Refractive Lens Surgery

40 D.D. Koch · L.Wang
corneal refractive surgery, the ELP calculated
with the flat postoperative corneal power values
will be artificially low, thereby estimating
that the IOL will sit more anteriorly; this results
in implantation of a lower power IOL
and a hyperopic postoperative refractive error
(Fig. 5.1).
Aramberri [1] proposed a modified IOL
formula, called double-K formula, in which
the pre-refractive surgery corneal power is
used to estimate the ELP and the post-refractive
surgery corneal power is used to calculate
the IOL power, in contrast with the traditional
method in which one corneal power (the
so-called single-K formula) is used for both
calculations. Holladay had previously recognized
this problem when developing the Holladay
2 formula. The magnitude of the error
in predicting ELP depends on the IOL formula
used, the axial length of the eye, and the
amount of refractive correction induced by
the refractive surgery. In general, the ELP-related
IOL prediction errors are the greatest
for the SRK/T formula, followed by Holladay
2, Holladay 1, and Hoffer Q formulas; this error
decreases in long eyes and increases with
increasing amount of refractive correction
[2, 6].
In a previous study, we confirmed the
greater accuracy of the double-K versions of
three third-generation (SRK/T, Holladay 1
and Hoffer Q) and the Holladay 2 fourth-generation
IOL calculation formulas, with decreased
chances of hyperopic surprises [7].
Tables for performing double-K adjustments
on third-generation formulas have been pub-
Fig. 5.1. Most third- and fourthgeneration
IOL formulas predict
the effective lens position (ELP)
using corneal power (a). If the
flattened corneal power after
myopic surgery is used, the
predicted ELP will be anterior
and lower IOL power will be
predicted, resulting in postoperative
hyperopia (b)
lished [2]. The Holladay 2 permits direct entry
of two corneal power values for the double-K
calculation. If the corneal power value
before refractive surgery is unknown, the
“Previous RK, PRK...” box should be checked,
which will instruct the formula to use 44 D as
the default preoperative corneal value. Another
option is to use the Haigis formula,
which does not use the corneal power for ELP
prediction [8].
5.2 Difficulties in Obtaining
Accurate Corneal
Refractive Power
Two factors cause the inaccurate estimation
of corneal refractive power:
1. Inaccurate measurement of anterior
corneal curvature by standard keratometry
or CVK. Standard keratometry or simulated
keratometry from CVK measures
only four paracentral points or small regions.
This is insufficient for the post-surgical
cornea, which can have wide ranges
of curvature even within the central 3-mm
region (Fig. 5.2).
2. Inaccurate calculation of corneal refractive
power from the anterior corneal curvature
by using the standardized value for
refractive index of the cornea (1.3375 in
most keratometers and CVK devices).
Based on the assumption that there is a
stable ratio of anterior corneal curvature
to posterior corneal curvature, the standardized
index of refraction has been used