Biomechanical Model of the Levator Ani Muscle: Identifying the Greatest Strain Areas in Its Subdivisions

During childbirth, the levator ani muscle (LAM) is subjected to significant mechanical stress, potentially causing damage and complications such as pelvic organ prolapse or incontinence. This study investigates previously undescribed defects in regions of the LAM beyond its pubic insertion, resulting from mechanical loading during childbirth, using a computational biomechanical model to identify areas of high fiber deformation.

A finite element model of the pelvic floor was developed from a 3 T MRI scan of a 23-year-old nulliparous woman. Pelvic structures were segmented, reconstructed, and modeled using isotropic hyperelastic materials. A 9 cm diameter sphere, simulating a fetal head, was passed along the curve of Carus to assess deformation in the puborectalis muscle (PRM) and pubovisceralis muscle (PVM).

The model demonstrated greater strain in the PRM compared to the PVM, with peak elongation at its pubic bone attachment. In contrast, the PVM experienced a more evenly distributed strain. Significant muscle deformation occurred at attachment sites of both muscles. Additionally, previously unreported muscle deformations in the dorsolateral region of the PVM were identified.

The computational model highlights high-strain regions in the LAM during childbirth, correlating with clinical observations of muscle injury. The PRM appears more susceptible to localized strain-induced damage, whereas the PVM experiences distributed strain. Notably, the model revealed previously unrecognized defects in lateral regions of the PVM, which current standard evaluations of LAM avulsions might overlook. These findings enhance our understanding of pelvic floor mechanics and could improve clinical assessments of pelvic floor dysfunction.