Peripapillary scleral stiffening increases optic nerve head shear deformation in porcine eyes during IOP elevation

Computational models predicted that peripapillary sclera (PPS) stiffening may reduce intraocular pressure (IOP) induced stresses and strains at the optic nerve head (ONH). Experimental studies in animal models of glaucoma however did not confirm such benefit. In this study, we used high-resolution ultrasound elastography to quantify PPS and ONH strains to evaluate biomechanical changes associated with PPS stiffening. Sixteen pairs of porcine globes were used. One globe in each pair was randomly assigned to the stiffening or the control group. Inflation tests were performed before and after treatment with 1.25 % glutaraldehyde (stiffening group) or saline (control group). IOP was raised from 5 to 30 mmHg while 2D cross-sectional images of the PPS and ONH were obtained using a 50 MHz ultrasound probe. PPS and ONH displacements and strains were calculated using a validated ultrasound speckle tracking algorithm. In the stiffening group, there was an increase in ONH shear strain (by 11.8 %, P = 0.025) and an increase in the posterior displacement of the ONH relative to PPS (P < 0.001). In contrast, ONH shear strain decreased (by 8.1%, P=0.006) and the relative ONH posterior displacement did not change in the control group. ONH tangential strains were reduced in both groups, but more so in the stiffening group. These results suggest that the biomechanical effects of PPS stiffening are complex. While a reduction of ONH tangential strain could be protective, the concomitant increase in ONH shear strain and ONH posterior displacement could be detrimental. Future studies are needed to optimize PPS biomechanical properties for neuroprotection.