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3D Finite Element Pelvic Floor Modeling

Manfred Maurer

office: LEE N 208
phone: +41 44 633 9228
email: maurer@imes.mavt.ethz.ch


Pelvic floor dysfunctions affect a wide range of women of all age groups, more than 10% of which require surgery. However the mechanisms resulting in such pathologies are still poorly understood.

   

In order to get a better understanding of the patient specific factors leading to pelvic floor weakness, we propose a new diagnostic tool conceptualized in a virtual 3D finite element model of the pelvic region.
This was built based on a finite element mesh of a segmented geometry from MRI data of a pregnant healthy woman, provided by the group of Prof. Brieu in Lille, France. It consists of the pelvic floor muscles, bladder, vaginal canal/uterus and several important ligaments. The applied boundary conditions are representative of physiological conditions.


Current methods to evaluate pelvic region kinematics in vivo (under MRI or ultra sound) consist mostly of applying a downwards pressure by either the patient itself (intra-abdominal pressure) or by pushing externally on the belly region. This is rather inconsistent and cannot be precisely mechanically controlled. We develop a new procedure with which intra vaginal boundary conditions are applied and controlled precisely. In a preliminary study the influence of the interface between the bladder and vaginal canal on the kinematics of the pelvic system has been investigated. This interface (fascias and connective tissue) was included as an individual model part with adjustable mechanical characteristics (see images, soft interface on the left, hard interface on the right).

 

 

 

 

Additional responsibility: Continuation of the project of Dr. Barbara Röhrnbauer:

 

Prosthetic meshes are implanted to repair weaknesses in the abdominal wall in case of hernia or to reconstruct the anatomy in case of pelvic organ prolapse. The structural supporting function of these meshes requires biocompatibility also in terms of their mechanical properties.

Objective: This study was aimed at providing experimental and theoretical methodologies to mechanically characterize prosthetic meshes and to evaluate aspects of their mechanical biocompatibility [8t].

In vivo mechanical characterization of the vaginal wall using the aspiration technique

A specific version of the aspiration device [14p] was used to assess in vivo mechanical properties of the anterior vaginal wall. Correlations were detected between stiffness like parameters and pre- and postoperative states, indicating vaginal wall stiffening due to a surgical intervention. In contrast, no correlations were found between mechanical properties and the stage of POP.


 

Combined uniaxial and biaxial mechanical characterization of prosthetic meshes in a rabbit model and using a non-biological model system

Different mesh types (light weight, heavy weight) ingrown in the host tissue in a rabbit model and according dry meshes embedded in an elastomeric matrix were mechanically characterized in uniaxial and biaxial (inflation [23p]) loading conditions [41p]. The findings show that a compliant-to-stiff ranking depends on the loading conditions and therefore procedures for mechanical characterization have to consider the specific medical application.

Mechanical characterization and modeling of a dry mesh including different length scales

A dry prosthetic mesh (large porous, light weight) was experimentally characterized in uniaxial and biaxial loading conditions, and in four different loading directions (Figure, only the two preferred material directions are shown).

A structural model of a representative mesh unit cell was developed based on the theory of multi-body systems (Figure). Geometry and force elements/laws were defined including physical considerations and observations.

A comparison between the experimental response and the model response show good descriptive capabilities of the model, both with respect to the force response and the kinematic response (Figure).

 

 

 

Partners
Prof. Brieu, Laboratoire de Mécanique de Lille, France
Prof. Dr. Jan Deprest, Katholieke Universiteit Leuven, Belgium
Dr. David Scheiner, Department of Gynecology, University Hospital Zurich
Prof. Dr. Caroline Maake, Institute of Anatomy, University of Zurich

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09/05/18 | Francesco Filotto | ZfM | ETH