Spotlight on the Corneal Back Surface Astigmatism: A Review

Abstract

Recent evidence indicates that the corneal back surface astigmatism (CBSA) contributes to the refractive state of the eye in cataract surgery, especially with the implantation of toric intraocular lenses. But this has been met with some scepticism. A review of key studies performed over the past three decades shows that the mean CBSA power ranges from 0.18(±0.16)D to 1.04(±0.20)D. The clinical assessment of CBSA is problematic. There is poor agreement between the current automated systems for assessment of CBSA and it is assumed that these systems directly measure the CBSA. But CBSA cannot be measured directly in vivo. A historical review of methods used to quantify the curvature of the posterior corneal surface reveals that CBSA estimated by current systems is based on values for corneal front surface astigmatism, corneal refractive index, central corneal thickness, corneal thickness at peripheral locations and the exact distance between the corneal apex and each one of these peripheral locations. Doubts and errors in these values, coupled with the precise details of the algorithm incorporated to estimate CBSA, are the likely sources of the lack of agreement between current systems. These systematic errors cloud the assessment of CBSA. Mean CBSA may be low, but it varies from case to case. There is a clear need for a realistic, practical procedure for clinicians to independently calibrate systems for estimating CBSA. This would help to reduce uncertainty and the discrepancies between instruments designed to measure the same parameter.

Evaluation of statistical correction strategies for corneal back surface astigmatism with toric lenses: a vector analysis

“…[6][7][8][9][10]14,16,17,[21][22][23] Currently, there are several strategies for considering the effect of the corneal back surface in tIOL power calculation: If a tomographer is available, both surfaces of the cornea can directly be measured, and the total corneal power can be derived by calculating the spherocylindrical equivalent power (generalized Gullstrand formula) or front vertex power of the cornea based on a thick lens model. 8,13 Most of the tomographers provide data on total/real/refractive power of the cornea where both surfaces are considered, but we have no insight as to the calculation concept of how these data are derived from the measured front and back surface curvature (because this information is mostly undisclosed by the manufacturers). If we are restricted to a corneal front surface measurement, for example, with a manual or automated plug-in keratometer in a biometer or simulated keratometry values of a topographer, we could consider a statistical correction for the corneal back surface.…”

“…Our data confirm the results of others that there is a systematic offset of this difference with an orientation against the rule, which is not explained by the keratometric astigmatism itself. 3,13,21,32 We subsequently applied classical nomograms, which are used in clinical routine to statistically correct for the effect of corneal back surface astigmatism (or any internal astigmatism resulting from any misalignment of the cornea or lens with respect to the visual axis). The Abulafia-Koch nomogram has been developed based on a regression analysis with a setup similar to this study.…”

“…Meanwhile, several articles have been published focusing on the contribution of the corneal back surface to the astigmatism of the entire eye. [6][7][8][9][10][11][12][13][14][15][16][17] Currently, in addition to the Baylor nomogram which describes the indication of lens toricity values for different ranges of with or against the rule keratometric astigmatism, there are the Abulafia-Koch formula, the Goggin nomogram, and the La Hood nomogram. 3,6,7,9,10,[18][19][20] In addition to using statistical models to predict the contribution of the corneal back surface to the total corneal astigmatism, there are currently several strategies for measuring the curvature of both corneal surfaces.…”

“…Meanwhile, several articles have been published focusing on the contribution of the corneal back surface to the astigmatism of the entire eye. 6–17 Currently, in addition to the Baylor nomogram which describes the indication of lens toricity values for different ranges of with or against the rule keratometric astigmatism, there are the Abulafia-Koch formula, the Goggin nomogram, and the La Hood nomogram. 3,6,7,9,10,18–20…”

The Effect of Pterygium on Front and Back Corneal Astigmatism and Aberrations in Natural-Light and Low-Light Conditions

“…2 Traditionally, corneal topography used the front indices to evaluate the total corenal indices without measuring the back corneal indices directly, which may result in error in the evaluation of total corneal power and astigmatism. [3][4][5][6][7] Visual performance and optical quality correlated with wavefront aberrations, 8 better understanding of the impact of pterygia on visual function should include the changes of corneal aberrations caused by pterygia. So far, there were only few researches studying the effect of pterygium on back corneal indices and corneal aberrations.…”