A novel numerical modelling approach for keratoplasty eye procedure.

Objective of the work is to investigate stress and deformation that conrneal tissue and donor graft undergo during endothelial keratoplasty. In order to attach the donor graft to the cornea, different air bubble pressure profiles acting on the graft are considered. This study is carried out by employing a three-dimensional nonlinear finite element methodology, combined with a contact algorithm. The ocular tissues are treated as isotropic, hyper-elastic and nearly-incompressible materials. The contact algorithm, based on the penalty-based node-to-surface approach, is used to model the donor graft-corneal interface region. First, the proposed computational methodology is tested against benchmark data for bending of the plates over a cylinder. Then, the influence of geometrical and material parameters of the graft on the corneal contact-structural response is investigated. The results are presented in terms of Von Mises stress intensity, displacement and mean contact force. Results clearly indicate that the air bubble pressure plays a key role in the corneal stress and strain, as well as graft stiffness and thickness.

A novel patient-oriented numerical procedure for glaucoma drainage devices.

The present work analyses the performance of four glaucoma drainage devices, by means of a novel patient-oriented numerical procedure. The procedure is based on the three-dimensional geometry reconstruction from the stacks of tomographic images of a human eye, at different angles, on meshing and on thermo-fluid dynamics modelling activities, carried out on the reconstructed computational domain. The current three-dimensional eye model considers anterior chamber (AC), trabecular meshwork, Schlemm's canal, and collector channels, making use of generalised porous medium approach for modelling ocular porous tissue and cavities. The intraocular pressure (IOP) management inside AC of human eye is analysed, by comparing the results obtained for four drainage devices implanted in a human eye for glaucoma treatment, ie, ExPRESS shunt, iStent inject, SOLX gold micro shunt, and the novel silicon shunt device. The numerical results allow predicting the effects of the installation of these implants on human eyes, in terms of IOP decrease, aqueous humour velocity, pressure, friction coefficient, and local Nusselt number, pointing out the clear distinction between pre-operative and post-operative eye conditions for different glaucoma surgical techniques.

A generalised porous medium approach to study thermo-fluid dynamics in human eyes.

The present work describes the application of the generalised porous medium model to study heat and fluid flow in healthy and glaucomatous eyes of different subject specimens, considering the presence of ocular cavities and porous tissues. The 2D computational model, implemented into the open-source software OpenFOAM, has been verified against benchmark data for mixed convection in domains partially filled with a porous medium. The verified model has been employed to simulate the thermo-fluid dynamic phenomena occurring in the anterior section of four patient-specific human eyes, considering the presence of anterior chamber (AC), trabecular meshwork (TM), Schlemm's canal (SC), and collector channels (CC). The computational domains of the eye are extracted from tomographic images. The dependence of TM porosity and permeability on intraocular pressure (IOP) has been analysed in detail, and the differences between healthy and glaucomatous eye conditions have been highlighted, proving that the different physiological conditions of patients have a significant influence on the thermo-fluid dynamic phenomena. The influence of different eye positions (supine and standing) on thermo-fluid dynamic variables has been also investigated: results are presented in terms of velocity, pressure, temperature, friction coefficient and local Nusselt number. The results clearly indicate that porosity and permeability of TM are two important parameters that affect eye pressure distribution. Graphical abstract Velocity contours and vectors for healthy eyes (top) and glaucomatous eyes (bottom) for standing position.

Temperature Effect on Rheological Behavior of Silicone Oils. A Model for the Viscous Heating.

The rheological behavior of silicone oils, (CH3)3SiO-[Si(CH3)2O]n-Si(CH3)3, and their mixtures is studied. Shear-stress measurements, in the temperature range of 293-313 K, reveal that this polymer family is a group of shear-thinning liquids with a yield stress below which no flow occurs. Experimental diagrams, i.e., shear stress versus shear rate, are satisfactorily described by the Casson fluid model over a wide range of shear rates. In order to monitor the effect of temperature on fluid properties, Casson's rheological model is reformulated using the fictitious shear rate, γ̇f, and the infinite-shear viscosity, η∞, as constitutive parameters. Due to low intermolecular forces and high chain flexibility, γ̇f varies very little when the temperature increases. For this reason, the apparent material viscosity depends on temperature only through η∞, which exponentially decreases until high shear rates are reached, and there is more alignment possible. Interestingly, the temperature sensitivity of this pseudoplastic behavior is the same for all of the silicone oils investigated; therefore, they can be classified according to their tendency to emulsify. Experimental results are then used to model the flow of silicone oils in a cylindrical pipe and estimate the temperature increase due to viscous heating. Numerical results show that the normalized temperature, i.e., ratio of fluid temperature to wall temperature, increases approximately 23%, and the apparent viscosity decreases drastically, going toward the center of the tube. The non-Newtonian nature of fluid is reflected in the presence of a critical region. In this region, the velocity and temperature gradients vanish. Since silicon oil is a surgical tool, we hope that the acquired physicochemical information can provide help to facilitate the removal of this material during surgical procedures.