Mechanical and finite element models of corneal keratoprostheses

Introduction. When developing ocular prostheses, a number of problems arise, one of which is the construction of the connection between the hard optical part and the soft corneal tissue. Their Young's modules can differ by three orders of magnitude. In this case, the problem arises of creating an intermediate layer, possibly with gradient properties, whose purpose is to exclude injury to soft biological tissues. Two types of keratoprostheses are considered: the first type with a support plate and the second type with an intermediate functionally gradient layer. The stress-strain state of the prosthesis is calculated for the first type. For the second type, analytical and finite element modeling of the interaction of a cylindrical optical prosthesis, an intermediate inhomogeneous layer, and the cornea was carried out in the elastic media. Two versions are considered: discounting the curvature (circular plate or plate) and with account of the curvature (spherical dome or shell). The work objective is to study the stress-strain state of the keraprosthesis and cornea in the contact area.

Materials and Methods. Mathematical models of the structures under consideration are the boundary value problems of the linear elasticity theory. The analytical solution is constructed for a simplified model in the form of a composite circular plate. Spatial three-dimensional problems and axisymmetric problems are solved by the finite element method. Finite element modeling of the considered structures was performed in the CAE package ANSYS and ACELAN. Results. CAD models of keratoprostheses with conditions of fixing and loading are constructed. The load acting on the keraprosthesis under the effect of intraocular pressure was determined. The stress-strain state of the keratoprosthesis and cornea elements was calculated. Special attention was paid to the area of its contact with the keratoprosthesis.

Discussion and Conclusions. The results of calculating the axial displacements and mechanical stresses in the first type of keratoprosthesis show that the selected geometric parameters meet the kinematic and strength requirements. The proposed models of the deformed state of soft biological tissues provide assessing their injury when using a keratoprosthesis of the second type, as well as selecting the geometric parameters and gradient properties of the intermediate layer.

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