Ординатура / Офтальмология / Английские материалы / Elevation Based Corneal Tomography 2nd_Belin, Khachikian, Ambrósio_2011

(FIGURES 15A - 15C) - These three figures illustrate a patient with anterior lentoconus (Alport’s syndrome). The first illustration is a standard slit photograph and the second a photograph against a dilated red reflex. The Scheimpflug image clearly demonstrates the anterior lens.

Figure 15A

Figure 15B

Figure 15C

(FIGURES 16A - 16B) - Scheimpflug image of post LASIK ectasia showing excessive central thinning secondary to changes seen on the posterior surface. This is confirmed by examination of the posterior elevation which reveals significant posterior changes.

Figure 16A

Figure 16B

INTRODUCTION

After corneal excimer laser refractive surgery (LRS), the direct use of measured topographic or keratometric (K) values without any correction results in inaccurate calculation of the intraocular lens (IOL) power required for cataract surgery compared to virgin eyes.1-6 After LRS for myopia there is typically an overestimation of the K readings with consequent underestimation of the IOL power required and a hyperopic outcome after surgery. Conversely, after LRS for hyperopia, there is often an underestimation of the K readings with consequent overestimation of the IOL power and a myopic outcome after surgery.

There are two main sources of error for biometry after LRS: 1) the erroneous corneal power estimation using corneal first-surface measurements by clinical keratometry or videokeratography; 2) the erroneous estimated IOL position due to the use of postLRS K values when using 3rd generation formulas. There are several reasons why these instruments provide inaccurate readings after LRS: 1) Keratometers measure only 4 points on the cornea at a paracentral region (typically 3.0 mm) ignoring flatter (or steeper) more central regions resulting from LRS (FIGURE 1); 2) Keratometers or videokeratographers also use a standardized keratometric index (1.3375 for most) to convert measurements of the anterior corneal curvature to an estimate of the refractive power of the entire cornea. The formula used for such conversion is based on two old assumptions that modern (Sheimpflug and OCT) technology has proven to be incorrect: the thickness of the cornea is constantly 500 microns as in the Gullstrand’s exact schematic eye; the relationship between the anterior and the posterior corneal curvature is always constant (approximately 0.82 in virgin corneas). This last assumption may still hold true following incisional procedures such as radial or astigmatic keratotomies, but no longer applies after LRS, where the anterior corneal curvature is selectively changed while the posterior curvature remains unaltered;7 3) Some studies have shown that also the refractive index of the cornea itself may change following LRS.1,2,8-11

Several methods have been developed to reduce the inaccuracy of IOL power calculation after LRS. Some of them are based on the need of (often unavailable) pre-refractive surgery information, while others are based on different mathematical regressions performed on various parameters starting from direct corneal measurements from different devices (keratometers, videokeratographers, or corneal topographers).

Figure 1. Oculus Pentacam’s anterior sagittal and elevation maps showing the centrally flattened (or steepened) optical zones resulting from either myopic (upper) or hyperopic (lower) LRS.

CORNEAL POWER CALCUATION FROM DIRECT CURVATURE MEASUREMENTS

We believe that the ideal method of calculating corneal power after LRS should entirely be based on direct corneal measurements, independently from any preoperative information. Knowing both the anterior and posterior corneal curvature and corneal thickness, it is possible to precisely estimate the “true net corneal power” either in untreated eyes or in eyes following any refractive procedure. In order to do so we need to use the thick lens equation, also known as the Gaussian Optic Formula (GOF):

Ftot = Fant + Fpost - (d/n) × (Fant × Fpost)

The equation can also be written as:

Ftot = [1/rant × (n1 – n0)] + [1/rpost × (n2-n1)] – (d/n1) × [1/ rant × (n1 – n0)] × [1/ rpost × (n2-n1)]

where Ftot, Fant, and Fpost are the powers of the total, anterior, and posterior corneal surfaces (respectively) in diopters (D); d is the thickness of the cornea (in meters); n is the corneal

refractive index (1.376); rant and rpost are the anterior and posterior corneal radii (in meters); n0 is the refractive index of air (1.000); n1 is the refractive index of the anterior corneal surface (1.376); and n2 is the refractive index of the aqueous humor (1.336).

The problem is that it is not possible to simply take the value calculated with the GOF and use it in current IOL power formulas as they are all calibrated using the standardized keratometric index of 1.3375 which does not precisely take into account the true posterior corneal curvature, therefore yielding an inaccurate result. This issue is even more striking in post-LRS eyes, where the relationship between the anterior and the posterior corneal curvature is no longer constant. In order to be able to use the “true net corneal power” of the GOF, we must first “adjust” it taking into consideration the actual posterior corneal curvature (in untreated eyes) and the altered anterior/posterior corneal curvature relationship (after LRS).

DEVELOPMENT OF THE BESSt_vc

In 200612 we studied the correlation between the K values obtained with the GOF and the K values obtained using a standard videokeratographer (Topcon KR-8100PA) in a group of 143 untreated eyes which had never undergone LRS. We found that there was a statistically significant correlation between the two values (r = 0.97; r2 = 0.95; p <0.05), although the GOF consistently underestimated corneal power by a mean of 1.30 ± 0.29 D compared to the videokeratographer (FIGURE 2). This fairly large difference in measurement may be explained

Figure 2. Correlation between K values obtained with the GOF and K values obtained using a standard videokeratographer (Topcon KR-8100PA). Outliers removed.