gms | German Medical Science

Dreiländertagung D-A-CH
24. Wissenschaftliche Jahrestagung der Deutschen Gesellschaft für Phoniatrie und Pädaudiologie e. V.

Deutsche Gesellschaft für Phoniatrie und Pädaudiologie e. V.

28. - 30.09.2007, Innsbruck, Österreich

Directionality of vocal fold elasticity

Vortrag

  • corresponding author presenting/speaker A.-K. Licht - Department of Voice, Speech and Hearing Disorders, University Medical Centre Hamburg-Eppendorf, Germany
  • author E. Goodyer - The Centre for Computational Intelligence - Bioinformatics Group, DeMontfort University, Leicester, UK
  • author F. Müller - Department of Voice, Speech and Hearing Disorders, University Medical Centre Hamburg-Eppendorf, Germany
  • author K. Püschel - Institute of Forensic Medicine, University Medical Centre Hamburg-Eppendorf, Germany
  • author M. Hess - Department of Voice, Speech and Hearing Disorders, University Medical Centre Hamburg-Eppendorf, Germany

Deutsche Gesellschaft für Phoniatrie und Pädaudiologie. Sektion Phoniatrie der Österreichischen Gesellschaft für HNO-Heilkunde, Kopf- und Halschirugie. Schweizerische Gesellschaft für Phoniatrie. Dreiländertagung D-A-CH, 24. Wissenschaftliche Jahrestagung der Deutschen Gesellschaft für Phoniatrie und Pädaudiologie e.V.. Innsbruck, Österreich, 28.-30.09.2007. Düsseldorf: German Medical Science GMS Publishing House; 2007. Doc07dgppV06

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Veröffentlicht: 28. August 2007

© 2007 Licht et al.
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Abstract

Introduction: The study of the visco-elastic characteristics of the vocal fold tissue is important for understanding the biomechanical properties of this tissue and their effect on vocal fold oscillation. The set goal of this work was to determine directional variations of the elastic properties of the vocal fold.

Materials and methods: 14 excised human larynges were split along the sagittal plane, mounted without tension, and measured each semilarynx using a Linear Skin Rheometer (LSR). The LSR-probe was attached primarily to the vocal fold epithelium mid-membranous between the anterior commissure and vocal process using a round suction catheter of 2.7 mm outer diameter. The elastic response was measured by applying stress at 5 different angles. Additional measurements were made in a similar procedure for epithelium, lamina propria, ligament and muscle. The successive exposure of different layers was correlated with histology.

Results: The distribution of elastic pliability in different directions will be demonstrated.

Conclusion: The range of shear modulus derived using this method is comparable to that obtained by our team in-vitro in a previous published study and by other research groups obtained in-vivo or in-vitro. In addition to the confirmation of previous results we highlight an anisotropic nature of the vocal fold tissue.


Text

Introduction

The biomechanical properties of the vocal fold have been reported in many studies observing solely excised laryngeal tissue. A better understanding of the oscillatory nature is only possible if we quantify the biomechanical properties of the tissue within its anatomical context and orientation. The authors have announced the successful development of a new, easy to use instrument to measure vocal fold elasticity in-vivo, the Laryngeal Tensiometer [1]. Preliminary results from 8 patients were presented in 2007 [2]. Parallel to the in-vivo studies we have used a Linear Skin Rheometer to obtain in-vitro biomechanical data from 34 excised human larynges. The methods employed were a direct application of a shear strain and the use of an indentometer. The preliminary results of the first 20 freshly excised cadaver larynges were published in 2007 [3]. The purpose of this work was to determine layer dependent and directional variations of the elastic properties of the vocal fold, in short the anisotropic behavior.

Materials and methods

Human cadaver larynges were obtained within 3 days (mean) post mortem. 14 larynges were evaluated in the following groups: 7 adult males (age range, 47–76 y) and 7 adult females (age range, 47–92 y). The larynges were extracted and split along the sagittal plane and mounted without tension. The LSR probe was attached primarily to the vocal fold epithelium mid-membranous between the anterior commissure and vocal process using suction catheter of 2 mm ID, and the results analyzed using the mathematical transformations detailed in our earlier publications . In order to quantify the anisotropic behavior the elastic response was measured at 5 different angles of applied stress at the complete vocal fold. Additional measurements were made in a similar procedure for lamina propria, ligament and muscle. The successive exposure of different layers was correlated with histology.

Results

Shear modulus

The mean reading for the shear modulus in the transverse direction is 1078 Pascals, with a range of 436 to 3398 Pascals.

Anisotropic nature of the vocal fold

The key advantage to measuring the biomechanical properties of the vocal fold using intact larynges is that it is possible to differentiate the elastic response with respect to the direction of the applied stress.

Relative shear modulus and angle of applied stress

From 5 different angles (0, 45, 90, 135 and 180 degrees) and from 2 different sides (left, right), values for the shear modulus were obtained. Figure 1a [Fig. 1] shows the overall results obtained only at the mid-position of the non-dissected vocal folds. The majority of larynges exhibited higher shear moduli if values were obtained at 180 degrees and 0 degrees. The median value is declining obtained at 135 and 45 degrees to a minimum at 90 degrees.

Relative shear modulus and layer

The relative shear modulus with respect to different layers of altogether 14 larynges shows an upwards tendency towards deeper layers (Figure 1b [Fig. 1]).

Ratio of elasticity in orthogonal directions

The value is always less than 1 what means a greater stiffness in longitudinal direction. This applies for all layers (Figure 2 [Fig. 2]). There is a slight upward tendency towards deeper layers in some measurements, which is not present in the median curve. In summary we can say the vocal fold requires all layers for a strong anisotropic behavior.

Discussion

The results for shear modulus of the vocal fold are in the same order of magnitude as found with our previous published results, and the results published by other research groups obtained either in-vivo or with whole excised larynges [4], [5], [6], [7], [8]. To the authors’ knowledge, a systematic study of the directional-specific mechanical response of human vocal fold is not available. By contrast, evaluation of mechanical properties of other soft tissues, for example blood vessel walls, revealed the presence of directionality [9]. But anisotropic mechanical properties for tubular specimens like blood vessels are not comparable with planar specimens like vocal fold, not to mention the diverse function. What is behind all this? With respect to the different functions of the vocal fold (mucosal wave and pitch adjustment) directionality could make sense: Different functions require different biomechanical properties. The vocal fold tissue shows higher pliability in the transverse direction. In addition, the trend is the whole structure is anisotropic, decreasing as you go down the layers. In contrast to these findings the commonly used phonosurgical materials inserted to restore appropriate shape and pliability show a clear isotropic material behavior.


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