Regional variations in the mechanical strains of the human optic nerve head

The mechanical behavior of the lamina cribrosa (LC), the connective tissue that supports the retinal ganglion cells (RGCs), plays an important role in the development of glaucoma. We have developed an in-vitro inflation test to measure the three-dimensional displacement and strain fields in the posterior 250 μm of the LC in response to controlled pressurization. Human eyes were obtained from a tissue bank and the posterior sclera mounted in a pressure-controllable fixture. A laserscanning microscope was used to acquire SHG images of the deforming tissue structure in response to pressure changes, and the fast-iterative digital volume correlation (FIDVC) algorithm developed by Bar Kochba et al. (Experimental Mechanics 55:261–274, 2015) was used to calculate 3D displacement fields from the SHG signal. Strains were calculated by fitting a high order polynomial function to the displacement field and taking the gradient. To compare for regional variation of the strains, the LC was divided about the central retinal artery and vein (CRAV) into the superior, nasal, inferior, and temporal quadrants and further divided into inner, middle, and outer radial regions centered on the CRAV and three equal regions through the thickness. These regions were statistically compared using a univariate repeated anova. We found that displacements and strains in the LC exhibited substantial variation in the plane but not through the thickness. The average shear strains were significantly smaller than the normal strains (p ¼ 0.0006, 45 mmHg). For all eyes, the LC appeared as an elliptical plate. The normal strains were greater along the major axis compared to the minor axis of the LC for all eyes tested (p = 0.03, 45 mmHg). The maximum principal strains were also found to be greater in the middle inferior and temporal regions than in the superior and nasal regions (p = 0.07, 45 mmHg). Eyes exhibited two kinds of radial variation. Three out of six specimens exhibited the stiffest behavior around the CRAV with higher principal strains towards the periphery, while the remainder were more compliant around the CRAV with lower principal strain on the periphery. These regional variations in pressure-derived strains in the LC may help to explain the progression of axonal damage observed in glaucoma.