2Optical Considerations for IOLs A number of factors must be taken into consideration when selecting IOLs,including :1.Image magnification2. Power selection3.Piggy back IOls4.Power after cornal refractive surgery
3Image MagnificationTheoretically , replacing a clear or cataractous crystalline lense with an artificial lens is optimal form of aphakic correction.The majority of aberrations and distortions produced by aphakic spectacles derive from their placement anterior to pupillary plane.These include:Image magnification,Ring scotoma,Peripheral distortion,jack-in-the box phenomen,decrease useful peripheral field.
4Image magnification – as much as 20% - 35% - is the major disadvantage of aphakic spectacles.Contact lens correction of aphakia magnifies the image 7%-12% ,IOLs magnify by 4% or less.Naturally ,a lens located in the posterior chamber produces less image magnification than a lens in the anterior chamber.Unilateral pseudophakic patients have better stereoacuity and less aniseikonia than aphakic patients whose vision is corrected with contact lens or spectacle.
5IOL Power SelectionNontoric IOLs provide spherical correction of the refractive error without any cylindrical component to this spherical correction should be calculated for each eye as accurately as possible.The optimal postoperative refraction depends on the situation and visual needs of the patient.
6For example, if a unilateral cataract is present and the fellow eye is more than 1.5 – 2 D hyperopic , the surgeon can consider making the operated eye slightly hyperopic as well.This strategy avoid inducing aniseikonia and anisophoria.In most settings , however it is desirable to produce emmetropia or slight myopia.
7Biometric Assumption In IOL Selection 1.Axial length :a) A scan ultrasonographyb) Partial coherence interferometry2.Kerotometry or corneal topography3.Anterior chamber depth or optical chamber depth(OCD)
8Ascan do not actually measure axial length Ascan do not actually measure axial length.They measure the time required for a sound pulse to travel through the ocular media , reflect from the retina , and return through the media.Sound moves faster through the crystalline lens than through the cornea , aqueous and vitreous.Even with in the lens itself , the speed of sound can varry according to the hardness of the cataract.
9Ascan tends to understimate the axial lens of short eyes and overstimate long eyes. 1 mm error in the measurement of axial length result in a refractive error of approximately 2.5 – 3 D . The two primary Ascan techniques – applanation (contact) and immersion – give different readings.Applanation method may give a shorter axial length measurement perhaps due to corneal indentation.
10Partial coherence interferometry measures the time required for infrared light to travel to the retina . This technique does not require contact with the cornea , so corneal compression artifacts are eliminated . In addition the patient must fixate a target thus , the length measured is the path the light takes to the fovea , the physiological axial length .
11However , the media must be clear enough to allow fixation and light transmission , in dense cataract , ultrasound axial length measurements may still be necessary . Partial coherence interferometry technique is probably more accurate and reproducible than ultrasound method , although some cases will still require ultrasound biometry .
12KeratometryKeratometry or corneal topography does not measure corneal power directly , keratometry measures only a small portion of central cornea , viewing the cornea as a convex mirror. Both front and back corneal surfaces contribute to corneal power and a keratometer measures only the front surface.
13Anterior Chamber Depth Formulas based on geometrical optics generally require a third parameter , anterior chamber depth or optical chamber depth (OCD).
14Power Prediction Formulas The first type of IOL formula based on geometrical optics:nvitKPIOL =AL - acd1 – [ K(acd) / naq ]PIOL = Power of the IOLK = DioptricAL = Axial lengthacd = anterior chamber depthnvit = index of refraction of the vitreausnaq = index of refraction of the aqueous
15Factors related to A constant: 1.lense position in the eye In the 1980 , Sanders , Retzlaff and Kraff took a different approach.formula turned out to be a simple linear equation , which was introduced as the SRK formula.P = A – (2.5 * axial length in mm) – ( 0.9 * average keratometry in diopter )A constant , which is provided by manufacturers for their lens ,is specific to each lens type .Factors related to A constant: 1.lense position in the eye2.haptic angulation3.lens shape
16Axial length (AL) Modified A constant 20 > AL A = A + 3 This formula was less accurate for long or short eyes. The power was too low in short eyes and too high in longer eyes . the formula was later modified as the SRKⅡ formula :Axial length (AL) Modified A constant20 > AL A = A + 321 > AL ≥ A = A + 222 > AL ≥ A = A + 124.5> AL ≥ A = A (No change)AL ≥ A = A − 0.5
17Second generation formulas added modifications based on AL for short or long. Third and fourth generation formulas added modifications for other factors such as corneal curvature , ACD , and so on .These formulas are too complex for convenient hand calculation.
18In the normal range of axial length (22 – 24 In the normal range of axial length (22 – 24.5 mm) almost all formulas function does not have discrepancies.But in medium long eye (24.5 – 26 mm) ,the Holladay 1 formula is the most accurate.
19– And is very long eyes ( > 26 mm) , the SRK/T is more accurate. – In short eyes ( < 22 mm ) the Hoffer Q formula is more accurate.– Holladay 2 formula equql the Hoffer Q in short eyes but is not as accurate as the Holladay 1 or Hoffer Q in average and medium long eyes.