Modulating phonation through alteration of vocal fold medial surface contour

Abstract

Objectives/hypothesis: 1) To test whether alteration of the vocal fold medial surface contour can improve phonation and 2) to demonstrate that implant material properties affect vibration even when implantation is deep to the vocal fold lamina propria.

Study design: Induced phonation of excised human larynges.

Methods: Thirteen larynges were harvested within 24 hours postmortem. Phonation threshold pressure (PTP) and flow (PTF) were measured before and after vocal fold injections using either calcium hydroxylapatite (CaHA) or hyaluronic acid (HA). Small-volume injections (median, 0.0625 mL) were targeted to the inferomedial aspect of the thyroarytenoid muscle. Implant locations were assessed histologically.

Results: The effect of implantation on PTP was material dependent. CaHA tended to increase PTP, whereas HA tended to decrease PTP (Wilcoxon test, P = .00013 for onset). In contrast, the effect of implantation on PTF was similar, with both materials tending to decrease PTF (P = .16 for onset). Histology confirmed implant presence in the inferior half of the vocal fold vertical thickness.

Conclusions: Taken together, these data suggested the implants may have altered the vocal fold medial surface contour, potentially resulting in a less convergent or more rectangular glottal geometry as a means to improve phonation. An implant with a closer viscoelastic match to vocal fold cover is desirable for this purpose, as material properties can affect vibration even when the implant is not placed within the lamina propria. This result is consistent with theoretical predictions and implies greater need for surgical precision in implant placement and care in material selection.

Figure 1

Glottal shape in coronal plane, defined with respect to the direction of air flow during phonation. Modified from Titze .

Figure 2

Change in contour of the medial vocal fold edge produced by injection. Coronal CT image of a right hemilarynx before (A) and after (B) injection of calcium hydroxylapatite. (C) shows superposition of the post-injection contour onto the pre-injection image. Previously unpublished data from work described in Mau and Courey .

Figure 3

Schematic of the concept of medial vocal fold contour alteration. A surgical intervention would bring the contour from the solid line to the dashed line, thereby increasing vertical thickness and creating a less convergent, more rectangular glottis.

Figure 4

Schematic of the excised larynx phonation bench apparatus.

Figure 5

Excised human larynx mounted on the induced phonation apparatus with posterior glottal width of (A) 2 mm before vocal fold injection, (B) 2 mm after injection, (C) 3 mm before injection, and (D) 3 mm after injection.

Figure 6

Change in PTP and PTF, individual larynx data. Each data point represents the mean value measured at different posterior glottal widths (0.5–3 mm) for a single larynx. Graphs on the left show data from larynges injected with CaHA, and graphs on the right show data from larynges injected with HA gel.

Figure 7

Mean PTP and PTF before and after injection, by implant material. The change produced by injection is compared between the two materials, as indicated by the P values from the Wilcoxon test.

Figure 8

Percentage change in PTP and PTF, by implant material. Solid bars represent data for CaHA, and open bars represent data for HA gel.

Figure 9

Location of injected implant in coronal sections of vocal folds. (A) Photograph of the cut surface at the mid-section of a vocal fold containing bolus of CaHA, after fixation and prior to decalcification and embedding. Ruler in the image contains markings in millimeters. (B) H & E-stained section taken from the same vocal fold. The CaHA implant has dissected into fibers of the TA muscle in its medial aspect. The implant is clearly deep to the vocal ligament. (C) Alcian blue-stained section containing bolus of HA gel in a different vocal fold. The scale bar equals 1 mm for (B) and (C).