Assessment of urethral support using MRI-derived computational modeling of the female pelvis

Abstract

Introduction and hypothesis: This study aimed to assess the role of individual anatomical structures and their combinations to urethral support function.

Methods: A realistic pelvic model was developed from an asymptomatic female patient's magnetic resonance (MR) images for dynamic biomechanical analysis using the finite element method. Validation was performed by comparing simulation results with dynamic MR imaging observations. Weaknesses of anatomical support structures were simulated by reducing their material stiffness. Urethral mobility was quantified by examining urethral axis excursion from rest to the final state (intra-abdominal pressure = 100 cmH2O). Seven individual support structures and five of their combinations were studied.

Result: Among seven urethral support structures, we found that weakening the vaginal walls, puborectalis muscle, and pubococcygeus muscle generated the top three largest urethral excursion angles. A linear relationship was found between urethral axis excursions and intra-abdominal pressure. Weakening all three levator ani components together caused a larger weakening effect than the sum of each individually weakened component, indicating a nonlinearly additive pattern. The pelvic floor responded to different weakening conditions distinctly: weakening the vaginal wall developed urethral mobility through the collapsed vaginal canal, while weakening the levator ani showed a more uniform pelvic floor deformation.

Conclusions: The computational modeling and dynamic biomechanical analysis provides a powerful tool to better understand the dynamics of the female pelvis under pressure events. The vaginal walls, puborectalis, and pubococcygeus are the most important individual structures in providing urethral support. The levator ani muscle group provides urethral support in a well-coordinated way with a nonlinearly additive pattern.

Keywords: Finite element method; Magnetic resonance imaging; Pelvic muscle; Stress urinary incontinence; Urethral hypermobility.

Figure 1

(a) Front and (b) back view of all pelvic muscles, ligaments and bones (Fats and organs hidden for better visualization). (c) Mid-sagittal view of the entire pelvic model. (d) Anterior and posterior supports to urethra from pubourethral ligament, vagina and perineal pouch muscles. Ligaments were modeled using connector elements. The uterosacral ligaments attach the cervix to the posterior pelvic wall. The cardinal ligaments attach the cervix to the lateral pelvic wall. The pubourethral ligaments attach the bladder neck to the symphysis pubis. (e) Posterior support to urethra from pelvic floor muscles. In (a) (b) and (e), muscles are shown in different colors (green: piriformis, orange: coccygeus, blue: iliococcygeus, yellow: obturator Internus, magenta: pubococcygeus, red: puborectalis)

Figure 2

Comparison (dynamic MR imaging vs. dynamic biomechanical analysis) of the pelvic structures of the female subject in the sagittal plane, at resting stage and at Valsalva stage. The black solid line in all pictures shows the location of urethra. The red curves in the dynamic MRI outline the bladder. (Abbreviations: Ut - Uterus, R - Rectum, B - Bladder, PB - Pubic Bone, V - Vagina, F - Fat, U - Urethra.)

Figure 3

Plots of urethral excursion angle against intra-abdominal pressure for (a) single tests and (b) group tests.

Figure 4

Deformation patterns of (a) intact test (b) weakened vaginal wall (c) weakened levator ani muscle and (d) weakened levator ani muscle together with vaginal wall.