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Viscoelastic Material Properties of the Peripapillary Sclera in Normal and Early-Glaucoma Monkey Eyes

¹From the LSU Eye Center and the ³Department of Orthopaedic Surgery, Louisiana State University Health Sciences Center, New Orleans, Louisiana; and the ²Department of Biomedical Engineering, Tulane University, New Orleans, Louisiana.

PURPOSE. To test the hypothesis that changes in the viscoelastic material properties of peripapillary sclera are present within monkey eyes at the onset of early experimental glaucoma detected by confocal scanning laser tomography (CSLT).

METHODS. Short-term (3–9 weeks), moderate (44 mm Hg) intraocular pressure (IOP) elevation was induced in one eye of each of eight male monkeys by lasering the trabecular meshwork. This procedure generated early experimental glaucoma, defined as the onset of CSLT-detected optic nerve head (ONH) surface change, in the treated eye. Scleral tensile specimens from the superior and inferior quadrants of the eight early-glaucoma eyes were subjected to uniaxial stress relaxation and tensile tests to failure and the results compared with similar data obtained in a previous study of 12 normal (nonglaucomatous) eyes. Linear viscoelastic theory was used to characterize viscoelastic material property parameters for each specimen. Differences in each parameter due to quadrant and treatment were assessed by analysis of variance (ANOVA).

RESULTS. Peripapillary sclera from the early-glaucoma eyes exhibited an equilibrium modulus (7.46 ± 1.58 MPa) that was significantly greater than that measured in normal eyes (4.94 ± 1.22 MPa; mean ± 95% confidence interval, P < 0.01, ANOVA). Quadrant differences were not significant for the viscoelastic parameters within each treatment group.

CONCLUSIONS. The long-term viscoelastic material properties of monkey peripapillary sclera are altered by exposure to moderate, short-term, chronic IOP elevations and these alterations are present at the onset of CSLT-detected glaucomatous damage to the ONH. Damage to and/or remodeling of the extracellular matrix of these tissues may underlie these changes in scleral material properties.

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