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Keith M Meek, Ahmed Abass, Thomas Sorensen, Sally Hayes; Transverse depth-dependent changes in corneal collagen fibril orientation. Invest. Ophthalmol. Vis. Sci. 2014;55(13):5137. doi: https://doi.org/.
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Following the suggestion that corneal surface topography may be stabilised by the angular orientation of out-of-plane lamellae that insert into the limiting membrane of the anterior cornea, this study aims to provide quantitative information about the angular orientations of these lamellae.
Four human donor eyes were perfusion-fixed under pressure. Following removal of the epithelium, the cornea with a 3mm scleral rim was dissected from each eye and a 1 mm wide cornea-scleral strip was cut from the centre (in the nasal-temporal direction). The scleral edges of each strip were clamped in a custom made, air-tight sample holder, designed to ensure that the corneal strip remained hydrated and in its natural curvature during data collection. A microfocus x-ray beam was used to collect wide-angle x-ray scattering data at 20 µm intervals throughout the cross-sectional thickness of the central and peripheral cornea and limbus. These data were analysed to produce quantitative information regarding the predominant direction of fibrillar collagen and the spread of fibrillar orientations about this direction as a function of tissue depth.
The predominant direction of the collagen remained parallel to the corneal surface at all tissue depths. However, a large spread of fibril angles were evident in the anterior-most 185 µm of the stroma, with the majority of lamellae being orientated up to ± 31° with respect to the corneal surface. Beyond this, in the mid and posterior stroma, the spread of fibrillar orientations decreased significantly, and the collagen was found to lie predominantly parallel to the corneal surface.
The relatively large angular spread of collagen lamellae in the anterior cornea likely contributes to enhanced shear strength and dynamic flexibility in this region. Incorporation of this quantitative information into finite element models will further improve the accuracy with which they can predict the biomechanical response of the cornea to pathology and refractive procedures.
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