Measurement of Posterior Corneal Surface Toricity

Clinical validation of an algorithm to correct the error in the keratometric estimation of corneal power in normal eyes

“…The k ratio, which is defined as the ratio of r 1c to r 2c , is not constant in the range of curvatures of the normal healthy cornea, which range from 1.157 to 1.295. [15][16][17] In a previous simulation study performed by our research group using the Gullstrand and Le Grand eye models, 1 we found that keratometric estimation can significantly overestimate or underestimate the Gaussian corneal power. This error (DP c ) was found to be dependent on both r 1c and r 2c , and therefore on the k ratio.…”

Regular and Irregular Refractive Powers of the Front and Back Surfaces of the Cornea

“…The corresponding ratio obtained by Royston et al was 1n214 for the vertical meridian using the Purkinje image method (Royston et al, 1990a). In a later study, a ratio of 1n236 for the vertical meridian and 1n195 for the horizontal meridian was reported (Royston et al, 1990b). Dunne et al calculated a ratio of 1n215 (a reciprocal of 0n823) (Dunne, Royston and Barnes, 1992).…”

“…The methods to model the posterior corneal surface from the anterior surface curvature and the corneal thickness profile were again confined to two meridians (Camellin, 1990 ;Patel, Marshall and Fitzke, 1993 ;Edmund, 1994). There have been only a few studies which measured the astigmatism of the posterior corneal surface (Royston, Dunne and Barnes, 1990b ;Dunne, Royston and Barnes, 1992 ;Dunne, Elawad and Barnes, 1996). Moreover, since these previous methods were based on the assumption that the corneal surface is spherical, assessment of the irregular astigmatism of the corneal back surface has not been possible at all.…”

Relationship between the corneal surface and the anterior segment of the cornea: An Asian perspective

“…To date, there are no reported concerns about the performance of the slit-scan topography system in normal virgin eyes. The ratio between posterior and anterior corneal curvature has been extensively measured in small populations with various techniques such as Purkinje imagery, 16 pachymetry, 17 photokeratoscopy and topographic phacometry algorithms, 18 and Scheimpflug photography. 19 Table 2 compares these findings with results in this study.…”