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By:
Master in Clinical Optometry, UKM
2015/2016
Introduction
Generation
Formula
 1st
and 2nd
generation formula
 3rd
generation formula
 4th
generation formula
 Hi...
 It is often said that cataract surgery is a refractive surgery.
 In old days the cataract was removed first and the spe...
 Since 1975, IOL power has been calculated using accurate
measurement of an eye’s corneal power and axial length (AL).
 ...
1ST
AND 2ND
GENERATION
FORMULA:
SRK, SRK II, HOFFER
By Leong ShinYi, 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 unde...
 Regression formula
 Empiric formulas generated by averaging large numbers of post-operative clinical results
(retrospec...
P= IOL power to be used (D)
A = IOL specific A constant
K = Average corneal refractive power (D)
L = Axial length of the e...
A constant
 Relates the P to K and L
 Depends on multiple variables
 IOL manufacturer
 Style
 Placement
 Used to cha...
16
Corneal refractive power
 Assumption;
 Thin spherical lens
 Fixed anterior and posterior corneal curvature ratio
 I...
 Corneal radius of curvature relates to corneal power with the equation
K = n – 1
r
r = 337.5
K
 Axial length of the eye
Distance between the anterior surface of the cornea and the fovea
 Most important factor in IOL...
 Based on regression analysis
 2nd
generation of SRK formula
 Optimized A constant based on axial length of the eye
 I...
P= IOL power to be used (D)
A = IOL specific A constant
K = Average corneal refractive power (D)
L = Axial length of the e...
A constant
Optimized based on axial length
A1 = (A – 0.5) for axial length greater than 24.5mm
A1 = (A) for axial length b...
Hoffer formula use the post-operative AC depth
A change in the true post-operative AC depth will affect the
refractive s...
The main feature of the 1st
generation theoretical formulae
was
 that position of IOL in the eye is fixed for each lens ...
2nd
generation theoretical IOL power formulae differ
from the 1st
generation because:
 Position of the IOL in the pseudo...
 Although the 1st
and 2nd
generation formulae are not used in present time,
they are all basis formulae developed or modi...
 Professor Dr. Jean. B., A Comparative Analysis of Methods for Calculation IOL Power: Combination of Three Corneal
Power ...
By Ling SookYee, LowYu 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 of
Axial length
K-reading
 Optimization of formula
to pre...
= distance from cornea to lens
 Explains position of the IOL postoperatively
 ELP is difficult to predict because:
a) IO...
 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...
P = f (A, K, Rx, pACD)
Axial length
Average corneal
refractive power
Previous refraction
Personalized ACD,
manufacturer’s ...
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 ...
Hyperopes (AL < 22 mm) (Kenneth Hoffer)
 Most accurate in short eyes < 22.0mm, confirmed in large study of 830 short eyes...
 Contribution of IOL power errors:
i. Inaccurate measurement/calculation of anterior corneal power (especially in those
r...
P= PTARG - 0.326 × RCC - 0.101
IOL power calculated by standard
IOL formulas
surgically induced
refractive change
This met...
 Double K formula
 K-reading before refractive surgery is used to estimate the ELP
 K-reading after refractive surgery ...
Myopic Correction
 Numbers in each row represent the
amount (D) that must be added to the
calculated IOL power
Hyperopic ...
 Produced by Jack Holladay in 1988
 Used axial length and keratometry to determine ELP
 Work best for eyes between 24.5...
 Using K, AL to predict IOL power
 No ACD input indicated
 Calculates predicted distance from cornea to iris plane + di...
 Distance between iris plane & IOL optic plane
 SF should be personalized
 A change in the true post-operative AC depth...
 For eyes with previous refractive surgery
 Use K value prior to surgery and change in manifest refraction resulting fro...
1. Regression formulas topped surgeon’s preferences, and one of the most successful was the SRK
formula. (Sanders D et al,...
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 <20
A+2 20-21
A+1 21-22
A 22...
 It can be calculated using the same A constants used with the original SRK
formula or with ACD estimates.
 SRK/T formul...
What is the effect of A-constant on IOL power?
 The term “A-constant” seems misleading because, it varies among IOL
model...
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 ...
Hoffer Q < 22mm
Holladay 1 24-26mm
SRK/T >26mm
•Holladay 1 formula - Uses “surgeon factor”
•Hoffer Q formula – uses “ Pseu...
 Wang, L., M.A. Booth, and D.D. Koch, Comparison of intraocular lens power calculation methods in eyes that have undergon...
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
...
PROBLEMS WITH 3RD GENERATION 2 VARIABLE
FORMULA (HOFFER Q, HOLLADAY 1, SRK/T)
The LARGER the IOL constant, the MORE IOL p...
 Assumption that anterior chamber dimensions are related to axial length: The
LONGER the axial length, the DEEPER the ant...
 d = Effective lens position
 ACD = Measured anterior chamber depth of the eye (corneal vertex to the anterior lens
caps...
 For the Haigis formula, the a constant moves the power prediction curve up, or down,˳
same way that the A-constant, Surg...
INTRAOCULAR LENS POWER CALCULATION USING
IOLMASTER AND VARIOUS FORMULAS IN SHORT EYES
 To evaluate the predictability of ...
Results
The constants used in the four formulas of the IOL Master in three intraocular lens (IOL) subtypes
Means and stand...
Proportion of the absolute errors (AE)
less than 1 diopter (D) according to
the intraocular lens formulas
The proportion ...
 MAE and PE results consistently showed that the Haigis formula was the most
accurate of the four formulas in eyes with a...
IOL power calculations were first developed over 100 years ago.
First generation: “single variable” formulas
Measuremen...
 In 1993, Dr Holladay led a worlwide study involved 34 cataract surgeons to
determine which of 7 variables were relevant ...
 The results from this study :
 led to the release of Holladay 2 formula.
 Invention of an easy-to-use program that all...
 There are now nine eye types – not just three – that could be used to classify a
given patient’s eye (Figure 2).
The WTW...
Holladay 2 formula determines Effective Lens Position (ELP) using 7
parameters :
All 7 parameters can be used to calcula...
Holladay IOL Consultant & Surgical Outcomes Assessment Program
(HIC.SOAP).
Traditionally, 5 variables can be measured wi...
 Holladay 2 formula has been considered as one of the most accurate
IOL formula today. (Srivannaboon et al. 2013)
 Holla...
•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.
 ...
 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 ...
SRK/T formula and the Holladay –
use corneal height (H), which is
calculated from the corneal
curvature and diameter.
Olse...
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 curvatu...
Conventional thick lens formula
Apply a total dioptric power from thick lens
formula, it results the refractive index as
f...
II) MEASUREMENT OF THE AXIAL LENGTH
 The AL measured by ultrasound ≠ true AL
 “retinal” spike originate from VR interfac...
III) THE ACD PREDICTION
 ACD prediction plays significant role in the IOL power calculation.
 Previously, lack of empiri...
 Olsen proposed his regression formula for the predicted postop ACD as follows:
 This formula apply to phakic eyes. The ...
IV) THE IOL OPTIC
 In order to calculate the power according to Gaussian Optics, it is
necessary to know the position of ...
 Defines the position of the IOL as a
fraction of capsular bag size.
 Predict the final IOL position from the
preoperati...
- Uses ray tracing to get the
preop lens thickness and ACD
to derive C, which can be
thought of as a fraction of the
preop...
IOLc = ACDpre + C x LTpre
IOLc = Center of the IOL
ACDpre = preop ACD (including corneal thickness)
LTpre = preop thicknes...
 Determine the phakic axial length with no axial length corrections.
 The greatest benefits of the Olsen formula for imp...
 Featured with the Olsen IOL calculation
formula for optimum prediction
accuracy.
 Pair with the innovative concept of t...
 Hill, W. E., & Mesa, A. (2002). The Haigis formula for IOL power calculation.Geriatric Ophthalmology, 1(1), 8.
 Charala...
 Using the correct IOL calculation formula is important for good
surgical outcomes.
 SRK I and II regression formulae ar...
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 m...
Biometry: Iol calculation
Biometry: Iol calculation
Biometry: Iol calculation
Biometry: Iol calculation
Biometry: Iol calculation
Biometry: Iol calculation
Biometry: Iol calculation
Biometry: Iol calculation
Biometry: Iol calculation
Biometry: Iol calculation
Biometry: Iol calculation
Biometry: Iol calculation
Biometry: Iol calculation
Biometry: Iol calculation
Biometry: Iol calculation
Biometry: Iol calculation
Biometry: Iol calculation
Biometry: Iol calculation
Biometry: Iol calculation
Biometry: Iol calculation
Biometry: Iol calculation
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-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.

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Biometry: Iol calculation

  1. 1. By: Master in Clinical Optometry, UKM 2015/2016
  2. 2. Introduction Generation Formula  1st and 2nd generation formula  3rd generation formula  4th generation formula  History  IOL calculation formula  Recommended formula usage
  3. 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. 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.
  5. 5. 1ST AND 2ND GENERATION FORMULA: SRK, SRK II, HOFFER By Leong ShinYi, Foo Hou Ling,Mohd Zharif
  6. 6. •Provided evidence for tolerance of a foreign body in the eye •Prospect of restoring functional vision.
  7. 7. •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
  8. 8.  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
  9. 9. 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
  10. 10. 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
  11. 11. 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
  12. 12.  Corneal radius of curvature relates to corneal power with the equation K = n – 1 r r = 337.5 K
  13. 13.  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
  14. 14.  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)
  15. 15. 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
  16. 16. 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
  17. 17. 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.
  18. 18. 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.
  19. 19. 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.
  20. 20.  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)
  21. 21.  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
  22. 22. By Ling SookYee, LowYu Chen, Nurul Akimi Abdullah 3RD GENERATION FORMULA : HOFFER Q, HOLLADAY 1, SRK/T
  23. 23. Merger of the linear regression methods with theoretical eye models Pseudophakic ACD Surgeon Factor A-constant
  24. 24.  Improved accuracy Better result & simple  Take into account of Axial length K-reading  Optimization of formula to predict the effective lens position ELP
  25. 25. = 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
  26. 26.  Errors in predicting the ELP caused: refractive surprise  Shallow AC -> sitting more anterior -> lower IOL power
  27. 27.  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.
  28. 28. 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
  29. 29. 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)
  30. 30. 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
  31. 31.  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
  32. 32. 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) Example IOL calculated 22.0D Change in Rx = +3.0D P = 22.0 – (0.326 x +3.0) – 0.101 = 21.0 D
  33. 33.  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
  34. 34. 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
  35. 35.  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)
  36. 36.  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)
  37. 37.  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.
  38. 38.  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.
  39. 39. 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)
  40. 40. 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 <20 A+2 20-21 A+1 21-22 A 22-24.50 A-0.5 >24.5
  41. 41.  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.
  42. 42. 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. 
  43. 43. 1:1 relationship with the A- constants: if A decreases by 1 diopter,  IOL power decreases by 1 diopter.
  44. 44. 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)
  45. 45. 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”
  46. 46.  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
  47. 47. By Ang Kai Li, Noor Munirah binti Awang Abu Bakar, Nurulhidayah Nordin
  48. 48.  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.
  49. 49. 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
  50. 50.  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
  51. 51.  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)˳
  52. 52.  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.
  53. 53. 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.
  54. 54. 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
  55. 55. 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
  56. 56.  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
  57. 57. 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
  58. 58.  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.”
  59. 59.  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).
  60. 60.  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.
  61. 61. 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).
  62. 62. 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
  63. 63.  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
  64. 64. •This formula has been found to be highly accurate for a large variety of patient eyes.
  65. 65.  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)
  66. 66.  Srivannaboon et al. 2013
  67. 67.  Srivannaboon et al. 2013 (cont.)
  68. 68.  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.
  69. 69. 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)
  70. 70. The Olsen formula addresses 4 area of concern ACD K AL IOL
  71. 71. 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
  72. 72. 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
  73. 73. 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
  74. 74. 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.
  75. 75.  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)
  76. 76. 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.
  77. 77.  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
  78. 78. - 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
  79. 79. 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:
  80. 80.  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.
  81. 81.  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.
  82. 82.  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
  83. 83.  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)
  84. 84. 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
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-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.

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