1.
INTRAOCULAR LENS (IOL)
CALCULATION
PRESENTER – IKECHUKWU VICTORIA
DATE – 23RD AUGUST, 2022
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2.
OUTLINE
• Introduction
• Measurement of Corneal Curvature
• Measurement of Axial Length
• Formulas
• Special circumstances
• Pediatric eyes
• Conclusion
• References
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3.
INTRODUCTION
When you walk into a store to purchase a pair of shoes, what comes to
your mind (a) first? Colour? Make?... SIZE! The size of the shoes you get
determines the outcome of fitting and good use for the wearer.
Everything that is a pair must have a measurement or a size. Whether it
is a pair of legs, of ears or the eyes etc, what you wear on them. Certain
measures or formulas must be carried out to determine their accurate
sizes – or power. Since it is inappropriate for one to have a power or
size that is totally different from the other. They must have the same or
slightly different measurement.
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4.
INTRODUCTION
• The history of intraocular lenses (IOLs) began in 1949, when English
ophthalmologist Harold Ridley implanted the first
polymethylmethacrylate (PMMA) IOL in London.
• Initially, other ophthalmologists strongly opposed the use of IOLs, and
it took years of development and perseverance for the IOL to become
the standard it is today.
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5.
INTRODUCTION
• Implantation of an IOL serves to eliminate the numerous difficulties
with aphakic spectacles correction which including:
image magnification,
 ring scotomata,
peripheral distortion,
a “jack-in-the-box” phenomenon
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6.
INTRODUCTION-IOL CALCULATION
• To obtain the desired IOL power some parameters of the eye are
measured calculated using the appropriate formulas. These
parameters include:
Measurement of Axial Length
Measurement of Corneal Curvature
Measurement of Anterior Chamber Depth
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7.
MEASUREMENT OF AXIAL LENGTH(AL)
• The AL is the most important factor in the formulas for IOL calculations.
• A 1-mm error in AL measurement results in a refractive error of
approximately 2.35 D in a 23.5-mm eye.
• The refractive error declines to only 1.75 D/mm in a 30-mm eye but rises to
3.75 D/mm in a 20-mm eye.
• Therefore, accuracy in AL measurement is more important in short eyes
than in long eyes
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8.
MEASUREMENT OF AXIAL LENGTH(AL)
• There basically two widely used method for measuring AL.
oUltrasonic measurement of axial length
Immersion (non contact) technique
Applanation (contact) technique.
oOptical measurement of axial length
optical coherence techniques.
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9.
ULTRASONIC MEASUREMENT OF
AXIAL LENGTH
• When A-scan ultrasonography is used to measure AL, we either assume a constant
ultrasound velocity through the entire eye or measure each of the various ocular
structures at its individual velocity.
• A-scans measure not distance but rather the time required for a sound pulse to travel
from the cornea to the retina.
• Sound travels faster through the crystalline lens and the cornea (1641 m/s) than it does
through aqueous and vitreous (1532 m/s). Even within the lens itself, the speed of sound
can vary in different layers of nuclear sclerosis.
• The measured sound transit time is converted to a distance by use of the formula d = tV
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10.
ULTRASONIC MEASUREMENT OF AXIAL
LENGTH- IMMERSION TECHNIQUE
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11.
ULTRASONIC MEASUREMENT OF AXIAL
LENGTH- APPLANATION TECHNIQUE
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12.
ULTRASONIC MEASUREMENT OF AXIAL
LENGTH
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13.
ULTRASONIC MEASUREMENT OF AXIAL
LENGTH
• The Applanation method may yield a shorter AL measurement that is also
inconsistent and unpredictable.
• An artificially shortened AL measurement occurs with inadvertent corneal
indentation.
• In the Immersion method, which is accepted as the more accurate of the 2
techniques, space is maintained between the probe and the cornea,
eliminating corneal indentation
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14.
ULTRASONIC MEASUREMENT OF AXIAL
LENGTH
• In eyes with AL values greater than 25 mm, staphyloma should be
suspected, especially when numerous disparate readings are obtained.
• Such errors occur because the macula is located either at the deepest part
of the staphyloma or on the “side of the hill.”
• To measure such eyes and obtain the true measurement to the fovea, the
clinician must use a B-scan technique.
• Optical methods (eg,IOL Master, Lenstar) are very useful in such cases
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15.
OPTICAL MEASUREMENT OF
AXIAL LENGTH
• IOLMaster method-1999. In 2008-Lenstar LS900 .
• Uses a partial coherence laser for AL measurement measures the time required for
infrared light to travel to the retina. Because light travels at too high a speed to be
measured directly, light interference methodology is used to determine the transit time
and thus the AL.
• Does not require contact with the globe, so corneal compression artifacts are eliminated.
• developed such that its readings would be equivalent to those of the immersion
ultrasound technique.
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16.
OPTICAL MEASUREMENT OF AXIAL LENGTH
• In dense cataracts (especially posterior sub capsular cataracts),
ultrasound biometry is still necessary (in 5%–8% of cataract patients).
• Compared with ultrasonography, this technique provides more
accurate, reproducible AL measurements.
• In addition, optical measurement is ideal in 2 clinical situations that
are difficult to achieve using ultrasonography: eyes with staphyloma
and eyes filled with silicone oil.
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17.
OPTICAL MEASUREMENT OF AXIAL
LENGTH
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18.
MEASUREMENT OF CORNEAL CURVATURE
(KERATOMETRY)
• The central corneal power, K, is the second most important factor in
the calculation formula
• A 1.0 D error in corneal power causes a 1.0 D postoperative refractive
error.
• Corneal power can be estimated by keratometry or corneal
topography, neither of which measures corneal power directly.
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19.
MEASUREMENT OF CORNEAL CURVATURE
(KERATOMETRY)
• Manual Keratometry
Von Helmholtz keratometer
The Javal-Schiotz keratometer
• Automated Keratometry
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20.
MEASUREMENT OF CORNEAL CURVATURE
MANUAL KERATOMETRY
• Manual keratometry is a method to determine the true central optical
power of the cornea by measuring the radius of curvature of the anterior
surface.
• It makes assumptions regarding the posterior surface (which cannot be
measured by the instrument) and then converts the radius into diopters (D)
by a simple equation:
• D = 337.5/ r Where D = diopters, r = radius of curvature
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21.
MEASUREMENT OF CORNEAL CURVATURE
MANUAL KERATOMETRY
• The standard manual keratometer measures only a small central
portion (from 2.8 to 3.2 mm diameter) of the cornea and views the
cornea as a convex mirror.
• Both front and back corneal surfaces contribute to corneal power, but
the keratometer power “reading” is based on measurement of the
radius of curvature of only the front surface and assumptions about
the posterior surface.
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22.
MEASUREMENT OF CORNEAL CURVATURE
MANUAL KERATOMETRY
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23.
MEASUREMENT OF CORNEAL CURVATURE
MANUAL KERATOMETRY
• 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.
• Routine of calibrating manual keratometers
• Average K reading
• contact lenses must off 2 weeks prior to the keratometry exam for IOL
power
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24.
MEASUREMENT OF CORNEAL CURVATURE
AUTOMATED KERATOMETRY
• The main advantage of automated over manual keratometry is
simplicity: joystick focusing and two clicks allow accurate and
repeatable measurements in a very short time
• Moreover, hand held autokeratometers allow the measuring of
patients in different postures—which can be useful for examination of
children under general anesthesia, disabled or mentally retarded
patients, etc.
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25.
MEASUREMENT OF CORNEAL CURVATURE
AUTOMATED KERATOMETRY
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26.
MEASURING CORNEAL POWER WITH THE
PENTACAM
• The Pentacam is a newer imaging system that uses a single Scheimpflug
camera to measure the radius of curvature of the anterior and posterior
corneal surfaces, as well as the corneal thickness, for the calculation of
corneal power.
• This technology can provide the following corneal measurements:
simulated keratometry (SimK), True Net Power, Equivalent K Reading (EKR),
and Total Refractive Power.
• In addition, it has been used to develop the BESSt formula.
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27.
MEASUREMENT OF ANTERIOR CHAMBER
DEPTH
• Anterior chamber depth (or ACD) because the optic of all IOLs in the
early era was positioned in front of the iris, in the anterior chamber,
or in the iris plane.
• Because almost all IOLs today are positioned behind the iris, new
terminology has been offered such as effective lens posit ion (ELP) by
Holladay1 and actual lens position (ALP) by the FDA.
• ELP is also used when referring to anterior chamber (AC) lenses,
which do sit in the ACD.
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28.
FORMULAS
• The original formulae were developed prior to 1980. They include the
theoretical formulae and regression formulae
• The theoretical formula are based on mathematic principles
revolving around the schematic eye.(1st generation)
• The regression formula is based on working on post operative
outcomes (2nd generation)
• Some others are mixed- 3rd & 4th generation.
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29.
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30.
FORMULA HISTORY
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31.
SRK FORMULA- SRK I
• It is a regression formulae and most popular, described by Sanders D,
Retzlaff J. and Kraff M.
• The formula is based on the following equation: P = A - 2. 5L - 0.9K
where P is the implant power for emmetropia,
 L the axial length in millimeters, and
K the average keratometric reading in diopters.
• A is a constant varies with the implant design and the manufacturer.
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32.
SRK FORMULA- SRK II
• The SRK II formula was introduced
for such 'too long' and 'too short'
eyeballs.
• 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.
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33.
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34.
SRK FORMULA- SRK /T (THEORETIC)
• In 1990, Retzlaff , modified the Holladay 1 formula to allow use of an
A constant instead of a SF, calling it the SRK/ T theoretic formula.
• SRK/T formula optimizes the prediction of postoperative ACD, retinal
thickness AL correction and corneal refractive index.
• It is recommended for eyes longer than 26.00mm.
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35.
HOLLADAY FORMULA-I
• Produced by Jack Holladay in 1988
• Used axial length and keratometry to determine ELP
• Takes into account AC depth, lens thickness and cornea radius
• Calculate predicted distance from cornea to iris plane + distance from iris plane to
IOL
• Uses surgeon factor for optimization of formula (specific for each lens)
• Work best for eyes between 24.5 to 26mm(medium long), useful for axial myopia
and high corneal curvature(>45)
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36.
HOLLADAY FORMULA-2
• Fourth generation formula
• Assumed anterior segment size was directly related to axial length but
resulted in surprise outcomes especially in small eye
• Findings
-horizontal white-to- white measurement emerged as the next most important variable
relate to ELP after AL and Ks.
-there was almost no correlation between AL and size of anterior segment in 80-90% of
eyes
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37.
HOLLADAY FORMULA-2
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38.
HOLLADAY FORMULA-2
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39.
HOLLADAY FORMULA-2
• Considered as one of the most accurate IOL formula
• It emerged as the state of the art IOL calculation formula
• The IOL master has the formula
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40.
HOFFER Q
• Introduced by Dr Kenneth Hoffer in 1993
• Was developed to predict the pseudophakic ACD
• It relies on a personalized ACD, AL and corneal curvature
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41.
HOFFER Q
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42.
HOFFER Q
• Most accurate in short eyes <22.0mm, confirmed in large study of 830
eyes
• Had the lower mean absolute error (MAE) for AL 20.0mm to
20.99mm
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43.
HAIGIS FORMULA
• Developed by Wolfgang Haigis, found in Zeiss IOL Master software.
• It has 3 constant and an accuracy close to that of Hoffer Q
• By regression analysis, the 3 constant are calculated to individually
adjust the IOL power prediction curve for each surgeon/IOL
combination in such a way as to closely reproduce observed results
over a wide range of AL and ACD.
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44.
HAIGIS FORMULA
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45.
HAIGIS FORMULA
• With three lens constants, the Haigis formula is able to make
adjustment adding or subtracting power when necessary, based on
actually observed results for a specific surgeon and the individual
geometry of an intraocular lens implant.
• Recommended for eyes with short axial length.
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46.
OLSEN FORMULA
• Developed by Thomas Oslen
• The Oslen formula addresses 4 area of concern
Calculation of the cornea power
Measurement of axial length
Anterior chamber depth prediction-method predict the postop ACD in a
given eye based on actual preop measurements of the eye.
IOL optics
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47.
OLSEN FORMULA
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48.
RANGE OF AL AND PREFERRED FORMULA
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49.
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..
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50.
SPECIAL CASES
• Silicone filled vitreous eyes- the sensitivity of the system should be
increased to visualize the retinal echo spike and the components of
the eye must be measured separately to reach an accurate result.
• The usage of a standard sound velocity can lead to an error of up to 8
mm in such eyes.
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51.
SPECIAL CASES
• 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.
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52.
SPECIAL CASES
• 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.
• 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 D postoperative
refraction as most sedentary elderly will prefer being near sighted.
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53.
SPECIAL CASES
• 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.
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54.
PEDIATRIC 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
• After the age of two years, a myopic shift of 4-6 D is expected
depending upon the age.
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55.
PAEDIATRIC IOL CALCULATION IN EEH KANO
PERCENTAGE METHOD
• Deduct a certain percentage from
the IOL depending on the patients
age
• 0-2 years- 20%
• 3-8 years-10%
• Example- <2yr IOLP=24.00D
20%=4.80
IOLP=24-4.8=19.20D
TARGET METHOD
AGE TARGET
1 +3.00
2 +2.25
3 +2.00
4 +1.75
5 +1.50
6-7 +1.25
8 +1.00
9-10 +0.75
11-14 +0.50
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56.
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57.
CONCLUSION
• Intraocular lens implantation today is a very important in cataract surgery visual
outcome. So the need for a good knowledge and accuracy of calculating IOL power can
not be overemphasized.
• The outcome of a successful cataract surgery is noticed in the post-cataract refraction.
• E.g IOL Power inserted-+18.50Dfor-0.20
• Refraction after surgery=-0.50-1.00*180-6/24.
• An error of -0.50D
• 4 weeks later,refraction is:plano-6/12.
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58.
THANK YOU FOR LISTENING
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59.
REFFERENCES
• AAO Clinical Optics 2016-2017
• Elkington, A.R And Frank H.J, Clinical Optics 3rd Edition
• Duane’s Ophthalmology
• Brad Bowling, Kanski Clinical Ophthalmology A Systematic Approach,
8th Edition,2016
• IOL Power By Kenneth J. Hoffer
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