Development And Mechanical Characterization Of A Novel Nitinol Corneal Implant For The Treatment Of Keratoconus

The GROSSO^® implant is the first-ever nitinol corneal implant aiming at reshaping deformed keratoconus corneas by restoring their physiological curvature by virtue of a dome-shaped design. The mechanical properties of the device have been investigated both experimentally and computationally, encompassing two bench tests that replicate potentially critical scenarios. The first is a longitudinal bending of two opposite edges of the implant, an operation enabling the minimally invasive implantation in the patient's intracorneal pocket. The second is a vertical crushing of the device to measure its overall strength and evaluate the effects of potential impacts on the eye with the GROSSO^® implant in it during daily use.

PoliToBIOMed Lab, Department of Mechanical and Aerospace Engineering, Politecnico di Torino. National Research Council, ICMATE Unit of Lecco. Recornea srl.

FINANCIAL DISCLOSURE: This activity is part of the project HUMANeye which has received funding from the European Union’s Horizon 2020 research and innovation program under grant agreement No 878719.This work is part of the project PNNR-NGEU, which has received funding from the MUR – DM 352/2022.

Uniaxial tensile tests were performed at 37°C with the Electropuls E3000 (Instrom) on dog bone-shaped specimens of the raw material to obtain stress-strain data in accordance with relevant standards. From these, the strain values at which plastic deformation occurs were determined. Moreover, bending and crush tests were performed on the device at 37 °C using a DMA Q800 (TA Instruments) with a thermal chamber and cell load of 18 N to obtain force-displacement curves. A numerical model able to replicate these tests virtually were set up in Abaqus (Dassault Systèmes Simulia Corp.), implementing a superelastic material constitutive law with parameters extracted from uniaxial tensile tests.

The GROSSO^® implant can be bent manually for the implantation with minimal effort, as the force required does not exceed 0.1 N. Excellent agreement was found between the experimental and computational bending tests in terms of force for all displacements (mean absolute error: 0.003 N). The computational simulation indicated a maximum principal strain value during bending well below the limit strain for plasticity of 12%. Force values for the complete crushing of the device were equal to 0.35 N in the numerical test and in the range 0.35-0.43 N when testing experimentally three different devices.

The developed computational framework allows to predict the influence of implant design characteristics on the resulting mechanical performance of the device itself and after implantation on eye tissues, as well as to evaluate the mechanical response in critical conditions. The preliminary validation of the computational model demonstrates its credibility, offering a cost-effective strategy for the assessment of the mechanical performance. Further investigation will be undertaken to understand and mitigate the discrepancies in the crush tests. Moreover, the anatomical fitting and the remodelling effect imposed by the GROSSO^® implant will be evaluated by simulating the implantation of the device in keratoconic corneal anatomies.