In-plane and out-of-plane deformations of gilt utero-sacral ligaments

Abstract

The uterosacral ligaments (USLs) are supportive structures of the uterus and apical vagina. The mechanical function of these ligaments within the pelvic floor is crucial not only in normal physiological conditions but also in reconstructive surgeries for pelvic organ prolapse. Discrepancies in their anatomical and histological description exist in the literature, but such discrepancies are likely due to large variations of these structures. This makes mechanical testing very challenging, requiring the development of advanced methods for characterizing their mechanical properties. This study proposes the use of planar biaxial testing, digital image correlation (DIC), and optical coherence tomography (OCT) to quantify the deformations of the USLs, both in-plane and out-of-plane. Using the gilts as an animal model, the USLs were found to deform significantly less in their main direction (MD) of in vivo loading than in the direction perpendicular to it (PD) at increasing equibiaxial stresses. Under constant equibiaxial loading, the USLs deform over time equally, at comparable rates in both the MD and PD. The thickness of the USLs decreases as the equibiaxial loading increases but, under constant equibiaxial loading, the thickness increases in some specimens and decreases in others. These findings could contribute to the design of new mesh materials that augment the support function of USLs as well as noninvasive diagnostic tools for evaluating the integrity of the USLs.

Keywords: Biaxial testing; Deformations; Digital image correlation; Optical coherence tomography; Pelvic floor support; Uterosacral ligaments.

Fig. 1.

(a) Location of USL specimens in the gilt with main direction (MD) and perpendicular direction (PD). (b) Experimental equipment including a custom-made biaxial machine, digital image correlation (DIC) system, and optical coherence tomography (OCT) system. (c) Testing protocol showing the loading profile over time in the MD (or PD).

Fig. 2.

(a) Representative in-plane image of one USL specimen. (b) Out-of-plane image at one marked location (yellow dotted line) in (a). (c) Filtered depth profile data and corresponding derivative at one marked location (yellow dotted line) in (b). Open circles indicate the locations of the top and bottom interfaces.

Fig. 3.

(a) Representative specimen speckled for DIC strain measurement, with reported MD and PD aligned along the loading axes. (b) Map of (normal, Lagrangian) strains and corresponding average strains in the MD at (nominal) stress levels of 0 kPa, 50 kPa, and 150 kPa in the MD. (c) Map of (normal, Lagrangian) strains and average strains in the PD at (nominal) stress levels of 0 kPa, 50 kPa, and 150 kPa in the PD.

Fig. 4.

Stress–strain curves in the (a) MD and (b) PD for a subset of tested specimens (n = 8). Data from the same specimen are reported using the same color. (c) Mean (± S.D.) strain in the MD (blue) and PD (purple) at equivalent stress levels of 50, 100, and 150 kPa in the MD and PD computed from n = 8 specimens. *, p < 0.05.

Fig. 5.

(a) Maps of maximum (top row) and minimum (bottom row) principal strains and corresponding average principal strains at (nominal and normal) stress values of 5 kPa, 50 kPa, and 150 kPa in the PD for one representative specimen. Small white arrows denote the directions of maximum (top row) and minimum (bottom row) principal strains. (b) Relative frequency of max principal strain angles and (c) mean (±S.D.) of the maximum and minimum principal strains at stress values of 50 kPa and 150 kPa in the PD computed from n = 8 specimens.

Fig. 6.

Maps of (normal, Lagrangian) strains and corresponding average strains in the (a) MD and (b) PD for one specimen at the start (t = 0 s) and at the end (t = 2400 s) of creep.

Fig. 7.

Moving average (dashed lines) and curve fitting (continuous lines) of (normal, Lagrangian) strain over time during creep testing in the (a) MD and (b) PD for n = 15 specimens. Data from the same specimen are reported using the same color. (c) Mean (± S.D.) values of the parameters, n and α, in the equation αtⁿ used to fit the creep data from n = 15 specimens.

Fig. 8.

Images of the cross-sections of one representative USL specimen during pre-creep subjected to stresses that increase from (a) 2.0 kPa in the MD and 1.8 kPa in the PD to (l) 198 kPa in the MD and 144 kPa in PD.

Fig. 9.

(a) Relative thickness change over time during creep at three in-plane locations across one representative USL specimen, as shown in the inset, with the mean relative thickness change. (b) Mean relative thickness change over time during creep for n = 8 USL specimens.

Fig. 10.

(a) Cross-sectional image of one USL specimen showing three sublayers (segments 1, 2, and 3) and one cavity at the beginning of creep. (b) Filtered depth profile data and corresponding derivative, showing the presence of three sublayers and one cavity. (c) Relative thickness change of each sublayer and cavity in (a) and the full tissue over time during creep.