<p>The Mechanical Impedance of the Human Skull via Direct Bone Conduction Implants</p>

Abstract

Point your SmartPhone at the code above. If you have a QR code reader, the video abstract will appear. Or use: https://youtu.be/zbv0NO6djwo Purpose: The mechanical skull impedance is used in the design of direct bone drive hearing systems. This impedance is also important for the design of skull simulators used in manufacturing, service, and fitting procedures of such devices. Patients and Methods: The skull impedance was measured in 45 patients (25 female and 20 male) who were using percutaneous bone conduction implants (Ponto system or Baha system). Patients were recruited as a consecutive prospective case series and having an average age of 55.4 years (range 18-80 years). Seven patients were treated in Gothenburg, Sweden, and 38 patients in Edmonton, Canada. An impedance head (B&K 8001), driven by an excitation transducer with emphasized low-frequency response, was used to measure the mechanical point impedance with a swept sine from 100 to 10k Hz. Results and Discussion: The skull impedance was found to have an anti-resonance of approximately 150 Hz, with a median maximum magnitude of 4500 mechanical ohms. Below this anti-resonance, the mechanical impedance was mainly mass-controlled corresponding to an effective skull mass of 2.5 kg at 100 Hz with substantial damping from neck and shoulder. Above the anti-resonance and up to 4 kHz, the impedance was stiffness-controlled, with a total compliance of approximately 450n m/N with a small amount of damping. At frequencies above 4 kHz, the skull impedance becomes gradually mass-controlled originating from the mass of the osseointegrated implant and adjacent bone. No significant differences related to gender or skull abnormalities were seen, just a slight dependence on age and major ear surgeries. The variability of the mechanical impedance among patients was not found to have any clinical importance. Conclusion: The mechanical skull impedance of percutaneous implants was found to confirm previous studies and can be used for optimizing the design and test procedures of direct bone drive hearing implants.

Review of Whole Head Experimental Cochlear Promontory Vibration with Bone Conduction Stimulation and Investigation of Experimental Setup Effects

“…Among the studies, different types of implants were used ( Table 1 ) but also different ways to attach to the implant, for example by screwing the transducer to the implant or by a snap-coupling. It has been shown that the connection between the transducer and the implant/skull introduces a compliance that can affect the high-frequency BC stimulation ( Hakansson et al, 2020 ). Since the impedance of the skull and the skull simulator used for calibration differ, and the connection to attach the BC transducer can be different between the skull and skull simulator, the high-frequency stimulation level can differ between the skull and the skull simulator.…”

“…This computation indicated, due to a shift in the BC transducer's resonance frequency at around 400 Hz, up to 5 dB higher output levels on the human mastoid compared with the artificial mastoid at frequencies below 400 Hz and around 5 dB lower output levels on the human compared with the artificial mastoid at frequencies above 400 Hz (Surendran & Stenfelt, 2021). A similar computation based on the impedances of a BC implant and the skull simulator used to measure the output force of a BC transducer for skull attachment (Hakansson et al, 2020), indicated 2.5–5 dB higher output levels on the human implant compared with the skull simulator at frequencies below 1 kHz. At frequencies above 1 kHz, the output on the human implant was 2.5 dB lower than that measured on the skull simulator.…”

Evaluating temporal bone column density for optimized bone conduction implant placement

“…For a frequency band-specific interpretation of our results, it can be helpful to consider the categorization of mechanical point impedance ( 20 , 38 ). The skull impedance exhibits an antiresonance at around 150 Hz ( 20 , 37 ). Below this antiresonance, the mechanical point impedance is predominantly governed by the mass of the skull.…”

“…The temporal bone is a highly complex structure containing air cells and composed of different types of bone tissue, such as cortical bone, cancellous bone, and diploë, with varying densities ( 19 ). The transmission of energy from the transducer to the temporal bone should be influenced by the mechanical point impedance ( 20 ). Different bone column densities should therefore affect the local mechanical impedance as well as the impedance matching between the transducer and the skull.…”

Effect of Driver Mass Loading on Bone Conduction Transfer in an Implantable Bone Conduction Transducer

“…However, the vibration does not propagate directly to the inner ear, but rather passes through the skull; it was thus important to measure the signal through the skull. We employed an artificial mastoid (type 4930; Brüel & Kjaer, Naerum, Denmark) commonly used to measure the forces of bone conduction devices [45], [46], [47], [48]. To this end, the coil vibration transducer was completely assembled including the titanium housing cap [Fig.…”