Учебники / Middle Ear Mechanics in Research and Otology Huber 2006

why the wave speed on the eardrum is slow in comparison to air and why there is delay on the eardrum [2, 9, 10].

Acknowledgments

WorksupportedbygrantNo.DC05960fromtheNIDCDofNIH.Wethank Chris Jacobs (Ph.D.) for use of the micro-CT scanner and Derek Lindsey for technical support.

References

1.Fay J.P., Puria S., and Steele C.R., The discordant eardrum. Proc Natl Acad Sci USA, 2006. 103(52): pp. 19743–19748

2.Fay J.P., Puria S., and Steele C.R., Three approaches for estimating the elastic

modulus of the tympanic membrane. J Biomech, 2005. 38(9): pp. 1807–1815

3. Decraemer W.F., Dirckx J.J., and Funnell W.R., Three-dimensional modelling of the middle-ear ossicular chain using a commercial high-resolution X-ray CT scanner. J Assoc Res Otolaryngol, 2003. 4(2): pp. 250–263

4.Kuypers L.C. et al., Thickness Distribution of Fresh Eardrums of Cat Obtained with Confocal Microscopy. J Assoc Res Otolaryngol, 2005: pp. 1–11

5.Funnell W.R., Decraemer W.F., and Khanna S.M., On the damped frequency response of a finite-element model of the cat eardrum. J Acoust Soc Am, 1987. 81(6): pp. 185–189

6.Amin S. and Tucker A.S., Joint formation in the middle ear: lessons from the mouse and guinea pig. Dev Dyn, 2006. 235(5): pp. 1326–1333

7.Vrttakos P.A., Dear S., and Sanders J.C., Middle ear structure in the chinchilla: a quantitative study. J Otolaryngol, 1988. 9: pp. 58–67

8.Weistenhöfer C. and Hudde H., Determination of the shape and inertia properties

of the human auditory ossicles. Audiol Neurootol, 1999. 4(3–4): pp. 192–196

2689. Puria S. and Allen J.B., Measurements and model of the cat middle ear: evidence of tympanic membrane acoustic delay. J Acoust Soc Am, 1998. 104(6): pp. 3463– 3481

10. Olson E.S., Observing middle and inner ear mechanics with novel intracochlear pressure sensors. J Acoust Soc Am, 1998. 103: pp. 3445–3463

INVESTIGATION OF

BONE CONDUCTION THRESHOLDS IN OTOSCLEROSIS

Andreas Arnold1,2, Tamer Fawzy2, Frank Böhnke2

1 Dept. of Otorhinolaryngology, Inselspital, Universität Bern, 3010 Bern, Switzerland, Email: andreas.arnold@insel.ch

2 Dept. of Otorhinolaryngology, Klinikum rechts der Isar, Technische Universität München, Ismaninger Straße 22, 81675 München, Germany

Keywords: otosclerosis, bone conduction, Carhart notch, finite element model, basilar membrane displacement

Introduction: Comparing bone conduction thresholds before and after stapes surgery inmostcasesrevealsareductionofpreoperativeboneconductionlossnotonlyaround 2 kHz (Carhart notch) but in the entire audio range. Postoperatively, a maximum threshold rise in the low and middle frequencies (500–3000 Hz) is seen.

The present study was devised to determine if there is an increase in bone conduction after removal of a fixed stapes footplate without a prosthesis in place. Methods: Stapedectomy under local anaesthesia was performed in 24 patients with otosclerotic stapes fixation. We compared the bone conduction of preoperative

audiograms with intra-operative thresholds measured directly after removing the 269 footplate using a bone conduction transducer at test frequencies of 0.5, 1, 2 and 4

kHz.

Results: Our data showed a clear improvement at 0.5, 1 and 2 kHz, but not at 4 kHz. Threshold rise is best at 2000 Hz, corresponding to the Carhart notch. Pure-tone averages (PTA) were calculated from the collected data and statistically analyzed. The improvement was significant for PTA from 0.5 to 2 kHz. No significant improvement was found for PTA from 0.5 to 4 kHz.

To investigate the underlying cochlear mechanisms, a finite element model of the cochlea was used. It suggests an increased amplitude of the basilar membrane displacement with an open oval window due to a lower impedance compared to the situation with a fixed footplate.

2. Patients and Methods

2.1 Clinical study

Stapedectomy under local anaesthesia was performed on 24 patients (14 female, 10 male, age 25–74) with stapes fixation due to otosclerosis. All patients underwent the regular preoperative audiological assessment with a pure-tone audiogram and speech tests using Aurical Audiometers (Madsen/GN Otometrics) calibrated to EN ISO 389 in a soundproof chamber. Intra-operative thresholds were measured directly after removing the footplate using a bone conduction transducer JKG 96 (CB-Elmec), certified for audiological measurements (Physikalische Technische Bundesanstalt) at test frequencies of 0.5, 1, 2 and 4 kHz. In order to avoid a bias, the first 10 patients were measured with both systems preoperatively. No di erence between the results was found. Masking of the contralateral ear during in- tra-operative testing showed no di erence compared to the results without masking on the same subject.

Finally, we compared the bone conduction of preoperative audiograms with intra-operative thresholds and did statistical analyses (Friedman test). The patients gave their written consent for the intra-operative measurement.

2.2 Finite element (FE) model

Using a FE model of the human cochlea, previously set up in ANSYS finite element software [2], the amplitude of the basilar membrane displacement was computed. In our model, the stimulation was done via bone conduction. A pressure signal with di erent frequencies (500–4000Hz) and constant sound pressure amplitude of 1 Pa equivalent to 94 dB (SPL) was applied to the total outer cochlear wall. The round window was presumed to be normal with mobile membrane structures. Basilar membrane displacement was calculated in a cochlea with a totally fixed stapes, in a cochlea

with a piston hole in the stapes footplate (stapedotomy) and in a cochlea 271 with a totally removed stapes (stapedectomy).

3. Results

Comparing the pre-operative and intra-operative averaged frequency val- uesfromall24patients,theriseinboneconductionthresholdsat500–2000 Hz is evident. This is not the case at 4 kHz. Corresponding to the Carhart notch, threshold rise is best at 2000Hz (Fig. 2). Pure-tone averages (PTA) werecalculatedfromthecollecteddataandstatisticallyanalyzed.Theintraoperative improvement was significant for PTA from 0.5 to 2 kHz. (Fig. 3) No significance was found for PTA from 0.5 to 4 kHz. (Fig. 4)

Fig. 2 Averaged frequency values from all 24 patients: the rise of intra-operative bone conduction thresholds at 500–2000 Hz is evident.

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Fig. 3 Change of bone conduction pure-tone averages (PTA) for every patient (=dot) after removal of the fixed stapes footplate: For PTA from 500 to 2000 Hz, a clear intraoperative improvement is visible and found statistically significant.

Fig. 4 Change of bone conduction pure-tone averages (PTA) for every patient (=dot) after removal of the fixed stapes footplate: For PTA from 500 to 4000 Hz, the overall tendency for intra-operative improvement is also visible but without statistical significance.

The investigation of the underlying cochlear mechanisms with the finite element model of the human cochlea computed an increased amplitude of the basilar membrane (BM) movement with an open oval window due to a lower impedance, compared to the situation with a fixed footplate. In the otosclerotic case with totally fixed footplate the BM displacement is about 0,141 nm for relevant frequencies between 1–4 kHz. In contrast to this the opened oval window (footplate removed) shows an increase of BM displacement by up to 0,523 nm. The increase of BM displacement is largest for the frequencies between 1–4 kHz.

The total removal of the stapes footplate produced a slightly greater 273 e ect than a piston hole. (Fig. 5)

Fig. 5 FE-model of the human cochlea: the increase of basilar membrane (BM) displacement after removal of the fixed stapes footplate is nearly 5-fold at 1000 Hz and is largest for the frequencies between 1–4 kHz.

4. Discussion

These results show that bone conduction thresholds increase immediately after removal of the fixed stapes footplate. This sudden rise in bone conduction threshold must have a mechanical cause (e.g. Carhart`s cochlear reserve)[1].Itisnotofsensorineuralorigin.Afixedfootplateleadstoadissipation of parts of the energy applied to the cochlear fluid system by bone conductionstimulation,evenwithamobileroundwindowmembrane.This gives evidence that optimal basilar membrane movement depends on the integrity of the cochlear windows. In other words, in a cochlea with totally fixed stapes there is “captured energy“ which cannot properly act on the basilar membrane during bone conduction stimulation. By removing the

274fixed footplate, additional energy becomes available for basilar membrane displacement, corresponding to a better bone conduction threshold. Physically, a cochlea with an opened oval window has a lower impedance.

The averaged intra-operative bone conduction frequency values improve up to 2000 Hz. The value at 4000 Hz shows an opposite trend. This may be caused by the known variability of bone conduction audiometry at higher frequencies [3]. For the same reason the values of PTA from 500 to 4000 kHz do not improve in a statistically significant manner.

This is the first investigation in which audiometry was performed with an opened oval window during stapedectomy. It directly explains the mechanical cause of parts of the depression of bone conduction in otosclerosis as already suspected by Carhart.

5. Conclusion

After total removal of the fixed stapes footplate, bone conduction thresholds improve immediately, corresponding to an increase of basilar membrane displacement.

References

1.Carhart R., Assessment of sensorineural response in otosclerotics. Arch Otolaryngol. 1960 Feb; 71: 141–149

2.Böhnke F., Arnold W., Bone conduction in a three-dimensional model of the cochlea. ORL J Otorhinolaryngol Relat Spec. 2006; 68(6): 393–396

3.Laukli E., Fjermedal O., Reproducibility of hearing threshold measurements. Supplementary data on bone-conduction and speech audiometry. Scand Audiol. 1990; 19(3): 187–190

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thresholds clinically; the transcranial transmission determines the amount of masking necessary to isolate the ear of interest. Also, it is important for BChearingaids,e.g.itprovidesanestimateforthee ectivenessofusingBC hearing aids for unilaterally deaf persons. So far, the transcranial transmission(sometimesreferredtoastranscranialattenuation)hasbeenestimated by BC hearing thresholds or skull bone vibration measurements [1,2]. The mostextensiveinvestigationofBChearingthresholdsreportedanearlyfrequency independent average transcranial transmission of around –10 dB [2]. The aim of this study is to estimate the transcranial transmission. We also investigated whether the transcranial transmission can be estimated from ear canal sound pressure (ECSP) measurements.

2. Method

20 normal hearing persons between 23 and 43 years old participated in the study. A BC transducer was used to stimulate at three positions of the skull bone, one at a time. The stimulation positions were at the center of the forehead and at the mastoid portion of both temporal bones. The BC hearing threshold of the test ear was obtained using a pulsed Békésy procedure, while the other ear was masked with pink noise 10 to 20 dB above the individuals’ hearing thresholds. The ECSP was measured with a probe microphone close to the eardrum. All measurements were conducted with open and occluded ear canals. The ear was occluded by a foam earplug.

The transcranial transmission was calculated as the BC hearing threshold or ECSP for contralateral stimulation relative to the same measure with ipsilateral stimulation (Fig. 1a). The transmission from the forehead was calculated as the BC hearing threshold or ECSP for stimulation at the forehead relative to the same measure with ipsilateral stimulation (Fig. 1b). Also obtained was the occlusion e ect that was calculated for the

three stimulation positions as the relation between BC hearing thresholds 277 or ECSP with an occluded and open ear canal.