Scleral shell stiffening in severe experimental glaucoma in the nonhuman primate (NHP)

posterior scleral shells of four NHPs in which severe experimental glaucoma had been induced in one eye. Methods: NHP posterior scleral shells were pressurized from 5 to 45 mmHg and the resulting full-field, three-dimensional, scleral surface deformations were measured using laser speckle interferometry. Scleral strain (local tissue deformation) was calculated directly from displacements assuming tissue incompressibility and zero strain at 5 mmHg. Maximum principal strain was averaged for the peripapillary and mid-peripheral regions, defined as the ~9 degree-wide-band adjacent to the ONH and a ~6 degree-wide-band immediately outside the peripapillary region, respectively. Results: Scleral tensile strain was higher in the peripapillary region than in the mid-periphery in the normal eyes of 3 of 4 NHPs (Table). In the two NHPs in which regional mean scleral tensile strain in the normal eye did not exceed ~1.5%, there was no appreciable change in tensile strain measured in the contralateral glaucoma eye for an IOP elevation from 5-45 mmHg. In the two NHPs in which mean scleral tensile strain in the normal eye exceeded ~1.5% when pressurized to 45 mmHg, there was a significant decrease in tensile strain measured in the contralateral glaucoma eye for an identical IOP elevation. Conclusions: These results indicate that structural stiffness of an individual eye when normal is critical in determining the magnitude of scleral remodeling that occurs in response to chronic IOP elevation. In eyes with compliant sclera, the sclera is stretched as IOP becomes elevated, which engenders a strong remodeling response that significantly stiffens the sclera as glaucoma progresses. Conversely, in stiff eyes that are resistant to scleral tensile strain at elevated IOP, the remodeling response (if present) does not result in an appreciable change in scleral stiffness as the eye is exposed to chronically elevated IOP. Taken together, these results suggest that the connective tissue remodeling response engendered by a given IOP differs widely between individual eyes, and is at least partially driven by the strain experienced by the tissues.