Drained secant modulus for human and porcine peripapillary sclera using unconfined compression testing

Glaucoma is an ocular disease characterized by damage of the optic nerve head (ONH) resulting in blindness. Recent research has identified the material properties of the sclera as being an important factor in the biomechanics of major load bearing tissues near the ONH. Most mechanical investigations performed on sclera have focused on the tensile behavior of this tissue, neglecting its compressive stiffness. The present study characterized the compressive moduli of peripapillary sclera using an unconfined compression (UCC) technique, for both human and porcine sources. UCC stress-relaxation tests were performed on human and porcine peripapillary scleral samples at 5%, 10% and 15% sequential compressive strain. Our results indicate a linearly decreasing drained equilibrium stress (at 5%) with age in male human samples, ranging from 79.4 Pa at 78 yrs to 40.1 Pa at 89 yrs of age. The drained secant modulus (E₅) of human and porcine sclera was found to be 1.1 ± 0.08 kPa and 3.9 ± 0.57 kPa, respectively. Our experimental results also reveal a non-linear increase in drained equilibrium stress with increasing compressive strain. The compressive stiffness of sclera, as reported here, provides important information on the mechanical response of peripapillary ocular tissues. This information will be useful in future computational simulations of the sclera, especially as they relate to understanding mechanical damage near the ONH. Furthermore, our results indicate that age-related changes in the biomechanical response of the sclera occur, suggesting that these factors may be playing a role in the increasing prevalence of glaucoma with age.