Учебники / Middle Ear Mechanics in Research and Otology Huber 2006

Fig. 3 Time history and spectra of desired elementary motions measured at the stapes head.Pistonmotiony(solidline),rockingxaboutshortaxis(dashedline)androcking z about long axis (dotted line). Left: Predominant rocking about short axis. Middle: Predominant piston motion. Right: Predominant rocking about long axis.

4. Conclusions

Mechanical excitation of the stapes o ers more detailed investigations in electrocochleography compared to classical acoustic stimulation. In particular,thedirectionofexcitationaswellasitsshapeandfrequencycontent canbevariedinawiderange.Thisrequiresamorecomplicatedexperimental setup, a very sensitive and time consuming preparation and a precise positioning of the subject. The developed setup with a three-axes actuator and 3D-LDV sensor is a system to excite and measure predefined motions of the stapes in guinea pigs. Elementary motions, i.e. pure translational pistonmotionandpurerockingmotion,canbedefinedandnearlyachievedin the experiments leading to predominant piston motions and predominant

128rockingmotionsofstapes.Thesubjectandactuatorcanbepreciselyaligned withoutgivingastaticalpreloadtothestapes.Shortclickswithanarbitrary shape can be applied to the prepared guinea pig. Di erent motion patterns can be imposed by external control while the complete experimental setup remains untouched.

References

1.Decraemer W.F. and Khanna S.M., Measurement, Visualization and Quantitative Analysis of Complete Three-Dimensional Kinematical Data Sets of Human and Cat Middle Ear. In Gyo K. and Wada H. (Eds.) Proceedings of the 3rd Symposium on Middle Ear Mechanics in Research and Otology. World Scientific, Singapore, 2004, pp. 3–10

2.Heiland K., Goode R., Asai M. and Huber A., A human temporal bone study of stapes footplate movement. Am J Otol, 20 (1999), pp. 81–86

3.Sequeira D., Breuninger C., Eiber A. and Huber A., The e ects of complex stapes motion on the response of the cochlea in guinea pigs. In Huber A. and Eib A. (Eds.) Proceedings of the 4th Symposium on Middle Ear Mechanics in Research and Otology. World Scientific, Singapore, 2007.

129

THE EFFECTS OF COMPLEX STAPES MOTION ON THE RESPONSE OF THE COCHLEA IN GUINEA PIGS

Damien Sequeira1, Christian Breuninger2, Albrecht Eiber2, Alexander Huber1

1 Department of Otorhinolaryngology, Head and Neck Surgery, University Hospital of Zurich, Switzerland

2 Institute of Engineering and Computational Mechanics, University of Stuttgart, Germany

Studiesintothevibrationmodesofthestapesinresponsetoacousticstimulationofthe normal ear have revealed a complex movement pattern of its footplate. These complex vibrations can be expressed as one translational displacement and two rotational movementsaroundthelongandshortaxesofthestapes,knownasthethreeelementary motions.Accordingtotheclassicaltheoryofhearing,therotationalmotionsinduceno volume displacement of cochlear fluid and, therefore, no cochlear activity (i.e. hearing sensation). It is the goal of this study to verify this hypothesis.

A custom-built, three-axis piezoelectric actuator, capable of eliciting any desired vibration mode, was coupled to the surgically prepared stapes superstructure of anesthetized guinea pigs. When producing di erent movement patterns, electrophysiological measurements of the cochlear potentials were simultaneously recorded.

Mechanical stimulation of the stapes according to the three elementary motions

130delivered three cochlear potentials of di erent amplitude. The greatest potential resulted from translational motions.

The results of the present study show a cochlear excitation in all performed movement patterns of the stapes. Hence, the prevailing hypothesis could not be verified.

1. Introduction

Based on recent studies into the vibration modes of the stapes, the normal ear, when responding to acoustical stimulation, exhibits a complex movement pattern of its footplate. Low frequency vibrations exhibit predomi- nantlypiston-likemotions,whichincreasinglyevolveintorocking-likemo-

tionsatmiddleandhighfrequencies[1,2].Thesecomplexvibrationscanbe decomposed into one translational and two rotational movements around thelongandshortaxesofthestapes.Inwardtranslationalmovementsresult in an increase of fluid pressure in the scala vestibuli, producing a pressure gradient between the scala vestibuli and the scala tympani, which contains the round window that is a flexible boundary to the ambient middle-ear air pressure.Thispressureequalizesbygeneratingafluidbulkshiftthatexcites the basilar membrane and launches a traveling wave [3].

However, the rotational components of the stapes motion produce no net volume displacement of cochlear fluid at the oval window, since the inward movement of a certain volume on one side of the stapes axis leads to outward movement of an equal volume on the other side. Therefore, no bulk pressure change is induced at a certain distance from the oval window. As the e ective stimulus to the cochlea is thought to be the pressure di erence between the oval and round windows [4], rotational motion of the stapes represents lost energy that is not transformed into a hearing sensation. However, this hypothesis has never been tested experimentally.

The present study aims to record the electrophysiological activity of the inner ear in response to mechanical excitation of the stapes, as discussed in [5]. This would allow verification of the present hypothesis of sound propagation from the middle ear to the inner ear.

2. Materials and Methods

2.1 Animal preparation

The study was performed in five specific-pathogen-free female guinea pigs showing normal Preyer reflexes, hence disease-free middle ears. A prior two-week acclimatization period to the new environment was implemented. General anesthesia was carried out by means of intramuscular appli-

cation of Ketamin (35mg/kg) and Xylacin (5mg/kg), as well as additional 131 inhalationnarcosiswithSevoflurane(0.4–2%).Theguineapigswereplaced within an animal specific head-holder. After prior tracheotomy, the stapes

was micro-surgically exposed as shown in Fig. 1. The animals were euthanized at the end of the experiment by an intraperitoneal barbiturate overdose.

Fig. 1 Left: Intraoperative view of the stapes exposure in a right ear [AC = anterior crus, PC = posterior crus, RW = round window, SH = stapes head]. Right: Stapes with the corresponding coordinate system.

2.2 Mechanical stimulation & 3D-Laser Doppler Vibrometry

A custom-built cylindrical steel actuator with a mounted needle containing a noose was used for exciting the stapes in the three di erent motion patterns. The actuator, mounted on a micromanipulator, was carefully advanced to the stapes head under direct control of an operation microscope, until firm contact was obtained. The noose enclosed in the needle was then fastened around the stapes head. The stimuli were very short clicks of alternating phase with an interclick-interval of 79ms (Fig. 2, A–C).

A Helium-Neon (HeNe) based CLV-3D compact three-dimensional laser vibrometer (Polytec GmbH, Waldbronn, Germany) was focused on the stapes head. Subsequently, this allowed for both the monitoring and measurement of stapes superstructure displacements in the nanometer range over a wide range of frequencies [5].

2.3 Electrophysiological measurements

132In the current experiment the recording of cochlear activity, namely the compound action potentials (i.e. CAP), was executed by means of a cus- tom-made, silver-ball electrode placed in the round window niche of the anesthetized guinea pigs. The measurements were averaged in order to increase the accuracy of these potentials. Additionally, the entire experiment was carried out within a made-to-measure test stand placed on a vibrationfree table and in a sound and electromagnetic-proof room.

The actuator stimulation and the recording of the resulting stapes motion and cochlear activity were driven by a computerized and analysis measurement system.

3. Results

The compound action potentials in response to mechanical stimulation, namely rarefaction and condensation clicks, were recorded over displacementsrangingupto200nm.Toavoidtraumatotheovalwindow,themaximum displacement level was 200nm.

Being able to generate predominant movement patterns from a single actuator, the stapes footplate vibrations were directly assessed using the three-dimensional laser Doppler vibrometer. The level of accompanying movements in the other two directions occurring while eliciting the desired prevalent vibration pattern was relatively small [5].

By repeating the stimulus for each specific displacement two hundred times,itwaspossibletoobtainarepresentativecochlearresponse.Theperformed experiment showed electrophysiological excitation in all cases of desired movement patterns of the stapes. Thus, the measured compound action potentials were influenced by both translational and rotational vibration patterns as shown in Fig. 2. Occurring with predominantly rotational movements pattern were, nevertheless, small amounts of translational movement. However, the corresponding compound action potential was much greater than the equivalent compound action potential for a predominantly translational movement of equal amplitude. The most

133

Fig. 2 Compound action potential [CAP] as a function of the displacement of the dominant movement pattern of the stapes. The upper three graphs depict the predominant movement pattern of a click of high and low intensity. A: x-motion inducing rocking-like movement around the short axis. B: y-motion inducing pistonlike movement. C: z-motion inducing rocking-like movement around the long axis. The lower graphs represent the corresponding CAP: the thick line in response to a click of high intensity, and the thin line in response to a click of low intensity.

influential vibration mode being the translational movement produced electrical potentials of the highest amplitude. All types of motion induced trends by which increased displacement caused increased compound action potential amplitudes.

A correlation between the translational component of the actual displacement of motion and its corresponding electrophysiological reaction in response to positive clicks was plotted on a logarithmic scale as shown in Figure 3. This correlation depicted the corresponding translational movement patterns with and without the rotational components. Accordingly, the subsequent addition of these rotational motions revealed greater compound action potential amplitudes than for amplitudes in response to translational movement patterns alone. A higher correlation was achieved among rotational motions around the short axis, due to larger volume displacements of cochlear fluid. Hence, a linear relationship between the logarithm of displacement or logarithm of velocity, respectively, and cochlear response amplitudes was found in all three motion types.

Fig. 3 Correlation between the piston-like velocity components of the actual displacement patterns of motion and its corresponding electrophysiological reaction in response to positive clicks.

4. Discussion

Inthepresentstudy,itwaspossibletogeneratethethreedesiredmovement patterns in a preponderant fashion. The main objective was to test whether only translational stapes movements influenced cochlear activity, while rotational movements elicited no reaction.

Having obtained cochlear excitation in all performed movement patterns ofthestapes,albeitofdi erentamplitudes,theprevailinghypothesiscould, therefore, not be verified. As a result of these findings, it can be concluded thatbothlocalpressuredi erencesintheperilymphproducedbyrotational motions and global pressure di erences produced by translational motions give rise to electrophysiological excitation from the stapes to the inner ear.

5. Conclusion

However, since the findings of the present study only represent a limited sampling size, these results need to be verified by a larger animal populationsize.Thiswouldthenallowfortheformulationofquantitativerelations between the three elementary motions and their corresponding cochlear response.

These results have particular clinical relevance in the development of passive and active middle-ear prostheses as well as in the surgical techniques used to position them [6]. Furthermore, basic research into mathematical models of the middle ear and cochlea will need to consider the data presented here, since almost all such models assume that only the translational stimulus mode is e ective at exciting the inner ear.

References

1.Decraemer W.F. and Khanna S.M., Three-dimensional vibration of the stapes measured with a heterodyne laser interferometer. In Tomasini E. (ed.) Vibration Measurements by Laser Techniques: Advances and Applications. SPIE, 3411 (1998), pp. 592–600

2.Heiland K., Goode R., Asai M. and Huber A., A human temporal bone study of

3.Dallos P., Cochlear Neurobiology. In Dallos P., Popper A.H. and Fay P.P. (eds.)

Handbook of auditory research: The Cochlea. Springer, New York, 1996, pp. 1–43

4.Voss S., Rosowski J. and Peake W., Is the pressure di erence between the oval and round windows the e ective acoustic stimulus for the cochlea? J Acoust Soc Am, 100 (1996), pp. 1602–1616

5.Breuninger C., Sequeira D., Huber A. and Eiber A., Mechanical Excitation of Complex Stapes Motion in Guinea Pigs. In this edition

6.Eiber A., Freitag H.-G., Burkhardt C., Hemmert W., Maassen M., Rodriguez Jorge J. and Zenner H.-P., Dynamics of Middle Ear Prostheses – Simulations and Measurements. Audiology & Neuro-Otology, 4 (1999), pp. 178–184

FACTORS AFFECTING MEASUREMENT OF EAR-CANAL PRESSURE

TO STAPES VELOCITY TRANSFER FUNCTION

Toshiki Maetani, M.D.1,2, Sunil Puria, Ph.D. 2,3,4, Richard L. Goode, M.D. 2,3

1 Department of Otolaryngology, Head and Neck Surgery, Ehime University School of Medicine, Shitsukawa, Toon, Ehime, 791-0295, Japan

Email: maeta3@m.ehime-u.ac.jp

2 Veterans Affairs Palo Alto Health Care System, 3801 Miranda Avenue, Palo Alto, CA 3 Department of Otolaryngology, Head and Neck Surgery, Stanford University School of Medicine, 801 Welch Road, Stanford, CA

4 Department of Mechanical Engineering, Stanford University, 496 Lomita Mall, Stanford, CA

Possible factors that a ect measurement of the middle ear transfer function were investigated using human cadaveric temporal bones. As done by many others in the past, a laser Doppler vibrometer measured the stapes velocity (Vst) and a probe-tube

136microphone measured pressure (Pec) in the external ear canal (EAC). The ratio of the two variables Vst/Pec is the stapes velocity transfer function (SVTF). Three di erent types of experiments were conducted: (1) artificial vs. natural ear canal, (2) di erent locations of probe tube microphone relative to the eardrum, and (3) varying levels of humidity. The mean SVTF with artificial EAC (A-EAC) was similar as previously reported data, and the di erences between SVTF with the natural EAC and that with A-EAC were less than 5.0 dB in magnitude up to 15 kHz and less than 45 degrees in phase angle. Significant di erences between SVTF magnitude slopes with N-EAC and A-EAC were not detectable. The position of the probe-tube microphone (PT mic) significantly changed the SVTF at high frequencies. At 8 kHz 10 mm di erence with the PT mic position caused 20 dB elevations in SVTF magnitude. The most influential factor that a ects on SVTF at high frequencies was the position of the probe-tube

microphone which is well known to be due ear canal acoustics. Dryness due to a decrease in humidity decreased the SVTF mainly at low frequencies.

1. Introduction

Laser Doppler vibrometry has played an important role for more than a decade for basic research of middle ear mechanics using cadaveric temporal bones or animals and for assessment of middle ear of living humans. This instrument is thought to be useful for diagnosis of conductive hearing impairment in some clinical cases with which conventional tympanometry cannot detect the cause of hearing impairment.

Recently stapes velocity (Vst) and/or umbo velocity (Vu) data has been reported in living and cadaver ears [1]–[7], [13]. Some measurements suggest that Vu of living humans is quite di erent from that with human cadaveric temporal bones at frequencies above 2 kHz. [6]. Even in cadaveric temporal bones there appear to be di erences in Vst at frequencies above 2 kHz arising from possible methodological di erences. For example, Aibara et al [8] report a SVTF magnitude slope of –7 dB/ octave while O’Connor and Puria [9] report a slope of –2.3 dB/octave at frequenciesabove2kHz.Recently,Chienetal[7]reportthatsomeofthese di erences may be due to di erences in the angle of the stapes velocity in relation to the translation axis of the footplate. The purpose of this study is to explore other methodological factors that may a ect laboratory measurements of SVTF.

2. Materials and Methods

2.1 Materials

Twelvehumancadaverictemporalboneswereusedthathadbeenextracted

saw; mean age was 72, ranging from 60 to 88 years old. These specimens were preserved with a 100 ppm merthiolate in normal saline solution at 5 degrees Celsius. Measurement on individual specimens was performed within 2 to 6 days after death. Abnormal and pathological bones were excluded by inspection using an operating microscope. Two ears were used to develop the procedures. Measurements from the remaining 10 ears are reported.