Elsevier

Journal of Biomechanics

Volume 48, Issue 9, 25 June 2015, Pages 1580-1586
Journal of Biomechanics

A multi-compartment 3-D finite element model of rectocele and its interaction with cystocele

https://doi.org/10.1016/j.jbiomech.2015.02.041Get rights and content

Abstract

We developed a subject-specific 3-D finite element model to understand the mechanics underlying formation of female pelvic organ prolapse, specifically a rectocele and its interaction with a cystocele. The model was created from MRI 3-D geometry of a healthy 45 year-old multiparous woman. It included anterior and posterior vaginal walls, levator ani muscle, cardinal and uterosacral ligaments, anterior and posterior arcus tendineus fascia pelvis, arcus tendineus levator ani, perineal body, perineal membrane and anal sphincter. Material properties were mostly from the literature. Tissue impairment was modeled as decreased tissue stiffness based on previous clinical studies. Model equations were solved using Abaqus v 6.11. The sensitivity of anterior and posterior vaginal wall geometry was calculated for different combinations tissue impairments under increasing intraabdominal pressure. Prolapse size was reported as pelvic organ prolapse quantification system (POP-Q) point at point Bp for rectocele and point Ba for cystocele. Results show that a rectocele resulted from impairments of the levator ani and posterior compartment support. For 20% levator and 85% posterior support impairments, simulated rectocele size (at POP-Q point: Bp) increased 0.29 mm/cm H2O without apical impairment and 0.36 mm/cm H2O with 60% apical impairment, as intraabdominal pressures increased from 0 to 150 cm H2O. Apical support impairment could result in the development of either a cystocele or rectocele. Simulated repair of posterior compartment support decreased rectocele but increased a preexisting cystocele. We conclude that development of rectocele and cystocele depend on the presence of anterior, posterior, levator and/or or apical support impairments, as well as the interaction of the prolapse with the opposing compartment.

Introduction

Pelvic floor dysfunction, including pelvic organ prolapse and stress urinary incontinence (SUI), leads one out of every ten women to undergo surgery in the U.S. (Olsen et al., 1997, Wu et al., 2014). Each year, over 200,000 operations are performed for prolapse (Boyles et al., 2003), with annual estimated cost for these operations exceeds US $1 billion (Subak et al., 2001).

A prolapse of the female pelvic organs can occur in one or both pelvic compartments. The anterior compartment contains the bladder and urethra while the posterior compartment contains the rectum and anus. The vagina separates these two compartments with its anterior and posterior vaginal walls. The upper portion of the vagina is suspended from the pelvic walls by cardinal ligament (CL) and uterosacral ligament (USL); these are actually mesenteric tissues that attach laterally to the pelvic sidewalls (DeLancey, 1992).

The underlying pathomechanics of cystocele, or anterior compartment prolapse, have begun to receive attention in terms of the associated geometric changes (Hsu et al., 2008a, Larson et al., 2010a, Larson et al., 2012a) as well as putative biomechanical changes (Chen et al., 2009). A knowledge gap remains, however, as to how and why a prolapse of the posterior compartment, a rectocele, forms. In addition, clinicians have recognized that there are important mechanical interactions between cystocele and rectocele. For example, surgical repair of one compartment is sometimes followed by development of new prolapse in the opposite compartment, despite its support appearing normal prior to the operation (Withagen et al., 2012). It is not clear why repairing one compartment unmasks weakness in the opposite compartment. Knowing how and why this occurs could lead to insights that could allow us to identify which women are risk for recurrence of prolapse following surgery so that the contralateral compartment could be reinforced at the time of the first operation.

The goal of this study, therefore, was to create a subject-specific 3-D anatomically-based finite element model which could develop either cystocele and/or rectocele depending on the particular impairments of the anterior and posterior pelvic compartment structural support systems. Of particular interest were the factors leading to the development of rectocele and comparing the results with observations in women with this condition. Furthermore, we sought to assess the model׳s ability to simulate rectocele interactions with the anterior vaginal wall under increasing intraabdominal pressures. If corroborated by others, this 3D finite element model could help design better pelvic floor surgical repairs.

Section snippets

Subject-specific model anatomy and finite element mesh

The finite element model was based on geometric data from a 45 year-old multiparous healthy woman, who was selected from an ongoing University of Michigan Institutional Review Board-approved (IRB # 1999-0395) case-control study of pelvic organ prolapse. Her BMI was 28.5 kg/m2, vaginal parity was 2 and she had not undergone any pelvic floor surgery. Her height (163 cm) was at the 50th percentile for women in United States (Fryar et al., 2012). This subject׳s vaginal dimensions (anterior vaginal

Results

To visualize the development of a typical rectocele, Fig. 3 shows the model simulation under systematically increasing values of intraabdominal pressure given 50% levator impairment, 70% apical impairment and 85% posterior support impairment, and with no anterior support impairment.

The model-generated simulation results for a rectocele are similar to those seen during the clinical examination of a patient with rectocele who is performing a Valsalva (Fig. 4B). The characteristic downward

Discussion

This subject-specific finite element model is based on the anatomy of a healthy woman who had no prolapse upon clinical examination, and includes the muscular and connective tissue structures that determine anterior and posterior compartment support. Hence it includes not just the organs but the important connecting structures such as the cardinal and uterosacral ligaments, both anterior and posterior tendineus arches, as well as the perineal body, perineal membrane that determine structural

Conflict of interest statement

Dr. John O. DeLancey and Dr. James A. Ashton-Miller do not have any conflicts of interest directly related to this study. The University of Michigan received funding from Johnson & Johnson, American Medical Systems, Kimberly-Clark Corporation, Proctor & Gamble, and Boston Scientific Corporation as partial salary support for research unrelated to the topic of this paper. They also received an honorarium and travel reimbursement for giving an invited research seminar at Johnson & Johnson.

Drs. Dee

Acknowledgments

We gratefully acknowledge support from the National Institute of Child Health and Human Development R01 HD 038665, the Office for Research on Women׳s Health SCOR on Sex and Gender Factors Affecting Women׳s Health P50 HD 044406, the National Center for Advancing Translational Sciences 2UL1TR000433 and BIRCWH K12 HD001438.

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