Control of Pre-phonatory Glottal Shape by Intrinsic Laryngeal Muscles

Abstract

Objectives: Surgical manipulations to treat glottic insufficiency aim to restore the physiologic pre-phonatory glottal shape. However, the physiologic pre-phonatory glottal shape as a function of interactions between all intrinsic laryngeal muscles (ILMs) has not been described. Vocal fold posture and medial surface shape were investigated across concurrent activation and interactions of thyroarytenoid (TA), cricothyroid (CT), and lateral cricoarytenoid/interarytenoid (LCA/IA) muscles.

Study design: In vivo canine hemilarynx model.

Methods: The ILMs were stimulated across combinations of four graded levels each from low-to-high activation. A total of 64 distinct medial surface postures (4 TA × 4 CT × 4 LCA/IA levels) were captured using high-speed video. Using a custom 3D interpolation algorithm, the medial surface shape was reconstructed.

Results: Combined activation of ILMs yielded a range of unique pre-phonatory postures. Both LCA/IA and TA activation adducted the vocal fold but with greater contribution from TA. The transition from a convergent to a rectangular glottal shape was primarily mediated by TA muscle activation but LCA/IA and TA together resulted in a smooth rectangular glottis compared to TA alone, which caused rectangular glottis with inferomedial bulging. CT activation resulted in a lengthened but slightly abducted glottis.

Conclusions: TA was primarily responsible for the rectangular shape of the adducted glottis with synergistic contribution from the LCA/IA. CT contributed minimally to vocal fold medial shape but elongated the glottis. These findings further refine laryngeal posture goals in surgical correction of glottic insufficiency.

Level of evidence: NA, Basic science Laryngoscope, 133:1690-1697, 2023.

Keywords: intrinsic laryngeal muscles; larynx; pre-phonatory posture; vocal fold medial surface.

Figure 1.

Experimental setup and generation of reconstructed surface and parameters. A: Calibration plate. B: Marked right hemilarynx with prism-generated stereoview. C: Identified landmarks in custom software of vocal fold. D: Reconstructed surface with prism. E: Reconstructed surface with coronal and horizontal section planes marked. F: Coronal sections generated across anterior, middle, and posterior vocal fold length. G: Axial sections generated across superior, middle, and inferior vocal fold medial surface.

Figure 2.

Effects of increasing LCA/IA activation at low levels of CT and TA (Level 1). A: LCA 2, B: LCA 3, C: LCA 4). D: Coronal sections at the posterior vocal fold with increasing LCA activation (CT 1, TA 1). LCA activation resulted in adduction of the posterior vocal fold (A-C) without change in the medial surface shape (D). Colormap demonstrates distance from vocal fold to prism.

Figure 3.

Effects of increasing TA activation at low levels of CT and LCA (level 1). A: TA 2, B: TA 3, C: TA 4, D: Coronal sections at the mid-membranous vocal fold. TA activation resulted in adduction at the mid-membranous vocal fold (A-C) and transition to a rectangular glottal shape (D).

Figure 4.

Effects of increasing TA activation at lower and higher LCA activations. A: LCA 1, B: LCA 4. Inferomedial bulging apparent with higher TA activation was diminished with higher LCA activation.

Figure 5.

Effects of increasing CT activation at high TA activation (Level 4) as a function of increasing LCA activation. A: LCA 1, B: LCA 2, C: LCA 3. CT activation at low LCA resulted in abduction of the superior edge of the vocal fold and the formation of a slightly divergent glottis (A, B). This effect was absent at increased LCA activation (C).

Figure 6.

Interactions between CT and TA at higher LCA activation (LCA 4). Increasing TA activation resulted in mid-membranous bulging, while increasing CT activation showed no effect on medial surface contour. Colormap demonstrates distance from vocal fold to prism.