Automated Indentation Mapping of Vocal Fold Structure and Cover Properties Across Species

Abstract

Objectives/hypothesis: Various animal models have been employed to investigate vocal fold (VF) and phonatory function. However, biomechanical testing techniques to characterize vocal fold structural properties vary and have not compared critical properties across species. We adapted a nondestructive, automated indentation mapping technique to simultaneously quantify VF structural properties (VF cover layer and intact VF) in commonly used species based on the hypothesis that VF biomechanical properties are largely preserved across species.

Study design: Ex vivo animal model.

Methods: Canine, leporine, and swine larynges (n = 4 each) were sagittally bisected, measured, and subjected to normal indentation mapping (indentation at 0.3 mm; 1.2 mm/s) with a 2-mm spherical indenter to quantify normal force along the VF cover layer, structural stiffness, and displacement at 0.8 mN; two-dimensional maps of the free VF edge through the conus elasticus were created for these characterizations.

Results: Structural stiffness was 7.79 gf/mm (0.15-74.55) for leporine, 2.48 gf/mm (0.20-41.75) for canine, and 1.45 gf (0.56-4.56) for swine. For each species, the lowest values were along the free VF edge (mean ± standard deviation; leporine: 0.40 ± 0.21 gf/mm, canine: 1.14 ± 0.49 gf/mm, swine: 0.89 ± 0.28 gf/mm). Similar results were obtained for the cover layer normal force at 0.3 mm. On the free VF edge, mean (standard deviation) displacement at 0.08 gf was 0.14 mm (0.05) in leporine, 0.11 mm (0.03) in canine, and 0.10 mm (0.02) in swine.

Conclusions: Automated indentation mapping yielded reproducible biomechanical property measurement of the VF cover and intact VF. Divergent VF structural properties across canine, swine, and leporine species were observed.

Level of evidence: NA Laryngoscope, 129:E26-E31, 2019.

Keywords: Larynx; indentation; mechanical testing; structural stiffness; vocal fold; voice.

Figure 1

A.) Swine larynx specimen in six-screw positioning template covered with PBS solution in Mach-1 mechanical tester with 2mm spherical indenter tip. B.) Leporine larynx affixed to specimen holder with calibration putty in place. C.) Swine larynx affixed to specimen holder. D.) Canine Larynx affixed to specimen holder. E.) Indentation mapping grid superimposed on canine hemilarynx fixed to specimen hold illustrating random order and area of indentations.

Figure 2

Sample normal force (gf) verses normal position (mm) graph. Black arrowhead identifies 0.08gf where displacement measures are determined. Black arrow pointing to 0.3mm location where force information is collected.

Figure 3

Heat maps illustrating a composite representation of each species by averaging normal force (gf), structural stiffness (gf/mm), and displacement at 0.08gf at each indentation mapping position (illustrated in the top row) for the 0.3mm indentation depth. Heat map reference values labeled along the left for each row of data. *Indicates that values above this are also colored red.

Figure 4

Heat maps illustrating a composite representation of each species by averaging normal force (gf), structural stiffness (gf/mm), and displacement at 0.08gf at each indentation mapping position (illustrated in the top row) at the 0.5mm leporine and 1.5mm canine and swine specimens. Heat map reference values labeled along the left for the leporine data and in between the first and second column for canine and swine data. *Indicates that values above this are also colored red.

Figure 5

Diagram of two specimens with same geometry, but different sizes. The black circle corresponds to a spherical indenter, the white line on the indenter indicating the same indentation amplitude for both specimens. Dashed brackets illustrate the thickness of the specimen under the indenter.