Indentation Testing of the Optic Nerve Head and Posterior Sclera

The mechanical behavior of the optic nerve head (ONH) and the surrounding sclera play important roles in the development of optic neuropathy. We assessed the indentation behavior of the ONH and the surrounding sclera in unfixed human autopsy eyes from individuals between 27 and 87 years of age. Our testing device utilized a round-tipped 250 µm diameter stainless steel probe to measure the force applied in discrete 50 µm steps from the surface of the tissues to a depth of 400 µm. Thirteen eyes from eight individuals were indented in various locations within and around the ONH and the posterior sclera. Tissue thickness, force and depth were recorded at each position tested. Data was analyzed by evaluating the recorded force as a function of the depth of indentation. The data showed that, for both the sclera and the ONH, the force rose exponentially with increased depth of indentation. Apparent stiffness values were estimated using 2 different equations that assess soft biologic tissues. Results showed that significantly higher levels of force were required with aging to indent the posterior sclera, with a corresponding increase in stiffness values. However, no significant increase in indentation force as a function of age was noted for the ONH. Interestingly, the data suggests that some individuals have relatively large differences in the stiffness between the ONH and the posterior sclera, while others show less difference regardless of age. This data supports the notion that some individuals have relatively softer ONH tissue, and as the sclera becomes stiffer with age, this difference is magnified. This difference may be important in understanding the biomechanical contribution to the individually different susceptibility to glaucoma.