Iol power And IOL power calculation

BIOMETRY&IOL POWER
CALCULATION
DR MD ALBAN UDDIN

OUTLINE
HISTORY
COMPONENTS OF IOL POWER CALCULATION
BIOMETRY
FORMULAE
VARIABLES
PROBLEMS AND ERRORS

HISTORY
1949-1st IOL Implantation-Sir Harold Ridley
Implantation was done on Feb 8,1950-secondary IOL
implantation.
Weight of the lens-17.4mg in water
Patient had a refractive surprise of nearly 20 D.
1967-Fyodorov et al-IOL power using vergence formulae.
1970s-”A “scans available-theoretical vergence formulae.
1980s-IDEM lens attempted.
1982-Gernet and Zorkendorfer-avg refractive power of
lens-+23.70D.
Biometry for Intra-Ocular Lens(IOL) Power Calculation-AAO

GENERATIONS OF INTRAOCULAR LENSES
GENERATION CHARACTERISTIC EXAMPLE
1st-1948 Experimental lens by Ridley Biconvex
2nd-1952 Anterior chamber angle fixated lens Baron’s Anterior chamber lens
Strampelli rigid tripod anterior
chamber IOL
Dannheim flexible closed loop
ACIOL
Barraquer J-Loop anterior chamber
IOL
3rd-1957 Iris supported,Iris clip lens Binkhorst iris clip lens
Fyrodov type 2 “Sputnik”iris –clip
IOL
Worst platina IOL
Copeland iris-plane IOL
4th-1956 Anterior chamber one piece lens Choyce Mark VIII rigid ACIOL
Strampelli angle supported ACIOL
5th Posterior chamber lenses Shearing J-loop posterior chamber
IOL

CRUDE IOL POWER CALCULATIONS IN
PAST
1980s-Guess work approach.
Concept-IDEM lenses
Standard lenses
Emmetropia lenses

IDEM LENSES:
Ideal emmetropic lenses
Pre-and post refractive refraction is same.
Recommended for emmetropic patients.
20D artificial lens in posterior chamber will restore preoperative refractive error.
Disorders of lens and cataract surgery-A K Khurana-pg 108

LIMITATION-Will restore the preoperative refractive error,only if the natural lens had
refractive power of 23.70 D which is not always true, as refractive power is a
combination of different factors-corneal curvature,depth of AC ,dioptric power of lens
and length of eye.

STANDARD LENS:
2D stronger than IDEM lens.
Renders eye pseudophakic about 1.5D myopic as compared to preoperative emmetropic
refraction.
80% of eyes-less than 1D nearsighted or farsighted-nearly correct in majority of patients.
Balanced distance and near vision.
Standard PCIOL-+22D
Standard ACIOL-+20D

EMMETROPIA LENS:
For restoring emmetropia in previously ammetropic eyes.
Power calculation-
HYPERMETROPIA-Power of IDEM lens+(Preoperative refractive error x 1.25).
MYOPIA-Power of IDEM lens-(Preoperative refractive error x 1.25).
History to be taken-kind of glasses patient wore previously,rely on refractive values only if
they come from reliable records,or if they match clinical findings.
Deviation of 2-3D is possible,can lead to significant errors.

DRAWBACKS:
Based on guess work approach.
1970s-’A’ scans were available-studies were conducted to establish various
theoretical vergence formulas.
It was found IOL power is determined by
Axial length
Keratometry
Effective IOL position

IOL POWER CALCULATION
BIOMETRY FORMULAS CLINICAL VARIABLES
• AXIAL LENGTH
• CORNEAL POWER
• EFFECTIVE LENS
POSITION
• GENERATIONS
• PERSONALISATION • SPECIAL
CIRCUMSTANCES
• PROBLEMS AND
ERRORS

BIOMETRY
AXIAL LENGTH(AL):
Most important factor in biometric calculations.
Distance from tear film to RPE.
1mm error in AL results in refractive error of 2.35D in a 23.5 mm eye,error being less
in case of longer eyes as compared to shorter eyes.
Methods of measurement of axial length-
Ultrasonic measurement
Optical measurement

ULTRASONIC MEASUREMENT:
Dr Karl Ossoinig-developed standardised A scans
Methods-Applanation method
Immersion method
Measures axial length from anterior surface of the cornea
to ILM of retina.
Measures axial length along anatomic axis.
Aphakic,pseudophakic and silicone oil filled eyes requires
adjustments for variation in velocity of sound in different
media.
Can cause variable corneal compression.
Mohan P, Chakrabarti A. Intraocular lens power calculation in 2019: The cutting edge. Kerala J Ophthalmol 2019;31:191-201.

CHARACTERISTICS OF A GOOD SCAN:
Corneal echo-single tall peak.
Aqueous chamber-No echo.
Anterior and posterior lens capsules-tall echoes.
Vitreous cavity-Few to no echoes.
Retina-tall,sharply rising echoes with no staircase at origin.
Orbital fat-medium to no echoes.
Anterior lens echoes-90% or more of maximum height.
Posterior lens echoes-50%-75% of maximum height.
Retinal echoes-75% or more of maximum height..
Obtain atleast 3 scans(ideally 5-10)which are within 15mm of each other,average 5-
10 most consistent results,giving lowest standard deviation(ideally <0.06).
Measure axial length of BE if:
AL is less than 22mm or more than 25mm in either eye.
Difference between two eyes is more than 0.3mm.

OPTICAL BIOMETRY:
Highly accurate non invasive automated method for measuring anatomical details of
eye.
Introduced in late 1990s.
1999-IOL Mater 500(Carl zeiss Meditec,Jena,Germany).
Technologies used:1.Partial coherence interferometry (PCI)
2. Optical low coherence reflectometry (OLCR)
3. Swept-source optical coherence tomography(SS-OCT)
Mohan P, Chakrabarti A. Intraocular lens power calculation in 2019: The cutting edge. Kerala J Ophthalmol 2019;31:191-201.

Partial coherence interferometry :
1986-Fercher and Roth.
Infrared light.
Tissue surfaces with differing refractive indices reflect this light.
Interferometric techniques are then used to measure ocular distances.
Optical low-coherence reflectometry:
Uses principle of a Michaelson interferometer.
Low coherence infrared light produced by a superluminescent diode is split into two
beams by a coupler.
One beam introduced into eye,other directed towards surface reference mirror.
Tissue interfaces reflects light.
Interference pattern detected by a detector.
Exact location from which the light was reflected from within the eye is determined
by scanning the reference beam.
Mohan P, Chakrabarti A. Intraocular lens power calculation in 2019: The cutting edge. Kerala J Ophthalmol 2019;31:191-201

Swept-source optical coherence tomography( SS-OCT):
Uses swift sweeping laser as the source.
The reference mirror is kept static.
The captured interference signal then undergoes Fourier transformation.

Nazm N, Chakrabarti A. Update on optical biometry and intraocular lens power calculation. TNOA J Ophthalmic
Sci Res 2017;55:196-210

SETTINGS OF A BIOMETER:
Calibration:-Check with the model eye provided from time to time for accuracy.
Gain/sensitivity settings:
Normal gain-70%
Dense cataracts,High Myopia,Ocular opacities-Increase gain
Silicone filled eyes,pseudophakic eyes-Decrease gain
Sound velocity settings:To be adjusted appropriately in
Normal
Cataractous
Aphakic
Pseudophakic
Mastering IOL-Ashok Garg pg 33

OPTICAL BIOMETERS:
IOLMaster 500
IOLMaster 700
Lenstar LS 900
Eyestar 900
OA-2000(Tomey)
Argos Advanced Optical Biometer(Movu)
Aladdin
AL-Scan (Nidek)
Galilei G6 Lens
Professional

IOLMaster 500:
First commercially available optical biometer from Carl Zeiss.
Uses 780nm infrared laser as source.
Can measure axial length to within 0.02 mm accuracy which is
5x more than that of an immersion ultrasound biometer.
Parameters measured:Axial length(AL)
Keratometry(K)
Anterior chamber depth(ACD)
White-to-white distance(WTW)
Formulae onboard-SRK 2
SRK-T
Haigis
Hoffer Q
Holladay-1
Haigis-L
Holliday 2

LENSTAR LS-900:
OLCR technology.
One of the first devices which could measure
Lens thickness(LT).
A low-coherent beam of light of 820 nm wavelength
produced from a superluminescent diode is used.
Can perform corneal topography-useful in Toric IOL
planning.
Parameters measured:Axial length(AL)
Keratometry(K)
Anterior chamber depth(ACD)
White-to-white distance(WTW)
Lens thickness(LT)

Formulae onboard:Holladay IOL Consultant Professional
Barrett Suite
Hill-radial basis function (RBF)
Masket
Modified Masket
Shammas
Additional software-Okulix and Olsen formulae also available.

Mohan P, Chakrabarti A. Intraocular lens power calculation in 2019: The cutting edge. Kerala J
Ophthalmol 2019;31:191-201

Mohan P, Chakrabarti A. Intraocular lens power calculation in 2019: The cutting edge. Kerala J Ophthalmol

INTRAOPERATIVE ABERROMETRY:
Can be performed during cataract surgery.
Aphakic and pseudophakic refractive measurements.
Can be attached to surgical microscope.
Provide real-time information on axis of placement of a toric IOL, position of limbal
relaxing incisions.
Intraoperative aberrometers are particularly beneficial in toric, multifocal, and
accommodative IOL implantation and also in cataract surgery in eyes postcorneal
refractive surgery.
The two commercially available intraoperative aberrometers are: Optiwave
Refractive Analysis (ORA) and Holos IntraOp.

OTHER USES OF OPTICAL BIOMETERS:
Toric IOL calculation.
IOL power calculation in eyes post corneal refractive surgery.
Lens centration
Central corneal thickness.

ADVANTAGES OF OPTICAL BIOMETRY:
Mohan P, Chakrabarti A. Intraocular lens power calculation in 2019: The cutting edge. Kerala J Ophthalmol
2019;31:191-201

R CECH et al:
The median of axial eye length measured using an optical biometer was 23.08 mm, and the
median of axial eye length measured using ultrasound biometry was 22.93 mm. The
difference between these values was 0.15 mm (150 microns).
Average variation of dioptric power of an implanted IOL from retrospectively established
optimum value of the IOLs optical power was 0.40 D lower with optical biometry and 0.16 D
lower with ultrasound biometry.
Unmbiguous preference of ultrasound biometry over optical biometry.
ChiaTMT et al:
Optical biometry AL and IOL power were not significantly different from ultrasound
measurements.
Cech R, Utíkal T, Juhászová J. Srovnání optické a ultrazvukové biometrie a zhodnocení užívání obou metod v praxi
[Comparison of optical and ultrasound biometry and assessment of using both methods in practice]. Cesk Slov Oftalmol.
2014 Feb;70(1):3-9. Czech. PMID: 24862369.

DISADVANTAGES OF OPTICAL BIOMETRY:
The cost of optical biometers is significantly more than ultrasound biometers.
Dense cataracts and corneal opacities may preclude accurate axial length
determination optical biometry.Immersion ultrasound biometry is preferred in such
cases, as it is more repeatable due to lack of variable corneal compression.
Many of these difficulties have, however, been overcome with the advent of swept-
source optical coherence tomography (SS-OCT) technology.

CORNEAL POWER:
Second most important factor.
1.0 D error in corneal power results in 1.0D
postrefractive error.
Methods of measurement:
Keratometry
Pentacam
Galilei
BAUSCH AND LOMB KERATOMETER
PENTACAM
GALILIE

POINTS TO BE CONSIDERED FOR KERATOMETRY:
Ideally be recalibrated after 20 cases for single observer.
Always do keratometry first before tonometry or any other corneal contact
procedures.
Re measure corneal curvature of both eyes if:
Corneal curvature is less than 40D or more than 47D.
The difference in corneal cylinder is more than 1D between eyes.
The corneal cylinder correlates poorly with the refraction cylinder
Mastering IOL-Ashok garg-pg36

EFFECTIVE LENS POSITION:
Historically referred as the anterior chamber depth(ACD).
ACD was found to be inaccurate for “in the bag” IOL.
Denotes the distance that the principal plane of IOL will sit behind the cornea.
1mm error in ELP leads to-0.25-0.5D of error.
Factors influencing effective IOL position:
Anatomical factors:Axial length,steepness of cornea,limbal white to white
measurement,preoperative anterior chamber depth and lens thickness.
IOL-related factors:Shape,length,flexibility,anterior angulation,material of haptic of
IOL
Surgeon-related factors:Surgeon’s individual surgical technique
Bag to sulcus shift:PCR,Loss of anterior capsule integrity-IOL needs to be placed in
ciliary sulcus-needs deduction (0.50-0.75D) from calculated IOL power .

Biometry for IOL-Power calculation-AAO

Using gaussian reduction equation,formula for calculating IOL power can be expressed in
accordance with refractive index-
P-Power of target IOL(In D)
K-Average dioptric power of central cornea(In D)
AL-Axial length(in mm)
C-ELP(in mm)
nv-refractive index of vitreous
Na-refractive index of aqueous
Biometry for IOL-Power calculation-AAO

FORMULAE
Based upon their derivation,the various IOL power formulae have been grouped
into-
THEORETICAL FORMULAE:Based on mathematical principles revolving around
‘schematic eye’.
REGRESSION FORMULAE:Arrived by looking at postoperative outcomes
retrospectively.
Artificial intelligence-based formulae: These formulae do use a form of regression,
but they rely on huge databases and employ artificial intelligence-based complex
statistical models to arrive identify relationships between variables.

Vergence formulae:Vergence formulae arrive at an IOL power using Gaussian
optics.
Gaussian optics makes an assumption that image vergence = object vergence + lens
vergence.
Most modern-day IOL power calculation formulae are based on the following
equation formulated by Fyodorov.
IOL power = (1336/[AL-ELP]) – (1336/[1336/{1000/([1000/ DPostRx] – V) + K} –
ELP])
K-Net corneal power ELP-Effective lens position V-Vertex distance
AL-Axial length DPostRx-Desired refraction
Vergence formulae are subclassified into 2-variable,3-variable,5-variable and
7-variable.

Ray tracing:
Ray tracing is a strategy to calculate the path that a light ray will travel when passed through
an optical system.
Gaussian optics assumes presence of a single optical axis and all rays of interest make only
very small angles to this.
3rd Generation vergence formulae assume that all components of optical system are thin
lenses
To describe the IOL position in this method, the radii of curvature of both anterior and
posterior corneal surfaces, their asphericity, central IOL thickness, and refractive index of
refraction are all used.
Gaussian optics formulae rely on postoperative refraction to back calculate ELP-less
accurate due to errors in postoperative refraction and incorrect IOL power labelling.
The final IOL position is a true geometrical position described by the distance between the
apex of the posterior surface of cornea and the apex of the anterior surface of the IOL
(postoperative ACD).

Advantages:
Postoperative ACD can be measured directly using PCI and compared with
preoperative estimated ACD.
Predict IOL power more accurately in abnormally short,long eyes and in eyes that
have undergone corneal refractive surgery.
Examples-Okulix,Olsen formula and PhacoOptics

CLASSIFICATION OF FORMULAE BASED ON GENERATIONS
1ST GENERATION 2ND GENERATION 3RD
GENERATION
4TH
GENERATION
5TH
GENERATION
THEORETICAL
FORMULAE
REGRESSI
ON
FORMULA
E
THEORETICA
L
FORMULAE
REGRESSION
FORMULAE
BINKHORST SRK-1 MODIFIED
BINKHORST
SRK-2 HOLLADAY-1 HOLLADAY-2 HAIGIS
COLENBRAND
ER-HOFFER
SRK-T OLSEN
GILL’S HOFFER’S Q
CLAYMAN’S
FYODOROV

1ST GENERATION FORMULAE:
1-THEORETICAL FORMULAE:
Based on 3 variables-AL of eye
K-reading
Estimated postoperative ACD
A-BINKHORST FORMULA: P=
𝟏𝟑𝟑𝟔(𝟒𝒓−𝒂)
(𝒂−𝒅)(𝟒𝒓−𝒅)
P-IOL power in diopters
r-corneal radius in millimeters(average)
a-AL in mm
d-assumed postoperative ACD plus corneal thickness

B-COLENBRANDER-HOFFER FORMULA:
P=
1336
𝑎−𝑑−0.05
-
1336
1336
𝐾
−𝑑−0.05
K-average keratometry in diopters
GILL’S FORMULA:
P=129.40+(-108 x K)+(-2.79 x AL)+(0.26 x LCL)+(0.38 x Ref)
P-desired IOL power
K-refractive power of cornea in diopters
AL-axial length in mm
LCL-distance of apex of anterior corneal surface to the apex of IOL in mm
Ref-desired postoperative refraction

CLAYMAN’S FORMULA:
Assume-
Emmetropizing iol=18D
Emmetropic AL=24mm
Emmetropic average keratometry value-42 D
1mm change in AL=3D of IOL power
1D in keratometry=1D of IOL power
FYODOROV FORMULA:P =
1336−𝐿𝐾
𝐿−𝐶 −
𝐶𝐾
1336
P-implant power for emmetropia
L-axial length(AL)in mm
K-corneal curvature in diopters
C-estimated postoperative anterior chamber depth(ACD)

ALGEBRAIC TRANSFORMATION OF THEORETICAL FORMULAE-
P=
𝑁
𝐿−𝐶
-
𝑁𝐾
𝑁−𝐾𝐶
P-Implant power for emmetropia
N-aqueous and vitreous RI
C-estimated postoperative ACD in millimeters
L-AL in mm
K-corneal curvature in diopters
DRAWBACKS:
Short eyes-large emmetropic value,Long eyes-too small value
Too cumbersome to apply
Guess of ACD is required
Were developed when iris-supported lenses were used
Based on theoretical simplistic assumption about optics of eye

2-REGRESSION FORMULAE:
Based on actual regression analysis of actual postoperative results of implant power
as a function of variables of corneal power and AL.
SRK-1 FORMULA:
Introduced by Sanders,Retzlaff and Kraff
Based on regression analysis,taking into account the retrospective analysis of
postoperative refractions.
Postoperative ACD was not included,replaced with A constant which was unique for
different type of IOL,determined by the manufacturer depending upon
material,position of eye,optic and haptic design.

FORMULA:
P=A-2.5L-0.9K
P-IOL power
A-constant specific for each lens
L-AL in mm
K-avg keratometry in diopters
Performs well for eyes with AL between 22.0 and 24.5mm
Short eyes-too small value,Long eyes-too large value.

SECOND-GENERATION FORMULAE:
1-THEORETICAL FORMULAE:
Modified Binkhorst formulae:1981-Used AL as scaling factor for effective lens
position and presented a formula to better predict ACD.
2-REGRESSION FORMULAE:
SRK-2 FORMULA: P=A-2.5L-0.9K
L<20mm A+3.0
L is 20.00-20.99mm A+2.0
L is 21.00-21.99mm A+1.0
L is 22.0-24.5mm A
L>24.5mm A-0.5

Modified SRK-2 Formula:
Based on AL,A-Constant is modified as:
If L<20mm A+1.5
L is 20-21mm A+1.0
L is 21-22mm A+0.5
L is 22.0-24.5mm A
L is 24.5-26.0mm A-1.0
L is >26mm A-1.5

3rd GENERATION FORMULAE:Requires 2 variables-K and AL
Holladay-1 formula:
Introduced in 1988
2 variable predictor(AL and Keratometry)could significantly improve the predictor of
effective lens position,particularly in unusual eyes.
Formula was proposed based on geometric relationship of the anterior segment.
Had better outcome for eyes between 22.00mm -26.00mm compared to other 3rd
generation.
SRK-T formula:
Non-linear theoretical optical formula,empirically optimised for postoperative
ACD,retinal thickness and corneal refractive index.
Combines advantages of both theoretical and empirical analysis.
Better outcomes for eyes>26.0 mm compared to other 3rd generation formulae

Hoffer-Q formula:
Variables required-K and AL
Better outcome for eyes <22.00mm,compared to other 3rd generation

4TH GENERATION FORMULAE:
Holladay-2 formula:
1993-Dr.Holladay led a worldwide study involving 34 cataract surgeons to determine which
of the 7 variables were relevant as predictors of ELP.
Led to concept of 9 types of eyes,not 3(short,medium and long)
7 Variables required-
1-K-Average K reading in diopters.
2-AL-Measured ultrasonic axial length in mm
3-ACD-Distance from corneal
4-LT-Lens thickness
5-Horizontal WTW-White to white corneal diameter
6-Age
7-Preoperative refraction
Allows calculation of IOL power in many different types of eyes but also honing of individual
result by personalising A constant
Available as a part of Holladay IOL consultant/surgical outcomes assessment programme

Olsen formula:
Variables required-
K
AL
ACD
LT
Horizontal WTW
Improved performance over 3rd generation formulae for eyes with AL 20.00-
26.00mm

5TH GENERATION FORMULAE:
Haigis:
Variables required
K
AL
ACD
Very good outcomes for eyes across the AL range and best reported outcomes for
eyes >28.00mm.For best results,constants need to be optimised requiring data from
atleast 500 eyes.

MODERN IOL FORMULAE:
LADAS SUPER FORMULA:
Devloped by Dr John G.Ladas,Albert Jun,Aazim Siddiqui,Uday Devgan.
Points of agreement and disagreement between Hoffer Q,Holladay 1,Holladay 1
with Wang-Koch adjustment,Haigis and SRK-T formulae for various ranges of axial
length were identified.
Ideal portions from four of five formulae were used to create super surface.
Dynamic formula-constantly being updated using inputs from surgeons worldwide.

BARRETT UNIVERSAL 2:
Devloped by Graham D Barrett.
Based on theoretical model eye in which ACD is related to axial length and
keratometry.
Can be unmodified in eyes of all axial lengths and for all IOL designs-universal.
5 Variables-AL,Keratometry,ACD,LT,Horizontal WTW corneal diameter to calculate
ELP along with A constant and desired postoperative refraction.
Predicts IOL power accurately in eyes with extremely long axial lengths.

HILL-RADIAL BASIS FUNCTION CALCULATOR:
Devloped by Warren E Hill with RBF calculator physician team.
Complex mathematical model-artificial neural network.
Has a property of adaptive learning.
Dynamic formula-becomes more accurate in its prediction as more data is fed into
it.
Optimised for Haag streit Lenstar in combination with Alcon SN60WF biconvex IOL
for powers from +30.00D to +6.00D and Alcon MA60MA meniscus design IOL
powers from +5.00D to -5.00D.

OLSEN FORMULA:
Devloped by Dr Thomas Olsen.
Paraxial and ray tracing optics.
Regression method used to determine ACD.ACD and LT used to determine ELP.
IOL position is described as a fraction of the capsular bag size using a new constant
called the C constant.
Variables used-ACD,AL,LT and keratometry.
power calculation in 2019: The cutting edge. Kerala J Ophthalmol 2019;31:191-201

PHACOPTICS:
Ray tracing.
Uses concept of Olsen’s C constant.
WANG-KOCH MODIFICATION:
Traditional IOL formulae for abnormally long eyes-postoperative residual hyperopia.
Modification for AL more than 25.2mm to avoid refractive surprise.

FULL MONTE INTRAOCULAR LENS:
Uses a method of analysis called the Markov Chain Monte Carlo process.
A probability instead of a single emmetropic power is displayed,surgeon can choose
appropriate IOL power.
Evolves continuously optimizing itself as new data is received.

TORIC CALCULATORS:
Correction of corneal astigmatism.
Components-1)Accurate K
2)Reliable method to calculate power of toric IOL.
Ideal characteristics:
It is comprehensive, that is, it can be used for preoperative planning, as well as for
refractive surprises
Software should preferably be a part of the biometric device
It should be applicable to all IOL types
There should be dynamic display of variables (e.g., dynamic adjustment of
astigmatic effects of phaco incision)
It should provide an alert for “axis-flip”
It should take into account posterior corneal astigmatism (PCA)
Nazm N, Chakrabarti A. Update on optical biometry and intraocular lens power calculation. TNOA J Ophthalmic Sci Res
2017;55:196-210

TORIC IOL CALCULATORS:
ASSORT TORIC CALCULATOR
BARRETT TORIC CALCULATOR
VERION
CALLISTO EYE
ASTIGMATISMFIX.COM

FUTURE OF INTRAOCULAR LENS POWER CALCULATION:
UniversIOL CALCULATOR:
Designed by Dr.Samir Sayegh.
Web based
Combines all high quality 3rd and 4th generation formulae with a toric IOL calculator.
Instead of proposing a new IOL power calculation formula, the surgeon is provided
with a choice of IOL power calculation formulae or combinations of formulae, a
choice of computational methods, and a choice of IOL ranking criteria.
Tells how much the formula differ from each other,gives surgeon an idea how close
she/he will be to the target.
Also contains all IOLs made so that an appropriate power as well as IOL can be
chosen.
2017;55:196-210

OKULIX:
Ray tracing formula.
Radius of curvature of anterior and posterior corneal surfaces need to be measured
using a corneal topographer.
IOL is described by radii of curvature of its anterior and posterior surfaces,thickness
and refractive index.

CLINICAL VARIABLES
PERSONALISATION:
Introduced by Retzlaff.
Used A constant to refine the formula based on surgeon’s past experience and data.
Parameters required:
Axial length(Pre-op)
Corneal Power(Pre-op)
IOL power
Postoperative refractive error
Same lens style required by one manufacturer implanted by one surgeon.
Same biometry and technician.
Eyes with postoperative surprises or acuity worse than 20/40 not included-poor
accuracy.
Mastering Intraocular Lenses-Ashok garg-pg no 42

Involves backsolving for the exact IOL position that would produce the resultant
refractive error with AL and K.
All the “ideal”IOL positions are averaged to arrive at personalised value for use in
future.
Can be easily done using Hoffer Programs or Holladay IOL Consultant computer
programs.

BIOMETRY IN SPECIAL CONDITIONS:
APHAKIA:
Required for secondary IOL implantation.
In aphakes,sound travels at speed of 1532m/s.
A Scan-2 lens spikes are replaced by anterior vitreous phase and posterior lens
capsule.
Immersion technique of biometry-method of choice.
Present biometers-aphakic mode is available

Mackool et al:
IOL power (D) = Aphakic refraction × 1.75
This method was found to be more accurate when anterior chamber IOLs were
implanted.
Ianchulev and Leccisotti:
IOL power (D) =0.07x 2 + 1.27x + 1.22, where x = aphakic refraction
This method was found to be more accurate for IOL placed posteriorly.

PSEUDOPHAKIA:
IOL exchange.
High spike at lens followed by artificial chain
of reduplication of echoes-can be misinterpreted as
spikes from retina.
Lower gain-eliminates artificial spikes and increases
retinal spikes.
Sound velocity depends upon type of IOL.
Modern biometers-options are available for phakic,
pseudophakic or aphakic mode.

PIGGYBACK IOL FOR PSEUDOPHAKIA:
Indicated in patients with significant residual refractive error following the primary
IOL implantation.
Surgically easier and more predictable optically rather than IOL exchange.
Formulae:
Holladay’s refractive formula
Gill’s nomogram-calculated from residence spherical based on axial length

VITRECTOMIZED EYE:
RI of silicon oil is much less than that of vitreous,acts as negative lens.
Silicon oil-sound attenuation-986m/s.
Increase the system gain and adjust sound velocity accordingly.
Result obtained is Apparent axial length(AAL)
which has to be converted to True
axial length(TAL)
TAL=1133/1550 AAL

PEDIATRIC BIOMETRY:
Dahan et al’ simplified approach based on axial length
AXIAL LENGTH IOL POWER
17mm 28D
18mm 27D
19mm 26D
20mm 24D
21mm 24D
22mm 23D
23mm 23D
>24mm Axial length-1D

Immersion A-scan preferred.
IOL power-SRK/T and Holladay 2 are preferred.
Undercorrection-
According to Dahan and Drusedau undercorrection of 20% in children <2 years
10% in children between 2 and 8 years
•Prost suggested 20% undercorrection between 1 and 2 years of age,
15% undercorrection between 2 and 4 years
10% between 4 and 8 years of age.
AAO-management of pediatric cataract.

POST KERATOREFRACTIVE SURGERY:
Standard methods of IOL power calculation-Inadequate.
Factors altered in corneal refractive surgeries:
Anterior corneal curvature
Ratio of anterior and posterior corneal curvature
Corneal asphericity
Modern formulae use the above factors to calculate IOL power-adds to error.

HAIGIS-L FORMULA:
The corneal radius of curvature (r corr) is modified as follows:
r - corr =
331.5
−5.1625 𝑥 𝑟 𝑚𝑒𝑎𝑠+82.2603−0.35
where r meas is the measured corneal radius of curvature.
SHAMMAS FORMULA:
Calculated corneal power, K = 1.14 ×TK −6.8
where TK is the post-LASIK corneal topography central K.

MASKET METHOD:
Regression formula-Samuel musket.
AL>23mm-Holladay 1.
AL<23mm-Hoffer Q.
Adjustment of IOL power-
(LSE × −0.326) +0.101 = Post-LASIK IOL power adjustment
Where LSE is the laser corrected spherical equivalent adjusted for vertex distance
MODIFIED MASKET METHOD:
(LSE × −0.4385) +0.0295 = Post-LASIK IOL power adjustment
Others:
ASCRS postrefractive calculator
Hoffer–Savini LASIK intraocular lens power tool
Barrett true-K formula
The McCarthy post refractive intraocular lens calculator

PROBLEMS AND ERRORS-IOL POWER
SURPRISE
AXIAL LENGTH:Most common source of error
Axial length >25mm,higher incidence of staphylomas.
Artificial shortening due to contact applanation ultrasound.
Technician inexperience.
Wrong average ultrasound velocity.
Poor IOL master readings-Dense cataracts.
Silicone filled eye.

CORNEAL POWER:Second most common cause.
Eyes with previous corneal refractive surgery.
Wrong calibration.
Not setting index of refraction(IR) in IOLMaster-leads to error in Hoffer Q formula.
Contact lens wear.
Corneal scarring.
EFFECTIVE POSITION OF IOL:Least effective factor
IOL in deeper or shallower position than predicted by the formula
ACD constants not personalised to individual IOL style,surgeon and clinic.

FORMULAS:
Regression formulas(SRK 1) for eyes outside the normal AL range of 22-24.5mm.
MISCELLANEOUS:
Manfacturer labelling error.
Insertion of wrong IOL.

PREVENTION OF ERRORS:
Immersion A-scan/IOLMater to measure AL.
Suspect staphylomas in eyes>25mm-Use IOLMaster.
Well-trained experienced technician.
Regularly calibrate manual keratometers.
Evaluate Biometer scans for reliability.
Keep contact lens out for 2 weeks prior to keratometry
Silicone oil-optical biometers/AL x 0.71
AL<22mm-Hoffer Q
AL 22-26mm-Holladay 1
AL >26mm-SRK/T
Avoid using SRK regression formulas(SRK 1 and 2)

Personalise ELP factors in the formulas.
Personally select IOL power for individual patient.
Set IR to 1.3375 in set-up screen of IOLMaster.
Post-refractive surgery-
Shammas method
Masket method
Haigis formula
Consider delaying IOL implantation until cornea has healed after a penetrating
keratoplasty rather than performing a “triple procedure”.

Iol power And IOL power calculation

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