Modeling of peri-implant corneal tissues and simulation of their interaction with a new intracorneal implant for keratoconus correction

Keratoconus is a progressive, non-inflammatory disease, characterized by stromal thinning and gradual protrusion of the cornea into a conical shape, resulting in irregular astigmatism and reduced visual acuity. Current treatment options available for the treatment of keratoconus suffer from major limitations, in particular Intra-Corneal Ring Segments (ICRS) offer scarce remodeling support to the cornea leading to unpredictable clinical results. The GROSSO® implant is the world's first nitinol corneal implant aiming at remodeling the whole cornea by virtue of a dome-shaped design. The mechanical performance of the GROSSO® implant was evaluated computationally in two potentially critical situations. The first situation consists of a 90° bending of two opposite edges of the implant occurring during its minimally invasive implantation in the patient's intracorneal pocket. The second is a vertical crushing of the device to test its mechanical strength and provide information on possible impacts occurring in daily use. The 3D model of the GROSSO® implant was developed from 2D geometry provided by Recornea, using Rhinoceros v.7.0 (Robert McNeel & Associates, Seattle, WA, USA) and meshed with tetrahedral elements in Hypermesh 2021 (Altair Engineering, USA). Using finite element (FE) analyses (ABAQUS Standard (Dassault Systemes Simulia Corp., Providence, RI,USA)), displacement-driven bending simulations and device crushing between two rigid plates with three different coefficient of friction (0, 0.1 and 0.57) were performed to obtain the maximum principal strain undergone by the device, verifying the absence of permanent deformations. Afterward, a mesh sensitivity analysis was carried out gradually reducing the size of mesh elements. The maximum principal strain (MPS) reached by the device in the bending simulation is 5.83 %, while the crushing simulation conducted with a friction coefficient of 0, 0.1 and 0.57 indicated a MPS of 3.372%, 3.527% and 3.473%, respectively. Additionally, a computational model of the cornea was developed to evaluate the interaction between the corneal tissues and the GROSSO® implant and the reshaping effect of the implant itself. The corneal model was created in Rhinoceros v.7.0 starting from references in the literature about the curvature and thickness of standard healthy corneas and then meshed with hexahedral elements in Hypermesh 2021. To allow the implantation of the device, a pocket with the same thickness and curvature was created. The mechanical behaviour of the corneal tissue was modelled by implementing two different hyperelastic constitutive laws with non-linear material parameters (Yeoh and Neo-Hookean), and a simple linear elastic constitutive law. The simulations showed that the MPS values, both in bending and crushing, never reached the permanent deformation range, consequently the super elasticity of the Grosso® implant is preserved. To investigate the behaviour of the different materials in response to an intraocular pressure of 15.5 mmHg and the device insertion, the MPS values were compared which are as follows: 3.117% for the linear elastic model, 7.318% for the Yeoh hyperelastic model and finally 5.584% for the Neo-Hookean hyperelastic model. The study could be further extended by taking into account the typical anisotropy of the corneal tissues with two families of orthogonal collagen fibres and including different patient-specific geometries.