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 & How does it e ff ect calculation? ๏ Which formula are best for which type of eye? ๏ What formula for patients with prior refractive ๏ surgery? www.medsalesacademy.co.uk
A Constants ๏ Each formula has a “number” or “constant” to represent the lens ๏ Takes into account - Lens placement - Curvature of Lens - Density of Lens ๏ Ultrasound and Optical measure slightly differently ๏ Use Ultrasound and Optical “A” constants www.medsalesacademy.co.uk
Measuring ALM Optical 23.5mm US 23.2mm www.medsalesacademy.co.uk
Calculating Lens Power Basic formula D = A - 2.5 ALM - 0.9 K D = IOL Power A = A constant ALM = Axial Length K = K Reading www.medsalesacademy.co.uk
Calculation • D = A - 2.5ALmm - 0.9KD – Eg SupaLens A constant 118 – K Reading 43 – ALM 23.5 optical and 23.2 with Ultrasound • D Opt = 118 - (2.5 x 23.5) - (0.9 x 43) – D Opt = 20.55 • D US = 118 - (2.5 x 23.2) - (0.9 x 43) – D US = 21.3 • Use the right A Constants! www.medsalesacademy.co.uk
ULIB ๏ Data from worldwide surgeons ๏ Out of date ๏ Surgeons recommended to optimise ๏ Contact IOL Manufacturer www.medsalesacademy.co.uk
Different Formula/ Different Term SRK/T - A Constant eg 118.0 ๏ Haigis - 3 Constants eg a0- 0.466, a1- 0.259, a2-0.135 ๏ Ho ff er Q - pACD eg 5.04 ๏ Holladay 1 - sf eg 1.27 ๏ 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 www.medsalesacademy.co.uk
Question Time www.medsalesacademy.co.uk
ELP E ff ective Lens Position ๏ Lens power varies with distance ๏ from pupil Knowing where the lens will sit ๏ improves outcomes www.medsalesacademy.co.uk
ELP ๏ 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 https://eyewiki.aao.org/Biometry_for_Intra-Ocular_Lens_(IOL)_power_calculation www.medsalesacademy.co.uk
Formula Guidelines ๏ 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 www.medsalesacademy.co.uk
Formula www.medsalesacademy.co.uk
SRK/T ๏ Most used formula ๏ Good for normal eyes ๏ Designed by Sanders, Retzlaff & Kraft ๏ Uses A Constant ๏ Needs K reading & Axial Length www.medsalesacademy.co.uk
HofferQ ๏ Designed by Ken Hoffer ๏ Good for short eyes ๏ Uses PACD (Personalised) ๏ Needs Axial Length measurement and K Reading www.medsalesacademy.co.uk
Haigis ๏ 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 www.medsalesacademy.co.uk
Holladay ๏ Jack Holladay ๏ Good for longer eyes ๏ Uses SF (Surgeon Factor) ๏ Needs K reading & Axial Length www.medsalesacademy.co.uk
Question Time www.medsalesacademy.co.uk
Prior Refractive Surgery K’s have been altered so any formula based on ๏ K’s will not work. Point to ASCRS Formula page http:// ๏ iolcalc.ascrs.org Drs may use one of the more modern formulas ๏ like Barratt or Olsen They should check topography as refractive ๏ surgery may make cornea irregular www.medsalesacademy.co.uk
Potvin Shammas Hill ‣ True Net Power ‣ Apex ‣ 4.0mm ‣ Zone www.medsalesacademy.co.uk
Holladay 2 ๏ 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) www.medsalesacademy.co.uk
Haigis L ๏ On modern IOL masters for Hyperopia and Myopic patients ๏ Works on a regression formula ๏ Only available on IOL Master www.medsalesacademy.co.uk
ASCRS ๏ Features many formula ๏ Can just type in as much information as you have ๏ www.ascrs.org https://iolcalc.ascrs.org www.medsalesacademy.co.uk
Today we have learnt About A Constants Optical and Ultrasound ✓ How to calculate IOL Power? ✓ Which constants used in di ff erent formula? ✓ What is ELP & How does it e ff ect calculation? ✓ Which formula are best for which type of eye? ✓ What formula for patients with prior refractive ✓ surgery? www.medsalesacademy.co.uk
Thank you! We will email you some questions. ๏ Please send your questions and comments to ๏ emma@medsalesacademy.co.uk We look forward to seeing you on the next course - ๏ Multifocal and Toric IOLs www.medsalesacademy.co.uk
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