Inflation test of the human optic nerve head using digital volume correlation

Understanding the mechanical behavior of the Optic Nerve Head (ONH) is important for understanding the pathophysiology of glaucoma. We have developed an inflation test that uses second harmonic generation (SHG) imaging and digital volume correlation (DVC) to measure the deformation response of the lamina cribrosa, the connective tissue structure of the ONH, to controlled pressurization. Human eyes were obtained from a tissue bank. The episclera and extraocular tissues were removed, the optic nerve was cut flush with the sclera, and the eye, secured to a holder, was pressurized using a saline solution injected through the anterior chamber. Pressure was controlled using a manometer. A laser-scanning microscope was used to acquire SHG images of the deforming tissue structure, and the DVC algorithm was used to post-process the SHG volumes to calculate the 3D displacement field. We evaluated the baseline error and effect of creep by applying the DVC algorithm to sequential z-stacks acquired at the same pressure separated by different hold times. The method was applied to measure the deformation response to pressure elevations representative of glaucoma.