Biomedical Engineering Reference
In-Depth Information
between its corresponding singular value squared and the arithmetic sum of
all squared singular values. PCA-derived models were generated by linearly
combining the parameters of the population mean and variation modes, which
were used for visualisation of the shape changes associated with individual shape
functions [
21
]. Back conversion from the Bezier to Hermite representations was
performed and the final PF models are in the Hermite interpolation scheme with the
same number of DOFs as the models for individual subjects.
2.4 Childbirth Simulations on PCA-Derived Models
The PCA-derived models were subsequently used to characterise the effect of
different shape modes on the mechanics of the second stage of labour. The most
commonly observed occipito-anterior presentation of the foetal head was simulated
in this study, where the head was allowed to rotate and negotiate its way through the
birth canal formed by the LA muscles.
2.4.1 Model Construction
The generated models were truncated to exclude the ES muscles, which do not
participate in vaginal delivery. Part of the obturator internus muscles and the pubic
bones where the pubococcygeus and puborectalis muscles are attached anteriorly
were also added to the model, to maintain numerical stability of the computation
framework and to restrict anterior opening of the LA muscles, respectively
(Fig.
3a
). The medial aspect of the iliococygeus were connected together in the
model since they are anatomically inserted into the coccyx medially through
anococcygeal raphe [
22
]. The order of interpolation for the PF models in the
mechanical simulations was increased from a bi-cubic Hermite-linear Lagrange
to a tricubic Hermite scheme, so that deformations across the muscle thickness
could also be captured. The PCA-derived PF models were rotated by 20
about the
y-axis to be aligned with the path of the foetal head descent.
The foetal skull model was constructed from laser-scanned surface data of a
foetal head replica produced by ESP Ltd [
23
], and scaled to 90% of its original size
to be accommodated by every PCA-derived model. It was interpolated using a
bicubic Hermite-linear Lagrange scheme since data were only available on the
interior and exterior surfaces. The face contour of the head was simplified to a
smooth surface for ease of computation.
2.4.2 Boundary Constraints and Mechanical Properties
Prior to the childbirth simulation, the foetal head was aligned centrally and superior
to the LA muscles (Fig.
3b
). Its descent through the birth canal was achieved by a
