IOL power calculation following keratorefractive surgery

M. Ghoreishi, MDIsfahan University of Medical

Sciences

case• A 56-year-old woman with LASIK for -11 D• Residual refraction: - 0.50D• Preop mean K : 44.5D • Postop mean K: 35.5 D• AL by A-scan: 30.1 mm• 13 D IOL implanted base on SRK/T formula• Postop refraction : +4.00 D•

IOL power calculations following keratorefractive surgery should not be carried out using standard keratometry combined with any one of several popular 2-variable third generation theoretic formulas, such as SRK/T, Holliday 1 and Hoffer Q without a special correction

Sources of errors• keratometric measurement• Calculating formula

FormulasInappropriate

K-readingsinaccurate

CAUTION

Determining Corneal Power after RK, PRK and LASIK

• The true corneal power following RK, ALK, PRK and LASIK is difficult to measure by any form of direct measurement, such as keratometry, or corneal topography

• Keratometry and topography assume a normal relationship between the anterior and posterior corneal curvatures, and measure the anterior corneal radius.

keratometric measurement errors

• Standard keratometry measures an intermediate area and extrapolates the central power based on some very broad assumptions.

• For this reason, keratometry, autokeratometry and simulated keratometry by topography will typically over-estimate central corneal power, following keratorefractive surgery for myopia.

• Failure to keep this important fact in mind will often result in an unexpected and unpleasant post-operative hyperopic surprise.

• Until recently, our attention for the post-keratorefractive eye has mainly been focused on accurately determining the central corneal power.

• It is now becoming more widely understood that a flattened central cornea not only renders keratometry inaccurate, but also causes problems with many IOL power calculation formulas that were previously old and trusted friends.

Variations within the IOL Power Calculation Formulas

• Third generation, 2-variable formulas such as SRK/T, assume that the anterior and posterior segments of the eye are mostly proportional and use a combination of axial length, and keratometric corneal power, to estimate the postoperative location of the IOL, known as the effective lens position (ELP).

• If the central corneal power is very low, as we see following keratorefractive surgery, the formula assumes that the anterior chamber is shallow.

Variations within the IOL Power Calculation Formulas

ELP

A major shortcoming of most 3rd generation, 2-variable formulas, such as SRK/T, is that they often assume that the anterior and posterior segments of the eye are mostly proportional and use only the axial length and keratometric corneal power to estimate the postoperative location of the IOL, known as the effective lens position (ELP).

Errors related to calculating formula

Calculating formula• Unless a specific correction is made for this situation,

the artifact of centrally flattened Ks following keratorefractive surgery will have these formulas assume a falsely shallow post-operative ELP

• The end result is that without a special correction, following LASIK these formulas will typically recommend less IOL power than is actually required.

• This is a second, and little recognized, source of unanticipated post-operative hyperopia following keratorefractive surgery for myopia.

Importance of ELP • If a +21.00 D IOL placed within the capsular bag

produces emmetropia, with only a 0.50 mm posterior displacement, that same lens has an effective power of +20.00 D. If that same lens is moved anterior by only 0.50 mm, it then would have an effective power of +22.00 D.

• With a low central corneal power the formula makes the assumption that the IOL following cataract surgery will end up sitting closer to the cornea than normal and call for less power.

• The flatter the cornea, the bigger a problem this becomes.

• Unless a correction is made for this situation, the artifact of centrally flattened Ks following keratorefractive surgery will have these formulas incorrectly assume a falsely shallow post-operative ELP.

• The end result is that without a special correction, 2-variable formulas following LASIK, PRK and RK will recommend less IOL power than is actually required.

• This is a second, and little recognized, source of unanticipated post-operative hyperopia following keratorefractive surgery for myopia.

Importance of ELP

Sugested corrective methods

• Double K" Formula Corrections Formula• Clinical History Method • Contact Lens Method • Corneal Bypass Method • Feiz-Mannis Method • Haigis-L Formula• Holladay IOL Consultant • Latkany Method • Masket Method • Modified Masket Method • Topographic Central Power • Wang Koch Maloney Method • Zeiss IOLMaster • Oculus Pentacam • Post Keratorefractive online Calculators

• Post Hyperopic LASIK • Post Radial Keratotomy

Aramberri "Double K" method,

• Uses preop Ks to calculate ELP and postop Ks to calculate IOL power

• Suggests IOL Power Adjustment Tables for SRK/T, Hoffer Q and Holladay l Formula

The numbers in each row of the myopic refractive correction represent the amount in diopters that must be added to the calculated IOL power when using the SRK/T Formula.

MYOPIC AL (mm)

Correction (D) 20 22 24 26 28 30

2 0.7 0.7 0.7 0.6 0.5 0.3 3 1.0 1.0 1.1 1.0 0.8 0.6 4 1.3 1.4 1.4 1.3 1.1 0.8 5 1.7 1.7 1.8 1.6 1.4 1.1 6 2.0 2.0 2.1 2.0 1.7 1.4 7 2.3 2.4 2.5 2.3 2.0 1.7 8 2.6 2.7 2.8 2.6 2.3 2.0 9 2.9 3.0 3.2 3.0 2.7 2.3

10 3.2 3.3 3.5 3.3 3.0 2.6

Aramberri "Double K" method After MYOPIC Keratorefractive Surgery IOL Power Adjustment Table for SRK/T Formula.

The numbers in each row of the HYPEROPIC refractive correction represent the amount in diopters that must be subtracted from the calculated IOL power when using the SRK/T Formula If no number appears, it means that the formula is not applicable for that combination of axial length and refractive correction.

HYPEROPIC AL (mm)

Correction (D) 20 22 24 26 28 30

2 0.7 0.7 0.7 0.6 0.4 0.0 3 1.1 1.1 1.1 0.9 0.5 0.0 4 1.4 1.4 1.5 1.2 -- -- 5 1.8 1.8 1.9 1.7 -- -- 6 2.2 2.2 2.5 -- -- --

Aramberri "Double K" method After Hyperopic Keratorefractive Surgery IOL Power Adjustment Table for SRK/T Formula

The numbers in each row of the myopic refractive correction represent the amount in diopters that must be added to the calculated IOL power when using the Hoffer Q Formula

Myopia AL (mm)

Correction(D) 20 22 24 26 28 30

2 0.4 0.3 0.2 0.2 0.1 0.0 3 0.6 0.5 0.3 0.3 0.2 0.0 4 0.8 0.6 0.5 0.3 0.2 0.0 5 1.0 0.8 0.6 0.4 0.3 0.0 6 1.2 0.9 0.7 0.5 0.4 0.1 7 1.4 1.1 0.8 0.6 0.5 0.1 8 1.6 1.2 1.0 0.7 0.6 0.2 9 1.7 1.3 1.1 0.8 0.7 0.2

10 1.9 1.5 1.2 1.0 0.7 0.3

Aramberri "Double K" method After MYOPIC Keratorefractive Surgery IOL Power Adjustment Table for Hoffer Q Formula

Hperopia AL (mm)

Correction (D)20 22 24 26 28 30

2 0.4 0.3 0.2 0.1 0.1 0.0

3 0.7 0.5 0.3 0.2 0.0 0.0

4 0.9 0.6 0.4 0.2 0.0 0.0

5 1.1 0.7 0.4 0.2 0.0 0.0

6 1.3 0.9 0.5 0.2 0.0 0.0

The numbers in each row of the HYPEROPIC refractive correction represent the amount in diopters that must be subtracted from the calculated IOL power when using the Hoffer Q Formula. If no number appears, it means that the formula is not applicable for that combination of axial length and refractive correction.

Aramberri "Double K" method After Hyperopic Keratorefractive Surgery IOL Power Adjustment Table for Hoffer Q Formula

Clinical History Method

The Clinical History Method for corneal power estimation was first described by Holladay and later by Hoffer as:

Ka = Kp + Rp – Ra Ka = the estimate of the central corneal power after

surgery.Kp = the average keratometry power before surgeryRp = the spherical equivalent before keratorefractive

surgeryRa = the stable spherical equivalent after surgery

Example:• Ks before LASIK were 45.37 D / 46.00 D• The refraction before LASIK was -6.00 +0.75 x 135

(Corrected for an estimated vertex distance of 12 mm)

• The stable refraction after LASIK was -0.25 sphere then...

• Ka = Kp + Rp - Ra • Ka = (45.69) + (-5.27) - (-0.25) = 40.67 D

Clinical History Method

Contact lens method

• Following all forms of ablative keratorefractive surgery (LASIK, PRK, etc.) a review of the literature now suggests that the hard Contact Lens method may be less accurate than originally thought.

• For this reason it is no longer recommended in this clinical setting

Contact Lens Method

Corneal Bypass Method.• Described by Walter et al in the March, 2006 issue of the Journal

of Cataract and Refractive Surgery. • The advantage of this method is that it is done without having to

calculate the post-LASIK corneal power. • IOL power is calculated using the post-LASIK axial length and the

pre-LASIK keratometry• The target refraction is set for the pre-LASIK spherical equivalent.

This "bypasses" the post-LASIK corneal power. Example: If the pre-LASIK keratometry was 45.37 D / 46.00 D, the spherical equivalent before LASIK was -5.62 D, andthe post-LASIK IOLMaster axial length is 26.32 mm, thenthe IOL power for a -5.62 D refractive result would be +17.27 D

Feiz-Mannis Method

• when good historical data is available. • least likely to result in a post-operative hyperopic surprise. • IOL power is calculated using the pre-LASIK keratometry, as

though the patient had never undergone keratorefractive surgery. • No special formula correction that needs to be employed • Calculated pre-LASIK IOL power is then increased by the amount

of refractive change at the spectacle plane divided by 0.7

• Example:• If calculated IOL power before LASIK is +9.18 D, and the

change in refractive power at the spectacle plane is -5.38 D, then:

• IOLpre - (ΔD / 0.7) = IOLpost • +9.18 D - (-5.38 / 0.7) = +16.87 D

Latkany Method

• A postoperative regression method developed by Robert Litany, et al.

• Only requires that the surgeon know the refraction prior to LASIK.

• Using the flattest K and the SRK/T formula • The calculated IOL power is then adjusted by the

formula:

-(0.47 [Pre-refractive surgery SEq] + 0.85) • The final IOL power is rounded to the nearest 0.50 D

Masket Method

• A postoperative regression method developed by Samuel Masket• There appears to be a linear relationship between the spherical

equivalent of the total amount of the stable laser vision correction (LSE) and the over-estimation of central corneal power by simulated keratometry.

• This works following both myopic and hyperopic LASIK. • By this method, the IOL power is calculated using the Holladay 1 formula

for axial lengths greater than 23.0 mm and the Hoffer Q formula for axial lengths less than 23.0 mm.

• The SRK/T formula is generally not recommended here as the artifact of very flat Ks may sometimes result in an under-correction.

• The IOL power is calculated based on Ks provided by simulated keratometry.

• The calculated IOL power is then adjusted by the vertex distance corrected laser vision correction spherical equivalent determined at four to six months after the procedure, multiplied by -0.326 with 0.101 added to this product.

Example• (LSE x -0.326) + 0.101 = Post-LASIK IOL power adjustment Where... • LSE = The vertex distance corrected laser vision correction

spherical equivalent.• The Masket method for the eye following LASIK would be carried

out as follows: • If the calculated IOL power is +15.26 D for a -0.25 D result, and ... • The stable laser vision correction spherical equivalent is -5.38 D,

then ...• (LSE x -0.326) + 0.101 = Post-LASIK IOL power adjustment (-5.05

D x -0.326) + 0.101 = +1.75 D = IOL power adjustment +1.75 D + 15.26 D = 17.01 D = Final adjusted IOL power

Masket Method

Modified Masket Method:• The calculated IOL power is multiplied by -0.4385 and 0.0295

added to this product.

Wang - Koch - Maloney Method

Described by Robert Maloney and subsequently modified by Doug Koch and Li Wang. Requires no historical data and has been reported to have a low variance when used with

either the Holliday 2 formula or a 2-variable formula combined with an Aramberri double K method correction nomogram published by Koch and Wang.

The central corneal power is estimated by placing the cursor at the exact center of the Axial Map of the Zeiss Humphrey Atlas topographer. This value is then converted back to the anterior corneal power by multiplying the Axial Map central topographic corneal power by 376.0/337.5, which is the same as 1.114. An assumed posterior corneal power of 6.1 D is then subtracted from this product.

(CCP x 1.114) - 6.1 = Post-LASIK adjusted corneal power Where...

CCP = the corneal power with the cursor in the center of the Axial Map of the Zeiss Humphrey Atlas topographer.

The Modified Maloney method for the eye following LASIK is carried out as follows: If the Axial Map central corneal power is 40.91 D, then... (CCP x 1.114) - 6.1 = Post-LASIK adjusted corneal power (40.91 D x 1.114) - 6.1 =

39.47 D

Haigis-L Formula - Post Myopic LASIK

Using an IOLMaster corneal radius measured in mm (r

meas), the Haigis-L algorithm generates a corrected

corneal radius (r corr), which is then used by the

regular Haigis formula to calculate the IOL power

• The corrected radius is calculated as follows: 331.5 r corr = ---------------------------------------------------

5.1625 x r meas + 82.2603 - 0.35

Example:• If IOLMaster keratometry in mm following myopic LASIK is

8.82 mm x 090 and 8.63 mm x 180, the calculation of the corrected corneal radius would be as follows:

• Average corneal radius = (8.82 + 8.63)/2 = 8.73 mm = r meas

331.5

• r corr = --------------------------------------------------- = 9.0 mm 5.1625 x 8.73 + 82.2603 - 0.35

To calculate “D” from “r” use formula: D = 337.5/r D = 337.5 / 9.0 = 37.5 D

• Haigis formula does not tie the effective lens position estimation to the central corneal power. Actual ELP is estimated by direct measurement of ACD

• Haigis-L measures corrected central corneal power based on a regression-derived algorithm. If this corrected is then entered into the regular Haigis formula, it will do a good job with the calculation

• The Haigis-L formula is now available on IOLMaster software version 4.xx and higher.

Haigis and Haigis-L Formula

Holladay 2 Formula

• IOL power calculations following any form of keratorefractive surgery can be carried out using the Holladay 2 formula (contained within the Holladay IOL Consultant)

• If your office does not have this software package, a trial version can be downloaded from the Internet at: Holladay IOL Constant

• Rotating Scheimpflug camera, generating information in three dimensions.

• Measures both anterior and posterior corneal curvatures• During the two second measuring sequence, a total of

25,000 data points are generated which includes the center of the cornea.

• All keratometers and corneal topographers do not see the corneal center and are forced to extrapolate this information, which gives the Pentacam this unique advantage in terms of accuracy.

Pentacam

The Equivalent K Reading (EKR)

• The Equivalent K Reading feature of the Pentacam uses information from both the anterior and the posterior cornea to generate a range of central corneal power values in keratometric diopters.

• Depending on the EKR zone selected, this value can then be used with the Holladay 2 Formula for IOL power calculations following keratorefractive surgery, or with one of the popular 3rd generation, 2-variable formulas combined with an Aramberri "Double K" method

Pentacam Holladay Report

• This new feature was developed by Oculus together with Dr. Jack T. Holladay to improve the measurement of the central corneal power for patients who have undergone prior keratorefractive surgery.

• For both myopic and even hyperopic LASIK, the 4.5 mm EKR zone has been shown to have a high correlation with the central corneal power calculated by the clinical history method.

• However, depending on the type of ablation employed, some surgeons are finding a better correlation for myopic LASIK using a smaller EKR area, such as the 3.0 mm EKR zone

Pentacam Holladay Report

IOLMaster• Uses partial coherence interferometry to

measure the length of the eye (as opposed to standard ultrasound measurements).

• It has an accuracy of +/- 0.02 mm or better! This represents a fivefold increase in measurement accuracy when compared to standard A-scan ultrasound (which has a resolution of +/- 0.1 to 0.12 mm).

• Optical AL measurement• IOLMaster K's measure a smaller 2.5mm zone.• Incorporated Haigis L formula

Topographic Central Corneal Power Adjustment Method

• Originated by Doug Koch and Li Wang• Based on determining the central power of the cornea using either the Zeiss

Humphrey Atlas topographer or the adjusted effective refractive power (EffRPadj) of the Holladay Diagnostic Summary of the EyeSys Corneal Analysis System.

• The 1, 2, 3 and 4 mm power values of the Numerical View of the Zeiss Humphrey Atlas topographer are averaged together and used as the central corneal power value (Ccp).

• As an alternative, the adjusted effective refractive power (EffRPadj) of the Holladay Diagnostic Summary of the EyeSys Corneal Analysis System can be used.

• This value is then reduced by 19% for every diopter of myopia corrected by LASIK. We have added the 3 mm and the 4 mm power values from the Numerical View of the Zeiss Humphrey Atlas topographer to this calculation based on the fact that Holladay has found that sampling the 4.0 mm central cornea for the post-LASIK eye has returned very good results with the Oculus Pentacam.

CCP + (ΔD x 0.19) = Post-LASIK adjusted corneal power Where... CCP = the EffRPadj, or the averaged Zeiss Atlas central corneal power, and... ΔD = the refractive change after LASIK at the spectacle plane.The topographic central corneal power adjustment method for the eye would be:

If the averaged topographic central corneal power is 41.97 D, and...

The change in refractive power at the spectacle plane is -5.38 D, then...

CCP + (ΔD x 0.19) = Post-LASIK adjusted corneal power 41.97 D + (-5.38 x 0.19) = 40.95 D

Topographic Central Corneal Power Adjustment Method

Hyperopic LASIK and PRK

• Determining Corneal Power after Hyperopic LASIK.

• For eyes that have undergone hyperopic LASIK, the central corneal power estimation technique is much easier than for myopic LASIK.

• This is thought to be due to the fact that the ablation takes place outside the central cornea.

• An interesting feature of hyperopic LASIK is that following this procedure, the ratio between the posterior and anterior corneal radii is increased, much like what is seen following radial keratotomy.

• For this reason, it is possible to use a similar method of central corneal power estimation. It is intriguing that a myopic incisional technique (RK) and a hyperopic ablative technique (hyperopic LASIK), would alter the posterior : anterior corneal radii ratio in much the same way.

• Based on the experience of Drs. Wang, Jackson and Koch, the average of the 1 mm, 2 mm and 3 mm annular power rings of the Numerical View of the Zeiss Humphrey Atlas topographer is sufficiently accurate to serve as a reasonable estimate of central corneal power following hyperopic LASIK.

• However, depending on the amount of hyperopic laser vision correction, a correction to the average of the 1 mm, 2mm, and 3mm annular power ring value must be made

Hyperopic LASIK and PRK

Hyperopic LASIK (DLASIK-InducedRefractive Correction (D)

Adjustment for Hyperopic LASIK (D)

Atlas AnnCP

0.5 -0.301.0 -0.201.5 -0.112.0 -0.012.5 0.083.0 0.183.5 0.274.0 0.374.5 0.465.0 0.565.5 0.656.0 0.75

• As an alternative, Drs. Wang, Jackson and Koch have shown that the effective refractive power (EffRP) of the EyeSys Corneal Analysis System also works well.¹ Notice that this technique is very similar to what is currently recommended for estimating the central corneal power following radial keratotomy. As well, for this method, a small correction needs to be made to the effective refractive power (EffRP).

• Remember that some form of correction is still required for IOL power calculations following hyperopic LASIK in order to avoid a refractive surprise.

• This can be done by checking the "Previous RK, LASIK, etc." box on the Holladay 2 formula (contained within the Hollday IOL Consultant), or by using a hyperopic "double K method" IOL power correction table based on the work of Aramberri for any of the three popular 3rd generation, 2-variable formulas.

• outcomes employing the Holladay 2 formula.

Hyperopic LASIK and PRK

Radial Keratotomy

• Unlike ablative forms of myopic keratorefractive surgery (LASIK and PRK) in which the ratio between the posterior : anterior corneal radii is decreased, for eyes that have previously undergone radial keratotomy, the ratio between the posterior : anterior corneal radii is increased.

• This allows for a direct estimation of the central corneal power using elevation data of the central 4.0 mm, if carried out in a certain way.

• For eyes with prior radial keratotomy, averaging the 1 mm, 2 mm, 3 mm and 4 mm annular power rings of the Numerical View of the 995, 994, and 993 Zeiss Atlas topographer (right) will typically give a useful estimate of central corneal power.

• For the Zeiss Atlas 9000 topographer, you would take average of the 1 mm, 2 mm, 3 mm and 4 mm ring (not zone) values.

• If the Zeiss Atlas topographer is not available, then the adjusted effective refractive power (EffRPadj) from the Holladay Diagnostic Summary of the EyeSys Corneal Analysis System can be used.

Zeiss Atlas TopographerAnnular Ring Power from the Numerical View feature. Use the average of the 1 mm, 2 mm, 3 mm and 4 mm annular power values.

Radial Keratotomy

Post Keratorefractive online Calculators

ASCRS POST KERATOREFRACTIVE ON-LINE CALCULATOR

for myopic LASIK, hyperopeic LASIK, and prior radial keratotomy

7/4/07 in Bologna

Ver 1.0 SURGEON

WARNING!!

Patient Name Date

James A**k*ki 7/12/2006 ODMYP: Lowest K HYP: Highest K

Pre-LASIK SE Rx = -6.00 Axial Length 27.55 Clinical History Method 36.60

PO LASIK SE Rx = -0.37 IOL A-con 118.50 Contact Lens Method 40.65 Emmetropic IOL c Pre-K 10.88

Rx Δ (RCc) 0mm = -5.23 Target PO Rx = -0.50 Maloney* Method 36.84 Target IOL c PO-K 14.92

Rx Δ (RCs) 12mm = -5.63 Koch/Maloney Method 37.07 TARGET IOL c PO Flat-K 15.49

Pre-LASIK K = 41.83 Using PO K 14.92 Hamed-Wang-Koch 37.92 18.92

PO MK/TK = 39.00 c "Low/High" K 17.58 Haigis Method 36.50 18.04

PO Topog EffRP 38.75 BESSt© Formula 38.67 17.46

Flattest Man PO K 38.50 CL Base Curve 38.00 Savini-Barboni-Zanini 38.47 17.47

PO Topog Central-K 38.00 Hard CL Power 0.00 Ronje Method 37.09 16.73

IOLMaster PO K 38.37 Refrx (SE) c CL -0.25 Shammas No History 37.66

IOLMaster IR 1.3375 Refrx (SE) Bare -3.00 Speicher (Seitz) Method 38.67 Lowest or Highest K 36.50 19.18

Pentacam = 37.30 Savini IR Method 38.42 PO K 39.00 15.68

Ant Radius r = 8.68 Camellin IR Method 37.62 AVG K 37.46 17.85

Post Radius r = 6.75 IANCHULEV IN OR 8.75 Jarade IR Method 38.09 Your Choice

of K NA NA

C Thickness = 489 MACKOOL PO 9.25 Ferrara IR Method 33.35 18.96

18.0 SA60AT Rosa Method 34.82

Enter DATA [or NA] Needed For Calculations *IOLs Calculated By SRK/T® (>24.5 mm)

Enter Powers

USING

Single K

Aramberri DOUBLE-K (Target)

Î HOFFER®—SAVINI Ê LASIK IOL POWER TOOL ©2007Jay Patel, MDCalculating IOL Power in Eyes with Previous Corneal Refractive Surgery

DATA

Eye For Surgery Calculation Results in Diopters

For K Reading For IOL Power

THIS SPREADSHEET WILL PRODUCE ERRONEOUS INFORMATION UNLESS THE LETTERS NA ARE TYPED INTO EACH CELL WHERE YOU DO NOT HAVE THE REQUESTED DATA.

WHITE cells are for Data Entry GRAY cells are for IOL Power Entry

Wake Forest Method

Aphakic Refrx Methods

APHAKIC REFRX

**Latkany Method [K]

IOL and POWER USED

Ianchulev Method

Latkany Method Flat K

My

op

es

On

ly

The IOLs must be calculated using other software and added in the 3 Cells BELOW .

IR M

ET

HO

DSMust Use Hard PMMA

TARGET IOL "Single-K"

Contact Lens Method

Masket Method

Feiz-Mannis Formula

Feiz-Mannis Method

UCVA 20/25+2 PO ACD Jarade Formula 38.62 17.73

BCVA 20/20-1 5.00 Pentacam = 37.30 16.29Mackool Method

Ianchulev Method

Transient hyperopia following cataract surgery and prior radial keratotomy

• Patients with previous 8-incision radial keratotomy will commonly show variable amounts of transient hyperopia in the immediate post-operative period following cataract surgery. This is felt to be due to stromal edema around the radial incisions, producing a temporary enhancement of central corneal flattening. While this central corneal flattening is usually transient, it can be as much as +4.00 D, and is further accentuated by greater than eight incisions, an optical zone of less than 2.0 mm, or incisions that extend all the way to the limbus.

• If a patient exhibits any of the above, significant unanticipated hyperopia may be seen in the immediate post-operative period, which should gradually resolve after eight to twelve weeks. Sometimes, due to a lack of corneal stability, the post-operative refraction can continue to slowly shift myopic over a several month period. We have seen several patients with myopic shifts as large a -5.00 D over a 12-week period.

• If the final post-operative refractive objective remains elusive, plans for an IOL exchange, or a piggyback IOL, should not be made until at least two months have passed and two consecutive refractions, two weeks apart (at the same time of the day), are stable (the "rule of twos.").

• Also, if more than six months passes before cataract surgery is required for the fellow eye, the corneal measurements should be repeated due to the fact that additional corneal flattening frequently occurs over time following radial keratotomy. For this reason, IOL power calculations are usually targeted for between -0.75 D and -1.00 D and are designed to make the operative eye more myopic than usual, so that five to ten years from surgery, the post-cataract surgery refractive error does not drift into hyperopia. This also helps to avoid hyperopic refractive results, which are quite common, in spite of every precaution being taken.

• Of all the various forms of keratorefractive surgery, we have had the best overall accuracy following radial keratotomy using the above technique.

•

Conclusions