Biaxial Mechanical Behavior of Swine Pelvic Floor Ligaments: Experiments and Modeling

Although mechanical alterations to pelvic floor ligaments, such as the cardinal and uterosacral ligaments, are one contributing factor to the development and progression of pelvic floor disorders, very little research has examined their mechanical properties. In this study, the first biaxial elastic and viscoelastic tests were performed on uterosacral and cardinal ligament complexes harvested from adult female swine. Biaxial elastic testing revealed that the ligaments undergo large strains and are anisotropic. The direction normal to the upper vagina was typically stiffer than the transverse direction. Stress relaxation tests showed that the relaxation was the same in both directions, and that more relaxation occurred when the tissue was stretched to lower initial strains. In order to describe the experimental findings, a threedimensional constitutive model based on the Pipkin-Rogers integral series was formulated and the parameters of such model were determined by fitting the model to the experimental data. In formulating the model, it was assumed that the tissues consist of a ground substance with two embedded families of fibers oriented in two directions and that the ligaments are incompressible. The model accounts for finite strains, anisotropy, and strain-dependent stress relaxation behavior. This study provides information about the mechanical behavior of female pelvic floor ligaments, which should be considered in the development of new treatment methods for pelvic floor disorders.