Учебники / Middle Ear Mechanics in Research and Otology Huber 2006

Fig. 5 Umbo FRF for simulation of malleus head fixation.

4. Discussion

The results of the simulations are in good agreement with some clinical studiesusingtheLaserDopplerVibrometry[1–3].Forotosclerosis,ISJlux- ation and malleus fixation similar characteristic changes in the umbo FRF could be observed. This indicates that the model seems to be suitable for simulations of pathological changes of the ossicular chain.

The following conclusions can be drawn from the simulation results: IMJ and ISJ fixation can hardly be distinguished from normal ears since the umbo FRF is within the normal range for both cases. These pathologies are also presumed to be without audiological findings because

228 there are only minor changes in the stapes footplate FRF.

Severe changes like chain interruption and malleus head fixation can be detected and distinguished.

In case of malleus head fixation measurements are very sensitive to the location of the measurement point. Only measurements at the umbo directly show a significant reduction of the magnitude across the whole frequency range, whereas measurements at short distance from the umbo (≈ 0.2 mm) might indicate a normal ossicular chain, especially in cases where the malleus head is not totally osseous fixed.

Otosclerosis does not necessarily show up in the umbo FRF. Results can range from almost unchanged to moderate decrease in the low frequency range (for the same degree of annular ligament sti ening). The

changes in the umbo FRF for otosclerotic ears depend very much on the particular morphology of the middle ear, especially the sti ness of the ISJ. A variation in ISJ sti ness by a factor of 10 determines whether otosclerosis shows up in the umbo FRF or not. These findings are in agreement with other investigations [8] where no correlation could be observed between LDV measurements on the umbo and the air-bone gap obtained from audiologicalinvestigations.Forclinicaldiagnosticsofotosclerosisthismeans that a comparison of amplitudes of the umbo FRF is insu cient.

References

1.Goode R.L., Ball G. and Nishihara S., Measurement of umbo vibration in human subjects – method and possible clinical applications. Am J Otol 14(5), (1993) pp. 247–251

2.Huber A., Schwab C., Linder T., Stoeckli S., Ferrazzini M., Dillier N. and Fisch U., Evaluation of eardrum laser doppler interferometry as a diagnostic tool. Laryngoscope 111(3), (2001) pp. 501–507

3.Rosowski J.J., Mehta R.P. and Merchant S.N., Diagnostic Utility of Laser-Doppler Vibrometry in Conductive Hearing Loss with Normal Tympanic Membrane. Otol Neurotol 24, (2003) pp. 165–175

4.Beer H., Bornitz M., Hardtke H., Schmidt R., Hofmann G., Vogel U., Zahnert T. and Hüttenbrink K., Modelling of components of the human middle ear and simulation of their dynamic behaviour. Audiology and Neuro-Otology 4(3–4), (1999) pp. 156–162

5.Bornitz M., Zahnert T., Hardtke H. and Hüttenbrink K., Identification of parameters for the middle ear model. Audiology and Neuro-Otology 4(3–4), (1999) pp. 163–169

6.Hudde H. and Engel A., Measuring and modeling basic properties of the human

7.Zahnert, T., Laser in der Ohrforschung. Laryngorhinootologie 82 Suppl 1, (2003) pp. 157–180

8.Merchant S.N., Nakajima H.H., Chien W. and Rosowski J.J., Six years of experience with measurements of tympanic membrane velocity by laser Doppler vibrometry as a clinical diagnostic tool. [published here]

BASILAR MEMBRANE DISPLACEMENT WITH OPENED AND OCCLUDED OVAL WINDOW AND BONE CONDUCTION

F. Böhnke1, A. Arnold2, T. Fawzy1

1 Hals-Nasen-Ohrenklinik und Poliklinik, Klinikum rechts der Isar der Technischen Universität München, Ismaningerstr. 22, D 81675 München, Germany

2 Univ. HNO-Klinik, Inselspital, 3010 Berne, Switzerland

PD Dr.-Ing. Frank Böhnke, Phone: +49 89 4140 4196, Fax: 49 89 4140 4971 Email: frank.boehnke@lrz.tum.de

Hearingsensationsarecausedbyairandboneguidedsound.Ofcourseotherbiological materials like tendons, muscles and tissue are also involved during conduction of sound. To study the influence of bone conduction (BC) a formerly developed finite element model was excited by harmonic pressure signals at the cochlea wall. Clinical findings during middle ear surgery, namely the increase of bone conduction sensitivity with removed footplate was confirmed.

Additionally the e ects of an immobile footplate and round window aplasia and bone conduction are studied. The maximum basilar membrane displacement with the closure of the oval and the round window and both with BC is examined numerically.

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1. Introduction

Thoughboneconduction(BC)hasbeenofclinicalandscientificinterestfor long [1], there are still open questions concerning this acoustical phenomenon. The term BC implies hearing by the transmission of sound through bone though it does not necessarily imply that the wave propagation is entirely bone because the sound is transmitted to the organ of Corti through the bone, periosteum, lymph and other biological material. To clarify open questions to the topic of BC the wave propagation along the basilar membrane (BM) with BC stimulation must be known. Though former workers already addressed this problem, the physics was mostly covered by onedimensional considerations [2]. These are not able to include the complex

geometry of the cochlea su ciently. Therefore we developed a 3D model of the cochlea and stimulated the bony wall surrounding the fluid by di erent pressure signals [3]. Herewith we are able to simulate di erent states in middle ear surgery.

A result, which confirms clinical findings in middle ear surgery is the increase of BM displacement with BC stimulation when the stapes footplate is removed during stapedectomy.

Anevaluationanddiscussionoftheinfluenceoftheclosedroundand oval window is given.

2. Modelling the Cochlea for Bone Conduction (BC)

To calculate the BC response we use a formerly developed finite element model of the cochlea [3]. Figure 1 shows the complete discretized finite element model with the stapes footplate and the round window at the base of the cochlea.

Fig.1 3D Finite Element Model of the human cochlea showing the oval and the round windows.

Fig. 2 The basilar membrane idealized as an elastic orthotropic shell with gradients of width and thickness.

The results shown in figs. 4 and 5 are the maximum displacements which occur with one stimulation frequency at di erent points along the BM. Instead of the usual stapes footplate stimulation of the middle ear the outer bony wall of the cochlea surrounding the perilymphatic fluid is stimulated (Fig. 3).

Fig. 3 Arrows represent the pressure loading of the outer cochlea wall with bone conduction.

The outer cochlea wall (OCW) is not represented by an extra material yet and therefore the pressure is applied to those areas, which confine the fluid elements. The stimulation by an external pressure is easy but the specificationofanexternalapplieddisplacementisnot.Toprescribeadisplacement at the OCW, in principle the number of local coordinate systems must be ashighasthenumberoffiniteelements.Becausethisisunrealisticanadditionallayerofbonemightbeinsertedaroundtheperilymphandstimulated on plane surfaces.

For this reason, at this point we are only able to stimulate the cochlea

232by pressure. According to Herzog and Krainz [4] we are merely able to study the influence of compressional BC because the middle ear is not yet included in the finite element model.

3. Results

3.1 Increase of basilar membrane displacement after footplate removal with bone conduction

During stapedectomy the complete stapes footplate is removed. Similar to the superior semicircular canal dehiscence (SCD) where BC sensitivity is increased for frequencies below 2 kHz [5] the BM displacement increases by 9 dB at f = 1500 Hz (Fig. 4) in the finite element model.

Fig. 4 The displacement of the basilar membrane is larger for an opened oval window,

LStim = 94 dB(SPL).

Thoughthereisadi erencebetweenthemodelpresentedhereandtheSCD, wherealargeamountofcerebrospinalfluidisconnected,thecomparisonis permitted because of an experimental finding with likewise coupled air to the lymph [6]. The increase of BM displacement is larger if the area of the opening increases as it is shown in fig. 4. The smaller piston (diameter 0.6 mm) area of A = 0.28 mm2 reduces the increase up to 2 dB, depending on thefrequency(Fig.4,pistonopenhole).BecausetheBMdisplacementwith a mobile elastic footplate does not di er considerably from the fixed state shownhereitisnotpresentedinfig.4.Themeasurementofthisdecreaseof BC threshold was done during stapedectomy with opened footplates [7].

The stimulation level of external pressure was 94 dB(SPL) in these cases. With the BC measurements the e ective level at the cochlea can be lower and irregular along the cochlea wall because of anatomical conditions.

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3.2Basilar membrane displacement with aplasia of the round window and both windows fixed stimulated by compressional bone conduction

There is still a diversity of opinion how the closure of the cochlear windowsa ectsbothairconduction(AC)andBC.Theresultsofthesimulation shown in Fig. 5 give a decrease of BM displacement by 7 to 12 dB for frequencies above 1 kHz with round window aplasia. Here the bone conduction stimulation level is 70 dB(SPL).

Fig. 5 The displacement of the basilar membrane for closed oval and round window, LStim = 70 dB(SPL).

In case of both windows fixed the BM displacement increases by 2 dB to 6 dB as a function of frequency. These results resemble the bone conduction thresholds of 15 to 35 dB with rare clinical findings of round window aplasia [11]. The di erences are due to the missing middle ear in our simulation. Furthermore a comparison to experimental data by Tonndorf and Tabor [12] is possible. Closure of both windows produced a relatively small e ect as to BC responses, which was not larger than that produced by closure of the oval window alone, unless the cochlear aqueduct was closed o simultaneously. Of course we are not able to study the influence of an open cochlear aqueduct with the model of the cochlea we used.

4. Discussion

234The simulation of the wave propagation on the BM in the cochlea with BC excitation confirms clinical findings. In this preliminary simulation the bone was not taken into account directly. Instead of this a homogeneous pressure application on the boundary walls of the cochlea was used. Our resultssupportrecentmeasurementsonfatsandrats,whichgaveadecrease ofBCthresholdby7.0dBwithsemicircularcanalfenestration[10],though transient stimulation was used in these experiment instead of our sinusoidal stimulation.

Thephysicalinterpretationoftheincreaseinresponsecanbethelowering of the acoustical impedance to fluid movements and therefore the increase of fluid movement and as a consequence the increase of BM displacement.

The actual stimulation at the outer bone is in phase for all areas and has an equal amplitude along the complete outer wall. The stimulating signal which arrives at the outer wall will have a frequency dependent phase shift and might di er in its amplitude from place to place. This will be regarded in future simulations.

In further steps of modelling the ear parts of the temporal bone and particularly components of the middle ear will be included. This will enable us to study the influence of inertial BC, i.e. the contribution of the middle ear structures to BC, and the osseotympanic BC, i.e. sound energy transmittedfromtheexternalearcanaltothetympanicmembrane,setting up an AC response [8]. In case of simulation of the otosclerotic ear with an immobile stapes footplate it was permissible to use the cochlea model withoutthemiddleearbecauseinthiscaseitdoesnotmatteriftheossicles are present or not.

The closure of both windows and stimulation by BC further on displaced the BM because of the compressibility of the fluid representing the lymph, which had the mechanical parameters of water. Further clinical comparisonsshouldnotbedrawnbeforeamoreelaboratemodeloftheear including the middle ear components is used.

References

1.Békésy, G. von, “Zur Theorie des Hörens bei der Schallaufnahme durch Knchenleitung (Hearing theory of acoustic perception by bone conduction),”Ann. Phys. (Leipzig), 1932, 13, 111–136

2.Tonndorf J., Compressional bone conduction in cochlear models. J. Acoust. Soc. Am. 1962; 34, 1127–31

3.Böhnke F., Arnold W., 3D-Finite Element Model of the Human Cochlea including

4.Herzog H., Krainz W., Das Knochenleitungsproblem. Theoretische Erwägungen.

Zeitschrift für Hals-, Nasenund Ohrenheilkunde 1926: 300–313.

5.Minor L.B., Superior canal dehiscence syndrome. Am. J. Otol. 2000; 21, 9–19

6.Songer J., personal communication, MEMRO2006, 4th International Symposium on Middle Ear Mechanics in Research and Otolgy, Zürich, July 2006

7.Arnold A., Fawzy T., Böhnke F. Investigations on bone conduction thresholds in otosclerosis, MEMRO2006, 4th International Symposium on Middle Ear Mechanics in Research and Otology, Zürich, 2006, P3

8.Tonndorf J., Bone Conduction. In Tobias J.V. ed., Foundations of Modern Auditory Theory, New York: Academic Press, 1972; 195–237

9.Sohmer H., Freeman S., Perez R., Semicircular fenestration – improvement of bone but not air conducted auditory thresholds. Hearing Research 2004; 34, 1127–31

10.Linder T., Furong M., Huber A., Round Window Atresia and its E ect on Sound Transmission, Otology & Neurootology 2003, 24, 259–263

11. Tonndorf J., Tabor J., Closure of the cochlear windows: Its e ect upon air and bone conduction, Ann. Otol. Rhinol. Laryngol., 71, 5–29

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DEVELOPMENT OF A NEW

CLIP-PISTON PROSTHESIS

FOR THE STAPES

G. Schimanski, Koenigsheide 9, 44536 Luenen, ENT-practice, Germany Email: g.schimanski@mittelohr.de

U. Steinhardt, Tuebinger Str. 3, 72144 Dusslingen, Kurz Medizintechnik, Germany Email: usteinhardt@kurzmed.de

A. Eiber, Pfaffenwaldring 9, 70569 Stuttgart, Institute of Engineering and Computational Mechanics, University of Stuttgart, Germany Email: eiber@itm.uni-stuttgart

275 inserted Clip-Pistons type “ÀWengen” within three years revealed di culties in 14.5% of the cases. In those cases it was necessary to make adjustments to the clip shape (plastic deformation) before insertion due to the individual dimension of the long incudal process. During 100 middle ear surgeries the cross sections of the long incudal processes where the clip is attached was measured. This resulted in data hitherto unknown. By virtue of a Finite Element Model (FEM) these data were used for optimizing the clip shape. Design criteria were a minimal variation of the contact force for di erent cross-sections and to minimize the force necessary to slide the clip overtheincudalprocess.Thenewcliphasalowersti nessandcanthereforebeapplied onto di erent incus diameters. The lower contact force reduces the risk of arrosion. Due to its optimized shape, the maximal stress in the clip is lowered preventing plastic

deformation during the application procedure. The application force was decreased by 237 up to 45% depending on the application points. This leads to easy and safe application reducing the risk of damaging the ossicular chain.

1. Introduction

For several years now, the titanium Clip-Piston type “ÀWengen” (manufacturer: KURZ Medizintechnik , Dusslingen, Germany) has been a tried- and-tested stapes replacement in otosclerosis surgery. Its advantage is its standardized manner of fixation to the long incudal process [1].

Otosurgical practice has shown that pushing the Clip-Piston “ÀWengen” (following called Clip“ÀW”) onto the incudal process can be prob-