Intracranial Pressure and Promontory Vibration with Soft Tissue Stimulation in
Cadaveric Human Whole Heads
Christof Roosli, MD1, Ivo Dobrev, PhD1, Jae Hoon Sim, PhD1, Rahel Gerig1, Flurin Pfiffner,
PhD1, Stefan Stenfelt, PhD2, Alexander M. Huber, MD1
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University of Zurich, Zurich, Switzerland
Department of Otorhinolaryngology, Head and Neck Surgery, University Hospital Zurich,
Department of Clinical and Experimental Medicine, Linköping University, Linköping,
12 13 14
Christof Roosli, MD
Department of Otorhinolaryngology, Head and Neck Surgery, University Hospital Zurich
CH-8091 Zurich, Switzerland
++41 44 255 47 67
++41 44 255 41 64
E-Mail: [email protected]
Running title: intracranial pressure with soft tissue stimulation
Conflicts of Interest and Source of Founding: Author SS is supported by the Swedish
Research Council. Fore the remaining authors none were declared.
Hypothesis: Soft tissue stimulation induces bone vibration to a similar extent like bone
conduction (BC) stimulation.
Background: A hearing sensation can be elicited by vibratory stimulation on the skin
covered skull, or by stimulation on soft tissue such as the neck. It is not fully understood
whether all these different stimulation sites induce skull vibration or whether other
transmission pathways are dominant. The aim of this study is to assess the correlation
between intracranial pressure and skull vibration on the promontory for different stimulation
Methods: Measurements were performed on 4 human cadaver heads. A BC hearing aid was
held in place with a 5-Newton steel headband at four locations (mastoid, forehead, eye, and
neck). While stimulating in the frequency range of 0.3 – 10 kHz, acceleration of the cochlear
promontory were measured with a Laser Doppler Vibrometer, and intracranial pressure at the
center of the head with a hydrophone.
Results: Promontory acceleration and intracranial pressure was measurable for all stimulation
sites. Its ratio is comparable between all stimulation sites.
Conclusions: These findings indicate that promontory acceleration and intracranial pressure
are involved to a similar extent for stimulation on the sites investigated. The transmission
pathway of sound energy is comparable for these four stimulation sites.
Key words: bone conduction, soft tissue stimulation, intracranial pressure, promontory
Bone conduction (BC) is an alternative pathway to air conduction (AC) for sound to
reach the cochlea. It is widely used in clinical audiometry as it is used to classify conductive
and sensorineural hearing loss. However, the exact mechanism of BC hearing has not been
completely understood, neither in normal hearing conditions nor in pathologic situations of
the middle ear. It is thought that a complex interaction of several different pathways causes
the hearing sensation with BC sound. The importance of the different pathways depends on
the stimulation frequency and on pathologies of the middle ear. Most of the pathways are
agreed on to involve bone vibrations: 1) Distortional vibrations of the cochlear shell cause
small deformations of the cochlear walls (1). These deformities cause fluid flow in the
cochlea that excites the basilar membrane. 2) Vibrations of the skull induce inertial forces of
cochlear fluid that result in pressure change in the inner ear exciting the basilar membrane (2,
3). A similar phenomenon is the relative motion of the skull and middle ear ossicles caused
by the inertia of the ossicles (2, 4, 5), which stimulates the inner ear through motion of the
stapes footplate in the oval window. 3) And finally, skull vibration causes vibration of the
osseous and cartilaginous external ear canal, resulting in a sound pressure in the external ear
canal and motion of the tympanic membrane and ossicular chain similar to AC stimulation.
This pathway is affected by changing the state of the ear canal from open to closed (occlusion
Another suggested pathway is dynamic sound pressure transmission from the skull
interior (brain tissue and cerebrospinal fluid) to the cochlea. Sohmer (9) coupled in an
experimental study the cranial cavities of two animals by a saline filled plastic tube. A bone
conduction click stimulus was applied to animal 1, while the brainstem evoked response
auditory (BERA) was recorded from the second animal. They found a correlation between
stimulation in animal 1 and BERA response in animal 2, and concluded that sound pressure
can be transmitted by a fluid pathway to the cochlea and stimulate it. In other experiments,
the acceleration of the bone was measured for stimulation at the forehead, eye or directly on
the brain. While BERA could be clearly recorded for all simulation sites, no acceleration of
the bone was measured for stimulation at the eye in human (9) or at the brain in animals (10).
On the contrary, others found bone vibration when measured directly at the teeth for
stimulation at the eye. Even so, they found no direct correlation between the BC threshold
and vibration of the teeth (11). However, it is not certain that the vibration of the teeth
corresponds to the vibrations of the bone surrounding the cochlea.
The aim of this study is to compare the relation between intracranial sound pressure
and bone vibrations measured at the cochlear promontory for stimulation on skin covered
bone (mastoid and forehead) and on soft tissue (eye and neck). We hypothesize that
intracranial pressure and skull vibrations are correlated and depend on the stimulation
position and frequency.
The experiments were reviewed and approved by the institutional Ethic Committee
(KEK-ZH-Nr. 2012-0136). In this study, four cadaveric human whole heads that were
conserved using a technique described by Thiel (12) were used. An endaural incision was
performed between the helix and the tragus to achieve access to the promontory. Then, a
tympanomeatal flap was elevated to expose the middle ear (13). Two self-retaining retractors
were placed to allow good visualization of both the endaural surgery and the later LDV
measurement. To enhance reflectivity of the laser beam, a small piece of retro-reflective foil
(1 x 1 mm) was placed onto the cochlear promontory near to the round window on the
measurement position. The skull was opened at the vertex and a tube of 5 mm diameter was
tightly sealed to the opening. Details regarding surgical preparation are described in our
previous work (14).
Measurements were performed on a stainless steel table to minimize vibrations from
unwanted sources. A hydrophone (Type 8103, Brüel & Kjær, Denmark) was inserted into the
intracranial space through the tube. The hydrophone was positioned at the center of the
cranial hemisphere and its position controlled by X-ray (Figure 1). The physiologic
intracranial pressure of 15cm H2O2 was maintained by a water column in the tube attached to
the skull of 15 cm (15).
The transducer of a BAHA Cordell II (Cochlear Company, Australia) was attached to
the head at 4 positions (mastoid, forehead, eye, and neck) using a 5-Newton steel-headband.
The coupling forces were controlled with a spring gauge (Light Line, Pesola, Switzerland).
The stimulus was directly routed to the transducer with a stimulus intensity of 1 Volt, that
was generated by the measurement system Audio Precision System One (Audio Precision
Inc., USA). The sound stimulus consisted of a stepped sine measurement procedure in the
frequency range of 0.3 – 10 kHz. The measurement frequencies were equally spaced on a
logarithm scale (50 frequency points per decade), resulting in 78 frequencies in the frequency
All motions of the cochlear promontory were measured at a single point using an
OFV-3001 Scanning Laser Doppler Vibrometry (SLDV) system (Polytec GmbH, Germany).
The sampling frequency was set at 51.2 kHz. The output of the SLDV was coupled to the
Audio Precision System One for further data processing. All of the measurement procedures
were controlled by the Audio Precision software (AP 2700 Control software) installed on a
personal computer. The acceleration of the cochlear promontory was calculated from the
recorded velocity. Simultaneously, intracranial pressure was recorded using the hydrophone
positioned in the cranial space (see above). The recorded pressures were routed to the Audio
Precision System One via a charge amplifier (Type 2635, Brüel & Kjær, Denmark).
The stimulation forces with the BAHA Cordell II transducer placed on the skin by the
steel-headband were calibrated using an artificial mastoid (Type 4930, Brüel & Kjær), and the
measured cochlear promontory accelerations and intracranial pressure changes were
normalized by the stimulation force. During the calibration of the transducer on the artificial
mastoid, the mechanical impedance of the artificial mastoid was measured, and the static
force between the transducer and the surface of the artificial mastoid was maintained at
approximately 5 Newton, which corresponds to the coupling force between the skin and the
transducer in the cadaver heads.
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The cochlear promontory accelerance, which is defined as acceleration normalized by
the stimulation force, is shown in figure 2 for all four heads separately and for all stimulation
sites. The cochlear promontory vibration varied among the skulls and the spread among
specimens was larger than the differences between different stimulation sites for one
specimen. For stimulation at the mastoid and forehead, the signal was above noise level for
most frequencies. There was a tendency for low signal to noise ratio (SNR) for stimulation at
the neck at frequencies above 2 kHz and for stimulation at the eye for frequencies above 3
kHz. Generally, acceleration of the cochlear promontory was largest with stimulation at the
mastoid. Lowest acceleration of the promontory was measured for stimulation at the neck.
Intracranial pressure for stimulation at all sites was measured in all four specimens
(Figure 3). Again, the variability among the heads was larger than the variability between
stimulation sites in one head. Therefore the results are shown for each head separately. A
SNR greater than 10 dB was achieved for all stimulation sites for frequencies below 2 kHz.
The SNR was less than 10 dB at frequencies above 2 kHz for stimulation at the mastoid,
forehead, and neck, while the signal was close to or in the noise for stimulation at the neck for
frequencies above 2 kHz. Largest intracranial pressure was measured for stimulation at the
mastoid and the forehead. Smallest intracranial pressure was measured when the stimulation
was at the neck.
The ratio between vibration of the cochlear promontory and intracranial pressure is
compared for all stimulation sites and for all specimens (Figure 4). For stimulation on the
neck and eye, this ratio is within the ratio of mastoid and forehead. Above 2 kHz, the ratio for
stimulation at the neck needs to be interpreted with care, because the promontory motion and
intracranial pressure was close to the noise level. For frequencies below 2 kHz, these
findings indicate that regardless of the site of stimulation, vibration of the cochlear
promontory and intracranial pressure was registered. These two components seem to be
related and are not mutually independent. The larger magnitude of the cochlear promontory
vibration and intracranial pressure when the stimulation is on the skin covered bone at the
mastoid and forehead indicates that this stimulation mode is more efficient than stimulating
on the head or neck.
This study investigates the relation between vibration of skull measured at the
cochlear promontory and intracranial pressure for BC stimulation at four different positions:
mastoid, forehead, eye and neck. The hypothesis is that skull vibration and intracranial
pressure correlate independent of the position of stimulation resulting in similar frequency
Measurements were performed in four cadaveric whole heads that give a close
approximation to the in vivo situation for the following reasons: the heads were stored
according to a technique described by Thiel (12) which shows no significant difference in
biomechanical testing as compared to fresh frozen material (16, 17). For measurements on
BC pathways, dry skulls, temporal bones, and animals have been used in previous studies (4,
5, 9, 18). Using cadaveric whole heads has the advantage of the presence of soft tissue
remaining attached to the head and a fluid filled intracranial space as compared to
measurements on dry skulls or temporal bones. Additionally, the presence of the whole
circumference of the head may model the complexity of BC pathways more adequately than
temporal bones or measurements on animals. The cochlear promontory vibrations are
comparable to the measurement of others (17, 19) who also stimulated on the skin covered
bone on Thiel fixed heads (17), or directly on the skull (19) (Figure 5).
Vibrations of the promontory were only measured in one direction which does not
exactly address the complexity of the three dimensional motion of the skull in BC
stimulation. However, cochlear promontory vibration levels in the three perpendicular
directions are normally within 5 dB (20). Therefore, we assumed that vibration of the skull
did not significantly differ in the three perpendicular directions and our method is a valid
One limitation of our measurement is that some data were close to, or below, the
noise level, especially for stimulation at frequencies above 4 kHz and for stimulation at the
neck and eye. One reason is the damping of the skin that affects the effective stimulation at
the frequencies above 4 kHz (21). This corresponds to worse hearing thresholds at these
positions compared with the mastoid position in normal hearing subjects (11, 22). A greater
driving voltage of the transducer could have been used but most likely would have caused
distortions of the stimulation.
Another finding was the large variability among the four heads. Averaging of the
measurements of the four cadaver heads was therefore not meaningful. Analysis of the
cochlear promontory vibration and intracranial pressure was done for each head individually.
The exact reason for this large variability is unclear. On reason might be differences in
anatomical structures such as thickness and circumference of the skull, or thickness and
structure of the soft tissue. Another possible contributor to differences is the coupling and
attenuation of the skin and soft tissues in the transducer-skull interface. This influence was
attempted to be minimized by controlling the static force of the coupling of the headband by a
It has been shown that direct stimulation of the soft tissues (i.e. eye or neck)
stimulates the cochlea and causes a hearing sensation (11, 23, 24). In the current
measurements, the absolute magnitude for both the cochlear promontory vibration and
intracranial pressure tended to be smaller for stimulation at the eye or neck compared with the
mastoid or the forehead. Such findings are consistent with worse hearing threshold for
stimulation at these locations as compared to stimulation at the mastoid. The lower magnitude
of promontory vibration by up to 10 dB for stimulation at the eye compared to stimulation at
the mastoid is consistent with the 10 to 15 dB lower hearing threshold for stimulation at the
eye (11, 22).
The results presented in this study indicate that the ratio of the cochlear promontory
vibration and intracranial pressure is comparable for stimulation on skin covered bone and
soft tissue without direct contact to bone. These findings can be explained by an interaction
between soft tissue, skull content and skull that results in vibration of the bone surrounding
the cochlea, and the intracranial space. It’s controversially discussed in the literature whether
vibration of the soft tissue is independent from bone vibration or not. Some reports claim that
soft tissue conduction does not involve bone vibration (9, 10, 24, 25). In an animal model,
Chordekar et al. (24) measured auditory brainstem responses (ABR) and vibrations of the
bony vestibule with an LDV for BC and soft tissue stimulation. For BC stimulation, ABR and
bone vibrations were measured, while for soft tissue stimulation only ABR were recorded for
low intensities. One possible reason for the difference to our findings is that vibrations may
have been too low to be detected, because low intensity stimulation was used. If bone
vibration did not contribute to BC hearing for stimulation at the neck, the ratio between
promontory vibration and intracranial pressure change is expected to be much lower than for
direct stimulation of the bone. Our data do not show such a difference in ratio, suggesting that
soft tissue stimulation involves bone vibration similarly to direct stimulation on the bone.
This similar ratio for stimulation on bone and on soft tissue indicates that the stimulation sites
investigated involve promontory vibration and intracranial pressure to a same extent, meaning
that bone vibration is considerably involved in soft tissue stimulation.
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The position of the hydrophone was checked by X-ray in two planes, lateral view (A), frontal
view (B) to make sure it is positioned in the center of the cranial space not touching the skull.
Promontory motion for all four heads (A-D).
Intracranial pressure for all four heads (A-D).
Ratio of promontory motion and intracranial pressure for all four heads (A-D).
Magnitude of promontory motion for all four heads compared to magnitude of promontory
motion with direct stimulation on the skull bone (19).