Учебники / Middle Ear Mechanics in Research and Otology Huber 2006

Fig. 1 Schematic representation of the measurement system.

2.2 Measurement system

As shown in Figure 1, all measurements were performed using SYSid 6.5 audio band measurement and analysis system (www.sysid.com, Berkeley, CA, U.S.A.). This software program in conjunction with a DSP-16+ processing board (Granbury, NJ, U.S.A.) produces an output signal used to drive a sound source, and synchronously measures and averages two channel input signals. The magnitude and phase angle at each frequency is obtained through fast Fourier transformation (FFT).

The sound stimuli were stepped tones swept over 200 logarithmicallyspaced frequency points from 0.1 to 25 kHz in a manner similar to that described previously [11].

138 OneoftheinputsignalswassoundpressureofEAC(Pec)measuredby a probe-tube microphone (ER-7C, Etymotic Research, Elk Grove Village, IL, USA) whose tip was placed within 2 mm from the tympanic membrane. In natural EAC experiments this tube was fed through the foam of the eartip into the canal space.

In the experiments with artificial EACs, a small hole was made near the medial end of the plastic tube, which was placed perpendicularly onto the bony tympanic ring as an artificial EAC, and an ER-7C tube was fed through this hole into the canal space, also within 2 mm from the edge of the tympanic membrane.

The second input is peak footplate velocity measured by a laser Doppler vibrometer (LDV) (HLV-1000, Polytec PI, Costa Mesa, CA, U.S.A.).

The LDV was set to the 5 ((mm/s)/V) velocity range, the 30 kHz low pass filter corner frequency, and with the high pass filter turned o .

PT mic and HLV laser calibrations have been previously described [7]. (Figure 2)

2.3 Data analysis

Pec values are RMS values and Vst was expressed with the peak values. SVTFswerecalculatedasVst/Pec anditsmagnitudeexpressedlogarithmicallydB units defined as:

TF dB = 20 * log10( (Vst/Pec) / 0.1 )

Here 0 dB is set to 0.1 (mm/s)/Pascal.

Since it is usually hypothesized that the distribution of TF is log-normal, the geometric mean was calculated as the mean SVTF and expressed with units of dB. In other words it is the mean of logarithmic values of SVTFs [12].

2.4 Measurement angle correction

The angles between laser beam and the normal vector of the footplate or piston axis were estimated to be 45 to 60 degrees. Vst was calculated by dividing the measured velocity by cosine of 55 degrees at all frequencies so that cosine correction is considered to be within 3 dB as Hato and Stenfelt argued [8]. In this paper a frequency-dependent method was not taken as argued by Chien et al [6].

3. Results

3.1 Signal to noise ratio (SNR)

The noise floor level was measured in all 10 bones. In those ears the SNR 139 of Pec magnitude was more than 60 dB up to 15 kHz and about 60 dB at frequencies from 15 kHz to 25 kHz. The SNR of Vst was more than 40 dB

up to 8 kHz, 35 dB from 8 kHz to 15 kHz and above 15 kHz the SNR was less than 20 dB.

3.2 Artifact levels

The promontory vibration was measured as the artifact level. It was more than 40 dB from 0.1 to 5 kHz, 20 to 40 dB from 5 to 15 kHz and less than 5 dB above 15 kHz.

3.3 Canal obliquity and SVTF

Fig. 2 The mean SVTF with the N-EAC and A-EAC (top figure: magnitude, bottom figure: phase). The magnitude standard deviations are shown above the 0 dB line, which corresponds to the reference of 0.1 mm/s/Pa.

Ineachofthetenbones,boththeSVTFwithnaturalEACandwithartificial EACweremeasuredwithina24-hourtimeinterval.Inexperimentalproce- dures, care was taken not to damage the ossicles, especially the footplate.

As shown in Fig. 2, di erences in SVTF magnitude with N-EAC and that with A-EAC is within 2.0 dB up to 6 kHz and at 6 to 12 kHz differences were within 5.0 dB. The standard deviation of TF magnitude dif-

140 ference was 4 dB up to 4 kHz and that at 4 to 12 kHz was within 10 dB. Di erences in SVTF phase angle with N-EAC and that with A-EAC

was less than 45 degrees below 12 kHz. The standard deviation of TF angle di erence was within 45 degrees up to 4 kHz and at frequencies from 4 to 12 kHz within 120 degrees. Mean, SD and range of both SVTF were quite similar and it seemed that the canal obliquity does not a ect on the SVTF.

3.4 PT microphone position and SVTF

Fig. 3 SVTF (upper figure: magnitude, bottom figure: phase) measured at 4 di erent positions of the PT microphone tip. The 4 positions are the PT mic position 0 mm from the TM (thick solid), 4 mm the TM (dashed line), 8 mm from the TM (dasheddot line), and 12 mm from the TM (thin-solid line).

To assess the e ect of the position of the PT mic, SVTFs of a single bone with an artificial EAC was measured with di erent position of the PT mic. The position of the PT mic was set at 0, 2, 4, 6, 8, 10, 12 and 14 mm from the tympanic membrane.

As the PT mic position was moved laterally, Pec magnitude decreased

in the 2 to 10 kHz range. Not surprisingly, the PT mic position didn’t af- 141 fect Vst. The position of PT mic drastically a ected on the measured SVTF values at high frequencies as Khanna and Stinson previously reported [9].

At 8 kHz the di erence SVTF magnitude was 20 dB and even at 4 kHz the di erencewasnearly10dB.Sincethequarterwavelengthat4kHzisabout 8.5 mm this result is consistent with basic acoustic theory. With this bone Vst at 7 kHz was quite low but the reason of this dip remains uncertain.

3.5 Humidity and SVTF

Fig. 4 Change in SVTFs under dry circumstances within 6 hours (top figure: 1, 2, 4, 6 hours; bottom figure: 12, 24, 48, 72, 96 hours). SVTF values were normalized by the initial values (SVTF|T=0). In this period it was sunny and the relative humidity decreased rapidly.

To assess the e ect of dryness on SVTF, measurements were repeated for 4 days with a single temporal bone.

This bone had been left in the lab without artificial application of humiditysource(i.e.,nosalinesprayorhumidifier)throughoutthemeasurement period. The measurements were made at 0, 1, 2, 3, 4, 6 hours (Figure

1424,top)and12,24,48,72,96hours(Figure4,bottom)afterthe initialmeasurement.

The room temperature was kept constant at approximately 23 degrees Celsius, but the relative humidity varied from 20 to 47 %. Apparently the room humidity was directly influenced by surrounding atmospheric humidity due primarily to changes in the weather. Since the experiment was performed in the rainy season the humidity was comparatively high and it increased when it rained hard (data not shown). Dryness decreased the SVTF by 15 dB at low frequencies by leaving the specimen in a room with 20 to 47 % relative humidity for 3 days. On the contrary, at 8 kHz dryness increased the SVTF by 10 dB for 3 days. These results indicate that in a six hour measurement period, the length of typical laboratory measurements,

dryness can a ect measurements of SVTF by up to +/– 5 dB if no external humidity is applied.

4. Discussion

Sincemanyfactorscana ectstapesvelocity(Vst)andtransferfunction(Vst/ Pec), they were classified into two categories. First is the subjective factor which is intrinsic to the bone itself. Pathological bones are excluded but there is certainly some inter-individual variation.

The other factors not intrinsic to the bone are subdivided into storage factors, bone-preparation factors, measurement factors (due to di erent methodologyin experimentalsettingsand instruments),andcomputation methods. In this paper factors of experimental setting or measurement factors were focused on.

4.1. Canal obliquity

The external ear canal has anatomically complex shape. We hypothesized that due to the oblique position of the eardrum in the ear canal, the medial tapering may amplify the sound energy that enters the EAC at high frequencies.ThephaseangleoftheanteriorpartoftheTMisdelayedinterms of that of the posterior part due to the di erence in the distance from the sound source. It might cancel or add due to EAC tapering. The results suggestthatourhypothesiswasincorrect.Theredoesnotappeartobeane ect due to canal obliquity.

4.2. Position of PT mic

Sound input parameters of auditory system are typically specified in terms of sound pressure level of a single point in the EAC.

When human cadaveric temporal bones are used for the research of auditory system, bony canal is drilled down and a plastic tube as an ar- 143 tificial ear canal is set onto the tympanic annulus perpendicularly and a probe-tube microphone is fixed near the tympanic membrane.

On the other hand in clinical settings it is hard to place the probetube microphone near the tympanic membrane. The distance between the annulus and the PT mic is estimated to be 5 to 10 mm since the isthmus of the EAC is located from 7 mm from the annulus. It may cause the difference or error in calculating the middle ear transfer function. In this experiment the e ect of the PT mic position was investigated. In this setting, the position of the reflectors (measurement point) and the ear plug, the angle of the laser beam was exactly identical so that the di erence in SVTF should be caused only by the di erence in the PT mic position.

The PT mic position has an e ect on the measured Pec values exclusively which resulted in changes to the calculated SVTF values.

Otolaryngologistsshouldkeepinmindthatsomefactorscana ectPec exclusively, the denominator of SVTF, and those values do not always reflect the actual sound energy that enters the EACat high frequencies. Thus it is also the case with clinical measurements of otoacoustic emissions.

4.3. Humidity

Although some authors [4] mentioned that dryness can a ect the SVTF drastically, analytic assessment has not been made. In this paper we measured the e ect of humidity but it seems is that this experiment was preliminary and still semi-quantitative. But the tendency that as the relative humidity decreased the SVTF magnitude at lowand mid-frequencies decreasedinamannerconsistentwithanincreaseinsti nessofthesofttissue (the tympanic membrane and the annular ligament).

5. Summary

Three possible factors that a ect the middle ear transfer function were investigated. Significant di erence between SVTF with natural EAC and artificial EAC were not measured. As expected the position of the probe-tube microphone and humidity appear to be two important factors.

Acknowledgments

Work supported in part by grant No. DC 05960 from the NIDCD of NIH.

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145

ACUTE TYMPANIC MEMBRANE PERFORATIONS HEAL WITHOUT SIGNIFICANT LOSS OF STRENGTH

M. von Unge, Karolinska University Hospital, Stockholm Email: magnus.von.unge@ki.se

J.J.J. Dirckx, Bimef, University of Antwerp, Belgium

A Rahman, Karolinska University Hospital, Stockholm, Email: Anisur.Rahman@ki.se M. Hultcrantz, Karolinska University Hospital, Stockholm

Email: Malou.Hultcrantz@karolinska.se

1. Introduction

The physical and mechanical properties of the tympanic membrane (TM) such as area size, mass, sti ness and 3D-shape defines its vibratory pattern in response to the sound waves and thereby modulates the perception of sound. Changes in some of these parameters will alternate the auditory function of the ear (1, 2). Inflammatory middle ear diseases cause structural aberrations of the TM resulting in changes in its mechanical properties (2–4).TheremaybechangesaswellintheseparametersafteraTMperfora- tion has healed. It has been stated that the healed wound does not recover its complete tensile strength as before although the perforation would heal e ciently (5). There are very few reports on the strength of the TM after

146 perforations have healed.

The aim of the present study was to adjust a moiré interferometry set up for displacement measurements on the Sprague-Dawley rat temporal bone, and to ascertain the degree of the functional and structural restitution of the healed TM following a traumatic fresh perforation.

2. Material and Methods

Ten healthy female Sprague Dawley young adult rats of approx. 300g BW were used. The animals were anesthetized with 100 mg ketamine hydrochloride and 10 mg xylazin hydrochloride intraperitoneally and myringotomy was performed in the right TM using a 0.2 mm KTP laser beam with

0.5 seconds single pulses of 1 Watt. Thereby a calibrated TM perforation was produced. The perforation was made in the upper posterior quadrant. The left TM was left untouched and was used as a normal control. Otomicroscopic inspection was performed thrice a week and the healing pattern was documented. The animals were sacrificed at 2 or 4 weeks (five in each group) after myringotomy. Measurements were made at the University of Antwerp, Belgium.

2.1 Moiré interferometry measurements/moiré principle

Moiré interferometry is an optic, non-contacting technique for shape or deformation measurements. A ruling of straight lines is projected onto the surface of an object and the image of these projected lines is recorded with a video camera. By electronically subtracting images obtained on di erent objectstates,interferencefringesareobtained.Thesefringesrepresentcontours of equal object shape or deformation. By subtracting images obtained on an object and on a flat plane, moiré fringes are obtained which correspond to contours of equal object height, and thus display the membrane shape like lines on a topographic map. From these so called “moiré topograms” full field deformation data are obtained. In order to get su cient moiréfringeimagequalitywhitechinainkisputbehindtheeardrum,from the ear canal side.

2.2 Physiological processing

Ten temporal bones were prepared for full-field moiré interferometry measurements of the TM: the tympanic bulla was opened widely, the tensor tympani muscle was cut and part of the medial wall of the tympanic cavity was removed. The incudo-stapedial joint was disarticulated and the stapes was removed. The malleus and incus with their ligaments were kept untouched.

Dehydrationoftheisolatedtemporalbonewaspreventedbyperform- 147 ing all the procedures under the air steam of an evaporator according the method of von Unge (6, 7). A plastic tube mimicking the external audi-

tory canal was glued to the bony meatus of the ear canal, through which sequencesofstaticpressureswereappliedtotheearcanal,therebyproducing a pressure gradient across the TM. In steps of 50 daPa a first positive sequencewasgiven,startingat0daPaincreasingupto350daPa,andfrom here back to 0 daPa in descending order. Then a similar sequence but with negative pressures was given. The time interval between each step was 10 seconds (6).