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# Effect of Type I Thyroplasty Implant Location and Stiffness on Voice Production

Weili Jiang, Mahdi Sangbori, Liran Goren, Charles Farbos, Ephraim Gutmark, Xudong Zheng, Qian Xue

<figure><img src="image_2.png" /></figure>

## To cite this version:

Weili Jiang, Mahdi Sangbori, Liran Goren, Charles Farbos, Ephraim Gutmark, et al.. Effect of Type I Thyroplasty Implant Location and Stiffness on Voice Production. SB3C2024 Summer Biomechanics, Bioengineering and Biotransport Conference, Jun 2024, Lake Geneva, WI, USA, United States. hal-04629222

HAL Id: hal-04629222

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Submitted on 28 Jun 2024

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# EFFECT OF TYPE I THYROPLASTY IMPLANT LOCATION AND STIFFNESS ON VOICE PRODUCTION

W. Jiang (1), M. Sangbori (1), L. Oren (2), C. de Luzan (2), E. Gutmark (3), X. Zheng (1), Q. Xue (1)

(1) Department of Mechanical Engineering, Rochester Institute of Technology, Rochester, NY, USA

(2) Department of Otolaryngology-Head and Neck Surgery, University of Cincinnati, Cincinnati, OH, USA

(3) Department of Aerospace Engineering, University of Cincinnati, Cincinnati, OH, USA

## INTRODUCTION

Thyroplasty Type I (TT1) is a common procedure for treating unilateral vocal fold paralysis. The procedure involves creating a window in the thyroid cartilage, allowing for the placement of an implant within the paraglottic space. The objective is to improve voice outcomes by narrowing the glottal gap between the paralyzed and healthy folds, thereby reducing leakage and threshold pressure. Traditionally, surgeons manually carve the implant, determining its specifications, including location, stiffness, and shape, through a trial-and-error approach. The impact of these implant characteristics on voice outcome remains an ongoing research topic.

The TT1 implant offers a range of stiffness options, from soft materials like Gore-Tex to harder one like Silastic. Reddy et al. [1] found that, among these materials, Silastic rectangular implant consistently improved voice quality in canine laryngeal experiments. Cameron et al. [2] reported similar observation that stiffer implant enhances acoustics at similar flow rates. Interestingly, Cameron et al. [2] also noted that softer implants excel at low flow rates. Additional studies by Zhang et al. [3], [4] and Smith et al. [5] suggested that a soft implant, with material properties closely resembling real vocal fold, may be preferable for its consistent acoustic outcomes across various conditions. Wu and Zhang [6] pointed out that the impact of implant stiffness also depends on vocal fold stiffness and paraglottic tissue, highlighting the need to consider individual variations. Regarding implant shape, divergent or rectangular implants are more effective [7] [1], but Zhang et al. [3] argued that implant insertion depth and stiffness have a significantly greater impact on acoustic improvement than implant shape.

Regarding insertion location, the implant window is usually made at the vocal fold level and about 5 mm posterolateral to the midline of the thyroid cartilage [3]. Interestingly, clinical observations indicate that medialization in the infraglottal region tends to yield better voice outcome than medialization in the glottal region, although the exact

reason remains unknown. In a study by Zhang and Chhetri [8], local medialization using a 2mm-diameter wood stick on excised human larynges showed improved glottal closure and higher-order harmonic excitation in voice when performed at a more inferior location. They hypothesized that inferior medialization contributed to a thicker vocal fold, but the medial surface's shape, including thickness, wasn't quantified, leaving the hypothesis unconfirmed. Zhang et al. [4] provided detailed structural changes before and after implant insertion using magnetic resonance imaging on ex vivo human larynges. They observed increased vertical thickness and significant stretching of the vocal fold tissue in the coronal plane, leading to a decrease in thyroarytenoid (TA) muscle dimension in the medial-lateral direction. However, since phonation experiments were not conducted, the direct relationship between structural changes and the produced sound is unclear.

This study aims to investigate the impact of TT1 implant location and stiffness on vocal fold dynamics and voice outcomes using numerical methods, establishing a direct link among the implant characteristics, vocal fold pre-phonatory conditions, and sound characteristics. Our hypothesis is that, in comparison to glottal-level medialization, infraglottal medialization will create a slightly divergent pre-phonatory shape and a more substantial vertical stiffness gradient, potentially enhancing the produced sound. Through parametric simulations, we will thoroughly explore how implant vertical location and stiffness influence pre-phonatory conditions (shape and stiffness), vocal fold vibration patterns, and sound characteristics.

## METHODS

**Geometry model.** The geometric models of the larynx and implant were created from computed tomography scans of an ex-vivo canine larynx. Both the cartilages and vocal fold were reconstructed. The vocal fold was represented as a two-layer structure consisting of the cover and body layers (Figure 1(a)).
<figure><img src="image_3.png" /><figcaption>**Figure 1.** Vocal fold and the implant: (a) Geometric model (b) Indentation locations.</figcaption></figure>

**Material property.** Fiber reinforced anisotropic material model was employed for both the cover and body layers [9], [10]. To determine material parameters for the body layer, we matched the strain-stress relationship obtained from a canine TA muscle [11]. For the cover layer, material parameters were determined by matching the indentation measurements on the medial surface of a canine vocal fold. Neo-Hookean material model was applied for the cartilages and implant, with implant stiffness provided in Table 1.

<table><caption>**Table 1.** Implant parameters in parametric simulations.</caption><thead><tr><td>Parameter</td><td>Values</td></tr></thead><tbody><tr><td>Height</td><td>The same and half with the vertical height of the fold.</td></tr><tr><td>Location</td><td>From superior to inferior side of the vocal fold medial surface.</td></tr><tr><td>Stiffness</td><td>Four values from 5 to 1400 kPa.</td></tr></tbody></table>

**Simulation setup.** The simulation comprises two phases. In the initial phase, we conduct a pre-phonatory posturing simulation. Tissue deformation will be modeled using finite element method. The anterior-posterior (AP) insertion location is determined following [3], and the insertion depth is adjusted to ensure appropriate fold adduction. Additional conditions, such as height and vertical location, are outlined in Table 1. Once the implant is inserted, we will assess the shape and stiffness distribution of the medial surface. In the second phase, we will perform a fluid-structure interaction (FSI) simulation to capture vocal fold oscillation, glottal flow waveform, and sound generation [9], [12]. The glottal flow will be modeled by a one-dimensional reduced-order flow model [13].

The stiffness distribution would be obtained through numerical indentation following a prior work [14]. Six indentation locations are denoted in Figure 1(b), distributed along three AP locations. In each AP location, indentation will be performed at the superior and inferior sides of the medial surface. The local stiffness is calculated by $pR/\Delta x_{max}$, where $p = 2kPa$ is the uniform pressure, $R = 0.6mm$ is the radius of the circular indentation area, $\Delta x_{max}$ is the displacement at the center of the circle. The stiffness gradient (SG) is calculated by $G = (E_{inf} - E_{sup})/dy$, where $E_{inf}$ and $E_{sup}$ are the stiffness at inferior and superior sides, respectively, $dy = 0.2cm$ is the vertical distance between the two indentation locations.

## RESULTS

In the baseline case, we inserted an implant with a height matching the vertical height of the fold at the glottic level. The insertion depth was adjusted so that 80% of the medial surface was adducted to the midline. The implant stiffness is 120 kPa.

Figure 2 illustrates the impact of the implant on the pre-phonatory conditions of the vocal fold at the three AP locations. Figure 2(a) overlays the medial surface profile before (dash) and after (solid) the implant insertion. It shows that the implant effectively adducted the fold toward the midline with only a small leakage in the posterior side. The shape of the medial surface profile does not show a significant

difference before and after the implant inserted except that it became slightly more convergent in the posterior side. Vertical thickness is larger in anterior than in posterior side.

Figure 2(b) shows the SG along the three AP locations, measured between the superior and inferior sides of the medial surface as denoted in Figure 2(a). It shows that a stiffness gradient exists before the implant insertion, with the largest value around the mid-coronal location and the smallest value in the anterior side. With the implant inserted, SG increased in all three locations with the largest increase occurred in the anterior side.

<figure><img src="image_4.png" /><figcaption>**Figure 2.** Influence of implant on pre-phonatory status: (a) medial surface profile; (b) stiffness gradient (SG).</figcaption></figure>

## DISCUSSION

The preliminary results show that the numerical model is able to simulate the impact of TT1 implant on vocal fold deformation and stiffness distribution before phonation. The simulation results of the baseline case show that when the implant is 120kPa and inserted at the glottis level, it adducted the vocal fold with only small effect on the shape of the medial profile. It increased SG in the whole AP range with the largest increase at the anterior side. In subsequent simulations and analysis, we will change the implant height, vertical location and stiffness as described in Table 1. The effects of these parameters on vocal fold pre-phonatory deformation and stiffness will be investigated. Additionally, FSI simulations will be performed on the obtained pre-phonatory conditions and the relationships among implant characteristics, vocal fold pre-phonatory conditions, vocal fold vibrations and voice acoustics will be studied. We will discuss the results by comparing them with the experimental measurements [8,15,16].

## ACKNOWLEDGEMENTS

The research was funded by NIH Grant No. R01DC009435 from the National Institute on Deafness and Other Communication Disorders (NIDCD).

## REFERENCES

[1] Reddy, N et al., 10.21437/Interspeech.2022-11158, 2022.

[2] Cameron, B.H. et al., *Laryngoscope Investig Otolaryngol*, 5(1): 82-89, 2020.

[3] Zhang, Z. et al., *J Voice*, 29(2):230-235, 2015.

[4] Zhang, Z. et al., *PLoS One* 5(1): e0228464, 2020.

[5] Smith, S.L. et al., *Laryngoscope*, 130(9): 2192-2198, 2020

[6] Wu, L. et al., *J Biomech*, 149: 111483, 2023.

[7] Orestes, M. I. et al., *Laryngoscope*, 124(12): 2791-2796, 2014.

[8] Zhang, Z. et al., *J Acoust Soc Am*, 146(5):EL412-EL417, 2019.

[9] Geng, B. et al., *J Acoust Soc Am*, 150(2):1176-1187, 2021.

[10] Dhondt, G. The Finite Element Method for Three-Dimensional Thermomechanical Applications. Wiley, 2004.

[11] Hunter, E.J. et al., *J Appl Physiol*, 103(1):206-219, 2007.

[12] Movahhedi, M. et al., *JASA-EL*, 1(12):125203, 2021.

[13] Li, Z. et al., *J Biomech Eng*, 142(2), 2020.

[14] Geng, B. et al., *J Acoust Soc Am*, 141(4): EL351-EL356, 2017.

[15] Maddox, A. et al., *Laryngoscope*, 133(3):621-627, 2023.

[16] Oren, L. et al., *Eur Arch Otorhinolaryngol*, in print, 2024.