Finite Element Analysis And Mechanical Mechanism Of Posterior Vaginal Prolapsed Based On MR Imaging

Objective:To investigate the mechanical mechanism of the motion of posterior vagina wall(PVW)by comparing the MRI from resting state to the maximal Valsalva state in different stages of posterior vagina prolapse(PVP),then explore the method to build the finite element model of the PVW and the related structures by means of different software,and discussed the mechanical mechanism of PVP by simulating the support structure damage and exploring the effects of the support structure on the maximal stress in the PVW support system through the finite element analysis method and orthogonal test design.Method:1.According to the standard of MRI,62 patients with different levels of PVP and 12 patients without pelvic organ prolapsed(POP)were selected.The variation of apex vagina,distal cervix,distal vagina,mid-perineal body(PB),9 average points distributed along PVW,length of PVW,straight-line distance of PVW,levator ani(LA)parameters and urogential hiatus(UH)were measured on the MR images from resting state to the maximal Valsalva.Data were expressed as means ±standard deviation(SD)and were tested for normality using the Kolmogorov-Smirnov test.Continuous variables were analyzed using one-factor analysis of variance(ANOVA)combined with a Post-Hoc multiple comparisons test.Correlation analysis was performed by determining Spearman's correlation coefficients.2.The medical reconstruction software 3D Slicer,the reverse engineering software Geomagic studio,the FE preprocessing software ANSA and the FE analysis software ANSYS were selected.A finite element model of PVW with support structure were reconstructed from the original MRI data of healthy and non-pregnant woman through the conversion between different formats in different softwares.3.The FEM of PVW with support structure were used for simulating the action of intra-abdominal pressure,and compared the location and trend of maximum stress with other model constructing under the similar condition in other study.4.Simulated damaged in different parts and different degrees of PVW support structure and observed the location of maximum stress under orthogonal test design.Results:1.The difference of point D and Bp was statistically significant at the POP-Q scores of the clinical gynecological examination between 5 different groups of PVP.Apex vagina,distal cervix,distal vagina,mid-PB,LA parameters(including levator hiatus length(LHL),levator hiatus width(LHW),M line),UH and 9 average points distributed along the PVW(except point 2 and 4)in stage ? of PVP were significantly different with other stages.The displacement of point 2 and 4 was the highest in actual.2.The displacement of apex vagina,distal cervix,LA parameters and prolapse diameter(PD)increased gradually from stage 0 to ?.From stage ? to ?,while the variation of levator plant angle(LPA),anorectal angle(ARA)and PD continue to increase,apex vagina,distal cervix,LHL,LHW and M line started to decline.The displacement of mid-vaginal,distal vaginal and mid-PB decreased slightly from stage 0 to ?,then increased from stage ? to ? and dropped again at stage ?.3.In the "recessive prolapse" phase,LA parameters and the second level of support structure were related to UH respectively,and the correlation coefficient of the former is greater than the latter.However the third level of support structure was not associated with UH.In the "dominant prolapse" phase,there was a correlationbetween the first level of support structure,LA,the second level of support structure,the third level of support structure and UH each other,and the correlation coefficient of the first level of support structure with other structures is the greatest.Prolapse grading was related to LPA,ARA,length of PVW and PD,but was not relate to straight-line distance of PVW.4.MR images of healthy pregnant women stored in the DICOM format were selected and imported into 3D Slicer software to reconstruct three-dimensional geometric model in *.stl format.After optimized into the *.iges format surface model by Geomagic Studio software,then meshed in the ANSA software and saved as a CAD format to generate a FEM.5.In this study,the FEM of PVW with support system was constructed and stress analysis was carried out without support structure impairment.It was found that the maximum stress was located in the middle and lower part of PVW,and has the same trend with other model established under the similar condition in other study,which proves the validity of the model and can be used for the subsequent numerical simulation.6.When setting the boundary of the cardinal-uterosacral ligament to zero,the maximum stress appears at the front end of LA.With the increase the impairment of LA,the maximum stress is transferred from cardinal-uterosacral ligament and uterine-vaginal junction to both sides of the PB.7.Utilizing 5 factors and 3 horizontal orthogonal test design table,force simulation was carried out under situation of impairment in different parts or different degrees of support structure,we found that on the role of the maximum stress,cardinal-uterosacral ligament,PB,LA,rectovaginal fascia(RVF)and paravagianl support gradually decreased.Conclusions:1.By comparing the differences between the resting state and the maximal Valsalva of MRI in patients with PVW,the movement patterns of PVW with related anatomical structures were found,and the mechanical factors that formed the transformation and the mechanical mechanism of PVP could be speculated.2.In the process of PVP,support structures and PVW itself are changing.The impairment of LA is the basis of PVP,among the three levels of support structure,the first level has the strongest effect in the support system,the second and the third level are weakened in turn.3.The reconstruction of the FEM by a series of software can be applicable to PVW with its support system,which could be used in other complex irregular human models.4.Combined with FE analysis and orthogonal test design,we found that the impact on the maximum stress of support structure,the first level,the second level and the third level descent successive,and the integrity of the structure and function of the LA is the important part of bearing the stress.