Simulation for Training Cochlear Implant Electrode Insertion

Xingjun Ma∗, Sudanthi Wijewickrema†, Yun Zhou†, Bridget Copson†,James Bailey∗, Gregor Kennedy‡ and Stephen O’Leary†

∗ Department of Computing & Information Systems, The University of Melbourne, Australia†Department of Surgery (Otolaryngology), The University of Melbourne, Australia‡ Center for the Study of Higher Education, The University of Melbourne, Australia

Email: {xingjunm@student., swijewickrem@, yun.zhou@, bcopson@student., baileyj@, gek@, sjoleary@}unimelb.edu.au

Abstract—Cochlear implant surgery is performed to restorehearing in patients with a range of hearing disorders. Tooptimise hearing outcomes, trauma during the insertion of acochlear implant electrode has to be minimised. Factors thatcontribute to the degree of trauma caused during surgeryinclude: the location of the electrode, type of electrode, andthe competence level of the surgeon. Surgical competencedepends on knowledge of anatomy and experience in a range ofsituations, along with technical skills. Thus, during training, asurgeon should be exposed to a range of anatomical variations,where he/she can learn and practice the intricacies of thesurgical procedure, as well as explore different implant optionsand consequences thereof. Virtual reality simulation offers aversatile platform on which such training can be conducted.In this paper, we discuss a prototype implementation for thevisualisation and analysis of electrode trajectories in relation toanatomical variation, prior to its inclusion in a virtual realitytraining module for cochlear implant surgery.

Keywords-Cochlear Implant Trajectory, Cochlear ImplantElectrode Insertion Training, Electrode Trajectory Visualisa-tion, Virtual Reality Simulation

I. INTRODUCTION

Cochlear implants are increasingly being used to restoreor augment hearing in patients with a variety of hearingdisorders [1], [2]. They are designed to replace the func-tionality of a damaged inner ear. The external portion of acochlear implant, placed behind the ear, typically consistsof a microphone and speech processor that capture soundand convert it into digital code. The (internal) implantcomprises a transmitter that converts the digital code toelectrical impulses, and an electrode array that transfers themto different regions of the auditory nerve.

It is evidenced that to optimise hearing outcomes fromcochlear implants, it is necessary to reduce the amountof trauma inflicted during the implantation process [3].Location of the electrode is an important factor in traumareduction [4], [5] which depends to a large degree onsufficient preparation of the temporal bone prior to theinsertion [6], [7], [8]. As such, it is imperative that surgeonsbe trained in proper surgical preparation and exposed to avariety of anatomical variations so that they understand howa surgical procedure can be tailored according to the patient.Torres et al. [9] demonstrated that experts inserted theelectrodes closer to the optimal insertion vector than fellows

and residents, confirming the importance of experience andtraining in cochlear implantation.

Training in cochlear implant surgery is currently relianton the apprenticeship model where a trainee observes andpractices under expert guidance in the operating theatre andcadaveric training programs [10], [11]. However, the soleuse of these training methods are becoming increasinglyinefficient in the current climate due to reasons such asscarcity of cadavers and reduction in time available forteaching [12], [13]. Computer based simulation technologiessuch as virtual reality (VR) offer an attractive alternativeto supplement traditional training methods. Use of VRsimulation for skill acquisition and translation in temporalbone surgery such as cortical mastoidectomy has been wellstudied [14], [15], [16], [17], [18]. Recent studies such asCopson et al. [19] have also addressed how more complexsurgeries such as preparation/dissection of the temporal bonefor cochlear implantation can be trained.

VR simulation also offers a convenient platform to trainsurgeons on the electrode insertion part of cochlear implantsurgery. One such example is Todd et al. [20], where visualand haptic feedback is provided to train electrode insertionto the cochlea. However, this simulation is quite limited inthat it does not address how the dissection of the temporalbone and anatomical structures such as the facial nerve andchorda tympani affect the insertion vectors.

Here, we discuss a prototype simulation module thatallows the trainee to visualise valid electrode trajectoriesalong with their contact locations on the cochlea at differentstages of temporal bone preparation. This will provide theopportunity to understand how the level of dissection affectsthe number of available trajectories, and how different strate-gies may be required for different anatomies. Integration ofthis system to a haptic-enabled VR simulator will allow thetrainee to practice the actual electrode insertion guided bythe possible trajectories thus generated.

The rest of the paper is organised as follows. SectionII briefly describes the VR simulation platform for whichthis prototype is developed, along with details of cochlearimplantation surgery. Sections III and IV discuss how theproblem is formulated and the design of the proposed solu-tion respectively. Section V shows examples of visualisation

and analysis of electrode trajectories using the proposedsystem. Section VI concludes the paper with a discussionof findings and future avenues of research.

II. SETTING

A. Simulation Platform

The prototype system discussed here is intended forthe University of Melbourne VR temporal bone surgerysimulator (see figure 1). The simulator consists of a PC (ora laptop), a haptic device, and a MIDI controller to provideinput. A virtual model of a temporal bone is displayed in3D with which the surgeon interacts using the haptic devicethat provides force/tactile feedback. Time since the start ofthe surgery is displayed on the screen for ease of assessmentand analysis.

Figure 1: The University of Melbourne VR temporal bonesurgery simulator.

Anatomical structures of the middle and inner ear thathave to be safely navigated when performing temporal bonesurgery are presented in the virtual model (see figure 2).

Figure 2: Anatomical structures of the middle and inner ear:A) facial nerve, B) sigmoid sinus, C) dura, D) ossicles, E)semicircular canals, F) round window, G) cochlea, and H)chorda tympani.

Virtual temporal bones are generated by processingMicro-CT scans of human cadaveric temporal bones. AMicro-CT scan is a 3D volume of grey scale data, fromwhich the temporal bone and the underlying anatomicalstructures are identified and segmented through a semi-automated process. The output of this step is a discretised 3Dvolume with labels to identify different structures. The struc-tures are then rendered in the virtual operating space usinga modified marching cubes algorithm [21]. This algorithmproduces a surface mesh consisting of a set of triangles foreach anatomical structure (and bone). Figure 3 illustrates theprocess of generating virtual models from microCT scans.

During a virtual surgery, the segment data is updated asmaterial is removed. The area being drilled is re-renderedin real-time using the updated segment data. In addition,the simulator records the timestamp and location of eachvoxel that has been removed, making it possible to trackthe sequence in which material is removed. In addition,other metrics such as specimen and drill position, specimenorientation, and distance of the drill to anatomical structuresare saved at a frequency of about 15Hz. This data can beused to recreate a surgery and identify which area of thebone is drilled during a given time interval.

B. Cochlear Implant Surgery

The operation undertaken to insert a cochlear implantconsists of a few stages. First, a cortical mastoidectomy isperformed to remove bone from the mastoid and uncoveranatomical structure landmarks such as the dura, sigmoidsinus, and facial nerve. Then, the bone between the facialnerve and the chorda tympani (facial recess) is removed in aposterior tympanotomy. There are typically two approachesto cochlear implantation in the next stage: through the roundwindow membrane or via a cochleostomy (an opening intothe cochlea) [22].

Although there are benefits to both approaches, sev-eral studies have shown that for straight electrodes, thecochleostomy approach may result in less trauma [5], [6],[23]. As such, in our system, we consider cochleostomyas the default approach to cochlear implantation. Figure4 shows the view of the temporal bone after the surgeryhas been conducted. Once this is achieved, the electrode isinserted into the cochlea through the cochleostomy.

III. PROBLEM FORMULATION

A. Representation of Regions and Structures of Interest

To obtain the relevant areas, we recorded the procedure ofan expert otologist performing a cochlear implant surgery onthe simulator. The surgeon was asked to fully drill the facialrecess and drill all possible locations for a cochleostomy,during which time, we noted the time intervals when eacharea was drilled. We then extracted the voxels correspondingto the facial recess and cochleostomy using the time intervals

(a) Micro-CT scan of a temporalbone.

(b) Segmented volume: eachanatomical structure is representedin a different colour.

(c) Bone and anatomical structuresrendered in 3D.

Figure 3: Generation of virtual models using MicroCT data.

and the timestamp data recorded by the simulator (seesection II-A for details of timestamp data).

To fill holes and obtain the full masks for the facial recessand cochleostomy, we used morphological operations suchas dilation and erosion [24]. Now, any voxels drilled in oneof these areas can be identified by intersecting the drilledvoxels with the area mask, in real-time if necessary, withoutthe need to note time intervals. The cochlea is representedas a mesh of triangles as discussed in section II-A). Figure5 shows an instance of a facial recess and cochleostomydrilling, along with the cochlea.

B. Considerations and Assumptions

Practical considerations in electrode insertion and assump-tions made in the problem formulation are listed below.Details of their usage in the solution will be discussed insection IV.• The cochlear implant electrode is a right circular cylin-

der with a radius r.• The distance between the entry point of the electrode

into the cochlea and the cochleostomy should be at

Figure 4: Cochlear implant surgery via cochleostomy.

most ethresh.• The distance between the entry point and the contact

point of the electrode with the internal wall of thecochlea should be at least dthresh.

• Any trajectories that are at a distance less than a givendistance fthresh at the facial recess and a distancecthresh at the contact point of the cochlea are consid-ered to be coincident.

IV. SYSTEM DESIGN

Once the drilled regions of the facial recess andcochleostomy are represented as a set of voxels and thecochlea is represented as a set of triangles (as discussed insection III-A), we calculate the resulting electrode insertiontrajectories as follows. A graphical representation of thenotations used in this section is given in figure 6.

A. Determination of Insertion Vectors

We generate all valid insertion vectors that pass throughthe drilled facial recess and cochleostomy, assuming that theelectrode is a line vector with negligible width. To this end,we determine the lines going through all the drilled voxelsin the facial recess and all those in the cochleostomy. Werepresent this set of lines L as:

Figure 5: Representation of regions and structures of interest.

L =

nF ,nC⋃i=1,j=1

PFi P

Cj (1)

where, PF and PC are drilled points in the facial recessand the cocheostomy respectively, and nF and nC are thenumber of drilled voxels for these two regions. The notationsof relevant points as well as their locations are illustrated inFigure 6.

We denote PFi P

Cj as lij for simplicity. For any line lij ∈

L we can define a point Pij on it as follows:

Pij = PFi + λij uij (2)

where, λij is a scalar parameter and uij =PC

j −PFi

‖PCj −PF

i ‖is the

unit vector on lij .As discussed above, the cochlea is a closed surface rep-

resented by a mesh of triangles. Each triangle is representedby three vertices. The intersection point Pijk between eachline lij ∈ L, and the plane formed by each kth trianglesatisfies the following (triple product) equation, as it lies onthe kth plane.

[(Tk2 − Tk1)× (Tk3 − Tk1)] · (Pijk − Tk1) = 0 (3)

where, Tk1, Tk2, and Tk3 are the vertices of the kth trianglein the surface mesh.

Solving equations 2 and 3, the scalar that represents theintersection point on the line lij can be obtained as:

λijk =nk ·

(Tk1 − PF

i

)nk · uij

(4)

where nk = (Tk2 − Tk1)× (Tk3 − Tk1) is the normal to theplane formed by the kth triangle and λijk is the scalar thatdefines the intersection point as: Pijk = PF

i + λijkuij .

Figure 6: Notations and locations of relevant points.

Note that if the plane and the line under consideration arecoplanar, the line is tangent to the surface of the cochlea andno unique intersection point exists. As we are not interestedin tangent points, the kth triangle is removed from furthercalculations for the ijth line.

Although this calculation provides the intersection pointbetween lij and the plane formed by the kth triangle of thecochlea surface, it does not guarantee that the intersectionpoint will be within the bounds of the triangle. Thus, todetermine if an intersection point is within the kth triangleor not, it is defined using barycentric coordinates as follows:

Pijk = aijkTk1 + bijkTk2 + cijkTk3 (5)

where, [aijk, bijk, cijk] are the barycentric coordinates of thepoint Pijk with respect to the triangle represented by thevertices Tk1, Tk2, and Tk3.

The barycentric coordinates for an intersection point Pijk

are obtained as shown in equation 6. If 0 ≤ aijk ≤ 1,0 ≤ bijk ≤ 1, and 0 ≤ cijk ≤ 1, Pijk is within the boundsof the triangle (including on the edges and vertices).

[aijk bijk cijk]T= [Tk1 | Tk2 | Tk3]−1 · Pijk (6)

By rejecting the those Pijk that are outside the boundsof the triangle, we obtain the set of intersection points forall lij ∈ L with the cochlear surface. As the cochlea is aclosed surface, multiple intersection points can exist for eachline. The intersection point with the least distance from thejth drilled point on the cochlea PC

j , is the point where theelectrode enters the cochlea (say PE

ij ). The intersection pointwith the second least distance is the point of contact of theelectrode with the internal surface of the cochlea (say PT

ij ).In practice, only those PE

ij that are close to thecochleostomy are considered as valid entry points. Thus,we reject lij as a valid trajectory if the distance from PE

ij tothe cochleostomy is greater than a given threshold ethresh.Further, depending on the type of electrode being used, for avalid trajectory, the distance between PE

ij and PTij should be

greater than or equal to a threshold dthresh. The remainingtrajectories are those that are valid for an electrode with anegligible width.

B. Collision Detection with Undrilled Bone and AnatomicalStructures

The next step is to consider the width of the electrodeand remove those trajectories that collide with anatomicalstructures or undrilled bone voxels. To avoid checking voxelsthat are not close to the area under consideration, we definethe areas that the electrode would occupy as that between(and around) the facial recess and cochleostomy masks usingmorphological operations such as dilation [24].

For each trajectory resulting from the previous step, wetest if at least one of the undrilled voxels in the definedarea fall within the radius of the electrode (r). As the

(a) Adequately drilled facial recess. (b) Partially drilled facial recess.

Figure 7: Visualisation of valid electrode trajectories for a cochleostomy and two facial recess drillings.

orthogonal projection point from a given undrilled voxel isperpendicular to the unit vector uij of trajectory lij , thefollowing equation is satisfied.

uij ·(PUm − PO

ijm

)= 0 (7)

where, PUm is the mth undrilled voxel in the area of interest,

and POijm is the orthogonal projection point of PU

m ontrajectory lij .

As the orthogonal projection point lies on lij , it alsosatisfies equation 2, giving us the scalar that represents iton lij as follows:

λijm = uij ·(PUm − PF

i

)(8)

where λijm the the scalar from which the orthogonal pro-jection point can be obtained as PO

ijm = PFi + λijm · uij .

The orthogonal distance from the undrilled voxel PUm to

the line lij can then be calculated as:

dorth(PUm , lij) =

∥∥PUm − PO

ijm

∥∥ (9)

If this distance is less than or equal to the electrode radiusr, the undrilled voxel collides with the electrode, and thusmakes the trajectory lij invalid.

C. Removal of Coincident Trajectories

Although all valid trajectories pertaining to a given com-bination of drilled facial recess and cochleostomy are foundin the previous steps, some of them may be too close toeach other to be considered separate trajectories. Thus, inthis step, we determine how close each trajectory is to allthe others at the end points (at the facial recess and contactpoints of the cochlea). If two trajectories lp and lq arecoincident, they satisfy the following conditions.

coincident(lp, lq) =∥∥PT

p − PTq

∥∥ ≤ cthresh AND(dorth(P

Fp , lq) ≤ fthresh OR

dorth(PFq , lp) ≤ fthresh

) (10)

where, dorth(PFp , lq) is the orthogonal distance between

the facial recess point PFp of trajectory lp and any other

trajectory lq , calculated using equation 9. PTp and PT

q are thecontact points of the trajectories lp and lq with the internalwall of the cochlea respectively.

Once all the trajectories have been checked for coin-cidence with all the other trajectories, we remove thosewith the highest number of coincidences first and repeat theprocess until no trajectories that are coincident are left. Theresulting list of trajectories satisfy all the required conditionsfor a given electrode, drilled facial recess, and cochleostomy.

Once all valid trajectories are calculated, they can bevisualised alongside anatomical structures such as the facialnerve and cochlea to analyse their distribution. Quantitativemeasures such as the number of available trajectories canalso be extracted. This form of analysis is useful in un-derstanding the relationship between the amount of voxelsdrilled in the facial recess and the location and size of thecochleostomy with respect to anatomical variations.

V. EXAMPLE ANALYSIS

Here, we visualise and analyse the valid electrode tra-jectories for partially and fully drilled facial recesses whencombined with the same cochleostomy on the same tem-poral bone specimen. The parameter values used in thiscalculation are: r=0.325mm, ethresh=2mm, dthresh=5.5mm,fthresh=0.325mm, and cthresh=0.1mm.

As can be observed in figure 7, the number of trajectoriesis reduced when the facial recess is not adequately drilled.For example, 203 valid trajectories are available for the casein 7a, while it is reduced to 110 in the case shown in 7b.

VI. CONCLUSION

We presented a method to visualise and analyse validcochlear electrode trajectories for different temporal bonedissections. This is important in training cochlear implantsurgery, as it allows the trainee to explore different strate-gies and observe their impact on the available electrode

trajectories. Further, as trainees are also able to visualisethe relationship of the anatomical structures to the electrodetrajectories, they are able to understand which strategy maybe used in which situation. For example, on some anatomicalvariations, it may not be possible to find valid trajectoriesfor straight electrodes through a cochleostomy. In such cases,alternative approaches such as insertion through the roundwindow membrane or the use of a contoured electrode maybe required.

In future work, we will implement this prototype systemin our VR temporal bone surgery simulator to provide real-time visualisation and analysis of electrode trajectories (aswell as haptic enabled electrode insertion simulation) tosurgical trainees. We will validate its effectiveness in trainingcochlear electrode insertion through user studies of surgicalresidents.

ACKNOWLEDGMENT

This work was funded through an Australian ResearchCouncil Linkage grant in collaboration with Cochlear Ltd.

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