Effect of transducer fixation in the human middle ear on sound transfer☆
Graphical abstract
Introduction
Hearing is one of the most important human senses. The ear is an organ which is responsible for sound transmission from the external environment to the cochlea and the brain. The human ear is composed of three parts: the outer, the middle and the internal (inner) ear. The middle ear consists of the tympanic membrane, three ossicles (i.e. the malleus, the incus and the stapes), as well as ligaments and tendons. The sound approaching to the outer ear as an acoustic pressure is transformed first into mechanical vibrations in the middle ear and then into an electrical signal in the inner ear. The middle ear is the smallest biomechanical system in the human body, therefore its treatment is particulary demanding and difficult. Generally, two types of hearing loss can be distinguished: conductive and sensorineural. The former is usually treated by means of different types of prostheses that are passive elements to connect damaged or missing ossicles. The latter can be treated by a cochlear implant which is a surgically implanted electronic device. The implant provides a sense of sound to a person with severe to profound hearing loss. As an alternative to conventional treatment method by means of cochlear implant, active middle ear implants, known as implantable middle ear hearing devices (IMEHDs) have been used in medical practice. IMEHDs can be applied in the case of both conductive and sensorineural hearing loss, therefore they are becoming more and more popular.
A typical IMEHD consists of three parts (Fig. 1): a microphone, a signal processing and a vibrating output transducer called floating mass transducer (FMT). Numerous procedures for implantation of hearing devices are currently used in clinical practice. An otolaryngologist must find both the best position for the FMT and the method of fixing it to the ossicular chain by a coupler (clip). The transducer is the main part of the IMEHD and provides a direct mechanical stimulation to the human middle ear (Liu et al., 2017; Hong et al., 2007) or to the cochlea by driving the round window (RW). This technique, known as RW stimulation, was developed in (Liu et al., 2017; Busch et al., 2017). Clinical studies on the IMEHD's applications have been reported by several authors (Lord et al., 2000; Morris et al., 2004; Williams and Lesser, 1990).
A review of the literature shows that the problem of the human middle ear with IMEHD (implanted middle ear - IME) is predominantly investigated experimentally or by the Finite Elements Method. Undoubtedly, there is a lack of nonlinear mechanical lumped mass models that would explain the behaviour of both the intact and the implanted human middle ear. Only several papers describe the middle ear as a lumped mass structure (Seong et al., 2009; Rusinek et al., 2019). The paper (Seong et al., 2009) offers a simple linear model without deep investigation while the model proposed in (Rusinek et al., 2019) is used here to analyse the effect of FMT fixation effect on sound transfer. However, the impact of coupler fixation is primarily investigated experimentally. Muller et al. (Müller et al., 2018) proposed a new design of the Hannover coupler to stimulate the round window. The authors showed that the Hannover coupler's stiffness and initial static preload affect the stapes motion. The effect of compliance of the incus long process coupler (ILPC) is analysed in (Schraven et al., 2016). For the flexible coupler, the velocity amplitude responses in temporal-bone preparations showed higher mean amplitudes at around 1 kHz (10 dB) and lower mean amplitudes between 1.8 and 6 kHz (13 dB on average between 2 and 5 kHz). Attachment of the FMT to the long process of the incus with the proposed coupler leads to generally good mechanical and functional coupling in temporal-bone preparations with a notable disadvantage between 1.8 and 6 kHz. Due to the elastic clip attachment, it is expected that the ILPC will reduce the risk of necrosis in the incus long process. A comparison of stapes footplate vibrations under active electromechanical stimulation is made for the ILPC, stapes-FMT-coupler and Bell-FMT-coupler on the stapes head (after incus and malleus removal). The results demonstrate that in contrast to the ILPC after the installation of the stapes-FMT-coupler and Bell-FMT-coupler in the middle ear, the average velocity amplitude of the stapes footplate is about 15 dB higher in the range between 1 and 6 kHz and 10 dB lower at about 0.5 kHz. Quantitatively, there is no significant difference between the stapes-FMT-coupler and the Bell-FMT-coupler. Therefore, the installation of the stapes-FMT-coupler or the Bell-FMT-coupler on the stapes head provides a considerable improvement in the middle ear mechanical and functional response, when compared to the ILPC results in the temporal bone experiments. Moreover, the installation of the Bell-FMT-coupler to the stapes head produces exactly the same footplate velocity responses as the stapes-FMT-coupler. Although the FMT is usually fixed to the incus long process, it can also be fixed to the incus short process (using an incus short process coupler ISPC), which may, in some medical cases, be more beneficial for the patient.
The literature review shows that it is important to explain the effects of coupler stiffness and transducer position. This paper investigates thoroughly the problem of coupler (clip) stiffness. The main goal of the paper is to investigate the dynamic behaviour of the stapes and floating mass transducer (FMT) that can be caused by variations in the clip's linear stiffness. Two sets of the damping coefficient are tested in order to model the normal and pathological middle ear.
The paper is organized as follows: Section 2 presents a nonlinear five-degree-of-freedom (5dof) model of the human middle ear with an implant. The model's shortened name is the IME, which stands for the implanted middle ear. In Section 3 the effect of linear clip stiffness variation on resonance amplitudes of the middle ear is described. The range covers frequencies up to 5 kHz, that are crucial for speech recognition. Next, a detailed analysis of polyharmonic vibrations and primary resonance oscillations is presented. Finally, Section 4 offers conclusions drawn from the research.
Section snippets
Model of the human middle ear with an implant
This study uses the nonlinear three-degree-of-freedom (3dof) model of the human middle ear proposed in (Rusinek et al., 2017) and extends its to the 5dof, where the ear is implanted with an IMEHD. The middle ear with an implant, shown in Fig. 1a is called for brevity as an implanted middle ear (IME). A detailed description of the model can be found in (Rusinek, 2020) where the similar one but with a different clip characteristic is analysed for the angle of excitation parameters. However, in
Coupler stiffness analysis
A typical coupler, shown in Fig. 1, has linear stiffness characteristic. However, the middle ear system is totally nonlinear, therefore it is expected that the coupler can affect the system dynamics and, finally the stapes vibration amplitude. The stiffness rate () is defined as the multiplier of linear stiffness of the clip . Amplitudes of both the stapes and the FMT elements are presented below as colourful 2-parameter maps.
Conclusions
It is important for the implanted ear to have the same or very close characteristics as the healthy ear. The research proves that the implanted human middle ear is also fully predictable with the periodic answer in the range of small excitation amplitude regardless of the excitation frequency. It is true in the case of normal ear, but in the pathological ear the region of non-harmonic motion appears near . When increasing the excitation amplitude, the non-harmonic vibrations area increases
Ethical Statement
Author states that the research was conducted according to ethical standards.
Declaration of competing interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgements
The research was financed in the framework of the project: Nonlinear effects in middle ear with active implant, no. , funded by the National Science Centre, Poland.
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This document is the results of the research project funded by the National Science Centre (Poland)under the grant "Nonlinear effects in middle ear with active implant", no.2018_29_B_ST 8_01293.