Finite-element models

		Reconstructed fibre architecture of a human uterus Finite Element mesh with applied boundary conditions. Initial uterine cavity. Predicted distended uterine cavity.

The aim of this project is to formulate an anatomically detailed numerical model of the human uterus that can serve as a benchmark reference for further developments, such as simplified real-time models.

The behaviour of the human uterus under an internal (intracavital) pressure of 150 mm Hg (20 kPa) was modelled. The application of such an intracavital or intrauterine pressure corresponds to the inflation procedure (hydrometra) performed at the beginning of hysteroscopy. The volume of the distended uterine cavity was predicted in three dimensional finite element model simulations implementing two hyperelastic material laws. Different material parameters were derived using both in- vivo aspiration experiments on human uteri (see experimental methods for soft-tissue characteristics), as well as from ex-vivo tensile test measurements on rabbit uteri for comparison purposes. The calculated results are in general agreement with in-vivo measurements performed on patients at the University Hospital of Zurich.

The fibre architecture of an organ can largely affect its mechanical behaviour. The fibre structure of the uterine wall (see picture 1.) was determined by magnetic resonance (MR) diffusion tensor imaging (DTI). For this type of measurement, uteri were fixed with agarose gel within a plastic box. MR DTI was performed using sensitivity encoding (SENSE). From the diffusion weighted images, the main diffusion vector of water molecules was determined which, under the assumption that the diffusion direction and the fibre orientation in the tissue coincide, allows the determination of the global fibre architecture.

Based on the results of the DTI measurements, an anisotropic constitutive model is being studied for adaptation of the fibre architecture into a FE model of the uterus. To this end, for each tetrahedral element of the model, a vector representing the main fibre direction must therein be defined. The non-fibrous components of the uterus in which the fibres are embedded are modelled as an isotropic matrix material.