Biometry: Iol calculation

-IOL formula
1st generation formula : SRK, Binkhost
2nd generation formula : SRK II
3rd generation formula: Hoffer Q, Holladay 1, SRK/T
4th generation formula: Haigis, Holladay 2, Olsen

-The Hoffer Q, Holladay I, and SRK/T formula are all commonly used.

1. By:
Master in Clinical Optometry, UKM
2015/2016

2. Introduction
Generation
Formula
1
st
and 2
nd
generation formula
3
rd
generation formula
4
th
generation formula
History
IOL calculation formula
Recommended formula usage

3. 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.

4. 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.

7. 1
ST
AND 2
ND
GENERATION
FORMULA:
SRK, SRK II, HOFFER
By Leong Shin Yi, Foo Hou Ling,Mohd Zharif

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

9. •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

13. 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

14. P= IOL power to be used (D)
A = IOL specific A constant
K = Average corneal refractive power (D)
L = Axial length of the eye (mm)
P = A – 0.9K – 2.5L

15. 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. 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

17. Corneal radius of curvature relates to corneal power with the equation
K = n – 1
r
r = 337.5
K

18. Axial length of the eye
Distance between the anterior surface of the cornea and the fovea
Most important factor in IOL calculation
1.0mm error  2.50D – 3.50D error
Measure by A-scan ultrasonography / optical coherence biometry
Suitable to use on axial length range : 22mm- 24.5mm

20. Based on regression analysis
2
nd
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)

21. P= IOL power to be used (D)
A = IOL specific A constant
K = Average corneal refractive power (D)
L = Axial length of the eye (mm)
P = A1 – 0.9K – 2.5L

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

24. 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.

25. The main feature of the 1
st
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.

26. 2
nd
generation theoretical IOL power formulae differ
from the 1
st
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 2
nd
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.

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

29. 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: 430-434
http://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

30. By Ling Sook Yee, Low Yu Chen, Nurul Akimi Abdullah
3
RD
GENERATION FORMULA :
HOFFER Q, HOLLADAY 1, SRK/T

32. Merger of the linear regression methods with
theoretical eye models
Pseudophakic ACD Surgeon Factor A-constant

33.  Improved accuracy
Better result & simple
 Take into account of
Axial length
K-reading
 Optimization of formula
to predict the effective lens position ELP

34. = distance from cornea to lens
Explains position of the IOL postoperatively
ELP is difficult to predict because:
a)IOL is thinner than cataract
b)ACD tends to increase with pseudophakia
c)Variable lens geometry across power range

35.  Errors in predicting the ELP caused: refractive surprise
 Shallow AC -> sitting more anterior -> lower IOL power

37. 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.

38. 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

39. P = 1336 -1.336
A – C – 0.05 [(1.336) – (C +0.05)]
K + R 1000
P = IOL power
A = Axial length
C = estimated post-op ACD
K = corneal power (in Diopters)
R = corneal radius (in mm)

40. 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

41. 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 power
Hyperopic-LASIK: overestimation of the IOL power

42. P= PTARG - 0.326 × RCC - 0.101
IOL 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)
ExampleIOL calculated 22.0D
Change in Rx = +3.0D
P = 22.0 – (0.326 x +3.0) – 0.101
= 21.0 D

43. 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

44. 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

46. 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)

47. 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)

49. 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.

50. 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.

52. 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)

53. SRK I – 1
st
gen
P = A – 0.9K – 2.5L
SRK II – 2
nd
gen
P = A1 – 0.9K – 2.5L
AI Axial
Length
A+3 <20
A+2 20-21
A+1 21-22
A 22-24.50
A-0.5 >24.5

54. 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.

55. 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 rule
IOL brand No. 1 : A-constant of 118.4 = +21.0 D
IOL brand No. 2: A-constant of 118.9 = +21.5 D
 to get the same plano postop refraction. 

56. 1:1 relationship with the A-
constants:
if A decreases by 1
diopter,
 IOL power decreases by
1 diopter.

57. 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)

58. Hoffer Q < 22mm
Holladay 1 24-26mm
SRK/T >26mm
•Holladay 1 formula - Uses “surgeon factor”
•Hoffer Q formula – uses “ Pseudophakic ACD)
•SRK/T formula – uses “ A- contstant”

59. 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

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

63. 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.

65. 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 measurements
In 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

66. 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

67. 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)
˳

68. 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.

70. 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.

71. 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 was
smallest in the Haigis formula
(0.37 ± 0.26 D), followed by
those 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

72. Proportion of the absolute errors (AE)
less than 1 diopter (D) according to
the intraocular lens formulas
The 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

73.  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

75. IOL power calculations were first developed over 100 years ago.
First generation: “single variable” formulas
Measurement of axial length
An 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

76. 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.”

77.  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).

78. 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.

79. 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).

80. 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

82. 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

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

84. The IOLMaster 500 by Carl Zeiss is the only instrument on the market that has the
Holladay 2 formula inside the unit.
IOL Master 500
The ZEISS IOLMaster
®
 500 is the gold standard in optical biometry.
It measures:
1.Axial length
2.Corneal radii/ power
3.White to white
4.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)

85. Srivannaboon et al. 2013

86. Srivannaboon et al. 2013 (cont.)

88. 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.

89. 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)

90. The Olsen formula addresses 4 area of concern
ACD
K AL
IOL

91. 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 1.333
Accurate 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

92. Conventional thick lens formula
Apply a total dioptric power from thick lens
formula, it results the refractive index as
follow:
Total dioptric power of
the thick lens
Dioptric power of
the front surface
Dioptric power
of the back surface

93. 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

94. 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.

95. 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 diameter
ACDpre = Preop ACD(mm)
T’ = Lens thickness (mm)
L = Axial length (mm)

96. 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.

97. 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

98. -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

99. IOLc = ACDpre + C x LTpre
IOLc = 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 representative
sample.
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:

100. 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.

101. 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.

102. 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

104. 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)

105. 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