a biomechanical ear model to evaluate middle-ear reconstruction

Research Article

International Journal of Audiology 2009; 48: 876–884

ISSN 1499-2027 print/ISSN 1708-8186 onlineDOI: 10.3109/14992020903085735„ 2009 British Society of Audiology, International Society of Audiology, and Nordic Audiological Society

Received:December 7, 2008Accepted:June 2, 2009

Hamidreza MojallalDepartment of Otorhinolaryngology, Medical University of Hannover, Carl-Neuberg-Str. 1, 30625 Hannover, Germany.E-mail: [email protected]

of the outer, middle, and inner ear the auditory system has attained an excellent level of sensitivity which enables audible even the extremely small displacement of the eardrum on the picometric scale (10−12 m) (Zahnert, 2003).

In middle-ear reconstruction, therefore, restoring such a sensitive transmission apparatus to its previous level of effectiveness repre-sents a considerable challenge. Findings published in some papers suggest a tendency for the reconstructed ossicular chain to have suboptimal transmission properties, so that—at least at some frequen-cies—an air-bone gap (ABG) up to 20 dB or more has been observed postoperatively (Hüttenbrink & Hudde, 1994; Rust et al, 1996; Gju-ric & Schargel, 1998; Merchant et al, 2003; Zahnert, 2005; Schrem-ber et al, 2006; Kwok & Jacob, 2006). Reasons for this might be the

Human sensory organs have adapted in the course of evolution to optimally suit their surroundings, and the auditory system is a good example of this. The middle ear, as part of this system, serves both transmission and impedance matching functions at the junction between two different media, i.e. the air in the outer ear and the fl uid in the inner ear. Physical laws dictate that up to 98% (about −35 dB) of the sound reaching the interface (in this case the middle ear) between air and fl uid is refl ected (Boehme & Welzl-Müller, 2005). To compen-sate this energy loss, amplifi cation effects of the eardrum-ossicular chain-apparatus namely the conical shape of the eardrum, hydraulic effect, and the lever function, cause frequency dependent acoustic amplifi cation of up to 25–30 dB at frequencies up to around 7000 Hz (Yost, 2006; Hüttenbrink, 1992). Overall due to amplifi cation effects

A biomechanical ear model to evaluate

middle-ear reconstruction

AbstractIn order to evaluate the effi ciency of middle-ear prosthe-ses in near-real conditions, an artifi cial model was devel-oped that approximately simulates the actual geometrical and biomechanical properties of the ear system (excluding the ossicular chain). The sound transmission characteris-tics of selected commercial middle-ear prostheses and of a synthetic test material were measured using laser Doppler vibrometry (LDV) in this model. The model’s realistic prop-erties enabled clinical tympanometry to be used to control the stiffness. In addition the infl uences of the implant mass on transmission characteristics were investigated. With an averaged displacement between 10 and 100 nm/Pa up to 2000 Hz, the transmission characteristic of the model was comparable with data obtained from the intact middle ear in temporal bone experiments. From the acoustical point of view, no signifi cant material-specifi c differences could be found. Increasing the mass of the implants to more than 50 mg results in poorer acoustic transmission. In general, changes to the stiffness involving compliance values greater than 3.5 ml and smaller than 0.2 ml led to poorer acoustic transmission.

Hamidreza Mojallal1

Martin Stieve1

Ilka Krueger2

Peter Behrens2

Peter P. Mueller3

Thomas Lenarz1

1Department of Otorhinolaryngology, Medical University of Hannover, Germany2Institute for Inorganic Chemistry, Leibniz University Hannover, Germany 3Helmholtz Centre for Infection Research, Braunschweig, Germany

Key WordsMechanical ear model, middle-ear reconstruction, middle-ear prosthesis, sound transmission characteristics, laser Doppler vibrometry, tympanometry, static compliance, air-bone gap

AbbreviationsABC: Air bone gapChi-HA: Chitosan-hydroxyapatitedaPa: DecapascaldB: DecibelFEM: Finite element modellingLDV: Laser Doppler vibrometryMEM: Mechanical ear modelPORP: Partial ossicular replacement

prosthesisSC: Static complianceSPL: Sound pressure levelTORP: Total ossicular replacement

prosthesisTM: Tympanic membrane

SumarioCon objeto de evaluar la efi ciencia de las prótesis de oído medio en condiciones cercanas a la realidad, se desarrolló un modelo artifi cial que simula de manera aproximada las propiedades geométricas y biomecánicas reales del sistema auditivo (excluyendo la cadena oscicular). Se midieron las características de transmisión del sonido de prótesis com-erciales seleccionadas de oído medio y de un material de prueba sintético, usando vibrometría laser Doppler (LDV) en este modelo. Las propiedades realistas del modelo permiti-eron el uso de la timpanometría clínica para controlar la rigi-dez. Además, se investigaron las infl uencias de la masa del implante en las características de transmisión. Con un despla-zamiento promedio entre 10 y 100 nm/Pa, hasta 2000 Hz, la característica de transmisión del modelo fue comparable con los datos obtenidos de un oído medio intacto en experimen-tos en hueso temporal. Desde el punto de vista acústico, no se pudieron encontrar diferencias signifi cativas relacionadas específi camente con el material. El incremento de la masa de los implantes a más de 50 mg, determina un empobrec-imiento de la transmisión acústica. En general, los cambios en la rigidez involucran valores de compliancia mayores a 3.5 ml y menores de 0.2 ml, que llevan a una transmisión acústica más pobre.

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Development of a Biomechanical Ear Model to Evaluate Middle Ear Reconstruction

Mojallal/Stieve/Krueger/Behrens/Mueller/Lenarz

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inner part, the middle-ear implants were to be used as transmit-ters. The implants could be inserted, and the air pressure balance could be maintained, via a sealable opening (see Figure 2).

• Membrane D: This area acts as the ‘oval window membrane’ and consists of the same material as membrane B. Serving as an opening into area C, its surface area is around 3.5 mm2.

• Area E: This cylinder, with a volume of some 60 μl, represents the inner-ear volume (Thorne et al, 1999). It was fi lled with saline solution in order to better simulate the cochlear resistance towards the middle ear.

• Membrane F: The other end of the cylinder, effectively serving as the ‘round window’— the second opening of the cochlea—is also covered with the biomembrane.

The membranes D and F were fi xed onto both ends of the cylinder E by two rubber rings with inner diameters of about 2 mm. The cylinder E was sealed by means of a simple valve system. This was realized by two further vents into the body of the cylinder which were sealed through silicon rods. Two cannulae were inserted into the cylinder through the vents. The fi rst cannula fi lled the cylinder with fl uid and the air was aerated by the second cannula.

Experimental procedures Clinical tympanometry was used to test the stiffness. Due to nearly comparable properties of the MEM with the actual auditory system the tympanometry can easily be applied to test the stiffness, or rather its reciprocal value (i.e. compliance). In clinical audiology the compli-ance is given in ‘mmho’ or in equivalent volume in ‘ml’. A further parameter in this terminology is the static compliance (SC), which is calculated as the difference between the compliance values between �200 daPa and 0 daPa (decapascal). The normal SC levels in adults are between 0.28 and 2.5 ml (Boehme & Welzl-Müller, 2005). Figure 3 shows recorded tympanograms with SC between 0.9 and 1 ml in the MEM with a middle-ear implant placed in position. To demonstrate the reproducibility of the procedure, tympanometry was performed three times in succession. In order to ensure that the infl uence of stiff-ness remained unaltered for each implantation, an attempt was made to insert all implants such that the compliance remained unchanged. A GSI Tympstar Middle-Ear Analyser manufactured by VIASYS Healthcare Inc, USA, was used for this experiment.

Vibration analysis was performed using laser Doppler vibrometry (LDV). The vibration velocity was measured at the exit point of the MEM, namely at the end of the third cylinder (see Figure 4). The

physical properties of middle-ear implants, surgical conditions, and postoperative interactions between the tissue and the implants (Hüt-tenbrink, 1992; Meister et al, 1998; Jahnke, 1998). Therefore, for an optimal evaluation of middle-ear prostheses it will be helpful to have appropriate experimental models, which simulate as much as possible the real characteristics of the reconstructed middle ear.

In an interdisciplinary effort with the overall aim of developing novel middle-ear implants, investigations about their sound transmis-sion characteristics were carried out involving three consecutive experiments. Firstly, a simple mechanical ear model (MEM) was developed which, using clinical and experimental procedures, served to assess the transmission properties of the implants. Further evalu-ative studies were performed involving temporal bone specimens and animal models. Findings obtained using the MEM will be described here. The objective of the present work was to investigate the sound transmission characteristics of implants in nearly-real conditions.

Materials and Methods

In this investigation an attempt was made to develop a simple ear model, with its geometrical and biomechanical properties as realistic as possible. To this end, the geometric conditions of the auditory sys-tem were used as underlying parameters. The MEM consisted of three parts, each designed to simulate the respective anatomical and biome-chanical properties of the external auditory canal, middle ear, and inner ear. Commercial middle-ear implants as well as experimental ones specifi cally synthesized for this purpose were used as sound transmit-ters between the external and internal parts. Three nested, sliding Tef-lon cylinders of different diameters were assembled for this model. A micro manipulator made possible a precise adjustment of these cylin-ders. Figure 1 shows the ear model with a profi le and three-dimensional plan generated in the technical drawing program AutoCAD.

• Area A: This area represents the external auditory canal. This cylinder therefore has a total volume of around 3 ml, with a diameter of 12.5 mm, and a length of 25 mm.

• Membrane B: This soft membrane simulates the tympanic membrane (TM). A biomembrane made from animal skin tis-sue, which is also used for musical instruments, was employed for this purpose, its thickness and elasticity being comparable with that of the TM. The membrane used weighed about 32 mg, and its surface area was around 1.2 cm2.

• Area C: With a volume of approximately 7 ml this area represents the middle-ear cavity. In this area, located between the outer and

Figure 1. Mechanical ear model with profi le and three dimensional plan.

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878 International Journal of Audiology, Volume 48 Number 12

Figure 2. Inserted middle-ear prosthesis (Aerial-gold) in the MEM.

Figure 3. Measured tympanograms in the MEM using an Aerial-gold; n � 3.

LDV system employed was the PSV-300 scanning laser Doppler vibrometer (SLDV) made by Polytec, Waldbronn, Germany. This system can perform measurements at one point as well as over spec-ifi ed portions of an object’s surface (as scanning mode). Both an oscillator for acoustic stimulation and a signal processing unit were integrated into the system.

Measurement set-up Figure 4 presents a schematic overview of the experimental set-up. After an implant had been placed between membranes B and D, compliance was measured using tympanometry. Through a fi ne adjustment of the position of the cylinders by the micro manipulator, a SC of around 1 ml was achieved, which remained constant over the entire series of measurements with different implants. It was also important to ensure that the membranes were kept suffi ciently moist to avoid changes in stiffness.

The MEM was acoustically stimulated in the outer part (area A) using an insert earphone (EARTONE 3A, Etymotic Research, IL, USA), which was placed in the outer cylinder using an EAR Link (ER3-14A disposable foam eartip, Etymotic Research, IL, USA). The loudspeaker was driven with an input voltage of about 300 mV using a chirp signal (100 to 10 000 Hz). A probe microphone (ER 7C, Etymotic Researech, IL, USA) was used to measure the sound pres-sure in the external cylinder in front of membrane B.

The vibrations were measured on membrane F using LDV. For these measurements, the central region of the membrane was defi ned and scanned, which involved fi ve points on the surface. The mea-sured vibration velocities were averaged and converted into corre-sponding displacement and, following Fourier transformation, shown in a frequency spectrum from 100 to 10 000 Hz. For the evaluation, 30 measurement frequencies between 100 and 8000 Hz were deter-mined and analysed. The transfer function was calculated as the quotient of displacement in μm and sound pressure in Pa shown in dB re μm/Pa (i.e. 0 dB � 1 μm/Pa).

In order to determine the reproducibility of these measurements, the test was fi rst performed fi ve times in succession on a specifi c implant (Aerial-gold TORP). The prosthesis was reinserted into the MEM each time. A fi ne adjustment of the position of the cylinders ensured that the same compliance was achieved on each occasion.

To determine the infl uence of stiffness, different compliances were achieved and recorded (using tympanometry) for the Aerial-gold TORP by shifting the cylinders through the micro manipulator. For the simulation of mass effects, the mass of the gold implant (22.5 mg) was increased progressively using modelling clay. For this the material was placed along the shaft of the prosthesis, so that its overall shape remained unchanged.

Finally, measurements on the various implants were carried out in fi ve series of assessments, with each series lasting about 15 min-utes per implant. The ‘student’s t-test’ was used for statistical anal-ysis. The confi dence level α was defi ned as signifi cant at p � 0.05 and as highly signifi cant at p � 0.001.

MaterialsAs specimens, commercially available passive middle ear prostheses and an experimental material specifi cally designed and synthesized for this study were used (see Table 1 and Figure 5).

Commercially available middle-ear prostheses were made of gold, titanium, and of Bioverit® II, a glass-mica composite material (Höland et al, 1990). The experimental material is an organic-inorganic com-posite material made of Chitosan and Hydroxyapatite (Chi-HA). The use of this material is basically inspired by the nature of natural bone as a nanocomposite made of collagen and apatite (Kong et al, 2005).

Results

The measured sound pressure level by the probe microphone in the outer part averaged approximately 70 �10 dB SPL. The reproduc-ibility of these measurements is indicated in Figure 6. The curve depicts the mean displacement with error bars at each frequency for the Aerial-gold TORP. The transfer function lies between −20 and

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−40 dB re μm/Pa (10–100 nm/Pa) at frequencies up to 2000 Hz. Above this frequency, the values decrease by about 15 dB/octave. Two resonance regions could be found, at around 500 Hz and 1200 Hz. The mean standard deviation (SD) was about 0.34 (p > 0.05; r � 0.99). The static compliance in these measurements was between 0.9 and 1.1 ml.

The infl uence of the stiffness on the transfer function is shown in Figure 7. Using the same implant as in the previous measurement, different degrees of stiffness were achieved by moving the position of the outer cylinder; compliance was determined by means of tym-panometry. The SC values (0.4 ml, 1.2 ml, and 3.1 ml) and the corresponding tympanograms are also shown in Figure 7. These measurements clearly indicated that the resonance regions shifted to higher frequencies when stiffness is increased (i.e. at low SC of about 0.4 ml) and vice versa. The differences in sound transmission were signifi cant between the measurements at 0.4 ml and 1.2 ml, as well as between 1.2 ml and 3.1 ml (P � 0.05), whereas the difference between the compliance values of 0.4 ml and 3.1 ml was found to be highly signifi cant (P � 0.001). For SC values above 3.5 ml and below around 0.2 ml, deterioration in transmission was observed over the entire frequency range.

Increasing the mass of a given implant resulted in a damping of the transfer function from around 1000 Hz upwards, and led to

the second resonance region shifting to a lower frequency (see Figure 8). Differences in transmission were observed for reso-nance regions around 10–15 dB. A highly signifi cant difference (P � 0.001) was calculated between two groups with masses of 22.5 mg and 80 mg. Comparison of the implant weighing 22.5 mg and that weighing 50 mg, however, revealed a high level of signifi -cance only in the 600–2000 Hz frequency range. Moreover, it was observed that the implants exhibited the same transmission charac-teristic at frequencies extending up to just above the fi rst resonance region (500 Hz). For this measurement, the SC for each positioning was maintained constant at around 1.1 ml.

Figure 9 shows the averaged transfer function of all implants tested using the ear model. The average displacement for different implants up to around 2000 Hz was between −20 and –40 dB re μm/Pa (10–100 nm/Pa), subsequently decreasing by about 15 dB/octave. In certain frequency ranges a transmission difference of up to around 10 dB was recorded between the different implants. At low frequencies up to around 500 Hz, the Aerial-gold TORP showed superior transmis-sion to the other implants. Above this frequency, up to around 1000 Hz, Bioverit® and Chi-HA yielded better results. From 1500 Hz upwards, the Aerial-titanium TORP exhibited superior transmission behaviour. Calculations of signifi cance incorporating all results over the entire frequency spectrum produced no signifi cance (P > 0.05).

Figure 4. Schematic overview of the experimental set-up in the MEM.

Table 1. Material, weight, and length of the implants used in the MEM

Implants Weight (mg) Length (mm)

Aerial-gold prosthesis; type ‘Dresden’ KURZ Medizintechnik, Dusslingen 22.5 6

Aerial-titanium prosthesis; type ‘Düsseldorf’ KURZ Medizintechnik, Dusslingen 5 5Middle ear implant: type ‘Jena’ 3di GmbH, Jena 15 3Chitosan-hydroxyapatite Developed in the present study 6.3 � 0.7 5-7

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However, signifi cant differences were found over certain frequency ranges. For example, a high level of signifi cance was determined between the gold and the titanium prosthesis between 100 and 600 Hz (P � 0.001). In addition, the transmission properties of the tita-nium implant was shown to be signifi cantly better than those of gold and Bioverit® prostheses above 2000 Hz (P � 0.05). The SC values varied between 1 and 1.1 ml.

Discussion

A simple mechanical ear model (excluding the ossicular chain) was presented closely simulating the geometric and mechanical properties of the human auditory system, in order to investigate the relevant parameters that infl uence middle-ear reconstruction. It is known from the literature that middle-ear prostheses built from different materials show comparable transmission characteristics (e. g. Meister et al,

Figure 5. Middle-ear implants used in the MEM. From left: Bioverit® II, Aerial-gold, Aerial-titanium, Chitosan-hydroxyapatite.

Figure 6. Averaged displacement in μm/Pa of an Aerial-gold with error bars (n � 5); mean SD � 0.34; p � 0.05; r: 0.99; SC � 0.9–1.1 ml.

1998, 1999; Jahnke, 1998; Zahnert, 2003). However it was very important for us, before starting with temporal bone and animal experiments, to know if the developed test material shows comparable acoustical properties to commercial middle-ear prostheses.

The authors of this paper don’t argue that the transmission char-acteristics of the MEM exactly correspond with those from the real ear system. There is no doubt that for example the function of the TM due to its conical shape, inclined position, and fi bre structure can not be exactly simulated through a fl at membrane. Regarding the fi nding in the literature (e.g. Wada et al, 1992; Fay et al, 2006), it will be clear how complex and important the special structure and anatomical properties of the TM are. The same comment can be made for the usage of a soft membrane instead of a hard plate as the footplate. However it must be considered, that the subject of this work was to evaluate the transfer function of various implants in closely realistic conditions, which has been indicated as realizable with our model. With regard to this it can be said that the overall result of the MEM was comparable to temporal bone and FE simu-lations (see below). A further advantage of the MEM was that the measurements are reproducible to a high degree.

The model we designed provides a means of applying a routinely used clinical procedure—i.e. tympanometry— to test the stiffness of the system. The advantage of usage of the tympanometry is that, while assessing this physical parameter (stiffness), a value can be obtained that is highly comparable with those achieved under real conditions. In accordance with this assumption, the static compli-ance levels we observed in our model are comparable with those for normal middle-ear systems. In other words tympanometry doesn’t deliver any additional information about the transmission properties of implants, but it indicated if the various implants had been inserted with the same stiffness in the MEM, which was very important for the comparable measuring of various implants.

A further application fi eld for the MEM would be in active mid-dle-ear implants-research (implantable hearing devices), where the

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affectivity or the transfer characteristics of these devices in nearly realistic situations can be evaluated.

However the MEM has shown some limitations. The main point is that the MEM is not able to simulate an intact middle-ear system with the ossicular chain. Consequently only a reconstructed middle ear can be simulated by the MEM. Furthermore the preparation of the MEM for a measuring set is complicated, e. g. the fi lling of the cylinder ‘E’ with fl uid, the accurate placement of the prosthesis with adequate stiffness, and to consistently pay attention that the mem-brane ‘B’ remains suffi ciently moist.

Infl uence of stiffnessThe transfer function determined for the three different SC values in the ear model confi rm the basic physical principle that increasing the stiffness (i.e. reducing the compliance) of an oscillating system results in improved transmission behaviour at higher frequencies (Lenhardt & Laszig, 2001). Meister et al (1999) reported a similar outcome with their mechanical middle-ear model. Gan et al (2002) reported having achieved—using their three dimensional FEM model—similar results with regard to the infl uence of stiffness on the sound transmission characteristic of the middle-ear system as a whole.

In our model with the aid of tympanometry can be demonstrated how (for example) a stiffened middle ear with a low SC infl uences

the sound transmission. As it is known from clinical practice, ossic-ular stiffening—as observed during the early stages of otosclerosis—leads to the middle-ear resonance shifting to higher frequencies (Shahnaz & Polka, 1997; Lenhardt & Laszig, 2001).

Infl uence of implant massThe results of these measurements indicated that an increase in mass leads to the resonance frequency shifting to lower frequencies. Although, in our investigation, the implants with increased mass showed slightly improved transmission, no alteration could be observed in the fi rst resonance frequency. The most probable reason for this is that altering the implant mass at low frequencies (below 500 Hz) had less impact on sound transmission. According to current knowledge in middle-ear mechanics, mass has a dominant infl uence rather above the resonance frequency of the middle ear, which lies between 600 and 1350 Hz (Colletti, 1977; Shanks et al, 1993; Shah-naz & Polka, 1997; Lenhardt & Laszig, 2001). Zahnert (2003) also showed in the temporal bone model, that increasing the mass of a titanium implant by adding additional masses of up to 40 mg can only cause signifi cant deterioration in transmission characteristics up to 10 dB, above 1000 Hz.

The second resonance frequency recorded in our model, however, suggested a dependency relationship with implant mass: resonance frequency was seen to become lower with increasing mass. Meister

Figure 7. Infl uence of stiffness on sound transmission with inserted Aerial-gold with corresponding tympanograms and different SC of 3.1, 1.2, and 0.4 ml.

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et al (1997) observed the same results with the middle-ear model that they had designed. On the basis of their model calculation, Rosowski & Merchant (1995) suggested that, in order to avoid signifi cantly infl uencing hearing performance, the mass of middle-ear prosthesis should not exceed 16 times the mass of the stapes (2–4 mg). Goode et al (1994) reported that masses of 10–40 mg are opti-mal for middle-ear prostheses. Our fi ndings from the ear model also confi rmed that increasing the implant mass above 50 mg results in signifi cant deterioration at frequencies between 1000 and 4000 Hz, which is very important for speech understanding.

In their temporal bone experiment Gan et al (2001) reported a general damping of the stapes displacement due to increasing the mass of the ossicular chain between 250 and 8000 Hz. They discovered a slight frequency-dependent decreasing of stapes displacement (about 1 to 6 dB change in displacement) above 1000 Hz due to a mass increasing of about 15 mg (from 22.5 to 37 mg), whereas the values under this frequency remained unchanged (Gan et al, 2002). However these results could not be exactly simulated in their fi nite element modelling (FEM). In contrast to temporal bone results the FEM showed for the same mea-

surement a frequency independent linear decreasing of displacement of about 3 dB over all frequencies due to the mass loading of 15 mg (Gan et al, 2002). However the results of the MEM confi rmed the frequency-dependent infl uence of the mass loading on the transfer function from the physical point of view. From their result Gan et al (2001) conclude a critically mass limit above that the transfer function deteriorated remarkably, which also was indicated by the results in the MEM.

Transmission properties of all implants There is not a clear explanation why two resonance areas at about 500 and 1200 Hz with each prosthesis were measured in the MEM. Calcu-lations showed that the natural frequencies of various segments in MEM were much higher than these frequencies. In addition, direct LDV-measurements at the membrane B showed similar resonance areas as measured in the whole system. For this reason, it is assumed that these resonances, with high probability, originated from the membrane B.

Overall, the transmission properties of different implants showed no signifi cant differences over the whole frequency spectrum. Similar

Figure 8. Infl uence of mass on sound transmission in the MEM.

Figure 9. Transmission characteristics of all implants in the MEM; stimulation signal: chirp signal 100 to 10 000 Hz.

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results to our fi ndings were obtained by Meister et al (1999) using their middle-ear model, and by Zahnert (2003) in temporal bone specimens. In our model, however, differences of up to 10 dB were observed in limited frequency ranges, although these are probabily due to varying implant mass as opposed to different implant materials.

Both our fi ndings and those published by other authors support the hypotheses that, in the frequency range relevant to the fi eld of audiology (100 to 10 000 Hz), material-specifi c properties, from the acoustic perspective, play a less prominent role, as long as the phys-ical parameters (i.e. stiffness and the mass of the implants) are com-parable (Meister et al, 1998; Janke, 1998). This is presumably the reason why similar results were obtained for the novel test implant made of Chi-HA as for other implants.

However, it should be considered that, apart from basic transmis-sion characteristics as they can be tested in a mechanical ear model, material properties will in addition infl uence acoustic transmission properties due to histological interactions and reactions with the sur-rounding tissue (Geyer, 1999; Turck et al, 2007). This means that the biocompatibility of implanted material construction is the fi rst pre-condition for optimal middle ear reconstruction.

Comparison with temporal bone experimentsAs discussed above, the objective of the present work was to investigate the sound transmission characteristics of implants in nearly realistic conditions. For this reason, the result obtained using our model is com-pared below with the data gathered in temporal bone experiments.

Numerous publications are available on the transmission character-istics of the middle ear (Asai et al, 1999; Huber et al, 2001; Lord et al, 2001; Hato et al, 2003; Zahnert, 2003; Gan et al, 2004; Nakajima et al, 2005; and own measurement (unpublished data)). In all of these investigations the displacement was measured at the stapes or stapes footplate by means of LDV. In order to make better compatibility all these measurements are converted in μm/Pa. Overall, almost all authors (with the exception of Huber et al, 2001) report a displace-ment of between 10 and 100 nm/Pa up to around 3000 Hz. Figure 10 provides a comparative overview of the fi ndings of some of these studies and the sound transmission characteristic of the MEM using the Aerial-gold prosthesis. Comparison of all relevant data reveals

differences of up to 25 dB (re μm/Pa) in transfer function, these being attributable to different conditions and methods of measurement. Assessment of the results obtained with the MEM revealed a good comparability with data acquired in temporal bone experiments. How-ever, in contrast to MEM, the measurements on temporal bone showed a fl at transmission characteristic up to about 1500 Hz (see Figure 10). However in some cases a slight increase in transmission was seen in the frequency range from 600 to 1500 Hz. This difference can be seen as an essential discrepancy between MEM and the real system.

Comparison with fi nite element simulationFirstly it must be mentioned that we did not want to replace the fi nite element modelling (FEM) with the MEM. With help of the MEM we wanted to achieve some information about the transfer function of middle-ear implants in comparable values with the clinical audiology.

Kelly et al simulated the transmission properties of four different middle-ear prostheses in their FE model, using two PORP and two TORP implants, and observed little difference in transmission proper-ties (Kelly et al, 2003). The comparison of these results with our data reveals similar transmission characteristics. For example, a TORP prosthesis (Kurz Aerial Tubingen) yielded results very similar to those with the Aerial-gold TORP prosthesis employed in our model.

Also, FE simulation of the middle ear by Wada et al (1992), Koike et al (2002), and Gan et al (2002) indicate comparable results with our MEM. In the MEM we measured the transfer function at the end point of the system (membrane F), which is comparable with the round window membrane in the ear system. But, in mentioned FE simulations, displacements are calculated for the stapes footplate, which however showed a good correlation with our results. An argu-ment for this could be that in a fl uid fi eld cylinder (like cylinder E), considerable damping would not be expected.

Conclusion

The general mechanical transmission behaviour of the ear system can be simulated using simple methods. Special new details in the construc-tion of this biomechanical ear model were for example the nearly real-istic geometrical characteristics, the use of biological membranes, and

Figure 10. Sound transmission characteristics of the intact middle ear measured in temporal bone model, and of an inserted Aerial-gold TORP in the MEM.

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of a liquid-fi lled part resembling the inner ear. The use of tympanom-etry to test the stiffness is also presented in this work for fi rst time and proved to be a reliable method. The development of such a model, with realistic biomechanical properties, enhances both the demonstration and understanding of the technical and clinical interrelationships involved. Use of this ear model enabled the infl uence of stiffness and implant mass on transmission properties to be evaluated. Moreover, from the acoustic point of view, signifi cant differences in transmission properties between various middle ear implants could not be observed.

Acknowledgement

This paper was supported by the German Research Foundation (Deutsche Forschungsgemeinschaft) as part of Collaborative Research Centre 599, ‘Biomedical Engineering’. The authors wish to thank the German Research Foundation for making this project possible.

Declaration of interest: The authors report no confl icts of interest. The authors alone are responsible for the content and writing of the paper.

References

Asai M., Huber A.M. & Goode R.L. 1999. Analysis of the best side on the stapes foot plate for ossicular chin reconstruction. Acta Otolaryngol (Stockh), 119, 356–361.

Boehme G. & Welzl-Müller K. 2005. Audiometrie Hörprüfungen im Erwachsenen und Kinderalter. (5th ed.) Bern: Hans Huber Verlag, pp. 25–28 & pp. 149–163.

Fay J.P., Puria S. & Steele C.R. 2006. The discordant eardrum. Proc Natl Acad Sci USA, 103, 19743–8.

Gan R.Z., Dyer R.K., Wood M.W. & Dormer K.J. 2001. Mass loading on the ossicles and middle ear function. Ann Otol Rhinol Laryngol, 110, 478–85.

Gan R.Z., Sun Q., Deyer R.K., Chang K.H. & Dormer K.J. 2002. Three-di-mensional modelling of middle ear biomechanics and its applications. Otology and Neurotology, 23, 271–280.

Gan R. Z., Wood M. W., Dormer K. J. 2004. Human middle ear transfer function measured by double laser interferometry system. Otol Neurotol,25, 423–435.

Geyer G. 1999. Materialien zur Rekonstruktion des Schallleitungsapparates. HNO, 47, 77–91.

Gjuric M. & Schargel S. 1998. Middle ear and mastoid disease. Am J Otol,19 (3), 273–276.

Hato N., Stenfelt S. & Goode R. L. 2003. Three-dimensional stapes foot-plate motion in human temporal bones. Audiol Neurootol, 8, 140–152.

Höland W., Vogel W., Schubert T., Schulze K.J., Carl G. et al. 1990. Structure and properties of Bioverit glass ceramics. In: Heimke G. (ed.) Bioceram-ics 2. Cologne: Deutsche Keramische Gesellschaft, pp. 97–104.

Huber A., Linder T., Ferrazzini M., Schmid S., Diller N. et al. 2001. Intra-operative assessment of stapes movement. Ann Otol Rhinol Laryngol,110, 31–35.

Hüttenbrink K.B. & Hudde H.1994. Untersuchung zur Schalleitung durch das Rekonstruierte Mittelohr mit einem Hydrophon. HNO, 42, 49–57.

Hüttenbrink K.B. 1992. Die Mechanik und Funktion des Mittelohres, Teil 1: Die Ossikelkette und die Mittelohrmuskeln. Laryngol Rhinol Otol,71, 545–551.

Jahnke K. 1998. Zur Biomechanik der rekonstruierten Gehörknöchelchen-kette. HNO, 3, 202–204.

Kelly D.J., Prendergast P.J. & Blayney A.W. 2003. The effect of prosthesis design on vibration of the reconstructed ossicular chain: A comparative fi nite element analysis of four prostheses. Otol Neurotol, 24, 11–19.

Koike T. & Wada H. 2002. Modelling of the human middle ear using the fi nite-element method. J Acoust Soc Am, 111, 1306–17.

Kong L., Cao Y., Gong Y., Zhao N. & Zhang X. 2005. Preparation and char-acterization of nano-hydroxyapatite / chitosan composite scaffolds. J Biomed Mater Res, 75A, 275–282.

Kwok P. & Jacob P. 2006. Mittelohrprothesen in der klinischen Anwendung. Z Audiol, 45, 128–136.

Lenhardt E. & Laszig R. 2001 (ed. 8). Praxis der Audiometrie. Stuttgart, New York: Thieme, pp. 31–39.

Lord R.M., Abel E.W. & Wang Z. 2001. Effects of draining cochlear fl uids on stapes displacement in human middle ear models. J Acoust Soc Am,110, 3132–39.

Meister H., Mickenhagen A., Walger M., Dück M., von Wedel H. et al. 2000. Standardisierte Messung der Schallübertragung verschiedener Mitte-lohrprothesen. HNO, 48, 204–208.

Meister H., Stennert E., Walger M., Klünter H.D., Mickenhagen A. 1997. Ein Messsystem zur Überprüfung des akustomechanischen Übertragungs-verhaltens von Mittelohrimplantaten. HNO, 45, 81–85.

Meister H., Walger M., Mickenhagen A. & Stennert E. 1998. Messung der Schwingungeigenschaften von Mittelohrimplantaten mit einem mecha-nischen Mittelohrmodel (erste Ergebnisse). HNO,46, 241–245.

Meister H., Walger M., Mickenhagen A., von Wedel H. & Stennert E. 1999. Standardized measurements of the sound transmission of middle ear im-plants using a mechanical middle ear model. Eur Arch Otorhinolaryngol,256, 122–127.

Nakajima H.H., Ravicz M.E., Mechant S.N., Peake W.T. & Rosowski J.J. 2005. Experimental ossicular fi xations and the middle ear’s response to sound: Evidence for a fl exible ossicular chain. Hear Res, 204, 60–77.

Rosowski J.J., Merchant S.N. 1995. Mechanicam and acoustic analysis of middle ear reconstruction. Am J Otol, 16, 486–497.

Rust K.R., Singleton G.T., Wilson J. & Antonelli P.J. 1996. Middle ear/mas-toid disease. Am J Otol, 17, 371–374.

Schmerber S., Troussier J., Dumas G., Lavieille J.P. & Nguyen D.Q. 2006. Hearing results with titanium ossicular replacement prosthesises. EurArch Otorhinolaryngol, 263, 347–354.

Thorne M., Salt A.N., DeMott J.E., Henson M.M., Henson O.W. et al. 1999. Chochlear fl uid space dimensions for six species derived from recon-struction of three-dimensional magnetic resonance images. Laryngo-scope, 109, 1661–68.

Turck C., Brandes G., Kreuger I., Behrens P., Mojallal H. et al. 2007. Histo-logical evaluation of novel ossicular chain replacement prostheses: An animal study in rabbits. Acta Oto-Laryngologica, 127, 801–808.

Wada H. & Metoki T. 1992. Analysis of dynamic behaviour of human middle ear using a fi nite-element method. J Acoust Soc Am, 92, 3157–68.

Yost W.A. 2006. Fundamentals of Hearing: An Introduction. (5th ed.) London: Elsevier, pp. 67–81.

Zahnert T. 2003. Laser in der Ohrforschung. Laryngol Rhino Otol, 82 Suppl. 1, 157–18.

Zahnert T. 2005. Gestörten Hörens 2, Chirurgische Verfahren. LaryngolRhino Otol, 84, Suppl. 1, 37–50.

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a biomechanical ear model to evaluate middle-ear reconstruction

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