Age and Racial Differences in Mechanical Energy Absorption in the Posterior Human Sclera

Abstract

 

Purpose

 

To establish the age and racial differences in the peripapillary human sclera’s capability to absorb mechanical energy from IOP-induced deformation in 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 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 was computed by analytical integration.

 

Results

 

In both the AD and ED groups, the ability of the peripapillary sclera to absorb IOP-induced deformation decreased significantly with age (p<0.001, Figure), and was more pronounced in the AD group (p<0.05).

 

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.

 

 

Figure (Top): strain-IOP response of the peripapillary sclera for 3 representative pairs of human eyes. Experimentally-derived maximum principal strain at 10 pressure levels (green dots) was fit to an asymptotic function (grey curves). The area under the curve (AUC, yellow) represents the IOP-induced ability of the peripapillary sclera to absorb mechanical energy per unit tissue volume. (Bottom): AUC values for the ED (blue) and AD (red) groups and of the predicted means for increasing age. The sclera’s capability to absorb IOP-induced deformations decreased with age in both the ED (p<0.001), and AD groups (p<0.001). The rate of change with age was more pronounced in the AD group (p<0.05)

 

Keywords: 708 sclera • 413 aging