Variation of the total mechanical energy density of the posterior human sclera with age and racial

Purpose: To quantify the variation with age of the peripapillary human sclera’s ability to absorb mechanical energy deformation due to the intra-ocular pressure (IOP) elevation in eyes form normal donors of African (AD) and European (ED) descent. Methods: Twenty-eight pairs of normal eyes from human donors (9 AD, 19 ED) aged 20 to 90 years were inflation tested within 48 hours post mortem. The intact posterior scleral shell of each eye was pressurized while the full-field, three-dimensional displacements of the outer scleral surface were measured using laser speckle interferometry (ESPI). By analytical differentiation of the displacement field, the mean maximum principal (tensile) strain was computed at 10 discrete pressure levels from 5 to 45 mmHg within the ~2-mm-wide band of peripapillary sclera surrounding the optic nerve head. An asymptotic functional form (strain = a + b*IOP + IOP/c) was used to fit the variation of strain at the increasing IOP (Figure; R2=0.99). The area under the strain-IOP curve, representing the total mechanical energy absorbed by the peripapillary sclera when inflated from 5 to 45 mmHg, was computed by analytical integration of the Max Principal Strain-IOP function. Results: In both the AD and ED groups, the capability of the peripapillary sclera to absorb IOP-induced deformation decreased significantly with age (p<0.001, Figure); this are-related loss of “toughness” was more pronounced in the AD group (p=0.0481). Conclusions: These results indicate 1) with advancing age, the peripapillary sclera suffers a significant loss in its ability to absorb mechanical energy from IOP-induced deformations. 2) AD eyes showed a significantly more rapid decline with age than ED eyes. These racial differences may increase the magnitude of transient IOP elevations and thereby contribute to the increased susceptibility of the elderly and persons of AD to glaucoma.