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Abstract: The Dumon silicone stent is widely used in human medicine for treating tracheal diseases. Although presenting many advantages, as its facility of positioning and removing and its relative low cost, commonly reported complications include stent migration, inflammatory granulation tissue formation, and obstruction. Taking into account this last aspect, in this work we propose an improved stent design which may help in preventing episodes of mucous accumulation. In particular, we changed the design of the stent extremities in order to improve the flow field which cyclically crosses the prosthesis. To evaluate the performances of this new design, a finite element model of a human stented trachea was developed using a fluid–structure interaction approach (FSI). The geometry of the trachea is obtained from computed tomography (CT) images of a healthy patient. The simulations were performed using a finite element-based commercial software code. The tracheal wall is modeled as a fiber reinforced hyperelastic solid material in which we introduced the anisotropy due to the orientation of the fibers. Different new prosthesis designs were tested. Results showed that linear and parabolic transitions of the top and bottom stent extremities reduce the local vorticity field preventing high local flow recirculations during breathing. [Copyright &y& Elsevier]

Abstract: Background and aims: Cartilage and smooth muscle constitute the main structural components of the human central airways, their mechanical properties affect the flow in the trachea and contribute to the biological function of the respiratory system. The aim of this work is to find out the mechanical passive response of the principal constituents of the human trachea under static tensile conditions and to propose constitutive models to describe their behavior. Methods: Histological analyses to characterize the tissues and mechanical tests have been made on three human trachea specimens obtained from autopsies. Uniaxial tensile tests on cartilaginous rings and smooth muscle were performed. Tracheal cartilage was considered an elastic material and its Young''s modulus and Poisson''s coefficient were determined fitting the experimental curves using a Neo-Hookean model. The smooth muscle was proved to behave as a reinforced hyperelastic material with two families of collagen fibers, and its non-linearity was investigated using the Holzapfel strain-energy density function for two families of fibers to fit the experimental data obtained from longitudinal and transversal cuts. Results: For cartilage, fitting the experimental curves to an elastic model, a Young''s modulus of 3.33MPa and ν =0.49 were obtained. For smooth muscle, several parameters of the Holzapfel function were found out (C 10 =0.877kPa, k 1 =0.154kPa, k 2 =34.157, k 3 =0.347kPa and k 4 =13.889) and demonstrated that the tracheal muscle was stiffer in the longitudinal direction. Conclusion: The better understanding of how these tissues mechanically behave is essential for a correct modeling of the human trachea, a better simulation of its response under different loading conditions, and the development of strategies for the design of new endotracheal prostheses. [Copyright &y& Elsevier]

Abstract: Background and aims: Nowadays interventions associated to the implantation of tracheal stents in patients with airway pathologies, are a very common surgery that in the post-operating period can bear many problems such as migration of the stent, development of granulation tissue at the edges of the stent with overgrowth of the tracheal lumen, or accumulation of secretions inside the prosthesis. Among the movements that trachea carries out, swallowing seems to drive harmful consequences for the tracheal tissues surrounding the prosthesis. In this work a finite element model of a human trachea has been developed and used to analyze its behavior during swallowing. Material and methods: In the present work, a complete human trachea finite element model based on experimental study was developed. The real swallowing movement of two patients before and after the implantation of Dumon prosthesis was used to simulate and then analyze the effect that the tracheal implant has on the stress response of the trachea and on the physiological capacity to swallow. Results: In both studied cases with an implanted Dumon prosthesis, patients showed a decrease of their ability to swallow; one lost 26.4% and the other one 18.9% of their tracheal ascending movements. Besides, the prosthesis implantation caused an increase of the stresses located in the superior contact border between the tracheal wall and the prosthesis. It could be seen that the resulting force equivalent to the elevating tracheal muscle forces for degluting, was around F =13.5N for the two patients both before and after the stent implantation. Conclusion: The implantation of a Dumon prosthesis modifies the mechanical response of the trachea altering its stress distribution and its ascending movement. [Copyright &y& Elsevier]

Abstract: Endotracheal stenting is a common treatment for tracheal disorders as stenosis, cronic cough or dispnoea episodes. However, medical treatment and surgery are still challenging due to the difficulties in overcoming potential prosthesis complications. In this work we analyze the response of the tracheal wall during breathing and coughing conditions under different stent implantations. A finite element model of a human trachea was developed and used to analyze tracheal deformability after prosthesis implantation under normal breathing and coughing using a fluid-structure interaction approach (FSI). The geometry of the trachea is obtained from computed tomography (CT) images of a healthy patient. A structured hexahedral-based grid for the tracheal wall and an unstructured tetrahedral-based mesh with coincident nodes for the fluid were used to perform the simulations with a finite element-based commercial software code. Tracheal wall is modeled as a fiber reinforced hyperelastic solid material in which the anisotropy due to the orientation of the fibers is taken into account. Deformations of the tracheal cartilage rings and of the muscle membrane, as well as the maximum principal stresses in the wall, are analyzed and compared with those of the healthy trachea in absence of prosthesis. The results showed that, the presence of the stent prevents tracheal muscle deflections especially during coughing. In addition, we proposed a methodology to evaluate, through numerical simulations, the predisposition of the stent to migrate. [Copyright &y& Elsevier]

Abstract: This work is focused on the analysis of the response of the tracheal wall to different ventilation conditions. Thus, a finite element model of a human trachea is developed and used to analyze its deformability under normal breathing and mechanical ventilation. The geometry of the trachea is obtained from computed tomography (CT) images of a healthy man. A fluid structure interaction approach is used to analyze the deformation of the wall when the fluid (in this case, air) moves inside the trachea. A structured hexahedral-based grid for the tracheal walls and an unstructured tetrahedral-based mesh with coincident nodes for the fluid are used to perform the simulations with the finite element-based commercial software code (ADINA R & D Inc.). The tracheal wall is modeled as a fiber reinforced hyperelastic solid material in which the anisotropy due to the orientation of the fibers is introduced. Deformation of the tracheal walls is analyzed under different conditions. Normal breathing is performed assuming a sinus shape of the pressure at the inlet and air speed at the outlet based on real data which represent the inspiration and the expiration processes respectively. Mechanical ventilation is simulated as smooth square shape velocity airflow considering positive values of pressure using data from a mechanical ventilation machine. Deformations of the tracheal cartilage rings and of the muscle membrane, as well as the maximum principal stresses in the wall, are analyzed. The results show that, although the deformation and stresses are quite small for both conditions, forced ventilation does not exactly imitate the physiological response of the trachea, since with always positive pressure values the trachea does not collapse during mechanical breathing. [Copyright &y& Elsevier]