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Moritz Winkler, Dongyul Chai, Shelsea Kriling, Chyong Jy Nien, Donald J. Brown, James V. Jester; Axial Anisotropy of Corneal Collagen Organization and Biomechanics. Invest. Ophthalmol. Vis. Sci. 2011;52(14):4202.
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© ARVO (1962-2015); The Authors (2016-present)
Visual acuity depends on corneal curvature, which is determined by its mechanical properties. We aim to characterize and quantify the lamellar organization of human corneas in three dimensions using non-linear optical High Resolution Macroscopy (NLO-HRMac), and to correlate these findings with mechanical data obtained by compression testing of corneal flaps, to establish a link between the supramolecular organization and corneal shape.
Vibratome sections, 200µm thick, from 5 donor eyes were cut along the vertical meridian from limbus to limbus (arc length: 12mm). Backscattered SHG signals from these sections were collected as a series of overlapping 3-D images, which were concatenated to form a single 8GB+ mosaic (resolution: 0.44µm/px lateral, 2µm/px axial). Lamellar intertwining was quantified by determining branching point density as a function of stromal depth. Lamellae were manually segmented to create 3-D reconstructions using Amira software. Mechanical testing was performed on corneal flaps from 5 additional eyes. Corneas were cut into 3 layers (anterior, mid, posterior) using an Intralase femtosecond surgical laser system and then underwent compression tests to determine the elastic modulus for each layer.
3-D reconstructions revealed a complex collagen fiber branching pattern in the anterior cornea with fibers extending from the anterior limiting lamina (ALL), intertwining with deeper fibers and reinserting back to the ALL forming ‘bowspring-like’ structures. Measured branching point density was 4 times higher in the anterior third of the cornea compared to the posterior third and decreased logarithmically with increasing distance from the ALL. Compression testing showed an 8-fold increase in elastic modulus in the anterior stroma.
The axial gradient in lamellar intertwining appears to be associated with an axial gradient in effective modulus of the cornea suggesting that collagen fiber intertwining and formation of ‘bowstring-like’ structures provide structural support similar to cross-beams in bridges and large-scale structures. We hypothesize that radial and axial structural/mechanical anisotropy may determine the characteristic parabolic shape of the cornea and play a role in astigmatism and other refractive errors.
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