Biomedical Engineering Reference
In-Depth Information
Fig. 1 Personalised pelvic floor muscle model and bony landmarks involved in definition of
an anatomically-based coordinate system.
Beige/light gray
, levator ani;
blue/dark gray
, external
sphincter;
red/dark gray spheres
, bony landmarks
since segmentation data were only available on muscle surfaces. Each element has
96 degrees of freedom (DOF) and the entire model has 4,344 DOF. To ensure the
reliability of the shape analysis, it is essential to assure that each node in the PF
muscle models represented similar anatomical locations across the population.
In light of this, the individually constructed meshes were transformed and fitted
systematically to yield PF muscle models with point-to-point correspondence.
2.1.1 Alignment with an Anatomically Meaningful Coordinate System
A new anatomically meaningful coordinate system, defined by positions of the
bony landmarks, was introduced to align the twelve individual PF models, which
eliminated difference in orientations of the pelvis in the MRI datasets.
In this new coordinate system, the y-axis, in the dorsal-caudal direction, was
established by connecting the midpoint of pubic symphysis and the base of the
coccyx. These points were combined with the positions of the two ischial spines to
define an angled axial plane with respect to the y-axis. The z-axis was defined to be
orthogonal to the angled axial plane. Finally, the x-axis was established to be
orthogonal to both the y- and z-axes (Fig.
1
). The origin of the coordinate system
for each model was set to the mean nodal position.
Sets of rigid body translation and rotation were then applied to the PF muscle
models to align them with the defined coordinate system and minimise variations in
