Refractive Lens Surgery

22 J.T. Holladay
4.1 Introduction
The indications for intraocular lens (IOL)
implantation following cataract or clear lensectomy
have significantly increased. These
expanded indications result in more complicated
cases such as patients with a scleral
buckle, silicone in the vitreous, previous
refractive surgery, piggyback IOLs in nanophthalmos,
positive and negative secondary
piggyback IOLs and specialty lenses, such as
multifocal and toric IOLs. Techniques for determining
the proper IOL and power are presented.
Several measurements of the eye are helpful
in determining the appropriate IOL power
to achieve a desired refraction. These measurements
include central corneal refractive
power (K-readings), axial length (biometry),
horizontal corneal diameter (horizontal
white to white), anterior chamber depth, lens
thickness, preoperative refraction and age of
the patient. The accuracy of predicting the
necessary power of an IOL is directly related
to the accuracy of these measurements [1, 2].
4.1.1 Theoretical Formulas
Fyodorov first estimated the optical power of
an IOL using vergence formulas in 1967 [3].
Between 1972 and 1975, when accurate ultrasonic
A-scan units became commercially
available, several investigators derived and
published the theoretical vergence formula
[4–9]. All of these formulas were identical
[10], except for the form in which they were
written and the choice of various constants
such as retinal thickness, optical plane of the
cornea, and optical plane of the IOL. These
slightly different constants accounted for less
than 0.50 diopters in the predicted refraction.
The variation in these constants was a result
of differences in lens styles, A-scan units,
keratometers, and surgical techniques among
the investigators.
Although several investigators have presented
the theoretical formula in different
forms, there are no significant differences except
for slight variations in the choice of retinal
thickness and corneal index of refraction.
There are six variables in the formula: (1)
corneal power (K), (2) axial length (AL), (3)
IOL power, (4) effective lens position (ELP),
(5) desired refraction (DPostRx), and (6) vertex
distance (V). Normally, the IOL power is
chosen as the dependent variable and solved
for using the other five variables, where distances
are given in millimeters and refractive
powers given in diopters:
IOL
AL ELP
DPostRx V
1336 1336
= −
−
1336
− ELP
1000
+ K
1000
−
The only variable that cannot be chosen or
measured preoperatively is the ELP. The improvements
in IOL power calculations over
the past 30 years are a result of improving the
predictability of the variable ELP. Figure 4.1
illustrates the physical locations of the variables.
The optical values for corneal power
(K opt ) and axial length (AL opt ) must be used
in the calculations to be consistent with current
ELP values and manufacturers’ lens constants.
The term “effective lens position” was recommended
by the Food and Drug Administration
in 1995 to describe the position of the
lens in the eye, since the term anterior chamber
depth (ACD) is not anatomically accurate
for lenses in the posterior chamber and can
lead to confusion for the clinician [11]. The
ELP for intraocular lenses before 1980 was a
constant of 4 mm for every lens in every patient
(first-generation theoretical formula).
This value actually worked well in most patients
because the majority of lenses implanted
were iris clip fixation, in which the principal
plane averages approximately 4 mm
posterior to the corneal vertex. In 1981,
Binkhorst improved the prediction of ELP by