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# Computational Analysis of Vocal Fold Posturing in Asymmetric Laryngeal Muscle Activations

Mahdi Sangbori, Weili Jiang, Biao Geng, Xudong Zheng, Qian Xue

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Mahdi Sangbori, Weili Jiang, Biao Geng, Xudong Zheng, Qian Xue. Computational Analysis of Vocal Fold Posturing in Asymmetric Laryngeal Muscle Activations. 13th International Conference on Voice Physiology and Biomechanics (ICVPB), Jul 2024, Erlangen, Germany. hal-04664195

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Submitted on 31 Jul 2024

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# COMPUTATIONAL ANALYSIS OF VOCAL FOLD PPOSTURING IN ASYMMETRIC LARYNGEAL MUSCLE ACTIVATIONS

Mahdi Sangbori<fnref n="1" />, Weili Jiang<fnref n="1" />, Biao Geng<fnref n="1" />, Xudong Zheng<fnref n="1" />, Qian Xue<fnref n="1" />

**Keywords:** Vocal Fold Paralysis; Vocal Fold Posturing; Flow-Structure Interaction

**Abstract:**

**Introduction:**

The human voice is produced through intricate neuromuscular contractions of the intrinsic laryngeal muscles (ILM), establishing the prephonatory posture of the vocal folds [1]. Deficiencies in either the recurrent laryngeal nerve (RLN), superior laryngeal nerve (SLN), or both can result in ILM malfunction, leading to laryngeal paresis and paralysis [2]. The conditions mostly occur on one side, resulting in asymmetric activation of ILMs, which can cause changes in the shape of the prephonatory posture, a significant contraction in pitch range, asymmetry in vocal fold vibratory phase, and acoustic aperiodicity. These factors ultimately contribute to an overall decline in vocal quality [3]. Hence, it is essential to comprehend the role of asymmetric muscle activation and its influence on the shape and vibrational patterns of the vocal folds.

Several studies have sought to investigate the impact of symmetric intrinsic laryngeal muscle activation on vocal fold prephonatory shape, vibration, flow, and acoustics [4][5][6]. Other investigations have delved into the role of laryngeal paresis and paralysis, along with asymmetric muscle activation, aiming to understand their effects on vibration patterns, acoustics, and perception through numerical and experimental methods [2][7][8]. However, the understanding remains incomplete. In particular, a systematic understanding of the effect of asymmetric ILM activations on vocal fold dynamics is incomplete. In this article, we propose to use high-fidelity computer modeling technique to investigate 3D vocal fold posturing in symmetric and asymmetric muscle activations, focusing on the relationships among muscle activity, vocal fold tension, stiffness, and geometry.

**Methods:**

**Geometry model.** A three-dimensional model of human larynx was reconstructed from high-resolution MRI scans of a 74-year-old female (Fig. 1). The model takes into account both left and right sides of all laryngeal cartilages—the thyroid, arytenoid, and cricoid cartilages—as well as the intrinsic muscles (CT, TA, lateral cricoarytenoid (LCA), interarytenoid (IA), posterior cricoarytenoid (PCA) muscles). These muscles are segmented into bellies, comprising the vertical (CTv) and oblique components (CTo) of the CT, as well as the thyromuscularis (TAm) and thyrovocalis (TAv) of the TA. The activation of muscles on each side can be independently regulated, allowing for both symmetric and asymmetric muscle activations. The vocal fold is a layered structure, encompassing the cover layer, ligament, conus elasticus (CE), TA, thyroid epiglottis (TE) muscle, paraglottic space, and cricoid space.

<figure><img src="image_2.png" /><figcaption>Figure 1. The geometric model of the human larynx reconstructed from the MRI scans of the larynx of a 74-year-old female. (Left) The whole larynx model for posturing simulation. (Right) The vocal fold model for FSI simulation.</figcaption></figure>

**Material property.** A fiber-reinforced anisotropic material model [9] is employed to represent the passive material characteristics of the vocal fold cover, ligament, conus elastics and all the muscles. The model parameters for different tissues were optimized by aligning with experiment measurements. The active stress in muscles is modeled using the Hill-based contractile model. Neo-Hookean material is applied to cricoid, thyroid, and arytenoid cartilages, as well as the paraglottic space and cricoid space.

**Simulation setup.** Each simulation consists of two phases. In Phase 1, the posturing of the complete laryngeal model is simulated in response to muscle activations. The displacements of nodes on the anterior, posterior, and lateral surfaces of the vocal fold are extracted from the simulation results. These displacements serve as the boundary conditions for Phase 2. In this phase, an asymmetric activation ratio of the muscles is employed to represent both SLN and RLN paralysis conditions. Phase 2 exclusively simulates the flow-structure interaction (FSI) of the vocal fold, incorporating the displacement boundary conditions obtained in Phase 1. The medial, inferior, and superior surfaces of the vocal fold are allowed to move in response to aerodynamic loading. Tissue deformation is modeled using the finite

<footnote n="1">¹Department of Mechanical Engineering, Rochester Institute of Technology, Rochester, NY, USA</footnote>
element method, and glottal flow dynamics is modeled using the one-dimensional Bernoulli equation. The three-dimensional geometries for both phases are illustrated in Fig. 1.

**Results:**

The changes in vocal fold geometry resulting from both asymmetric and symmetric ILM activations will be examined in detail. Fig. 2 shows preliminary results, including the variations in vocal fold length and vertical thickness in response to symmetric CT and TA activations. The baseline scenario represents a fully adducted condition with zero CT and TA muscle activations. The vocal fold strain resulting from the isolated activation of TA and CT muscles align with typical values reported in previous studies, suggesting that our simulation result is within the physiological range. CT activation leads to vocal fold thinning, while TA activation results in vocal fold thickening. This influence of CT-TA activation on vertical thickness is in line with the prior study in the canine model [4] and corresponds to experimental observations [10]. Asymmetric ILM activations will be conducted to understand their impacts on vocal fold pre-phonatory geometry and tension as well as vibration patterns. Fig. 3 shows two preliminary simulations of asymmetric activations. In both simulations, the CT, TA, IA, and LCA are fully activated on the left side. In Fig. 3a, the activation level of LCA and IA was reduced to 50% while keep other muscles the same as the left side. In Fig. 3b, the activation level of TA was reduced to 50% while keep other muscles the same as the left side. The results indicate a slight left-right asymmetry in vocal fold shapes.

<figure><img src="image_3.png" /><figcaption>Figure 2. The variation of the vocal fold length and vertical thickness relative to the baseline condition in response to CT and TA activations.</figcaption></figure>

<figure><img src="image_4.png" /></figure>

<figure><img src="image_5.png" /><figcaption>Figure 3. Asymmetric activation of (a) LCA/IA and (b) TA muscles</figcaption></figure>

**Conclusions:**

A high-fidelity computer model of the human larynx, encompassing all cartilages and intrinsic laryngeal muscles on both sides, is reconstructed using MRI data. The model is used to quantify the 3D properties of vocal fold prephonatory posture during symmetric and asymmetric muscle activations representing SLN and RLN paralysis. The relationship between the asymmetric ratio of muscle activations and the asymmetric ratio of posture properties will be investigated. The FSI simulation is also employed to investigate the vibration patterns.

**Acknowledgements:**

The research was funded by NIH Grant No. 1R15DC019229.

**References:**

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