Corneal Elevation Topography: Best Fit Sphere, Elevation Distance, Asphericity, Toricity, and Clinical Implications

Abstract

Purpose To describe the effect of the corneal asphericity and toricity on the map patterns and Best Fit Sphere (BFS) characteristics in elevation topography. Methods The corneal surface was modeled as a biconic surface of principal radii and asphericity values of (r1, r2) and (Q1, Q2), respectively. The apex of the biconic surface corresponded to the origin of a polar coordinates system. Minimization of the squared residuals was used to calculate the values of the radii of the best fit spheres (BFS) and apex distance (A-values :z distance between the corneal apex and the BFS) of the modelled corneal surface for various configurations relating to commonly clinical measured values of apical radius, asphericity and toricity. Results Increased apical radius of curvature and increased prolateness (negative asphericity) led to an increase in BFS radius, but had opposite effects on the A-value. Increased prolateness resulted in increased BFS radius and A-value. Increasing toricity did not alter these findings. Color-plot elevation maps of the modelled corneal surface showed complete ridge patterns when toricity was increased and showed incomplete ridge and island patterns when prolateness was increased. Conclusions High A-values in patients with corneal astigmatism may result from steep apical curvature and/or high prolateness (negative asphericity). The BFS radius may help in distinguishing between these two causes of increased A-values. Increased prolateness and decreased apical radius of curvature (often seen in keratoconus) have opposite effects on the BFS radius but similar effects on the apex distance.

Comparison of dual rotating Scheimpflug–Placido, swept-source optical coherence tomography, and Placido–scanning-slit systems

“…Thus, the dual rotating Scheimpflug-Placido device and swept-source OCT device detected an equivalent posterior cornea shape and could be considered interchangeable. 22 Overall, while the dual rotating ScheimpflugPlacido device and swept-source OCT showed differences in posterior elevation values, eccentricity and K showed high agreement even in different mechanical properties within the same cornea as reference line measurements for each device. Therefore, we concluded that the measurements obtained using the 2 devices were highly comparable.…”

Analysis of age, refractive error and gender related changes of the cornea and the anterior segment of the eye with Scheimpflug imaging

“…Evaluation of the elevation maps is important when considering for refractive surgery to avoid post-LASIK ectasia. Increased corneal asphericity leads to an increase in the best fit sphere [35]. It is reported that anterior elevation values greater than 5 and posterior elevation values greater than 13 at the thinnest point should raise suspicion of corneal irregularity [36].…”

Evaluation of corneal elevation and thickness indices in pellucid marginal degeneration and keratoconus

“…21,22 Therefore, an aconic reference surface, which is aspheric and toric, fits closer to the corneal surface and provides an approximation of the overall corneal shape. The scanning-slit topographer software generates an aconic surface of reference from the elevation data of the anterior and posterior corneal surfaces and provides information about the mean apical curvature, asphericity, and toricity values that a spherical reference surface cannot provide ( Figure 2).…”