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Marine Gobbe, Timothy J. Archer, Dan Z. Reinstein; Comparison Between Stromal And Corneal Elevation Topography In The Normal Cornea. Invest. Ophthalmol. Vis. Sci. 2011;52(14):5902.
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© ARVO (1962-2015); The Authors (2016-present)
To compare the anterior stromal surface topography with the anterior corneal surface topography in a population of normal eyes. Having previously characterised the epithelial thickness profile in a population of normal eyes and demonstrated that it is asymmetric, this study aimed at investigating whether the underlying anterior stromal surface was asymmetric or whether the asymmetry was on the anterior corneal surface.
Elevation of the anterior corneal surface was measured using the Orbscan (Bausch and Lomb) in 110 normal eyes. Epithelial thickness profile was measured by Artemis 1 (ArcScan Inc) very high-frequency (VHF) digital ultrasound scanning across the central 10-mm corneal diameter in the same eyes. Elevation of the anterior stromal surface was calculated for each eye as the difference between the elevation of the anterior corneal surface and the epithelial thickness profile. The height data for both the corneal and stromal surface were averaged across all eyes. This average elevation for each surface was displayed by importing the data into VolPro (Sarver and Associates) as a best-fit 7.8mm sphere (BFS) elevation map. The irregularity of the BFS elevation maps was described using asymmetry indices (AI) calculated as the difference in BFS elevation with reference to the 7.8-mm BFS, between 2 points equidistant from the corneal vertex and in opposite directions along the 180, 90, 45 and 135 degree meridians, at the 4, 3, 2, 1.5, 1 and 0.5-mm radii.
The anterior stromal BFS elevation was greater than the corneal BFS elevation superiorly and nasally. There was no difference inferiorly. AIs were higher for the anterior stromal BFS elevation than for the corneal BFS elevation in all meridians. In the 45, 90 and 135° meridians, AIs varied between 1.0 and 4.8 µm for the anterior stromal BFS elevation, and between 0.2 and 0.6 µm for the corneal BFS elevation. In the horizontal meridian, AIs varied between 0.2 and 1.3 µm for the stromal BFS elevation and between 0.0 and 0.3 µm for the corneal surface BFS elevation. The average stromal surface astigmatism was 1.27 D at 100° while the average corneal surface astigmatism was 1.06 D at 96°.
In a population of normal eyes, the asymmetric epithelial thickness profile appears to compensate for the asymmetry of the anterior stromal surface, reducing the asymmetry of the anterior corneal surface.
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