Racioethnic Differences in Human Posterior Scleral and Optic Nerve Stump Deformation

Abstract

Purpose: The purpose of this study was to quantify the biomechanical response of human posterior ocular tissues from donors of various racioethnic groups to better understand how differences in these properties may play a role in the racioethnic health disparities known to exist in glaucoma.

Methods: Sequential digital image correlation (S-DIC) was used to measure the pressure-induced surface deformations of 23 normal human posterior poles from three racioethnic groups: African descent (AD), European descent (ED), and Hispanic ethnicity (HIS). Regional in-plane principal strains were compared across three zones: the optic nerve stump (ONS), the peripapillary (PP) sclera, and non-PP sclera.

Results: The PP scleral tensile strains were found to be lower for ED eyes compared with AD and HIS eyes at 15 mm Hg (P = 0.024 and 0.039, respectively). The mean compressive strains were significantly higher for AD eyes compared with ED eyes at 15 mm Hg (P = 0.018). We also found that the relationship between tensile strain and pressure was significant for those of ED and HIS eyes (P < 0.001 and P = 0.004, respectively), whereas it was not significant for those of AD (P = 0.392).

Conclusions: Our results suggest that, assuming glaucomatous nerve loss is caused by mechanical strains in the vicinity of the optic nerve head, the mechanism of increased glaucoma prevalence may be different in those of AD versus HIS. Our ONS strain analysis also suggested that it may be important to account for ONS geometry and material properties in future scleral biomechanical analysis.

Figure 1

Posterior scleral pole showing (A) the dividing plane based on ONS saddle ring points and the division into (B) ONS, PP scleral, and non-PP scleral zones, (C) S, T, I, and N regions, and (D) 12 spatial locations defined by regions and zones.

Figure 2

Microscope image of full optic nerve cross section used for semiautomated axon counting. Dark sections indicate bundles of axons separated by lighter connective tissue. Scale bar denotes 500 μm.

Figure 3

A representative example of E1 and E2 strain maps at 45 mm Hg from each racioethnic group. Four views are included for each sample: top view (upper left), isoparametric view (upper right), back side view (lower left), and front side view (lower right). Note: the color map for the E2 strain maps has been reversed compared with the E1 strain maps to emphasize absolute value.

Figure 4

Mean E1 and E2 value estimate zonal comparisons for all inflation pressures for each racioethnic group. *P < 0.05. Error bars denote SE as estimated by the statistical model.

Figure 5

Regression plots of mean E1 and E2 values as a function of pressure for all racioethnic groups within each zone.

Figure 6

Regression plots of mean E1 and E2 values as a function of optic nerve axon count at 45 mm Hg within each zone.

Figure 7

ONS volume (left), diameter (middle), and length (right) comparisons between racioethnic groups. Error bars denote 95% confidence interval.

Figure 8

Regression plots of mean E1 and E2 values as a function of ONS volume for all inflation pressures within each zone.