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Bone conduction in Thiel-embalmed cadaver heads
Hearing Research 306 (2013) 115e122
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Research papers
Bone conduction in Thiel-embalmed cadaver heads Jérémie Guignard a, b, Christof Stieger c, Martin Kompis d, Marco Caversaccio a, d, Andreas Arnold a, d, * a
ARTORG Center, Artificial Hearing Research, University of Bern, Switzerland Graduate School for Cellular and Biomedical Sciences, University of Bern, Switzerland c Department of Otorhinolaryngology, University Hospital Basel, Switzerland d Department of Otorhinolaryngology, Head and Neck Surgery, Inselspital, University of Bern, Freiburgstrasse, 3010 Bern, Switzerland b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 17 April 2013 Received in revised form 30 September 2013 Accepted 3 October 2013 Available online 23 October 2013
Introduction: Sound can reach the inner ear via at least two different pathways: air conduction and bone conduction (BC). BC hearing is used clinically for diagnostic purposes and for BC hearing aids. Research on the motion of the human middle ear in response to BC stimulation is typically conducted using cadaver models. We evaluated middle ear motion of Thiel-embalmed whole-head specimens in terms of linearity, reproducibility, and consistency with the reported middle ear motion of living subjects, fresh cadaveric temporal bones, and whole-heads embalmed with a Non-Thiel solution of salts. Methods: We used laser Doppler vibrometry to measure the displacement of the skull, the umbo, the cochlear promontory, the stapes, and the round window in seven ears from four human whole-head specimens embalmed according to Thiel’s method. The ears were stimulated with a BahaÒ implanted behind the auricle. Results: The Thiel model shows promontory velocity similar to that reported in the literature for wholeheads embalmed with a Non-Thiel solution of salts (0- to 7-dB difference). The Thiel heads’ relative velocity of the stapes with respect to the promontory was similar to that of fresh cadaver temporal bones (0- to 4-dB difference). The velocity of the umbo was comparable in Thiel-embalmed heads and living subjects (0- to 10-dB difference). The skull and all middle ear elements measured responded linearly to different stimulation levels, with an average difference less than 1 dB. The variability of repeated measurements for both short- (2 h; 4 dB) and long-term (4e16 weeks; 6 dB) repetitions in the same ear, and the difference between the two ears of the same donor (approximately 10 dB) were lower than the interindividual difference (up to 25 dB). Conclusion: Thiel-embalmed human whole-head specimens can be used as an alternative model for the study of human middle ear mechanics secondary to BC stimulation. At some frequencies, differences from living subjects must be considered. Ó 2013 Elsevier B.V. All rights reserved.
1. Introduction Two pathways through which the inner ear can be stimulated by sound are air conduction (AC) and bone conduction (BC). BC is the vibratory excitation of the inner ear structures secondary to
Abbreviations: AC, air conduction; BC, bone conduction; LDV, laser Doppler vibrometer; RW, round window; STD, standard deviation; TM, tympanic membrane; ST, stapes; CP, cochlear promontory; SK, skull; RMSD, root-mean-square deviation * Corresponding author. Department of Otorhinolaryngology, Head and Neck Surgery, Inselspital, University of Bern, Freiburgstrasse, 3010 Bern, Switzerland. Tel.: þ41 31 632 83 78; fax: þ41 31 632 49 00. E-mail addresses: [email protected], [email protected] (A. Arnold). 0378-5955/$ e see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.heares.2013.10.002
vibrations of the skull resulting in sound perception. The phenomenon of BC is routinely exploited in audiology to determine whether deafness is related to pathologies in the inner ear or the middle ear. Despite BC’s clinical significance, its fundamental mechanisms are still not entirely understood (Kim et al., 2011). As recently outlined by Röösli et al. (2012), at least four components are involved in BC: (a) the inertia of the ossicles and cochlear fluid (Stenfelt and Goode, 2005a); (b) the generation of sound pressure in the external auditory canal, resulting in AC (Stenfelt et al., 2003); (c) the compression of the bone surrounding the cochlea (Békésy, 1980); and (d) the sound transmission via the cerebrospinal fluid (Sohmer and Freeman, 2004). The therapeutic application of the BC pathway is the use of BC hearing aids to treat conductive or mixed hearing loss. Such hearing
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aids can bypass a middle ear that has been damaged beyond repair and use skull vibrations to excite a functioning inner ear. BC hearing aids are also indicated to treat patients with single-sided deafness (Hol et al., 2004; Pfiffner et al., 2010; Wazen et al., 2010). The increasing use of implantable BC hearing aids calls for intensified study of the BC hearing pathway in order to minimize the size of the device and to improve the patients’ hearing by optimizing the placement and mode of the excitation. The established method for measuring the displacement of middle ear components in response to sound stimulation is laser Doppler vibrometry (LDV) (Vlaming and Feenstra, 1986; Goode et al., 1994; Rosowski et al., 2003, 2008; Stasche et al, 1994; Voss et al., 2000). In live human subjects, LDV measurements are possible at the TM (Goode et al.,1996; Huber et al., 2001a); additional measurements at the middle ear structures can be performed during surgery (Huber et al., 2001b, 2003). It has been shown that stapes velocity is similar in live ears and fresh cadaveric temporal bones, if the measurement’s angle and location are similar (Chien et al., 2009). Studies have reported the use of LDV specifically to characterize BC transmission at the cochlear promontory (Eeg-Olofsson et al., 2008; Stenfelt and Goode, 2005b) and at middle ear structures (Stenfelt et al., 2002). Analogous to AC research, a considerable part of experimental BC research relies on LDV measurements using human cadaveric temporal bones. Measurements of the velocity of the cochlear promontory in response to BC stimulation have been reported in fresh whole cadaver heads (Stenfelt and Goode, 2005b), and appear similar to measurements in live subjects (Håkansson et al., 2008; Stenfelt and Goode, 2005a). The use of human whole-head specimens also implies the presence of the brain and the soft tissues, which may offer an advantage over isolated temporal bone specimens, especially in the study of semicircular canal dehiscence (Chien et al., 2007) or for studies of soft tissue or fluid-conduction pathways. Chien et al. reported limitations of temporal bone preparations to investigate the mechanics of BC hearing loss in SCD and suggested the use of whole-heads, which would better correspond to the clinical situation. The fixation method published by Walter Thiel (Thiel, 1992) is known to preserve the mechanical properties without the hardening and shrinkage of soft tissue, while conserving the specimen for long periods, similarly to formaldehyde. The Thiel method uses a watery solution of salts (e.g. ammonium nitrate, potassium nitrate), ethylene glycol, boric acid and a small amount of formaldehyde. Our group has evaluated and established the use of Thiel heads as a model for measurements of human middle ear motion in response to AC stimulation as an alternative to temporal bones (Stieger et al., 2012). The results were similar to tympanic membrane, stapes and round window motion measurements reported for living subjects and for fresh temporal bones. We have shown that a single head can be used for multiple AC experiments with reproducible results. The variability of repeated measurements in the same head is smaller than the inter-individual variability. Even long-term experiments of 20 h can be performed. The strong disinfectant properties of Thiel solution against bacteria (Staphylococcus aureus, Pseudomonas aerogiosa, Mycobacterium tuberculosum), fungi (aspergillus niger) and viruses (HIV) were tested and reported by Walter Thiel (Thiel, 1992). Therefore, the risk of infection is minimized. The aim of this study is to validate the use of Thiel-embalmed heads for BC experiments using LDV. We compared the displacement of the middle ear components in response to BC between Thiel heads and similar measurements reported in the literature for whole-heads, fresh temporal bones, and live subjects. Furthermore,
we addressed the following points: the linearity of the Thiel heads in response to BC, the reproducibility of the results in the same ear after short (2 h) or long (4e16 weeks) time intervals, and the disparity between the two ears of the same specimen. 2. Methods 2.1. Specimens We conducted measurements on a total of seven normal ears from four human cadaver heads, all embalmed according to Thiel’s method (Thiel, 1992). One ear of one cadaver head was excluded because its eardrum was damaged. The details of the specimens are listed in Table 1. All donors consented to the post-mortem use of their body for science according to the terms of the human body donation program of the Institute of Anatomy, University of Bern, Switzerland. The local ethics committee approved the use of Thielembalmed head specimens for research (KEK-BE 030/08). The specimens were conserved in Thiel solution at a temperature of 5 C before and between experiments. The heads had been fixed with Thiel solution for several months at the time of the experiments; storage time has been shown to have no influence on the quality of measurements (Stieger et al., 2012). 2.2. Preparation of specimens The specimens were removed from the fluid at least 2 h prior to each measurement (Stieger et al., 2012). We used suction to remove fluid and cerumen from the ear canal. We placed a BahaÒ implant with an abutment (Cochlear, Sidney, Australia) in the temporal region according to the surgical protocol provided by the manufacturer. In particular, we followed the manufacturer’s recommendations regarding the distance and angle relative to the ear canal. We placed small pieces of reflecting tape (approximately 0.5 0.5 mm) on the skull (SK) 2 cm anterior to the implant and on the umbo of the tympanic membrane (TM). The use of reflecting tape increases the SNR of the LDV without adding significant mass (Voss et al., 2000). After we measured the velocity of the SK and the TM, a tympanomeatal flap was elevated to expose the cochlear promontory (CP), the stapes footplate (ST), and the round window membrane (RW). We slightly widened the posterior bony ear canal with a surgical drill. We placed reflecting tape on the ST, the CP, and the RW (Fig. 1). The laser beam was aimed at the middle ear structures through the ear canal. The mechanical integrity of the cochlea was assessed by gently pushing the incus with a surgical needle and observing the resulting movement of the RW’s surface. The specimens were placed on a soft, stabilizing ring cushion to attenuate ambient vibrations. 2.3. Test signal generation and signal processing We used a Baha IntensoÒ (Cochlear, Sidney, Australia) sound processor to provide the BC stimulus. The output force of the sound Table 1 List of specimens. Specimen
Age (years)
Sex
Side
1
50
F
2
83
M
3
78
F
4
91
F
Right Left Right Left Right (discarded) Left Right Left
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capitals are used for complex values) was calculated. The means of the magnitudes and their standard deviations (STD) were calculated on a logarithmic scale, whereas phase means and their STDs were calculated in the linear domain. For linearity assessment, the differences D1[f] ¼ H30dBV[f] H20dBV[f] and D2[f] ¼ H40dBV[f] H20dBV[f] were calculated for each f on a logarithmic scale for the magnitude and on a linear scale for the phase. The combined average and STD of D1 and D2 were computed across the measured frequencies for each location of velocity measurement (Table 2). Data points at f ¼ 0.1 and 0.17 kHz were ignored because they often showed an SNR below 6 dB. The root-mean-square deviation (RMSD) in magnitude and phase between repeated measurements, left and right ear of a given specimen, and between left ears of multiple specimens was calculated at each frequency for the SK, CP, ST and RW as follows: Fig. 1. After elevation of a tympanomeatal flap, small pieces of reflective tape were placed at the cochlear promontory (CP) and the stapes footplate (ST). Another piece of reflective tape was placed on the surface of the round window (dashed circle, RW). IN: Incus.
processor, represented with the complex value F hereafter, was measured with a TU 1000 Skull Simulator (Håkansson and Carlsson, 1989). Only the electrical input of the sound processor was used; the built-in microphone was disabled at all times (the processor’s input was set to “E”). The volume was set to the minimum position. The analog generator of an audio analyzer (R&S UPV, Rohde & Schwarz, Germany) was used to generate the electrical stimulation signal. The signal consisted of 37 pure sinus tones; 30 were logarithmically spread between 0.1 and 10 kHz, and an additional seven where standard audiogram frequencies (0.25, 0.5, 0.75, 1, 1.5, 2, 3, and 4 kHz). The velocity of the SK, the TM, the CP, the ST, and the RW was measured with a single-point laser Doppler vibrometer (LDV; HLV1000, Polytec, Waldbronn, Germany). The LDV was attached to the operating microscope, and a joystick-driven mirror (HLV MM2, Polytec, Waldbronn, Germany) was used to aim the laser beam. We used the signal quality indicator of the LDV device and its acoustic output to monitor the signal-to-noise ratio before and during measurements. We used the analog signal processor of the audio analyzer to record the LDV signal. For each individual frequency, the magnitude and the phase were recorded after a settling period of up to 5 s. 2.4. Experiment protocol We measured the velocity of the SK, CP, ST, and RW in all seven ears. The velocity of the TM was measured in six specimens. Each series was repeated twice at 20, 30, and 40 dBVrms to assess linearity. For two of the ears, the measurements were repeated after a latency period of 2 h (short-term reproducibility). For four of the ears, the setup was renewed and measurements were repeated after a storage period of 4e16 weeks (long-term reproducibility). The background noise was measured prior to each series by running the spectral analysis once with the stimulation muted.
rffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi . ffi X 2 n 0 n RMSD½f ¼ ðV½f V ½f Þ where n is the number of measurements, and V and V0 the compared velocities. We compared our results with the following measurements reported in the literature: (1) The magnitude of CP motion measured with LDV in 14 ears of seven Non-Thiel embalmed cadaver heads after the removal of the TM, malleus, and incus. The heads were embalmed with a water solution containing sodium chloride, potassium nitrate, calcium chloride, chloral hydrate, formalin, glycerol, and Lysoformin, which has no specific name and is referred to “a Non-Thiel solution of salts” throughout this paper. Stepped sine tones between 0.1 and 10 kHz were applied using a Baha Classic. LDV measurements were performed with reflective glass spheres (Eeg-Olofsson et al., 2008). We compared the averages and the overall range of the measurements. (2) The ST-to-CP velocity ratio measured with LDV in 26 fresh temporal bones in response to stimulation with a minishaker. The stimulus was a swept sine signal with a logarithmically spaced resolution of 50 frequencies per decade. Frequencies between 0.1 and 10 kHz were reported. LDV measurements were performed with reflective micro spheres (Stenfelt et al., 2002). We compared the averages and the overall range of the measurements. (3) the TM velocity in response to BC stimulation with a Radioear B71 measured with LDV in 20 healthy ears of human subjects. Nine tones between 0.3 and 6 kHz were applied. LDV measurements were performed at the umbo (Röösli et al., 2012). Statistical analysis (ManneWhitney test) was possible for the TM data of Röösli et al., where STD was known. 3. Results 3.1. Linearity
2.5. Data analysis For each situation, a trace file containing a total of 222 (37 frequencies 3 stimulation levels 2 repetitions) points was saved for both the magnitude and phase. We used GraphPad Prism 5 (GraphPad Software, La Jolla, CA, USA) for data visualization and analysis. Data points at frequencies above 7.2 kHz were ignored as they are outside the Baha’s working frequency range. The transfer function H ¼ V/F (velocity V divided by the stimulation force F; bold
A typical velocity measurement series is plotted in Fig. 2. The signal-to-noise ratio was higher than 6 dB in frequencies above 0.2 kHz, which was the case for all locations of velocity measurements. The force-to-input voltage transfer function of the Baha device itself shows nonlinearity for stimulation smaller than 30 dBVrms (Fig. 2A). A similar difference in amplitude of the CP velocity can be seen (Fig. 2B). The constancy of the velocity-toforce ratio with varied stimulus level (Fig. 2C) suggests the
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Table 2 Difference between stimulation at 30 or 40 dBVrms and stimulation at 20 dBVrms. SK
CP
ST
RW
Phase (cycles)
Mag. (dB)
Phase (cycles)
Mag. (dB)
Phase (cycles)
Mag. (dB)
Phase (cycles)
Mag. (dB)
Phase (cycles)
0.43 1.75
<0.01 0.13
0.21 2.52
0.02 0.19
0.59 2.06
0.01 0.13
0.24 1.97
<0.01 0.16
0.72 4.62
<0.01 0.21
0 -20 -40 -60 1000
CP motion @ -20, -30 and -40 dBV stim. -20
-40
-60
-80 100
10000
C
1000
CP motion @ 1N 10 0 -10 -20
10000
100
0.5
0.5
0.3
0.0
0.0
0.2
-0.5 -1.0 -1.5 -2.0 100
1000
frequency (Hz)
10000
phase (cycles)
phase (cycles)
-80 100
B
magnitude (dBmm/s/N)
BAHA mechanical output @ -20, -30 and -40dBV stim.
magnitude (dB mm/s)
magnitude (dB N)
A
phase (cycles)
jAVGj STD
TM
Mag. (dB)
-0.5 -1.0 -1.5
1000
10000
1000
10000
0.1 0.0 -0.1 -0.2
-2.0 100
1000
10000
-0.3 100
frequency (Hz)
frequency (Hz)
Fig. 2. One representative cochlear promontory (CP) velocity measurement. (A) Force output of the Baha measured with a skull simulator. (B) Corresponding raw CP velocity (2 repetitions; the grey area shows the typical background noise). (C) Average ratio with the standard deviation range (thick black line and dashed lines, respectively). The lower graphs show the corresponding phases.
nonlinearity in 2A and 2B comes from the stimulator and not the temporal bone. When considering all seven specimens, the average difference between the transfer function V/F at 30 and 40 dBVrms stimulation compared with 20 dBVrms stimulation is reported in Table 2. The average D value was near zero dB in magnitude and zero cycle in phase for all locations of velocity measurement, and the STD was typically between 1 and 2.5 dB in magnitude, with the exception of the RW, for which it was of 4.6 dB. The STD in phase was between 0.13 and 0.21 cycles for all locations of velocity measurement. 3.2. Velocity-to-force ratio The transfer function V/F for the SK, TM, CP, ST, and RW for all seven ears are depicted in Fig. 3. For low frequencies, on average, the magnitude decreased with a slope of approximately 20 dB/ decade to a minimum for all of the locations of velocity measurement. The minimal magnitude was observed near 0.5 kHz for the SK and near 1 kHz for the CP, ST, and RW. The velocity of the TM showed a local minimum in the 0.5- to 1-kHz range and decreased again in the higher frequencies. The individual variability is high for frequencies above the local minimum. Two of the TM measurements showed a different behavior than the others; in those cases, the velocity was 10e15 dB higher above 0.25 kHz. The phase at the SK was near zero on average for all frequencies. For the other locations of velocity measurement, it was near zero up to the frequency of minimal magnitude. In the higher frequencies, on average, the phase decreased to 0.2 cycles.
3.3. Reproducibility The short-term, long-term reproducibility, the difference between the two ears of the same specimen, and the difference between specimens are plotted in Fig. 4. For short-term reproducibility (Fig. 4A), the RMSD in magnitude for the SK and the CP was below 3 dB, and it was below 5 dB for the ST and the RW, one spike above 5 dB is seen for the ST. For all locations of velocity measurement, the RMSD in phase was smaller than 0.1 for frequencies above 0.2 kHz, and spikes up to 0.5 cycles were observed between 1.5 and 4 kHz for the ST and RW. For the long-term reproducibility (Fig. 4B), the RMSD was in the range of 3e5 dB for the SK and CP ans 4e7 dB for the ST and RW. The RMSD in phase was below 0.1 cycles for the SK and CP, with spikes up to 0.2 and 0.6 cycles, respectively, at higher frequencies. The RMSD in magnitude between the two ears of the same specimen (Fig. 4C) was 3e10 dB for the ST and the CP, and 3e12 dB for the SK and the RW. The RMSD in phase was up to 0.6 cycles, especially at high frequencies. The RMSD in magnitude between left ears of the different specimens (Fig. 4D) was between 5 and 12 dB with spikes up to 15 dB and the RMSD in phase showed spikes up to 0.8 cycles at higher frequencies. 3.4. Comparison with other models The average magnitude of the velocity at the CP measured in the Thiel heads and in heads embalmed with a Non-Thiel solution of salts as reported by (Eeg-Olofsson et al., 2008) is depicted in Fig. 5A. The means were similar in shape. In the Thiel heads, the average
J. Guignard et al. / Hearing Research 306 (2013) 115e122
phase (cycles)
magnitude (dB mm/s/N)
A
B
SK
C
TM
D
CP
119
E
ST
RW
20
20
20
20
20
10
10
10
10
10
0
0
0
0
0
-10
-10
-10
-10
-10
-20
-20
-20
-20
-20
-30
-30
-30
-30
-30
-40 100
-40 100
-40 100
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10000
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10000
-40 100
0.4
0.4
0.4
0.4
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0.2
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0.2
0.2
-0.0
-0.0
-0.0
-0.0
-0.0
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-0.2
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-0.4
100
1000 10000 frequency (Hz)
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1000 10000 frequency (Hz)
100
1000 10000 frequency (Hz)
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1000 10000 frequency (Hz)
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1000 10000 frequency (Hz)
Fig. 3. The velocity with respect to force (V/F). The skull (SK; A), umbo of TM (TM; B; dashed: outliers), cochlear promontory (CP; C), stapes footplate (ST; D) and round window (RW; E) are represented in magnitude and phase. Thin black lines show individual specimens; the thick black line is the average of all measurements.
0 100
1000
10000
SK CP ST RW
10
0.6
0.4
1000
1000 frequency (Hz)
10000
15
10
0.8
0.6
0.4
0.0 100
SK CP ST RW
1000
1000
10000
1000 10 100 00 00
10000 10 100 00
1000
10000
1.0 SK CP ST RW
0.8
0.6
0.4
0.0 100
10
0 100 00
10000
SK CP ST RW
0.6
0.4
0.2
0.2
frequency (Hz)
SK CP ST RW
5
1.0 SK CP ST RW
inter-specimen variation 20
0 100
10000
0.2
0.2
D
5
0 100
0.8 RMSD (cycles)
RMSD (cycles)
15
1.0
1.0
0.0 100
SK CP ST RW
5
5
left-to-right reproducibility 20
RMSD (dB mm/s)
15
10
0.8
C
RMSD (dB mm/s)
SK CP ST RW
RMSD (dB mm/s)
RMSD (dB mm/s)
15
long-term reproducibility 20
RMSD (cycles)
B
short-term reproducibility 20
RMSD (cycles)
A
1000 frequency (Hz)
10000
0.0 100
frequency (Hz)
Fig. 4. Short-term (2 h, A; n ¼ 2 ears), long-term (4 weeks or more; B; n ¼ 4 ears), and left-to-right (C; n ¼ 3 specimens) reproducibility of the measurements, as well as interspecimen variations (D) for the skull (solid black; SK), cochlear promontory (dashed grey; CP), stapes (grey; ST) and round window (light grey; RW). Dashed grey: overall range. Top: RMSD in magnitude; bottom: RMSD in phase.
velocity was 6e7 dB higher up to 0.7 kHz. Above 0.4 kHz, the average reported by Eeg-Olofsson was within the range of the Thiel measurements. The averages overlap at 0.9, 1, 2, and 5 kHz. Fig. 5B shows the ST-to-CP velocity ratio in Thiel heads and in fresh temporal bones. The ST-to-CP ratio is near zero dB up to 0.5 kHz in both our measurements and in the literature. In both temporal bones and Thiel heads, the velocity of the ST was slightly higher (2e5 dB) than the velocity of the CP between 0.5 and 2 kHz and lower (to a maximum 3 dB) for higher frequencies. The range measured in
Thiel heads and the one reported in temporal bones overlap for most frequencies. The velocity of the TM in four of the Thiel heads’ ears (two were omitted for the comparison with live ears because of their noncharacteristic behavior, although the TM were normal on visual inspection) is represented in Fig. 5C along with the TM velocity in living subjects. The means were not significantly different at 0.7, and 3 kHz (p > 0.05). The means were significantly different at 1 and 4 kHz (p < 0.05) although they appear rather similar on the
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A
VST/VCP
B
CP motion
C
TM motion
15 Thiel (greyed area: STD)
Thiel (greyed area: range)
Non-Thiel embalmed heads (Eeg-Olofsson et al. 2008, dashed lines: range)
temp. bones (Stenfelt et al., 2002; thin dashed lines: range)
10
0
-20
20
VTM (dB mm/s/N)
Thiel (greyed area: range)
dB ref CP
VCP (dB mm/s/N)
20
5
0
living subjects (Röösli et al. 2012; thin dashed lines: STD)
0
-20
-5
-40 100
-40
1000 frequency (Hz)
10000
-10 100
1000 frequency (Hz)
10000
100
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10000
frequency (Hz)
Fig. 5. The Thiel-embalmed head model compared with other reported models. (A) Average velocity of the cochlear promontory (CP) in seven ears of Thiel heads (solid line; grey area: range) and in 14 ears of heads embalmed with a Non-Thiel solution of salts (dashed line; thin dashed lines: range). (B) Average stapes (ST)-to-CP velocity ratio (VST/VCP) in seven ears of Thiel heads (solid line; grey area: range) and in 26 fresh temporal bones (dashed line; thin dashed lines: range). (C) Average velocity of the umbo of the tympanic membrane (TM) in four ears of Thiel heads (solid line; grey area: STD) and in 20 ears from living subjects (dashed line; thin dashed lines: STD).
graph. For frequencies below the minimum in magnitude (0.3; 0.5 kHz) and at 2 kHz, the average was significantly lower in living subjects than in Thiel heads (p < 0.05). 4. Discussion Our results suggest that whole head preparation according to Thiel’s method may be useful and valid for studies of the ossicular and RW biomechanics in response to sound transmitted through BC. Our results from Thiel-embalmed heads are comparable with reported data in cadaver heads embalmed with a Non-Thiel solution of salts (average velocity of the CP), fresh cadaver temporal bones (average ST-to-CP velocity ratio), and living human (average velocity of the umbo of the TM). Additionally, the Thiel-embalmed head model is linear in the range of clinically relevant stimulation levels. The short-term, longterm, and left-to-right variability is lower than the inter-individual variability. 4.1. Linearity The force-to-input voltage transfer function of the Baha device itself is not amplitude-independent for stimulation smaller than 30 dBVrms (Fig. 2A). We chose to use a Baha implant to reproduce the clinical situation as closely as possible. The reported equivalent hearing level with a Baha Intenso stimulated at 20 dBVrms is between 40 and 70 dB HL in the 0.25e4 kHz range (Arnold et al., 2010). We can estimate that our stimulation steps roughly covered a clinically relevant range from 20 dB HL to 70 dB HL. The transfer function V/F, however, is independent from the stimulation force (Fig. 2C). For all locations of velocity measurement, the average difference between the stimulation at 30 and 40 dBVrms and at 20 dBVrms of the LDV measurement is near zero for magnitude and phase (Table 2), indicating a linear behavior for the explored amplitude and frequency range. The linearity of BC transmission through the SK in vivo was previously reported in one living subject (Hakansson et al., 1996). This work shows that BC transmission is linear for all the measured components of the middle ear, and for the cochlear fluid estimated by RW movement (Table 2). The small STD of the repeated measures shows the relatively high immediate reproducibility. The exception is for the RW where
the STD is twice as high as for the other middle ear structures measured. High variability of RW motion is likely due to different compliance of the RW across ears and different patterns of RW motion especially at higher frequencies. The influence of the surgical angle and aiming location on LDV signal at the RW has been previously discussed (Stenfelt et al., 2004; Stieger et al., 2012). Another explanation may be an alteration of the RW response caused by accumulation of fluid, although we cleared the niche every time before RW measurements. 4.2. Velocity-to-force ratio Our current understanding of BC transmission includes: (1) the combined effect of the relative motion of the ossicles and the cochlear fluid resulting from inertia; (2) the generation of sound pressure in the EAC resulting in AC; (3) the compression of the petrous bone; and (4) sound transmission via the cerebrospinal fluid. We measured the TM velocity in response to BC sound, which we assume to be an expression of the ossicles and cochlear fluid inertia, and the sound pressure generated in the EAC (effects 1 and 2 above). Opening the TM results in a large reduction in the transmission of ear canal sound pressures generated by both vibration of the head, and airborne sound generated by the BAHA stimulator. Thus, consecutive measurements reflect the petrous bone compression, the sound transmission through the cerebral fluid, and a residual ossicular inertia effect. Two TM measurements showed nontypical shape with higher magnitudes. As the reflective tape placed at the umbo may have had firm contact to the pars tensa of the TM at its edges, measurements may have captured components of complex motion not identical to that of the umbo (Cheng et al., 2010). Furthermore, the displacement pattern of the TM is reported to be more complex at higher frequencies, which may explain the higher variability of TM measurements above the local minimum observed in our study. The TM measurements were always performed before the outer ear canal was widened. Therefore inter-individual differences in outer ear canal shape may have caused variations in laser beam angle. Differences in velocity measurements have been explained by different LDV setups, particularly the angle of the laser beam (Huber et al., 2001b; Chien et al., 2006). The other points measured showed similar responses in all specimens.
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We observed the two modes of transmission of motion reported in the literature (McKnight et al., 2013; Stenfelt and Goode, 2005b); (1) whole-body motion in the lower frequencies showing a magnitude decrease of approximately 20 dB/decade and a phase of zero up to a local minimum in magnitude, and (2) wave transmission at the higher frequencies. The vibration is transmitted with little delay up to the frequency of minimum magnitude. The phases more significantly differ from zero in the higher frequencies. Generally, the inter-individual variation is higher at the frequencies above the minimum in magnitude; within that range, the model behaves in a more complex manner (Stenfelt et al., 2002). 4.3. Reproducibility A single head can be used for multiple experiments with reproducible results. The variability of repeated measurements for both short- (4 dB) and long-term (6 dB) repetitions in the same ear is less than the inter-individual variability (up to 25 dB; Fig. 3). The difference between the two ears of the same donor (approximately 10 dB; Fig. 4C) is lower than the inter-individual difference (up to 25 dB; Fig. 3), but it is higher than repeated measurements in the same ear. The short- and long-term stability of the measurements is a practical advantage for experimentation, as it allows a long series of manipulations while maintaining consistent behavior. 4.4. Comparison with other models Ideally, comparison would have been performed with data of specimens, which are measured fresh, subsequently embalmed according to Thiel and re-measured when conserved. This was not possible because local protocol requires embalming to be initiated as soon as the corpse reaches the institute of anatomy, which is typically within 24 h post-mortem (Stieger et al., 2012) Moreover, changing specimen properties with e.g. drilling to open the middle ear prior to embalming adds another factor to be considered when comparing the measurements. We compared our results with similar measurements reported in the literature. The comparison was possible, but the available data is sparse. To our knowledge, the body of literature contains no other reports of the velocity of all the consecutive points of the middle ear transmission chain secondary to BC stimulation. Middle ear LDV measurements in living subjects with BC stimulation are rare, and no mention of the intraoperative measurement of intratympanic structures excited by BC was found in the literature. The motion of the temporal bone that we measure in Thiel heads is similar to that measured in Non-Thiel embalmed cadaver heads (Fig. 5A) by Eeg-Olofsson (Eeg-Olofsson et al., 2008). We did not expect a dramatic difference because the components of the NonThiel-embalming solution described by Eeg-Olofsson et al. are, to some extent, also used in the Thiel solution (potassium nitrate and sodium salts, alcohols such as ethylene glycol or glycerol, formaldehyde). The averages and range are similar above the minimum in magnitude for the two models. In the lower frequencies, the velocity of the CP was higher in our specimens. This result may be explained by different tissue properties resulting from the embalming technique. It has been reported that when bone is in contact with formalin, a chemical reaction of aldehydes with primary amine groups of the collagen chains leads to an increased number of inter- and intrafibrillar cross-links. In consequence, an alteration of the mechanical properties of the bone is to be expected (Unger et al., 2010; Currey et al., 1995). We assume that a different concentration of formaldehyde between the Thiel- and Non-Thiel-embalming solutions (although the concentration of formaldehyde in the Non-Thiel solution is unknown) might lead to a difference in bone stiffness.
121
There are differences between our setup and the one reported in Stenfelt et al. (2002) because we stimulated a whole head with a Baha device whereas Stenfelt stimulated a temporal bone with a shaker; however, the comparison of the magnitude of relative motion (ratio VST/VCP) is possible. We observed that the VST/VCP in Thiel heads and fresh temporal bones are comparable (Fig. 5B). The average curves show a similar shape and the ranges overlap to a large extent. An explanation for the high variability of ST/CP above 1 kHz may be the complex motion of the stapes at high frequencies caused by an increase in anterior-posterior rocking motion (Heiland et al., 1999; Hato et al., 2003). In addition the variability may be caused by multidirectional movement of the CP reported at higher frequencies (Stenfelt and Goode, 2005a). Furthermore, we observed a similar magnitude and phase for the motion of the RW and the ST (Fig. 3D and E), an observation that Stenfelt et al. (2004) also reported for temporal bone. On average, the shape and the magnitude level of the TM velocity were similar in our experiments and those conducted on living humans. It is known that stapes fixation has minimal effect on umbo velocity (during AC) because of the compliance of the ossicular joints (Jakob et al., 2009). That live and Thiel measurements are similar points to interpretation that generally Thiel has little effect on umbo velocity because it has little effect on the ossicular joint and TM material properties. The amplitudes for low frequencies and the frequency of minimal magnitude were higher in Thiel heads. We assume that this is a combined effect of inter-individual variation in the frequency of minimal magnitude and the attenuation of the skin in the case of transcutaneous stimulation with the B71 as opposed to the direct mechanical coupling of the Baha. Additionally, the velocity in Thiel heads was significantly higher at 2 kHz. We have two hypotheses to explain this observation. First, the Baha generates a higher audible sound pressure than the B71 at 2 kHz. The velocity of the TM could be higher in response to this sound emission. Second, the active behavior of the transmission chain, known as the acoustic reflex (Reger, 1960; Smith, 1943), is absent in post-mortem specimens. Another difference between the Röösli experiment and our study is that the external ear canal was open in our setup. The statistical significance of the mean differences must be interpreted with caution because of the difference in sample size in both datasets. Our setup constrains the measurement of the CP velocity in roughly the same plane as the stimulation. It is known that the cochlea moves in multiple directions in response to BC stimulation. Components perpendicular to the axis of the laser beam are not visible; therefore, a portion of the signal energy is not present in the measurements.
4.5. Conclusion Anatomical human whole-head preparations embalmed according to Thiel’s method are an alternative model for the study of human middle ear mechanics secondary to BC stimulation. Thiel heads have the advantage of long-term preservation, which allows these specimens to be used for multiple experiments over long periods of time with reproducible results. At some frequencies, differences from living subjects must be considered.
Disclosure The authors declare that there are no conflicts of interest.
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Acknowledgments We would like to thank John Rosowski and David Chhan for sharing their umbo velocity data. This work was partly funded by a grant from the Swiss Commission for Technology and Innovation (CTI 12593). References Arnold, A., Kompis, M., Candreia, C., Pfiffner, F., Häusler, R., Stieger, C., 2010. The floating mass transducer at the round window: direct transmission or bone conduction? Hear. Res. 263, 120e127. Békésy, G.V., 1980. Experiments in Hearing. Krieger, Huntington. Cheng, J.T., Aarnisalo, A.A., Harrington, E., Hernandez-Montes, Mdel S., Furlong, C., Merchant, S.N., Rosowski, J.J., 2010. Motion of the surface of the human tympanic membrane measured with stroboscopic holography. Hear. Res. 263, 66e 77. Currey, J.D., Brear, K., Zioupos, P., Reilly, G.C., 1995. Effect of formaldehyde fixation on some mechanical properties of bovine bone. Biomaterials 16, 1267e1271. Chien, W., Ravicz, M.E., Merchant, S.N., et al., 2006. The effect of methodological differences in the measurement of stapes motion in live and cadaver ears. Audiol. Neurootol. 11, 183e197. Chien, W., Ravicz, M.E., Rosowski, J.J., Merchant, S.N., 2007. Measurements of human middle- and inner-ear mechanics with dehiscence of the superior semicircular canal. Otol. Neurotol. 28, 250e257. Chien, W., Rosowski, J.J., Ravicz, M.E., Rauch, S.D., Smullen, J., Merchant, S.N., 2009. Measurements of stapes velocity in live human ears. Hear. Res. 249, 54e61. Eeg-Olofsson, M., Stenfelt, S., Tjellström, A., Granström, G., 2008. Transmission of bone-conducted sound in the human skull measured by cochlear vibrations. Int. J. Audiol. 47, 761e769. Goode, R.L., Killion, M., Nakamura, K., Nishihara, S., 1994. New knowledge about the function of the human middle ear: development of an improved analog model. Am. J. Otol. 15, 145e154. Goode, R.L., Ball, G., Nishihara, S., Nakamura, K., 1996. Laser Doppler vibrometer (LDV) e a new clinical tool for the otologist. Am. J. Otol. 17, 813e822. Håkansson, B., Carlsson, P., 1989. Skull simulator for direct bone conduction hearing devices. Scand. Audiol. 18, 91e98. Hakansson, B., Carlsson, P., Brandt, A., Stenfelt, S., 1996. Linearity of sound transmission through the human skull in vivo. J. Acoust. Soc. Am. 99, 2239e2243. Håkansson, B., Eeg-Olofsson, M., Reinfeldt, S., Stenfelt, S., Granström, G., 2008. Percutaneous versus transcutaneous bone conduction implant system: a feasibility study on a cadaver head. Otol. Neurotol. 29, 1132e1139. Hato, N., Stenfelt, S., Goode, R.L., 2003. Three-dimensional stapes footplate motion in human temporal bones. Audiol. Neurootol. 8, 140e152. Heiland, K.E., Goode, R.L., Asai, M., Huber, A.M., 1999. A human temporal bone study of stapes footplate movement. Am. J. Otol. 20, 81e86. Hol, M.K.S., Bosman, A.J., Snik, A.F.M., Mylanus, E.A.M., Cremers, C.W.R.J., 2004. Bone-anchored hearing aid in unilateral inner ear deafness: a study of 20 patients. Audiol. Neurootol. 9, 274e281. Huber, A., Schwab, C., Linder, T., Stoeckli, S., Ferrazzinin, M., Dillier, N., Fisch, U., 2001a. Evaluation of eardrum laser doppler interferometry as a diagnostic tool. Laryngoscope 111, 501e507. Huber, A., Linder, T., Ferrazzini, M., Schmid, S., Dillier, N., Stoeckli, S., Fisch, U., 2001b. Intraoperative assessment of stapes movement. Ann. Otol. Rhinol. Laryngol. 110, 31e35.
Huber, A., Koike, T., Wada, H., Nandapalan, V., Fisch, U., 2003. Fixation of the anterior mallear ligament: diagnosis and consequences for hearing results in stapes surgery. Ann. Otol. Rhinol. Laryngol. 112, 348e355. Jakob, A., Bornitz, M., Kuhlisch, E., Zahnert, T., 2009. New aspects in the clinical diagnosis of otosclerosis using laser Doppler vibrometry. Otol. Neurotol. 30, 1049e1057. Kim, N., Homma, K., Puria, S., 2011. Inertial bone conduction: symmetric and antisymmetric components. J. Assoc. Res. Otol. 12, 261e279. McKnight, C.L., Doman, D.A., Brown, J.A., Bance, M., Adamson, R.B.A., 2013. Direct measurement of the wavelength of sound waves in the human skull. J. Acoust. Soc. Am. 133, 136e145. Pfiffner, F., Kompis, M., Flynn, M., Åsnes, K., Arnold, A., Stieger, C., 2010. Benefits of low-frequency attenuation of BahaÒ in single-sided sensorineural deafness. Ear Hear. 32, 40e45. Reger, S.N., 1960. Effect of middle ear muscle action on certain psycho-physical measurements. Ann. Otol. Rhinol. Laryngol. 69, 1179e1198. Röösli, C., Chhan, D., Halpin, C., Rosowski, J.J., 2012. Comparison of umbo velocity in air- and bone-conduction. Hear. Res. 290, 83e90. Rosowski, J.J., Nakajima, H.H., Merchant, S.N., 2008. Clinical utility of laser-Doppler vibrometer measurements in live normal and pathologic human ears. Ear Hear. 29, 3e19. Rosowski, J.J., Mehta, R.P., Merchant, S.N., 2003. Diagnostic utility of laser-Doppler vibrometry in conductive hearing loss with normal tympanic membrane. Otol. Neurotol. 24, 165e175. Smith, H., 1943. Audiometric effects of voluntary contraction of the tensor tympani muscles. Arch. Otolaryngol. 38, 369e372. Sohmer, H., Freeman, S., 2004. Further evidence for a fluid pathway during bone conduction auditory stimulation. Hear. Res. 193, 105e110. Stasche, N., Foth, H.-J., Hörmann, K., Baker, A., Huthoff, C., 1994. Middle ear transmission disorders e tympanic membrane vibration analysis by laser-dopplervibrometry. Acta Otolaryngol. 114, 59e63. Stenfelt, S., Goode, R.L., 2005a. Transmission properties of bone conducted sound: measurements in cadaver heads. J. Acoust. Soc. Am. 118, 2373e2391. Stenfelt, S., Goode, R.L., 2005b. Bone-conducted sound: physiological and clinical aspects. Otol. Neurotol. 26, 1245e1261. Stenfelt, S., Hato, N., Goode, R.L., 2002. Factors contributing to bone conduction: the middle ear. J. Acoust. Soc. Am. 111, 947e959. Stenfelt, S., Hato, N., Goode, R.L., 2004. Round window membrane motion with air conduction and bone conduction stimulation. Hear. Res. 198, 10e24. Stenfelt, S., Wild, T., Hato, N., Goode, R.L., 2003. Factors contributing to bone conduction: the outer ear. J. Acoust. Soc. Am. 113, 902e913. Stieger, C., Candreia, C., Kompis, M., Herrmann, G., Pfiffner, F., Widmer, D., Arnold, A., 2012. Laser Doppler vibrometric assessment of middle ear motion in thielembalmed heads. Otol. Neurotol. 33, 311e318. Thiel, W., 1992. [The preservation of the whole corpse with natural color]. Ann. Anat. 174, 185e195. Unger, S., Blauth, M., Schmoelz, W., 2010. Effects of three different preservation methods on the mechanical properties of human and bovine cortical bone. Bone 47, 1048e1053. Vlaming, M.S.M.G., Feenstra, L., 1986. Studies of the mechanics of the normal human middle ear. Clin. Otolaryngol. 11, 353e363. Voss, S.E., Rosowski, J.J., Merchant, S.N., Peake, W.T., 2000. Acoustic responses of the human middle ear. Hear. Res. 150, 43e69. Wazen, J.J., Ess, M.J.V., Alameda, J., Ortega, C., Modisett, M., Pinsky, K., 2010. The Baha system in patients with single-sided deafness and contralateral hearing loss. Otolaryngol. Head Neck Surg. 142, 554e559.
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Research papers
Bone conduction in Thiel-embalmed cadaver heads Jérémie Guignard a, b, Christof Stieger c, Martin Kompis d, Marco Caversaccio a, d, Andreas Arnold a, d, * a
ARTORG Center, Artificial Hearing Research, University of Bern, Switzerland Graduate School for Cellular and Biomedical Sciences, University of Bern, Switzerland c Department of Otorhinolaryngology, University Hospital Basel, Switzerland d Department of Otorhinolaryngology, Head and Neck Surgery, Inselspital, University of Bern, Freiburgstrasse, 3010 Bern, Switzerland b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 17 April 2013 Received in revised form 30 September 2013 Accepted 3 October 2013 Available online 23 October 2013
Introduction: Sound can reach the inner ear via at least two different pathways: air conduction and bone conduction (BC). BC hearing is used clinically for diagnostic purposes and for BC hearing aids. Research on the motion of the human middle ear in response to BC stimulation is typically conducted using cadaver models. We evaluated middle ear motion of Thiel-embalmed whole-head specimens in terms of linearity, reproducibility, and consistency with the reported middle ear motion of living subjects, fresh cadaveric temporal bones, and whole-heads embalmed with a Non-Thiel solution of salts. Methods: We used laser Doppler vibrometry to measure the displacement of the skull, the umbo, the cochlear promontory, the stapes, and the round window in seven ears from four human whole-head specimens embalmed according to Thiel’s method. The ears were stimulated with a BahaÒ implanted behind the auricle. Results: The Thiel model shows promontory velocity similar to that reported in the literature for wholeheads embalmed with a Non-Thiel solution of salts (0- to 7-dB difference). The Thiel heads’ relative velocity of the stapes with respect to the promontory was similar to that of fresh cadaver temporal bones (0- to 4-dB difference). The velocity of the umbo was comparable in Thiel-embalmed heads and living subjects (0- to 10-dB difference). The skull and all middle ear elements measured responded linearly to different stimulation levels, with an average difference less than 1 dB. The variability of repeated measurements for both short- (2 h; 4 dB) and long-term (4e16 weeks; 6 dB) repetitions in the same ear, and the difference between the two ears of the same donor (approximately 10 dB) were lower than the interindividual difference (up to 25 dB). Conclusion: Thiel-embalmed human whole-head specimens can be used as an alternative model for the study of human middle ear mechanics secondary to BC stimulation. At some frequencies, differences from living subjects must be considered. Ó 2013 Elsevier B.V. All rights reserved.
1. Introduction Two pathways through which the inner ear can be stimulated by sound are air conduction (AC) and bone conduction (BC). BC is the vibratory excitation of the inner ear structures secondary to
Abbreviations: AC, air conduction; BC, bone conduction; LDV, laser Doppler vibrometer; RW, round window; STD, standard deviation; TM, tympanic membrane; ST, stapes; CP, cochlear promontory; SK, skull; RMSD, root-mean-square deviation * Corresponding author. Department of Otorhinolaryngology, Head and Neck Surgery, Inselspital, University of Bern, Freiburgstrasse, 3010 Bern, Switzerland. Tel.: þ41 31 632 83 78; fax: þ41 31 632 49 00. E-mail addresses: [email protected], [email protected] (A. Arnold). 0378-5955/$ e see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.heares.2013.10.002
vibrations of the skull resulting in sound perception. The phenomenon of BC is routinely exploited in audiology to determine whether deafness is related to pathologies in the inner ear or the middle ear. Despite BC’s clinical significance, its fundamental mechanisms are still not entirely understood (Kim et al., 2011). As recently outlined by Röösli et al. (2012), at least four components are involved in BC: (a) the inertia of the ossicles and cochlear fluid (Stenfelt and Goode, 2005a); (b) the generation of sound pressure in the external auditory canal, resulting in AC (Stenfelt et al., 2003); (c) the compression of the bone surrounding the cochlea (Békésy, 1980); and (d) the sound transmission via the cerebrospinal fluid (Sohmer and Freeman, 2004). The therapeutic application of the BC pathway is the use of BC hearing aids to treat conductive or mixed hearing loss. Such hearing
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aids can bypass a middle ear that has been damaged beyond repair and use skull vibrations to excite a functioning inner ear. BC hearing aids are also indicated to treat patients with single-sided deafness (Hol et al., 2004; Pfiffner et al., 2010; Wazen et al., 2010). The increasing use of implantable BC hearing aids calls for intensified study of the BC hearing pathway in order to minimize the size of the device and to improve the patients’ hearing by optimizing the placement and mode of the excitation. The established method for measuring the displacement of middle ear components in response to sound stimulation is laser Doppler vibrometry (LDV) (Vlaming and Feenstra, 1986; Goode et al., 1994; Rosowski et al., 2003, 2008; Stasche et al, 1994; Voss et al., 2000). In live human subjects, LDV measurements are possible at the TM (Goode et al.,1996; Huber et al., 2001a); additional measurements at the middle ear structures can be performed during surgery (Huber et al., 2001b, 2003). It has been shown that stapes velocity is similar in live ears and fresh cadaveric temporal bones, if the measurement’s angle and location are similar (Chien et al., 2009). Studies have reported the use of LDV specifically to characterize BC transmission at the cochlear promontory (Eeg-Olofsson et al., 2008; Stenfelt and Goode, 2005b) and at middle ear structures (Stenfelt et al., 2002). Analogous to AC research, a considerable part of experimental BC research relies on LDV measurements using human cadaveric temporal bones. Measurements of the velocity of the cochlear promontory in response to BC stimulation have been reported in fresh whole cadaver heads (Stenfelt and Goode, 2005b), and appear similar to measurements in live subjects (Håkansson et al., 2008; Stenfelt and Goode, 2005a). The use of human whole-head specimens also implies the presence of the brain and the soft tissues, which may offer an advantage over isolated temporal bone specimens, especially in the study of semicircular canal dehiscence (Chien et al., 2007) or for studies of soft tissue or fluid-conduction pathways. Chien et al. reported limitations of temporal bone preparations to investigate the mechanics of BC hearing loss in SCD and suggested the use of whole-heads, which would better correspond to the clinical situation. The fixation method published by Walter Thiel (Thiel, 1992) is known to preserve the mechanical properties without the hardening and shrinkage of soft tissue, while conserving the specimen for long periods, similarly to formaldehyde. The Thiel method uses a watery solution of salts (e.g. ammonium nitrate, potassium nitrate), ethylene glycol, boric acid and a small amount of formaldehyde. Our group has evaluated and established the use of Thiel heads as a model for measurements of human middle ear motion in response to AC stimulation as an alternative to temporal bones (Stieger et al., 2012). The results were similar to tympanic membrane, stapes and round window motion measurements reported for living subjects and for fresh temporal bones. We have shown that a single head can be used for multiple AC experiments with reproducible results. The variability of repeated measurements in the same head is smaller than the inter-individual variability. Even long-term experiments of 20 h can be performed. The strong disinfectant properties of Thiel solution against bacteria (Staphylococcus aureus, Pseudomonas aerogiosa, Mycobacterium tuberculosum), fungi (aspergillus niger) and viruses (HIV) were tested and reported by Walter Thiel (Thiel, 1992). Therefore, the risk of infection is minimized. The aim of this study is to validate the use of Thiel-embalmed heads for BC experiments using LDV. We compared the displacement of the middle ear components in response to BC between Thiel heads and similar measurements reported in the literature for whole-heads, fresh temporal bones, and live subjects. Furthermore,
we addressed the following points: the linearity of the Thiel heads in response to BC, the reproducibility of the results in the same ear after short (2 h) or long (4e16 weeks) time intervals, and the disparity between the two ears of the same specimen. 2. Methods 2.1. Specimens We conducted measurements on a total of seven normal ears from four human cadaver heads, all embalmed according to Thiel’s method (Thiel, 1992). One ear of one cadaver head was excluded because its eardrum was damaged. The details of the specimens are listed in Table 1. All donors consented to the post-mortem use of their body for science according to the terms of the human body donation program of the Institute of Anatomy, University of Bern, Switzerland. The local ethics committee approved the use of Thielembalmed head specimens for research (KEK-BE 030/08). The specimens were conserved in Thiel solution at a temperature of 5 C before and between experiments. The heads had been fixed with Thiel solution for several months at the time of the experiments; storage time has been shown to have no influence on the quality of measurements (Stieger et al., 2012). 2.2. Preparation of specimens The specimens were removed from the fluid at least 2 h prior to each measurement (Stieger et al., 2012). We used suction to remove fluid and cerumen from the ear canal. We placed a BahaÒ implant with an abutment (Cochlear, Sidney, Australia) in the temporal region according to the surgical protocol provided by the manufacturer. In particular, we followed the manufacturer’s recommendations regarding the distance and angle relative to the ear canal. We placed small pieces of reflecting tape (approximately 0.5 0.5 mm) on the skull (SK) 2 cm anterior to the implant and on the umbo of the tympanic membrane (TM). The use of reflecting tape increases the SNR of the LDV without adding significant mass (Voss et al., 2000). After we measured the velocity of the SK and the TM, a tympanomeatal flap was elevated to expose the cochlear promontory (CP), the stapes footplate (ST), and the round window membrane (RW). We slightly widened the posterior bony ear canal with a surgical drill. We placed reflecting tape on the ST, the CP, and the RW (Fig. 1). The laser beam was aimed at the middle ear structures through the ear canal. The mechanical integrity of the cochlea was assessed by gently pushing the incus with a surgical needle and observing the resulting movement of the RW’s surface. The specimens were placed on a soft, stabilizing ring cushion to attenuate ambient vibrations. 2.3. Test signal generation and signal processing We used a Baha IntensoÒ (Cochlear, Sidney, Australia) sound processor to provide the BC stimulus. The output force of the sound Table 1 List of specimens. Specimen
Age (years)
Sex
Side
1
50
F
2
83
M
3
78
F
4
91
F
Right Left Right Left Right (discarded) Left Right Left
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capitals are used for complex values) was calculated. The means of the magnitudes and their standard deviations (STD) were calculated on a logarithmic scale, whereas phase means and their STDs were calculated in the linear domain. For linearity assessment, the differences D1[f] ¼ H30dBV[f] H20dBV[f] and D2[f] ¼ H40dBV[f] H20dBV[f] were calculated for each f on a logarithmic scale for the magnitude and on a linear scale for the phase. The combined average and STD of D1 and D2 were computed across the measured frequencies for each location of velocity measurement (Table 2). Data points at f ¼ 0.1 and 0.17 kHz were ignored because they often showed an SNR below 6 dB. The root-mean-square deviation (RMSD) in magnitude and phase between repeated measurements, left and right ear of a given specimen, and between left ears of multiple specimens was calculated at each frequency for the SK, CP, ST and RW as follows: Fig. 1. After elevation of a tympanomeatal flap, small pieces of reflective tape were placed at the cochlear promontory (CP) and the stapes footplate (ST). Another piece of reflective tape was placed on the surface of the round window (dashed circle, RW). IN: Incus.
processor, represented with the complex value F hereafter, was measured with a TU 1000 Skull Simulator (Håkansson and Carlsson, 1989). Only the electrical input of the sound processor was used; the built-in microphone was disabled at all times (the processor’s input was set to “E”). The volume was set to the minimum position. The analog generator of an audio analyzer (R&S UPV, Rohde & Schwarz, Germany) was used to generate the electrical stimulation signal. The signal consisted of 37 pure sinus tones; 30 were logarithmically spread between 0.1 and 10 kHz, and an additional seven where standard audiogram frequencies (0.25, 0.5, 0.75, 1, 1.5, 2, 3, and 4 kHz). The velocity of the SK, the TM, the CP, the ST, and the RW was measured with a single-point laser Doppler vibrometer (LDV; HLV1000, Polytec, Waldbronn, Germany). The LDV was attached to the operating microscope, and a joystick-driven mirror (HLV MM2, Polytec, Waldbronn, Germany) was used to aim the laser beam. We used the signal quality indicator of the LDV device and its acoustic output to monitor the signal-to-noise ratio before and during measurements. We used the analog signal processor of the audio analyzer to record the LDV signal. For each individual frequency, the magnitude and the phase were recorded after a settling period of up to 5 s. 2.4. Experiment protocol We measured the velocity of the SK, CP, ST, and RW in all seven ears. The velocity of the TM was measured in six specimens. Each series was repeated twice at 20, 30, and 40 dBVrms to assess linearity. For two of the ears, the measurements were repeated after a latency period of 2 h (short-term reproducibility). For four of the ears, the setup was renewed and measurements were repeated after a storage period of 4e16 weeks (long-term reproducibility). The background noise was measured prior to each series by running the spectral analysis once with the stimulation muted.
rffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi . ffi X 2 n 0 n RMSD½f ¼ ðV½f V ½f Þ where n is the number of measurements, and V and V0 the compared velocities. We compared our results with the following measurements reported in the literature: (1) The magnitude of CP motion measured with LDV in 14 ears of seven Non-Thiel embalmed cadaver heads after the removal of the TM, malleus, and incus. The heads were embalmed with a water solution containing sodium chloride, potassium nitrate, calcium chloride, chloral hydrate, formalin, glycerol, and Lysoformin, which has no specific name and is referred to “a Non-Thiel solution of salts” throughout this paper. Stepped sine tones between 0.1 and 10 kHz were applied using a Baha Classic. LDV measurements were performed with reflective glass spheres (Eeg-Olofsson et al., 2008). We compared the averages and the overall range of the measurements. (2) The ST-to-CP velocity ratio measured with LDV in 26 fresh temporal bones in response to stimulation with a minishaker. The stimulus was a swept sine signal with a logarithmically spaced resolution of 50 frequencies per decade. Frequencies between 0.1 and 10 kHz were reported. LDV measurements were performed with reflective micro spheres (Stenfelt et al., 2002). We compared the averages and the overall range of the measurements. (3) the TM velocity in response to BC stimulation with a Radioear B71 measured with LDV in 20 healthy ears of human subjects. Nine tones between 0.3 and 6 kHz were applied. LDV measurements were performed at the umbo (Röösli et al., 2012). Statistical analysis (ManneWhitney test) was possible for the TM data of Röösli et al., where STD was known. 3. Results 3.1. Linearity
2.5. Data analysis For each situation, a trace file containing a total of 222 (37 frequencies 3 stimulation levels 2 repetitions) points was saved for both the magnitude and phase. We used GraphPad Prism 5 (GraphPad Software, La Jolla, CA, USA) for data visualization and analysis. Data points at frequencies above 7.2 kHz were ignored as they are outside the Baha’s working frequency range. The transfer function H ¼ V/F (velocity V divided by the stimulation force F; bold
A typical velocity measurement series is plotted in Fig. 2. The signal-to-noise ratio was higher than 6 dB in frequencies above 0.2 kHz, which was the case for all locations of velocity measurements. The force-to-input voltage transfer function of the Baha device itself shows nonlinearity for stimulation smaller than 30 dBVrms (Fig. 2A). A similar difference in amplitude of the CP velocity can be seen (Fig. 2B). The constancy of the velocity-toforce ratio with varied stimulus level (Fig. 2C) suggests the
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Table 2 Difference between stimulation at 30 or 40 dBVrms and stimulation at 20 dBVrms. SK
CP
ST
RW
Phase (cycles)
Mag. (dB)
Phase (cycles)
Mag. (dB)
Phase (cycles)
Mag. (dB)
Phase (cycles)
Mag. (dB)
Phase (cycles)
0.43 1.75
<0.01 0.13
0.21 2.52
0.02 0.19
0.59 2.06
0.01 0.13
0.24 1.97
<0.01 0.16
0.72 4.62
<0.01 0.21
0 -20 -40 -60 1000
CP motion @ -20, -30 and -40 dBV stim. -20
-40
-60
-80 100
10000
C
1000
CP motion @ 1N 10 0 -10 -20
10000
100
0.5
0.5
0.3
0.0
0.0
0.2
-0.5 -1.0 -1.5 -2.0 100
1000
frequency (Hz)
10000
phase (cycles)
phase (cycles)
-80 100
B
magnitude (dBmm/s/N)
BAHA mechanical output @ -20, -30 and -40dBV stim.
magnitude (dB mm/s)
magnitude (dB N)
A
phase (cycles)
jAVGj STD
TM
Mag. (dB)
-0.5 -1.0 -1.5
1000
10000
1000
10000
0.1 0.0 -0.1 -0.2
-2.0 100
1000
10000
-0.3 100
frequency (Hz)
frequency (Hz)
Fig. 2. One representative cochlear promontory (CP) velocity measurement. (A) Force output of the Baha measured with a skull simulator. (B) Corresponding raw CP velocity (2 repetitions; the grey area shows the typical background noise). (C) Average ratio with the standard deviation range (thick black line and dashed lines, respectively). The lower graphs show the corresponding phases.
nonlinearity in 2A and 2B comes from the stimulator and not the temporal bone. When considering all seven specimens, the average difference between the transfer function V/F at 30 and 40 dBVrms stimulation compared with 20 dBVrms stimulation is reported in Table 2. The average D value was near zero dB in magnitude and zero cycle in phase for all locations of velocity measurement, and the STD was typically between 1 and 2.5 dB in magnitude, with the exception of the RW, for which it was of 4.6 dB. The STD in phase was between 0.13 and 0.21 cycles for all locations of velocity measurement. 3.2. Velocity-to-force ratio The transfer function V/F for the SK, TM, CP, ST, and RW for all seven ears are depicted in Fig. 3. For low frequencies, on average, the magnitude decreased with a slope of approximately 20 dB/ decade to a minimum for all of the locations of velocity measurement. The minimal magnitude was observed near 0.5 kHz for the SK and near 1 kHz for the CP, ST, and RW. The velocity of the TM showed a local minimum in the 0.5- to 1-kHz range and decreased again in the higher frequencies. The individual variability is high for frequencies above the local minimum. Two of the TM measurements showed a different behavior than the others; in those cases, the velocity was 10e15 dB higher above 0.25 kHz. The phase at the SK was near zero on average for all frequencies. For the other locations of velocity measurement, it was near zero up to the frequency of minimal magnitude. In the higher frequencies, on average, the phase decreased to 0.2 cycles.
3.3. Reproducibility The short-term, long-term reproducibility, the difference between the two ears of the same specimen, and the difference between specimens are plotted in Fig. 4. For short-term reproducibility (Fig. 4A), the RMSD in magnitude for the SK and the CP was below 3 dB, and it was below 5 dB for the ST and the RW, one spike above 5 dB is seen for the ST. For all locations of velocity measurement, the RMSD in phase was smaller than 0.1 for frequencies above 0.2 kHz, and spikes up to 0.5 cycles were observed between 1.5 and 4 kHz for the ST and RW. For the long-term reproducibility (Fig. 4B), the RMSD was in the range of 3e5 dB for the SK and CP ans 4e7 dB for the ST and RW. The RMSD in phase was below 0.1 cycles for the SK and CP, with spikes up to 0.2 and 0.6 cycles, respectively, at higher frequencies. The RMSD in magnitude between the two ears of the same specimen (Fig. 4C) was 3e10 dB for the ST and the CP, and 3e12 dB for the SK and the RW. The RMSD in phase was up to 0.6 cycles, especially at high frequencies. The RMSD in magnitude between left ears of the different specimens (Fig. 4D) was between 5 and 12 dB with spikes up to 15 dB and the RMSD in phase showed spikes up to 0.8 cycles at higher frequencies. 3.4. Comparison with other models The average magnitude of the velocity at the CP measured in the Thiel heads and in heads embalmed with a Non-Thiel solution of salts as reported by (Eeg-Olofsson et al., 2008) is depicted in Fig. 5A. The means were similar in shape. In the Thiel heads, the average
J. Guignard et al. / Hearing Research 306 (2013) 115e122
phase (cycles)
magnitude (dB mm/s/N)
A
B
SK
C
TM
D
CP
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E
ST
RW
20
20
20
20
20
10
10
10
10
10
0
0
0
0
0
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-40 100
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-40 100
0.4
0.4
0.4
0.4
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0.2
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0.2
-0.0
-0.0
-0.0
-0.0
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-0.4
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-0.4
100
1000 10000 frequency (Hz)
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Fig. 3. The velocity with respect to force (V/F). The skull (SK; A), umbo of TM (TM; B; dashed: outliers), cochlear promontory (CP; C), stapes footplate (ST; D) and round window (RW; E) are represented in magnitude and phase. Thin black lines show individual specimens; the thick black line is the average of all measurements.
0 100
1000
10000
SK CP ST RW
10
0.6
0.4
1000
1000 frequency (Hz)
10000
15
10
0.8
0.6
0.4
0.0 100
SK CP ST RW
1000
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1000 10 100 00 00
10000 10 100 00
1000
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1.0 SK CP ST RW
0.8
0.6
0.4
0.0 100
10
0 100 00
10000
SK CP ST RW
0.6
0.4
0.2
0.2
frequency (Hz)
SK CP ST RW
5
1.0 SK CP ST RW
inter-specimen variation 20
0 100
10000
0.2
0.2
D
5
0 100
0.8 RMSD (cycles)
RMSD (cycles)
15
1.0
1.0
0.0 100
SK CP ST RW
5
5
left-to-right reproducibility 20
RMSD (dB mm/s)
15
10
0.8
C
RMSD (dB mm/s)
SK CP ST RW
RMSD (dB mm/s)
RMSD (dB mm/s)
15
long-term reproducibility 20
RMSD (cycles)
B
short-term reproducibility 20
RMSD (cycles)
A
1000 frequency (Hz)
10000
0.0 100
frequency (Hz)
Fig. 4. Short-term (2 h, A; n ¼ 2 ears), long-term (4 weeks or more; B; n ¼ 4 ears), and left-to-right (C; n ¼ 3 specimens) reproducibility of the measurements, as well as interspecimen variations (D) for the skull (solid black; SK), cochlear promontory (dashed grey; CP), stapes (grey; ST) and round window (light grey; RW). Dashed grey: overall range. Top: RMSD in magnitude; bottom: RMSD in phase.
velocity was 6e7 dB higher up to 0.7 kHz. Above 0.4 kHz, the average reported by Eeg-Olofsson was within the range of the Thiel measurements. The averages overlap at 0.9, 1, 2, and 5 kHz. Fig. 5B shows the ST-to-CP velocity ratio in Thiel heads and in fresh temporal bones. The ST-to-CP ratio is near zero dB up to 0.5 kHz in both our measurements and in the literature. In both temporal bones and Thiel heads, the velocity of the ST was slightly higher (2e5 dB) than the velocity of the CP between 0.5 and 2 kHz and lower (to a maximum 3 dB) for higher frequencies. The range measured in
Thiel heads and the one reported in temporal bones overlap for most frequencies. The velocity of the TM in four of the Thiel heads’ ears (two were omitted for the comparison with live ears because of their noncharacteristic behavior, although the TM were normal on visual inspection) is represented in Fig. 5C along with the TM velocity in living subjects. The means were not significantly different at 0.7, and 3 kHz (p > 0.05). The means were significantly different at 1 and 4 kHz (p < 0.05) although they appear rather similar on the
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A
VST/VCP
B
CP motion
C
TM motion
15 Thiel (greyed area: STD)
Thiel (greyed area: range)
Non-Thiel embalmed heads (Eeg-Olofsson et al. 2008, dashed lines: range)
temp. bones (Stenfelt et al., 2002; thin dashed lines: range)
10
0
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Thiel (greyed area: range)
dB ref CP
VCP (dB mm/s/N)
20
5
0
living subjects (Röösli et al. 2012; thin dashed lines: STD)
0
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1000 frequency (Hz)
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frequency (Hz)
Fig. 5. The Thiel-embalmed head model compared with other reported models. (A) Average velocity of the cochlear promontory (CP) in seven ears of Thiel heads (solid line; grey area: range) and in 14 ears of heads embalmed with a Non-Thiel solution of salts (dashed line; thin dashed lines: range). (B) Average stapes (ST)-to-CP velocity ratio (VST/VCP) in seven ears of Thiel heads (solid line; grey area: range) and in 26 fresh temporal bones (dashed line; thin dashed lines: range). (C) Average velocity of the umbo of the tympanic membrane (TM) in four ears of Thiel heads (solid line; grey area: STD) and in 20 ears from living subjects (dashed line; thin dashed lines: STD).
graph. For frequencies below the minimum in magnitude (0.3; 0.5 kHz) and at 2 kHz, the average was significantly lower in living subjects than in Thiel heads (p < 0.05). 4. Discussion Our results suggest that whole head preparation according to Thiel’s method may be useful and valid for studies of the ossicular and RW biomechanics in response to sound transmitted through BC. Our results from Thiel-embalmed heads are comparable with reported data in cadaver heads embalmed with a Non-Thiel solution of salts (average velocity of the CP), fresh cadaver temporal bones (average ST-to-CP velocity ratio), and living human (average velocity of the umbo of the TM). Additionally, the Thiel-embalmed head model is linear in the range of clinically relevant stimulation levels. The short-term, longterm, and left-to-right variability is lower than the inter-individual variability. 4.1. Linearity The force-to-input voltage transfer function of the Baha device itself is not amplitude-independent for stimulation smaller than 30 dBVrms (Fig. 2A). We chose to use a Baha implant to reproduce the clinical situation as closely as possible. The reported equivalent hearing level with a Baha Intenso stimulated at 20 dBVrms is between 40 and 70 dB HL in the 0.25e4 kHz range (Arnold et al., 2010). We can estimate that our stimulation steps roughly covered a clinically relevant range from 20 dB HL to 70 dB HL. The transfer function V/F, however, is independent from the stimulation force (Fig. 2C). For all locations of velocity measurement, the average difference between the stimulation at 30 and 40 dBVrms and at 20 dBVrms of the LDV measurement is near zero for magnitude and phase (Table 2), indicating a linear behavior for the explored amplitude and frequency range. The linearity of BC transmission through the SK in vivo was previously reported in one living subject (Hakansson et al., 1996). This work shows that BC transmission is linear for all the measured components of the middle ear, and for the cochlear fluid estimated by RW movement (Table 2). The small STD of the repeated measures shows the relatively high immediate reproducibility. The exception is for the RW where
the STD is twice as high as for the other middle ear structures measured. High variability of RW motion is likely due to different compliance of the RW across ears and different patterns of RW motion especially at higher frequencies. The influence of the surgical angle and aiming location on LDV signal at the RW has been previously discussed (Stenfelt et al., 2004; Stieger et al., 2012). Another explanation may be an alteration of the RW response caused by accumulation of fluid, although we cleared the niche every time before RW measurements. 4.2. Velocity-to-force ratio Our current understanding of BC transmission includes: (1) the combined effect of the relative motion of the ossicles and the cochlear fluid resulting from inertia; (2) the generation of sound pressure in the EAC resulting in AC; (3) the compression of the petrous bone; and (4) sound transmission via the cerebrospinal fluid. We measured the TM velocity in response to BC sound, which we assume to be an expression of the ossicles and cochlear fluid inertia, and the sound pressure generated in the EAC (effects 1 and 2 above). Opening the TM results in a large reduction in the transmission of ear canal sound pressures generated by both vibration of the head, and airborne sound generated by the BAHA stimulator. Thus, consecutive measurements reflect the petrous bone compression, the sound transmission through the cerebral fluid, and a residual ossicular inertia effect. Two TM measurements showed nontypical shape with higher magnitudes. As the reflective tape placed at the umbo may have had firm contact to the pars tensa of the TM at its edges, measurements may have captured components of complex motion not identical to that of the umbo (Cheng et al., 2010). Furthermore, the displacement pattern of the TM is reported to be more complex at higher frequencies, which may explain the higher variability of TM measurements above the local minimum observed in our study. The TM measurements were always performed before the outer ear canal was widened. Therefore inter-individual differences in outer ear canal shape may have caused variations in laser beam angle. Differences in velocity measurements have been explained by different LDV setups, particularly the angle of the laser beam (Huber et al., 2001b; Chien et al., 2006). The other points measured showed similar responses in all specimens.
J. Guignard et al. / Hearing Research 306 (2013) 115e122
We observed the two modes of transmission of motion reported in the literature (McKnight et al., 2013; Stenfelt and Goode, 2005b); (1) whole-body motion in the lower frequencies showing a magnitude decrease of approximately 20 dB/decade and a phase of zero up to a local minimum in magnitude, and (2) wave transmission at the higher frequencies. The vibration is transmitted with little delay up to the frequency of minimum magnitude. The phases more significantly differ from zero in the higher frequencies. Generally, the inter-individual variation is higher at the frequencies above the minimum in magnitude; within that range, the model behaves in a more complex manner (Stenfelt et al., 2002). 4.3. Reproducibility A single head can be used for multiple experiments with reproducible results. The variability of repeated measurements for both short- (4 dB) and long-term (6 dB) repetitions in the same ear is less than the inter-individual variability (up to 25 dB; Fig. 3). The difference between the two ears of the same donor (approximately 10 dB; Fig. 4C) is lower than the inter-individual difference (up to 25 dB; Fig. 3), but it is higher than repeated measurements in the same ear. The short- and long-term stability of the measurements is a practical advantage for experimentation, as it allows a long series of manipulations while maintaining consistent behavior. 4.4. Comparison with other models Ideally, comparison would have been performed with data of specimens, which are measured fresh, subsequently embalmed according to Thiel and re-measured when conserved. This was not possible because local protocol requires embalming to be initiated as soon as the corpse reaches the institute of anatomy, which is typically within 24 h post-mortem (Stieger et al., 2012) Moreover, changing specimen properties with e.g. drilling to open the middle ear prior to embalming adds another factor to be considered when comparing the measurements. We compared our results with similar measurements reported in the literature. The comparison was possible, but the available data is sparse. To our knowledge, the body of literature contains no other reports of the velocity of all the consecutive points of the middle ear transmission chain secondary to BC stimulation. Middle ear LDV measurements in living subjects with BC stimulation are rare, and no mention of the intraoperative measurement of intratympanic structures excited by BC was found in the literature. The motion of the temporal bone that we measure in Thiel heads is similar to that measured in Non-Thiel embalmed cadaver heads (Fig. 5A) by Eeg-Olofsson (Eeg-Olofsson et al., 2008). We did not expect a dramatic difference because the components of the NonThiel-embalming solution described by Eeg-Olofsson et al. are, to some extent, also used in the Thiel solution (potassium nitrate and sodium salts, alcohols such as ethylene glycol or glycerol, formaldehyde). The averages and range are similar above the minimum in magnitude for the two models. In the lower frequencies, the velocity of the CP was higher in our specimens. This result may be explained by different tissue properties resulting from the embalming technique. It has been reported that when bone is in contact with formalin, a chemical reaction of aldehydes with primary amine groups of the collagen chains leads to an increased number of inter- and intrafibrillar cross-links. In consequence, an alteration of the mechanical properties of the bone is to be expected (Unger et al., 2010; Currey et al., 1995). We assume that a different concentration of formaldehyde between the Thiel- and Non-Thiel-embalming solutions (although the concentration of formaldehyde in the Non-Thiel solution is unknown) might lead to a difference in bone stiffness.
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There are differences between our setup and the one reported in Stenfelt et al. (2002) because we stimulated a whole head with a Baha device whereas Stenfelt stimulated a temporal bone with a shaker; however, the comparison of the magnitude of relative motion (ratio VST/VCP) is possible. We observed that the VST/VCP in Thiel heads and fresh temporal bones are comparable (Fig. 5B). The average curves show a similar shape and the ranges overlap to a large extent. An explanation for the high variability of ST/CP above 1 kHz may be the complex motion of the stapes at high frequencies caused by an increase in anterior-posterior rocking motion (Heiland et al., 1999; Hato et al., 2003). In addition the variability may be caused by multidirectional movement of the CP reported at higher frequencies (Stenfelt and Goode, 2005a). Furthermore, we observed a similar magnitude and phase for the motion of the RW and the ST (Fig. 3D and E), an observation that Stenfelt et al. (2004) also reported for temporal bone. On average, the shape and the magnitude level of the TM velocity were similar in our experiments and those conducted on living humans. It is known that stapes fixation has minimal effect on umbo velocity (during AC) because of the compliance of the ossicular joints (Jakob et al., 2009). That live and Thiel measurements are similar points to interpretation that generally Thiel has little effect on umbo velocity because it has little effect on the ossicular joint and TM material properties. The amplitudes for low frequencies and the frequency of minimal magnitude were higher in Thiel heads. We assume that this is a combined effect of inter-individual variation in the frequency of minimal magnitude and the attenuation of the skin in the case of transcutaneous stimulation with the B71 as opposed to the direct mechanical coupling of the Baha. Additionally, the velocity in Thiel heads was significantly higher at 2 kHz. We have two hypotheses to explain this observation. First, the Baha generates a higher audible sound pressure than the B71 at 2 kHz. The velocity of the TM could be higher in response to this sound emission. Second, the active behavior of the transmission chain, known as the acoustic reflex (Reger, 1960; Smith, 1943), is absent in post-mortem specimens. Another difference between the Röösli experiment and our study is that the external ear canal was open in our setup. The statistical significance of the mean differences must be interpreted with caution because of the difference in sample size in both datasets. Our setup constrains the measurement of the CP velocity in roughly the same plane as the stimulation. It is known that the cochlea moves in multiple directions in response to BC stimulation. Components perpendicular to the axis of the laser beam are not visible; therefore, a portion of the signal energy is not present in the measurements.
4.5. Conclusion Anatomical human whole-head preparations embalmed according to Thiel’s method are an alternative model for the study of human middle ear mechanics secondary to BC stimulation. Thiel heads have the advantage of long-term preservation, which allows these specimens to be used for multiple experiments over long periods of time with reproducible results. At some frequencies, differences from living subjects must be considered.
Disclosure The authors declare that there are no conflicts of interest.
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