Anterior Surface-Based Keratometry Compared With Scheimpflug Tomography-Based Total Corneal Astigmatism

Abstract

Total corneal astigmatism is overestimated by CASim-K in eyes with with-the-rule astigmatism and cannot be predicted safely in eyes that do not have with-the-rule astigmatism by anterior measurements only.

Changes in Anterior, Posterior, and Total Corneal Astigmatism after Descemet Membrane Endothelial Keratoplasty

“…Anterior corneal astigmatism (AA) is the astigmatism value arising from the anterior corneal surface alone and calculated as the difference in simulated keratometry values (calculated using the standard keratometric index (1.3375) and the radius of anterior corneal curvature) between the steepest and the flattest meridians [13–16]. …”

“…The posterior corneal astigmatism is defined as the difference in keratometry values between the flattest and steepest meridians, which were calculated using the radius of posterior corneal curvature and the refractive indices of the cornea (1.376) and aqueous humor (1.336) [13–16]. …”

“…Total corneal astigmatism is defined as the difference in total corneal refractive power between the steepest and flattest meridians (calculated by ray tracing through the anterior and posterior corneal surfaces, according to Snell's law) [13–16]. …”

Total Corneal Astigmatism Measurement

“…We read with interest the article by Tonn et al 1 on corneal astigmatism. At the beginning of the article, the authors indicated that: ''Scheimpflug tomographers provide a map of the anterior chamber and therefore a more accurate model of the cornea, its thickness, and posterior surface.''…”

“…Undoubtedly, we still have a long way to go to get truly reliable posterior corneal measurements and to determine the best way to analyze those data (there are several different complex vectorial methods available, which are not either straightforward or universally accepted) [5][6][7][8][9] ; but at least we have taken the first crucial steps in the right direction, and clinical studies such as the one performed by Tonn et al 1 will help us reach the goal of better results when implanting toric intraocular lenses. However, as Alpins et al 10 recently stated, posterior corneal astigmatism does not seem to be the only significant factor in the equation of final manifest refractive cylinder and noncorneal contributors likely include other sources of intraocular astigmatism (in pseudophakic eyes, IOL tilt or decentration) and processing in the visual cortex.…”