Background: Induction of labour is poorly understood even though it is performed in
20% of births in the United States. One method of induction, the balloon dilator
applied with traction to the interior os of the cervix, engages a softening process,
permitting dilation and effacement to proceed until the beginning of active labour.
The purpose of this work is to develop a simple model capable of reproducing the
dilation and effacement effect in the presence of a balloon.
Methods: The cervix, anchored by the uterus and the endopelvic fascia was modelled
in pre-labour. The spring-loaded, double sliding-joint, double pin-joint mechanism
model was developed with a Modelica-compatible system, MapleSoft MapleSim 6.1,
with a stiff Rosenbrock solver and 1E-4 absolute and relative tolerances. Total
simulation time for pre-labour was seven hours and simulations ended at 4.50 cm
dilation diameter and 2.25 cm effacement.
Results: Three spring configurations were tested: one pin joint, one sliding joint and
combined pin-joint-sliding-joint. Feedback, based on dilation speed modulated the
spring values, permitting controlled dilation. Dilation diameter speed was maintained
at 0.692 cm · hr−1 over the majority of the simulation time. In the sliding-joint-only
mode the maximum spring constant value was 23800 N · m−1. In pin-joint-only the
maximum spring constant value was 0.41 N·m· rad−1.With a sliding-joint-pin-joint pair
the maximum spring constants are 2000 N · m−1 and 0.41 N · m · rad−1, respectively.
Conclusions: The model, a simplified one-quarter version of the cervix, is capable of
maintaining near-constant dilation rates, similar to published clinical observations for
pre-labour. Lowest spring constant values are achieved when two springs are used, but
nearly identical tracking of dilation speed can be achieved with only a pin joint spring.
Initial and final values for effacement and dilation also match published clinical
observations. These results provide a framework for development of electro-mechanical
phantoms for induction training, as well as dilator testing and development.
Smith, J. A. (2013). A simplified cervix model in response to induction balloon in pre-labour. Theoretical Biology & Medical Modelling, 10(1), 58-58. doi:10.1186/1742-4682-10-58