Regional variations in mechanical strain in the posterior human sclera

Purpose: To establish regional variability of the mechanical behavior of peripapillary and mid-peripheral sclera in normal eyes of elderly human donors. Methods: Ten pairs of normal eyes from human donors aged 57 to 90 years old were mechanically inflation tested within 48 hours post mortem as follows. The intact posterior scleral shell of each eye was pressurized from 5 to 45 mmHg while the full-field three-dimensional displacements of the scleral surface were measured using laser speckle interferometry. A customized B-spline system was used to fit the displacement field to continuous and differentiable analytical functions, from which the full strain tensor of the outer sclera surface was calculated assuming tissue incompressibility. Mean maximum principal (tensile) strain was computed overall and for eight circumferential sectors (45° wide) withi n the peripapillary and mid-peripheral regions surrounding the optic nerve head (ONH). The peripapillary region was defined as a ~9 degreewide- band adjacent to the ONH and the mid-peripheral region as a ~6 degree-wide-band immediately outside the peripapillary region.Results: On average, the peripapillary sclera exhibited significantly higher tensile strain (1.2%) than mid-peripheral sclera (0.95%) for a 40 mmHg IOP elevation (p<0.001). Regional differences are presented in the Figure. In the peripapillary region, the inferior sector exhibited the highest tensile strain (1.35%) while the superotemporal had the lowest strain (1.12%). In the mid-peripheral region, the temporal sector had the highest tensile strain (1.07%) while the superonasal sector exhibited the lowest (0.86%). Conclusions: Human posterior sclera exhibits highly complex mechanical behavior in response to acute IOP elevations from 5 to 45 mmHg. These results indicate 1) the peripapillary sclera is subjected to significantly higher tensile strain than the midperipheral sclera, and 2) there is a tendency for strain to concentrate inferiorly in the peripapillary sclera, which could contribute to the increased susceptibility of focal glaucomatous damage seen in the inferior ONH.