FINITE ELEMENT MODEL FOCUSED ONSTRESS DISTRIBUTION IN THE LEVATOR ANI DURING VAGINAL DELIVERY IN THEPERSISTENCE OF AN INITIAL OCCIPUT POSTERIOR POSITION.
L. KROFTA 1, L. HAVELKOVA 2,I. URBANKOVA1, M. KRCMAR 1, O. GOJIS 1,L. HYNCIK 2, J. FEYEREISL 1;
1Inst.for the care of mother and child, Prague, Czech Republic, 2NewTechnologies – Res. Ctr., Univ. of West Bohemia, Plzen, CzechRepublic.
Introduction: Vaginal deliveryseverely affects pelvic floor anatomy and function. Occiput posteriorposition of the fetus is found in about 5% of deliveries, but it isassociated with an increased risk of maternal perineal trauma andanal sphincter injury.
Objective: Firstly, we simulatedchanges in the levator ani muscle (LAM) induced by the fetus that isinitially in the occiput posterior position and whose movementsrespect the cardinal movements of the labor spec. persistence of theocciput posterior position (OPP). Secondly, observed intrapartumchanges in the LAM were compared to our previous simulation of thefetus is in the occiput anterior position (OAP).
Methods: Asubject-specific 3D finite element model of the female pelvic floorand fetal head was developed. The model geometry was based on MRIdata from a healthy young nulliparous woman and a 1-day-old child.The structures included the pelvic bones, the internal obturatormuscle, and subdivisions of the LAM. The pelvis was fixed in alldegrees of freedom. The fetal head trajectory was given by the curveof Carus and followed the cardinal movements of labor. Cardinalmovements corresponded to the rotation and trajectory during deliveryof either occiput posterior or anterior position of the fetus. Thebones were modeled by rigid bodies, the soft tissues were representedby viscoelastic nonlinear Ogden material model. Surroundingstructures, as well as supporting systems, were replaced by boundaryconditions. For the stations, the pelvis was divided above and belowthe ischial spines into 5 levels. At the level of the ischial spines,the internal rotation was completed.
Results: The maximalstress values in the iliococcygeus portion of the LAM at station +5were 9.8 - 12.4 MPa for OPP compared to 3.7 - 5.8 MPa in OAP. In thepubovisceral and puborectal attachments on the pubic bone, themaximal values were 10.8 - 52.8 MPa (OPP) and 2.5 - 22.1 MPa (OAP).The highest stress values were induced in the posteromedial sectionof the puborectalis portion, and reached up to 28.2 - 108.3MPa in OPPas compared to 6.9 - 18.3 MPa in OAP. During the delivery of thefetus in occiput posterior position, the midsagittal length of thelevator ani tripled compared to its initial resting position. In theOAP muscle elongated 2.5-fold (Figure 1).
Conclusions: Thedistribution of the stretch ratio along the MLA is heterogeneous. Thecardinal movements of labor significantly affect the subsequentstress distribution in the levator ani. The absolute stress valueswere highest during vaginal occiput posterior delivery in thepersistence of an initial OPP in comparison of vaginal delivery ofthe head in the optimal initial OAP.
References: Ref -