Biomedical Engineering Reference
In-Depth Information
Fig. 5 Parameters quantifying the progress of vaginal delivery, including (a) normalised pelvic
floor reaction forces and (b) principal stretch ratio at the right LA attachment, for the mean and
PCA-derived models at different levels of foetal head descent
five cases. It also advanced the peak force to occur at a slightly earlier head descent.
According to the results from the statistical shape analysis, a positive standard
deviation of the first mode reduced the circumference of the PF muscles and
increased the muscle thickness, which potentially made the PF models require
larger forces to accommodate the foetal skull due to the increase in muscle
resistance.
The second shape mode also affected the magnitude of the peak reaction forces
although less significantly. One standard deviation below the mean increased the
peak reaction force; in addition, it shifted the foetal head crowning to be at a greater
descent displacement and the foetal head needed to descend further before its bi-
parietal diameter passed the orifice of the PF muscles (Fig.
5a
). This observation is
likely to be attributed to the increased axial span of the PF muscle in the model with
a negative standard deviation.
The maximum principal stretch ratios were found to be at the right attachment
point of the pubococcygeus to the anterior pubic bone in all cases. Addition of the
first and second mode shapes to the population mean increased the stretch ratio
magnitudes (Fig.
5b
). When correlating the stretch ratios to the shape analysis
results, it was found that factors including increases in muscle thickness, reduction
in PF circumference and elongated axial span of the PF muscles, increased the
maximum stretch of the PF muscles during the second stage of labour, potentially
leading to a complete or partial detachment of the muscles from the pubic bones.
3.3 Model Limitations
First, the boundaries between the LA and ES muscles were not clearly identifiable
from the MRI images, thus their segmentation may have been subjective. Never-
theless, both muscles were considered as a single entity in the statistical shape
analysis and the ES muscles were only discarded from the PCA-derived models
when conducting the childbirth simulations. Second, effects of only two mode
