Mechanical Characterization of Vocal Fold Tissue: A Review Study
Introduction
Involving flow-induced vibrations in the head and neck, the human voice is one component in the production of speech. The primary source of voice production is self-sustained oscillations of the vocal folds,1 traditionally known as “vocal cords,” located within the larynx (Figure 1). Many instances of voice abuse can lead to benign or malignant voice disorders. It is estimated that 3–9% of the general population, including children and adults, suffers from a voice-related problem at any given point in time.2 Estimates are considerably higher for at-risk people, such as teachers and professional singers.
Phonotrauma is believed to be a result of high impact stresses between two colliding vocal fold membranes,3 although the inaccessibility of impact stress in vivo prevents verification of the hypothesis. Impact stress stands for the contact force per unit area of the colliding surface. The limited knowledge of voice biomechanics affects the success of clinical treatments, such as laryngoplastic surgeries.4 Moreover, the stiffness of the vocal fold tissue determines the wave motion within the mucosal layer.5 The efficiency of computational simulations of vocal fold vibrations is thus associated with the accuracy of the input mechanical properties.6 The study of tissue biomechanics in pathological states of the larynx may help in understanding their etiology. The tissue stiffness is believed to be the single measure of choice for many treatments.7
The dynamics of the glottal airflow, the geometry of vocal folds, and their biomechanical properties may define voice parameters, such as the fundamental frequency of phonation (FFP). Many studies have investigated the effects of geometry on vocal fold modal properties (eg, Cook and Mongeau8) and the effects of tissue biomechanics on vibratory motion (eg, Berry and Titze9). Computational models of the elastic deformation of the mucosa require proper constitutive equations that relate tissue strain to the associated mechanical stress.10 In this review, different approaches used to determine the mechanical properties of the vocal fold tissue are classified. The scale and type of tissue deformation in each testing method constitute the criteria of the classification. The present work intends to review the existing mechanical testing methods and constitutive models to help researchers to choose better characterization methods to evaluate tissue mechanical properties.
This article first covers an overview of vocal fold biomechanics then examines the mechanical testing methods used to characterize the vocal fold tissue. The methods are reviewed considering the type of loading and the scale of deformation. From a structure-function perspective, vocal folds act at the macroscale (ie, millimeter range). The vocal fold ligaments elongate 5–15 mm, and their mucosas oscillate with an amplitude of 0.1–0.5 mm.11 The ultrastructure, including the extracellular matrix (ECM), mostly interacts at the micrometer scale, known as the mesoscale. The reader is referred to Goodyer et al12 to review the functionality and applicability of the testing methods and the related instrumentation. The relevant constitutive models of the human and animal vocal folds, which fulfill data analysis needs, are presented and categorized from the perspective of their applications in computational simulations. A discussion regarding the range of values reported as vocal fold stiffness will conclude this review.
Section snippets
Anatomy and physiology of the vocal folds
Vocal folds are a pair of soft mucosal membranes stretched across the larynx located between the trachea and the pharynx, as depicted in Figure 1. Each fold has a length of 10–20 mm along the anterior-posterior direction, a length of 8–12 mm along the medial-lateral direction, and a thickness of 3–10 mm.1 The anatomy of the human vocal fold can be divided into the epithelium, lamina propria, and vocalis muscle, as demonstrated by a coronal cross section in Figure 2. The lamina propria is mainly
Uniaxial traction testing
The goal of uniaxial traction testing is to determine the normal (ie, Young) modulus of the vocal fold tissue along the anterior-posterior direction. Traction testing involves pulling the tissue along a predefined straight direction, as depicted in Figure 4. Although biaxial testing is ideal for biological soft tissues,33 performing multiaxial testing on dissected vocal folds is very challenging because of their large length-to-width ratio and structural heterogeneity. The vocal folds can
Traction testing
A miniaturized setup was built to perform unidirectional traction testing on rat vocal folds with an effective length of nearly 1 mm.64 A custom-designed acrylic-plastic clamp attached to a load sensor of 100 μN resolution and a linear actuator of 1 μm resolution was molded. The average Young modulus was reported as 50 kPa. The methodology suffered from slippage of tissue samples under the clamp and edge effects on uniaxial tension conditions.
Indentation testing
The first indentation-like experiment on the vocal
Lumped-mass models
There are two approaches to simulate the biomechanical behavior of the vocal folds: continuum models and discrete or lumped-mass systems. The latter group provides low-cost computations to understand the physics of phonation. They seem unsuitable for practical applications because of the lack of direct correspondence to directly measurable properties of the vocal folds. Cveticanin29 reviewed the lumped-mass models of vocal fold phonations; thus, they are excluded from the current article.
Continuum models
Concluding remarks
A wide range of values for the linear elastic modulus of the vocal fold tissue has been reported in the literature (Figure 4). The traction testing elucidated a Young modulus of 15–40 kPa for 15% strain (in vitro length reference), whereas the indentation modulus was obtained as 2–5 kPa. The rheometry data led to 0.5–2 kPa (assuming incompressibility of the tissue) for low frequencies. The unidirectional traction testing yields the modulus along the axis of anisotropy, in which the vocal folds
Acknowledgments
I dedicate this work to Prof Luc Mongeau (Department of Mechanical Engineering, McGill University) who was a great source of inspiration for my research on biomechanics of vocal folds.
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