Three-dimensional posture changes of the vocal fold from paired intrinsic laryngeal muscles

Abstract

Objectives/hypothesis: Although the geometry of the vocal fold medial surface affects voice quality and is critical in the treatment of glottic insufficiency, the prephonatory shape of the vocal fold medial surface is not well understood. In this study, we activated intrinsic laryngeal muscles individually and in combinations, and recorded the temporal sequence and precise three-dimensional configurational changes of the vocal fold medial surface.

Study design: In vivo canine hemilarynx model.

Methods: A hemilaryngectomy was performed in an in vivo canine model and ink was used to mark the medial surface of the in situ vocal fold in a grid-like fashion. The thyroarytenoid (TA), lateral cricoarytenoid (LCA), cricothyroid (CT), and posterior cricoarytenoid (PCA) muscles were stimulated individually and in combinations. A right-angle prism whose hypotenuse formed the glottal midline provided two distinct views of the medial surface for a high-speed digital camera. Image-processing package DaVis (LaVision Inc., Goettingen, Germany) allowed time series cross-correlation analysis for three-dimensional deformation calculations of the vocal fold medial surface.

Results: Combined TA and LCA activation yields an evenly adducted rectangular glottal surface. Addition of thyroarytenoid to cricoarytenoid adducts the vocal fold from inferior to superior in a graded fashion allowing formation of a divergent glottis. Posterior cricoarytenoid has a bimodal relationship with thyroarytenoid favoring abduction. Cricothyroid and lateral cricoarytenoid yield unique glottal postures necessary but likely not conducive for efficient phonation.

Conclusions: Understanding the three-dimensional geometry of the vocal fold medial surface will help us better understand the cause-effect relationship between laryngeal physiology and phonation.

Level of evidence: NA Laryngoscope, 127:656-664, 2017.

Keywords: Larynx; canine; intrinsic laryngeal muscle; pre-phonatory posture; vocal fold; voice.

Fig. 1

Glottal shape for the interaction between maximal TA activation and graded LCA activation. Columns correspond to temporal sequence of medial surface shape changes following ILM stimulation starting at the left-most column, 0 ms, rest. Each subsequent column is then some time interval later as denoted by the title above each column. Total recorded duration was 50 ms. Each row corresponds to a particular ILM combination. All rows have full TA activation (grade 5) but LCA grade varies by row with grade 0, or no LCA, for row 1 and increasing LCA grades for subsequent rows as denoted by the title beside each row; ‘LCA 0’ corresponds to LCA activation grade 0. Each plot is a color-coded contour plot of the vocal fold medial surface. Left edges is posterior (vocal process), right edge is anterior (anterior commissure), inferior edge is inferior, superior edge is superior. Colorbar denotes surface height or glottal width; the distance from the vocal fold medial surface to the glottal midline; midline is denoted as ‘0’. Each subsequent figure is designed in this exact same format except a different muscle with be held at maximum stimulation while another muscle is added in a graded fashion. TA provides robust near complete adduction of the mid membranous vocal fold (96%, row 1). This level of adduction is maintained with combined graded LCA activation. Further, TA shortens the vocal fold by 5% and thickens it in the vertical dimension by 8%. Thickening is more evenly distributed in the glottal anteroposterior axis with LCA activation and adduction is overall more complete for the entire medial surface. Posterior glottal closure is seen for LCA grades 3-6 and is observed as early as 20 ms. Final posture is rectangular with full TA and LCA activation. (inf – sup = inferior to superior, LCA = lateral cricoarytenoid, ms = millisecond, post – ant = posterior to anterior, TA = thyroarytenoid).

Fig. 2

Glottal shape for the interaction between maximal LCA activation and graded TA activation. Design layout is same as Fig.1 except now LCA is held at maximum stimulation for all rows (grade 6), while TA is added in a graded fashion from grade 0 (‘TA 0’, row 1) to grade 5 (‘TA 5’, row 6). Increased adduction is apparent as early as 13 ms for TA stimulation grade 3 and above. TA increases overall mid-point adduction from 82% with LCA alone to 95% with full TA grade 5 activation. LCA alone (row 1) results in 3.5% vocal fold shortening and a 4-7% vertical height narrowing. Added TA shifts vertical height thickening to the mid-to-anterior vocal fold with increased grade of activation. (inf – sup = inferior to superior, LCA = lateral cricoarytenoid, ms = millisecond, post – ant = posterior to anterior, TA = thyroarytenoid).

Fig. 3

Glottal shape for the interaction between maximal TA activation and graded CT activation. Design layout is same as Fig.1 except now TA is held at maximum stimulation for all rows (grade 5), while CT is added in a graded fashion from grade 0 (‘CT 0’, row 1) to grade 6 (‘CT 6’, row 7). CT is a slower acting ILM and as such duration of recording was extended to 117 ms to capture complete posture change. TA maintains a 95% vocal fold adduction irrespective of CT activation. The effects of CT appear to be evident beginning at grade 3 and a latency of 83 ms. TA shortens the vocal fold by 11% while CT activation neutralizes this effect. TA alone thickens the anterior vocal fold by 18%. With maximum CT added activation the anterior medial surface is net 20% thinner than baseline in the vertical dimension. (CT = cricothyroid, inf – sup = inferior to superior, ms = millisecond, post – ant = posterior to anterior, TA = thyroarytenoid).

Fig. 4

Glottal shape for the interaction between maximal CT activation and graded TA activation. Design layout is same as Fig.1 except now CT is held at maximum stimulation for all rows (grade 6), while TA is added in a graded fashion from grade 0 (‘TA 0’, row 1) to grade 5 (‘TA 5’, row 6). CT activation alone (row 1) results in 25% abduction, 8% lengthening, and 15% narrowing of the mid cord vertical height. TA activation provides dramatic 95-98% adduction beginning at grade 1. CT combined with TA grades 1-3 yields preferential adduction of the inferomedial vocal fold resulting in a divergent glottal configuration. Greater TA activation yields more uniform adduction of the full vertical thickness of the vocal fold. (CT = cricothyroid, inf – sup = inferior to superior, ms = millisecond, post – ant = posterior to anterior, TA = thyroarytenoid).

Fig. 5

Glottal shape for the interaction between maximal TA activation and graded PCA activation. Design layout is same as Fig.1 except now TA is held at maximum stimulation for all rows (grade 5), while PCA is added in a graded fashion from grade 0 (‘PCA 0’, row 1) to grade 6 (‘PCA 6’, row 7). TA provides robust 95% adduction of the mid vocal fold, which is countered by PCA grade 6 abducting the mid-to-posterior cord back to its near baseline glottal width. The PCA also counteracts vocal fold shortening by the TA and improves the distribution of vertical height thickness in the anteroposterior glottal axis while mildly thinning the vocal fold relative to full independent TA activation. (inf – sup = inferior to superior, ms = millisecond, PCA = posterior cricoarytenoid, post – ant = posterior to anterior, TA = thyroarytenoid).

Fig. 6

Glottal shape for the interaction between maximal PCA activation and graded TA activation. Design layout is same as Fig.1 except now PCA is held at maximum stimulation for all rows (grade 6), while TA is added in a graded fashion from grade 0 (‘TA 0’, row 1) to grade 5 (‘TA 5’, row 6). PCA abducts the mid vocal fold by 75%, lengthens it by 5% and maintains near baseline to subtle increase in vertical height thickness. Mid-level TA activation (grade 2-3) results in 45-60% adduction, near completely reversing the abductory effects of PCA. Higher-level TA activation (grade 4-5) fails to reverse the abductory effects of the PCA suggesting a priority for vocal fold abduction or airway patency with high level laryngeal neuromuscular stimulation. (inf – sup = inferior to superior, ms = millisecond, PCA = posterior cricoarytenoid, post – ant = posterior to anterior, TA = thyroarytenoid).

Fig. 7

Glottal shape for the interaction between maximal CT activation and graded LCA activation. Design layout is same as Fig.1 except now CT is held at maximum stimulation for all rows (grade 6), while LCA is added in a graded fashion from grade 0 (‘LCA 0’, row 1) to grade 6 (‘LCA 6’, row 7). Duration of recording is 67 ms in order to capture full spectrum of posture changes. CT alone (row 1) results in 16% abduction, 12% lengthening, and 4-13% thinning of the vocal fold. LCA activation adducts the mid vocal fold to 63%. Increased LCA activation also augments vocal fold thinning throughout the anteroposterior glottal axis to as much as 18%. Adductory effect of LCA with full CT activation yields overall a more homogenous adduction in the anteroposterior glottal axis as opposed to its usual more posterosuperiorly based adduction with LCA acting alone. (CT = cricothyroid, inf – sup = inferior to superior, LCA = lateral cricoarytenoid, ms = millisecond, post – ant = posterior to anterior).

Fig. 8

Glottal shape for the interaction between maximal LCA activation and graded CT activation. Design layout is same as Fig.1 except now LCA is held at maximum stimulation for all rows (grade 6), while CT is added in a graded fashion from grade 0 (‘CT 0’, row 1) to grade 6 (‘CT 6’, row 7). The LCA itself does not change the length of the vocal fold, however with increasing CT activation the vocal fold is lengthened up to 7%. LCA alone (row 1) causes 88% adduction which is reduced by 25% (to 63%) with full grade 6 CT activation. LCA results in vertical thickness thinning by 5-7%. This thinning is further augmented by CT activation to 19%. Again, CT activation helps distribute the posterosuperior effect of LCA more evenly along the glottal axis. (CT = cricothyroid, inf – sup = inferior to superior, LCA = lateral cricoarytenoid, ms = millisecond, post – ant = posterior to anterior).