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D. Gatinel, J. Malet, T. Hoang–Xuan, International Corneal Modeling Group (ICMG); Elevation Topography of the Corneal Anterior and Posterior Surfaces : Influence of Apical Radius, Toricity and Asphericity on the Best Fit Sphere and Elevation Maps Characteristics . Invest. Ophthalmol. Vis. Sci. 2006;47(13):577.
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© ARVO (1962-2015); The Authors (2016-present)
To describe the effect of the anterior and posterior corneal surfaces apical curvature, toricity and negative asphericity on the map patterns and Best Fit Sphere (BFS) characteristics in elevation topography.
The corneal surface was modelled as a biconic surface of principal radii and asphericity values of (R1, R2) and (Q1, Q2), respectively. First, the separate influence of apical radius and asphericity on the best fit circle radius (BFC–r) and distance to the apex (BFC–d) of a particular corneal cross section corresponding to a conic section was investigated. Second, the value of the radius of the best fit sphere (BFS–r) and apical distance to the apex of the modelled corneal surface (BFS–d) were calculated analytically by minimization of the squared residuals (MSR) for different configurations relating to commonly clinical measured values of apical radius, negative asphericity and toricity. This allowed to generate colour plots of the modeled corneal elevation data against the BFS.
The decrease in apical radius of curvature of a meridian (steepening) resulted in a linear decrease in the BFC–r and exponential increase in the BFC–d. The decrease in asphericity value of a meridian (increased prolateness) resulted in an increase in the BFC–r and BFC–d values. The apical curvature influenced the value of BFS–d and the number of colours plotted in 3 and 7 mm central concentric areas. The"complete ridge"–like patterns were obtained when increasing the toricity over negative asphericity, whereas "incomplete ridge" and "island"–like patterns were obtained when increasing negative asphericity over toricity.
The representation in elevation of biconic anterior and posterior corneal models against a BFS corresponded to common symmetric elevation features observed in corneal elevation topography. These results may explain the differences in the elevation map characteristics of the anterior and posterior corneal surfaces. Increased prolateness and decreased apical radius of curvature have opposite effects on the BFS–r but similar effects on the BFS–d. This suggests that indexes of detection based on the BFS–d values may be better indicators in early keratoconus detection than indexes based on the value of the BFS–r value.
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