In silico mechanical characterization of a new corneal implant for the treatment of keratoconus

The work presented in this thesis is the result of a collaboration between the start-up Recornea and Politecnico di Torino. The project aims to develop, validate clinically, and commercialize a new corneal implant for the restoration of the physiological shape of corneas affected by keratoconus (KC) and, above all, study its mechanical characterization. This disease involves the progressive irregular thinning and gradual bulging of the cornea into a conical shape, leading to detrimental refractive defects, including astigmatism, myopia, and a poor quality of life for keratoconus patients. Currently, the main treatments for corneal ectasia vary according to the structural changes of the cornea, the stage of KC, and the refractive error of the patient. These treatments range from non-invasive options capable of providing short-term and palliative results to invasive techniques for more long-term outcomes. The GROSSO® implant is a patented Nickel-Titanium (Nitinol) corneal implant intended to reshape deformed corneas. The purpose of this work was, firstly, to assess the mechanical behaviour of the newly developed design, numerically reproducing its characteristics in terms of material and geometry and, secondly, to evaluate the effect of the manufacturing process on real samples by comparison of the ideal and real geometries, after electropolishing. Within this context, finite element (FE) analysis represents a useful tool to address these challenges, aiming to predict material and device behaviours as well as their evolution. For these reasons, different FE model were developed to test the implant response under bending (resembling surgical implantation procedure) and compression configurations. Initially, 3D and 1D ideal models were developed (Rhinoceros v.7.0 (Robert McNeel & Associates, Seattle, WA, USA)). They were meshed using tetrahedral and beam elements, respectively(Hypermesh 2021 (Altair Engineering, USA)). In addition, different meshing element sizes were adopted to carry out a sensitivity analysis. Then, a “real” model was obtained from tomographic acquisitions and meshed with tetrahedral elements, allowing geometrical comparisons with the previously developed ideal ones (CloudCompare(GPL software, v. 2.12 beta, 2022)). Using FE analysis on all the generated models (ABAQUS Standard (Dassault Systemes Simulia Corp., Providence, RI, USA)), the distribution of maximum principal strains was evaluated, additionally verifying the risk of permanent deformations due to bending; also gaining information on the most stressed areas and deformed points due to reaction forces generated by the crushing setting. During bending, all models showed similar critical areas which corresponded to the zones where the device is bent, without reaching permanent deformations. The maximum strain values are 7.3% for the real model, and 6. 2% for ideal ones. Moreover, crushing simulations allow estimating reaction forces to test how the device responds to compression and the possible effects on the patient’s tissues. Their values are, in accordance with before, 0.88 N ,0.68 N and 0.44 N. Results have revealed good agreement between different models, confirming, in addition, the computational advantage provided by the use of 1D elements. The gained knowledge can be informative for future mechanical tests to be conducted in the laboratory to compare the results presented in this thesis and experimental data.