Regional elastic performance of the human cornea

The regional mechanical performance of the cornea and limbus was studied in vitro by pressure loading of 18 intact human eyes. The pressure-induced (2-100 mmHg) meridional and circumferential tangential mechanical strains of the epi-and endothelial side of the cornea were measured at the centre, the para-centre, the periphery and the limbus. Strains were computed from digital measures of distances between tiny mercury droplet markers fixed on the corneal surfaces. Corresponding in-plane membrane stresses were calculated from measurements of the regional meridional and circumferential radius of curvature and from measurements of the corneal thickness of the four regions. Young's moduli of elasticity were computed for each region in the meridional and circumferential direction assuming orthotropic elastic behaviour of the corneal stroma. A power function was used to fit the non-linear elastic stress-strain relationships. The pressure-induced meridional strains were smallest at the corneal paracentre and periphery, and largest at the limbus. The circumferential strains varied less between regions with the para-centre straining most. In the meridional direction, Young's modulus of elasticity was highest at the central and para-central corneal regions, whereas the highest circumferential elastic modulus was found at the limbus. This study supports the notation of circumferentially orientated reinforcing structures in human limbal tissue. The para-central region of the human cornea was found stiffer in the meridional direction compared with the circumferential direction, suggesting a meridionally orientated reinforcement of the para-central parts of the human cornea. These findings may have important implications for mechanical modelling of keratorefractive procedures.