Table 1a.

Individual topographic indices in keratoconus.

Individual indices	Description	Probable keratoconus	Clinical keratoconus
Central keratometry (K central)	Average corneal power for ring diameters of 2, 3, and 4 mm	47.2–48.7	>48.7 [18]
Inferior–superior value (I–S)	Power difference between 5 points of the superior and inferior hemisphere at 3 mm from the corneal apex with spatial intervals of 30°. A positive value indicates a higher inferior curvature and a negative value indicates a higher superior curvature	1.4–1.8	>1.8 [19]
Skew of steepest radial axis (SRAX)	Angle between superior semi-meridian and inferior semi-meridians, determined by averaging powers of rings from 5 to 16 mm		>21° [20]
Apex curvature (AK)	Instantaneous curvature in the corneal apex	48–50 D	>50 D [20]
Asphericity coefficient (Q)	Index describing corneal curvature change from central region to the peripheral region. A normal cornea has a average Q value of −0.2 ± 0.12 (77) and represents that the cornea is more curved in the center. The value depends on the diameter of the study, which is usually set at 4–5 mm		1. −0.84 for anterior surface and −1.10 for the posterior surface at 8 mm [14] 2. −0.65 to −1.18 for the anterior surface and −1.17 to −0.6 for the posterior surface [15]
Corneal irregularity measurement (CIM)	Degree of corneal surface irregularity by quantifying standard deviation between corneal surface and best-fit reference	0.69–1 μm	1.10–5.00 μm [21]
Calossi–Foggi Apex curvature gradient (ACG)	Average difference per length unit of the corneal power relative to the apical power	1.5–2.0 D	>2.0 D [21]
Calossi–Foggi Top–Bottom Index	Vertical asymmetric index indicating difference in average power between superior and inferior area. A positive value indicates an inferior area that is more curved	1.5–2.0 D	>2.0 D [21]