Depth-dependent biaxial biomechanical characterization of the corneal stroma: an experimental and numerical study.

The World Health Organization reports that over 2.2 billion people worldwide are affected by visual impairments, most of which occur in low- and middle-income countries. While vision problems are not life-threatening, they significantly impact the quality of life. Common refractive errors, such as myopia, hyperopia, presbyopia, and astigmatism, can be corrected using laser refractive surgery, which reshapes the patient’s cornea. However, challenges remain, including the risk of suboptimal postoperative results and ectasia, partly due to insufficient personalization of the cornea's mechanical properties in surgical planning. The cornea provides approximately two-thirds of the eye's refractive power, so even minor alterations in its shape can significantly affect vision. As the cornea maintains a constant mechanical equilibrium with intraocular pressure, small variations in its stiffness can disrupt this balance, altering its shape and thus affecting the patient’s vision. Therefore, an accurate characterization of corneal biomechanics is important to improve refractive interventions. The first objective of this work is to develop a protocol for biaxial tensile testing of corneal stroma at different depths. Then, this protocol was used to evaluate the biomechanical properties of the porcine cornea at various depths. The collected experimental data were finally used to define material parameters in a numerical model of corneal behavior. Corneal tissue is highly anisotropic, heterogeneous, and multilayered, with its mechanical properties varying by depth. To account for this, samples were extracted at three distinct depths: anterior, central, and posterior. Each specimen measured 7 mm × 7 mm, with a thickness of 150 μm. Eighteen corneas were biaxially tested, with six samples in each depth group. Alongside the experimental work, simulations were performed using FEBio software. The corneal strips were modeled in line with their actual dimensions and loading conditions from testing. The Holzapfel-Gasser-Ogden material model was employed, accounting for the extracellular matrix and the dispersion of collagen fibers. Experiments revealed that the mechanical behavior of the cornea is similar between the anterior and central layers. In contrast, the posterior layer shows a decrease in stiffness, indicating significant depth-dependent variations. A good fit was obtained between the results of the numerical simulations and the experimental data. Improved models can enhance predictions of how the cornea will respond to surgical interventions, refining both pre-operative planning and long-term outcome predictions. This study confirms depth's impact on the cornea's mechanical properties and underscores the advantages of biaxial tests, which better reflect physiological loads than uniaxial methods. Furthermore, the knowledge obtained through biaxial testing could be used to develop new instruments or diagnostic techniques that assess the biomechanics of an individual patient's cornea before surgery. Due to the scarcity of human corneal samples, porcine corneas were used to develop and evaluate the testing protocol. Future research should apply the developed testing methodology to human tissues. Our preliminary tests have already validated the suitability of this protocol on a small number of human specimens. While this work lays the foundation for understanding human corneal mechanics, further studies are necessary to refine the parameter estimation process and validate the findings in human corneas.