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The study to validate the method was conducted using the Holladay 2 formula. Here it was deter-
mined that after LASIK the best correlation with the traditional method, with a mean prediction
error of -0.06 D ± 0.56 D, is obtained using a mean zonal EKR for the 4.5 mm zone. For post-RK
patients, the mean prediction error is –0.04 D ± 0.94 D [2].
3.5 Equivalent Keratometer Readings power map
This map (Figure 5) was designed to take into account the refractive effects of both the anterior
and the posterior surface. Another requirement was that it should output power values which
in normal cases (no Lasik) would be comparable with simulated K (SimK) values, which are
usually derived from sagittal curvature map. Its output is therefore also referred to as Equivalent
Keratometer Readings (EKR). It calculates power according to Snell’s law using the refractive
indices of corneal tissue and aqueous humour and aggregating the values for anterior and posterior
power. Then the output is shifted such that for a normal eye (posterior corneal radius 82% of
anterior corneal radius) its values (EKR) are identical to those of SimK readings from a sagittal map.
In other words, the EKR map is corrected by adding the error that would be created by a refractive
index of 1.3375 in a sagittal map. In this way it provides equivalent K-values (EKR) that can be
used in IOL formulas that correct for n=1.3375. The EKR map thus takes into account effects "A",
"B" and "C".
Figure 5: EKR power map calculated by twospheric surfaces of
anterior r = 8 mm and posterior r = 6.58 mm
3 Differences between the various topography maps of Pentacam
®
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