Biomedical Engineering Reference
In-Depth Information
2.2
Actions of the Jaw-Closing Muscles
This section presents a 3D method analyzing and quantifying the muscle forces
exerted on the mandible under loadings. The experimental protocol is described and
data from four volunteers are presented.
A sensor had been designed to simulate and register the bite force between two
teeth, when placed successively between two incisors, two premolars and then two
molars.
We were able to describe contacts between the mandible condyles and the
temporal bone and also the insertions of the jaw closing-muscles after carrying
out dissections. The directions of the muscular forces were thus determined in a
morphological coordinate system. Using in vivo MRI and electromyography data,
we were able to evaluate the magnitudes of six muscle forces on each side of
the face.
2.2.1
Biomechanical Model and Volunteers
The morphological symmetry of the face was admitted with respect to the sagittal
plane.
Reference Coordinate System
Most papers analyzing actions exerted on the mandible exploit an undefined or
exotic coordinate system to express the results. This study used a functional and
anatomical reference coordinate system attached to the Camper's plane (Fig.
5
).
This coordinate system (noted
S
c
), like the one used to study the jaw motion,
was associated to the three morphological points defining the plane: point
N
,the
sub nasal point, points
L
and
R
, the centers of the left and right condyles in the
intercuspidation position. Using the 3D video analysis to determine the co-ordinates
of these three points, the coordinate system
S
c
was the following:
-
The origin
C
was the middle of the segment defined by the two condyle centers,
-
The x-axis passed through the two TMJ centers (positive x-axis towards the left),
-
The z-axis was defined by points
C
and
N
(positive z-axis from
C
towards
N
),
-
The y-axis completed the system (perpendicular to the Camper's plane, positive
y-axis upward). The basis associated to these three axes was noted
b
.
Jaw-Closing Muscles
Three pairs of muscles on the left and right sides of the face mainly rise up from
the mandible and contribute to transmitting the forces that are used for speaking or
for chewing: the masseter muscles, the medial pterygoid muscle and the temporal
