Structural analysis of muscles elevating the hyolaryngeal complex

Abstract

A critical event of pharyngeal swallowing is the elevation of the hyolaryngeal complex to open the upper esophageal sphincter. Current swallowing theory assigns this function to the submental and thyrohyoid muscles. However, the attachments of the long pharyngeal muscles indicate that they could contribute to this function, yet their role is uninvestigated in humans. In addition, there is evidence the posterior digastric and stylohyoid contribute to hyoid elevation. A cadaver model was used to document the structural properties of muscles. These properties were used to model muscle groups as force vectors and analyze their potential for hyolaryngeal elevation. Vector magnitude was determined using physiological cross-sectional areas (PCSAs) of muscles calculated from structural properties of muscle taken from 12 hemisected cadaver specimens. Vector direction (lines of action) was calculated from the three-dimensional coordinates of muscle attachment sites. Unit force vectors in the superior direction of submental, suprahyoid (which includes the submental muscles), long pharyngeal, and thyrohyoid muscles were derived and compared by an analysis of variance (ANOVA) to document each muscle's potential contribution to hyolaryngeal elevation. An ANOVA with Tukey HSD post hoc analysis of unit force vectors showed no statistically significant difference between the submental (0.92 ± 0.24 cm(2)) and long pharyngeal (0.73 ± 0.20 cm(2)) muscles. Both demonstrated greater potential to elevate the hyolaryngeal complex than the thyrohyoid (0.49 ± 0.18 cm(2)), with P < 0.01 and P < 0.05, respectively. The suprahyoid muscles (1.52 ± 0.35 cm(2)) demonstrated the greatest potential to elevate the hyolaryngeal complex: greater than both the long pharyngeal muscles (P < 0.01) and the thyrohyoid (P < 0.01). The submental and thyrohyoid muscles by convention are thought to elevate the hyolaryngeal complex. This study demonstrates that structurally the long pharyngeal muscles have similar potential to contribute to this critical function, with the suprahyoid muscles having the greatest potential. If verified by functional data, these findings would amend current swallowing theory.

Fig. 1

a Illustrated hyolaryngeal complex including (1) hyoid bone, (2) thyroid cartilage, (3) cricoid cartilage, and (4) upper esophageal sphincter comprising the (a) inferior portion of the thyropharyngeus and (b) cricopharyngeus. b Illustration of the current theory of hyolaryngeal elevation by the (1) submental (mylohyoid, geniohyoid, anterior digastric) and (2) thyrohyoid muscles. This action is thought to open the upper esophageal sphincter. c Illustration of the two-sling theory with (1) the submental muscles as an anterior sling, (2) the thyrohyoid muscle, and (3) the long pharyngeal muscles (stylopharyngeus, palatopharyngeus, and salpingopharyngeus) as a posterior sling. d Illustration of the two-sling theory with (1) the suprahyoid muscles group (submental, posterior digastric, and stylohyoid) as an anterior sling, (2) the thyrohyoid muscle, and (3) the long pharyngeal muscles as a posterior sling

Fig. 2

a, b Means and standard deviations of superior PCSA force unit vectors (cm²) of a submental versus thyrohyoid versus long pharyngeal and of b suprahyoid versus thyrohyoid versus long pharyngeal. An analysis of variance with Tukey HSD of the unit force vector shows that the long pharyngeal muscles have as much potential to elevate the hyolaryngeal complex as the submental muscles, though the suprahyoid muscles as a group have the greatest potential for force in hyolaryngeal elevation

Fig. 3

Mean PCSA (cm²) versus mean fascicle length (cm) of muscles in sample (n = 12), with trend line indicating an inverse relationship between the potential for maximum tetanic tension and the potential for excursion. While in general the long pharyngeal muscles demonstrate less potential for force, they show greater potential for excursion given their length advantage