Biometry- Iol power and calculation final ppt.pptx

Biometry- IOL power formulae and calculations
This presentation describes about different generations of IOL formulae and newer formulae. It also gives information how to calculate IOL power in special situations

1. Biometry –IOL formulas and Calculation Presenter : Dr. Kervi

2. HISTORY -

3. IDEM LENSES In the 1980s IDEM lenses(ideal emmetropia lenses) with them pre and post refraction was similar. Done in patients who were emmetropic before cataract development. The power of this lens was mathematically deduced to be +17.0 D for an AC lens +19.0D for an iris fixated lens +21.0D for a posterior chamber lens.

4. STANDARD LENSES The standard lens is approx. 2D stronger than the IDEM lens, thereby rendering the Pseudophakic eye myopic as compared to preoperative emmetropic refraction. Lens power calculated by adding +1.25D to the calculated power of an IDEM lens to restore emmetropia.

5. EMMETROPIA LENS Done in previous ammetropia patients To restore the patient to an emmetropic status after cataract surgery Take care of pre-existing refractive error Power calculated by multiplying the pre existing refractive error with a conversion factor of 1.25 and algebraically adding it on to the IDEM lens power (hypermetropia patients) Subtracting in myopic patients

6. For example: if a patient had an axial myopia of 3.0D before the onset of cataract The power of the emmetropia lens may be calculated thus: 3.0 x 1.25 = 3.75 Idem lens power for a Posterior chamber IOL = 21.0D Emmetropia lens power = 21.0 + (-3.75) = +17.25D

7. DUAL LENS SYSTEM The human eye forms a homo centric complex lens system.   The primary or the objective lens as well as the distance of the focusing screen are fixed – then the effective power of this system of lenses will depend on the power and the position of the second lens.

8. SCHEMATIC EYE According to GAUSS concept eye can be resolved into 6 cardinal points

9. LISTINGS REDUCED EYE DONDERS REDUCED EYE

10. GAUSS THEOREM For a system of two or more lenses in succession : the image produced by the first lens acts as an object to the second lens The image produced by the second Lens acts as an object for the third lens and so on.

11. According to the GAUSS theorem for a system of homo centric Lens, there exist three pairs of cardinal points 2 focal points 2 Principal points and 2 nodal points which are all situated on the principal axis of the system

12. Having known all the three pairs of cardinal points of a thick lens, it is easier to calculate the power of the lens

13. Using the corneal refractive indices and radius of curvature as given by Gullstrand, the net corneal power can be derived by applying Gaussian optics Da is the anterior corneal power; Dp is the posterior corneal power; ra is anterior radius of curvature of cornea; rp is posterior radius of curvature of cornea; Dnet is the total/net power of cornea

14. PRE-REQUISITES FOR IOL POWER CALCULATION FORMULA 3 Essential parameters needed for accurate IOL power calculation 2 Measured parameters- Keratometry measurement of corneal curvature and Axial Length 1 predicted parameter- Post op anterior chamber depth

15. TYPES OF IOL POWER FORMULAE Theoretical formulae Regression formulae

16. THEORETICAL FORMULAE Based on an optical model of the eye An optics equation is used to determine the IOL power Complex calculations Assumptions are made Examples of Theoretical formulae- Binkhorst Formula, colenbrander , Gill formula

17. Different theoretical formulae, make different assumptions about,  Refractive index of the cornea Distance of the Cornea to the IOL Distance of the IOL to the retina All these theoretical equations make simplifying assumptions about the optics of the eye. Therefore they do not provide a perfect prediction of the IOL power. Obsolete

18. REGRESSION/EMPIRICAL FORMULAE These are based on retrospective analysis of actual post operative refractive data. Large number of IOL implantation plotted with respect to Corneal power Axial length of the eye The best fit equation is then determined by statistical regression analysis of data

19. No assumptions are made about the optics of the eye The regression equations are only as good as the accuracy of the data used to derive them Example is SRK formula, SRK 2 , Hoffer formula

20. VERGENCE FORMULAE Are 3rd and 4 th generation formulae which incorporate theoretical and regression analysis Accurate calculation of Effective lens position (ELP) is possible when both theoretical and regression analysis are combined Sub classified based on number of biometry variables Holladay 1 , hoffer Q , SRK T , T 2 : 2 variables Haigis formula : 3 variables Barrett universal 2 : 5variables Holladay 2 : 7 variables

21. RAY TRACING FORMULAE Individual rays path through different refracting surfaces is traced It calculates the postoperative lens position as a fraction of the crystalline lens thickness and the ACD. This allows accurate calculation of lens position Prediction of IOL position with C constant Example- Olsen formula

22. Ray tracing formula that takes in account both paraxial as well as marginal rays of light entering the eye.

23. ARTICIFICAL INTELLIGENCE FORMULA Researchers in Boston and Los Angeles have developed an artificial intelligence (AI) neural network to cal­culate IOL power. The Hill-radial basis function (RBF) uses artificial intelligence and regression analysis of a very large database of actual postsurgical refractive outcomes to predict the IOL power.

24.  Method of pattern recognition If the anatomic characteristics of a particular eye do not match with many of the eyes in the Hill-RBF database, then the IOL prediction will be less accurate calculator will acknowledge this limitation by showing an out-of-bounds notification. Kane formula incorporates artificial intelligence with theoretical optics for IOL power prediction

25. A Constant A-constant is actually highly variable depending upon multiple factors IOL dependent: type, material, position surgeon dependent: technique of incision, placement of incision K (keratometry) and AL (axial length) measurement adjustments It approximately varies with a ratio of 1:1 with the IOL power.

26. Changing from one IOL design to Another The difference in A-constant between the two IOL types is the same as the adjustment needed to the IOL power.  For example, IOL power +20 D and A-constant of 119.2 in the capsular bag, but switch with A-constant 118.7 in the capsular bag, we need to drop the IOL power by 0.5 D to +19.5 D The difference of A-constants (119.2 – 118.7 = 0.5) is the same as the difference in the IOL powers (20.0 – 19.5 = 0.5).

27. Choosing the AC IOL power The original IOL for in-the-bag placement of power of +20 D with an A-constant of 119.2 but we have to implant an AC IOL with an A-constant of 115.7 we need to drop the anterior chamber IOL power to +16.5 to have the same refractive result. This drop from +20 to +16.5 is 3.5 D, which is the same as the difference in A-constants (119.2 -115.7).

28. Sulcus IOL placement- The sulcus IOL will need to have a power lower than the same IOL placed in the capsular bag. This “Rule of 9s” says that IOL powers can be grouped into groups, split at IOL powers 9, 18 and 27. The IOL power is reduced by 0.5 D, 1 D, and 1.5 D.

29. EFFECTIVE LENS POSITION Coined by Dr. Jack Holladay The position of the lens in the eye- specifically the distance that the principal plane of the IOL will sit behind the cornea. The strength and predictability of the various IOL formulae depends upon their ability to accurately predict the ELP “One of the most important and challenging tasks in IOL power calculation is to predict the ELP for a given eye,” - Dr. Haigis

30. FACTORS INFLUCENING ELP Anatomical factors- Axial length steepness of the cornea (average K) limbal white to white measurements preoperative anterior chamber depth lens thickness 1st and 2nd generation IOL formulae tried to predict the ELP based on anatomical factors alone. IOL and surgery related factors- The shape, the length, the flexibility, the anterior angulation if any and the material of the haptic of the IOL will affect the ELP. Individual Surgeon’s Technique Bag to sulcus shift- Requires reduction of 0.50-0.75 D from base power of IOL.

31. The difference is the manner in which Estimated lens position(ELP) / Estimated post operative anterior chamber depth(ACD) is calculated- Original formulas – ELP is a constant value Modified formulas – varies with axial length ( decreases in shorter eye and increases in longer eyes) Modern formulas- ELP varies with axial length as well as corneal curvature

32. The ELP appeared with different names in different formulae SRKT- A Constant Holladay 2- S factor Haigis – a0, a1, a2 Hoffer Q- pACD

33. Various generations are grouped as- First generation formulae - SRK 1 and BINKHORST formula Second generation formulae- SRK 2, Hoffer Third generation formulae- SRK T, Holladay 1 , hoffer Q Fourth generation formulae- H olladay 2, Haigis Fifth generation formulae – Hoffer H 5

34. A. First generation formulae- These formulae based on three variables- The AL of eyeball K-reading and, The estimated postoperative ACD BINKHORST FORMULA ( Theoretical formulae) - P= 1336 (4r-a) (a-d)(4r-d) Where, P is the IOL power in diopters R is corneal radius in millimetres , a is AL in millimetres , d is postoperative ACD plus corneal thickness.

35. FIRST GENERATION FORMULA SRK 1

36. It was introduced by Sanders, Retzlaff and kraff (SRK) in 1980 The postoperative ACD is not included but was replaced with A constant which is unique to each different type of IOL. Suitable to use on axial length range- 22mm-24.5mm Erratic outside this range

37. RECOMMENDED FORMULA USAGE The main feature of the 1 ST generation theoretical formula was – Position of IOL in the eye is fixed for each lens type This assumption was true that time when cataract surgery was represented by ICCE and ACIOL implantation.

38. DRAWBACKS OF FIRST GENERATION- They tend to predict too large an emmetropic value in short eyes less than 22mm and too small values in long eyes more than 24.5mm. They are too cumbersome to apply without the use of calculator. Require a guess about the ACD, ultimate results depends on the accuracy of the guess.

39. SECOND GENERATION FORMULA SRK 2

40. SRK 2 FORMULA - The basic equation of the formula is same i.e , P= A 1 -2.5 L-0.9K , But the A constant is modified on the basis of AL as follows- IF L is <20 mm : A+ 3.0 IF L is 20.00-20.99: A+2.0 IF L is 21.00-21.99: A+1.0 IF L is 22.0-24.5: A IF L is >24.5 A-0.5

41. MODIFIED SRK- 𝛱 FORMULA – In this formula, based on the AL , A constant is modified as given: IF L is <20 mm : A+ 1.5 IF L is 20-21mm: A+1.0 IF L is 21-22mm: A+0.5 IF L is 22.0-24.5mm: A IF L is >24.5-26.0mm: A-1.0 IF L is >26mm: A-1.5

42. RECOMMEMDED FORMULA USAGE 2 nd generation theoretical IOL formula differ from 1 st generation because- Position of IOL in pseudophakic eye is not fixed but changes based on 2 variables : Axial length and corneal curvature or corneal power of eye

43. 3 rd GENERATION FORMULAE HOFFER Q, SRK/T, HOLLADAY 1

44. SRK/T FORMULA

45. SRK/T – T for theoretical Third generation formula, described in 1990 by John Retzlaff and Donald Sanders. Benefits of both the theoretical and regression formulae Theoretical element- predicted post op ACD, Retinal thickness adjusted axial length, refractive indices of cornea Regression element- optimise A Constant Useful in Normal length and moderately long and very long eyes(> 26mm)

46. The SRK T formula has made the SRK 2 formula obsolete since it combines all the advantages of the SRK 2 formula and also enables you to optimize the A-Constant What is optimization - optimization is achieved by analyzing post operative outcomes with respect to the targeted refraction for a given surgical technique and a specified model or design of IOL as well as for a given range of axial lengths This optimization is then added on to the ‘A constant’ to make the formula more predictable.

47. HOFFER Q

48. Third generation formula Was described by Dr. Kenneth Hoffer in 1993 P = f (A, K,Rx , pACD ) A: axial length K: average corneal refractive power (K-reading) Rx: Previous refraction pACD personalized ACD (ACD-constant) Hoffer –Q formula was such that it was extremely reliable in short eye balls with an axial length of less than 22.0mms

49. Uses personalised post operative ACD as A constant Used for AXL < 22.00 mm . Accurate prediction with Hoffer Q when compared with SRK T and Holladay 1 Hoffer KJ. The Hoffer Q formula: a comparison of theoretic and regression formulas. J Cataract Refract Surg. 1993;19:700–712.

50. FORMULA REQUIREMENTS

51. In medium AL range, 3 rd generation formulas are equally accurate In an analysis of >13,000 surgeries , all formulas, including the third generations, had prediction errors within 0.1D of the predicted refraction when used for medium length eyes (AL 23 – 25 mm). * Moving outside of this range, the prediction errors increase widely among the formulas. Few other studies demonstrate similar overall mean absolute error for SRK/T, Holladay 1, and Hoffer Q, with a slightly lower absolute error for Holladay 1. ** *Melles RB, Holladay JT, Chang WJ. Accuracy of intraocular lens calculation formulas. Ophthalmology. 2018;125:169–178. **Darcy K, Gunn D, Tavassoli S, Sparrow J, Kane JX. Assessment of the accuracy of new and updated intraocular lens power calculation formulas in 10 930 eyes from the UK National Health Service. J Cataract Refract Surg. 2020;46:2–7.

52. WHEN TO USE HOFFER Q? Hyperopes (AL < 22 mm) Most accurate in short eyes < 20.0mm, 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

53. HOLLADAY 1

54. 3rd generation formula Produced by Jack Holladay in 1988 Require only 2 variables : AXL and K for IOL power calculation but also requires optimisation of equation for more accurate prediction of ELP Work best for eyes between 24.5-26mm(medium long eyes)

55. ACD = CT + (CORNEAL ENDO + iris plane) + ( iris + IOL position) Iris plane + IOL position = surgeon factor (this is known post operative only) hence , varies with lens type and requires optimisation SURGEON FACTOR- Distance between iris plane and IOL optic plane . A change of 1mm of post operative AC depth causes 1.5 D change in final refraction.

56. SURGEON FACTOR (SF) 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. SF constants must be personalized to accommodate any consistent shift that might affect IOL power calculation

57. FOURTH GENERATION FORMULAE HAIGIS,OLSEN , HOLLADAY 2

58. HAIGIS

59. 4th generation formula A/k/s GOW70 The versatility of the formula lies in the three individualized A constants namely a0, a1 and a2 The a0 is linked to the manufacturers lens constant The a1 is linked to the pre operative ultrasonically measured anterior chamber depth (this has a default value of 0.4) a2 which is linked to the axial length measurements and which has a default value of 0.1

60. 3 a constants : can customise each component of IOL Work well across entire range of axial length

61. The Melles et al’s analysis - Haigis formula demonstrated low variability in prediction error across the range of AL (21–28 mm) and ACD (2.25–4.25 mm) analyzed , suggesting that the Haigis formula may be good for a wide range of eyes* * Melles RB, Holladay JT, Chang WJ. Accuracy of intraocular lens calculation formulas. Ophthalmology. 2018;125:169–178.

62. The three ‘A constants’ enable to customize each component of the IOL formula. When fully optimized this formula will work across the entire range of axial length values and you may not need to use different formulae for different axial lengths.

63. HOLLADAY 2

64. 4th Generation Formula Dr Jack T Holladay has attempted to increase its accuracy and predictability by incorporating seven different parameters into the formula which contribute accurate estimation of the ELP. Axial length Central corneal power (K) Anterior chamber depth Lens thickness measurement Limbal white to white measurement Age of the patient Previous refraction of the patient

65. The ‘nine types’ of eyes model by Dr. Holladay The assumption that there was a constant relationship between the central corneal power (K), the pre operative anterior chamber depth and the axial length measurement.

66. No direct correlation between the axial length measurements and the anterior chamber size. This model overcame the discrepancies in all the other IOL formulae

67. It is available as part of a package called the ‘Holladay IOL consultant’ which also provides the necessary information to optimize every component of the surgeon factor(SF).

68. T2 formula In SRK T formula a corrected Axial length is used in calculations This corrected axial length used to calculate imaginary ACD – source of error In T2 formula, corrected AXL is not used. T2 Formula is better than SRK /T for AXL 24.5 to 26 mm Although T2 Improved on SRK /T , it still has limitations of being based on 2 variables Kane JX, Van Heerden A, Atik A, Petsoglou C. Intraocular lens power formula accuracy: comparison of 7 formulas. J Cataract Refract Surg. 2016;42:1490 – 1500.

69. OLSEN

70. OLSEN FORMULA Olsen formula uses both marginal and paraxial ray tracings of optical light through the refractive media in the eye, including the specific optics of a particular IOL, to derive the postoperative position of that lens.

71. More precise and specific than theoretical formulas Fewer number of surgical cases are needed to validate or optimize the C constant Study comparing the Olsen ray tracing formula with Haigis , Hoffer Q formulas showed no significant improvement with the Olsen* * Jin H, Rabsilber T, Ehmer A, et al. Comparison of ray-tracing method and thin-lens formula in intraocular lens power calculations. J Cataract Refract Surg. 2009;35:650–662.

72. BARRET UNIVERSAL II

73. Universal – highly accurate over wide range of axial lengths and different type of IOL materials Vergence formula (Theoretical and regression model) Theoretical model of the eye in which the anterior chamber depth (ACD) is related to the AL and corneal curvature (K) Regression model of the eye predicts the distance from the iris root to the second principal plane of the lens denoted by an individualized lens constant known as the lens factor.

74. The Lens Factor, is the distance from the Iris plane to the second principal plane of the IOL A relationship between the A-constant and a "lens factor" is also used to determine ACD.

75. The important difference between the Barrett formula and other formulas is that the location of the Secondary principle plane of refraction of the IOL is retained as a relevant variable in the formula so its UNIVERSAL

76. Parameters used for ELP prediction The effective lens position (ELP) is calculated with the help of ACD and a lens factor (LF), which itself is dependent on five variables: Keratometry (K), Axial length (AL), Anterior chamber depth(ACD), lens thickness (LT), and Horizontal white-to-white (W-W) Recommended eye type The Barrett formula is recommended for short – long eyes

77. The formula can be accessed in the online form in Asia Pacific Association of Cataract and Refractive Surgeons website.   https://www.apacrs.org/barrett_universal2/

78. Retains positive correlation of AXL and K values and ACD Accuracy is because of : incorporation of principle plane of the IOL formula Most accurate when compared with Other 3 rd and 4 th gen formulae* *C ooke DL, Cooke TL. Comparison of 9 intraocular lens power calculation formulas. J Cataract Refract Surg. 2016;42:1157–1164.

79. Barrett universal 2 is accurate across wide range of AL and ACD compared to earlier formulae.* Least refractive surprise when compared to other earlier formulae Xia, T., Martinez, C.E. and Tsai, L.M. (2020) “Update on intraocular lens formulas and calculations,”  Asia-Pacific Journal of Ophthalmology , 9(3), pp. 186–193. Available at: https:// doi.org /10.1097/apo.0000000000000293. 

80. HILL RBF

81. Pattern recognition of AI accounts for the errors caused due to “undefined factors” Limitation : as it is based on database , type of data and eye characteristics from which it is derived Algorithm continuously evolves as increasing data is fed Hill-RBF 2.0 (2018) has been released, which is derived from a larger data-set with expanded “in-bounds” biometry ranges

82. RBF- Radial Basis Function Came in 2016, which was the first formula based on artificial intelligence (AI) It is pure data based IOL Calculation approach and therefore it is free of the limitation of the effective lens position. Artificial intelligence + regression analysis It can be used for all IOL from -5 D to +30D independent of eyes anatomy ( short/ medium/ large)

83. Darcy K, Gunn D, Tavassoli S, Sparrow J, Kane JX. Assessment of the accuracy of new and updated intraocular lens power calculation formulas in 10 930 eyes from the UK National Health Service. J Cataract Refract Surg. 2020;46:2–7. Barrett universal 2 : larger overall mean error compared with Olsen Hill RBF Comparable with AL adjusted holladay 2 When analysed with different categories of AL , Barrett had less error with long AXL ( >26.0mm) And equivalent to Olsen in medium eyes ( 22.0 -26.0mm)

84. Kane formula Artificial intelligence with theoretical optics of IOL power prediction Required parameters : AL , corneal power, ACD, gender, and A constant In the 2020 study of 10,930 eyes, the Kane formula was the most accurate formula for all ranges of ALs, with the smallest absolute error for long eyes, AL >26.0 mm. * *Darcy K, Gunn D, Tavassoli S, Sparrow J, Kane JX. Assessment of the accuracy of new and updated intraocular lens power calculation formulas in 10930 eyes from the UK National Health Service. J Cataract Refract Surg. 2020;46:2–7.

85. Ladas formula: Works by combining most accurate portions of IOL formulae to make “super formula” Depending on AL and k values , it will choose among available formula and combine these SRK T, Hoffer Q, Holladay 1 , Holladay with Wang Koch adjustment, Haigis

86. Wang Koch adjustment A Wang-Koch (WK) adjustment can be applied to some third- and fourth-generation IOL formulas to optimize the calculation for AL >25 mm Holladay 2 formula can be improved by this adjustment* Results of such optimised Holladay 2 formulae was comparable with Barrett universal 2 and better than Holladay 1* Darcy K, Gunn D, Tavassoli S, Sparrow J, Kane JX. Assessment of the accuracy of new and updated intraocular lens power calculation formulas in 10 930 eyes from the UK National Health Service. J Cataract Refract Surg. 2020;46:2–7.

87. WK adjustment shift refractive outcomes in long eyes from hyperopic to myopic Can be considered as an adjunct to the use of the Holladay 1, Hoffer Q, SRK/T, and Haigis formulas in long eyes Adjusted axial length = 0.8453 × measured axial length + 4.0773 mm

88. INTRA OPERATIVE ABERROMETRY ORA SYSTEM WaveTec Vision Intraoperative Wavefront Aberrometry - ORA System Optiwave refractive analysis To allow to take both aphakic and pseudophakic refractive measurements in the operating room

89. Fits on bottom of surgical microscope Improves accuracy in IOL Power calculations including post refractive surgery patients More precise in Toric IOL placement Consistency in LRI procedures

90. Summary

91. FORMULA YEAR FORMULA CLASSIFICATION VARIABLES ADVANTAGES DISADVANTAGES SRK/T 1990 Vergence AL K A-constant Post-operative refractive target Accurate in eyes with normal axial lengths and mean keratometry values Accurate in axial myopes compared to holladay 1 and 2 hoffer Q Wang-Koch adjustment can be easily applied to further enhance outcomes in axial myopes Pre-installed on: AL-Scan ( Nidek ) Aladdin (Topcon) Anterion EQ Workplace (Zeiss) Galilei G6 ( Zeimer ) IOLMaster 700 (Zeiss) OA-2000 ( Tomey ) Less accurate in long eyes than modern vergence-based formulas ( BU-II, EVO, Hill-RBF, Kane) Assumes normal ACD

92. FORMULA YEAR FORMULA CLASSIFICATION VARIABLES ADVANTAGES DISADVANTAGES H offer Q 1993 Vergence AL K pACD Post-operative refractive target Accurate in short eyes Pre-installed on: AL-Scan ( Nidek ) Anterion (Heidelberg Engineering) EQ Workplace (Zeiss) Galilei G6 ( Zeimer ) IOLMaster 700 (Zeiss) OA-2000 ( Tomey ) Veracity Surgery Planner (Zeiss) Vision Planner (Alcon) No use of anatomic ACD, so theoretically less reliable in anatomically abnormal anterior segments Recommended to be replaced by Hoffer QST

93. FORMULA YEAR FORMULA CLASSIFICATION VARIABLES ADVANTAGES DISADVANTAGES Holladay 1 1988 Vergence AL K SF Post-operative refractive target Accurate for short eyes Unique relationships of AL and K adjust for anterior segments accurately Pre-installed on: AL-Scan ( Nidek ) Aladdin (Topcon) Anterion EQ Workplace (Zeiss) Galilei G6 ( Zeimer ) IOLMaster 700 (Zeiss) OA-2000 ( Tomey ) Less accurate in long eyes (hyperopic results) Holladay 2 1995 Vergence AL K ACD LT WTW CCT Age A-constant/ACD/SF Post-operative refractive target Open Access Calculator Button on website permits Forward and Back Calculation of Holladay 2 Formula Along with BU, accurate in pediatric populations Pre-installed on: EQ Workplace (Zeiss) IOLMaster 700 (Zeiss) Veracity Surgery Planner (Zeiss) Vision Planner (Alcon) Less accurate in long eyes(hyperopic results)

94. FORMULA YEAR FORMULA CLASSIFICATION VARIABLES ADVANTAGES DISADVANTAGES Haigis 2004 Vergence AL K ACD 3 constants: a0 A1(measured ACD) A2(measured AL) Post operative refractive target Accurate in short eyes (AL<22mm)  Accurate for stage III keratoconus eyes  Pre-installed on: Aladdin (Topcon) Anterion (Heidelberg Engineering) EQ Workplace (Zeiss) Galilei G6 ( Zeimer ) IOLMaster 700 (Zeiss) OA-2000 ( Tomey ) Less accurate in long eyes Resulting hyperopic outcomes in long eyes using Haigis alone, which can be addressed using Haigis with Wang-Koch adjustment Less accurate in eyes with extreme LT values

95. FORMULA YEAR FORMULA CLASSIFICATION VARIABLES ADVANTAGES DISADVANTAGES Barrett Universal (BU) Version I: 199 Version II: 2010 Vergence AL K ACD LT (optional) WTW (optional) LF/DF or A-constant Post-operative refractive target Accurate in long eyes  Accurate in normal ranges AL Along with Holliday 2, accurate in pediatric populations Best in eyes with mild-moderate keratoconus Easily accessible for free online Pre-installed on: AL-Scan ( Nidek ) Aladdin (Topcon) Anterion (Heidelberg Engineering) EQ Workplace (Zeiss) IOLMaster 700 (Zeiss) OA-2000 ( Tomey ) Veracity Surgery Planner (Zeiss) May be less accurate in short eyes (i.e. AL ≤22.0mm However, BU-II can still provide excellent refractive outcomes with AL <22.5mm or AL 20.8mm-22.0mm

96. FORMULA YEAR FORMULA CLASSIFICATION VARIABLES ADVANTAGES DISADVANTAGES Hill-RBF Version 2: 2018 Version 3: 2020 Artificial Intelligence AL K ACD LT (optional) WTW (optional) CCT (optional) A-constant Post-operative refractive target P rediction accuracy continues to improve as more data is analysed May outperform BU-II Haigis and Hill-RBF V.2.0 were significantly influenced by LT, independently of the ACD myopic shift with thin lenses and a hyperopic shift with thick lenses

97. FORMULA YEAR FORMULA CLASSIFICATION VARIABLES ADVANTAGES DISADVANTAGES Kane 2017 Blended (Vergence, Regression, and Artificial Intelligence-based) AL K ACD LT (optional) CCT (optional) Gender A constant (developed to be similar to SRK/T A-constant) Post-operative refractive target Accurate in short eyes  Accurate in long eyes ( AL ≥26mm)  Prediction accuracy continues to improve as more data is analysed Accurate in extreme ACD ( ≤3.0mm) Easily accessible for free online Pre-installed on: Veracity Surgery Planner (Zeiss) EQ Workplace (Zeiss) Ladas Super formula 2015 Artificial Intelligence most ideal calculations from other formulas (SRK/T, Hoffer Q, Holladay 1, Holladay with WK adjustment, Haigis Post-operative refractive target Prediction accuracy continues to improve as more data is analysed Newer versions including Toric and Post-LASIK calculators

98. Optimisation of IOL formula Making a formula more predictable by refining manufacturers lens constant Optimisation achieved by : analysis of post operative outcome with respect to target refraction ( for a specific surgeon, specific IOL and given range of Axial length) This optimisation is then added to A constant to make it more specific

99. Why to optimise Most IOL formulae accurate for AXL 22 – 26 mm Outside this range : inaccuracy increases This is because A constant is computed based on AVERAGE axial length of 23.50 mm Formulae ASSUME direct proportional relationship between AXL and Post op ACD and corneal steepness.

100. HOW TO OPTIMISE (FOR SRK/T) Minimum of 25 eyes Same surgical technique to be used A scan and k values by same person Post operative 6 weeks results to be analysed: to look for difference between target refraction and actual post op result Enter data in electronic spreadsheet: https://doctor-hill.com/ Optimised A constant then can be downloaded Optimisation for specific axial length range : Short eyes 20 - 22.0 mm Normal: 22 – 24.5 mm Long eyes 24.5mm – 26.00

101. But 4 th gen IOL formulae can be optimised over the entire range of AXL. Haigis formula has constants linked to manufacturers lens constant, POST OP ACD, AXL 4 th generation formulae require 200 cases or more for optimization of IOL power website: www.augenklinik.uni-wuerzburg.de / ulib

102. SPECIAL SITUATIONS

103. APHAKIC EYES- Speed of travel of sound is altered (1532m/s) – Slower speed than phakic eyes(1550m/s).  Two lens spikes in A-scan are Replaced by a single spike of the anterior vitreous face and posterior lens capsule Immersion method is preferred Optical biometers with aphakic mode Depending on type of IOL deductions should be done.

104. VELOCITY CONVERSION EQUATION correct measurement when an inappropriate sound velocity is used during the examination correct value = ( Vc / Vm ) X measurement The formulae are as follows:  1 IOL power (D) = Aphakic refraction x 2.01 2 IOL power (D) = Aphakic refraction x 1.75 3 IOL power (D) = 0.07x(2) + 1.27x + 1.22, where x = aphakic refraction Ianchulev et al., J. Cataract Refract. Surg. 31 (2005) 1530 Mackool et al., J. Cataract Refract. Surg.32 (2006) 435 Leccisotti , Graefes Arch. Clin . Exp. Ophthalmol.246 (2008) 729

105. All the formulae (Holladay 1, Hoffer Q, SRK/T, and SRK II) showed hyperopic shift, SRK/T showed the best accuracy Biometry-based formulae (Holladay 1, Hoffer Q, SRK/T, and SRK II) formulae were superior to Ahakic Refraction-based formulae in accuracy of IOL power calculation when IOL was implanted in the sulcus or in the bag

106. PSEUDOPHAKIC EYES Required in - unexpected postoperative refractive surprise, IOL Exchange, for comparison while doing biometry for other eye. correct setting on the machine measurement performed in the phakic or cataract mode will produce erroneous results Reducing gain to decrease the artificial spikes and make retinal ones more prominent.

107. Manual mode is preferred Best way to record axial length in the pseudophakic eye is to use an optical biometer Optical Biometry is preferred which offers more accurate correction of the AL by correction factor (CF) which varies as per the lens type and thickness. 

108. In biphakic eye (pseudophakia with phakic IOL) -Thickness (T) and material specific correction factor (C) of the implanted phakic IOL

109. Apparent Axial Length(AAL) – AL measured in pseudophakic eye with sound velocity of 1550m/s which is standard setting for cataractous eye

110. SECONDARY PIGGYBACK IOL FOR PSEUDOPHAKIA It is often easier to surgically implant secondary piggyback IOL and leave primary IOL in place to achieve desired refraction. Gill’s nomogram for residual hyperopia and residual myopia-

111. PIGGY-BACK IOLS Post IOL refractive surprise or in those with large dioptric requirement, a piggy back IOL in sulcus can be placed along with the primary implant. Myopic correction: P = 1.0 x Error Hyperopic correction: P = 1.5 x Error Where P = the needed power in the piggyback lens Error = the residual refractive error that needs to be corrected   Findl O , Menapace R. Piggyback intraocular lenses [letter]. JCataract Refract SLlrg . 2000;26(3): 308~30 9.   Findl O , Menapace R, Rainer G, Georgopoulos M. Contact zone of piggyback acryliCintraocular lenses. , Cataract Refract Surg. 1999;25(6):860- 862

112. SHORT AND LONG EYES Haigis , Hoffer Q, and Holladay 2 formulas are the best for intraocular lens power prediction in short eyes (<22 mm). In long eyes (>26 mm), the Barrett Universal II, Haigis (with optimized constants), Olsen, and SRK/T formulas provide the most accurate outcomes.

113. Greater accuracy - Hoffer Q formula in eyes shorter than 22 mm  Holladay 2 was equally as accurate as the Hoffer Q in eyes shorter than 22 mm, and that it was less accurate than the Holladay 1 in eyes between 22 and 26 mm

114. SILICONE FILLED EYE Low sound velocity (987m/s) and difficulty in identifying retinal spike Error in Axial length measurements occur as ultrasound travels longer time to reach the probe which is interpreted as longer measurements- post operative hyperopic results Optical biometer with silicone oil mode is preferred Usually IOL required is 2-3 D stronger than indicated by standard power calculation

115. True vitreous length =980/1532x Apparent Vitreous Length Corrected axial length=True vitreous length+ ACD+ Lens thickness Corrected Axial Length= Correction factor(0.71) x measured AL

116. True axial length (AL) of the silicone oil-filled (viscosity 1300 centistokes) eye can be estimated from the measured AL (MAL) obtained on A and/or B scan echography, by multiplying MAL by a conversion factor of 0.71. IOL power can then be calculated using current biometry formulae (SRK/T).

117. POSTERIOR STAPHYLOMA Myopic eyes poses a challenge because of the localization of the fovea The best method of ensuring that the optical path length is measured is to use optical biometry

118. B Scan can be used to demonstrate shape of posterior ocular wall and relationship of macula to the staphyloma Probes with fixation lights are preferable Optical biometer preferred Barret universal II formula preferred Optimised axial length/or optimized IOL constants minimizes error

119. CORNEAL ECTASIA Patients with corneal ectasias, such as keratoconus, pellucid marginal degeneration and post-refractive ectasia Irregular astigmatism disease progression Among devices- The reproducibility is best with the Pentacam because it incorporates posterior corneal curvature compared to optical biometry. The Pentacam also tends to measure flatter keratometry values when compared with optical biometry and avoid hyperopic outcomes.

120. K Values- Prefer to utilize standardized K value (43.25D) or utilize the Barrett formula and aim at least 3 dioptres more myopic than the actual targeted refractive outcome * IOL Formulae- SRK/T was found to have the smallest absolute error when compared with other formulas such as SRK II, Haigis , HofferQ , and Barrett Universal II ** *Watson MP, Anand S, Bhogal M, et al. Cataract surgery outcome in eyes with keratoconus. Br J Ophthalmol . 2014;98:361–364. ** Savini G, Abbate R, Hoffer KJ, et al. Intraocular lens power calculation in eyes with keratoconus. J Cataract Refract Surg. 2019;45:576–581.

121. Wang et al with 73 eyes compared SRK/T, Hoffer Q, Holladay I and II, Haigis and Barrett Universal II and demonstrated that for mild and moderate keratoconus- Barrett Universal II had the smallest prediction error. For severe keratoconus, all formulas performed poorly but Haigis had the smallest error. Wang KM, Jun AS, Ladas JG, Siddiqui AA, Woreta F, Srikumaran D. Accuracy of intraocular lens formulas in eyes with keratoconus. Am J Ophthalmol . 2020;212:26–33.( MOST RECENT STUDY)

122. No studies on the newer artificial intelligence-based algorithms to corneal ectasias

123. POST REFRACTIVE SURGERY EYES Refractive surgery alters the corneal curvature and introduces error into both the measurement of corneal power and the prediction of ELP  underestimation of IOL power in eyes with myopic refractive surgery and an overestimation IOL power in eyes with previous hyperopic refractive surgery.

124. Laser vision correction(LASIK/SMILE/LASEK or PRK), the anterior surface is affected while the posterior surface remains unaltered So ratio changes Radial keratotomy f lattens both the anterior and posterior corneal surfaces, but only in a small central optical zone 

125. Errors in ELP Estimation Third-generation formulae link the ELP estimation to the keratometry reading Myopia – flattening of cornea without changing anterior chamber depth False low estimate of ELP, with the formulae predicting a more anteriorly placed IOL Reversed occurs in hyperopic eyes

126. CLINICAL HISTORY METHOD- Refractive status prior to the refractive surgery and the post-correction refractive status The corneal power is calculated by subtracting the change in manifest refraction at the corneal plane induced by the refractive surgical procedure from the corneal power values obtained before refractive surgery.

127. Clinical history method is not suitable for RK because of unstable corneal power (Post RK cornea typically flattens progressively over many years)

128. CONTACT LENS OVER-REFRACTION METHOD Corneal power is calculated as the sum of the contact lens base curve, power, and over-refraction minus the spherical equivalent of the manifest refraction without a contact lens. Suitable for both post LASIK and RK corneas

129. TOPOGRAPHY-BASED POST-LASIK ADJUSTED KERATOMETRY Based on analysis of post-LASIK corneal topography central Ks (TK) in LASIK eyes. True corneal power is predicted using only the single central postoperative reading TK. They are based on LASIK data and are not suitable for post-RK cases.

130.  *Koch, D. and L. Wang, Calculating IOL power in eyes that have had refractive surgery. J Cataract Refract Surg , 2003. 29: p. 2039 - 2042. *Shammas, H.J., et al., Correcting the corneal power measurements for intraocular lens power calculations after myopic laser in situ keratomileusis. Am J Ophthalmol , 2003. 136(3): p. 426-32

131. CENTRAL RING TOPOGRAPHY METHOD Corneal refractive power after RK was best described by averaging the topographic corneal power of the central 3.0 mm area. Applying this method, together with a double-K IOL formula, achieved excellent IOL power predictability. Not suitable for LVC eyes

132. NET CORNEAL POWER MEASUREMENT Solution to obtaining accurate corneal power is to directly measure both anterior and posterior corneal curvature and thereby calculate the net corneal power.  Several instruments (orbscan 2 videokeratography , pentacam, optical coherence tomography) can directly measurement of both anterior and posterior corneal surfaces.

133. IOL Power Formulae for Post-Refractive Surgery Eyes Double k formula Hoffer Q formula Haigis L formula Masket formula

134. DOUBLE K FORMULA- In “double-K” version of IOL formula, the post-refractive surgery corneal power reading is used in the vergence calculation while the pre-refractive surgery corneal power (or an estimate) is used in the ELP prediction formula. Double-K versions of SRK/T, Hoffer Q and Holladay II formulae are available. The double-K Holladay II formula allows both a post-RK and a post-LVC setting.

135. HOFFER Q FORMULA ELP calculation is less sensitive to corneal power variation. So less error in post-refractive surgery eyes than other single-K formulae If double-K formulae are not available, the single-K Hoffer-Q formula may be useful HAIGIS-L FORMULA Built-in software of IOLMaster Corneal power is calculated by inputting IOL-Master biometry as follows: axial length (AL), anterior chamber depth (ACD), keratometry only suitable for post-LVC cases, not post-RK cases (based on LASIK data)

136. MASKET FORMULA They recommend using the SRK/T formula for myopic ALs and the Hoffer Q for hyperopic ALs.

137. KOCH AND WANG NOMOGRAM ADJUSTMENT Separate nomograms for both post myopic and hyperopic refractive surgeries Easy to use by just look up the axial length of the patient and add or subtract the adjusted IOL power to the IOL power calculated using the SRK/T, Hoffer Q, and Holladay 1 formulas Choose higher IOL power or select lower corneal power estimation to use in IOL calculation

138. IOL Calculators- ASCRS website (website based post-LVC and post-RK IOL calculator):  https://ascrs.org/tools Post-LVC IOL calculator:  https://www.eyelab.com/ IOLMaster reference:  https://doctor-hill.com/iol-power-calculations/

139. POST KERATPLASTY PATIENTS- IOL implantation can be a part of TRIPLE PROCEDURE or in prior grafted eyes. For triple procedure better keep aphakic , 4-8 months later can plan secondary IOL provided all sutures removed. Biometry from fellow eye Central corneal power values input from topography I ndian J Ophthalmol . 2010 Mar-Apr; 58(2):115-8

140. Optical biometer preferred 3 th and 4 th generation formulae suggested Toric IOLs can also preferred to correct high astigmatism provided stable refraction after complete suture removal

141. PAEDIATRIC IOL POWER CALCULATION-  Increased errors in AL measurement, which compounds the final IOL power errors due to shorter AL. AL and K value must be measured under general anaesthesia. The IOL power chosen should allow good vision in growing age to prevent amblyopia and ideally also give emmetropia in adult age.  All infants above two years are advised IOL implantation

142. RULE OF 7- Enyedi proposed “the rule of 7 ” where the sum of postoperative refractive goal and age of the child is 7 and target refraction is decided accordingly. American Journal of Ophthalmology, Volume 174 - Feb 1, 2017

143. Khokhar SK, Tomar A, Pillay G, Agarwal E. Biometric changes in Indian pediatric cataract and postoperative refractive status. Indian J Ophthalmol . 2019 Jul;67(7):1068-1072.

144. INFANT APHAKIA TREATMENT STUDY Overall, SRK/T was found to give the minimum average prediction error (0.3 D) and Hoffer Q the highest error (2.3 D) They concluded that that SRK/T and Holladay 1 yield good results in infants less than 2 years or with AL ≤21 mm Whereas Barrett and Haigis formulas were better in patients older than 2 or with AL >21 mm Infant Aphakia Treatment Study Group; Lambert SR, Buckley EG, Drews- Botsch C, DuBois L, Hartmann E, Lynn MJ, Plager DA, Wilson ME. The infant aphakia treatment study: design and clinical measures at enrollment . Arch Ophthalmol . 2010 Jan;128(1):21-7

145. Study of 20 Saudi paediatric patients included the Barrett Universal II and Olsen formulas in its comparison with the formulas (SRK 2, SRK/T Holladay 1 and Holladay 2 , Hoffer Q Both the Barrett and Olsen had larger prediction Error compared with all other formulas except for the Haigis SRK II was most accurate An- Nakhli FR. Accuracy of new and standard intraocular lens power calculations formulae in Saudi pediatric patients. Taiwan J Ophthalmol . 2019;9:37–42 ( RECENT STUDY)

146. Grouped as - short (%22.0 mm), medium(>22.0 to <24.5 mm), medium long (24.5 to <26.0 mm), and long (>26.0 mm) 3241 patients , 5 years duration Barrett Universal II formula had the lowest mean absolute prediction error over the entire AL range No statistically significant difference in the short AL subgroup J Cataract Refract Surg 2016; 42:1490–1500 Q 2016 ASCRS and ESCRS

147. Thank you