Intravelar and Extravelar Portions of Soft Palate Muscles in Velic Constrictions: A Three-Dimensional Modeling Study

Abstract

Purpose This study predicts and simulates the function and relative contributions of the intravelar and extravelar portions of the levator veli palatini (LVP) and palatoglossus (PG) muscles in velic constrictions. Method A finite element-based model of the 3-dimensional upper airway structures (palate, pharynx, tongue, jaw, maxilla) was implemented, with LVP and PG divided into intravelar and extravelar portions. Simulations were run to investigate the contributions of these muscles in velopharyngeal port (VPP) closure and constriction of the oropharyngeal isthmus (OPI). Results Simulations reveal that the extravelar portion of LVP, though crucial for lifting the palate, is not sufficient to effect VPP closure. Specifically, the characteristic "bulge" appearing in the posterior soft palate during VPP closure ( Pigott, 1969 ; Serrurier & Badin, 2008 ) is found to result from activation of the intravelar portion of LVP. Likewise, the intravelar portion of posterior PG is crucial in bending the "veil" or "traverse" ( Gick, Francis, Klenin, Mizrahi, & Tom, 2013 ) of the velum anteriorly to produce uvular constrictions of the OPI ( Gick et al., 2014 ). Conclusions Simulations support the view that intravelar LVP and PG play significant roles in VPP and OPI constrictions.

Figure 1.

A diagram of the upper airway with key components labeled and fiber directions of the extravelar portions of LVP and PG illustrated (adapted from image by Patrick J. Lynch, medical illustrator [CC BY 2.5, http://creativecommons.org/licenses/by/2.5], via Wikimedia Commons).

Figure 2.

A midsagittal view of the biomechanical model providing an overview of the upper airway components included in the computational model. The yellow line indicates the location of the cross-section used to calculate closure areas.

Figure 3.

Front (left) and sagittal (right) views of the soft palate and pharynx models. Half of the palate is rendered semitransparent to show internal muscle fibers, including the levator veli palatini (green), anterior palatoglossus (magenta), posterior palatoglossus (dark magenta), tensor veli palatini (cyan), palatopharyngeus (orange), and musculus uvulae (purple).

Figure 4.

The model describing the passive and active force generation capabilities of soft palate muscles.

Figure 5.

Posterior oblique views of the model with the pharynx rendered semitransparent to show internal muscle fibers, including the superior constrictor (green), middle constrictor (magenta), inferior constrictor (purple), cricopharyngeus (red), salpingopharyngeus (cyan), stylopharyngeus (yellow), and palatopharyngeus (orange, left image).

Figure 6.

Muscle activations (top), velopharyngeal cross-sectional area (middle), and closure force (bottom) versus time for different stiffnesses of the soft palate under concurrent activation of extravelar (ext) and intravelar (int) levator veli palatini. This simulation reveals the rapid reduction of velopharyngeal cross-sectional area with levator veli palatini activation and the stronger closure force of a stiffer soft palate.

Figure 7.

Muscle activations (top), velopharyngeal cross-sectional area (middle), and closure force (bottom) versus time for sequential activations of extravelar (ext) and intravelar (int) levator veli palatini (LVP). This simulation reveals that, although extravelar LVP alone contributes to the majority of velopharyngeal port closure, the intravelar portion of LVP is required for complete closure.

Figure 8.

Midsagittal cutaway (left) and transverse cutaway (right) views of the model at rest (top) with extravelar levator veli palatini (LVP) activation (middle) and with extravelar and intravelar LVP activation (bottom). With extravelar LVP activation, the velopharyngeal port appears closed midsagittally, but the transverse view reveals that the closure is not complete off the midsagittal plane (middle). Additional activation of intravelar LVP results in complete velopharyngeal port closure (bottom).

Figure 9.

Oblique posterior view of the soft palate at rest (left) and with 50% levator veli palatini activation, illustrating the bulge formed as a result of levator veli palatini activation (right).

Figure 10.

Muscle activations (a), velopharyngeal cross-sectional area (b), closure force (c), and anterior uvula movement (d) versus time for sequential activation of levator veli palatini (LVP) and palatoglossus (PG). First, (a) LVP is activated to create velopharyngeal port (VPP) closure, followed by PG; the portions of PG that are activated affect the VPP closure as illustrated by (b) the resulting VPP area function, (c) the resulting VPP closure forces, and (d) the distance moved anteriorly by the uvula. The intravelar posterior PG (PGP) has little effect on VPP area or closure force but results in the largest uvula motion. Dist = distance; ext = extravelar; int = intravelar.

Figure 11.

Midsagittal view of the model, showing the oropharyngeal isthmus and velopharyngeal port (VPP) consequences of activating portions of palatoglossus (PG). Arrows indicate motion due to PG. Model shown at rest (a), with levator veli palatini activated alone (b), with PG activated (c), with posterior PG (PGP) activated (d), with extravelar PGP activated (e), and with intravelar PGP activated (f). After VPP closure is achieved with levator veli palatini (b), activating PG (c) or only PGP (d) results in anterior uvula motion but also diminishes VPP closure quality (see Figure 10). Extravelar PGP alone produces no noticeable uvula motion (e), but intravelar PGP produces uvula motion with little compromise of VPP closure quality.

Figure 12.

Front view of the oral cavity at rest (left) and after posterior palatoglossus is activated (right). The anterior faucial pillars enlarge after posterior palatoglossus activation but are not distinctive, suggesting a shortcoming in the design of the current model.