A 3D finite element model of anterior vaginal wall support to evaluate mechanisms underlying cystocele formation

Abstract

Objectives-To develop a 3D computer model of the anterior vaginal wall and its supports, validate that model, and then use it to determine the combinations of muscle and connective tissue impairments that result in cystocele formation, as observed on dynamic magnetic resonance imaging (MRI).Methods-A subject-specific 3D model of the anterior vaginal wall and its supports was developed based on MRI geometry from a healthy nulliparous woman. It included simplified representations of the anterior vaginal wall, levator muscle, cardinal and uterosacral ligaments, arcus tendineus fascia pelvis and levator ani, paravaginal attachments, and the posterior compartment. This model was then imported into ABAQUS™ and tissue properties were assigned from the literature. An iterative process was used to refine anatomical assumptions until convergence was obtained between model behavior under increases of abdominal pressure up to 168 cmH 2 O and deformations observed on dynamic MRI.Results-Cystocele size was sensitive to abdominal pressure and impairment of connective tissue and muscle. Larger cystocele formed in the presence of impairments in muscular and apical connective tissue support compared to either support element alone. Apical impairment resulted in a larger cystocele than paravaginal impairment. Levator ani muscle impairment caused a larger urogenital hiatus size, longer length of the distal vagina exposed to a pressure differential, larger apical descent and resulted in a larger cystocele size.Conclusions-Development of a cystocele requires a levator muscle impairment, an increase in abdominal pressure, and apical and paravaginal support defects.

Biomechanics of pelvic organ prolapse

“…The results showed that if the pubovisceral impairment reached at a certain level, the anterior vaginal wall would prolapse; at 60% pubovisceral muscle impairment, a 90% impairment of apical support caused the prolapse to increase from 0.3 to 1.9 cm. In 2009, a three-dimensional (3D) finite element model of the anterior vaginal wall and its support system were developed by Chen et al [8]. Effects of the apical impairment and/or levator ani muscle impairment on prolapse size were evaluated.…”

“…The anterior vaginal wall was usually modeled as a separated part from the posterior compartment, which was entirely made up of the posterior vaginal wall and rectum. In addition, the ligaments were simulated by passive elastic springs [7] or 3D truss elements [8]. The oversimplification made it impossible to evaluate the shear stress in the lateral vaginal walls, which is, however, eagerly needed clinically.…”

“…In previous studies, the pelvis was usually assumed to be fixed. The origins of the levator ani muscle and the cardinal and uterosacral ligaments from the pelvic wall were set stationary in 2D simulation [7], rotatable but not translatable in 3D simulation [8]. The problem located in the perineal body, which was usually assumed to be stationary [7].…”

“…In Chen's 3D simulation of POP, the distal vagina was set fixed since there is no support of the perineal body in this model. However, the distal vagina was sometimes observed moving downward clinically [8]. According to the anatomy, the lower portion of the perineal body is attached to the ischiopubic rami by superficial transverse perineal muscles.…”

“…The existing FE studies of POP [7,8] applied a constant pressure on the anterior vaginal wall to simulate Valsalva maneuver. However, abdominal pressure is filling the whole pelvic floor in reality.…”