In silico characterization of the interaction of corneal tissue with a nitinol intracorneal implant for the treatment of keratoconus

Keratoconus is a progressive bilateral and asymmetric eye disease of the cornea. It causes progressive corneal thinning and a cone-shaped corneal protrusion, resulting in irregular astigmatism and decreased visual acuity. The management of keratoconus depends on the degree of progression of the pathology. Today, the options available include both non-surgical treatments (e.g., glasses and contact lenses) and surgical treatments (e.g., corneal collagen crosslinking treatments (CXL), corneal ring segment implantation, and keratoplastic). However, these procedures can only limit the progression of the disease, but fail in restoring the physiological curvature of the cornea. The GROSSO® implant is an innovative corneal implant in nitinol that aims to restore the physiological curvature of the cornea. The device can be implanted with a minimally invasive surgical procedure inside a corneal pocket created using a femto-laser. The three-dimensional (3D) model of the GROSSO® implant was developed starting from a two-dimensional (2D) design provided by Recornea Srl, using the software Rhinoceros 3D v.7.0 (Robert Mcneel & Associates, Seattle, WA, USA). Subsequently, the model was imported into Hypermesh 2021 (Altair Engineering, USA) and was meshed with hexahedral elements. Finite-elements simulations were carried out in ABAQUS Standard (Dassault Systèmes Simulia Corp., Providence, RI, USA) adopting a superelastic constitutive law with non-linear material parameters obtained from experimental tensile tests performed at 37°C. In addition, an idealized 3D corneal model was created, using Rhinoceros 3D v.7.0 software (Robert Mcneel & Associates, Seattle, WA, USA) using literature data. The corneal model was meshed with hybrid hexahedral elements using Hypermesh 2021. The implantation of the device inside the corneal model was performed by creating a pocket inside the corneal model with the same thickness and curvature as the device. The mechanical behavior of the developed corneal model was modeled by implementing two hyperelastic constitutive laws with nonlinear material parameters (Yeoh and Neo-Hookean) and a linear elastic constitutive law. The behavior of the different corneal materials was evaluated by reproducing the physiological conditions of the cornea by applying an intraocular pressure of 0.002066 MPa (15.5 mmHg) and considering a coefficient of friction between the device and the corneal tissue of 0.1. A sensitivity analysis was carried out gradually reducing the size of mesh elements both for the device and the corneal model. The Maximum Principal Strain % values obtained for the different corneal material models are as follows: 5.407% for the linear elastic model, 10.039% for the hyperelastic Neo-Hookean model and finally, 11.999% for the hyperelastic Yeoh model. Moreover, a preliminary digital keratoconus cornea was developed starting from a healthy 3D corneal model. The mechanical behaviour of the keratoconus corneal tissue was reproduced using a hyperelastic constitutive law (Yeoh) for the entire model, but the elastic modulus of the corneal apex was reduced to simulate the weakening effect of keratoconus. The developed models allow for the detailed investigation of the interaction between device and tissue, the functional assessment of the novel corneal implant by evaluating its anatomical fitting, ultimately paving the way to personalized implants according to the “in silico clinical trial” paradigm.