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Dongyul Chai, Philip Ngai, Bryan E. Jester, James V. Jester, Donald J. Brown; Measurement of Elastic Modulus of the Optic Nerve Head (ONH) and Peripapillary Sclera. Invest. Ophthalmol. Vis. Sci. 2011;52(14):6250.
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Finite element models suggest that the material properties of the lamina cribrosa (LC) and peripapillary sclera have a significant impact on the deformation of the ONH associated with intraocular pressure (IOP), a major risk factor for primary open angle glaucoma (POAG). The purpose of this study was to measure the material properties of the ONH and peripapillary sclera using an indentation method.
An indentation testing device was assembled composed of two translation stages (Newport, Irvine, CA), and a 250 µm diameter probe with rounded tip fixed to a force transducer, set normal to the XY plane. Using hydrogels of varying material properties, effective elastic moduli (E) were calculated using indenting force and validated by comparison to tensile and compressive elastic moduli. Samples containing the ONH and peripapillary sclera from 5 human eyes were attached to a metal plate with a 12.7 mm diameter central hole anchoring the ONH at the center. Indenting force measurements were made at 500 µm intervals in a grid pattern over the ONH and peripapillary sclera. Following mechanical testing, samples were evaluated for collagen content using second harmonic generated imaging.
Elastic modulus values derived from hydrogels showed a linear relationship among the three methods used (r2 ~ 0.95) and there was no statistical difference between E and compressive modulus (p = 0.14). E values ranged from 5 to 100 kPa within ONH with the variation dependent on the density of collagen (r2 ~ 0.95) and from 100 to 500 kPa in peripapillary sclera with large variation between positions.
Indentation testing can be used to measure the material property of tissues at specifically defined regions. E-values, calculated using this method, over the ONH varied in association with the collagen content of LC. These results suggests that indentation testing can be used to provide better understanding of the biomechanical behavior of the ONH and peripapillary sclera under elevated IOP and provide insights to the development of POAG.
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