Luyun Chen

Interaction among apical support, levator ani impairment, and anterior vaginal wall prolapse.

OBJECTIVE: To use a biomechanical model to explore how impairment of the pubovisceral portion of the levator ani muscle, the apical vaginal suspension complex, or both might interact to affect anterior vaginal wall prolapse severity. METHODS: A biomechanical model of the anterior vaginal wall and its support system was developed and implemented. The anterior vaginal wall and its main muscular and connective tissue support elements, namely the levator plate, pubovisceral muscle, and cardinal and uterosacral ligaments were included, and their geometry was based on midsagittal plane magnetic resonance scans. Material properties were based on published data. The change in the sagittal profile of the anterior vaginal wall during a maximal Valsalva was then predicted for different combinations of pubovisceral muscle and connective tissue impairment. RESULTS: Under raised intra-abdominal pressure, the magnitude of anterior vaginal wall prolapse was shown to be a combined function of both pubovisceral muscle and uterosacral and cardinal ligament (“apical supports”) impairment. Once a certain degree of pubovisceral impairment was reached, the genital hiatus opened and a prolapse developed. The larger the pubovisceral impairment, the larger the anterior wall prolapse became. A 90% impairment of apical support led to an increase in anterior wall prolapse from 0.3 cm to 1.9 cm (a 530% increase) at 60% pubovisceral muscle impairment, and from 0.7 cm to 2.4 cm (a 240% increase) at 80% pubovisceral muscle impairment. CONCLUSION: These results suggest that a prolapse can develop as a result of impairment of the muscular and apical supports of the anterior vaginal wall. LEVEL OF EVIDENCE: II-2

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

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 were 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 cm H(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.

Anterior vaginal wall length and degree of anterior compartment prolapse seen on dynamic MRI

The objective of the study was to determine the relationship between midsagittal vaginal wall geometric parameters and the degree of anterior vaginal prolapse. We have previously presented data indicating that about half of anterior wall descent can be explained by the degree of apical descent present (Summers et al., Am J Obstet Gynecol, 194:1438–1443, 2006). This led us to examine whether other midsagittal vaginal geometric parameters are associated with anterior wall descent. Magnetic resonance (MR) scans of 145 women from the prior study were suitable for analysis after eight were excluded because of inadequate visibility of the anterior vaginal wall. Subjects had been selected from a study of pelvic organ prolapse that included women with and without prolapse. All patients underwent supine dynamic MR scans in the midsagittal plane. Anterior vaginal wall length, location of distal vaginal wall point, and the area under the midsagittal profile of the anterior vaginal wall were measured during maximal Valsalva. A linear regression model was used to examine how much of the variance in cystocele size could be explained by these vaginal parameters. When both apical descent and vaginal length were considered in the linear regression model, 77% (R2 = 0.77, p < 0.001) of the variation in anterior wall descent was explained. Distal vaginal point and a measure anterior wall shape, the area under the profile of the anterior vaginal wall, added little to the model. Increasing vaginal length was positively correlated with greater degrees of anterior vaginal prolapse during maximal Valsalva (R2 = 0.30, p < 0.01) determining 30% of the variation in anterior wall decent. Greater degrees of anterior vaginal prolapse are associated with a longer vaginal wall. Linear regression modeling suggests that 77% of anterior wall descent can be explained by apical descent and midsagittal anterior vaginal wall length.

Measurement of the pubic portion of the levator ani muscle in women with unilateral defects in 3-D models from MR images

Develop a method to quantify the cross‐sectional area of the pubic portion of the levator ani muscle, validate the method in women with unilateral muscle defects, and report preliminary findings in those women.

On pelvic reference lines and the MR evaluation of genital prolapse: a proposal for standardization using the Pelvic Inclination Correction System

Five midsagittal pelvic reference lines have been employed to quantify prolapse using MRI. However, the lack of standardization makes study results difficult to compare. Using MRI scans from 149 women, we demonstrate how use of existing reference lines can systematically affect measurements in three distinct ways: in oblique line systems, distances measured to the reference line vary with antero-posterior location; soft issue-based reference lines can underestimate organ movement relative to the pelvic bones; and systems defined relative to the MR scanner are affected by intra- and interindividual differences in the pelvic inclination angle at rest and strain. Thus, we propose a standardized approach called the Pelvic Inclination Correction System (PICS). Based on bony structures and the body axis, the PICS system corrects for variation in pelvic inclination, at rest of straining, and allows for the standardized measurement of organ displacement in the direction of prolapse.

Vaginal thickness, cross-sectional area, and perimeter in women with and those without prolapse

OBJECTIVE: Use axial magnetic resonance imaging to test the null hypothesis that no difference exists in apparent vaginal thickness between women with and those without prolapse. METHODS: Magnetic resonance imaging studies of 24 patients with prolapse at least 2 cm beyond the introitus were selected from an ongoing study comparing women with prolapse with normal control subjects. The magnetic resonance scans of 24 women with prolapse (cases) and 24 women without prolapse (controls) were selected from those of women of similar age, race, and parity. The magnetic resonance files were imported into an experimental modeling program, and 3-dimensional models of each vagina were created. The minimum transverse plane cross-sectional area, mid-sagittal plane diameter, and transverse plane perimeter of each vaginal model were calculated. RESULTS: Neither the mean age (cases 58.6 years ± standard deviation [SD] 14.4 versus controls 59.4 years ± SD 13.2) nor the mean body mass index (cases 24.1 kg/m2± SD 3.3, controls 25.7 kg/m2± SD 3.7) differed significantly between groups. Minimum mid-sagittal vaginal diameters did not differ between groups. Patients with prolapse had larger minimum vaginal cross-sectional areas than controls (5.71 cm2± standard error of the mean [SEM] 0.25 versus 4.76 cm2± SEM 0.20, respectively; P = .005). The perimeter of the vagina was also larger in the prolapse group (11.10 cm ± SEM 0.24) compared with controls (9.96 cm ± SEM 0.22) P = .001. Subgroup analysis of patients with endogenous or exogenous estrogen showed prolapse patients had larger vaginal cross-sectional area (P = .030); in patients without estrogen group differences were not significant (P = .099). CONCLUSION: Vaginal thickness is similar in women with and those without pelvic organ prolapse. The vaginal perimeter and cross-sectional areas are 11% and 20% larger in prolapse patients, respectively. Estrogen status did not affect differences found between groups.

See it in 3D!: Researchers examined structural links between the cardinal and uterosacral ligaments

However, important structural specifics of the cardinal ligament’s attachments and its relationship with the uterosacral ligament, as seen in MR imaging of living women, have not been fully established. We used MR cross-sectional imaging and 3-dimensional (3D) modeling to study characteristic features of the cardinal ligament. At the same time, we examined anatomicdistinctionsandstructuralrelationshipsbetweenthe cardinalanduterosacralligamentsrelevanttotheirroleinapical support.

Magnetic resonance imaging-based three-dimensional model of anterior vaginal wall position at rest and maximal strain in women with and without prolapse.

Introduction and hypothesisTwo-dimensional magnetic resonance imaging (MRI) demonstrates apical support and vaginal length contribute to anterior wall prolapse (AWP). This paper describes a novel three-dimensional technique to examine the vagina and its relationship to pelvic sidewalls at rest and Valsalva.MethodsTwenty women (10 with AWP and 10 with normal support) underwent pelvic magnetic resonance imaging at rest and Valsalva. Three-dimensional reconstructions of the pelvic bones and anterior vaginal wall were created to assess morphologic changes occurring in prolapse.ResultsIn women with AWP, Valsalva caused downward translation of the vagina along its length. A transition point separated a proximal region supported by levator muscles and a distal, unsupported region no longer in contact with the perineal body. In this latter region, sagittal and frontal plane “cupping” occurs. The distal vagina rotated inferiorly along an arc centered on the inferior pubis.ConclusionDownward translation, cupping, and distal rotation are three novel characteristics of AWP demonstrated by this three-dimensional technique.

Quantification of Levator Ani Cross-Sectional Area Differences Between Women With and Those Without Prolapse

OBJECTIVE: Compare levator ani cross-sectional area as a function of prolapse and muscle defect status. METHODS: Thirty women with prolapse and 30 women with normal pelvic support were selected from an ongoing case-control study of prolapse. For each of the two groups, 10 women were selected from three categories of levator defect severity: none, minor, and major identified on supine magnetic resonance scans. Using those scans, three-dimensional (3D) models of the levator ani muscles were made using a modeling program (3D Slicer), and cross-sections of the pubic portion were calculated perpendicular to the muscle fiber direction using another program, I-DEAS. An analysis of variance was performed. RESULTS: The ventral component of the levator muscle of women with major defects had a 36% smaller cross-sectional area, and women with minor defects had a 29% smaller cross-sectional area compared with the women with no defects (P<.001). In the dorsal component, there were significant differences in cross-sectional area according to defect status (P=.03); women with major levator defects had the largest cross-sectional area compared with the other defect groups. For each defect severity category (none, minor, major), there were no significant differences in cross-sectional area between women with and those without prolapse. CONCLUSION: Women with visible levator ani defects on magnetic resonance imaging had significantly smaller cross-sectional areas in the ventral component of the pubic portion of the muscle compared with women with intact muscles. Women with major levator ani defects had larger cross-sectional areas in the dorsal component than women with minor or no defects. LEVEL OF EVIDENCE: II-2

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

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.