Assessment of local vocal fold deformation characteristics in an in vitro static tensile test

Abstract

Voice quality is strongly dependent on vocal fold dynamics, which in turn are dependent on lung pressure and vocal fold biomechanics. Numerical and physical models are often used to investigate the interactions of these different subsystems. However, the utility of numerical and physical models is limited unless appropriately validated with data from physiological models. Hence a method that enables analysis of local vocal fold deformations along the entire surface is presented. In static tensile tests, forces are applied to distinctive working points being located in cover and muscle, respectively, so that specific layer properties can be investigated. The forces are directed vertically upward and are applied along or above the vocal fold edge. The resulting deformations are analyzed using multiple perspectives and three-dimensional reconstruction. Deformation characteristics of four human vocal folds were investigated. Preliminary results showed two phases of deformation: a range with a small slope for small deformations fading into a significant nonlinear deformation trend with a high slope. An increase of tissue stiffness from posterior to anterior was detected. This trend is more significant for muscle and in the mid-anterior half of the vocal fold.

Figure 1

Left: Split view by the prism of a vocal fold with sewn position markers and calibration cube. The pulling markers are sewn above the vocal fold edge and the first marker is pulled. The forces are induced via strands to the fold. Right: 3D reconstruction of the vocal fold surface on basis of the reconstructed position markers of the split view (left). The circle indicates the pulled marker (here M1), the arrow the induced force direction. The boundary points have been extrapolated and are necessary for post processing (Ref. 9). The marker and boundary points are connected by patches, thus forming the reconstructed vocal fold surface.

Figure 2

Force vs normalized deformation for experiments E1 to E4. The symbols represent the deformation data of the pulled sutures M1 to M5. The graphs are fitted [f(x) = a·(e^b·x – 1)]. The vertical line marks the boundary between range I that is characterized by a small slope and range II that presents a nonlinear deformation characteristic with a high slope. All deformations are normalized to the maximum measured deformation relating to experiments E1 to E4.

Figure 3

Deformation vs marker position. The deformations are normalized to the maximum measured deformation relating to experiments E1 to E4. The marker positions are given in distance from the vocal process normalized to the vocal fold length, 100% match with the anterior commissure. The plots are separated for the applied forces. The symbols represent the data measured in experiments E1 to E4.

Figure 4

Stiffness vs normalized deformation for experiments E1 to E4. The symbols represent the local stiffness computed by the measured data at M1 to M5. The graphs show the stiffness computed by the values of the fitted lines in Fig. 2. All deformations are normalized to the maximum measured deformations in the particular experiments E1 to E4.