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Chapter - 3

IOL : POWER CALCULATION & SELECTION

Precise IOL power calculation is essential for optimal benefits of implant surgery. Prior to1975, IOL power was calculated on the basis of clinical history, i.e. pre-operative refractive error prior to development of cataract. This led to errors in over 50% of cases. However, a number of formulae are now available to accurately calculate the IOL power required in a patient. All these formulae are based on an accurate measurement of the corneal power and the axial length. FORMULAE IN USE The original formulae were developed prior to 1980. They include the theoretical formulae and regression formulae. The commonly used formulae are the regression formulae, of which the most popular one is the SRK formula described by Sanders D, Retzlaff J. and Kraff M. The formula is based on the following equation: P = A ­ BL - CK where P is the implant power for emmetropia, L the axial length in millimeters, and K the average keratometric reading in diopters. A, Band C are constants. The value of B is 2.5 and that of C is 0.9 Thus P = A - 2. 5L - 0.9K The constant A varies with the implant design and the manufacturer. Be sure of the constant value of the IOL you are using while making the calculations. The SRK formula has been found to be reasonably accurate for eyes with axial lengths between 22mm and 24.5mm. These eyes constitute approximately 75% of cases, while 14% of cases have axial lengths greater than 24.5 mm, and 10% have axial lengths less than 22mm. The modified formulae were developed to correct for errors in these formulae occurring in long and short eyes. It is for such 'too long' and 'too short' eyeballs that SRK II formula was introduced. The SRK II formula is a modification of the original SRK formula with the addition of a correction factor that increases the lens power in short eyes and decreases it in long eyes.

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The suggested method of modification of SRK to SRK II is shown below: L (mm) Less than 20.00 20.00 - 20.99 21.00 - 21.99 Greater than 24.50 Modern formulae for emmetropia: These formulae are more complex than the original and the modified formulae. The most striking difference is the manner in which the estimated anterior chamber depth (ACD) value is calculated. The ACD value is a constant value in the original formulae. It varies with the axial length in the modified formulae (decreases in the shorter eye and increases in the longer eye). In the modern formulae, ACD value varies not only with axial length, but also with corneal curvature (being more with steeper cornea and deep AC and vice versa). The commonly used modern formulae are the Holladay formula, the SRK-T formula and the Hoffer-Q formula. Add to 'A' constant +3 +2 +1 -0.5

KERATOMETRY Manual keratometry is the most commonly used method to measure corneal curvature. It is fast, easy and is very accurate in most cases. Keratometry should be done before axial length measurement, and for both eyes. Remember to calibrate the eyepiece for your refraction before recording measurements. The procedure of keratometry using the common Bausch and Lomb keratometer is given here. The patient is seated behind the keratometer, with the chin well positioned in the chin rest and the head resting on the head band. The keratometer is directed towards the eye to be examined and the other eye is occluded. The keratometer is focused on the central portion of the cornea using the focusing knobs. The instrument is now rotated to align the (-) signs in the same vertical meridian and the (+) signs in the same horizontal meridian. This will determine the axis of the pre-existing astigmatism. The left drum is rotated to superimpose the (+) signs and the horizontal measurement is read out. The right drum is now rotated to superimpose the (-) signs and the vertical measurement reading is recorded. The Javal-Shiötz keratometer utilizes two mires to achieve the end point. IOL power calculation formulae use the average corneal

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power, K = average of the horizontal and the vertical readings. It is important to remember that the keratometer has to be calibrated every 6 months.

It is advisable to repeat measurement if the a. b. c. Average keratometry (K) in either eye is less than 40 D or greater than 47 D. Difference in K between the two eyes is greater than 1 D. Corneal cylinder does not correlate well with the refractive cylinder.

In certain situations, like irregular corneal contour or previous refractive surgery, or when the surgeon wants to better evaluate the astigmatism, corneal topography may be utilized.

AXIAL LENGTH MEASUREMENTS The measurement of the axial length is best done with A-scan ultrasonography. It can be performed by an immersion technique or a contact technique. The machine should have a screen showing the spikes for ensuring correct measurement. Always take measurement for both eyes.

Technique With the contact technique, a drop of local anesthetic is instilled into each eye. The patient is examined in the seated position. The probe is positioned in front of the eye and the patient is asked to fixate on the red light in the probe. The probe is then brought forward to gently touch the cornea. Particular attention and care must be taken to ensure that the probe is not indenting the cornea. The probe is moved slightly up and down or to the side to optimize the echospikes displayed on the machine. Either the operator or the machine selects the optimum pattern and the reading is obtained. The immersion technique is performed with the patient in the supine position. Topical anesthetic is instilled and a proper scleral shell is chosen. The 20 mm shell fits most eyes. The flared edges of the scleral shell are placed between the lids and

Good A-scan. Echos from left to right : cornea, anterior lens capsule, posterior lens capsule, retina, sclera, orbital fat

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the cup is filled with fluid, preferably gonioscopic solution. The ultrasound probe is immersed in the solution but kept 5-10 mm away from the cornea. The patient is asked to look with the fellow eye at a fixation point on the ceiling. The probe is then gently moved till it is aligned with the optical axis of the eye and the a-scan echogram on the panel is adequate. The reading is then taken.

The contact technique usually yields shorter measurement than the immersion technique for various reasons. Most modern biometers calculate the axial length based on separate sound velocities for different eye components (cornea, anterior chamber, lens, vitreous cavity).

It is recommended that measurements be repeated if the a. b. c. Measured axial length is less than 22.0 mm or more than 25.0mm Difference between the two eyes is more than 0.5mm. Axial length value seems wrong when compared with refraction.

All measurements should be repeated if following exist: a. Calculated emmetropic implant power is more than 3D from the average for the specific lens style used. b. Difference in emmetropic implant power between the two eyes is more than 1D.

A new device, the IOL Master, yields accurate axial length measurements using optical coherence techniques. The A constant Formulae in use currently utilize constants, which are based on various factors that affect the refractive state of the eye post-operatively. The Binkhorst and the Hoffer formulae use the post-operative AC depth, the SRK II and SRK-T formulae use the A-constant and the Holladay formula uses the S-factor. The A- constant encompasses multiple variables including the implant manufacturer, implant style, surgeon's technique, implant placement within the eye, and measuring equipment. Because of its simplicity, the A constant has become the value by which an implant is characterized. The most common A constants used are-

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! ! !

Anterior chamber lenses

- 115.0-115.3

Posterior chamber lenses in the sulcus - 115.9-117.2 Posterior chamber lenses in the bag - 117.5-118.8

In most cases the power of the IOL for emmetropia varies in a 1:1 relationship with the A constant.

The S-factor used in the Holladay formula is the distance between the iris plane and the IOL optic plane. The S-factor should be personalized by solving the formula in reverse. A change in the true post-operative AC depth will affect the refractive status of the eye. A change in 1 mm causes a 1.5 D change in the final refraction. Hence, these constants must be personalized to accommodate any consistent shift that might affect IOL power calculation. Each constant has to be back calculated for at least 20 cases, with care to ensure that the same person takes the measurements.

SPECIAL CASES Intumescent cataracts will yield a 0.15 mm longer axial length resulting in a +0.4 -+0.5 hyperopia postoperatively. For aphakic eyes being planned for ACIOL or scleral fixated IOL, the appropriate A constant must be used and the mode of the machine changed to compensate for the change in speed of the sound waves. In eyes with silicone filled vitreous, the sensitivity of the system should be increased to visualize the retinal echospike and the components of the eye must be measured separately to reach an accurate result. The usage of a standard sound velocity can lead to an error of upto 8 mm in such eyes. Usually a factor of 0.72 gives a rough estimate of the IOL power. It is better to refer the patient to a centre capable of separate measurements for more accurate assessment. After corneal refractive surgery, the K reading may not truly reflect the corneal power. Hence the refractive history method or the contact lens method must be used to obtain corrected K value. In eyes with high myopia, a B-scan examination is recommended to rule out a posterior staphyloma or other retinal pathologies. Identification of the posterior pole may be difficult. The problems are compounded in unilateral cases. While selecting the IOL power for a myope several factors are to be kept in mind. The surgeon should aim for a -0.50 D to -1.00 D 25

postoperative refraction as most sedentary elderly will prefer being near sighted. In the presence of monocular cataract in a myopic eye when the other eye is emmetropic, emmetropia should be aimed for if the myopia was induced by the cataract. However, if the patient has been functioning with monocular vision using the emmetropic eye for distance and the myopic eye for near, it is better to leave the operative eye myopic. In patients with hypermetropia the aim should be to achieve emmetropia. Here, the use of linear formulae can result in large errors in IOL power calculation in small eyes. In children, it is wise practice to remove the cataract and use contact lens correction if the surgery is being performed within the first two years of life, because growth of the eye will result in a large myopic shift if IOL has been implanted with intraoperative K and axial length measurements. When surgery is being performed after the age of two years, a myopic shift of 4-6 D is expected depending upon the age. Undercorrecting the IOL power by around 3 D partially compensates for this. A greater undercorrection can lead to anisometropia and difficulty in amblyopia correction. Residual myopia in adulthood can easily be corrected by spectacles, contact lenses or refractive surgery. As expected, biometry in children is difficult and may require general anesthesia. Postoperative refraction (R) for a given IOL power (I) can be computed as given below: · · · · For P less than 14.00 R = P-I

For P greater than 14.00 R = (P-I)/1.25 For P less than 14.00 I = P-R

To calculate the IOL power which would produce a given refraction:

For P greater than 14.00 I = P - (R x 1.25)

Choice of IOL Power The following factors should be considered:· · · · The refraction and presence/absence of cataract in the fellow eye. Relevance of emmetropia, isometropia & iseikonia. Lifestyle of patient: active patients prefer near emmetropia; sedentary patients may prefer myopia. Hedging: it has been found from experience that it is preferable to hedge towards myopia.

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It is important to remember that a myopic patient would be very unhappy if he is made hypermetropic. Also, the final refraction results may be +/- 1D either way from the calculated power. IOL DESIGN FEATURES: A variety of design features incorporated in modern IOLs make them very safe and reduce adverse phenomena and late complications after cataract surgery. The modern modified C-loop design ensures maintenance of centration and the square edge design significantly retards the opacification of the posterior capsule. Plate haptic lens manufacturing has improved and now lenses with a very good surface can be fashioned. Various modifications of the edge have been tried to reduce glare and improve contrast sensitivity. A recent development has been the introduction of multi-focal lenses which are designed to give three zones (distance, intermediate and near) of clear vision. Still in the research stage are accommodative lenses which mimic the change in refractive status of the natural lens with accommodation. IOL MATERIALS: IOL materials PMMA Advantages High optical quality Large optical centre Proven biocompatibility Possibility of surface modification Good laser resistance Foldable Controlled unfolding Good laser resistance Good biocompatibility Good optical quality Good laser resistance Good biocompatibility Good optical quality Easy handling Good biocompatibility Less CME Disadvantages Large incision wound Not autoclavable Mild foreign body reaction

Soft acrylic

Hydrogel

Limited experience Possible damage during implantation Sticky surface can adhere to instruments Lack of long term experience

Silicone

Irreversible adherence to silicone oil Can tear Slippery when wet Limited control during implantation ?Long term discoloration

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IOL Calculation

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