Biomedical Engineering Reference
In-Depth Information
predictive power in musculoskeletal models, as well as by a desire to simulate more
complex anatomical structures.
Tissue-scale muscle models, based on the Finite-Element (FE) method, can
capture details regarding internal tissue strain, the wrapping of muscles around bones
and other structures, and the transmission of force by muscles with internal connec-
tive tissue and broad attachments [
2
]. The FE modeling paradigm can also be used
to represent highly deformable muscular structures such as the face, lips, tongue,
and pharyngeal tract, where changes in shape of the structures have functional sig-
nificance. The face and vocal tract have received less attention by computational
biomechanists than the limbs and whole-body. This is likely due to the additional
biomechanical complexity of the head and neck anatomy. However, it is this very
complexity that makes face and vocal tract systems excellent candidates for simula-
tion in order to elucidate the unique biomechanics of these structures in breathing,
feeding, and speaking.
Biomechanical face models have a long history. The earliest physically-based face
model was reported by Terzopoulos and Waters [
3
]. The model was composed of a
linear spring-mass mesh and was used to generate compelling animations of facial
expression for its time. More recent models have used FE methods to improve the
representation of facial tissue and muscle mechanics. Sifakis et al. [
4
] developed a
detailed FE model and simulated speech movements using a kinematically driven
jaw. Hung et al. [
5
] have recently developed a FE face model targeted for visual
effects in film. Few previous models have integrated the craniofacial and vocal tract
components into a unified simulation. For this reason, much of our modeling efforts
have targeted the integration of face and vocal tract anatomy in dynamic simulations
[
6
-
11
].
Tissue-scale simulations of face and vocal tract biomechanics can be applied in a
wide range of domains, including computer animation, medicine, and biology. Much
of the previous face modeling work has come out of the computer graphics com-
munity in an attempt to create realistic simulations of facial appearance for visual
effects in films [
3
,
4
,
12
]. Robotic and computer generated faces suffer from the
so-called “uncanny valley” phenomenon [
13
]. This phenomenon, first postulated by
Mori in the 70s based on his work building humanlike robots, suggests that as artificial
faces become closer to reality they become more eerie and repulsive to a perceiver.
Biomechanics-based face simulations have the potential to surpass the uncanny val-
ley, as the facial movements would theoretically mimic the real physical system
perfectly. Motion-capture techniques for face animation have improved dramatically
for use in computer generated films [
14
]; however, biomechanics-based face anima-
tions have not yet achieved the same level of realism. Synthesizing face animations
through simulation without an actor remains an attractive research direction with
the potential to reduce the production costs and constraints of motion-capture driven
animation.
Biomedical applications of face and vocal tract modeling are also numerous.
Dysfunctions in breathing (e.g. obstructive sleep apnea) and feeding (e.g. dyspha-
gia) are thought to involve combined deficits to both the tissue mechanics and neural
control of patients. Also, maxillofacial surgeries can benefit from tissue-level cranio-
