sheep as a large animal model for middle and inner ear ...€¦ · lized temporal bone lab....

Sheep as a Large Animal Model for Middle andInner Ear Implantable Hearing Devices: A Feasibility

Study in Cadavers

*Johannes Schnabl, *Rudolf Glueckert, †Gudrun Feuchtner, †Wolfgang Recheis,*Thomas Potrusil, ‡Volker Kuhn, *Astrid Wolf-Magele, *Herbert Riechelmann,

and *Georg M. Sprinzl

Departments of *Otolaryngology, !Radiology, and "Traumatology, University Hospital,Medical University Innsbruck, Innsbruck, Austria

Objective: Currently, no large animal model exists for surgical-experimental exploratory analysis of implantable hearing de-vices. In a histomorphometric study, we sought to investigatewhether sheep or pig cochleae are suitable for this purpose andwhether device implantation is feasible.Methods: Skulls of pig and sheep cadavers were examinedusing high-resolution 128-slice computed tomography (CT) tostudy anatomic relationships. A cochlear implant and an activemiddle ear implant could be successfully implanted into thesheep’s inner and middle ear, respectively. Correct device pla-cement was verified by CT and histology. The cochlear anatomyof the sheep was further studied by micro-CT and histology.Results: Our investigations indicate that the sheep is a suitableanimal model for implantation of implantable hearing devices.

The implantation of the devices was successfully performed byaccess through a mastoidectomy. The histologic, morphologic,and micro-CT study of the sheep cochlea showed that it is highlysimilar to the human cochlea. The temporal bone of the pig wasnot suitable for these microsurgical procedures because the mid-dle and inner ear were not accessible owing to distinct soft andfatty tissue coverage of the mastoid.Conclusion: The sheep is an appropriate large animal modelfor experimental studies with implantable hearing devices, whereasthe pig is not. Key Words: Active middle ear implantVAnimalmodelVCochleaVCochlear implantationVMicro-CTVSheepVVibrant Soundbridge.

Otol Neurotol 33:481Y489, 2012.

In view of the rapid progress in the development ofnew technical features and new surgical techniques forimplantable hearing devices, an animal model would bedesirable for testing device capabilities/functionality beforeusing them in humans.

Various small animal models for the development ofcochlear implants (CIs) are described in the literature,including a mouse model (1), a gerbil model (2Y5), a catmodel (6Y10), a guinea pig model (11Y19), and a rat model(20). What these models have in common is that the ana-tomic proportions are much smaller than in humans, andthis implies that a lot of forward-looking analysis withhuman CI and Vibrant Soundbridge (VSB) devices can-not be made. To date, no large animal model for a surgical-experimental exploratory analysis of implantable hearingdevices has been established.

We hypothesized that, given their similarities withhuman anatomy, the temporal bones of the sheep and thepig might be suitable for implantation with CIs and ac-tive middle ear implants such as the Vibrant Soundbridge(VSB). Hence, our study results would provide some basicinformation for the further development of an in vivo largeanimal model.

Several publications have described the sheep ear asan appropriate model for surgical trainingVfor example,for middle ear proceduresVowing to similarities in sheepand human external and middle ear anatomy (21Y24).

Lavinsky and Goycoolea (22) were the first to describethe sheep as a possible animal model for otologic surgerybecause of these significant similarities in ear anatomy.

Gurr et al. (25) proposed lamb and pig temporal bonesas alternatives in ENT education.

In a computed tomography (CT) study by Seibel et al.(26), computed tomographic scans from inner ear struc-tures in sheep were compared with human cases. Theydescribed inner ear anatomic structures that were similarto but smaller than those of humans.

Address correspondence and reprint requests to Georg M. Sprinzl,M.D., AnichstraQe 35, 6020 Innsbruck, Austria; E-mail: [email protected] authors disclose no conflicts of interest.

Otology & Neurotology33:481Y489 ! 2012, Otology & Neurotology, Inc.

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Copyright © 2012 Otology & Neurotology, Inc. Unauthorized reproduction of this article is prohibited.

To our knowledge, no report on the feasibility of im-planting hearing devices in the sheep or pig has beenpublished. Furthermore, no exact data on the lengths andvolumes of sheep and pig cochleae, or on the areas ofthe round windows, exist. This information is importantfor planning new surgical techniques and for determiningwhether the sheep is an appropriate large animal modelfor implantable hearing devices.

The primary aim of our study was to investigate whetherthe placement of a CI or a VSB in a sheep or pig skull isfeasible. We used endoscopic and high-resolution 128-slice CT to check correct device placement.

The secondary aim of this study was to perform amorphometric-anatomic study of the sheep cochleaVincluding performing measurements related to the feasi-bility of placing implantable hearing devicesVby usingmicro-CT and histology as reference methods.

MATERIALS AND METHODS

Study DesignSkulls from the cadavers of a sheep (weighing 22 kg) and a

pig (weighing 12 kg) were obtained from the local slaughter-house. All skulls from the slaughterhouse are so-called ‘‘by-products,’’ hence no animal was killed for this study.First, the skulls of the lamb and the pig were scanned with

a 128-slice dual-source CT scanner (Definition Flash; Siemens,Erlangen, Germany) for anatomic study purposes.Next, 3 additional skulls from sheep aged between 4 and

8 months were obtained from the same slaughterhouse under

circumstances identical to those described above. Within a fewminutes after the sheep’s deaths, the skulls were split into 6skull halves. Three of those halves were used for histology andmicro-CT studies. Two of the halves were implanted with aVSB and then with a CI, then a 128-slice CT examination anda micro-CT were performed, and finally, the 2 cochleae wereanalyzed histologically. One skull half was destroyed during thepreparation.The preparation and implantation was done in our specia-

lized temporal bone lab.

Preliminary Anatomic-Morphometric Study of theSheep and Pig Temporal Bones by CT

To study the anatomy, we scanned the skulls of a sheepand a pig with 128-slice dual-source CT (Siemens DefinitionFlash). The computed tomographic scan parameters were asfollows: detector collimation, 128! 0.6 mm; gantry rotation time,0.28 seconds; tube voltage, 120 kV; image reconstruction, 0.7 mmeffective slice width at 0.5 increment; ultra high-resolution mode(0.2-mm spatial resolution), 512 ! 512 pixel matrix; sharpconvolution kernel, U 70. On a dedicated external workstation,3-D volume-rendering reconstruction, multiplanar reformationsof the temporal bone of the sheep and pig were made. Thefollowing distances were measured with a digital caliper by2 independent observers and repeated after 6 weeks: 1) length ofexternal auditory channel (EAC), 2) angulation of EAC to thesagittal plane, and 3) thickness of the mastoid soft and fattytissue.The mean value was taken for final data presentation.

Preparation of the Sheep Skull and Implantation ofthe Hearing Devices

The skulls were fixed in a special tray in our temporal bonelaboratory, and a surgical microscope was used. The first stepwas the removal of soft tissue surrounding the temporal bone.After exposure of the temporal bone and the external auditorycanal, we prepared the temporal line, the mastoid tip, and themastoid portion.We performed the mastoidectomy between the temporal

line and the posterior canal wall with a conventional burr.After opening the middle ear, the anatomic structures wereexposed and documented with a 0-degree endoscope attached to

TABLE 1. Anatomic study of the sheep and pig cochlea withhigh-resolution 128-slice CT

Sheep Pig

EAC, mmLength 11 33Minimal diameter 4.9 3.8

Angulation of EAC to sagittal plane, degrees 62 28Mastoid soft and fatty tissue coverage, mm 13 52

FIG. 1. Computed tomographic scans: coronal view of the temporal bones from pig (A) and sheep (B) showing longer EACs with a higherangulation and a thicker mastoid (M) and soft tissue coverage (asterisk) in the pig (A).

482 J. SCHNABL ET AL.

Otology & Neurotology, Vol. 33, No. 3, 2012


a 3-megapixel digital camera (Coolpix 995; Nikon; Dusseldorf,Germany).The next step was the preparation of the promontorium and

the exposure of the round window. A floating mass transducer(FMT) from the VSB device (Vibrant MED-EL, Innsbruck,Austria) normally used for surgical training was prepared by re-moving the clip and placing it on the round window. This spe-cial FMT is slightly smaller in size (diameter, 1.65 mm; length,2.21 mm) than the normal FMT (diameter, 1.8 mm; length,2.3 mm) used in humans. This round window Vibroplasty wasdocumented with the camera. After removing the FMT, theround window membrane was opened, and a cochlear implan-tation was performed with a FlexEAS electrode (MED-EL) forsurgical practice. The FlexEAS had a length of 24 mm and adiameter of 0.8 mm.A mastoidectomy was performed on another skull half, and

an FMT for surgical training was clipped on to the long pro-

cess of the incus, after bending the clip by 90 degrees. Thisincus Vibroplasty was also documented with the camera. In thiscase, too, the FMT was removed, and after opening the roundwindow membrane, a cochlear implantation with a Standardelectrode fromMED-EL was performed. The Standard electrodehas a length of 31.5 mm and a maximum diameter of 1.3 mm.After the 2 CIs were implanted, a 128-slice CT was performed

to ascertain correct device placement. The same scan parametersas described above were used, and the cochlea with the insertedelectrode was reconstructed with multiplanar reformations andin 3-D volume-rendering reconstruction.The cochleae with the inserted electrodes were then excised

and used for further detailed anatomic-histologic studies bymicro-CT and histology (see the sections on micro-CT and histology).

Micro-CTVSheepMicro-CTs have system-inherent limitations in signal-to-

noise ratio performance owing to their small voxel size andrelatively low x-ray exposure level but can be used effectivelyto scan bony structures such as the cochlea.In our case, micro-CT was performed with a SCANCO Viva

(Scanco Medical AG; Bruettisellen, Switzerland) 40 KCT with10.5 and 19 Km isotropic voxel size (depending on the size of thespecimen) and with 70 kV, 43 KA tube current, 380-millisecondexposure time, and 1,000 projections. The acquired image stackshad an average size of 1,300 ! 1,300 ! 1,500 voxels with 16-bitgray value resolution.Data were transferred to a high-performance 64-bit PC with

8-GB RAM featuring Amira 5.3.3 (Visage Imaging, Richmond,Australia) and ImageJ containing macros developed in-housefor segmentation and measuring purposes (27).Five cochlea samples were scanned following the protocol

described; in 2 cochleae, CIs (FlexEAS electrode and Standardelectrode; MED-EL) were inserted.Owing to the lack of reference data, 2 coworkers (T.P. and

W.R.) independently segmented and measured the cochlea lengths,round window areas, and volumes of the scala tympani.

Length Measurement. The lengths of the cochleae weremeasured manually on each slice along the visually recogniz-able borders of the basilar membrane using ImageJ (Fig. 6).

Surface Measurements. The round windows were seg-mented manually using Amira. The membrane of the roundwindow was determined by choosing a narrow windowing of themicroYcomputed tomographic scans offering the possibility ofmanual segmentation. Furthermore, the normal projection of the

FIG. 3. A, The promontorium with a round window Vibroplasty. B, The incus with an incus Vibroplasty on the long process.

FIG. 2. Endoscopic view of the ossicles and promontorium aftera mastoidectomy. We can see the malleus (M), the head of the mal-leus (HM), the incus (I)with the longprocess (Ilp) and theshort process(Isp), the stapes (S), the ligament of the stapes (SL), the chordatympani (CT), the promontorium (P), and the round window (RW).

483SHEEP AS A MODEL FOR MIDDLE/INNER EAR IMPLANTS



round window was used to determine the diameters and nor-mal surface areas. For visualization (Fig. 6), data were smoothedby producing a surface from a resampled label field.

Volume MeasurementVSegmented Scala Tympani.The scala tympani were segmented with Amira by using thebasilar membrane as the boundary between the scala tympaniand scala vestibuli (Fig. 6). Scala volumes were calculated fromthe segmented data set.

HistologyVSheepTemporal bones of 2 healthy sheep (approximately 3.5 mo

old) were extracted in the course of slaughtering as describedabove. The head was removed and cut sagitally into 2 halves.After removal of the brain, the temporal bones were excisedwith bone forceps.The round and oval windows from the inner ears of 1 indi-

vidual were immediately penetrated with a needle and 4%phosphate-buffered salineYbuffered formaldehyde freshly made

FIG. 5. Micro-CT: 3-D visualization of the sheep cochlea. A, Three-dimensional top view of the sheep cochlea demonstrating 23 turns.B, Three-dimensional visualization of the round window of the sheep.

FIG. 4. A, Endoscopic view of the promontorium showing the inserted FlexEAS electrode. B, Three-dimensional reconstruction of the 128-slice computed tomographic scan from the same cochlea with the inserted FlexEAS electrode.C, Endoscopic view of the promontorium withthe inserted Standard electrode. D, Three-dimensional reconstruction of the computed tomographic scan from the cochlea with the insertedStandard electrode.




from paraformaldehyde was gently flushed with a plastic pi-pette into the inner ear to ensure good fixation. The cochleaewere fixed in this solution for 24 hours at 4-C. After scanningwith micro-CT, the cochleae were decalcified with 20% EDTAat pH 7.4 for 6 weeks. After thoroughly washing the tissue inphosphate-buffered saline, the specimens were prepared forcryoembedding (28) and serially sectioned radial to the mod-iolus (10 Km thick). Standard hemalaun-eosin staining servedto differentiate the structures. The temporal bones of anotherindividual were stored at 4-C for approximately 60 minutesduring CI implantation. After implantation, the inner ears werefixed through the oval window as described above and scannedby micro-CT. After decalcification in EDTA, the cochleae wereembedded in gelatin (29) and postfixed with 8% paraformalde-hyde for 1 week. This procedure fixed the gelatin proteins andensured a solid matrix of the inner ear except for the parts ofthe cochlea filled by the electrode. Removal of the implantresulted in an electrode cast within the tissue. The gelatin blockwas then sectioned with a Leica VT1000S vibratome (LeicaMicrosystems; Wetzlar, Germany) (200 Km section). Stainingwith hemalaun-eosin also stained the gelatin matrix and delin-eated the space filled by the electrode. This made possible anevaluation of the electrode position at light microscopic level.Images were acquired with a Zeiss AxioImager M1 (Zeiss;

Jena, Germany) equipped with an AxioCam HRc at 3900 !3090 pixels with AxioVision 4.8. The spiral ganglion diameter(the arithmetic mean of the longest and shortest diameters ofneurons with a clearly visible nucleus, taken from 50 cells

counted in the basal turn and apical turn) was estimated with theZeiss software and evaluated withMicrosoft Excel 2007 (MicrosoftCorp., Redmond, WA, USA).

RESULTS

Preliminary Anatomic-Morphometric Study of Sheepand Pig Temporal Bones by CT

The 128-slice CT measurements are presented in Table 1.The length of the osseous external auditory canal was

higher in the pig, with a mean of 33 mm, than in the sheep,with a mean length of 11 mm. The minimal luminal dia-meter of the external ear canal in the pig and in the sheepwere, on average, 3.8 and 4.9 mm, respectively. The ex-ternal auditory canal angulation to the sagittal plane was28 degrees for the pig and 62 degrees for the sheep. Themastoids of the pig and the sheep were covered with a layerof mixed soft tissue/fatty tissue of 52 and 13mm thickness,respectively (Fig. 1, A and B).

Preparation of the Sheep and Implantation ofHearing Devices

A mastoidectomy was performed posterior to theexternal ear canal (on the mastoid plane) for the surgicalapproach to the middle and inner ear structures.

There was very poor mastoid pneumatization, and themastoid was filled with adipose tissue. After the explo-ration of the posterior ear canal wall, during the mastoid-ectomy, the middle ear was opened (Fig. 2). In general, theossicles were connected to the walls of the tympanic cavityby ligaments. The incus is composed of a large body andvery similar in appearance to the human incus. The longand the short processes of the incus were standing at thesame level and at right angles to each other. On visualinspection, the processes appeared to be of the same length.The stapes that articulated with the long process of theincus was located beneath the incus. The stapes was con-nected by its ligament to the wall of the tympanic cavity.Furthermore, the facial nerve, the chorda tympani, thepromontorium, and the round window were identified, andtheir morphometric locations and courses were similar tothose in humans.

TABLE 2. Micro-CT: cochlea length

Cochlea no. Length, mm Variance, mm

1 32.72 0.862 35.64 0.673 37.8 0.974 32.94 0.565 31.46 0.44

FIG. 6. Three-dimensional visualization of the round window, thescala tympani, and the length of a sheep cochlea from micro-CTdata sets, which were used to measure the round window area,cochlea length, and scala tympani volume.

TABLE 3. Micro-CT: round window area

Cochlea no.Projected area,

mm2Variance,mm2

Curved area,mm2

Variance,mm2

1 2.64 0.22 8.1 0.762 3.3 0.14 9.23 1.13 5.091 0.56 10.02 1.23

TABLE 4. Micro-CT: volume of scala tympani

Cochlea no. Volume, Kl Variance, Kl

1 24.2 1.42 28.02 1.63 22.9 1.5




For a better visualization of the round window, thetympanic cavity was enlarged with a diamond burrand the window niche was drilled. Then the specialFMTVwhich is normally used only for surgical trainingpurposesVwas implanted on the round window aftercutting and removing the clip.

It was observed that the round window was largeenough for placement of the FMT on the RW membranewith the best possible contact (Fig. 3A).

After opening the round window membrane, a Flex-EAS electrode (Fig. 4A) was successfully inserted.

A postoperative computed tomographic scan showedthe successful implantation of the electrode with completeinsertion (Fig. 4B).

In another temporal bone, a special FMT was clippedonto the long process of the incus. Because the youngsheep was not fully grown and had tight anatomicstructures, this incus Vibroplasty was only possible bybending the clip upward (Fig. 3B).

After the removal of the FMT, the round window wasexposed, and after opening the round window membrane,a MED-EL Standard electrode was implanted (Fig. 4C).

FIG. 7. Histology of the sheep cochlea. A, Section through the temporal bone parallel to the modiolar plane. Neurons of the spiral ganglion(SG) are located within the Rosenthal canal. ST indicates scala tympani; SV, scala vestibuli. B andC, More highly magnified views of bipolarSGNs. D and E, Sensory organ with hair cells and supporting cells in the basal turn (D) and the apical turn (E). F, The stria vascularis of thebasal turn showing typical tissue structure.




The accurate positioning of the electrode was shown bya computed tomographic scan including a 3-D recon-struction (Fig. 4D).

Micro-CTVSheepThe sheep cochlea had 23 turns, as shown in Figure 5A.

Length of the CochleaThe average length of the 5 cochleae was 34.1 mm with

a variance of 0.7 mm. The lengths of all cochleae arepresented in Table 2.

Round Window AreaThe round window area could be measured in 3 of the

5 cochleae scanned with micro-CT (Figs. 5B and 6). Twoimplanted cochleae could not be measured because ofmissing or destroyed (during cochlear implantation)round window membranes. The mean area of the roundwindow was 3.7 mm2 for the projected area and 9.11 mm2

for the curved area (Table 3).

Volume of the Scala TympaniThe mean volume of the scala tympani of the same 3

cochleae mentioned in a previous paragraph was 25.04 Kl(Table 4).

HistologyVSheepSimilar to human anatomy, the otic capsule was found

to be completely fused with the surrounding temporalbone and showed the typical convoluted substructure ofa woven-type bone with rich vasculature and more orless basophilic osteons and acidophilic areas in between(Fig. 7).

The cochlea contained 23 turns, with a central bonymodiolus housing mainly spiral ganglion neurons(SGNs), nerve fibers, and blood supply. A distinct bonychannel filled with SGNs, referred to as Rosenthal canal,runs spirally in the distal wall of the modiolus and ends upin a bulge in the upper second turn (Fig. 7A).

Spiral ganglion neuron density is higher in the upperturns than in the basal turn (Fig. 7, B and C). However,the size of the somata is equal (mean [SD] diameter: basalturn, 15.7 [1.67] Km; apical turn, 15.7 [1.63] Km). Theorgan of Corti presents the typical arrangement of haircells and supporting cells (Fig. 7, D and E). In addition,the same trend of tonotopical differences in the size ofcells within the sensory organ is obvious. Hensen andtectal cells form the prominent distal ridge in the moreapical organ of Corti, whereas it is more flat in the basalarea. Outer hair cells are around 30 Km in length in theapical turn (as measured in Fig. 7) and only 22 Km in thebasal part of the cochlea. Compared with other mammals,the stria vascularis does not display any deviations intissue composition (3 types of cells, rich vasculature) andhas a profuse appearance of melanocytes that give thetissue a brownish color. Figure 8 shows the cochlea withthe electrode, positioned in the scala tympani.

DISCUSSION

Our study demonstrates for the first timeVto the bestof our knowledgeVthat the sheep is an appropriate ani-mal model for the implantation of hearing devices, asproven by surgery, histology, high-resolution CT, andmicro-CT. The sheep’s temporal bone is an adequateanimal model for implantation of implantable hearingdevices because of its anatomic similarity to the humantemporal bone. In contrast, pigs are not suitable models.

FIG. 8. A and B, Histology of the cochlea after implantation,confirming the placement of the electrode (E) into the scala tympani(ST). The scala media (SM) and scala vestibuli (SV) are shownadjacent.

TABLE 5. Comparison of sheep and humancochlea proportions

Sheep Human

Cochlea turns 23 22Cochlea length, mm 34.1 33.0Y45.0 (30)

28.0Y42.0 (34)Round window (projected area), mm2 3.7 2.29 (32)

2.14 (33)Volume of scala tympani, Kl 25.04 29.22 (31)




Our measurements of the external ear canal of thesheep and the pig are similar to those of previous stud-ies (24,25). Our study also shows that the soft and fattytissue layers of the mastoid are significantly thicker inthe pig than in the sheep, inhibiting a successful surgicalapproach. However, we could successfully implant boththe VSB and the CI in the sheep, which provides funda-mental knowledge regarding the most promising directionfor the future development of an in vivo study for es-tablishing a large animal model. We have demonstratedthat the pig is less suitable as an animal model for im-plantable hearing devices. The approach through the largesoft and fatty tissue coverage of the mastoid is moredemanding in the pig and could be associated with morecomplications.

The findings and anatomic descriptions establishedduring preparation of the sheep temporal bones, espe-cially the middle ear, confirm the results of previousstudies (22,25). We find that the middle ear of the sheep ismorphologically equivalent to the human middle ear,although some structures in the sheep’s middle ear weresmaller than the corresponding structures in human ears.

We have proven for the first time, by high-resolutionCT and micro-CT imaging, that it is possible to perform acochlear implantation and an incus and round windowVibroplasty in the sheep. Moreover, histology confirmedcorrect electrode placement within the scala tympani.

In addition, we are the first research group to perform aseparate study of the morphometric dimensions of thesheep cochlea with specific regard to the planning of CIand VSB, by using the most accurate imaging modalityavailable for our research group: micro-CT.

Our results, therefore, expand the knowledge obtainedfrom previous CT morphologic studies (26). In particular,we measured the following dimensions that are impor-tant for cochlear implantation and round window Vibro-plasty: the average curved length of the sheep cochlea, at34.1 mm, is slightly smaller than the human cochlealength of 42.0 mm (30). This length can be regarded assufficient for cochlear implantation because the MED-ELStandard electrode has a maximal insertion depth of31.5 mm, and the MED-EL FlexEAS electrode has amaximal insertion depth of 24.0 mm. For more exactsizing, we calculated the volume of the scala tympaniusing 3-D micro-CT data sets, revealing an averagevolume of 25.04 Kl. This volume is similar to that of thehuman mean of 29.22 Kl (31).

With respect to the planning of round window Vibro-plasty, we measured 2 areas from the round window ofthe sheep: first, the projected area for comparison withprevious literature; and second, the curved area, whichgives a more accurate estimate of its real dimensions. Ourstudy found the average projected area to be 3.7 mm2.This is larger than that in humans, which has beenreported as 2.29 mm2 (32) and 2.14 mm2 (33). Theaverage curved area, at 9.11 mm2, was also larger than inhumans. In short, the area of the round window of thesheep is large enough for FMT placement because theFMT has an area of 2.54 mm2.

In summary, we found surprising and impressive mor-phometric and morphologic similarities between humanand sheep cochlear anatomy. The length of the cochlea andthe number of turns are nearly equal. Individual variationin cochlea length is known to be high in humans, havingbeen reported as 33 to 45 mm (30) and 28 to 42 mm(34,35). This may also hold true for sheep, but we cannotdraw any conclusions on this issue because the number ofspecimens investigated in our study was too small. Thedimensions of the scala tympani allow the use of standardelectrodes (for human use) for insertion via the roundwindow. To underline the similarities of the sheep and thehuman cochlea, a comparison is shown in Table 5.

However, there are also evident differences betweenhuman and sheep anatomy. In sheep, cochlear neuronsreach a mean diameter of around 15 to 16 Hm, which isabout half of the size measured in humans (36). Thisimplies differences in electrical stimulation.

A bipolar cochlear neuron can be described in anequivalent circuit model containing resistance and capa-citors. Charging the somatic capacitance is the mainbarrier for the action potential traveling from the hair cellto the nervus cochlearis. Rattay et al. (37) have shown in acomputer simulation using a modified Hodgkin-Huxleymodel that the somatic delay of a myelinated cat neuronwith a diameter of 20 Km is 118 Ks. Because of the factthat the current needed to charge the somatic capacitanceis proportional to the somatic surface (38), it can beassumed that an action potential will traverse a sheepneuron faster than that of a cat.

Regarding the electrode positioning, we can demon-strate that the electrodes are positioned in the scala tym-pani. Particularly for purposes of EAS treatment, anatraumatic insertion and electrode positioning in the scalatympani are important (39,40).

A possible placement for the implantation of the CI orVSB receiver in an in vivo animal model could be theparietal or intraparietal bone of the sheep’s skull, but thismust first be investigated in future experiments.

In conclusion, the sheep’s temporal bone is an idealanimal model for implantable hearing devices because ofits similarity to human middle and inner ear structures.The implantation of both a VSB and a CI in the sheep isfeasible. Our results provide the basics for the develop-ment of an in vivo large animal model. Furthermore, thetemporal bones of sheep are a good alternative for sur-gical training in implantation of both CIs and VSBs.Acknowledgments: The authors thank Michael Verius for

supporting cochlea length measurement and Mario Bitsche andAnneliese Schrott-Fischer for supporting the neurotology work.In addition, the authors thank Marek Polak, Michael Sinding,and Noelani Peet for medical writing assistance.

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sheep as a large animal model for middle and inner ear ...€¦ · lized temporal bone lab....

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