The document discusses intraocular lens (IOL) power calculation and selection. Precise calculations are important for optimal outcomes after cataract surgery. Various formulas are now used that require accurate measurement of corneal power and axial length. The most commonly used regression formula is the SRK formula, which relates IOL power to axial length and corneal curvature. Modifications like SRK-II adjust the formula for shorter or longer eyes. Proper technique is described for taking keratometry and axial length measurements, which are input into formulas to determine the appropriate IOL power.

1. IOL : POWER CALCULATION & SELECTION
Precise IOL power calculation is essential for optimal
benefits of implant
surgery. Prior to1975, IOL power was calculated on the
basis of clinical history, i.e.
pre-operative refractive error prior to development of
cataract. This led to errors in
over 50% of cases. However, a number of formulae are
now available to accurately
calculate the IOL power required in a patient. All these
formulae are based on an
accurate measurement of the corneal power and the
axial length.
FORMULAE IN USE
The original formulae were developed prior to 1980.
They include the
theoretical formulae and regression formulae. The
commonly used formulae are the
regression formulae, of which the most popular one is
the SRK formula described by
Sanders D, Retzlaff J. and Kraff M. The formula is based
on the following equation:

2. P = A – BL - CK
where P is the implant power for emmetropia, L the
axial length in millimeters, and K
the average keratometric reading in diopters. A, Band C
are constants. The value of
B is 2.5 and that of C is 0.9
Thus P = A - 2. 5L - 0.9K
The constant A varies with the implant design and the
manufacturer. Be sure
of the constant value of the IOL you are using while
making the calculations. The
SRK formula has been found to be reasonably accurate
for eyes with axial lengths
between 22mm and 24.5mm. These eyes constitute
approximately 75% of cases,
while 14% of cases have axial lengths greater than 24.5
mm, and 10% have axial
lengths less than 22mm. The modified formulae were
developed to correct for errors
in these formulae occurring in long and short eyes.
It is for such 'too long' and 'too short' eyeballs that SRK
II formula was

3. introduced. The SRK II formula is a modification of the
original SRK formula with the
addition of a correction factor that increases the lens
power in short eyes and
decreases it in long eyes.22
The suggested method of modification of SRK to SRK II
is shown below:
L (mm) Add to 'A' constant
Less than 20.00 + 3
20.00 - 20.99 + 2
21.00 - 21.99 + 1
Greater than 24.50 -0.5
Modern formulae for emmetropia:
These formulae are more complex than the original
and the modified
formulae. The most striking difference is the manner in
which the estimated anterior
chamber depth (ACD) value is calculated. The ACD
value is a constant value in the
original formulae. It varies with the axial length in the
modified formulae (decreases

4. in the shorter eye and increases in the longer eye). In
the modern formulae, ACD
value varies not only with axial length, but also with
corneal curvature (being more
with steeper cornea and deep AC and vice versa). The
commonly used modern
formulae are the Holladay formula, the SRK-T formula
and the Hoffer-Q formula.
KERATOMETRY
Manual keratometry is the most commonly used
method to measure corneal
curvature. It is fast, easy and is very accurate in most
cases. Keratometry should be
done before axial length measurement, and for both
eyes. Remember to calibrate
the eyepiece for your refraction before recording
measurements. The procedure of
keratometry using the common Bausch and Lomb
keratometer is given here. The
patient is seated behind the keratometer, with the chin
well positioned in the chin rest
and the head resting on the head band. The
keratometer is directed towards the eye

5. to be examined and the other eye is occluded. The
keratometer is focused on the
central portion of the cornea using the focusing knobs.
The instrument is now rotated
to align the (-) signs in the same vertical meridian and
the (+) signs in the same
horizontal meridian. This will determine the axis of the
pre-existing astigmatism. The
left drum is rotated to superimpose the (+) signs and
the horizontal measurement is
read out. The right drum is now rotated to
superimpose the (-) signs and the vertical
measurement reading is recorded. The Javal-Shiötz
keratometer utilizes two mires to
achieve the end point. IOL power calculation formulae
use the average corneal23
power, K = average of the horizontal and the vertical
readings. It is important to
remember that the keratometer has to be calibrated
every 6 months.
It is advisable to repeat measurement if the -
a. Average keratometry (K) in either eye is less than 40
D or greater than 47 D.

6. b. Difference in K between the two eyes is greater than
1 D.
c. Corneal cylinder does not correlate well with the
refractive cylinder.
In certain situations, like irregular corneal contour or
previous refractive surgery, or
when the surgeon wants to better evaluate the
astigmatism, corneal topography may
be utilized.
AXIAL LENGTH MEASUREMENTS
The measurement of the axial length is best done with
A-scan ultrasonography. It
can be performed by an immersion technique or a
contact technique. The machine
should have a screen showing the spikes for ensuring
correct measurement. Always
take measurement for both eyes.
Technique
With the contact technique, a drop of local anesthetic
is instilled into each eye.
The patient is examined in the seated position. The
probe is positioned in front of the

7. eye and the patient is asked to fixate on the red light in
the probe. The probe is then
brought forward to gently touch the cornea. Particular
attention and care must be
taken to ensure that the probe is not indenting the
cornea. The probe is moved
slightly up and down or to the side to optimize the
echospikes displayed on the
machine. Either the operator or the machine selects
the optimum pattern and the
reading is obtained.
The immersion technique is
performed with the patient in the
supine position. Topical anesthetic is
instilled and a proper scleral shell is
chosen. The 20 mm shell fits most
eyes. The flared edges of the scleral
shell are placed between the lids and Good A-scan.
Echos from left to right : cornea,
anterior lens capsule, posterior lens capsule,
retina, sclera, orbital fat24

8. the cup is filled with fluid, preferably gonioscopic
solution. The ultrasound probe is
immersed in the solution but kept 5-10 mm away from
the cornea. The patient is
asked to look with the fellow eye at a fixation point on
the ceiling. The probe is then
gently moved till it is aligned with the optical axis of
the eye and the a-scan
echogram on the panel is adequate. The reading is
then taken.
The contact technique usually yields shorter
measurement than the
immersion technique for various reasons. Most
modern biometers calculate the axial
length based on separate sound velocities for different
eye components (cornea,
anterior chamber, lens, vitreous cavity).
It is recommended that measurements be repeated if
the -
a. Measured axial length is less than 22.0 mm or more
than 25.0mm
b. Difference between the two eyes is more than
0.5mm.

9. c. Axial length value seems wrong when compared with
refraction.
All measurements should be repeated if following
exist:
a. Calculated emmetropic implant power is more than
3D from the average for the
specific lens style used.
b. Difference in emmetropic implant power between
the two eyes is more than 1D.
A new device, the IOL Master, yields accurate axial
length measurements using
optical coherence techniques.
The A constant
Formulae in use currently utilize constants, which are
based on various
factors that affect the refractive state of the eye post-operatively.
The Binkhorst and
the Hoffer formulae use the post-operative AC depth,
the SRK II and SRK-T
formulae use the A-constant and the Holladay formula
uses the S-factor.

10. The A- constant encompasses multiple variables
including the implant
manufacturer, implant style, surgeon’s technique,
implant placement within the eye,
and measuring equipment. Because of its simplicity,
the A constant has become the
value by which an implant is characterized. The most
common A constants used are-25
! Anterior chamber lenses - 115.0-115.3
! Posterior chamber lenses in the sulcus - 115.9-117.2
! Posterior chamber lenses in the bag - 117.5-118.8
In most cases the power of the IOL for emmetropia
varies in a 1:1 relationship
with the A constant.
The S-factor used in the Holladay formula is the
distance between the iris
plane and the IOL optic plane. The S-factor should be
personalized by solving the
formula in reverse. A change in the true post-operative
AC depth will affect the
refractive status of the eye. A change in 1 mm causes a
1.5 D change in the final

11. refraction. Hence, these constants must be
personalized to accommodate any
consistent shift that might affect IOL power calculation.
Each constant has to be back
calculated for at least 20 cases, with care to ensure
that the same person takes the
measurements.
SPECIAL CASES
Intumescent cataracts will yield a 0.15 mm longer axial
length resulting in a
+0.4 -+0.5 hyperopia postoperatively. For aphakic eyes
being planned for ACIOL or
scleral fixated IOL, the appropriate A constant must be
used and the mode of the
machine changed to compensate for the change in
speed of the sound waves. In
eyes with silicone filled vitreous, the sensitivity of the
system should be increased
to visualize the retinal echospike and the components
of the eye must be measured
separately to reach an accurate result. The usage of a
standard sound velocity can

12. lead to an error of upto 8 mm in such eyes. Usually a
factor of 0.72 gives a rough
estimate of the IOL power. It is better to refer the
patient to a centre capable of
separate measurements for more accurate
assessment.
After corneal refractive surgery, the K reading may not
truly reflect the
corneal power. Hence the refractive history method or
the contact lens method must
be used to obtain corrected K value. In eyes with high
myopia, a B-scan
examination is recommended to rule out a posterior
staphyloma or other retinal
pathologies. Identification of the posterior pole may be
difficult. The problems are
compounded in unilateral cases. While selecting the
IOL power for a myope several
factors are to be kept in mind. The surgeon should aim
for a -0.50 D to -1.00 D26
postoperative refraction as most sedentary elderly will
prefer being near sighted. In

13. the presence of monocular cataract in a myopic eye
when the other eye is
emmetropic, emmetropia should be aimed for if the
myopia was induced by the
cataract. However, if the patient has been functioning
with monocular vision using
the emmetropic eye for distance and the myopic eye
for near, it is better to leave the
operative eye myopic. In patients with hypermetropia
the aim should be to achieve
emmetropia. Here, the use of linear formulae can
result in large errors in IOL power
calculation in small eyes. In children, it is wise practice
to remove the cataract and
use contact lens correction if the surgery is being
performed within the first two years
of life, because growth of the eye will result in a large
myopic shift if IOL has been
implanted with intraoperative K and axial length
measurements. When surgery is
being performed after the age of two years, a myopic
shift of 4-6 D is expected

14. depending upon the age. Undercorrecting the IOL
power by around 3 D partially
compensates for this. A greater undercorrection can
lead to anisometropia and
difficulty in amblyopia correction. Residual myopia in
adulthood can easily be
corrected by spectacles, contact lenses or refractive
surgery. As expected, biometry
in children is difficult and may require general
anesthesia.
Postoperative refraction (R) for a given IOL power (I)
can be
computed as given below:
• For P less than 14.00 R = P-I
• For P greater than 14.00 R = (P-I)/1.25
To calculate the IOL power which would produce a
given refraction:
• For P less than 14.00 I = P-R
• For P greater than 14.00 I = P - (R x 1.25)
Choice of IOL Power
The following factors should be considered:-

15. • The refraction and presence/absence of cataract in
the fellow eye.
• Relevance of emmetropia, isometropia & iseikonia.
• Lifestyle of patient: active patients prefer near
emmetropia; sedentary patients
may prefer myopia.
• Hedging: it has been found from experience that it is
preferable to hedge
towards myopia.27
It is important to remember that a myopic patient
would be very unhappy if he
is made hypermetropic. Also, the final refraction
results may be +/- 1D either way
from the calculated power.
IOL DESIGN FEATURES:
A variety of design features incorporated in modern
IOLs make them very
safe and reduce adverse phenomena and late
complications after cataract surgery.
The modern modified C-loop design ensures
maintenance of centration and the

16. square edge design significantly retards the
opacification of the posterior capsule.
Plate haptic lens manufacturing has improved and now
lenses with a very good
surface can be fashioned. Various modifications of the
edge have been tried to
reduce glare and improve contrast sensitivity. A recent
development has been the
introduction of multi-focal lenses which are designed
to give three zones (distance,
intermediate and near) of clear vision. Still in the
research stage are accommodative
lenses which mimic the change in refractive status of
the natural lens with
accommodation.
IOL MATERIALS:
IOL materials Advantages Disadvantages
PMMA High optical quality
Large optical centre
Proven biocompatibility
Possibility of surface
modification

17. Good laser resistance
Large incision wound
Not autoclavable
Mild foreign body
reaction
Soft acrylic Foldable
Controlled unfolding
Good laser resistance
Good biocompatibility
Good optical quality
Limited experience
Possible damage during
implantation
Sticky surface can
adhere to instruments
Hydrogel Good laser resistance
Good biocompatibility
Good optical quality
Easy handling
Lack of long term

18. experience
Silicone Good biocompatibility
Less CME
Irreversible adherence to
silicone oil
Can tear
Slippery when wet
Limited control during
implantation
IOL calculation using the SRK II formula
The SRK II formula is based on the SRK I formula:
SRK I: P = A - 0.9 K - 2.5 L (1)
where:
P : IOL power for emmetropia
K : corneal refractive power (K-reading)
L : axial length
A : A-constant
Adjusting the A-constant to different axial length ranges, the SRK II formula is
obtained:
SRK II: P = A1 - 0.9 K - 2.5 L (2)
The difference between SRK I and SRK II is given by the A1 constant. A1 is
related to the A-constant A according to
A1 = A + 3 for L < 20
A1 = A + 2 for 20 < = L < 21
A1 = A + 1 for 21 < = L < 22
A1 = A for 22 < = L < 24.5

19. A1 = A - 0.5 for 24.5 < = L
Whereas SRK I and II only give the IOL power for emmetropia, another SRK
formula has to be used to derive the power I necessary to produce a desired postop
refraction R :
I = P - cr R (3)
P is again given by SRK II (2), and cr is another empirical constant defined as
cr = 1 für P < = 14
cr = 1.25 für P > 14
?Long term discolorationChoosing the Proper Formula
for Accurate IOL Calculations
BY FARRELL "TOBY" TYSON, M.D.
Accurate and reproducible axial length (AL) measurements are only the first step to IOL power
selection. IOL calculation formulas have now become the limiting factor to achieving predictable
postoperative outcomes. We have quite a menu of formulas to choose from, but how do we choose
the right one?
First, we must understand how these formulas have evolved. The original formulas were
mathematical-regression formulas. The most well known of the first-generation formulas are the SRK I
by Sanders, Retzlaff and Kraff and the Binkhorst II.
The SRK I is well known for its simplicity and ease of use where P = A – 0.9K – 2.5L.
P = the IOL power for emmetropia
K = the corneal refractive power
L = the axial length
A = the A-constant
This formula works well for average ALs but is less accurate for long and short eyes.
To increase predictability, the SRK II formula emerged as a second-generation formula, where P =
A1 – 0.9K – 2.5L. The A constant was then modified into 6 subtypes based on AL. This resulted in:
A1 =(A–0.5) for axial lengths greater than 24.5
A1 =A for axial lengths between 22 and 24.5
A1 =(A+1) for axial lengths between 21 and 22
A1 =(A+2) for axial lengths between 20 and 21
and A1 =(A+3) for axial lengths less than 20
Predictability improved markedly, but spectacle correction was still necessary.
The Holladay I, Hoffer Q and the SRK/T emerged as the third-generation formulas. These
formulas were a merger of the linear regression methods with theoretical eye models. This

20. allowed for greater accuracy, but the reliance on theoretical assumptions led to the differences
between the three Since 1975, IOL power has been calculated using accurate
measurement of an eye’s corneal power and axial length (AL). Prior to
that, the power of the IOL was calculated using clinical history alone—in
other words, the preoperative refractive error prior to cataract
development.
The earliest IOL power calculation formulas, in the late 1970s and early
1980s, were either theoretical or regression formulas. Regression formulas
topped surgeons’ preferences, and one of the most successful was the SRK
formula devised by Donald R. Sanders, PhD, MD; John A. Retzlaff, MD;
and Manus C. Kraff, MD.1 ,2
The SRK formula uses the following equation to calculate IOL power: P =
A – BL - CK, where P is the implant power for emmetropia; L is the axial
length (mm); K is the average keratometry (D); and A, B, and C are
constants. The values of B and C are 2.5 and 0.9, respectively, and the
value of A varies with the IOL design and the manufacturer. With this
information, the formula can be written as follows: P = A – 2.5L - 0.9K.
Over the years, surgeons discovered that the SRK formula is best used in
eyes with average AL, between 22.00 and 24.50 mm; a subsequent
formula, the SRK II, was developed for use in long and short eyes.3 In this
formula, a correction factor was added to increase the lens power in short
eyes and decrease it in long eyes: P = A1 – 0.9K -2.5L. For eyes with AL of
less than 20.00 mm, a numerical value of 3.00 is added to the A constant;
a numerical value of 2.00 is added if the AL measures between 20.00 and
20.99, a numerical value of 1.00 if the measurement is between 21.00 and
21.99, and -0.50 if the AL is greater than 24.50 mm.

21. Even more customized formulas are required today to calculate anterior
chamber depth (ACD) based on AL and corneal curvature. The SRK/T (T for
theoretical) is one such formula, representing a combination of linear
regression method with a theoretical eye model.4 Based on the nonlinear
terms of the theoretical formulas, the SRK/T also incorporates empirical
regression methodology for optimization, resulting in greater accuracy. The
SRK/T and other third-generation formulas work best for near-schematic eye
measurements; specifically, the SRK/T is best for eyes longer than 26.00 mm.
With this generation, which used an iterative process on five data sets
consisting of 1,677 procedures, the SRK/T formula optimizes the prediction of
postoperative ACD, retinal thickness AL correction, and corneal refractive
index. It can be calculated using the same A constants used with the original
SRK formula or with ACD estimates.4 This calculation, however, does not
account for effective lens positionIntraocular Lens Power Calculation After
Corneal Refractive Surgery
Vahid Feiz
Author information ► Copyright and License information ►
Abstract
Go to:
INTRODUCTION
An increasing number of patients undergo corneal surgical procedures to decrease
dependence on glasses or contact lense. These procedures alter corneal effective power.
Excimer laser keratectomy has quickly become the modality of choice for corneal refractive
surgery, replacing older incisional surgeries such as radial keratotomy (RK).1,2
As surgeons gain experience with cataract extraction in postrefractive surgery patients, they
are finding that standard intraocular lens (IOL) formulas and keratometry can lead to
“refractive surprises.” The most common observation is underestimation of IOL power and
unexpected hyperopia after cataract surgery in patients who have undergone corneal
refractive surgery for correction myopia, regardless of the procedure.3–11 Moreover, these
refractive surprises seem to be directly related to the amount of keratectomy performed.
Clinically, this means that greater refractive corrections correlate with greater errors of IOL
power.12–14
Experience with IOL power determination after corneal surgery to correct hyperopia remains
limited. A few reported cases of cataract surgery after hexagonal keratectomy (now
abandoned) resulted in myopic surprises.15 As procedures like hyperopic LASIK/PRK have
gained wider acceptance, surgeons can expect to encounter different refractive surprises after
cataract surgery in this population.

22. IOL power determination IOL power calculation relies on three measurements: axial length,
corneal power and anterior chamber depth, which are not independently measured. An error
in any of these three parameters can lead to a possible refractive surprise.
Historically, axial length measurements have been the source of most refractive surprises,
although refinements in biometry techniques and instruments have decreased these
errors.16,17 Assuming accurate biometry, axial length measurements are unlikely to contribute
significantly to IOL power errors after corneal refractive surgery. Two studies analyzing axial
length before and after RK and excimer keratectomy found no significant differences.18,19
Effective lens position (ELP) or anterior chamber depth affects post-cataract surgery
refraction so that a greater myopic shift is observed with more anterior IOL position. Anterior
chamber depth cannot be independently measured because even after in-the-bag implantation,
it is hard to predict the exact distance between the cornea and the IOL. If corneal surgery
significantly changes anterior chamber depth and therefore the ELP, the result can effectively
change post-cataract surgery refraction. Several investigators have looked at anterior chamber
depth after refractive surgery. One study reported a small forward shift of the posterior
cornea after myopic LASIK. This observation, however, has not been confirmed in a similar
study.20,21 These changes, even if real, appear too small to account for changes in refraction
and therefore probably do not significantly contribute to IOL power errors after myopic
treatments.
Corneal power calculations rely on determining the radius of curvature of the anterior cornea
in meters (r), which is converted into a diopteric power (P) using an index of refraction (n)
utilizing the following formula.
P=(n−1)/r
Radius of curvature is measured by manual keratometry, automated keratometry or
topography. Two assumptions regarding topography or keratometry are that: (1) the cornea is
a true spherical surface and (2) the power of the cornea's para-central 3–4 mm is not
significantly different from that of the central cornea. These assumptions are clinically
acceptable in most normal eyes. In reality, however, the cornea is a prolate, aspheric
refractive media with progressive flattening toward the periphery.
Go to:
SOURCES OF ERROR IN CORNEAL POWER DETERMINATION
Considering that different types of refractive surgery fundamentally alter corneal shape and
power, the usual assumptions no longer apply and may be the sources of error in determining
corneal power. In this review of possible error sources, we have divided corneal refractive
surgery into RK and excimer keratectomy (PRK, LASIK, LASEK).
RK

23. RK steepens the peripheral cornea and flattens the central cornea, resulting in a hyperopic
shift and a proportionally greater flattening of the cornea in the center compared with the
paracentral cornea.22 This creates an abrupt change from treated to untreated cornea. Because
keratometry and topography units measure radius of curvature in the cornea's para-central 3–
4 mm, the measured diopteric power is significantly steeper than the central cornea. The
measured zone also increases in size further from the central cornea as the cornea becomes
flatter, resulting in overestimation of cornea power.23,24
Myopic excimer keratectomy
The ability of large optical zones to decrease post-operative glare and halos has become
evident with increased LASIK and PRK experience, and optical zones >5−6 mm are now
considered routine. As a result, the para-central radius of curvature would be expected to
closely approximate central corneal curvature. In clinical experience, however, when the
radius of curvature is converted to diopteric power, this calculated value overestimates
central corneal power.4–12 This occurs for two main reasons:
First, after excimer keratectomy, the anterior corneal surface changes but the posterior
corneal surface remains unaltered. Sonergo-Krone et al. found small changes in the posterior
corneal power after LASIK but large changes in the anterior–posterior power
ratio.25 Changing the anterior–posterior power alters the cornea's effective refractive index in
direct relation to the amount of keratectomy. In the original Gullstrand model, for every 9%
change in ratio, the effective corneal power is changed by 0.5 diopters.26
The second factor is the variation in corneal refractive index of the different layers of the
cornea. This was shown by Patel et al., who found the index of refraction to be slightly
different in different layers.27Because excimer laser selectively removes anterior stromal
layers and leaves the posterior stroma intact, it changes the cornea's total refractive index.
Removing more tissue is also expected to produce a greater change in the refractive index.
This is supported by the observed correlation between depth of ablation and error in IOL
power after myopic PRK.12,28
Hyperopic excimer keratectomy
Little, if any, experience with cataract surgery after hyperopic excimer keratectomy has been
reported. Because these treatments cause steepening of the central cornea with large optical
zones, para-central radius of curvature, measured by manual keratometry or topography,
should be a fairly accurate estimation of central curvature. As in myopic treatments, the
anterior–posterior corneal power ratio is expected to change, although in the opposite
direction. Therefore, using the standard refractive index would theoretically underestimate
corneal power and result in unexpected myopia after IOL implantation.
In our center, we analyzed eight eyes after hyperopic LASIK, using pre-LASIK keratometry
and amount of hyperopic treatment to predict a fictitious post-LASIK IOL power. In each
case, the predicted IOL power was lower than the IOL power determined by standard post-

24. LASIK keratometry.13 Despite a lack of actual implantation, this study indicated that using
post-hyperopic LASIK standard keratometry could theoretically result in IOL power
overestimation and unexpected myopia.
Summary
Manual keratometry after myopic L ASIK, PRK and RK overestimates corneal power and
underestimates IOL power. The causes differ for RK and LASIK/PRK. In LASIK/PRK, error
is directly proportional to the amount of keratectomy. Manual keratometry after hyperopic L
ASIK and PRK theoretically underestimates corneal power and results in IOL power
overestimation, also in direct proportion to the amount of correction.
Go to:
METHODS TO IMPROVE IOL POWER DETERMINATION
Several methods can improve IOL power accuracy after corneal refractive surgey. No single
approach has been studied in a large sample, and some are based purely on theory. Most
cases also require knowledge of pre-refractive surgery data that may not be available to
cataract surgeons. Proposed methods include use of topography to measure central corneal
power, advanced IOL calculation formulas, contact lens over-refraction, clinical history,
nomogram-based adjustment, corneal power determination by directly determining posterior
curvature and intentional overcorrection targeting for myopia.
Topography
Topography-measured corneal power has been suggested to improve central corneal power
measurements in post-refractive surgery eyes. Hussein et al. developed the topography
method to calculate the corneal power within the pupil.28 The study showed that the average
central power differed from standard keratometry in post-refractive surgery eyes having small
optical zones and large attempted corrections. Theoretically, this method offers advantages in
eyes with small optical zones.
By contrast, Seitz et al. found manual keratometry to be superior to topography-derived
values in post-myopic PRK eyes.12,29
In summary, using topography to determine central corneal power may be beneficial after RK
with small optical zones. However, topography has not been found to be superior to standard
keratometry in post-PRK/LASIK corneas, and its reliability and accuracy have not been
verified.
Using advanced formulas
Modern theoretic optical formulas (Holladay, Hoffer Q, SRK-T) may offer improved
accuracy of IOL power determination in post-refractive surgery eyes. Koch et al.4 found the
Binkhorst and Holladay formulas to be superior to SRK II in post-RK eyes. Odenthal et

25. al. noted that using the Hoffer Q formula after myopic LASIK decreased, but did not
eliminate, IOL power underestimation.30
Another popular formula proposed by Aramberri, know as the double K method, utilizes pre-refractive
surgey Ks to estimate an ELP and post-refractive surgery Ks are used to determine
IOL power taking into account the ELP.31
A number of other formulas have been proposed by other authors. Some include Haigis-L,
Latkany formula, etc. A review of all these is beyond the scope of this article.
Although these studies offer no clear-cut conclusions regarding the accuracy of different
modern theoretic formulas, their use is probably advantageous in post-refractive surgery eyes.
Contact lens over-refraction
This method uses a hard contact lens of known power and base curve to determine true
corneal power. After patients have undergone refraction, a plano hard contact lens is placed
on the eye and over-refraction is performed. If no difference exists between refractions,
corneal power is the same as the contact lens base curve. If over-refraction is more myopic
than refraction without the contact lens, the lens is steeper than the cornea. The change in
refraction is subtracted from the contact lens base curve to yield corneal power. If over-refraction
is more hyperopic than the contact lens refraction, the cornea is steeper than the
lens. Change in refraction is added to the contact lens base curve to calculate corneal power.
Contact lens-derived corneal powers have been shown to correlate well with manual
keratometry in normal corneas when visual acuity is better than 20/70.32 Once the visual
acuity is lower than 20/70, which may be the case in many patients with cataract, the
correlation is poor. The accuracy of this technique is not established in post-refractive surgery
eyes.
Clinical history
Originally proposed by Holladay to determine corneal power after RK, this method was
advocated by Hoffer for use in post LASIK/PRK eyes.33,34 Using this method requires
knowledge of keratometry prior to refractive surgery as well as induced refractive change
before the development of cataract. These values are used to determine a calculated corneal
power as follows:
For post-myopic (post-RK/myopic excimer) procedures: Corneal diopteric power = pre-refractive
surgery Ks – change in SE.
For post-hyperopic (post-hyperopic excimer) procedures: Corneal diopteric power = pre-refractive
surgery Ks + change in SE.
The major shortcomings of this approach are that accuracy and reliability have not been
established in large series and that it requires knowledge of keratometry values prior to
refractive surgery, which cataract surgeons may not have. Its major flaw, however, is

26. assuming a one-to-one relation between corneal diopteric power and refraction (i.e., if
corneal power changes by one diopter, refraction changes by one diopter). Studies by Patel et
al. and Hugger et al. analyzed changes in refraction and corneal power after refractive
surgery in a large sample.35,36 Both studies found less change in corneal power than in
refraction and concluded that this was due to a change in the cornea's effective refractive
index. This indicates that the clinical history method reduces IOL power errors but the degree
of accuracy is not yet established.
Nomogram-based correction
By analyzing eyes after myopic and hyperopic LASIK, we developed a theoretic nomogram
to correct IOL power after these procedures.13 The nomogram is based on four established
clinical premises:
1. IOL power after myopic corneal surgery has to be higher than before surgery.
2. IOL power after hyperopic corneal surgery is expected to be lower than before
surgery.
3. To maintain emmetropia, the difference between IOL powers before and after
refractive surgery must compensate for refraction changes.
4. For every diopter of change in IOL power, refraction at the spectacle plane with a
vertex distance of 12.5 mm changes by only 0.67 diopters.37
These formulas allowed the development of a nomogram to adjust IOL power based on post-
LASIK standard keratometry [Tables [Tables11 and and2]2] and eliminated the need for pre-
LASIK keratometry. Compared with the clinical history method, this nomogram gave a
higher IOL power after myopic LASIK and lower IOL power after hyperopic LASIK.
Table 1
Nomogram for intraocular lens (IOL) power adjustment for emmetropia after myopic LASIK
This nomogram has been tested and appears to be reliable in a limited number of
studies.13 Further prospective data of this method's accuracy are currently being collected.
Optical formula corneal power calculations
Using Gaussian optics, the cornea's true power can theoretically be determined regardless of
previous surgical procedures. Thisapproach considers the cornea to have two refractive
surfaces, anterior and posterior. The theoretic power of the cornea is calculated using corneal

27. thickness and refractive indexes of air, cornea and aqueous humor through a series of
formulas.
Hamed et al. used this method to look at 100 post-myopic LASIK eyes. The authors used a
mathematical optical formula to directly calculate corneal refractive power.38
Good theoretical correlation was noted between this calculated corneal power and the clinical
history method. To our knowledge, no actual IOL implantations based on this formula have
been performed.
Direct corneal power measurements
The major shortcoming with all the above-mentioned techniques is the need to know the pre-refractive
surgery values, such as refraction and keratometry. An ideal method would
determine corneal power accurately without these values. True corneal power could be
determined regardless of the refractive status if anterior and posterior corneal curvatures
could be directly measured. However, direct measurement of the posterior curvature was not
possible until recently.
Introduction of slit-beam scanning combined with placido-disk topography Orbscan allows
posterior power measurements.
This technology also allows analysis of central optical zones as small as 1–2 mm.39
Sonego-Krone et al. as well as Seitz et al. used this technology for post-myopic LASIK,
comparing refractive changes at the corneal level induced by LASIK with Orbscan-measured
central total powers within the central 2-mm zone.25,40 They found a good correlation between
expected central diopteric power and measured values, and recommended using central 2-mm
power measured by Orbscan for IOL power determination after myopic LASIK. Qazi et
al. also used a similar method for post-myopic LASIK patients with good results.41
Although a promising technology, the accuracy and applicability of these power
measurements have not been established clinically.
Go to:
TARGETING MYOPIA
When regular keratometry is performed after myopic refractive surgery, selective choice of
an IOL to target myopia when other data are not available may reduce refractive surprises. In
analyzing eyes undergoing cataract surgery after RK, Chen et al. found that selecting an IOL
targeting –1.50 in post-RK eyes reduced the frequency of post-cataract hyperopia by 60%.
Some initial hyperopia immediately after cataract surgery also regresses over several weeks,
possibly because of inherent instability of the post-RK cornea.42–44
Go to:
CONCLUSION

28. Current methods of IOL power determination after corneal refractive surgery are limited by a
lack of actual clinical experience on a large scale and by the theoretic nature of all the
calculation methods. However, based on accumulated clinical experience, several useful
guidelines can be followed.
In addition to the recommendations below, refractive surgeons should consider providing
patients with pre-refractive surgery keratometry and refraction and having them keep these
records for possible cataract surgery in the future.
1. If only pre-and post-corneal surgery refraction are available, use post-refractive
surgery keratometry and axial length and adjust IOL power using a theoretic
nomogram [Tables [Tables11 and and22].
Table 2
Nomogram for IOL power adjustment for emmetropia after hyperopic LASIK
2. If pre-refractive surgery keratometry values and refraction are available, predict IOL
power theoretically using clinical history or nomogram-based methods. If using the
clinical history method, determine changes in spherical equivalent at the spectacle
plane rather than the corneal level.
3. If data are not available and patients have visual acuity >20/70, consider the contact
lens method.
4. If data are not available and patients have visual acuity <20/70, consider targeting
−1.50 to −2.00 for post-myopic refractive surgery patients and +1.00 for post-hyperopic
refractive surgery patients.
5. Some hyperopia in the immediate post-cataract surgery can regress in RK patients, so
delay intervention through lens exchange or further refractive surgery until the
refraction is stable.
6. Inform patients who have had previous corneal refractive surgery of limitations in
accurate IOL power calculations. As part of their informed consent for cataract
surgery, specifically discuss the possible need for corrective refractive aids, repeat
corneal refractive surgery or IOL exchange.
Go to:
Footnotes

29. Source of Support: Nil
Conflict of Interest: None declared.
formulas. These formulas work best near schematic eye measurements and are based on central
corneal power and AL.
Over time, it became apparent that the Hoffer Q formula was best for eyes shorter than 22 mm, the
Holladay I formula performed best with eyes between 24 mm and 26 mm and the SRK/T formula was
best for eyes longer than 26 mm. The assumption that the anterior chamber depth (ACD) was a
proportion of the AL and not a true measurement, led to IOL surprises with post -refractive patients.
This is because the third-generation formulas do not account for effective lens position.
Adjusting the Formulas
All three formulas allowed for optimization by adjusting a factor of
the formula. This factor is called the "Surgeon Factor" for the
Holladay I formula, the "ACD" for the Hoffer Q formula and the "A
Constant" for the SRK/T formula. Advances in computer technology
allow for the quick optimization of these formulas for a surgeon's
patient population. Most immersion ultrasound systems and the
IOLMaster allow for optimization after approximately 25 cases.
On the surface, optimization of one's cases sounds ideal, but one has to remember "garbage in
equals garbage out." To correctly optimize any of the formulas, no complicated cases should be
entered. Ideally, you should leave out any cases that have concurrent limbal relaxing or astigmatic
incisions. This basic optimization will provide you with excellent results the majority of the time.
Optimization brings any of the three formulas to your population average. For example, if your patient
population was primarily small hyperopic eyes, you would expect that the Hoffer Q would be the ideal
formula. After optimization of the Holladay and SRK/T formulas for the same population, the results
would be extremely similar to the Hoffer Q. The downside of the optimization in this scenario would be
that the three optimized formulas for the small AL population would not be as accurate as the
unoptimized SRK/T formula when dealing with large ALs. So, effectively, optimization raises or lowers
the curve but does not affect its shape. Ideally, one would have separate optimizations for separate
AL subgroups.
The Haigis Formula
In 1991, the Haigis formula evolved as one of two fourth-generation formulas in order to overcome
these shortcomings. The Haigis formula does not depend on assumptions for the ACD and requires
real measurement of it. In addition, the Haigis formula does not have just one "a Constant" but three
(a0, a1, a2) derived by multi-variable regression analysis.
a0 constant moves the power prediction curve up or down
a1 constant is tied to the measured anterior chamber depth
a2 constant is tied to the measured axial length

30. By using three adjustable constants, the surgeon can not only raise or lower the prediction curve, but
also adjust its shape. This allows for optimization over a larger range of ALs. This also requires a
much larger number of cases for optimization – 200 eyes. A note of caution: Most built-in optimization
programs in immersion units and the IOLMaster only optimize a0, making it effectively a third-generation
formula. These units usually allow for manual adjustment of a0, a1 and a2. The optimization
of all three constants can be performed on an Excel spreadsheet easily found on the Internet.
The inclusion of measured ACD into the Haigis formula has allowed for potentially increased
accuracy. On my Accusonic A-Scan, I always use the Haigis formula because I know that my ACD
measurements are precise. The optically-measured ACD on the IOLMaster is usually very accurate,
but a little less reproducible. Therefore, I use an optimized third-generation formula on my IOLMaster.
The Holladay II Formula
The Holladay II formula, derived in 1998, is the other fourth-generation formula. It attempts to be a
predictable formula for AL axial lengths by incorporating as much measured information as possible. It
requires seven different variables to be measured (white to white, corneal diameter, ACD, lens
thickness, patient's age, preop Rx and axial length). This information effectively works as a pattern-recognition
system. This formula has also been found to be highly accurate for a large variety of
patient eyes.
The use of immersion A-scan or the IOLMaster has become the standard of care in cataract surgery,
and so has the use of third-generation formulas. In order to make the leap into refractive cataract
surgery and lens exchange optimization, adoption of third-generation formulas is necessary, and use
of fourth-generation formulas is preferable. The time spent optimizing your formula of choice will be
well spent and leads to happy patients and fewer surprises.
Farrell Tyson, M.D., practices refractive cataract and glaucoma surgery in Cape Coral, Fla. He
obtained his biomedical engineering degree from Johns Hopkins University and completed his
ophthalmology residency at the Storm Eye Institute in Charleston, S.C.