Left 0.45 0.325 Right 0.24 0.136 Right 0.41 0.19

Mean � SD 0.31 � 0.10 0.15 � 0.09

frames. Drilling was performed on six (n � 6) cadaverictemporal bone specimens. The target was achieved withoutdamage to vital anatomy. The facial nerve and ossicles werepreserved in all six specimens with satisfactory distancebetween the drill and the structure. The chorda tympani waspreserved in four specimens and sacrificed during planningand drilling in two specimens.

The achieved drilled path was close to the planned pathwith a deviation of 0.31 � 0.10 mm at the target. Thisaccuracy is dependent on the rigidity of the fiducial systemrelative to the patient’s head and the microtable. If thesystem is rigid, the image to physical registration will resultin lower error at the target.14 The anchors are bone-im-planted, enabling rigid attachment of the microstereotacticframe to the skull. Spherical fiducial markers are attached tothe anchors, and the microtable is rigidly attached to thespherical fiducial markers. With this rigid system, the tar-geting accuracy of the microtable at the cochlea has beenshown to be 0.37 � 0.18 mm by phantom studies.2 Thecurrent results of drilling, 0.31 � 0.10 mm at the cochlea,are comparable to this prior work, showing that our drillsystem does not add significant error to the system.

The microtable has been clinically validated for access-ing the cochlea.4 With the success of PCI drilling shown inthis study, we are in a position to begin PCI drilling in vivo.The procedure for this in vivo PCI drilling would be thesame as that mentioned in the Methods section, with theonly difference being that the microtable must be sterilizedafter fabrication (Step 6) and before attachment to the mark-ers (Step 7). Immediately after fabrication, the microtablewill be transported to the sub-sterile room near the operatingroom for sterilization. Approximate time to sterilize bysteam is 15 minutes. Once sterilized, the microtable will bemade available to the surgeon with a total time from end ofintraoperative CT scan to beginning of drilling of 30 min-utes.

For the proposed technique, a CNC machine is required.The one utilized in our laboratory costs approximately$25,000. We are in the process of building a miniaturized

Table 2

Closest distance of the mid-axis of the drilled

trajectory to critical structures

Specimen

Distance to structures (mm)

Facialnerve

Chordatympani Ossicles

1 1.06 0.83 1.562 1.34 0.88 1.343 1.23 1.60 1.354 1.50 1.53 1.405 1.40 1.65 2.456 1.12 1.34 1.45

Mean � SD 1.28 � 0.17 1.31 � 0.36 1.59 � 0.43

version that can be located in the operating theater and

426 Otolaryngology–Head and Neck Surgery, Vol 142, No 3, March 2010

would cost approximately $15,000. This initial capital ex-pense may be offset by the time savings afforded by the newtechnique, which we estimate to be consistently performedin less than 60 minutes. This is a significant time savingsover reports of traditional cochlear implantation.15,16

We anticipate performing PCI drilling in an incrementalfashion to ensure safety to patients, after which we willutilize a customized insertion tool (Schurzig D, et al. Insubmission.)8 to allow electrode insertion through the PCIdrilled path. Our next step is drilling the lateral part of thepath, stopping well short of the facial recess, after which anintraoperative CT scan will be acquired to confirm theaccuracy of the achieved path. With successful lateral dril-ling, we will begin drilling through the facial recess withfacial nerve monitoring. While this PCI approach seemsradical, it is based on sound engineering concepts and datademonstrating submillimetric accuracy.

Author Information

From the Departments of Otolaryngology (Drs. Balachandran and Laba-die), Mechanical Engineering (Mr. Mitchell and Mr. Blachon), and Elec-trical Engineering and Computer Science (Drs. Fitzpatrick and Dawant,and Mr. Noble), Vanderbilt University, Nashville, TN.

Corresponding author: Ramya Balachandran, PhD, Research AssistantProfessor, Department of Otolaryngology, Vanderbilt University MedicalCenter, 7209 Medical Center East, South Tower, 1215 21st Ave South,Nashville, TN 37232.

E-mail address: ramya.balachandran@vanderbilt.edu.

This article was presented at the 2009 AAO–HNSF Annual Meeting &OTO EXPO, San Diego, CA, October 4-7, 2009.

Author contributions

Ramya Balachandran, study design, data acquisition, analysis, and inter-pretation, manuscript editing; Jason E. Mitchell, study design, data ac-quisition, manuscript editing; Grégoire Blachon, study design, data ac-quisition, manuscript editing; Jack H. Noble, study design, dataacquisition, analysis, and interpretation, manuscript editing; Benoit M.Dawant, study design, manuscript editing; J. Michael Fitzpatrick, studydesign, data acquisition, analysis, and interpretation, manuscript editing;Robert F. Labadie, study design, data acquisition, analysis, and interpre-tation, manuscript editing.

Disclosures

Competing interests: Jason E. Mitchell, patent applications on variouscomponents of the technology pending; Jack H. Noble, patent applicationson various components of the technology pending; Benoit M. Dawant,patent applications on various components of the technology pending; J.Michael Fitzpatrick, patent applications on various components of thetechnology pending; Robert F. Labadie, patent applications on various

components of the technology pending.

Sponsorships: The project described was supported by Award NumberR01DC008408 from the National Institute on Deafness and Other Com-munication Disorders. The content is solely the responsibility of the au-thors and does not necessarily represent the official views of the NationalInstitute on Deafness and Other Communication Disorders or the NationalInstitutes of Health.

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