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Biomechanical and Structural Characterization of Bridging Veins

Boek - Dissertatie

Traumatic brain injury (TBI) is the fourth leading cause of death and a major cause of disability in the Western world. This dissertation focuses on a specific type of TBI, i.e. acute subdural hematoma caused by bridging vein (BV) rupture. This occurs when the relative motion between the brain and the skull causes a BV, that connects or "bridges" these two structures, to rupture. The etiology of this pathology can be investigated through finite element (FE) modelling which is a powerful tool that is becoming more and more popular in head impact research due to the increase of cheap computational power. Increasing the biofidelity of BV representation in FE head models is paramount in order to investigate the etiology of BV rupture under linear and rotational accelerations due to impacts. This in turn, will provide a trustworthy tool in the design optimization of protective devices such as helmets. Therefore the goal of this dissertation is to provide more biofidelic data on the material and geometrical properties of BV. The first step in this dissertation was to microscopically investigate the structure of BV and provide quantitative data on collagen fibre orientation of human BV in a way that can be used to model this tissue. The microscopical analysis is done both when the tissue is at 0% and at 50% strain in order to visualize the collagen structure in its reference state and under what is considered by literature to be its ultimate strain. The second step was to perform a multiaxial characterization of the mechanical behaviour of BV. Due to the presence of collagen the mechanical behaviour was considered to be nonlinear and transversely isotropic. During a BV rupture the material experiences both tensile and shear deformations. Therefore a planar biaxial tester was modified to apply a combination of tensile and shear deformations and was equipped with a triaxial load cell to measure the resulting forces. Human BV were tested with this set up and the Gasser Ogden Holzapfel (GOH) material model was fitted on the data to obtain nonlinear transversely isotropic material parameters. As a third step, the direction of inflow from the BV into the superior sagittal sinus (SSS) was adapted in the KTH FE head model to assess the effect of this geometrical parameter when simulating previously performed acute subdural hematoma cadaver experiments. The entry angle to the SSS is adapted according to CT angiogram data from 78 patients where the mean, the maximum and the minimum observed entry angle of the BV to the SSS was obtained. The results of the simulations both from the original model and from the models with the updated angle datasets were compared to assess possible improvement in BV rupture predictability. Results showed improvement in estimating the location of the rupture along the cortex with the new dataset. Finally, as a last part for this dissertation the same CT angiogram method was used to determine the distribution of BV along the cortex and the differences in BV diameters in different brain lobes in humans. The hypothesis that BV location and BV diameter are related, was verified. This can be applied in existing FE head models that have a geometrical representation of BV and further improve the performance of the models. To summarize, BV representation in the current FE head models is oversimplified and this has an impact in terms of their rupture predictability performance. Throughout this work, data have become available that can be used to improve the representation of BV in the model both mechanically and geometrically.
Jaar van publicatie:2019
Toegankelijkheid:Open