Biaxial Mechanical Evaluation of Uterosacral and Cardinal Ligaments

The uterosacral ligament (USL) and the cardinal ligament (CL) are two major suspensory tissues that provide structural support to the vagina/cervix/uterus complex. These ligaments have been studied mainly due for their role in the surgical repair for pelvic organ prolapse (POP). POP, which is the descent of a pelvic organ from its normal place towards the vaginal walls and into the vaginal cavity, affects an estimated 3.3 million women in the United States annually. Despite their important mechanical function, little is known about the elastic and viscoelastic properties of the USL and CL due to ethical concerns with in vivo testing of human tissues and the lack of accepted animal models.

The goal of this first study is to help establish an appropriate animal model for studying the mechanics of these pelvic supportive ligaments. To achieve this, the first rigorous comparison of histological and planar equi-biaxial mechanical properties of the swine and human USLs was completed. Relative collagen, smooth muscle, and elastin contents were quantified from histological sections and the USL was found to have similar components in both species, with a comparable relative collagen content. Using the digital image correlation (DIC) method to calculate the in-plane Lagrangian strain, no differences in the peak strain during precon- ditioning/cyclic loading tests, secant modulus of the pre-creep/elastic response, and strain at the end of creep tests were detected in the USLs from the two species along both axial loading directions (the main in vivo loading direction and the direction that is perpendicular to it).

Because these ligaments are subjected to repeated constant loads in vivo, the effect of re- peated biaxial loads at three different load levels (1 N, 2 N, or 3 N) on elastic and creep properties of the swine CL was investigated. The results showed that CL was elastically anisotropic, as statistical differences were found between the mean strains along the two axial loading directions for specimens at all three different load levels. The increase in strain over time by the end of the 3rd creep test was comparable along the axial loading direc- tions. The greatest mean normalized strain (or, equivalently, the largest increase in strain over time) was measured at the end of the 1st creep test, regardless of the equi-biaxial load magnitude or loading direction.

Overall, these experimental findings validate the use of swine as an appropriate animal model and offer new knowledge of the mechanical properties of the USL and CL that can guide the development of better treatment methods such as surgical reconstruction for POP.