The Mechanical Mechanism Of Pelvic Floor Morphology On Pelvic Organ Prolapse

Objective: To assess the morphological features of the levator ani and bony pelvis in Chinese women with different grades of prolapse by measuring the parameters relevant to the levator ani and bony pelvis based on reconstructed three-dimensional models, and then to establish different patterns of finite element models of the levator ani, visualize the deformation of them, and explore the mechanical mechanism of the pelvic floor disorders caused by defects of the levator ani muscle.Methods: Ninety-three Chinese women with different grades of pelvic organ prolapse(POP) were studied, 18 asymptomatic nulliparous volunteers as normal controls. Resting supine T2-weighted magnetic resonance(MR) images were obtained from the subjects. Three-dimensional(3-D) models which included pelvic bones, levator ani, pubic symphysis, and the sacrococcygeal bone, were generated from these images. The width and length of the levator hiatus, levator symphysis gap, width of the iliococcygeus, and iliococcygeal angle were measured to assess the levator ani morphology; while the intertuberous diameter, interspinous diameter, angle of the pubic arch, and pubococcygeal line were measured on the 3-D models to characterize the bony pelvis morphology. Four subject-specific finite element models, which included one left defect, one right defect, one bilateral defect, and one intact levator ani model, were reconstructed based on magnetic resonance imaging(MRI) data from four live subjects, and then were validated by comparing the experimental data with that of the dynamic MRI data retrieved from the same subject. By means of the finite element method, four simplified models were used to simulate the deformation under a gradually increasing abdominal pressure from 0 to 4k Pa with 1k Pa increments. The value and color maps of displacement were recorded for each model.Results: 1. Four patterns of the levator ani morphology of Chinese women with and without prolapse were presented, the intact levator ani, left avulsion, right avulsion, and bilateral avulsion levator ani, which were classified on the basis of the integrity of the pubovisceral muscle. 2. Among the bony pelvic parameters of the anterior and middle compartment, the pubococcygeal line was found to be significantly different between the subjects with prolapse and normal controls, also among subjects with different grades of prolapse, but not significantly different among the subjects of the posterior compartment with different grades of prolapse. 3. Significant differences were seen in the width and length of the levator hiatus and levator symphysis gap among women with different grades of prolapse in the anterior and middle compartment, and the four parameters increased with the increasing grade of prolapse. 4. Women with prolapse in the study group were more likely to have levator defects; avulsion rates in different stages of prolapse in the anterior and middle compartment were significant, but not significant in the posterior compartment. 5. The four finite element models of the levator ani muscle were created and were validated by comparing the displacement value under abdominal pressure of 4k Pa with that measured from the dynamic MRI of the same subject performing the same maneuver. 6. The four models showed different deformation and displacement distributions under the same material properties and the same load. The maximum displacement value and deformation were observed in the avulsion area of the levator ani muscle, and asymmetric distribution of displacement occurred in the left and right defect models.Conclusions: 1. The bony pelvis and levator ani morphological features in Chinese women with different grades of prolapse can be intuitively observed in vitro based onreconstructed three-dimensional models. 2. Not all the women with prolapse have levator ani avulsion, compared with iliococcygeal muscle morphology, changes in pubovisceral muscle morphological features are more likely to accompany prolapse. 3. The three-dimensional finite element model can be build based on medical imaging data by using the software of 3DSlicer, Geomagic studio and ANSYS. 4. A levator defect could result in the decreased capability of the pelvic support system for carrying abdominal pressure, and cause the discontinuity of pressure transmission along the pubovisceral portion of the levator ani muscle. The loss of stability of the pelvic support system is an etiology of the pelvic floor disorders from a mechanical point of view.