6. A Constants and IOL Formula www.medsalesacademy.co.uk - - PowerPoint PPT Presentation
6. A Constants and IOL Formula www.medsalesacademy.co.uk www.medsalesacademy.co.uk Today we will learn About A Constants, Optical and Ultrasound How to calculate IOL Power? Which constants used in di ff erent formula? What is ELP
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About A Constants, Optical and Ultrasound
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How to calculate IOL Power?
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Which constants used in different formula?
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What is ELP & How does it effect calculation?
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Which formula are best for which type of eye?
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What formula for patients with prior refractive surgery?
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๏ Each formula has a “number” or “constant” to represent
the lens
๏ Takes into account
๏ Ultrasound and Optical measure slightly differently ๏ Use Ultrasound and Optical “A” constants
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Optical 23.5mm US 23.2mm
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Basic formula D = A - 2.5 ALM - 0.9 K D = IOL Power A = A constant ALM = Axial Length K = K Reading
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–Eg SupaLens A constant 118 –K Reading 43 –ALM 23.5 optical and 23.2 with Ultrasound
–DOpt = 20.55
–DUS = 21.3
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๏ Data from worldwide surgeons ๏ Out of date ๏ Surgeons recommended to optimise ๏ Contact IOL Manufacturer
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SRK/T - A Constant eg 118.0
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Haigis - 3 Constants eg a0- 0.466, a1- 0.259, a2-0.135
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Hoffer Q - pACD eg 5.04
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Holladay 1 - sf eg 1.27
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If Haigis a1=0.40 a2=0.10 then this has not been optimised, Haigis formula will not give as good results. Use fully optimised
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Effective Lens Position
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Lens power varies with distance from pupil
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Knowing where the lens will sit improves outcomes
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https://eyewiki.aao.org/Biometry_for_Intra-Ocular_Lens_(IOL)_power_calculation
๏ First - Before 1980’s ELP was a constant 4mm for every
eye and IOL (Mainly AC)
๏ Second - In 1980’s Binkhorst used AL as a scaling factor
for ELP
๏ Third - In 1988 K&AL improved scaling accuracy of ELP ๏ Fourth - In 1995 Olsen et al improved accuracy by
adding 2 more variables PreOp ACD & Lens Thickness
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๏ SRK, SRK II and Binkhorst are considered obsolete ๏ All modern formula (Haigis, SRK/T, Holliday 1 and Hoffer Q) are considered
good for all eyes, but Hoffer Q and Haigis may be better for short eyes
๏ Optimising A constants can improve predictability by as much as 20% the
percentage of eyes within +/- 1D
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๏ Most used formula ๏ Good for normal eyes ๏ Designed by Sanders, Retzlaff & Kraft ๏ Uses A Constant ๏ Needs K reading & Axial Length
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๏ Designed by Ken Hoffer ๏ Good for short eyes ๏ Uses PACD (Personalised) ๏ Needs Axial Length measurement and K Reading
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๏ Designed by Professor Haigis ๏ Uses Anterior Chamber depth as well as K
reading and Axial Length
๏ Accurate over a wide range of lenses if optimised
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๏ Jack Holladay ๏ Good for longer eyes ๏ Uses SF (Surgeon Factor) ๏ Needs K reading & Axial Length
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K’s have been altered so any formula based on K’s will not work.
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Point to ASCRS Formula page http:// iolcalc.ascrs.org
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Drs may use one of the more modern formulas like Barratt or Olsen
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They should check topography as refractive surgery may make cornea irregular
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๏ Jack Holladay designed this software which is
good for ALL odd eyes, very short, post LASIK etc
๏ Pay for use ๏ Needs normal measurements plus Lens Thickness ๏ Holladay also has some solutions using the
Pentacam (EKR) and OPD Scan (ECCP)
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๏ On modern IOL masters for Hyperopia and
Myopic patients
๏ Works on a regression formula ๏ Only available on IOL Master
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๏ Features many formula ๏ Can just type in as much information as you have ๏ www.ascrs.org
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About A Constants Optical and Ultrasound
✓
How to calculate IOL Power?
✓
Which constants used in different formula?
✓
What is ELP & How does it effect calculation?
✓
Which formula are best for which type of eye?
✓
What formula for patients with prior refractive surgery?
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We will email you some questions.
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Please send your questions and comments to emma@medsalesacademy.co.uk
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We look forward to seeing you on the next course - Multifocal and Toric IOLs