Elsevier

Medical Engineering & Physics

Volume 33, Issue 9, November 2011, Pages 1136-1146
Medical Engineering & Physics

Effect of anterior tympanomeatal angle blunting on the middle ear transfer function using a finite element ear model

https://doi.org/10.1016/j.medengphy.2011.05.005Get rights and content

Abstract

The anterior tympanomeatal angle (ATA) blunting is clinically defined as a certain degree of the ATA obliteration due to excessive fibrous tissue formation, which is a relatively common complication of external auditory canal (EAC) related operations. The aim of this study was to examine the effect of ATA blunting on the middle ear transfer function using a finite element (FE) model. Results showed that the displacements at the tympanic membrane (TM), at the manubrium and at the stapes footplate, and also the ratio of stapes footplate velocity to the sound pressure in the EAC were decreased to various degrees from ATA blunting of Grades 1–4. This was more significant with TM thickening at the frequencies below 3.2 kHz, particularly in Grades 3 and 4 when analyzing the anterior region of the TM. The phase differences of TM and stapes footplate increased with the ATA blunting from Grades 1 to 4 in relation to normal ATA. It is noteworthy that the vibration mode of the malleus does not show obvious change, compared to the displacement reduction at the TM with ATA blunting Grades 1–4. These results suggest that FE analysis of ATA blunting effect appears to be effective.

Introduction

The tympanic membrane (TM), located at the medial end of the external auditory canal (EAC), collects the sound energy and vibrates the middle ear ossicles, which in turn excite the inner ear. The TM is an irregular round and slightly cone-shaped plane, with its apex pointing medially to the tip of the handle of malleus. In the adult ear, along the axis of the handle of malleus, the vertical diameter of the TM is from 8.5 to 10 mm, the horizontal diameter is from 8 to 9 mm, and the TM is approximately 0.1 mm thick [1]. It is angled at approximately 140° with respect to the superior wall of the external auditory canal. Anatomically, the antero-inferior wall of the EAC is 6 mm longer than its postero-superior wall, so that the TM is inclined from supero-lateral to infero-medial [2]. Consequently, the EAC and TM form the anterior tympanomeatal angle (ATA) antero-inferiorly with an acute angle of 45–50° [3] (Fig. 1a).

The effective TM vibration area is approximately 20 times greater than the oval window area, whereas the length of the malleus is 1.3 times longer than the long process of the incus. When the acoustical energy encounters the TM, through the ossicles, the sound pressure at the stapes is increased. Therefore, the TM plays an important role in the middle ear transfer function. Disruption to the TM structure will cause a reduction of the efficiency of the TM transfer vibration, resulting in a conductive hearing loss [4].

Pathological changes in the TM are commonly seen in Ear, Nose & Throat (ENT) clinics. With a decrease in the vibration area of the TM, the area ratio decreases, and consequently the effective sound transfer function of middle ear is likely to be compromised. For example, in the case of TM perforation caused either by trauma or chronic otitis media, conductive hearing loss occurs because less acoustical energy is transmitted to the inner ear. In cases with severe pathological changes to the TM, surgical treatment is necessary in order to improve hearing sensitivity by reconstructing the TM structure (called tympanoplasty). However, the surgical procedure is complicated, and several serious complications may occur. Of these, excessive fibrous tissue formation in the ATA region results in the blunting of the ATA, which is caused by (1) incorrect placement of EAC flaps on the bony canal wall in the ATA region; (2) poor wound healing or (3) inappropriate graft packing near the ATA which retards the healing process. Clinically, the blunting of the ATA is defined as a certain degree of ATA obliteration due to the thickness of the anterior part of TM. It is a relatively common complication of EAC-related operations with an prevalence of about 1.3% [5]. Fig. 1 shows examples of four grades of ATA blunting, indicating its severity.

Various studies have shown that the normal structure of the ATA is important in terms of the middle ear transfer function. For example, Tos's study showed that various degrees of ATA blunting could compromise the middle ear transfer function [6]. Moreover, Wigand [7] also showed that the effective vibration area of the TM was reduced in patients with a blunted TM, and consequently its acoustical energy transfer function was impaired. In some cases of severe ATA blunting, a large area of the anterior part of the TM is involved causing moderate to severe conductive hearing loss- and consequently, revision surgery for ATA reconstruction is necessary.

In views of the fine structure of this region, it is difficult to directly measure the relationships between any change in the ATA structure and its effect on middle ear function Therefore, an appropriate method needs to be used to examine the influence of the ATA blunting on the middle ear transfer function in order to minimize the conductive hearing loss resulting from such surgical complications.

Finite element (FE) analysis is characterized by simulation of any complex structures and their functions, and this theoretical method has been widely employed over the last 15 years to simulate the middle ear structure and calculate its transfer function [8], [9], [10], [11], [12]. For example, the human middle ear FE models created by Wada et al. [13] and further modified by Koike et al. [8], and the FE model developed by Gan et al. [9], [14], [15] have been used to investigate middle ear transfer functions in a variety of pathologies. In the studies by Gan et al. [9] and Koike [8], the thickness of the TM was also simulated using a FE middle ear model. Their results showed that with increasing thickness of the TM, the middle ear transfer function decreased at the low frequencies. Thus, FE analysis of the structure and function of the ATA appears to be a useful tool to simulate the condition and consequently calculate its effects. However, their FE model made no mention of the combined influence of local thickness of the TM and of the EAC on the middle ear transfer function, such as in ATA blunting.

To our knowledge, there have been no investigations simulating ATA blunting and examining its effects on the middle ear transfer function using the FE method. The aim of the present study was to develop a middle ear FE model using Micro-CT scanning data of the temporal bone, including the EAC, TM, middle ear cavity, ossicles, incudo-malleolar joint, incudo-stapedial joint, and ligaments/tendons. We consequently simulated four ATA blunting conditions in order to explore their effects on the middle ear transfer function.

Section snippets

The development of FE middle ear model

In this study, a temporal bone from a 74-year-old man with no history of otological disorders was used to establish a geometric model. The temporal bone was scanned using a Micro-CT system (Institute of High Energy Physics, Chinese Academy of Sciences). The slice thickness was 13 μm; tube voltage was 80 kV as 188 μA. All the images were reconstructed using software developed by Institute of High Energy Physics (Chinese Academy of Sciences), and a 2048 × 2048 matrix was established. All matrix data

The working FE model validation

The working FE model developed in the present study was evaluated and validated by comparing it with published experimental data. We selected appropriate undetermined parameters [18], [27] and compared the displacements and phases of the umbo and of the stapes footplate calculated using our working model with the experimental data obtained from human temporal bones [28]. In the present study, a uniform sound pressure of 90 dB was applied to the lateral side of the TM, and the displacement and

Discussion

The aim of the present study was to provide information on the effect of ATA blunting on the middle ear transfer function using a middle ear FE model. To develop the geometric model of the middle ear, the traditional histological method is unlikely to provide the 3D anatomical information of the ligaments and tendons of the middle ear easily and accurately due to problems with the sample preparation and the sophisticated procedures needed. Consequently inaccurate results would be inevitable [18]

Conclusion

In conclusion, we developed FE models of four degrees of ATA blunting based on the pathological grades. Our results suggest that this method of investigating the effects of ATA blunting on the middle ear transfer function using a FE model analysis appears to an be effective and valid approach. The displacements at the TM, manubrium and at the stapes footplate, together with the SVTF, were decreased dramatically in conditions of ATA blunting, particularly in the severe cases (e.g., G3 and G4 ATA

Conflict of interest

This is to confirm that we have no conflict of interest to declare for publishing this paper.

Acknowledgements

We would like to thank Dr. Sheng Li, Dalian University of Technology and Dr. Cunfeng Wei, Institute of High Energy Physics, Chinese Academy of Sciences who assisted with the experiments. We also would like to thank the editor and three anonymous reviewers for their helpful suggestions. This work was supported by a grant from the 2010 Beijing Higher Education International Joint Graduate Training Base.

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