Posterior Corneal Astigmatism: Basics and Clinical Implications

5 Posterior Corneal Astigmatism: Basics and Clinical Implications

Fernando Antonio Faria-Correia, José Carlos Ferreira Mendes, and Renato Ambrósio Jr.

Abstract

Until recently, corneal power calculations were based on the measurement of anterior corneal surface values taking into consideration a fixed posterior-to-anterior curvature ratio. Presence of posterior corneal astigmatism prompts to some of the unexpected surprises met after cataract surgery, as astigmatism is an important and often a visually significant optical aberration. This chapter discusses the importance of taking into consideration the posterior corneal power to optimize the postcataract surgery outcomes.

Keywords: posterior corneal astigmatism, corneal astigmatism, posterior corneal curvature, Scheimpflug imaging, simulated keratometry, with-the-rule astigmatism, against-the-rule astigmatism

5.1 Introduction

In the intraocular lens (IOL) power calculation setting, optical modeling has moved from thin-lens paraxial vergence calculations to thick-lens exact ray-tracing calculations. This is a direct consequence of improvements either in measuring devices or in calculation models and formulae. In the theoretical third-generation (i.e., Sanders–Retzlaff–Kraff [SRK]/T, Holladay 1, Hoffer Q, and Haigis) and fourth-generation (i.e., Holladay 2) formulae, cornea and IOL are thin lenses. Corneal power is calculated from the measured anterior radius using the keratometric index of refraction, 1.3375, which renders a figure that estimates the divergent or negative lens power effect of the unmeasured posterior surface. The accuracy of this method is based on two main implicit assumptions: anterior-to-posterior radius ratio fits the normal proportion (around 1.21) and posterior corneal toricity follows the anterior pattern with a constant and proportional magnitude.

Keratometers, both manual and automated, and front-surface reflection topographers have been providing these keratometric and simulated measurements for decades with acceptable clinical results in terms of spherical equivalent dioptric results in IOL surgery. However, the last 10 years have revealed the weaknesses of this method as clinical demand has risen with the emergence of refractive lens surgery, mostly related to the advent of multifocal and toric IOLs. New technologies, capable of measuring both the anterior and posterior corneal curvatures, have emerged, permitting the surgeon to target more precisely the total corneal power and astigmatism. With this development, there is a need to properly distinguish topography from tomography,1 and Scheimpflug imaging, optical coherence tomography (OCT), and novel reflection-based devices are now competing for the position of the most accurate method to calculate these parameters. Knowledge of the effect of posterior corneal astigmatism (PCA) on the total has also allowed for the development to more accurate formulae.

5.2 Measuring the Posterior Corneal Astigmatism

In 2012, Koch and coworkers published a relevant work highlighting the contribution of PCA to total corneal astigmatism (TCA). In a retrospective study, 715 eyes from 435 patients were examined by Galilei dual Scheimpflug analyzer to calculate the TCA using a ray-tracing formula over the central 1 to 4 mm of the cornea. Mean magnitudes were as follows: TCA, + 1.07 ± 0.71 diopters; anterior corneal astigmatism (ACA), + 1.20 ± 0.79 diopters; corneal astigmatism from simulated keratometry (CA Sim K), + 1.08 ± 0.71 diopters; and PCA, −0.30 ± 0.15 diopters. Vertical meridian was steeper in 86.8% of PCA, compared to 50.9% of ACA. Almost 5% had a vector difference of more than 0.50 diopters.² Later, Savini and coworkers published a similar work in eyes with more than 1.00 diopter of CA Sim K. PCA excessed 0.50 diopters in more than 55.4% of eyes. More than 16% presented a difference in astigmatism magnitude between TCA and CA Sim K higher than 0.50 diopters. Compared with TCA, CA Sim K overestimated with-the-rule (WTR) astigmatism (mean 0.22 ± 0.32 diopters); underestimated against-the-rule (ATR) astigmatism (mean 0.21 ± 0.26 diopters); and overestimated oblique astigmatism (mean 0.13 ± 0.37 diopters).³

5.3 Impact of Posterior Corneal Astigmatism on Toric Intraocular Lens Calculation

To validate their theory, Koch and coworkers tried to measure the impact of PCA in toric IOL calculation. Pre- and postoperative corneal astigmatism were assessed with different technology devices: automated keratometry, IOL Master, Lenstar LS900, Placido corneal topography, Atlas system, manual keratometer, Bausch and Lomb, Placido dual Scheimpflug analyzer, and Galilei. In vectorial analysis calculation, predicted error was not significant from zero only in Placido dual Scheimpflug analyzer. Corneal astigmatism was overestimated (0.5–0.6 diopters) in WTR by all devices and underestimated (0.2–0.3 diopters) in ATR by all devices, except the Placido dual Scheimpflug analyzer. The authors proposed a nomogram for toric IOL selection, based on preoperative corneal astigmatism, designated as “Baylor nomogram” ( Fig. 5.1).⁴ In the Baylor nomogram, the planning in correction ATR astigmatism is toward an overcorrection, while the WTR astigmatism is calculated to be on target or undercorrected ( Table 5.1). In this estimation, the Baylor nomogram does not consider posterior cornea in determining the IOL axis alignment.⁴

The Pentacam HR, has also been used in similar works. Tonn and coworkers analyzed 3,818 healthy eyes comparing CA Sim K and TCA. The latter was measured in the 3-mm zone by ray tracing. Again, CA Sim K overestimated TCA in eyes with WTR astigmatism. CA Sim K could not predict TCA in eyes that did not have WTR astigmatism. In eyes with WTR anterior astigmatism, posterior astigmatism was vertical too in 97%; in eyes with ATR anterior astigmatism, 18% of the cases also presented a horizontally oriented posterior astigmatism.⁵ Table 5.1 summarizes the results presented in the scientific studies that consider corneal astigmatism measurements.

5.4 Toric Intraocular Lens Calculation in Clinical Practice

Toric IOL calculations must consider the influence of the posterior cornea not relying just on the keratometric (Sim K) astigmatism anymore. This can be done using the measured posterior curvature in a thick-lens calculation model or using an algorithm that calculates that effect. The former method is particularly important in eyes that had previous corneal refractive surgery as the anterior/posterior astigmatism proportion has been changed, especially if any astigmatic correction was done in the anterior cornea. New-generation Scheimpflug and OCT tomographers can measure the TCA accurately ( Fig. 5.2). The Cassini (I-Optics, The Hague, The Netherlands) can measure the posterior cornea from LED (light-emitting diode) reflection analysis and calculates the total corneal power and astigmatism by ray tracing. Some of these devices incorporate IOL calculation software, which takes into account the different refractive effect of both corneal surfaces. Another option is to input the anterior and posterior curvatures in ray tracing thick-lens models like Okulix and PhacoOptics.