Viscous effects in pelvic floor muscles during childbirth: A numerical study

Abstract

During vaginal delivery, women sustain stretching of their pelvic floor, risking tissue injury and adverse outcomes. Realistic numerical simulations of childbirth can help in the understanding of the pelvic floor mechanics and on the prevention of related disorders. In previous studies, biomechanical finite element simulations of a vaginal delivery have been performed disregarding the viscous effects present on all biological soft tissues. The inclusion of the viscoelastic behaviour is fundamental, since it allows to investigate rate-dependent responses. The present work uses a viscohyperelastic constitutive model to evaluate how the childbirth duration affects the efforts sustained by the pelvic floor during delivery. It was concluded that viscoelasticity adds a stiffness component that leads to higher forces comparing with the elastic response. Viscous solutions are rate dependent, and precipitous labours could be associated to higher efforts, while lower reaction forces were denoted for normal and prolonged labours, respectively. The existence of resting stages during labour demonstrated the capability of the tissue to relax and recover some of the initial properties, which helped to lower the forces and stresses involved. The present work represents a step further in achieving a robust non-invasive procedure, allowing to estimate how obstetrical factors influence labour and its outcomes.

A numerical study on fetal head molding during labor

“…The surface represented in black (rigid quadrilateral shell elements) in Figure 2 delimits the anterior region, imposing the limits that are anatomically insured by the birth canal, replacing the anterior portion of the pelvic bones. This surface allows the fetal head to translate in the anterior–posterior direction and ensures a smooth contact surface between the fetus head and the replaced pelvic bones, improving the convergence of the simulations, as used in previous works 22,23 …”

“…A User‐Defined Material Subroutine (UMAT) was implemented with the Holzapfel‐Gasser‐Ogden (HGO) constitutive model with two families of fibers 24 . This model was adapted to a state of plane stress based on the work of Prot et al, 25 and the viscoelastic component implemented by Vila Pouca et al 23 was added to obtain a visco‐hyperelastic constitutive model.…”

“…The hyperelastic constitutive parameter of the Neo‐Hookean model was calibrated from a stress–strain experimental curve of a 2‐month‐old donor's suture, obtained by Coats and Margulies 27 . The viscoelastic properties used to characterize the sutures were based on those adopted for the pelvic floor muscles in Vila Pouca et al, 23 and then adapted. Although these parameters were used to characterize a different tissue, the model was implemented with a similar purpose, that is, studying the effect of viscoelasticity on soft tissues.…”

“…Further information on the viscoelastic contribution and the modified form of the Martins constitutive model can be obtained in References 21 and 23, respectively.…”

“…This surface allows the fetal head to translate in the anterior-posterior direction and ensures a smooth contact surface between the fetus head and the replaced pelvic bones, improving the convergence of the simulations, as used in previous works. 22,23…”