The distribution of strain in the human cornea

Experimental data on the mechanics of human cornea is meager and sometimes flawed. Moreover, questions regarding the correct material symmetry and the role of the fibrous microstructure are usually glossed over when mechanical models of the cornea and corneal shape changing procedures are presented. Accordingly, the deformation of 14 intact human corneas was measured for five pressures in the physiologic range (0, 5, 10, 25 and 45 cmH2O) by tracking small, self-adherent particles placed on their anterior surfaces. The meridional strains, calculated in five regions assuming axisymmetric deformation, are small; the average strain in the apical region being 1.14% at 45 cmH2O. Results also indicate that the strain distribution is unexpectedly nonuniform with statistically significant (p < 0.01, typical) variations between regions and a minimum occurring approximately half-way between apex and limbus. To better understand these results, a finite-element model (FEM) of the cornea was constructed and used to simulate the experiment. The heterogeneous model shows that our data may reflect the changing fiber orientation along a meridian suggested in the literature. The implications of a link between microstructure and mechanics are discussed in light of clinical procedures, such as radial keratotomy, the outcomes of which are dependent on corneal mechanical properties.