Determining the center of a keratoconus: Comparison of different tomographic parameters and impact of disease severity

Abstract

PurposeThere exists remarkable variation in definitions for the location of the center of a keratoconus. The objective of this study was to analyze deviations between locations obtained by different tomographic maps for that purpose. Furthermore, it was investigated whether these deviations are influenced by disease severity.MethodsIn 162 eyes with keratoconus, corneal tomographic maps derived by Scheimpflug technology were retrospectively analyzed to determine the cone location with 5 different methods: maximum axial curvature of the front surface (Kmax), maximum tangential curvature of the front surface (tKmax), minimum pachymetry (Pachymin), maximum elevation of the front surface (ELEF), and maximum elevation of the back surface (ELEB). Distances between the locations were calculated and tested for a correlation with keratoconus severity and distance between cone and corneal vertex.ResultsCone locations derived from the curvature maps (Kmax, tKmax) showed the lowest agreement with the locations determined by pachymetry or elevation maps. The largest distances were found between Kmax and Pachymin [Median and Interquartile range: 1.19 mm (0.87, 1.60)], Kmax and ELEB [1.12 mm (0.79, 1.41)], and Kmax and ELEF [0.97 mm (0.64, 1.27)]. Low distances (<0.5 mm) were calculated between ELEB and ELEF, and ELEB and Pachymin. All of the calculated distances between the locations showed a significant negative correlation with keratoconus severity and most of them increased significantly with a more peripheral position of the cone (p < 0.05).ConclusionsThere was low consistency between different methods for describing the location of a keratoconus. Curvature-based determinations of the cone center (Kmax, tKmax) showed the highest deviations and should not be used for that purpose. However, the discrepancies between different cone location methods diminished with increasing disease severity and more central position of the cone.

Keratoconus cone location influences ocular biomechanical parameters measured by the Ocular Response Analyzer

“…The difference is expected since the axial curvature algorithm is the mathematical average of the tangential curvature algorithm, so the cone will be represented more peripherally in an axial map than a tangential map. This difference is also consistent with the literature, which reports varying cone location for different map types and for different instruments [ 13 , 42 ]. It did not have a marked effect on our results, however, as there were significant differences in p1area, p2area, h1, and h2 between the peripheral-cone and central-cone cohorts for both types of curvature.…”

“…The location of the corneal cone apex differs between patients with keratoconus, usually ranging from the central cornea to its inferior quadrants [ 12 , 13 ]. The exact location of the conical apex has clinical implications, influencing the design of gas-permeable contact lenses [ 14 ], the placement of intracorneal ring segments [ 15 ], and the location of corneal cross-linking therapy [ 16 ].…”

Longitudinal Changes of Cornea Volume Measured by Means of Anterior Segment-Optical Coherence Tomography in Patients with Stable and Progressive Keratoconus

“…At the same time, statistically significant differences were detected regarding CV at 5 mm and 8 mm (but not at 3 mm) from T0 to T1. This discrepancy may be attributed to the fact that a larger diameter allows for a higher likelihood of analyzing the corneal area in which the cone is typically located, which is situated in the middle periphery rather than the central region [35]. Similarly, Cui et al analyzed 48 patients, a control group of 29 individuals with myopic astigmatism and 19 patients who had subclinical keratoconus.…”