Computational simulation of the interaction between corneal tissue and a novel implantable Nitinol-based device for the treatment of keratoconus

Keratoconus is a progressive, asymmetric condition characterized by corneal thinning and cone-shaped corneal protrusion, leading to a decline of the visual acuity. The therapeutic choice depends on the severity of the pathology. In moderate or severe cases, corneal cross-linking or intrastromal corneal ring segment implantation (ICRS) is commonly used. However, these fail in the predictable recovery of corneal curvature. A new approach that aims to address the current limitations of keratoconus treatments has been developed by the medtech company Recornea and involves implanting the Grosso® Reshaper device inside the affected cornea. This Nitinol-based device is designed to restore the physiological curvature of the cornea and enhance the patient's quality of vision. The device can be implanted with a minimally invasive surgical procedure inside a corneal pocket created using a femto-laser. The finite element simulation of the surgical procedure was carried out in ABAQUS Standard (Dassault Systèmes Simulia Corp., Providence, RI, USA). First, the 3D model of the device was generated in Rhinoceros 3D software v.7.0 (Robert McNeel & Associates, Seattle, WA, USA), starting from the 2D design provided by Recornea. The 3D model of the cornea, on the other hand, was created starting from point clouds of the anterior and posterior surfaces of rabbit corneas acquired in vivo with an OCULUS Pentacam®. These surfaces were reconstructed with Matlab R2024a software (MathWorks, Natick, MA, USA) using a combination of Zernike polynomial fitting and quadratic surface fitting and then imported into Rhinoceros to obtain the complete 3D model. A tetrahedral mesh was chosen for the computational models of the cornea and the device, and a corneal pocket was created to insert the device. A superelastic constitutive law with non-linear material parameters obtained from experimental tensile tests performed at 37°C was adopted for the device and a fiber-reinforced hyperelastic Holzapfel-Gasser-Ogden model to characterize the corneal tissue. The parameters of the corneal material were obtained by fitting the stress-strain curve provided by Corvis ST device with the constitutive model using Hyperfit software. Then, a zero-pressure algorithm was implemented to obtain the stress-free configuration of the cornea. The cornea was then repressurized with an intraocular pressure of 9.12 mmHg. Finally, the device was implanted in the corneal pocket. The results were evaluated in terms of Maximum Principal Strain (MPS) and keratometric parameters. In the repressurized model of the cornea, the implantation of the device within the intracorneal pocket produced a MPS equal to 9.75%, in correspondence of the corneal area in contact with the apical region of the device. The mean keratometry (Kmean), defined as the average of the K1 (flattest meridian) and K2 (steepest meridian) values, was determined to be 48.618 D prior to insertion of the device into the intracorneal pocket, and 41.215 D post-insertion. The developed tool is able to evaluate the interaction between the device and corneal tissue, aiding the pre-surgical evaluation of the reshaping effect of the device and its biomechanical impact. The validation of numerical results will be conducted through comparison with experimental data obtained from the rabbit animal study and the ongoing patients study.