Micro-structural and Biaxial Creep Properties of the Swine Uterosacral-Cardinal Ligament Complex

The uterosacral ligament and cardinal ligament (USL/CL) complex is the major suspensory tissue of the uterus, cervix, and vagina. This tissue is subjected primarily to bi-axial forces in-vivo that significantly alter its structure and dimension over time, compromising its support function and leading to pelvic floor disorders. In this study, we present the first rigorous characterization of the collagen fiber microstructure and creep properties of the swine USL/CL complex by using scanning electron microscopy and planar biaxial testing in combination with three-dimensional digital image correlation. Collagen fiber bundles were found to be arranged into layers. Although the fiber bundles were oriented in multiple directions, 80.8% of them were aligned within +/- 45 to the main in-vivo loading direction. The straightness parameter, defined as the ratio of the end-to-end distance of a fiber bundle to its length, varied from 0.28 to 1.00, with 95.2% fiber bundles having a straightness parameter between 0.60 and 1.00. Under constant equi-biaxial loads of 2 and 4 N, the USL/CL complex exhibited significant creep both along the main in-vivo loading direction (the parallel direction) and along the direction perpendicular to it (the perpendicular direction). Specifically, over a 120-min period, the mean strain increased by 20-34 in the parallel direction and 33-41 in the perpendicular direction. However, there was no statistically significant difference in creep strains observed after 120 min between the parallel and perpendicular directions for either the 2 or 4 N load case. Creep proceeded slightly faster in the perpendicular direction under the equi-biaxial load of 2 N than under the equi-biaxial load of 4 N (). It proceeded significantly faster in the parallel direction under the equi-biaxial loads of 2 N than under the equi-biaxial loads of 4 N (). Overall, our findings contribute to a greater understanding of the biomaterial properties of the USL/CL complex that is needed for the development of new surgical reconstruction methods and mesh materials for pelvic floor disorders.