By:Master in Clinical Optometry, UKM2015/2016

IntroductionGenerationFormula

1st and 2nd generation formula 3rd generation formula 4th generation formula

History IOL calculation formula Recommended formula

usage

It is often said that cataract surgery is a refractive surgery.

In old days the cataract was removed first and the spectacle prescription given last, the situation today is reversed.

We prescribe an IOL to obtain a certain refractive effect Aims to reduce spectacle dependency

Therefore, IOL calculation plays an important role to determine the refractive outcome after the surgery.

Since 1975, IOL power has been calculated using accurate measurement of an eye’s corneal power and axial length (AL).

Power of the IOL was calculated using clinical history alone.

Or the preoperative refractive error prior to cataract development.

Today, we can customize the power of the lens implanted during cataract surgery.

Even patients who are highly myopic or hyperopic can achieve a near plano result after IOL implantation.

1ST AND 2ND GENERATION FORMULA:SRK, SRK II, HOFFER

By Leong Shin Yi, Foo Hou Ling,Mohd Zharif

•Provided evidence for tolerance of a foreign body in the eye•Prospect of restoring functional vision.

•A- constant= specific constant for each type of IOL, which is determined empirically on the large sample of patients underwent cataract surgery.

•A-constant is calculated for each lens type based on the refractive outcomes

Regression formula Empiric formulas generated by averaging large numbers of post-operative clinical results

(retrospective computer analysis of data obtained from a great number of patients who have undergone surgery)

1980s; popular because it was simple to use

Power error often resulted from the use of these formulas

P= IOL power to be used (D)A = IOL specific A constantK = Average corneal refractive power (D)L = Axial length of the eye (mm)

P = A – 0.9K – 2.5L

A constant Relates the P to K and L Depends on multiple variables

IOL manufacturer Style Placement

Used to characterize the IOL implants Intended location Orientation within the eye

Provided by the manufacturer of IOL

16

Corneal refractive power Assumption;

Thin spherical lens Fixed anterior and posterior corneal curvature ratio Index of refraction of 1.3375

Measured by keratometry / corneal topography

Corneal radius of curvature relates to corneal power with the equation

K = n – 1 r

r = 337.5 K

Axial length of the eyeDistance between the anterior surface of the cornea and the fovea

Most important factor in IOL calculation1.0mm error 2.50D – 3.50D error

Measure by A-scan ultrasonography / optical coherence biometry

Suitable to use on axial length range : 22mm- 24.5mm

Based on regression analysis 2nd generation of SRK formula Optimized A constant based on axial length of the eye

Increase the A constant for shorter eye Decrease the A constant for longer eye

The new SRK II formula was more accurate than the original SRK and Binkhorst II formulae. 80% of the eyes has less than 1D error and one eye 0.3% had an error of more than 3D

(Dang et al.1989)

P= IOL power to be used (D)A = IOL specific A constantK = Average corneal refractive power (D)L = Axial length of the eye (mm)

P = A1 – 0.9K – 2.5L

A constantOptimized based on axial length

A1 = (A – 0.5) for axial length greater than 24.5mmA1 = (A) for axial length between 22 and 24.5mmA1 = (A + 1) for axial length between 21 and 22mmA1 = (A + 2) for axial length between 20 and 21mmA1 = (A + 3) for axial length less than 20mm

Hoffer formula use the post-operative AC depth

A change in the true post-operative AC depth will affect the refractive status of the eye. A change in 1 mm causes a 1.5D change in the final refraction.

Hence, these constants must be personalized to accommodate any consistent shift that might affect IOL power calculation.

The main feature of the 1st generation theoretical formulae was that position of IOL in the eye is fixed for each lens type.

This assumption was true at that time, when cataract surgery was represented by ICCE and ACIOL implantation: the ACIOL was assumed to have a defined position in relation to the anterior plane of the cornea.

2nd generation theoretical IOL power formulae differ from the 1st generation because: Position of the IOL in the pseudophakic eye; is not fixed but changes based on 2 variables: axial length and corneal curvature or, corneal power of the eye.

The 2nd generation regression formulae were designed to improved accuracy has been shown to reduce the prediction error of the original SRK formula in short (<22mm) and long(≥24.5mm axial length) eyes.

Although the 1st and 2nd generation formulae are not used in present time, they are all basis formulae developed or modified for newer generation formulae (3rd and 4th generations).

SRK formula recommended used in cases such as ICCE ACIOL Emmetropic eye

SRK II formula recommended used in cases such as ECCE Phacoemulsification PCIOL Axial length (too long or too short than normal)

Professor Dr. Jean. B., A Comparative Analysis of Methods for Calculation IOL Power: Combination of Three Corneal Power and Two Axial Length Measuring Techniques, (2008).https://publikationen.unituebingen.de/xmlui/bitstream/handle/10900/45350/pdf/stanbekova.pdf?sequence=1

Masket. S. MD, Masket, S.E., PhD, Simple Regression Formula For Intraocular Lens Power Adjustment in Eyes Requiring Cataract Surgery After Excimer Laser Photoablation (2006), J Cataract Refract Surg, Vol: 32, Pg: 430-434http://www.unisinucartagena.edu.co/biblioteca/oftalmologia/REVISION_TEMA/SEGMENTO_ANTERIOR/CATARATA/FACOEMULSIFICACION/ARTICULOS/Articulos_Calculo_de_LIO/3.pdf

Dang, M. S., and Raj, P.P.S., SRKII Formula In The Calculation of Intraocualr Lens Power, (1989), British Journal of Ophthalmology, Vol. 73, Pg: 823-826.http://bjo.bmj.com/content/73/10/823.full.pdf

Olsen. T, Calculation Of Intraocular Lens Power: A Review, (2007), Acta Ophthalmologica Scandinavica, Vol. 85, Pg: 472-485. http://onlinelibrary.wiley.com/doi/10.1111/j.1600-0420.2007.00879.x/full

Apple, D.J., MD, and Sims, J. MD., Harold Ridley And The Invention Of The Intraocular Lens, (1996), Survey Of Ophthalmology, Vol. 40, No.4. http://www.rayner.com/skin/frontend/mtcolias/default/pdf/Invention_of_the_IOL.pdf

By Ling Sook Yee, Low Yu Chen, Nurul Akimi Abdullah

3RD GENERATION FORMULA : HOFFER Q, HOLLADAY 1, SRK/T

Merger of the linear regression methods with theoretical eye models

Pseudophakic ACD

Surgeon Factor

A-constant

Improved accuracy

Better result & simple

Take into account ofAxial length K-reading

Optimization of formula

to predict the effective lens position ELP

= distance from cornea to lens

Explains position of the IOL postoperatively

ELP is difficult to predict because:a) IOL is thinner than cataractb) ACD tends to increase with pseudophakiac) Variable lens geometry across power range

Errors in predicting the ELP caused: refractive surprise

Shallow AC -> sitting more anterior -> lower IOL power

Introduced by Dr Kenneth Hoffer in 1993

Was developed to predict the pseudophakic anterior chamber depth (ACD)

Being optimized from Hoffer formula by personalizing the ACD

It relies on a personalized ACD , axial length and corneal curvature.

P = f (A, K, Rx, pACD)Axial length

Average corneal refractive power

Previous refraction

Personalized ACD, manufacturer’s ACD-

constant

ACD-constant = 0.58357 * A-constant – 63.896

P = 1336 - 1.336 A – C – 0.05 [(1.336) – (C +0.05)]

K + R

1000

P = IOL powerA = Axial lengthC = estimated post-op ACDK = corneal power (in Diopters)R = corneal radius (in mm)

Hyperopes (AL < 22 mm) (Kenneth Hoffer) Most accurate in short eyes < 22.0mm, confirmed in large study of 830 short

eyes Had the lowest mean absolute error (MAE) for AL 20.0mm to 20.99mm Hoffer Q and Holladay 1 had lower MAE than SRK/T for AL 21.0mm to 21.49mm

In post corneal refractive surgery

Contribution of IOL power errors:i. Inaccurate measurement/calculation of anterior corneal power (especially in

those remove corneal tissue i.e PRK)ii. Incorrect estimation of ELP

Flat central corneal power after LASIK, the formula assumes that the AC is shallow

Myopic-LASIK:  underestimation of the IOL powerHyperopic-LASIK: overestimation of the IOL power

P= PTARG - 0.326 × RCC - 0.101IOL power calculated by standard IOL formulas

surgically induced refractive change

This method adjusts the power of the IOL, using the knowledge of the surgically induced refractive change.

Masket S and Masket SE (2006)

Example IOL calculated 22.0DChange in Rx = +3.0DP = 22.0 – (0.326 x +3.0) – 0.101

= 21.0 D

Double K formula K-reading before refractive surgery is used to estimate the ELP K-reading after refractive surgery is used to calculate the IOL power

Tradition method: Single K formula K-reading is used for both calculations Tends to underestimate the IOL power in myopic LASIK eyes

Myopic Correction Numbers in each row represent the

amount (D) that must be added to the calculated IOL power

Hyperopic Correction Numbers in each row represent the

amount (D) that must be subtracted to the calculated IOL power

Produced by Jack Holladay in 1988 Used axial length and keratometry to determine ELP Work best for eyes between 24.5 to 26 mm (medium long) Takes into account ac depth, lens thickness and corneal radius Useful for axial myopia and high corneal curvature (>45)

Using K, AL to predict IOL power No ACD input indicated Calculates predicted distance from cornea to iris plane + distance from iris

plane to IOL Uses surgeon factor for optimization of formula (specific for each lens)

Distance between iris plane & IOL optic plane SF should be personalized 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 refraction SF 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.

For eyes with previous refractive surgery Use K value prior to surgery and change in manifest refraction resulting

from LASIK or PRK

IOL power is calculated using the Aramberri double-K method uses corneal power prior to refractive surgery to estimate effective lens

position value of 43.86 D is used when corneal power pre refractive surgery not

available.

1. Regression formulas topped surgeon’s preferences, and one of the most successful was the SRK formula. (Sanders D et al,1983)

2. Over the years, surgeons discovered that the SRK formula is best used in eyes with average AL, between 22.00 and 24.50 mm.

3. A subsequent formula, the SRK II, was developed for use in long and short eyes. ( Dang MS et al, 1989)

4. 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. (Retzlaff JA,1990)

SRK I – 1st gen

P = A – 0.9K – 2.5L

SRK II – 2nd gen

P = A1 – 0.9K – 2.5L

AI Axial Length

A+3 <20A+2 20-21A+1 21-22

A 22-24.50A-0.5 >24.5

It can be calculated using the same A constants used with the original SRK formula or with ACD estimates.

SRK/T formula optimizes the prediction of postoperative ACD, retinal thickness AL correction, and corneal refractive index.

Recommended formula usage : best for eyes longer than 26.00 mm.

What is the effect of A-constant on IOL power? The term “A-constant” seems misleading because, it varies among IOL models and even among surgeons.

“A-constant” is adjustable & depends on multiple variables including IOL manufacturer, style and placement within the eye.

Different model of IOL , has different A-constant. Eg ~ 1:1 ruleIOL brand No. 1 : A-constant of 118.4 = +21.0 DIOL brand No. 2: A-constant of 118.9 = +21.5 D to get the same plano postop refraction. 

1:1 relationship with the A-constants: if A decreases by 1 diopter, IOL power decreases by 1 diopter.

Research shown there was no significant difference between the predictive abilities of SRKII or SRK/T.

However, there are differences in the predictability of refractive outcomes between different IOL.

( M J ELDER, 2002)

Hoffer Q < 22mmHolladay 1 24-26mm

SRK/T >26mm

•Holladay 1 formula - Uses “surgeon factor”•Hoffer Q formula – uses “ Pseudophakic ACD)•SRK/T formula – uses “ A- contstant”

Wang, L., M.A. Booth, and D.D. Koch, Comparison of intraocular lens power calculation methods in eyes that have undergone LASIK. Ophthalmology, 2004. 111(10): p. 1825-31.

Masket, S. and S.E. Masket, Simple regression formula for intraocular lens power adjustment in eyes requiring cataract surgery after excimer laser photoablation. J Cataract Refract Surg, 2006. 32(3): p. 430-4.

Aramberri J. Intraocular lens power calculation after corneal refractive surgery: Double K method. J Cataract Refract Surg 2003; 29(11): 2063-2068.

Eom Y, Kang S-Y, Song JS, Kim YY, Kim HM. Intraocular Lens Power Calculation According to the Anterior Chamber Depth in Short Eyes. American Journal of Ophthalmology, April 2014, Vol 157, Issue 4, pp 818-824.

Hoffer KJ. The Hoffer Q formula: A comparison of theoretic and regression formulas. Journal of Cataract and Refractive Surgery, November 1993.

Sanders DR, Retzlaff J, Kraff MC. Comparison of empirically derived and theoretical aphakic refraction formulas. Arch Ophthalmol. 1983;101(6):965-967.

Dang MS, Raj PP. SRK II formula in the calculation of intraocular lens power. Br J Ophthalmol. 1989. Retzlaff JA, Sanders DR, Kraff MC. Development of the SRK/T intraocular lens implant power calculation formula. J Cataract Refract Surg.

1990 IOL power calculation retrieved http://www.rajswasthya.nic.in/RHSDP%20Training%20Modules/Ophthalmologist/Cataract%20Surgery

%20with%20IOL.Pdf/03%20IOL%20calculation.pdf Findl, O. Biometry and intraocular lens power calculation. Current Opinion in Ophthalmology 2005, 16:61–64 Parmar, M. (2008). IOL power calculation. Retrieved from http://www.eophtha.com/eophtha/ppt/IOL%20power%20calculation.html

By Ang Kai Li, Noor Munirah binti Awang Abu Bakar, Nurulhidayah Nordin

Developed by Wolfgang Haigis, director, Department of Biometry, University of Würzburg Eye Hospital, Würzburg, Germany

Found in Zeiss IOLMaster software

The Haigis formula (3 constants) has an accuracy close to that of the Hoffer Q (two-variable formulas)

By regression analysis, the 3 constants are calculated to individually adjust the IOL power prediction curve for each surgeon/IOL combination in such as way as to closely reproduce observed results over a wide range of axial lengths and anterior chamber depths.

PROBLEMS WITH 3RD GENERATION 2 VARIABLE FORMULA (HOFFER Q, HOLLADAY 1, SRK/T)

The LARGER the IOL constant, the MORE IOL power each formula will recommend for the same set of measurements; the SMALLER the IOL constant, the LESS IOL power the same formula will recommend for the same set of measurementsIn reality, two eyes with the exact same axial length and the same keratometry may require completely different IOL powers for emmetropia.

IOL power prediction curve is mostly fixed and is moved up or down depending on the IOL constant

Do not take into account the individual geometry of each IOL model

Assumption that anterior chamber dimensions are related to axial length: The LONGER the axial length, the DEEPER the anterior chamber, and the SHORTER the axial length, the SHALLOWER the anterior chamber

However, 80% of short eyes have large crystalline lenses but a normal anterior chamber anatomy in the pseudophakic state

Assumption that anterior chamber dimensions are related to cornea power: Eyes with STEEP corneas have DEEP anterior chambers and eyes with FLATTER corneas have SHALLOW anterior chambers

Relying on the axial length and the central corneal power to predict the postoperative position of the IOL implant

d = Effective lens position ACD = Measured anterior chamber depth of the eye (corneal vertex to the

anterior lens capsule) AL = axial length of the eye ( the distance from the cornea vertex to the

vitreoretinal interface) a˳ = Moves the power prediction curve up/ down a1 = Measured anterior chamber depth a2 = Measured axial length

d = a˳ + (a1 × ACD) + (a2 × AL)

For the Haigis formula, the a˳ constant moves the power prediction curve up, or down, same way that the A-constant, Surgeon Factor, or ACD does for the SRK/T, Holladay and Hoffer Q

Both the a1 and the a2 constants are used to vary the shape of the power prediction curve, changing the power based on the central corneal power, anterior chamber depth, axial length and individual lens geometry.

Importance of ACD: An error of 1 mm affects the postoperative refraction by approx. 1.0 D in myopic eye, 1.5 D in emmetropic eye and up to 2.5 D in hyperopic eye

The geometry of many IOL models may not be the same for all powers. When this is the case, it would be helpful if a formula was able to take this information account.

With three lens constants, the Haigis formula is able to make adjustments adding or subtracting power when necessary, based on actual observed results for a specific surgeon and the individual geometry of an intraocular lens implant.

INTRAOCULAR LENS POWER CALCULATION USING IOLMASTER AND VARIOUS FORMULAS IN SHORT EYES To evaluate the predictability of intraocular lens (IOL) power calculations using the IOLMaster and four different

IOL power calculation formulas (Haigis, Hoffer Q, SRK II, and SRK/T) for cataract surgery in eyes with a short axial length (AL)

Included 25 eyes with an AL shorter than 22.0 mm that underwent uneventful phacoemulsification with IOL implantation from July 2007 to December 2008 at Seoul National University Boramae Hospital.

Preoperative AL and keratometric power were measured by the IOLMaster. Postoperative refractive errors two months after surgery were measured using automatic refracto-keratometry (Nidek)

and were compared with the predicted postoperative power. The mean absolute error (MAE) was defined as the average of the absolute value of the difference between actual

and predicted spherical equivalences of postoperative refractive error. The differences in the MAE according to the four IOL calculation formulas in the three IOL groups were analyzed

Purpose

Methods

Roh, Y. R., Lee, S. M., Han, Y. K., Kim, M. K., Wee, W. R., & Lee, J. H. (2011). Intraocular lens power calculation using IOLMaster and various formulas in short eyes. Korean Journal of Ophthalmology, 25(3), 151-155.

Results

The constants used in the four formulas of the IOL Master in three intraocular lens (IOL) subtypes

Means and standard deviations of the absolute errors the four intraocular lens calculation formulas•The MAE wassmallest in the Haigis formula (0.37 ± 0.26 D), followed bythose of the SRK/T (0.53 ± 0.25 D), SRK II (0.56 ± 0.20 D),and Hoffer Q (0.62 ± 0.16 D) formulas

Proportion of the absolute errors (AE) less than 1 diopter (D) according to the intraocular lens formulasThe proportion of AE less than 1 D was greatest in the Haigis formula (96%), followed by those in the SRK II (88%), SRK-T (84%), and Hoffer Q (80%) formulas

Means and standard deviations of the mean predicted errors (PE) of the four intraocular lens calculation formulas•PE showed several myopic shifts and was smallest in the Haigis formula (-0.21 ± 0.22 D), followed by those of the SRK II (-0.41 ± 0.28 D), SRK/T (-0.45 ± 0.28 D), and Hoffer Q (-0.59 ± 0.28 D) formulas

MAE and PE results consistently showed that the Haigis formula was the most accurate of the four formulas in eyes with an AL shorter than 22.0 mm

Conclusion

IOL power calculations were first developed over 100 years ago.

First generation: “single variable” formulas Measurement of axial lengthAn assumed anterior chamber depth (ACD) of 4.5 mm

Third generation: 1988-Holladay 1 formula added keratometry to offer the first “two

variable” formula, which helped improve accuracy in short and long eyes.Holladay 1, Hoffer Q, SRK-T :

Assumed anterior segment size was directly related to axial length resulted in “surprise” outcomes esp in small eye

In 1993, Dr Holladay led a worlwide study involved 34 cataract surgeons to determine which of 7 variables were relevant for predictors of effective lens position (ELP).

A large data set of from 34,000 eyes was collected and analyzed to determine relative significance of each variable, as shown in Figure 1.

Findings:1. “We were surprised to learn that

horizontal white-to-white measurements emerged as the next most important variable relate to ELP after axial length and Ks,” remarked Dr. Holladay.

2. “We also proved that there is almost no correlation between axial length and size of the anterior segment in 80-90% of eyes.”

The results from this study :

led to the release of Holladay 2 formula.

Invention of an easy-to-use program that allowed for data entry of the new variables and instant calculation of Effective Lens Position (ELP) and the appropriate IOL power selection (aka HIC.SOAP).

Led to a new paradigm of evaluating eyes by both their axial length (short, normal, long) and their anterior segment size (small, normal, large).

There are now nine eye types – not just three – that could be used to classify a given patient’s eye (Figure 2).

The WTW measurements demonstrated that:

•Short axial length eyes (<21 mm), 80% would be considered normal and 20% would be considered small in terms of anterior segment size.

•Normal axial length eyes (21-26 mm) had an equal distribution of eyes being of either large (2%) or small (2%) anterior segment size.

•Long axial length (>27 mm). 90% would be considered normal and 10% considered as large in terms of anterior segment size.

Holladay 2 formula determines Effective Lens Position (ELP) using 7 parameters :

All 7 parameters can be used to calculate IOL power by input into Holladay IOL Consultant & Surgical Outcomes Assessment Program (HIC.SOAP).

Holladay IOL Consultant & Surgical Outcomes Assessment Program (HIC.SOAP).

Traditionally, 5 variables can be measured with:ACD, LT & AL : Standard ultrasound biometry.K & WTW : Autokeratometer or corneal topography

Holladay 2 formula has been considered as one of the most accurate IOL formula today. (Srivannaboon et al. 2013)

Holladay 2 has emerged as the “state of the art” IOL calculation formula and today is the leading formula used by US surgeons. (Hill, 2005)

Holladay 2: Currently most sophisticated formula Accuracy Predictability

•This formula has been found to be highly accurate for a large variety of patient eyes.

The IOLMaster 500 by Carl Zeiss is the only instrument on the market that has the Holladay 2 formula inside the unit.

IOL Master 500The ZEISS IOLMaster® 500 is the gold standard in optical biometry.It measures:

1. Axial length2. Corneal radii/ power3. White to white4. AC depth

Formula: Holladay 1, Holladay 2, Haigis, SRK 2, SRK-T, Hoffer

IOL measurement instruments need to transfer the data to an external computer as well as purchase of a separate software package for Holladay 2 calculation. (Mahdavi, 2011)

Srivannaboon et al. 2013

Srivannaboon et al. 2013 (cont.)

Developed by Thomas Olsen from University Eye Clinic, Aarhus Hospital, Aarhus, Denmark in the late 1980s at a time when the regression formulas were dominant.

The Olsen formula uses paraxial & exact ray tracing based on physical data to avoid the errors of the ‘thin lens’ formula.

The true net power of the cornea is calculated and it is not necessary to fudge the effective lens plane (ELP)

Use the information of the exact IOL position from C-constant directly in the formula.

SRK/T formula and the Holladay – use corneal height (H), which is calculated from the corneal curvature and diameter.

Olsen – from preop ACD and lens thickness (LT)

The Olsen formula addresses 4 area of concern

ACD

K AL

IOL

I) CALCULATION OF CORNEAL POWER

METHODS CONVENTIONAL KERATOMETRY

GULLSTRAND BINKHORST

Curvature Only measure front curvature

Assume P proportional to A surface (6.8 / 7.7 = 0.833)

Use value of 4/3

Physiological n Use ficititious n 1.376 -

Equivalent n 1.3375 1.3315 1.3333

The difference in calculated power almost 1D – might introduce a prior error of IOL calculation

POWER DETERMINATION OF AN IOL IN SITU 1.3315 1.333Accurate estimation of front lens surface could be obtained with no significant off-set error

Result a significant off-set error

DETERMINATION OF EFFECTIVE CORNEAL POWER

Conventional thick lens formula

Apply a total dioptric power from thick lens formula, it results the refractive index as follow:

Total dioptric power ofthe thick lens

Dioptric power ofthe front surface

Dioptric powerof the back surface

II) MEASUREMENT OF THE AXIAL LENGTH The AL measured by ultrasound ≠ true AL

“retinal” spike originate from VR interface Compression of the cornea (contact technique)

So, the term ‘retinal thickness’ was introduced as a corrective term in order to eliminate error.

Previously, large error raised in extreme short & long eye due to velocity assumption. The avg velocity from cornea to retina is 1550 m/s Avg velocity in extreme myopia (increase) & hyperopia change To correct AL acc to shift of velocity, the AL can be corrected with equation:

RealAx = Ax/MeanVel – Lthick / LensVel) x AqueousVel + LThick

III) THE ACD PREDICTION ACD prediction plays significant role in the IOL power calculation.

Previously, lack of empirical data on postop position of the implant (postop ACD) – tend to result myopic error (overest IOL power) in short eye.

The method to predict the postop ACD in a given eye based on the actual preop measurements of the eye.

Olsen proposed his regression formula for the predicted postop ACD as follows:

This formula apply to phakic eyes. The coefficient will change in pseudophakia and aphakic eyes.

ACDpost = ACDmean + 0.12H + 0.33 ACDpre + 0.3T’ + 0.1L – 5.18

ACDpost = Expected postop ACD of the IOL (in mm)ACDmean = Average postop ACD of the IOL (in mm) H = Height of cornea seg based on keratometry and corneal diameterACDpre = Preop ACD(mm)T’ = Lens thickness (mm)L = Axial length (mm)

IV) THE IOL OPTIC In order to calculate the power according to Gaussian Optics, it is necessary to know the position of the principal plane of the IOL optic.

This position is important in determining the effective power of the lens within the eye.

All the dioptric power of a planoconvex lens is on one surface and thus that surface represents the effective lens plane.

With a biconvex lens, the effective lens plane is ‘inside’ the lens.

Defines the position of the IOL as a fraction of capsular bag size.

Predict the final IOL position from the preoperative ACD and lens thickness.

Produce better results of accurate predictions for both short and long eyes compared to Haigis.

It works in any type of eye including post-LASIK eyes!

C - Constant

- Uses ray tracing to get the preop lens thickness and ACD to derive C, which can be thought of as a fraction of the preoperative lens thickness.

- This C constant is then used to determine where the IOL will come to rest in the eye

IOLc = ACDpre + C x LTpreIOLc = Center of the IOL

ACDpre = preop ACD (including corneal thickness)

LTpre = preop thickness of the crystalline lens

C = A constant related to the IOL type determined as the mean value in a representativesample.

Based on the observation after standardized lens surgery and in-the-bag implantation, the IOL tends to locate itself in a defined manner that is predictable according to the formula:

Determine the phakic axial length with no axial length corrections.

The greatest benefits of the Olsen formula for improving power prediction accuracy compared with the other formula were noted especially in the extreme short & long eyes.

Perform consistently well in short, normal, and long eyes, having a lower bias with axial length compared with the conventional formula.

Featured with the Olsen IOL calculation formula for optimum prediction accuracy.

Pair with the innovative concept of the C-constant, so the surgeon gets a sophisticated tool for accurate IOL prediction in all kind of human eyes.

Measured all intraocular distances, including CCT, ACD, lens thickness in one shot laser.

Hill, W. E., & Mesa, A. (2002). The Haigis formula for IOL power calculation.Geriatric Ophthalmology, 1(1), 8. Charalampidou, S., Cassidy, L., Ng, E., Loughman, J., Nolan, J., Stack, J., & Beatty, S. (2010). Effect on refractive

outcomes after cataract surgery of intraocular lens constant personalization using the Haigis formula. Journal of Cataract & Refractive Surgery, 36(7), 1081-1089.

Mahdavi, S. 2011.IOLMaster 500 and Integration of the Holladay 2 Formula for IOL Calculations. Available at www.sm2strategic.com.

Mahdavi, S. The IOLMaster and its Role in Modern Cataract Surgery, November 2011, available at www.sm2strategic.com.

Srivannaboon, S. Chirapapaisan, C. et al. Accuracy of Holladay 2 Formula Using IOLMaster Parameters in the Absence of Lens Thickness Value. Graefe's Archive for Clinical and Experimental Ophthalmology. November 2013, Volume 251, Issue 11, pp 2563-2567.

http://www.haag-streit.com/de/product/biometry/olsen-formula-and-lens-thickness.html http://ophthalmologytimes.modernmedicine.com/ophthalmologytimes/news/modernmedicine/modern-medicine-

news/biometry-iol-power-formulae-improve-outc http://www.medscape.com/viewarticle/820900_4 http://haag-streit-usa.com/customer-support/olsen-formula-download.aspx http://www.reviewofophthalmology.com/content/i/3592/c/59832/#sthash.E6LV4naF.dpuf

Using the correct IOL calculation formula is important for good surgical outcomes. SRK I and II regression formulae are now regarded as obsolete. The Hoffer Q, Holladay I, and SRK/T formulae are all commonly used. More recent formulae: the Holladay II, Haigis or Olsen ,are not currently built into most biometry software, but available in certain equipment like IOL Master 500. 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 (Tyson, 2006)

Axial length (mm) Formula

< 20 mm Holladay II

20-22 mm Hoffer Q

22-24.5 mm SRK/T / Hoffer Q/Holladay (average)

> 24.5-26 mm Holladay I

> 26 mm SRK/T

Astbury & Ramamurthy, 2006