Iol master

IOL Master

Moderator: Dr. A.Y. Yakkundi

Presenter: Dr. Arushi Prakash

4th March ‘15

History• The optics of the eye

represents one of the oldest fields in ophthalmology

• The history of IOL power calculation began in 1949 when Sir Harold Ridley implanted the first IOL. 2Department of

Ophthalmology, JNMC

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• He used the human lens as his model and selected similar radii of curvature to create a biconvex disc while using approximately half the thickness and weight (∼5 mm thick and 230 mg for the human lens).

• One of his original lenses made by Rayner, a 23.00 diopter (D), was measured at 8.5 mm in diameter and 2.4 mm thick, with a weight of 108 g

History

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Department of Ophthalmology, JNMC

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• The Ridley lens was placed in the posterior chamber after ECCE.

• The anterior capsulectomy of the day was very large, and thus zonular support was poor. Some Ridley lenses dislocated into the vitreous because of poor zonular support,and partially because of their weight, which was approximately eight times that of current IOLs.

History

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• Because of the difficulty with posterior chamber IOL placement, pioneering surgeons spent the next two decades trying to find a better place to fixate the IOL.

• The AC lens, pupil-fixated IOL, iris-fixated IOL, and iridocapsular IOL were be placed in large numbers, only to return to the posterior chamber in the 1970s.

History

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• A major breakthrough came in the late seventies with Doctors Binkhorst and Worst in Europe and Dr. Shearing in America

• They began putting their implants “in the bag”. Instead of removing the entire cataract, they scooped out the inside of the lens, leaving the capsule, the outer envelope of the lens intact.

• They then implanted their lenses into this cavity which gave the lens implant a natural support system. Success dramatically improved.

History

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• While this was going on, Dr. Charles Kelman, was developing phacoemulsification, a radically new method of removing cataracts.

• A small probe is passed into the eye, and ultrasonic vibrations were used to break up the cataract into tiny particles, easily removed through the small probe.

• This allowed the cataract to be removed through a small opening. This left a problem: the opening was too small to allow the insertion of the intraocular lens, so wound had to be enlarged.

• Enter the foldable implant. First made of silicone, these lenses could be folded in half, inserted through a small opening, and then unfolded inside the eye to their original shape, all this taking place “within the bag”.

History

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• Implant materials and designs continued to improve through the late 20th century and early 21st century. Implants were developed that could be rolled instead of just folded, allowing insertion through smaller and smaller incisions.

• The next major leap forward was the development of specialty lenseswith optics that could allow the patient to see both distance and near through the same lens.

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History

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• Some patients have a large amount of astigmatism.

• This defect can be optically corrected with proper glasses. After cataract surgery, the shape of the cornea does not change much.

• Therefore, patients with astigmatism will still need glasses for distance and near to see clearly.

• Enter the toric implant. This implant is constructed with astigmatism of various powers built in, and allows clear vision, often without glasses.

History

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http://www.flei.com/vision-correction-lasik/conditions/astigmatism/

http://www.flei.com/cataract-lens-implants/premium-lens-options/toric-lens-implants/

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Department of Ophthalmology, JNMC 10

“Accurate and precise biometry is one of the key factors in obtaining a good refractive outcome after cataract surgery.”

An error of only 1.0 mm in axial length will results in a post-operative refractive error of three dioptres

Ocular Biometry

Ophthalmic Ultrasonography

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Non- invasive, efficient and inexpensive diagnostic tool to detect and differentiate various ocular and orbital pathologies

Indispensible tool for calculation of IOL power, evaluation of posterior segment behind dense cataract / vitreous haemorrhage, diagnosis of complex vitreoretinal conditions and the differentiation of ocular masses

Physics of Ultrasonography

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• Based on propagation, reflection and attenuation of sound waves

• Ultrasound are high frequency sound waves (> 20,000 kilohertz)

• Those used for diagnostic ophthalmic ultrasound have a frequency of 7.5 to 12 megahertz

Physics of Ultrasonography

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• Speed of ultrasound depends on medium through which it passes

• As the ultrasound passes through tissues, part of the wave may be reflected back towards the probe, this reflected wave is referred to as an echo.

• Echoes are produced at the junction of media with different sound velocities

• Greater the difference in the sound velocites of the media at the interface, stronger is the echo

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Examiner dependent Needs high level of skill and expertise Dynamic test

A scan ultrasound biometry is a contact method and is operator dependent. Experience has shown that excessive corneal indentation compresses the eye, in the anterior-toposterior direction. This produces an artificially short eye, producing the myopic refractive results

Limitation

Measurement of Corneal Power

• Corneal power accounts for about 2/3rd s of the total dioptric power of the eye and is an important component of the ocular refractive system.

• If the corneal power is inaccurate, it will induce error propagation and have profound consequences on the remaining steps in the calculation of IOL power. 15

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• Unfortunately calculation of corneal power is not a straight forward process

• No keratometer measures corneal power directly.

• What is measured is the size of the image reflected from the convex mirror constituted by the tear film of the corneal surface 16


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• A magnification is calculated from the image size which is directly related to the radius of curvature of the reflecting corneal surface.

• To do this, the cornea is normally assumed to be a sperocylinder,

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Measurement of Axial Length

• Measurement of axial length remains one of the most crucial steps in IOL power calculation.

• As a 0.1 mm error is axial length is equivalent to an error of abut 0.27 D in the spectacle plane (assuming normal eye dimensions), accuracy of within 0.1mm is necessary

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Variable Error Refractive Error

Corneal Radius

1.0 mm 5.7 D

Axial Length 1.0 mm 2.7 DPostoperative AC Depth

1.0 mm 1.5 D

IOL Power 1.0 D 0.67 D20

• Deviation from the mean values of different variables and corresponding refraction errors

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a = cornea spike b = anterior lens spike c = posterior lens spike d = retinal spike e = orbital spike

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Acoustic Biometry• What is really

measured by ultrasound is the transit time taken by the ultrasonic beam to travel through the ocular media while it is deflected from the internal structures of the eye.

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Acoustic Biometry• The best signal is obtained when

the ultrasonic beam strikes a surface at normal incidence that gives rise to a steep spike on the echogram

• With good alignment along the ocular axis, it is possible to detect a corneal signal (sometimes a double spike), the front and back surfaces of the lens and the retina at the same time

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• The ‘retinal’ spike is generally assumed to arise at the internal limiting membrane of the retina

• This may call for correction to account for retinal thickness when the readings are to be used in an IOL power formula.

• It is important to know the velocity of ultrasound in order to calculate the distances in question 23

Acoustic Biometry

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• For the normal phakic eye, velocity is generally assumed to be 1532/second for the anterior chamber and vitreous and 1641m/second for the lens (Jansson & Knock)

• In an average eye, this is equivalent to 1550 m/second for the whole eye.

• However, if we assume a constant lens thickness, this average velocity is lower in a long eye and higher in a short eye, and should be corrected to obtain an unbiased prediction in these unusual eyes

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The pitfalls of ultrasound measurements are numerous

•Readings should be coaxial with the ocular axis

•This requires a steep spike from the retina as well as good spikes from the anterior and posterior surfaces of the lens 25

Acoustic Biometry

limitations

• Some eyes do not have perfectly parallel structures, however,

• readings can be difficult to obtain in eyes with dense cataracts

• and eyes with posterior staphyloma. • Care should be taken not to indent

the cornea if contact measurements are used .

• For this reason immersion readings are generally considered more accurate than contact measurements

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Acoustic Biometry

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Optical Biometry

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• The introduction of optical biometry using partial coherence interferometry significantly improved the accuracy with which axial length can be measured.

• The fact that the retinal pigment epithelium is the end– point of an optical measurement, whereas the interface between the vitreous and the neuro retina is the endpoint of an ultrasonic measurement, makes measurements by PCLI longer than those taken with ultrasound

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OPTICAL BIOMETRY

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• However, just as distance measurements taken with ultrasound are dependent on the assumed ultrasound velocity, optical biometry is dependent on the assumed group refractive indices of the phakic eye.

• The indices used by the Zeiss IOL Master were estimated by Haigis and were partly based on extrapolated data.

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Optical Biometry –

Uses Optical Low-Coherence Reflectometry, a similar technology that is used in OCT devices. This technology results in highly accurate measurements of the eye using light in comparison to sound. The added benefit is that this technology is also non contact and can be performed with the patient sitting comfortably in a chair without the need for any topical anaesthesia, and without the risk of damage to the cornea.

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Principle of Michelson Interferometer

Xiaoyu Ding

Albert Michelson (1852~1931)

the first American scientist to receive a Nobel prize, invented the optical interferometer.The Michelson interferometer has been widely used for over a century to make precise measurements of wavelengths and distances.

Albert Michelson

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Principle of Michelson Interferometer

A Michelson Interferometer for use on an optical table

Xiaoyu Ding

1)Separation2)Recombination3)Interference

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IOL master employs the principle of optical coherence biometry (OCB)It uses partially coherent infrared light beams of 780nm diode laser light emitted is split up into two beams in a Michelson interferometer one mirror of the interferometer is fixed and the other is moved at constant speed making one beam out of phase with the other. Both beams are projected in the ye and get reflected at cornea and retina .The light reflected from the cornea interferes with that reflected by the retina as the optical paths of both the beams are equal

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This interference produces a light and dark band pattern which is detected by a photo detector The signals are amplified, filtered and recorded as a function of the position of the interferometer mirror.An optical encoder is used to convert the measurements into axial length measurements

In interferometer, the eye needs to be absolutely stable so as not to disturb interference patterns

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The Down SideSince optical Biometry uses light there is a higher probability of the “scatter” effect. Meaning that if the light beam is reflected prior to the RPE then the signal returning to the device sensor will be very weak if detected at all. This will result in low SNR. Patients with Dense PSC, Extreme Corneal Abnormalities, or White Cataracts are very tough to measure.

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IOL Master (Carl Zeiss)Lenstar LS 900 (Haag-Streit)

Manufacturers

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IOL MasterA combined biometry instrument. It measures parameters of the human eye needed for intraocular lens calculation.

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1.Joystick with release button2.Display3.Red eye level marks4.Lock knob5.Connector panel6.Mouse connector 7.Keyboard connector 8.keyboard

PARTS


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1.DVD Drive2.Adjustment of headrest3.Chin rest4.Holding pins for paper pads5.Forehead rest6. aperture for diode laser7. device control connector

PARTS


IOL Master 4th March 2015


It measures quickly and precisely : 1. Axial length 2. Corneal curvature 3. Anterior chamber depth (ACD) 4. White-To-White (optional) 5. IOL power

It measures quickly and precisely

Axial length : Based on partial coherence interferometry ( Michelson interferometer)

Corneal curvature is determined by measuring the distance between reflected light images.

ACD : as the distance between the optical sections of the crystalline lens and the cornea produced by lateral slit illumination.

White to white is determined from the image of the iris.

IOL power calculation : by software incorporating internationally accepted calculation formulae. 41

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How do we operate IOLmaster ?

After switching on the device, patient manager screen will appear

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Screen layout

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Axial length measurement(al

m)Activate the axial length measurement mode by clicking on ALM icon.

Switching to ALM mode will automatically change the magnification ratio: a smaller section of the eye becomes visible with the reflection of the alignment light and a vertical line.

The patient should look at the red fixation point in the center . A crosshair with a circle in the middle will appear on the display.

Fine align the device so that the reflection of the alignment light appears within the circle. 44

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Note : The patient should be asked if he or she sees the fixation point. If the patient fails to fixate properly, the visual axis will not be correctly recognized, which may result in measuring errors.

In the case of poor visual acuity/high ametropia (> 4 D) it is advisable to measure through the spectacles. If the procedure is followed correctly, no measuring errors will be produced. Measurements should not be taken while a patient is wearing contact lenses, as this will result in measuring errors.

The corresponding display field next to the video image will show the measured axial length. 45

Axial length measurement

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The IOL Master requires five measurements to be taken. The message Measure again will thus appear. Only then will the composite signal be calculated and displayed as a blue measurement curve following the red individual measuring signal.

With stronger lens opacities, it may be advisable to defocus the device. Defocusing and shifting the reflection within the circle will have no effect on the result, because interferometric axial length measurement is completely independent of distance.

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Axial Length Modes

1.Phakic2.Pseudophakic3.Aphakic4.Silicone filled eyes

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IOLMaster produces a primary maxima (narrow, well-defined, centered peak identified by a circle above it), secondary maxima (discrete lower peaks, sometimes disappearing into the baseline), and a baseline (which is low and even, but may become high and uneven with decreasing signal-to-noise ratio (SNR)).

Triple peak curve

SNR categories :SNR is a measure of accuracy and decreases with increasing cataract density.

The SNR is automatically analyzed while the system is internally calculating the axial length from the interference signal. SNR display at GREEN reading is valid. SNR display at YELLOW reading is uncertain SNR display at RED reading should not be used

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Keratometric measurementAsk the patient to relax and look at the

fixation light. If the patient cannot see the fixation light, he or she should look straight ahead into the device. When adjusting the device, make sure that all 6 peripheral points are visible and located in the field between the two auxiliary circles, as closely as possible to the center of the display. The images of the measuring marks on the display must be optimally focused by varying the distance between patient and device. 50

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Focus points

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The IOLMaster reflects six points of light, arranged in a 2.3 mm diameter hexagonal pattern (measured by digital callipers), from the air/tear film interface. The separation of opposite pairs of lights is measured objectively by the instrument’s internal software and the toroidal surface curvatures calculated from three fixed meridians

Keratometric measurement


Acd measurementAsk the patient to relax and look at the

fixation light. If the patient cannot see the fixation light, he or she should look straight ahead into the device. When the anterior chamber depth mode is turned on, the system automatically activates the lateral slit illumination. The illumination always originates from a temporal direction.

An image similar to that of a slit lamp (optical section through the anterior segment of the eye) is visible on the display. Align the device to the patient’s eye by lateral adjustment using the joystick until: The image of the fixation point appears optimally focused in the green square on the display.

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The image of the anterior crystalline lens is visible in the pupil. The image of the fixation point may not lie in the image of the lens or cornea.

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Acd measurement

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Measuring errors The "Error" message may have two basic causes: • The results of the five internal individual measurements vary by more than 0.15 mm (very rare), or • The images produced (optical sections) do not contain relevant structures (normally without the edge of the crystalline lens) or disturbances are preventing their detection. 55

Acd measurement

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Ask the patient to relax and look at the fixation light.

Focus on the iris, not on the light spots.

After the image has been taken, the operator should check if the software has correctly detected the edge of the iris. If the circle segments drawn in the image do not define the iris correctly, the result must be discarded.

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WTW measurement

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Once all measurements have been taken (depending on the IOL calculation formula), options can be generated for intraocular lenses to be implanted.

Start the calculation by: clicking on IOL Click on the appropriate tab to select the desired formula. The IOL Haigis, HofferQ, Holladay, SRK II, and SRK®/T formulae are implemented as standards.

After refractive corneal surgery the Haigis-L formulae may be selected.

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IOL CALCULATION

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result

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Measuring ranges Axial length : 14 – 40 mmCorneal radii : 5 – 10 mmDepth of anterior chamber : 1.5 – 6.5 mm White-to-white : 8 – 16 mm

formulas SRK® II, SRK®/T, Holladay, Hoffer Q, Haigis Haigis-L for IOL calculation for eyes after myopic/hyperopic LASIK/PRK/LASEK Optimization of IOL constants

Line voltage 100 – 240 V +/– 10% (self sensing)

Line frequency 50 – 60 Hz

Power consumption max. 90 VA

Technical data

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Biometry Formulas

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IOL Formulas1st Generation – The first theoretical formula (based on Geometric Optics as applied to schematic eye models) was developed in 1967. These formulas were very primitive and usually resulted in large amounts of post-cataract surgery refractive errors. (Regression)

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IOL formulas1st generation

•Most are based on regression formula developed by Sander ,Retzlaff & Kraff•Known as SRK formula.•P = A - 2.5(L) - 0.9(K)•P=lens implant power for emetropia•L= Axial length (mm)•K=average keratometric reading (diopters)•A= lens constant 64

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IOL formulas2nd Generation – With an extreme need for increased IOL Calculation, the second generation formulas (Hoffer, SRK II) listed manual correction factors for long or short eyes. (These formulas are now considered obsolete.) (Regression)

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IOL formulas• IOL FORMULA 2nd generation

• SRK II formula• modification of SRK• works on ELP• P = A1 – 2.5L – 0.9K

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IOL formulas3rd Generation – In 1988 Dr. Holladay published a formula (Holladay I)that predicted the AC Depth on the basis of K Height and the distance from the iris plane to the IOL optical plane called the “Surgeon Factor”. This change in the physics greatly increased the visual outcomes for Cataract Surgery. This generation also includes the SRK/T and Hoffer Q.

4th Generation – Consist of Holladay II as well as the modern post-refractive formulas

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IOL formulas• IOL FORMULA 3rd generation• Third generation formulas-• SRK/T -very long eyes >26mm• Holladay I -long eyes 24-26 mm• HofferQ -Short eyes<22mm

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IOL formulas• IOL FORMULA 4th generation• Holladay II• Haigis formula-• d = a0 + (a1 * ACD) + (a2 * AL)• ACD is the measured anterior chamber depth• AL is the axial length of the eye• The a0, a1 and a2 constants are set by optimizing• a set of surgeon- and IOL-specific outcomes for a wide• range of ALs and ACDs.• SRK/T formula — uses "A-constant“• Holladay 1 formula — uses "Surgeon Factor“• Holladay 2 formula — uses "Anterior Chamber Depth“• Hoffer Q formula — uses "Anterior Chamber Depth"

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IOL formulas• When capsular tear does not allow bag

placement of the lens → change IOL power for sulcus placement

• >=28.5 D Decrease by 1.5 D• +17 To 28 D Decrease by 1.0 D • +9 To 17 D Decrease by 0.5 D• <+ 9 D

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Lens ConstantsA-Constants are used with all IOL formulas, and are determined by the anticipated position within the eye.

Surgeon Factor – is used with the first Holladay formula, and is determined by the distance from the Iris plane to the Optical plane of the implant.

Effective Lens Position (ELP) is used for the Holladay II formula, and is based on the depth of the AC following Cataract surgery with the new IOL in place.

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Types of FormulasRegression formulas are based upon mathematical analysis of a large sampling of post-operative results. The most familiar regression formula is the SRK formula. The basic SRK formula works well for eyes in the "average" measurement range; 22.5 to 25.0 mm in axial length, with certain combinations of K readings. The formula does not work well for "long" (>25 mm) or "short" (<22.5 mm) eyes.

Advantage - relatively simple to calculate. A factor can be added to a simple regression formula to compensate for a long or a short eye

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Types of FormulasTheoretical formulas are optical formulas based on the optical properties of the eye. They do a better job of predicting post-op outcomes for long and short eyes.

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Formula Requirements

Haigis Hoffer Q SRK/2 SRK/T HOLLADAY 1 HOLLADAY 2 Olsen

Axial Length YES YES YES YES YES YES YES

ACD YES NO NO NO NO YES* YES

Keratometry YES YES YES YES YES YES YES

Lens Thickness NO NO NO NO NO YES YES

Corneal Thickness NO NO NO NO NO NO NO

White to White NO NO NO NO NO YES YES

Pupil Size NO NO NO NO NO NO NO

Visual Axis NO NO NO NO NO NO NO

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Formula Preferences

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Short Eyes (<22.0mm)

Hoffer Q / Holladay 2

Average Eyes (22.1-24.4mm)

Hoffer Q / Holladay I / SRK/T

Medium-Long Eyes (24.5-25.9mm

Holladay I / Hoffer Q

Long Eyes (25.0mm +)

SRK/T / Holladay I

(Holladay II All eye lengths.)(Haigis All eye lengths w/o optimization)

Post-Refractive Surgery Patient

One of the most challenging problems facing modern Cataract Surgery is the Post-Refractive patient. Following refractive surgery (RK, PRK, LASIK, ect) accurate K readings cannot be obtained from topography, automated or manual keratometry because the central cornea has been flattened causing the mires of the measuring device to measure roughly 4.5mm versus 3.0mm for which they were designed. This causes erroneous K readings compromising the effectiveness of all modern IOL formulas. 76

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Post Refractive FormulasHaigis L - The Haigis-L formula offers predictable outcomes after laser refractive surgery for myopia based only on current measurements without refractive history.)

Masket Method - The Masket Method of post-LASIK corneal power estimation is a postoperative regression method developed by Samuel Masket and Clinical History Method – Is usable when both the pre-op and post-op Keratometry values are known.

Contact Lens Method - The Contact Lens Method, originally outlined by Dr. Holladay is considered a helpful way to estimate the average central corneal power following radial keratotomy. This technique required a special PMMA contact lens, of a known base curve and power.

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Shammas – Used when no pre-op data is available such as refraction and keratometry.

Double K SRK/T – Utilizes pre-op refraction and keratometry

Post Refractive Formulas

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Advantages# Learned very quickly (User Friendly)

# Extensive integrated safety features

# Non-contact measurements.

# It gives the true refractive length than anatomical axial length

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Advantages# Accuracy of IOL Master is 0.02 µm

which is operator independent

# It is upright, non contact, ultra high resiltuion biometry

# Highly ametropic patient can wear glasses while sitting on the IOL master which aids in fixation

# It has the advantage of measuring fovea in cases of posterior staphyloma80

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Limitations# Cannot measure axial length in

media opacities like corneal opacities, dense cataract, nuclear sclerosis grade IV, posterior Polar Cataract

# Cannot measure axial length in cases of vitreous haemmorrhage

# Difficulty in measuring axial lengths in infants, small children and mentally handicapped patients

# Patients with poor fixation 81

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Do not throw away old ultrasound machine

Immersion ultrasound

IOL master

Posterior staphylomaSilicone oilPseudophakia4++brunescent lensCentral PSC plaqueVitreous hemorrhageCentral corneal scar

DifficultDifficultVariable•Yes•Yes•Yes•Yes

•Yes•Yes•YesNoNoNoNo

Lenstar 900

4th March 2015 Department of Ophthalmology, JNMC

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Lenstar 900Lenstar features a unique dual zone keratometer with a total of 32 marker points on two concentric rings of 1.65 and 2.3 mm in diameter for improved refractive outcomes with toric lense.

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Lenstar 900It has been complemented with an optional T-Cone topography add-on and an optional toric surgery planning platform.

The T-Cone enables the Lenstar to provide true Placido topograph of the central 6 mm optical zone.

The toric surgery planning platform allows planning and optimization of the surgical

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Lenstar 900• The toric planner shows the implantation

axis, the incision location and user-defined guiding meridians in the real patient image.

• Incision optimization tools allow for precise placement of the incision to minimize the residual astigmatism based on the surgically induced astigmatism.

• Planning of the operation on real eye images allows the user to define recognizable, additional guiding lines to anatomical landmarks in the intraoperative view.

• They either serve as a base line point for the intraoperative orientation or as a fallback strategy if external marking is not successful.

• The planning sketch can easily be printed and hung near the microscope

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• The LENSTAR LS 900 ® measures:·· Axial eye length·· Corneal thickness·· Anterior chamber depth·· Aqueous depth·· Lens thickness·· Radii of curvature of flat andsteep meridian·· Axis of the flat meridian··White-to-white distance·· Pupil diameter


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Lenstar 900

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Olsen formula calculates the postoperative lens position as a fraction of the crystalline lens thickness and the ACD. This approach allows accurate calculation of the lens positionindependent of the corneal status of the eye. The lens position is then used to calculate the IOL power based on ray tracing, the same technology that physicists use to design telescopes and camera lenses.

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Post -refractive IOL calculation


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• Shammas No-History and Masket – for premium results

• The Lenstar EyeSuite software provides the user with a comprehensive set of cutting-edge IOL calculation formulae for normal eyes. IOL Power calculation in patients with prior LASIK or PRK, presenting with no history, is easily achieved with the on-board Shammas No-History method.

• If the change in refraction is known, then the Masket and modified Masket formulae may also be used.

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Optical Biometer Properties

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Feature DeviceIOL Master Lenstar

Axial Length X XWhite to White X XKeratometry X XACD X XPachymetry XLens Thickness XRetinal Thickness

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Pupillometry XVisual Axis X

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Summary

With state of the art technology and modern IOL calculation formulas, excellent refractive outcomes can be achieved after IOL implantation in challenging eyes, that approach the benchmarks postulated for normal eyes.

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05/02/23 07:41 PM Dept. of Ophthalmology, JNMC 92

Iol master

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