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Andrew Shin, Joseph Park, Ayesha Baig, Alan Le, Michael Jang, Vadims Poukens, Joseph L Demer; Biomechanics of Human Optic Nerve (ON) & Sheath Correlate With Connective Tissue Composition. Invest. Ophthalmol. Vis. Sci. 2017;58(8):3160.
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© ARVO (1962-2015); The Authors (2016-present)
The ON sheath consists of inner & outer layers (Baig et al, ARVO'17). It has been proposed that tension exerted on the globe by the ON & sheath may be an intra-ocular pressure (IOP) independent mechanism of optic neuropathy in glaucoma. However, the biomechanics of these tissues are poorly characterized. This study correlated biomechanics with histology in human ON & sheath.
Human eyes with contiguous ONs 26 to 82 years of age were obtained within 3 days of death. Uniaxial tensile loading was used to characterize Young’s modulus of ON & ON sheath (whole, inner, & outer layer). Specimens were elongated at constant rate during tension recording in a load cell under physiological conditions with simultaneous cross section imaging using optical coherence tomography. Compressive testing was performed by microindentation with small, hard spheres imposing small deformations. Intact ON & sheath samples from each specimen were formalin fixed, paraffin embedded, sectioned at 10 mm, & stained with Masson trichrome to demonstrated collagen, & van Giesson’s elastin (EVG).
Young’s modulus (YM), a measure of tensile stiffness of an elastic material, was calculated for each ON & sheath in 10 eyes. Mean (± SEM) YM for ON was 3.6±0.5 MPa, markedly less than for ON sheath at 17.9±1.0 MPa. For both ON & sheath, YM correlated positively with specimen age, with sheath YM for the oldest 2.5 times that of the youngest. Tensile YM of the inner sheath layer at 26.6±5.0 MPa significantly exceeded 9.0±3.4 MPa for the outer layer (P<0.05). Compressive modulus for the inner sheath at 8.6±0.5 kPa significantly exceeded the 6.0±0.3 kPa value for the outer sheath (P<0.04). Histology showed marked age-related increases in collagen & elastin in both ON & sheath, correlating closely with tensile & compressive moduli of the same specimens.
Human ON & sheath stiffness increases with age, probably due to the correlated increase in collagen & elastin in these tissues. At all ages, maximum tensile modulus occurs in the ON sheath inner layer that inserts at the perimeter of the lamina cribrosa (LC). ON tethering in adduction would produce progressively increased strain on the LC & peripapillary sclera because of this age-related increase in ON & sheath stiffness, perhaps accounting for the progressive, age-related increase in prevalence of glaucomatous optic atrophy in older people who have normal IOP.
This is an abstract that was submitted for the 2017 ARVO Annual Meeting, held in Baltimore, MD, May 7-11, 2017.
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