Abstract
Purpose :
Chemical crosslinking (CXL) produces a local stiffening of the cornea, which results in a modification of ocular optical properties. As the CXL agent is delivered to the tissue using passive diffusion, the resulting spatial distribution of the stiffening remains poorly described. We hypothesize that the effect of CXL is heterogeneous and that it doesn’t only modifies the tissue stiffness, but also its permeability and viscoelasic properties.
Methods :
Using a bioindenter from Anton Paar TriTec (Austria), nanoindentation creep tests were carried out in 11 control and 11 CXL donor eyes. A spherical ruby tip (R=1mm) was used to measure in three different corneal locations: central (C:0-1mm), para-central (B:1-2.5mm), and peripheral (A:2.5-4mm). Force-controlled experiments were performed; a load of 50µN was gradually applied with a ramp of 30s, followed by a plateau of constant force for 180s.
A grid-based optimization using finite element modelling was carried out to identify the poroviscoelastic response of the corneal tissue (E: Young’s modulus; k: permeability; γ: viscoelastic coefficient; τ: relaxation time; ν=0.075). These mechanical parameters were calibrated using 10,000 simulations for each region (A, B, C) of the control and CXL samples.
Results :
Control and CXL corneas showed a heterogeneous distribution of the material properties. The E was higher in the centre and decreasing towards the periphery. Also, CXL increased E up to 3 times in the centre and up to 2 times at the mid-periphery. On the other hand, the CXL doesn't affect the properties at the peripher (Fig.1). The remaining parameters were also affected: k decreased towards the periphery for control, while decreased dramatically at the centre for CXL; γ remained constant for both cases; and τ increased in the region affected by CXL while remained constant for control.
Conclusions :
Nanoindentation measurement was able to characterize the spatial modification in the corneal biomechanics associated with CXL treatment. Based on the parameters identified for the different region, we were able to determine the region of the cornea affected by CXL. Results of this study also indicate that CXL not only significantly increase the Young’s modulus, but also induces a decrease of the permeability of the tissue.
This is a 2020 ARVO Annual Meeting abstract.