Craniofacial Muscles

242 Z.-J. Liu
skeletal muscular tissue and lacks an internal bony support for motor function. While
the extrinsic tongue muscles arise externally from bony structures, the intrinsic
tongue muscles originate and terminate within the tongue proper (see Sokoloff and
Burkholder, Chap. 9 ). By contracting these complex muscular structures, the tongue
performs motor function and exerts force through various shape changes (Mu and
Sanders 1999 ) . As a “muscular hydrostat,” the tongue kinematics and biomechanical
effects (displacement, deformation, and load production) are produced by complex
changes of its shape in various dimensions. For example, the tongue is capable
of simultaneously lengthening and shortening in different regions. These kinematic
features may produce a great variety of nonlinear movements and deformations
without altering its tissue volume (Kier and Smith 1985 ; Kier et al. 1989 ; Nishikawa
1999 ; Sokoloff 2004 ). It has been recently advocated that the tongue neuromuscular
organization and motor control are no longer entirely muscle-based but use grouped
motor unit- or segmented structure unit-based strategies (Slaughter et al. 2005 ;
Sokoloff 2004 ; Takemoto 2001 ) .
14.2 Tongue Kinematics
Because of anatomical complexity and inaccessibility, tissue attributes, and functional
precision and diversity, studying tongue kinematics and biomechanics by
examining its shape changes (deformation) or position changes (movement) during
manipulation and function has been an ongoing challenge (Napadow et al . 1999a, b ;
Takemoto 2001 ; Sawczuk and Mosier 2001 ) . In addition to various imaging techniques
including video fl uoroscopy, cineradiography, X-ray microbeam, and MRI,
ultrasonography has been extensively used to study tongue kinematics in feeding
and speech. However, even with the availability of 3D reconstruction, this technique
can only trace movements represented by surfaces of the tongue, and deformational
changes remain undetectable. Due to the incompressibility and complex
fi ber structure, studying tongue biomechanics from changes of its overall tissue
shape or displacements may not be appropriate. Rather, internal muscular deformation
should be examined and analyzed (Napadow et al . 1999a, b ) . Therefore, tongue
internal kinematics are a key component of tongue biomechanics.
For years, our group has developed an innovative approach of digital microsonometrics
to study real-time tongue kinematics by measuring the changes in tongue
shape in multiple dimensions as well as tongue regional volume during various
functions. By implanting 6 ultrasonic crystals in the anterior 2/3 of the tongue blade
and body (Fig. 14.1a ) or 8 in the posterior 1/3 tongue base (Fig. 14.1b ) in a minipig
model, together with other techniques such as high-speed jaw movement tracking,
wire electromyography (EMG), in vivo loading, and respiratory monitoring, the
normal kinematics of the tongue and ensuing biomechanical consequences have
been extensively investigated.