Biometry

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2.The distensible tip helps to separate the corneal reflection from the signal sent out from the front surface of the transducer, i.e. it makes it more accurate to determine exactly where the front surface of the cornea is, and when it is not in direct contact with the transducer.

Keratometric Measurements

The keratometric measurements can be done through a keratometer or through an autokeratometer. Many biometers (Fig. 4.2) have provision for connecting the autokeratometer to their computer so that once the keratometer reading is taken automatically, the value is entered into the biometer, and one does not have to feed it in again.

IOL Formula

There are two major categories of IOL formulae.

FIGURE 4.2 Biometer

Theoretical Formula

Introduction

This formula is based on an optical model of the eye. An optics equation is solved to determine the IOL power needed to focus light from a distant object onto the retina. In the different formulae, different assumptions are made about the refractive index of the cornea, the distance of the cornea to the IOL, the distance of the IOL to the retina as well as other factors. These are called theoretical formulae because they are based on a theoretical optical model of the eye. All of these theoretical equations make simplifying assumptions about the optics of the eye, and hence, provide a good (but not perfect) prediction of IOL power.

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The most popular formula in this group is the Binkhorst formula. This is based on sound theory. All the theoretical formulae can be algebrically transformed into the following

P=[N/(L−C)]−[NK/(N−KC)]

where,

P = Dioptric power of the lens for emmetropia, N = Aqueous and vitreous refractive index,

L = Axial length (mm)

C = Estimated postoperative anterior chamber depth (mm), and K = Corneal curvature [D].

Binkhorst Formula

Binkhorst has made a correction in his formula for surgically induced flattening of the cornea, using a corneal index of refraction of 1.333. Binkhorst also corrects for the thickness of the lens implant by subtracting approximately 0.05 mm from the measured axial length. Thus with the Binkhorst formula, 0.25 mm is added to the measured axial length to account for the distance between the vitreoretinal interface and the photoreceptor layer, and 0.05 mm is subtracted for lens thickness, resulting in a net addition of 0.20 mm to the measured axial length. The Binkhorst’s formula is:

D=1336 (4r−a)/(a−d) (4r−d)

where,

D = Dioptric power of IOL in aqueous humor, 1336 = Index of refraction of vitreous and aqueous,

r = Radius of curvature of the anterior surface of the cornea, a = Axial length of the globe (mm), and

d = Distance between the anterior cornea and the IOL.

Disadvantages

The problem in the theoretical formula is in the axial length measurement. The reason why it is difficult to measure the axial length accurately is that one must know the exact velocities of the ultrasound as it travels through the various structures of the eye. Because of the variation of the acoustic density of a cataract, these velocities cannot be known exactly. As a result, when cataractous lenses are much more acoustically dense than the average lens, the sound wave will move more rapidly through the lens and return to the transducer much more quickly than would have been expected for a given axial length. As a result of the velocity error, the eyes appear to be shorter. The formula consequently calculates an IOL power for an axial length which is too short. The patient then becomes overminused (too myopic). Theoretical formulae help the surgeon to anticipate what should result, not what will result from implantation.

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Regression Formula (Empirical formula)

Introduction

The regression formulae or empirical formulae are derived from empirical data and are based on retrospective analysis of postoperative refraction after IOL implantation. The results of a large number of IOL implantations are plotted with respect to the corneal power, axial length of the eye, and emmetropic IOL power. The best-fit equation is then determined by the statistical procedure of regression analysis of the data. Unlike the theoretical formulae, no assumptions are made about the optics of the eye. These regression equations are only as good as the accuracy of the data used to derive them.

Advantages

Implant power calculations can be made much more accurately through the use of regression formulae that are based on the analysis of the actual results of many uncomplicated IOL implantations in previous cataract surgeries. Since regression analysis is based on the results of actual operations, it includes the vagaries of the eye and measuring devices, vagaries that theoretical formulae attempt to address with correction factors.

Sanders-Retzlaff-Kraff (SRK) Formula

The most popular regression formula is the SRK formula which was developed by Sanders, Retzlaff and Kraff in 1980. This is

P=A−2.5 L−0.9 K

where,

P = Implant power to produce emmetropia, L = Axial length (mm),

K = Average keratometer reading, and

A = Specific constant for each lens type and manufacture.

The SRK formula calculates the IOL power by linearly regressing the results of previous implants. As this is a linear formula, it will underestimate the power of high-powered lenses and it will overestimate the power of the low-powered lenses compared to the theoretical calculation. For example, if the Binkhorst formula predicts that a 28-diopter lens should be used, the SRK formula will predict that a 26-diopter lens should be used. In lenses with low power, if the Binkhorst formula predicts that a 10-diopter lens is necessary, the SRK will predict that a 12-diopter lens should be used.

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Relation of Equipment to Specific Formulae

Most of the instruments calculate the desired power for the IOL at least by three different methods including a regression formula and a theoretical formula. It is the responsibility of the doctor to select which of the formulae he or she wants to use. Rarely, between 18 and 22 diopters, is there a significant difference between the calculated lens powers. But outside this range, there will be a progressive increase in difference between that determined by the theoretical formula and the one calculated by the regression formula. Since the regression formula has turned out to be statistically more accurate, 5 percent at these extremes, it is presently more reliable than the theoretical formulae. The manufacturers vary as to which programs they provide. One should anyway make sure that both the regression and theoretical formulae are included so that one has the opportunity to personally select the most reliable technique for one’s surgery.

Targeting IOL Postoperative Refraction

The question that comes to one’s mind next is “How to predetermine what postoperative refraction the patient should have?” This is the one parameter which the doctor has to decide upon and feed into the computer. The other parameters like axial length, etc. we have no control over. The answer depends on whether we are doing a monocular or binocular correction.

Monocular Correction

If we are considering only one eye (i.e. if the other eye has cataract or is amblyopic), targeting the postoperative refraction for approximately −1.00 diopter is probably the best choice. This is usually best because most people have visual needs for both distance and near. This means that the patient wants to be able to drive and to read without wearing glasses. If we target the postoperative refraction to −1.00D, it will allow the patient to perform most tasks with no glasses. At times, when they need finer acuity, they can wear regular bifocals, which will correct them for distance and near.

The second reason for targeting the postoperative refraction to −1.00D is that statistically, between 70 percent and 90 percent of the patients will fall within +1.00D error of the desired postoperative refraction. The errors, as mentioned earlier are due to our inexact measurements. Therefore, the patient will fall between piano and −2.00D 90 percent of the time. This will assure most patients of useful vision without glasses. Hence, the error of the ultrasound is best handled by choosing the postoperative refraction to −1.00D. If we would target for piano, then 90 percent of the patients will be between −1.00 and +1.00D. When the patient’s refraction is on the +1 side he or she has no useful vision at any distance because he or she is hyperopic and does not have the ability to accommodate. Consequently, because it is very undesirable to have a hyperopic correction, targeting for −1.00D not only optimizes the best vision at all distances, but

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also minimizes the chance for hyperopia that can result from inaccurate ultrasonic measurements.

Binocular Correction

When the vision in the other eye is good, its refraction must be considered for binocular vision. One overriding rule when prescribing glasses is that one should never prescribe spectacles which gives the patient a difference in the power between the right and left lens greater than 3D. The reason for this is that even though the patient may have 6/6 vision in primary gaze, when the patient looks up or down, the induced vertical prism difference in the two eyes is so great that it will create double vision. In a patient who has good vision in the nonoperative eye, one must target the IOL power for a refraction within 2D of his or her present prescription in the nonoperative eye. Two dipoters, not three, due to our 1D A-scan variability. For example, if we have a patient who is hyperopic and has +5D correction in each eye, we cannot target the IOL for a postoperative refraction of −1.00D because this would produce a 6D difference between the two lenses resulting in double vision. We must therefore select the IOL power to obtain a refraction which is approximately 2D less than the nonoperative eye. Consequently, on our patient who is +5D in both eyes, we should target the postoperative refraction in the eye with the cataract for +3D, so that there is a 90 percent probability that there will be less than a 3D difference.

In contrast, if the patient were highly myopic in each eye, for example, −10D in both eyes, we should target the IOL power to produce refraction of approximately −8D. Again, we have limited the difference in the spectacles lenses to a 2D difference in the final prescription. Again, target, for a 2D difference not a 3D, because there is approximately a 1D tolerance in the accuracy of the ultrasonic measurement.

If the operation on the second eye is to be done shortly after the first, the preoperative spectacles refraction can be ignored, and the patient is treated as if he or she were monocular.

Factors Affecting Accuracy of IOL Power Calculation

Many factors can affect the accuracy of the power of the IOL calculated.

Keratometry

Keratometers only measure the radius of curvature of the anterior corneal surface. This measurement must be converted to an estimate of the refracting power of the cornea in diopters, using a fictitious index (the true corneal refractive index of 1.376 could be used only if both the anterior and posterior corneal radii of curvature were known). The variability can alter calculated corneal dioptric power by 0.7D.

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Axial Length Measurement

As explained earlier, indentation of the cornea by the A-scan instrument tip can alter the axial length affecting the accuracy of the power of the IOL.

Axial Length Correction Factor

The distance from the vitreoretinal interface to the photoreceptor layer has been estimated to be about 0.15 to 0.5 mm. This distance can affect the accuracy of the IOL power calculated.

Site of Loop Implantation

Posterior chamber IOLs may be implanted with both loops in the ciliary sulcus or in the capsular bag, or with one loop in the sulcus and one loop in the capsular bag. Positioning the implants within the capsular bag places the implant further back in the eye and decreases the effective power of the lens. There is usually a 0.5 to 1.5D loss of effectivity by placing the implant in the capsular bag as opposed to the ciliary sulcus. A higher power lens should therefore be used when the implant is placed in the capsular bag.

Orientation of Planoconvex Implants

Some surgeons implant planoconvex posterior chamber lenses with the piano surface forward.

FIGURE 4.3 Ultrasonic reading in dense cataract

Such flipping of the implant decreases the effective power of the lens by 0.75D even if the position of the lens is unchanged. An additional 0.5D loss of effectivity occurs because the principal plane of the lens is usually displaced further back into the eye. Thus, a total loss in effectivity of 1.25D is expected by turning the lens around.

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Postoperative Change in Corneal Curvature

Suturing of a cataract incision has a tendency to steepen the vertical meridian. These changes affect the postoperative refraction of the patient.

Density of the Cataract

The density of the cataract also makes a difference. In a dense cataract (Fig. 4.3), the ultrasonic waves travel faster whereas in an early cataract (Fig. 4.4) the ultrasonic waves travel slower.

IOL Tilt and Decentration

When a lens is tilted, its effective power increases and plus cylinder astigmatism is induced about the axis of the lens tilt. The tilting of the lens occurs if one loop is in the capsular bag and the other in the sulcus (Fig. 4.5). Alternatively, residual cortex being left behind can cause an inflammatory response which causes contraction and pulling unequally on parts of the loops and the optic.

FIGURE 4.4 Ultrasonic reading in early cataract

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FIGURE 4.5 Captive iris syndrome

Pseudophakic Lasik

If a patient has had a wrong biometry then the solution can be to remove the IOL and replace it with a correct powered IOL. Another alternative is to perform LASIK and correct the problem. Figure 4.6

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FIGURE 4.6 Topograph of a patient in whom a wrong power IOL was implanted

FIGURE 4.7 Topograph of the same patient as in Figure 4.6 after LASIK

is the topograph before LASIK of a patient who had a power of −10.0 dioptres after IOL implantation. The patient was referred to us and we did a LASIK as the patient was operated a year back, We felt that the IOL might be fixed firmly in the bag. Figure 4.7 is the topograph after LASIK.

5

IOL Power Calculation After Corneal

Refractive Surgery

Jairo E Hoyos

Melania Cigales

J Hoyos-Chacón

Corneal refractive surgery corrects refractive errors by modifying the anterior surface of the cornea. The problem appears years later when, as a result of the normal aging process, these patients develop cataracts and require lens extraction surgery and intraocular lens (IOL) implantation. Figure 5.1 shows one eye with cataract surgery after corneal refractive surgery. The question arises about which should be the basis for calculating this lens implant. Being able to determine the accurate power of the IOL to be implanted in a patient undergoing cataract surgery is a big challenge, even more so when the patient has had prior refractive surgery. A patient with prior refractive surgery is very special both medically and psychologically because he or she will not want to use glasses permanently after having spent time without them.

FIGURE 5.1 (Hoyos). High myope patient operated on with “in situ” queratomileusis and radial keratotomy for the residual error, who developed cataract 10 years later. A phacoemulsification technique was

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performed and an IOL foldable was implanted

Formulas for calculating the IOL power include multiple variables, but with the current standardized cataract surgery techniques, these are reduced to only three: lens constant, corneal diopter power and axial length of the eye. The lens constant is standard for each lens model; refractive corneal surgery does not change axial length; however, postrefractive surgery produces a significant change in corneal curvature. At present, the best system for measuring corneal curvature is computerized corneal topography, although this method overestimates the central diopter power of a flattened cornea resulting from corneal refractive surgery. Consequently, the IOL power calculation will depend primarily on how accurately the cornea’s central refractive power is calculated.

In this chapter we will analyze the information from the patient’s refractive surgery history, the current systems for measuring corneal power and the proposed methods for patients with previous corneal refractive surgery. In addition to this, the scan measurement of axial length and the IOL power calculation formulas we also be reviewed.

Ocular History

The refractive success of cataract surgery in eyes with prior refractive corneal surgery will depend mainly on the ability to calculate the current keratometric power of the cornea accurately. This requires knowledge of the patient’s ocular history before and after refractive surgery, as well as of the current ocular status.

a.The history prior to refractive surgery will provide information about refraction, corneal power and axial length before refractive surgery.

b.The postrefractive surgery history will provide information about stable refraction obtained following the procedure.

c.The current ocular history must include biometry and computerized corneal topography besides the complete ophthalmological examination.

Keratometric Readings

The refractive change of the anterior surface of the cornea as measured by topographic and keratometric readings has been and continues to be the basis for a large number of surgical techniques designed to correct refractive errors. Everything started with the early observations by Christopher Scheiner in 1619, based on the reflection of the grid of windowpanes on the cornea, which later led to the use of simple versatile keratoscopy and keratometry. Until the development of the modern computerized equipment for corneal topography, ophthalmologists used different instruments for studying the anterior corneal surface whose sensitivity and specificity have improved with time and as a result of technological breakthroughs.1–3

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Corneal Power Calculation Methods

The next challenge for the surgeon is to determine the keratometry to be used for calculating the intraocular lens power. The main methods which are normally used to calculate corneal power after refractive surgery are: the calculation method or refractive history method;4 the hard contact lens method5; and corneal topography.

A.Calculation method or refractive history method: This is the most accurate method and requires knowing three parameters: K-readings and refraction before the keratorefractive procedure, and stabilized postoperative refraction (before any myopic shifts from nuclear cataract occur). The main concept is to subtract the refractive change on the corneal plane due to the keratorefractive procedure, from the original K- readings before the procedure, to arrive at a calculated postoperative K-reading.

Step I: Calculate the spheroequivalent refraction for refraction on the corneal plane (SEQc) on the basis of the spheroequivalent refraction on the spectacle plane (SEQs) at a given vertex.

Step II: Calculate the change in refraction on the corneal plane, where refraction change is equal to preoperative SEQc minus postoperative SEQc.

Step III: Determine the calculated postoperative corneal refractive power, where mean postoperative K is equal to the mean preoperative K minus the change in refraction on the corneal plane. The value obtained is the calculated central power of the cornea following the keratorefractive procedure.

Once the preand postrefractive surgery refraction (spherical equivalent) is known, the refractive change induced on the corneal plane is determined and subtracted from the K- reading present before the corneal refractive surgery. Refraction must be measured on the corneal plane. For this purpose, distance-to-vertex conversion tables or the following formula may be used: Rc=Rg/[1−(d×Rg)], where Rc is the refraction on the corneal plane, Rg is the refraction on the spectacle plane and d is the distance to the vertex in meters (0,012).

It is important to determine the stable residual refraction in patients who have undergone prior refractive surgery since it may change with time as a result of cataractinduced myopization of the index or due to an increased axial length in some cases.

B.Trial hard contact lens method: This method requires a piano hard contact lens with a known base curve and a patient with a cataract, which allows us to see the retinoscopy, shadows during the refraction.

Step I: The patient’s spherical equivalent refraction is determined by normal refraction.

Step II: The refraction is repeated with a hard contact lens in place. If the spheroequivalent refraction does not change with the contact lens, the

patient’s cornea must have the same power as the base curve of the piano contact lens (Fig. 5.3A). If there is a hyperopic shift in the refraction, then the base curve of the contact lens is weaker than the cornea by the shift amount (Fig. 5.3B). If the patient has a myopic shift, then the base curve of the contact lens is stronger than the cornea by the shift amount (Fig. 5.3C).

Step III: The appropriate algebraic formula for each case is then made, taking into account that spheroequivalent values greater than ±4.00 D must be converted to corneal plane. This method is limited by the accuracy of the

FIGURE 5.3 Trial hard contact lens method: A If the spheroequivalent refraction does not change with the contact lens, the power of the patient’s cornea must be the same as the base curve of the planocontact lens. B When there is a hyperopic shift in the refraction, then the base curve of the contact lens is weaker than the cornea by the amount of the shift. C If the patient exhibits a myopic shift, then the base curve of the contact lens is stronger than the cornea by the amount of the shift

refraction, which is in turn limited by the amount and type of the cataract, and it requires good visual acuity of 20/80 or better.

C.Corneal topography method: Corneal topography allows for an accurate determination of the anterior surface of the cornea. For this method, the central corneal power following keratorefractive surgery must be known. This one, like the keratometry

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method, is very inaccurate. Current topographers do not allow us to determine an objective central K-reading and only provide Sim K and Min K values which are always overestimated in patients with prior surgery for myopia and underestimated in cases of hyperopic surgery. If those keratometric values were used for calculating the intraocular lens, postoperative results would be very far from ametropia (Fig. 5.4).

Biometry

A-mode ultrasound biometry enables us to measure the eye’s visual axis through echo generation representing the reflection of the ultrasound beams on the different interfaces of the ocular tissues. Corneal ablation used for correcting the refractive

FIGURE 5.4 IOL power calculation using different corneal powers: Sim K, Min K, Calculated K and Central K

error is minimal (less than 150 mµ) and has negligible effects in terms of modifying the axial length after refractive surgery.6 Care must be taken when performing biometry in order to avoid errors caused by the loss of alignment with the optic axis, and excessive pressure of the transducer on the cornea which leads to short readings of the axial length (especially in eyes with a thinned-out cornea). It is also important to ensure that a standard deviation of less than 0.1 is obtained for several measurements. In high myopia, the presence of a posterior staphyloma may give rise to inconsistent recordings, but this may be avoided by asking the patient to fix on the transducer light.7,8

During biometry, it is important to ensure that the transducer is applied without indenting the cornea, which may be significantly thinner in high myopes as a result of the refractive technique. A 1 mm error in the axial length determination results in a 3-diopter error, while a 1 mm error in the preoperative determination of the anterior chamber depth will induce a refractive error of 1.4 diopters. Consequently, we recommend using the biometer in automatic mode with five measurements and a standard deviation of less than 0.1. Also, whenever possible, the patient must be made to fix on the transducer light.

In our patients, whenever we find variations between the current axial length and the one existing before refractive surgery, we always use the higher value because of the

possibility of a post-operative increase of the axial length with increased myopia, or because of the possibility of introducing a biometric error as a result of indenting a thin cornea.

IOL Power Calculation Formulas

A number of theoretical and regression formulas are used. It is generally accepted that both theoretical and regression-derived formulas perform well for eyes of average axial lengths between 22 mm and 24.5 mm. Even in extremely long eyes of more than 28.4 mm in length, excellent accuracy can be achieved with the Holladay and the SRK-T formulas. Regression formulas such as the SRK II, which perform very well in eyes of usual length, should not be used in these extremely long eyes.9 Hoffer10 recommends using his own formula for eyes with axial lengths of less than 22 mm, and the SRK-T and Holladay for axial lengths greater than 24.5 mm.

Koch11 discovered that the modern theoretical formulas were far better than regression formulas when evaluating IOL power for radial keratotomy eyes. Therefore, the first thing to do to improve accuracy is to refrain from using a regression formula (SRK, SRK II, etc) to calculate the IOL power for these eyes. A modern third-generation theoretical formula such as the Holladay, the Hoffer Q or the SRK-T should be used; their accuracy is further improved when they are individualized.

It is very important to use third-generation formulas, which take into consideration axial length and anterior chamber depth (e.g. Holladay, SRK-T, or Hoffer-Q) since the old ones like SRK I, SRK II and Binkhorst may give rise to significant errors. Accurate power calculation constants are absolutely essential for the effective implant power calculation; the SRK II formula utilizes A-constants, the SRK-T theoretical formula utilizes either A constants or ACD constants, the Holladay formula utilizes the Surgeon Factor (SF) constant, which is an offset of the ACD constant. Manufacturer published constants are first determined for specific IOLs by the manufacturer through closed studies or are estimated by comparison with existing IOLs. Each surgeon should become familiar with, and use the same type of lens as long as the operative conditions allow it,

because that would allow the surgeon to determine personal variations and establish a personal constant (A, ACD or SF).12,13

For calculating the power of the intraocular lens in high myopia, it is better to aim for −0.75D, because we have noticed that these patients benefit significantly from their near vision. In many of our patients, despite using an intraocular lens calculated to induce a myopia of −0.75 to −1.00, we have found that there is a minor residual defect or a tendency to emetropization; this observation should be taken into account because the tendency to hyperopia is undesirable in these patients who, aside from having been myopes, have no accommodation.

FIGURE 5.5C Clinical case: Data 3 years after LASIK surgery when the eye shows cataract

topographic values were: Sim K [32.7D×55°/32.3 D×145°] (mean 32.50), Min K [32.1D×173°] and Central K [30.30D] (Fig. 5.5C).

Corneal Power Calculation

We used the refractive history method to calculate the corneal power.

1.Pre-LASIK refraction was −16/−4×170°. The spherical equivalent (SE) was −18 D and the same value towards the cornea was −14.75D.

2.Post-LASIK refraction was −0.50D.

3.Refractive change was the pre-LASIK SE (−14.75) minus the post-LASIK SE (−0.50), that means −14.25D.

4.Preoperative keratometry average was 44.35D (Sim K [46.3D×82°/42.4D×172°]).

5.Calculated corneal power was the preoperative keratometry average (44.35D) minus the refractive change induced by LASIK (−14.25), that equals 30.1D.

Axial Length

There are two axial length values for this patient: 31.9 mm before LASIK and 31.06 mm before cataract surgery the difference being 0.84 mm. The reduction in axial length is attributed to corneal indentation, which may occur during measurement despite every precaution, especially in very high myopes with thin corneas resulting from refractive surgery. As mentioned previously, axial length does not change statistically Therefore, the axial length selected in this patient was the preoperative axial length of 31.9 mm.

IOL Power Calculation

The IOL power calculation, using the SRK-T formula, for an IOL with A constant of 118.8 and a −0.75D of desired refraction, was +16.50D.

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Result

Uncorrected visual acuity three months after cataract surgery was 20/25 and pianorefraction. (Fig. 5.5D).

Summary

Refractive corneal surgery induces changes on the corneal surface and, consequently, in topographic

FIGURE 5.5D Clinical case:

Postcataract surgery data

readings. After refractive corneal surgery, corneal power may be calculated using the data of the patient’s refractive history.

Comparisons between the keratometric values obtained with the refractive history method (“calculated K”) and the topographic data, allow us to conclude that the central keratometric value shown by topography (“central K”) is the closest to the calculated value. Sim K and Min K values correspond to rings 6, 7 and 8 on the outer limit of the three central millimeters of the corneal fixation center. For this reason, they cannot reflect the true value of the central cornea or, in other words, the keratometric value of the center of the cornea. Therefore, in calculating the IOL, these topographic values would lead to an undercorrected power calculation, thus inducing a final hyperopic refractive error.

Although the calculation method will continue to be used until the computerized system of modern videokeratoscopes allows us to obtain objective central keratometric values, topographic “central K” may be useful when the refractive history is not available.

We recommend recording refraction values pre-and postrefractive surgery, corneal topography values and axial length measurements in all patients who will be subjected to corneal refractive surgery, in order to facilitate calculation of intraocular lens power in the future, when the patient requires cataract surgery.

As discussed previously, third-generation formulas (SRK-T, Holladay and Hoffer Q) are much more accurate than previous formulas for the more unusual eyes. Old formulas such as SRK, SRK-II and Binkhorst should not be used in these patients (Fig. 5.6). None

of these formulas provide the desired result if the central corneal power is measured incorrectly.

We generally use phacoemulsif ication bi-manual technique with foldable intraocular lens implantation through a 3.5 mm clear cornea incision. On the first day following cataract surgery, patients usually exhibit a hyperopic shift primarily due to transient postoperative corneal edema and intraocular pressure changes. These patients also exhibit

FIGURE 5.6 IOL power calculation using different formulas: SRK, SRKII, SRK-T, Holladay, Binkhorst

the same daily fluctuation during the early postoperative period after cataract surgery. Because refractive changes are expected and vary significantly among patients, no lens exchange should be considered until after the first postoperative week or until after the refraction has stabilized, whichever is longer.

References

1.Rabinowitz YS, Wilson SE, Klyce SD: Color atlas of corneal topography: Interpreting Videokeratography (1st ed). New York: Igaku-Shoin Medical Publishers Inc, 115, 1993.

2.Sanders DR, Koch DD et al: Atlas of Corneal Topography. (1st ed). Thorofare, NJ: Slack Incorporated, 209, 1993.

3.Klyce SD, Dingeldein SA: The topography of normal corneas. Arch Ophthalmol 107:512–18, 1989.

4.Holladay JT: IOL calculations following radial keratotomy surgery. Refractive & Corneal Surg (Question & Answer) 5(3): 36A, 1989.

5.Hoffer KJ: Intraocular lens power calculation for eyes after refractive keratotomy. J Refract Surg 11:490–3, 1995.

6.Hoffer K: Accuracy of intraocular ultrasound lens calculation. Arch Ophthalmol 99:1819–23, 1981.

7.Olsen, Thim, Cory don: Accuracy of the newer generation IOL power calculation formulas in long and short eyes. J Cataract Refract Surg 17:187–93, 1991.

8.Steele CE, Crabb DP, Edgar DF: Effects of different ocular fixation conditions on A-Scan ultrasound biometry measurements. Ophthal Physiol Opt 12:491–5, 1992.

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9.Retzlaff J, Sanders DR, Kraff MC: Development of the SRK/T intraocular lens implant power calculation formula. J Cataract Refractive Surg 16(3):333–40, 1990.

10.Hoffer KJ: The Hoffer Q formula: a comparison of theoretical and regression formulas. J Cataract Refract Surg 19:700–12, 1993.

11.Koch DD, Liu JF, Hyde LL et al: Refractive complications of cataract surgery after radial keratotomy. Am J Ophthalmol, 108:676–82, 1989.

12.Holladay JT, Praeger TC, Chandler TY, Musgrove KH: A three-part system for refining intraocular lens power calculations. J Cataract Refract Surg 14:17–24, 1988.

13.Retzaff J, Sanders DR, Kraff MC: A manual of implant Power Calculation: Medfort, Oregon. Retzlaff, Sanders and Kraff, 1982, 1985, 1988.