gms | German Medical Science

Measurement and analysis of vocal fold oscillation is necessary to evaluate and refine current theories of voice production. In recent years, many in vivo studies have been performed using digital high-speed imaging in combination with an endoscope to study vocal fold oscillation [Ref. 2]. These studies have helped to improve our understanding of normal and pathological voice production [Ref. 3]. Since endoscopy only allows a superior view of the vocal folds, it was not possible to do a complete analysis of the medial surface dynamics of the vocal folds, where vocal fold opening and closing take place and where the sound is generated within the glottis [Ref. 4]. In contrast, direct imaging the medial surface would reveal the origin of the mucosal wave (e.g., the mucosal upheaval) and its propagation along the medial surface of the folds. Different experimental approaches have been used to analyze the medial surface dynamics of the vocal folds including computational models [Ref. 5], and laboratory hemi-larynx techniques which have utilized either canine or human excised larynges [Ref. 4], or in vivo canine larynges [Ref. 6].

An excised human larynx was obtained from the autopsy unit of the UCLA Medical Center. The creation of a hemi-larynx required the removal of one (i.e., right) vocal fold (Figure 1 [Fig. 1]). To modify the degree of vocal fold adduction, a suture pierced the arytenoid cartilage at the muscular process, where varying weights (10 g, 20 g, 50 g) were attached (Figure 1 [Fig. 1]). Another suture with constant weight (10 g) was attached anteriorly at the thyroid cartilage, to elongate the vocal fold (Figure 1 [Fig. 1]). The trachea was mounted over a stainless steel cylindrical tube. A glass plate was attached at the top of the tube (Figure 1 [Fig. 1]). A rubber glove was used to mount the hemi-larynx against this plate. Vacuum grease was applied between the anterior and posterior regions of the larynx and the glass plate to prevent air leaks.

To track the movements of the medial surface of the vocal fold, 30 surgical micro-sutures with a diameter of 0.034 mm were mounted. They were arranged at 5 vertical rows with 6 sutures per row. The distance between the sutures was between 1.7 mm and 2 mm. To avoid any disturbance of the natural dynamics of the vocal fold, the sutures were positioned to penetrate only the mucosal epithelium.

Vibrations of the vocal fold were induced by passing a constant airflow (200, 320, 400 ml/s) up through the trachea and through the area between the glass plate and the vocal fold (i.e. the hemi-glottis). The vibrations were imaged with a high-speed digital camera at 4000 frames/s and a pixel resolution of 512x512 (Figure 1 [Fig. 1]). Three 150W lamps served as light sources.

A right-angle prism was place at the glass plate, opposite to the vocal fold, to simulating two camera views, a necessary condition to compute three-dimensional coordinates from a two-dimensional recording [Ref. 1], see Figure 1 [Fig. 1]. For later calibration, a brass cube was glued to the glass plate superior to the vocal fold. The tracking of suture positions and the computation of the physical coordinates was performed using a previous described linear transformation method [Ref. 1]. The three-dimensional coordinates were smoothed using empirical eigenfunctions [Ref. 4].

Nine recordings with different stimulation levels were performed (i.e., varying air-flow and muscular process pull). The investigated time window was 100 ms or approximately 12-14 oscillation cycles. The fundamental frequency was between 115 Hz and 140 Hz. The subglottal pressure ranged between 2.17 kPa and 3.17 kPa. The computed maximal displacement and velocity values did not change significantly across the different phonatory conditions. The averaged values are shown in Table 1 [Tab. 1] which coincide with theoretical assumptions [Ref. 5] and former studies [Ref. 4]. Within the upper medial part the highest dynamical behavior was computed. The lowest dynamical range was observed in the inferior part of the vocal fold and for the most anterior and posterior mounted sutures, see Figure 2 [Fig. 2].

Vocal fold dynamics were investigated across different phonatory conditions. The dynamics were quantified and visualized across the entire medial surface of the vocal folds, and parts of the superior surface (Figure 2 [Fig. 2]). Variation in adductory force and applied air-flow did not yield unique changes in the investigated dynamics. In the future, higher forces of adduction and elongation should be applied (e.g., up to 150 g), which may reveal more significant differences between the phonatory conditions.

However, dynamics across recordings were highly correlated and areas of different behavior and vibrational amplitudes could be identified across the vocal fold surface (Figure 2 [Fig. 2]). The longitudinal displacements were found to be half the size of the vertical displacements (Table 1 [Tab. 1]). Further, the lateral displacements were almost double the size of the vertical displacements (>1 mm), which shows the importance to investigate and include the vertical displacements of the vocal folds in further mathematical studies.