A Biomechanical Model of Sagittal Tongue Bending

Vitaly J. Napadow Department of Health Sciences and Technology, Massachusetts Institute of Technology, 77 Massachusetts Ave., Cambridge, MA 02139 Roger D. Kamm Department of Mechanical Engineering, Massachusetts Institute of Technology, 77 Massachusetts Ave., Cambridge, MA 02139 Richard J. Gilbert 1 Department of Mechanical Engineering, Massachusetts Institute of Technology, 77 Massachusetts Ave., Cambridge, MA 02139 A Biomechanical Model of
Sagittal Tongue Bending The human tongue is a structurally complex and extremely exible organ. In order to
better understand the mechanical basis for lingual deformations, we modeled a primitive
movement of the tongue, sagittal tongue bending. We hypothesized that sagittal bending is
a synergistic deformation derived from co-contraction of the longitudinalis and transver-
sus muscles. Our model of tongue bending was based on classical bimetal strip theory, in
which curvature is produced when one muscle layer contracts more so than another.
Contraction was modulated via mismatched thermal expansion coefcients and tempera-
ture change (to simulate muscular contraction). Our results demonstrated that synergistic
contraction produced curvature and strain results which were in better correspondence to
empirical results derived from tagging MRI than were the results of contraction of the
longitudinalis muscle alone. This fundamental reliance of tongue bending on the syner-
gistic contraction of its intrinsic bers supports the muscular hydrostat theory of tongue
function. DOI: 10.1115/1.1503794 Keywords: Muscle Analog Model, Lingual Deformation Introduction The human tongue is a versatile, lithe muscular organ, which is critical to such physiological tasks as mastication, swallowing and
speech. In this paper, we model a primitive movement of the
tongue, sagittal tongue bending. This deformation can be consid-
ered a foundation of lingual deformations seen in oral phase swal-
lowing, wherein the tongue undergoes sagittal bending to place
the tip posterior to the top row of teeth, forming an accommoda-
tion pouch in which a bolus of food is held. The musculature of the anterior tongue Fig. 1 is composed of intrinsic muscles, myober populations wholly contained in the
body of the tongue and unconnected to any external bony attach-
ments 1 . In contrast, extrinsic muscles have a bony attachment
outside the tongue proper. One of the intrinsic muscles, the supe-
rior longitudinalis muscle, runs parallel to the dorsal surface of the
tongue, below the thick epithelium. Below this myober popula-
tion, lies the intrinsic core of the tongue which contains sequential
sheets in the coronal plane of superior-inferiorly and medial-
laterally directed bers; the verticalis and transversus muscles,
respectively. Below this layer is the inferior longitudinalis muscle
followed by connective tissue, the lingual gland, epithelium, and a
thin strip of tissue called the frenulum, which connects the ante-
rior tongue to the oor of the mouth. The transversus, verticalis,
and longitudinalis muscles also extend to the posterior tongue.
The posterior tongue contains a central region of bers originating
at the mental spine of the mandible and projecting in a fan-like
manner in the superior, lateral, and posterior directions corre-
sponding to the extrinsic genioglossus muscle . There are two
major laterally inserted ber populations, the rst directed poste-
rior and inferior the extrinsic hyoglossus muscle and the second
directed posterior and superior the extrinsic styloglossus muscle . Sagittal bending is one of several primitive deformations as- sumed by the tongue during swallowing. Classically, superior-
directed tongue bending is considered to result from contraction
of the superior longitudinalis muscle alone 2 . In contrast, we
hypothesized that sagittal bending may be better depicted by the combined contractions of the transversus and the longitudinalis
muscles. This theory converges well with the depiction of the
tongue as a muscular hydrostat 2 , an organ that functions by
deforming in a plane orthogonal to the contraction axis by con-
servation of volume. One of the goals of this modeling effort was
to evaluate the extent to which combined contractions of the trans-
versus and longitudinalis muscle account for sagittal tongue bend-
ing. In order to validate our model, we compared the theoretical
results to previously published empirical measurements obtained
by Tagging MRI 3 . The theoretical model of tongue bending was based on classical bimetal strip theory 4,5 , which is typically applied in electrical
thermostat switch design. Bimetal strip theory applies to the case
in which two metals of different thermal expansion coefcient are
bonded together at an interface and are subjected to a temperature
change 6 . Under these conditions, the thermal expansion re-
sponse of each layer differs, yet the bond between the layers re-
mains xed. Thus, the only way for the strip to accommodate the
differing expansions is to bend. In the case of sagittal tongue
bending, a beam curvature is produced when one muscle layer specically the longitudinalis muscle contracts more so than the other layers. Hence, if muscular contractile strain can be consid-
ered analogous to thermal contractile strain, the layered muscular
sandwich within the tongue can be adequately modeled by a clas-
sical bimetal strip. One strip layer was composed of the superior
longitudinalis muscle and was bonded to the other layer, com-
posed of the transversus and verticalis muscles. Once the strip was
subjected to a temperature change, contraction of the longitudina-
lis and transversus muscles was accomplished by specifying the
directionally dependent thermal expansion coefcients for the dif-
ferent layers. Previous models of lingual muscular activity have emphasized relatively simple geometry and few elements, incorporating real-
istic activation and dynamics properties 7 , as well as multi-
element nite element models, incorporating correct three-
dimensional geometry and viscoelastic constitutive properties 8 .
The geometry of the former model was particular to certain rep-
tilian tongues and signicantly divergent from human tongues.
The latter models complexity provided greater exibility of de-
formation, yet introduced model instability when setting optimi-
zation functions. Still other models have attempted to model two-
dimensionally the intrinsic and extrinsic behavior of the human 1 Corresponding author: Richard J. Gilbert, Department of Mechanical Engineer- ing, Massachusetts Institute of Technology, 77 Massachusetts Avenue, 41-211,
Cambridge, MA 02139, Phone: 617-254-6875, Fax: 617-253-2249, E-mail: rgilbert@mit.edu Contributed by the Bioengineering Division for publication in the J OURNAL OF B IOMECHANICAL E NGINEERING . Manuscript received March 2001; revised manu- script received June 2002. Associate Editor: M. S. Sacks. Copyright