Evolution of Middle Ear Modelling Techniques: A Review

This review article attempts to analyze the various research studies conducted in developing the models to evaluate the anatomy of the middle ear, its biomechanics, and the applications of these models in normal and diseased states. Various studies conducted over the past 50-60 years have been critically analyzed. We also discuss the various advantages and disadvantages of different methods of measurement of middle ear parameters. Beginning from anatomical modelling to histopathological sections and the latest three-dimensional (3D) reconstruction with finite element modelling, various methods of middle ear measurements have been critically analyzed. At the end of this review, we have concluded that the best and most effective method of middle ear modelling is the 3D reconstruction using high-resolution computed tomography and finite element modelling.


Introduction And Background
The middle ear is a rather complex structure that largely influences hearing, disease processes, and reconstruction of hearing mechanisms [1]; hence, understanding the middle ear physiology is important. Various middle ear models have been developed to assess acoustics of the middle ear, and continuous refinements have been done in their design over years. Initially, static models were developed to understand the physiology of hearing and the role of the middle ear in amplification. However, these had limitations in terms of reproducibility of results in live subjects, as middle ear structures are affected by stress, strain, and differences in the thickness of the tympanic membrane at various sites.
Surgical techniques and materials have been constantly revised by the surgeons, based on their experience and existing knowledge of physiology. However, the knowledge of middle ear physiology and biomechanics is based mainly on the experiments conducted by engineers, who may not be aware of the medical challenges. Therefore, an otolaryngologist's opinion on this issue was highly warranted; hence, the current review was undertaken to discuss the evolution of middle ear modelling techniques and their role in studying the biomechanics of the middle ear, in normal and diseased states.

Search strategy
The present review was performed addressing the Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) guidelines ( Figure 1) to assess the literature based on the evolution of middle ear modelling techniques and their clinical applications. Manual and electronic data resources were accessed and articles published before December 2021 were included for this review. The following databases were used in searching the literature: PubMed, Scopus, Web of Science, and Google Scholar. The articles written in English were included in this review. The words used for search strategy were: "middle ear model" {Medical Subject Heading terms} OR "Malleus" OR "Incus" OR "Stapes" AND "Computed Tomography".

Selection criteria
The selection criteria for the present review included randomized as well as non-randomized controlled trials, cohort studies, and prospective studies based on live humans as well as cadaveric temporal bones along with retrospective radiological studies. Technical reports, animal studies, letters to the editor, and case reports were excluded from the present review.

Middle ear modelling
Middle ear modelling comprises two main steps: reproducing the dimensions of the middle ear cavity and making the model.

Reproducing the Dimensions of the Middle Ear Cavity
Reproducing the dimensions of the middle ear cavity requires static and dynamic measurements. Static measurements include dimensions of middle ear cavity, tympanic membrane, and ear ossicles by various methods as depicted in Table 1. These are known as boundary conditions. Requires prior knowledge of the shape, density of the middle-ear ossicles.

MRI temporal
bones [5] The middle ear cleft and cavity are filled with contrast dye that has a high absorptive capacity of magnetic resonance and then the ossicles will light up as islands within a fluidfilled cavity.
The definition can be compared to that of Xray micro-CT.
Care has to be taken not to introduce any bubbles of air. Around five to 10 times more costly than an X-ray scanning machine for this purpose [6]. HRCT, high-resolution computed tomography.
Dynamic measurements include the measurement of the velocity of the tympanic membrane, which is measured by laser Doppler vibrometer, stress, strain, and measurements of immittance of the ossicles and tympanic membrane, as depicted in Table 2. These are known as loading conditions.

Method Description Advantage Disadvantage
Laser Doppler vibrometer [6] Promptly  Material data of each part are shown in Table 3. These data are determined by referring to the research of Higashimachi et al. [4] and Koike et al. [9] to give a combined analysis of Young's modulus, density, and Poisson's ratio of the tympanic membrane, ossicles, and the ligaments.  The malleus, incus, and stapes had similar Young's modulus and density, whereas the tympanic membrane was found to be less dense. These parameters were comparable for lateral, superior, and anterior malleolar ligaments; however, these values were much lower for the anterior and superior incudal and stapedial annular ligaments.

Making a Middle Ear Model
Static modelling: Various methods have been used by scholars to create static models of the middle ear, beginning from serial histological sections from cadaveric temporal bones to three-dimensional (3D) modelling from CT images, as shown in Table 4. These can be used to make dedicated geometrical models for their utilization in middle-ear biomechanical research studies [10].

Technique Advantages Disadvantages
Physical model [10] The full-size physical model is an artificial representation of the tympanic membrane, the auditory ossicles, and three ligaments.
The structure of the model is made up of silicone for bony parts and silicone sheets for the tympanic membrane.
Reproduces the basic characteristics of a real human middle ear relatively accurately, also in terms of 3D effects [10].
Immobile model, cannot reproduce movements of the tympanic membrane and ossicular chain.
3D model [11] The mastoid X-rays are compared with temporal bone CT scans by synchrotron radiation, using fluorescence optical sectioning, magnetic resonance microscopy, and physical serial sections, and a 3D middle ear model is constructed [4].
Used in the making of design of mathematical models of parts of the ear and teaching models.
Requires expensive software for conversion of CT and MRI images into 3D images. Dynamic modelling: The creation of a dynamic model needs the incorporation of some extra parameters for the creation of the middle ear model. These include vibratory properties of the tympanic membrane, stress, and strain of ossicles and TM, measured by Young's modulus ( Table 5).

Technique Advantages Disadvantages
Finite element model [12] The object of interest is divided into numerous small simplified mesh elements. The applied forces and mechanical properties are depicted by functions defined over each and every element known as mesh particles and the mechanical response of the system as a whole is computed.
Takes into account the phase-shift moiré shape dimensions to accurately define the shape of the tympanic membrane.
Requires expensive software for conversion of CT images into finite element model.
Acoustic modelling [13] Formulation of a circuited lumped-element model of the adult middle-ear of human beings for biomechanics, according to the comparisons taken from measuring air-conduction information.
Incorporates the acoustic effects of the middle ear cavity, antrum, and aditus, as well as third-window effects, which are not included in any of the previously described models.
Requires highly skilled professional assistance for the development and analysis for working of the model.

Conclusions
Through this review article, we have attempted to analyze the different methods of middle ear modelling, which have been done in the form of physical, acoustic, finite element modelling, and 3D reconstruction using X-ray micro-CT.
These can be converted into finite element models for clinical applications to assess the properties in normal ears as well as to assess the status of the tympanum and ossicles and middle ear volume in diseased conditions.
It is also important to identify which method of modelling is the best to assess the hearing gain/loss after the ear surgeries. The best method would be a combination of 3D reconstruction with finite element modelling using high-resolution computed tomography scans of the temporal bones, for analysis of the ossicular chain and its various properties.

Conflicts of interest:
In compliance with the ICMJE uniform disclosure form, all authors declare the following: Payment/services info: All authors have declared that no financial support was received from any organization for the submitted work. Financial relationships: All authors have declared that they have no financial relationships at present or within the previous three years with any organizations that might have an interest in the submitted work. Other relationships: All authors have declared that there are no other relationships or activities that could appear to have influenced the submitted work.