Li Wang, MD, PhD,
Douglas D. Koch, MD
Contents
|
• |
Factors Causing Intraocular Lens Power Errors |
|
|
• |
Approaches Improving the Accuracy of Intraocular Lens Power Calculation |
|
|
• |
Pearls in Selecting Intraocular Lens Power |
|
|
• |
Post-radical-keratotomy Eyes |
|
|
• |
Patient Education |
|
|
• |
Conclusion |
|
CHAPTER HIGHLIGHTS |
|||||||||
|
It is difficult to determine the intraocular lens (IOL) power in eyes that have undergone corneal refractive surgery.[1,][2] IOL power errors in these eyes can be attributed primarily to two factors: (1) inaccurate determination of the true corneal refractive power, and (2) incorrect estimation of the effective lens position (ELP) by the third- or fourth-generation IOL power-calculation formulas when the postoperative corneal powers are used. Several methods have been proposed to improve the accuracy of the IOL power calculation in eyes following corneal refractive surgery.
This chapter will discuss the factors causing the IOL power errors, the methods and techniques available to improve the accuracy of the IOL power calculation in these eyes, and pearls in selecting IOL power in these challenging cases.
Factors causing intraocular lens power errors
Inaccurate determination of the true corneal refractive power
Corneal powers are routinely measured using keratometers or computerized videokeratography (CVK). Keratometry is accurate for measuring normal unoperated corneas, but it is inaccurate for eyes that have undergone corneal refractive surgery. These errors are caused primarily by two issues: (1) inaccurate measurement of anterior corneal curvature, and (2) inaccurate calculation of the net corneal refractive power.
Inaccurate Measurement of Anterior Corneal Curvature
Corneal refractive surgery alters corneal asphericity and induces wider ranges of curvature values within the central 5mm zone. Standard keratometry or simulated keratometry from CVK measures only four points on the anterior cornea in a paracentral region, which misses the region where dramatic corneal curvature changes exist (Figure 5-1).
|
Figure 5-1 A map displays the four points in a paracentral region measured by standard keratometry or simulated keratometry from CVK. Central area with dramatic corneal power changes has been missed by the four points. |
Mean values for central corneal powers overcome the limitation of measurements by the four points. Some CVK devices provide mean values over certain central areas, such as the EffRP (effective refractive power) displayed by the EyeSys Corneal Analysis System (Houston, TX). These central corneal values can be used in eyes that have undergone radial keratotomy (RK), but they are inaccurate in eyes following ablative corneal refractive surgery due to the inaccuracy of using 1.3375 as a standardized value for corneal refractive index, which is discussed below.
Inaccurate Calculation of Total Corneal Refractive Power
In order to compensate for posterior corneal curvature, keratometers and CVK use a standardized index of refraction to convert measurements of anterior corneal curvature to the refractive power of the entire cornea. In most keratometers and CVK devices, a value of 1.3375 is used. Because ablative corneal refractive surgery (e.g., excimer laser photorefractive keratectomy (PRK) or laser in-situ keratomileusis (LASIK)) alters the relationship between the front and back surfaces of the cornea,[3] use of the standardized index of refraction of 1.3375 is no longer valid.
This problem will ultimately be solved by the development of devices that directly and accurately measure anterior and posterior corneal curvatures or, perhaps preferably, actual corneal refractive power. One currently available technology is Scheimpflug imaging. Studies have shown that posterior corneal curvatures cannot be reliably measured with the Orbscan system (Bausch & Lomb, Inc.).[4] Early data from studies of the Pentacam (Oculus, Inc.) are promising,[5] but we are unaware of peer-reviewed studies evaluating the accuracy of this and a new device, the Galilei dual scheimpflug analyzer (Ziemer Ophthalmic Systems, Port, Switzerland), in measuring posterior corneal power in post-LASIK eyes.
Tang and colleagues[6] investigated the repeatability of a high-speed corneal and anterior segment optical coherence tomography (OCT) prototype in measuring anterior and total corneal powers in 32 eyes before and 3 months after myopic LASIK. They found that the repeatability of the anterior corneal power measurement using the OCT was worse than that using the corneal topography (Atlas) in both unoperated corneas and corneas following LASIK (0.79D and 0.73D with the OCT, vs. 0.24D and 0.28D with the topography for virgin corneas and post-LASIK corneas, respectively). For total corneal measurements, the repeatability with OCT was 0.71D preoperatively and 0.66D postoperatively; the repeatability with the hybrid method, which combined the anterior corneal map from Placido ring corneal topography and corneal thickness map from OCT, was 0.24D and 0.26D, respectively.
Inaccurate estimation of effective lens position
Third- and fourth-generation formulas have greatly improved the accuracy of IOL power calculations, particularly in atypical eyes. Most of these formulas predict the postoperative location of the lens or ELP based on the corneal power (one exception being the Haigis formula). If postoperative corneal power is used to calculate the ELP, then the calculated lens position will be more anterior than will likely occur. This will cause the formula to select an IOL of insufficient power, resulting in postoperative hyperopia (Figure 5-2).
|
Figure 5-2 Some of third- and fourth-generation IOL power calculation formulas predict the effective lens position (ELP) based on the corneal power (A). In eyes following myopic surgery, if the flattened postoperative corneal power is used to calculate the ELP, the predicted ELP will be falsely shallow and result in underestimated IOL power (B). |
Aramberri[7] proposed the so-called double-K method to overcome this problem. In this method, the preoperative K reading is used to predict the ELP and the postoperative K reading is used in the vergence formula to calculate the IOL power. Clinical case series have demonstrated that hyperopic surprises have been dramatically decreased with the double-K version of the IOL formulas.[7–9] This approach had previously been incorporated by Holladay in the Holladay II formula.
In previous studies, we investigated the ELP-related prediction error of SRK/T, HofferQ, Holladay I and Holladay II formulas. The magnitude of this error varies according to the particular formula, the amount of refractive correction, and the axial length (AL).[10,][11] In general, a greater ELP-related prediction error is found for the SRK/T formula than for the Holladay I and Hoffer® Q formulas. The error increases linearly with increasing myopic and hyperopic corneal refractive surgical correction.
We have developed nomograms to adjust the IOL power when the modified corneal powers and the standard Holladay I, Hoffer® Q and SRK/T formulas are used (Tables 5-1 and 5-2).[11] With the Holladay II formula, one can check the “Previous RK, PRK or LASIK” box, and either enter the K reading before corneal refractive surgery or use the formula's default K value (43.86D) to predict the ELP.
Table 5-1 -- Nomogram for IOL power adjustment (D) following myopic surgery
|
Axial length (mm) |
||||||||||||
|
Refractive correction (D) |
19 |
20 |
21 |
22 |
23 |
24 |
25 |
26 |
27 |
28 |
29 |
30 |
|
2 |
0.7 |
0.7 |
0.7 |
0.7 |
0.7 |
0.7 |
0.7 |
0.6 |
0.6 |
0.5 |
0.4 |
0.3 |
|
0.5 |
0.4 |
0.4 |
0.3 |
0.3 |
0.2 |
0.2 |
0.2 |
0.1 |
0.1 |
0 |
0 |
|
|
0.4 |
0.5 |
0.5 |
0.5 |
0.5 |
0.5 |
0.5 |
0.4 |
0.4 |
0.3 |
0.2 |
0.1 |
|
|
3 |
1.0 |
1.0 |
1.0 |
1.0 |
1.1 |
1.1 |
1.0 |
1.0 |
0.9 |
0.8 |
0.7 |
0.6 |
|
0.7 |
0.6 |
0.5 |
0.5 |
0.4 |
0.3 |
0.3 |
0.3 |
0.2 |
0.2 |
0.1 |
0 |
|
|
0.7 |
0.7 |
0.7 |
0.7 |
0.7 |
0.7 |
0.8 |
0.7 |
0.6 |
0.4 |
0.3 |
0.2 |
|
|
4 |
1.3 |
1.3 |
1.3 |
1.4 |
1.4 |
1.4 |
1.4 |
1.3 |
1.2 |
1.1 |
0.9 |
0.8 |
|
1.0 |
0.8 |
0.7 |
0.6 |
0.5 |
0.5 |
0.4 |
0.3 |
0.3 |
0.2 |
0.1 |
0 |
|
|
0.9 |
0.9 |
0.9 |
1.0 |
1.0 |
1.0 |
1.1 |
0.9 |
0.8 |
0.6 |
0.5 |
0.4 |
|
|
5 |
1.7 |
1.7 |
1.7 |
1.7 |
1.7 |
1.8 |
1.7 |
1.6 |
1.5 |
1.4 |
1.2 |
1.1 |
|
1.2 |
1.0 |
0.9 |
0.8 |
0.7 |
0.6 |
0.5 |
0.4 |
0.4 |
0.3 |
0.2 |
0 |
|
|
1.1 |
1.2 |
1.2 |
1.2 |
1.2 |
1.3 |
1.3 |
1.2 |
1.0 |
0.8 |
0.7 |
0.5 |
|
|
6 |
2.0 |
2.0 |
2.0 |
2.0 |
2.1 |
2.1 |
2.1 |
2.0 |
1.8 |
1.7 |
1.5 |
1.4 |
|
1.4 |
1.2 |
1.0 |
0.9 |
0.8 |
0.7 |
0.6 |
0.5 |
0.5 |
0.4 |
0.3 |
0.1 |
|
|
1.4 |
1.4 |
1.4 |
1.5 |
1.5 |
1.6 |
1.6 |
1.5 |
1.2 |
1.0 |
0.8 |
0.7 |
|
|
7 |
2.3 |
2.3 |
2.3 |
2.4 |
2.4 |
2.5 |
2.4 |
2.3 |
2.2 |
2.0 |
1.8 |
1.7 |
|
1.6 |
1.4 |
1.2 |
1.1 |
0.9 |
0.8 |
0.7 |
0.6 |
0.6 |
0.5 |
0.3 |
0.1 |
|
|
1.6 |
1.6 |
1.7 |
1.7 |
1.8 |
1.8 |
1.9 |
1.7 |
1.5 |
1.2 |
1.0 |
0.9 |
|
|
8 |
2.6 |
2.6 |
2.6 |
2.7 |
2.7 |
2.8 |
2.8 |
2.6 |
2.5 |
2.3 |
2.2 |
2.0 |
|
1.8 |
1.6 |
1.4 |
1.2 |
1.1 |
1.0 |
0.8 |
0.7 |
0.7 |
0.6 |
0.4 |
0.2 |
|
|
1.8 |
1.9 |
1.9 |
2.0 |
2.0 |
2.1 |
2.2 |
2.0 |
1.7 |
1.5 |
1.2 |
1.0 |
|
|
9 |
2.9 |
2.9 |
2.9 |
3.0 |
3.1 |
3.2 |
3.1 |
3.0 |
2.8 |
2.7 |
2.5 |
2.3 |
|
2.0 |
1.7 |
1.5 |
1.3 |
1.2 |
1.1 |
1.0 |
0.8 |
0.8 |
0.7 |
0.5 |
0.2 |
|
|
2.1 |
2.1 |
2.2 |
2.2 |
2.3 |
2.4 |
2.5 |
2.3 |
2.0 |
1.7 |
1.4 |
1.2 |
|
|
10 |
3.1 |
3.2 |
3.2 |
3.3 |
3.4 |
3.5 |
3.4 |
3.3 |
3.1 |
3.0 |
2.8 |
2.6 |
|
2.2 |
1.9 |
1.7 |
1.5 |
1.3 |
1.2 |
1.1 |
1.0 |
0.8 |
0.7 |
0.6 |
0.3 |
|
|
2.3 |
2.4 |
2.4 |
2.5 |
2.6 |
2.7 |
2.8 |
2.6 |
2.2 |
1.9 |
1.7 |
1.4 |
Reprinted from Koch DD, Wang L: Calculating IOL power in eyes that have had refractive surgery, J Cataract Refract Surg 29:2039–2042. Copyright (2003) Elsevier. With permission from Elsevier.
|
For third-generation formulas, nomogram for IOL power adjustment (D) in eyes following myopic surgery when modified corneal powers are used to calculate IOL power. The numbers in the first, second and third rows of each cell represent the amounts that need to be added to the IOL power calculated using the SRK/T, Hoffer Q and Holladay 1 formulas, respectively. |
Table 5-2 -- Nomogram for IOL power adjustment (D) following hyperopic surgery
|
Axial length (mm) |
||||||||||||
|
Refractive correction (D) |
19 |
20 |
21 |
22 |
23 |
24 |
25 |
26 |
27 |
28 |
29 |
30 |
|
2 |
0.7 |
0.7 |
0.7 |
0.7 |
0.7 |
0.7 |
0.7 |
0.6 |
0.5 |
0.4 |
0.2 |
0 |
|
0.5 |
0.4 |
0.4 |
0.3 |
0.3 |
0.2 |
0.2 |
0.1 |
0.1 |
0.1 |
0 |
0 |
|
|
0.4 |
0.4 |
0.4 |
0.4 |
0.4 |
0.5 |
0.5 |
0.4 |
0.3 |
0.2 |
0 |
0 |
|
|
3 |
1.1 |
1.1 |
1.1 |
1.1 |
1.1 |
1.1 |
1.0 |
0.9 |
0.7 |
0.5 |
0.2 |
0 |
|
0.8 |
0.7 |
0.5 |
0.5 |
0.4 |
0.3 |
0.2 |
0.2 |
0.1 |
0 |
0 |
0 |
|
|
0.6 |
0.6 |
0.6 |
0.7 |
0.7 |
0.7 |
0.7 |
0.5 |
0.3 |
0.2 |
– |
– |
|
|
4 |
1.4 |
1.4 |
1.4 |
1.4 |
1.4 |
1.5 |
1.4 |
1.2 |
0.9 |
– |
– |
– |
|
1.1 |
0.9 |
0.7 |
0.6 |
0.5 |
0.4 |
0.3 |
0.2 |
0.1 |
0 |
0 |
0 |
|
|
0.9 |
0.9 |
0.9 |
0.9 |
0.9 |
0.9 |
0.9 |
0.6 |
0.4 |
0.4 |
– |
– |
|
|
5 |
1.8 |
1.8 |
1.8 |
1.8 |
1.8 |
1.9 |
1.8 |
1.7 |
– |
– |
– |
– |
|
1.4 |
1.1 |
0.9 |
0.7 |
0.6 |
0.4 |
0.3 |
0.2 |
0.1 |
0 |
0 |
0 |
|
|
1.1 |
1.1 |
1.0 |
1.0 |
1.0 |
1.0 |
1.0 |
0.7 |
0.3 |
– |
– |
– |
|
|
6 |
2.2 |
2.2 |
2.2 |
2.2 |
2.2 |
2.5 |
– |
– |
– |
– |
– |
– |
|
1.7 |
1.3 |
1.1 |
0.9 |
0.7 |
0.5 |
0.3 |
0.2 |
0 |
0 |
0 |
0 |
|
|
1.1 |
1.1 |
1.1 |
1.1 |
1.1 |
1.1 |
1.1 |
0.7 |
0.3 |
– |
– |
– |
Reprinted from Koch DD, Wang L: Calculating IOL power in eyes that have had refractive surgery, J Cataract Refract Surg 29:2039-2042. Copyright (2003) Elsevier. With permission from Elsevier.
|
For third-generation formulas, nomogram for IOL power adjustment (D) in eyes following hyperopic surgery when modified corneal powers are used to calculate IOL power. The numbers in the first, second and third rows of each cell represent the amounts that need to be subtracted to the IOL power calculated using the SRK/T, Hoffer Q and Holladay 1 formulas, respectively. The corresponding formula is not applicable. |
Copyright © 2010 Elsevier Inc. All rights reserved. Read our Terms and Conditions of Use and our Privacy Policy.
For problems or suggestions concerning this service, please contact: online.help@elsevier.com
Close
Print Page
Close
Print Page
Approaches improving the accuracy of intraocular lens power calculation
Various methods and techniques have been proposed to improve the accuracy of the IOL power calculation in eyes following corneal refractive surgery. These methods can be categorized into three groups:
|
1. |
Methods relying entirely on historical data |
|
|
2. |
Methods using historical data and current corneal measurements |
|
|
3. |
Methods only using current measurements. |
To illustrate these methods, an example of IOL power calculation in an eye following myopic LASIK will be used, and in Table 5-3 each of the methods will be applied to these data.
Table 5-3 -- Case example of intraocular lens power calculation in an eye with prior myopic LASIK from the authors’ clinical series
|
Pre-cataract surgery data: |
|
Pre-LASIK data: |
|
Pre-LASIK refraction: −4.75 D |
|
Pre-LASIK mean keratometry: 43.40 D |
|
Post-LASIK data: |
|
Post-LASIK refraction: plano |
|
EffRP: 40.03 D |
|
Central topographic power (Humphrey Atlas): 39.90 D |
|
IOLMaster mean keratometry: 40.00 D |
|
Flat K value with Humphrey Atlas: 40.00 D |
|
Post-cataract surgery data: |
|
IOL implanted: Alcon SN60WF lens with power of 21.0 D |
|
Refraction after cataract surgery: −1.00 D. |
|
Corneal refractive power estimation: |
|
Clinical history method: |
|
Refraction correction at corneal plane (vertex distance: 12.5 mm): 0 − (−4.75) / {1− [0.0125 x (−4.75)]} = 4.48 D |
|
Corneal power = 43.40 − 4.48 = 38.92 D |
|
Adjusted EffRP: |
|
Adjusted EffRP = 40.03 − 0.15 x 4.48 − 0.05 = 39.31 D |
|
Modified Maloney Method: |
|
Corneal power = 39.90 x (376/337.5) − 5.51 = 38.94 D |
|
IOL power calculation with the Holladay 1 formula except Haigis-L formula (aiming at refraction of −1.00 D): |
|
Clinical history method: |
|
IOL power using corneal power obtained from the clinical history method and the double-K Holladay 1 formula: 21.56 D |
|
Adjusted EffRP: |
|
IOL power using Adjusted EffRP and double-K Holladay 1 formula: 21.00 D |
|
Modified Maloney method: |
|
IOL power using corneal power obtained from the Modified Maloney method and double-K Holladay 1 formula: 21.54 D |
|
Feiz-Mannis IOL power adjustment method: |
|
IOL power using pre-LASIK K: 14.81 D |
|
IOL power after LASIK: 14.81 + 4.48 / 0.7 = 21.21 D |
|
Masket IOL power adjustment method: |
|
IOL power using post-LASIK IOLMaster K: 19.08 D |
|
IOL power after LASIK: 19.08 + 4.48 x 0.326 + 0.101 = 20.64 D |
|
Corneal bypass method: |
|
IOL power using pre-LASIK K and pre-LASIK refraction: 21.16 D |
|
Latkany formula: |
|
IOL power for flat Humphrey Atlas K: 19.08 D |
|
IOL power with Latkany formula = 19.08 − [0.47 x (−4.75) + 0.85] = 20.46 D |
|
Haigis-L formula: |
|
IOL power after LASIK: 20.97 D |
|
IOL power prediction error using different methods (Implanted − Predicted): |
|
Double-K clinical historical method: −0.56 D |
|
Double-K Adjusted EffRP: 0.00 D |
|
Double-K Modified Maloney method: −0.54 D |
|
Feiz-Mannis IOL power adjustment method: −0.21 D |
|
Masket IOL power adjustment method: +0.36 D |
|
Corneal bypass method: −0.16 D |
|
Latkany formula: +0.54 D |
|
Haigis-L formula: +0.03 D |
|
A 54-year-old lady underwent cataract extraction and posterior-chamber IOL implantation in her right eye. (Since Pentacam data were not obtained, methods using this device are not illustrated.) |
Approaches that rely entirely on historical data
Methods relying entirely on historical data are entirely dependent on the accuracy of the prior data and in our experience are often less accurate than the other two approaches (described below). The problem is twofold: (1) historical data were typically acquired elsewhere and, therefore, may not be accurate, and (2) there is a one-to-one error if any datum is incorrect, including the pre-LASIK/PRK data and the post-LASIK/PRK refraction. To maximize accuracy, one should validate the historical data and use the most recent refraction obtained before the cataract began to develop.
Clinical History Method
The clinical history method requires pre-LASIK/PRK keratometry, pre-LASIK/PRK refraction and post-LASIK/PRK stable refraction. Using this method, the corneal refractive power is derived by subtracting the change in refraction at corneal plane from the preoperative keratometric value:where:
LASIK = laser in-situ keratomileusis
PRK = photorefractive keratectomy
RC = the amount of refractive correction induced by the surgery (prior to the development of the cataract).
This approach has been tentatively confirmed by studies involving small numbers of eyes and has been used as gold standard in studies comparing the accuracy of different methodsfor IOL power calculation in eyes with prior corneal refractive surgery. However, studies involving cases following cataract surgery and IOL implantation have shown that the accuracy of this method varied largely, primarily due to less accurate historical data. In a previous study,[8] with the double-K Holladay I formula, the mean arithmetic IOL prediction error was −0.67 ± 1.53D (range −3.50–2.38D) with the clinical history method, compared to −0.62 ± 0.87D (range −1.87–0.56D) with the adjusted EffRP; note the higher standard deviation and range with the clinical history approach. In another set of cases, with the double-K Holladay I formula, the mean arithmetic IOL prediction error was −0.65 ± 1.05D (range −3.30–0.36D) with the clinical history method, −0.37 ± 0.74D (range −1.97–0.56D) with the adjusted EffRP, and +0.06 ± 0.69D (range −0.96–1.31D) with the modified Maloney method (unpublished data).
Feiz–Mannis Method
The Feiz–Mannis method requires pre-LASIK/PRK keratometry and the amount of refractive correction.[12] With this technique, the IOL power is first calculated using the pre-LASIK/PRK corneal power and currently measured axial length as though the patient had not undergone keratorefractive surgery. This pre-LASIK/PRK IOL power is then increased by the amount of refractive change at the spectacle plane divided by 0.7, by assuming that every diopter of change in IOL produces 0.7D of change in refraction at the spectacle plane. The formula is:where:
IOL = intraocular lens
LASIK = laser in-situ keratomileusis
PRK = photorefractive keratectomy
RC = the amount of refractive correction induced by the surgery (prior to the development of the cataract).
An advantage of this method is that it overcomes the problems of both inaccurate corneal power measurement and ELP estimation when the post-LASIK/PRK keratometric values are used. However, in a study that compared several techniques for calculating IOL power in 10 eyes after LASIK, this method was found to be less accurate than clinical history method or contact lens method.[13] In another study of 11 eyes after myopic LASIK, this method had the largest variance in the IOL prediction errors.[8]
Corneal Bypass Method
The corneal bypass method proposed by Walter and coauthors[14] requires pre-LASIK/PRK keratometry, pre-LASIK/PRK refraction and post-LASIK/PRK stable refraction. Using the current axial length and pre-LASIK/PRK keratometry, the IOL power calculation is targeted as the pre-LASIK/PRK refraction or the net refractive correction if the post-LASIK/PRK refraction is not plano.
Like the Feiz–Mannis method, this approach avoids the problems of both inaccurate corneal power measurement and ELP estimation when the post-LASIK/PRK keratometric values are used. In nine eyes that had cataract surgery after LASIK, Walter and colleagues[14] found that this method consistently chose the most accurate and precise IOL power compared with other methods. Further studies are desirable.
Approaches that use a combination of prior data and current corneal measurements
Some methods use current corneal measurements and then modify the measured corneal power or calculated IOL power based on the amount of refractive correction. The modifiers for the corneal powers range from 15% to 24% of refractive correction, and for the IOL powers 33% to 47%. Therefore, the errors due to incorrect historical data are significantly reduced compared to the one-for-one error involved in the approaches relying entirely on historical data.
Modified Computerized Videokeratography
There are several approaches to modifying current post-LASIK/PRK corneal power measurements:
|
• |
Adjusted EffRP: The EffRP is displayed on the Holladay Diagnostic Summary of the EyeSys Corneal Analysis System. This value samples all points within the central 3 mm and takes into account the Stiles–Crawford effect. The adjusted EffRP can be obtained using the following formulas in eyes after myopic LASIK/PRK or hyperopic LASIK/PRK, respectively: [3,][15] |
|
|
• |
Adjusted K: If the EffRP or other CVK values are not available, for myopic LASIK/PRK eyes, the current keratometry readings may be modified by a modifier of 24% of the refractive correction: [3]Because of the aforementioned poorer accuracy of K readings in these eyes, the adjusted K approach is not as accurate as the adjusted EffRP method. |
|
|
• |
Adjusted Annular Corneal Power: Some topography devices provide values for corneal power at incremental annular zones, e.g., 1mm, 2mm, etc. The accuracy of corneal power estimation in hyperopic LASIK/PRK eyes can be improved by adjusting the annular corneal power by a modifier of 19% of refractive correction:[15]where AnnCP is the average of powers at the center and the 1, 2 and 3mm annular zones from the numerical view map of Humphrey Atlas device. |
Latkany Formula
This method requires pre-LASIK/PRK manifest refraction. With the SRK/T formula, IOL power is first calculated using the current flat K reading or average K reading, and then adjusted using the following formulas:[16]
Masket Formula
With this approach,[17] in both myopic and hyperopic LASIK/PRK eyes, IOL power is calculated in the standard way and then modified by around 33% of the refractive correction. In the study in which this method was developed, the IOL master K values were used:
Approaches that require no prior data
Ideally, the optimal techniques for IOL power calculation in eyes following corneal refractive surgery are those that do not require prior data, eliminating the concern about accuracy or availability of historical data. Several methods have been developed:
Contact Lens Over-refraction
With this method, manifest refraction is performed first; a hard contact lens of known base curvature and power is then placed over the eye, and refraction is repeated. The corneal power is estimated as the sum of contact lens base curvature, contact lens power and the difference between the refractions with and without the contact lens.
Zeh and Koch[18] evaluated this method in cataract patients who had normal corneas and found acceptable accuracy for eyes with Snellen visual acuity of 20/70 or better. Recent studies suggest that the hard contact lens method is less accurate than other approaches.[8,][19] Contact lens designs with posterior curvatures that better fit the surgically modified corneal surface may improve the accuracy of this method. [20,][21]
Modified Maloney Method
In this approach, the central corneal power measured by the Humphrey Atlas topographer is converted back to the anterior corneal power by multiplying this value by 376.0/337.5 or 1.114. The central corneal power is obtained by placing the cursor at the exact center of the axial map. An assumed posterior corneal power of 5.51D is then subtracted from the anterior corneal power to yield the total corneal power. Corneal power is, therefore, calculated as follows:When used with either the Holladay II formula or third-generation formulas combined with the “double-K method,” this technique produced significantly smaller variances in IOL prediction error than did the clinical history method, indicating that this method may produce more consistent results. In the previous study,[8] we proposed a posterior corneal power of 6.1D to eliminate any hyperopic surprises following cataract surgery in eyes with prior myopic LASIK/PRK. In a separate series of 11 cases that had cataract surgery with prior myopic LASIK, as expected, this technique produced myopic outcome in 10 out of 11 eyes, with average refractive prediction error of −0.57D (unpublished data). A posterior corneal power of 5.51D would give a mean refraction error of zero, and we, therefore, recommend use of this value in this formula. Again, the modified Maloney method yielded more consistent results than did the clinical history method.
Haigis-L Formula
The Haigis-L formula is implemented in the Zeiss IOL Master. This formula uses a regression approach to adjust corneal power values measured by the IOLMaster® based on corneal powers derived from the history method. With this modified value, IOL power is calculated with the Haigis formula, and a small correction factor is then applied (www.augenklinik.uni-wuerzburg.de/kurse/ascrs2006/haigis-l.pdf).
Performance of the Haigis-L formula was evaluated in 77 cases from 30 surgeons (www.augenklinik.uni-wuerzburg.de/kurse/ascrs2006/haigis-l.pdf). The mean absolute refractive prediction error was 0.62 ± 0.55D (range 0.01–2.40D), and 51.9% of eyes were within ±0.50D, 79.2% within ±1.0D, and 96.1% within ±2.0D of prediction error.
Intraoperative Refraction
Two approaches have been proposed to do intraoperative refraction in these challenging eyes:
|
• |
Optical refractive biometry method: With this technique, after the removal of the cataract and just prior to implanting IOL, aphakic retinoscopy is performed with a portable autorefractor. The IOL power is then calculated based on the intraoperative refraction:[22]Axial length and K reading are not required by this method. Although early data on six post-LASIK eyes were promising, further studies are desirable. |
|
|
• |
Aphakic refraction technique: After removal of the cataract, the patient is removed from the operating room. Manifest refraction is performed 30 min later, and the IOL power is then determined using the following formula:[23] |
We believe that larger studies are required before these approaches are more widely applied; we have seen a hyperopic surprise of 1.75D with the second approach, and, because of the multiplier, both are subject to large errors if the refraction is not correct.
Gaussian Optics Formula
The Gaussian lens formula can be used to calculate the total corneal power when the anterior and posterior corneal powers are available. Using the Pentacam device, equivalent K-readings for central 4mm zone and the BESSt formula are proposed to estimate the total corneal power:
|
• |
Equivalent K-readings (4mm zone): On the Holladay report display of the Pentacam system, equivalent K-readings for the central 4mm zone can be used for IOL power calculation in eyes following corneal refractive surgery. We are unaware of studies evaluating the accuracy of these values in IOL power calculation in these eyes. |
|
|
• |
BESSt formula: In 143 eyes that had wavefront-guided myopic LASIK or LASEK, Borasio and colleagues[24] developed the BESSt formula to estimate the total corneal powers in eyes after corneal refractive surgery. In 143 virgin corneas, they found that the corneal power produced by the Gaussian optics formula consistently underestimated topographic keratometry values (Topcon), and the BESSt_vc (vc = virgin corneas) formula for virgin corneas was obtained by compensating for these discrepancies. Using the clinical history method as gold standard, postoperative corneal powers estimated with the BESSt_vc formula were refined against the corneal powers calculated with the clinical history method to develop the BESSt formula. The formulas are:where K-values Gaussian optics are K values obtained with the Gaussian optics formula. |
The accuracy of the BESSt formula was then tested in 13 eyes that had cataract surgery with prior keratorefractive surgery. The target refractions calculated with the BESSt formula were significantly closer to the refraction following cataract surgery than those calculated with double-K history method and contact lens method, with 46% of eyes within 0.5D and 100% within 1.0D of intended refraction.
Copyright © 2010 Elsevier Inc. All rights reserved. Read our Terms and Conditions of Use and our Privacy Policy.
For problems or suggestions concerning this service, please contact: online.help@elsevier.com
Close
Print Page
Close
Print Page
Pearls in selecting intraocular lens power
When calculating IOL powers in eyes that had undergone corneal refractive surgery, we recommend that surgeons use several methods.
Estimate corneal powers with various approaches
|
• |
Adjusted EffRP with the EyeSys topography |
|
|
• |
Modified Maloney method with the Humphrey Atlas topographer |
|
|
• |
Clinical history method if historical data are available. |
Calculate intraocular lens powers with different techniques
|
• |
Masket formula |
|
|
• |
Latkany formula |
|
|
• |
Haigis-L formula |
|
|
• |
Feiz–Mannis method |
|
|
• |
Corneal bypass method |
|
|
• |
Double-K method with the estimated corneal values. When modified corneal values are used to calculate the IOL power, the double-K version of the IOL power calculation formulas is required. With the SRK/T, HofferQ and Holladay I formulas, the calculated IOL powers should be modified by referring to the double-K tables (Tables 5-1 and 5-2). With the Holladay II formula, check the “Previous RK, PRK or LASIK” box, insert the pre-LASIK/PRK keratometry or the default average preoperative K value (43.86D) can be used to predict the ELP. |
In our practice, we use some of these methods as primary approaches and some as back-up options:
Primary Approaches
|
• |
Adjusted EffRP |
|
|
• |
Modified Maloney method |
|
|
• |
Clinical history method |
|
|
• |
Masket formula. |
Back-up Options
|
• |
Feiz–Mannis method |
|
|
• |
Corneal bypass method |
|
|
• |
Contact lens over-refraction. |
Copyright © 2010 Elsevier Inc. All rights reserved. Read our Terms and Conditions of Use and our Privacy Policy.
For problems or suggestions concerning this service, please contact: online.help@elsevier.com
Close
Print Page
Close
Print Page
Post-radical-keratotomy eyes
Since the RK procedure does not remove corneal tissue, eyes that have previously undergone RK experience flattening of both the anterior and posterior corneal radii. The relationship between the anterior and posterior corneal powers in virgin eyes may be applied in these post-RK eyes. Therefore, any map that provides a representative average of anterior corneal power over the central 2–3mm gives a fairly accurate estimation of corneal refractive power. Examples include the EffRP from the Holladay Diagnostic Summary of the EyeSys Corneal Analysis System and averaging the 0mm, 1mm and 2mm annular rings of the Numerical View of the Zeiss Humphrey Atlas topographer.
It is important to note that compensation for potential errors in ELP is still needed by using the Holladay II formula or the double-K approach with third-generation formulas as described above. Unfortunately, even with these measures, refractive surprises can still occur. We suspect that this is because there are some posterior curvature changes that deviate from those estimated by using the standardized index of refraction. Because of this relative inaccuracy of IOL calculations in post-RK eyes and their tendency to experience a long-term hyperopic drift, we usually target IOL power calculations for −1.00D.
Copyright © 2010 Elsevier Inc. All rights reserved. Read our Terms and Conditions of Use and our Privacy Policy.
For problems or suggestions concerning this service, please contact: online.help@elsevier.com
Close
Print Page
Close
Print Page
Patient education
It is important to explain to patients that IOL power calculations following all forms of corneal refractive surgery are problematic. Patients should be warned of reduced accuracy of IOL power calculation and of the possible need for additional surgery. Additional surgeries include LASIK or PRK enhancement, IOL exchange, or piggyback IOL. The possible costs associated with the additional surgery should also be discussed with the patient.
Copyright © 2010 Elsevier Inc. All rights reserved. Read our Terms and Conditions of Use and our Privacy Policy.
For problems or suggestions concerning this service, please contact: online.help@elsevier.com
Close
Print Page
Close
Print Page
Conclusion
The methodology for accurately calculating IOL power in eyes following corneal refractive surgery has improved dramatically in recent years. However, refractive surprises still occur. To effectively use the types of approaches described in this chapter, further progress is needed in the methods of measuring corneal power and in predicting ELP.
The “Holy Grail” in this field may be an adjustable IOL, which could facilitate correction, or at least reduction of residual spherical and astigmatic refractive errors, and residual higher-order aberrations. Ideally, such an IOL could be modified from time to time to adapt to the patient's changing visual needs and to compensate for aging changes of the cornea.[25] This and other future advances bode well for solving this challenging clinical problem.
Copyright © 2010 Elsevier Inc. All rights reserved. Read our Terms and Conditions of Use and our Privacy Policy.
For problems or suggestions concerning this service, please contact: online.help@elsevier.com
Close
Print Page
Close
Print Page
References
[1]. Koch D.D., Liu J.F., Hyde L.L., Rock R.L.: Emery JM. Refractive complications of cataract surgery after radial keratotomy. Am J Ophthalmol 1989; 108:676-682.
[2]. Seitz B., Langenbucher A., Nguyen N.X., Kus M.M., Kuchle M.: Underestimation of intraocular lens power for cataract surgery after myopic photorefractive keratectomy. Ophthalmology 1999; 106:693-702.
[3]. Hamed A.M., Wang L., Misra M., Koch D.D.: A comparative analysis of five methods of determining corneal refractive power in eyes that have undergone myopic laser in situ keratomileusis. Ophthalmology 2002; 109:651-658.
[4]. Maldonado M.J., Nieto J.C., Diez-Cuenca M., Pinero D.P.: Repeatability and reproducibility of posterior corneal curvature measurements by combined scanning-slit and placido-disc topography after LASIK. Ophthalmology 2006; 113:1918-1926.
[5]. Ciolino J.B., Belin M.W.: Changes in the posterior cornea after laser in situ keratomileusis and photorefractive keratectomy. J Cataract Refract Surg 2006; 32:1426-1431.
[6]. Tang M., Li Y., Avila M., Huang D.: Measuring total corneal power before and after laser in situ keratomileusis with high-speed optical coherence tomography. J Cataract Refract Surg 2006; 32:1843-1850.
[7]. Aramberri J.: Intraocular lens power calculation after corneal refractive surgery: double-K method. J Cataract Refract Surg 2003; 29:2063-2068.
[8]. Wang L., Booth M.A., Koch D.D.: Comparison of intraocular lens power calculation methods in eyes that have undergone LASIK. Ophthalmology 2004; 111:1825-1831.
[9]. Chan C.C., Hodge C., Lawless M.: Calculation of intraocular lens power after corneal refractive surgery. Clin Experiment Ophthalmol 2006; 34:640-644.
[10]. Koch D.D., Wang L., Booth M.: Intraocular lens calculations after LASIK. In: Probst L., ed. LASIK – advances, controversies and custom, Thorofare, NJ: Slack Inc; 2004:259-267.
[11]. Koch D.D., Wang L.: Calculating IOL power in eyes that have had refractive surgery. J Cataract Refract Surg 2003; 29:2039-2042.
[12]. Feiz V., Mannis M.J., Garcia-Ferrer F., et al: Intraocular lens power calculation after laser in situ keratomileusis for myopia and hyperopia: a standardized approach. Cornea 2001; 20:792-797.
[13]. Randleman J.B., Loupe D.N., Song C.D., Waring 3rd G.O., Stulting R.D.: Intraocular lens power calculations after laser in situ keratomileusis. Cornea 2002; 21:751-755.
[14]. Walter K.A., Gagnon M.R., Hoopes Jr P.C., Dickinson P.J.: Accurate intraocular lens power calculation after myopic laser in situ keratomileusis, bypassing corneal power. J Cataract Refract Surg 2006; 32:425-429.
[15]. Wang L., Jackson D.W., Koch D.D.: Methods of estimating corneal refractive power after hyperopic laser in situ keratomileusis. J Cataract Refract Surg 2002; 28:954-961.
[16]. Latkany R.A., Chokshi A.R., Speaker M.G., Abramson J., Soloway B.D., Yu G.: Intraocular lens calculations after refractive surgery. J Cataract Refract Surg 2005; 31:562-570.
[17]. Masket S., Masket S.E.: Simple regression formula for intraocular lens power adjustment in eyes requiring cataract surgery after excimer laser photoablation. J Cataract Refract Surg 2006; 32:430-434.
[18]. Zeh W.G., Koch D.D.: Comparison of contact lens overrefraction and standard keratometry for measuring corneal curvature in eyes with lenticular opacity. J Cataract Refract Surg 1999; 25:898-903.
[19]. Haigis W.: Corneal power after refractive surgery for myopia: contact lens method. J Cataract Refract Surg 2003; 29:1397-1411.
[20]. Joslin C.E., Koster J., Tu E.Y.: Contact lens overrefraction variability in corneal power estimation after refractive surgery. J Cataract Refract Surg 2005; 31:2287-2292.
[21]. Gruenauer-Kloevekorn C., Fischer U., Kloevekorn-Norgall K., Duncker G.I.: Varieties of contact lens fittings after complicated hyperopic and myopic laser in situ keratomileusis. Eye Contact Lens 2006; 32:233-239.
[22]. Ianchulev T., Salz J., Hoffer K., Albini T., Hsu H., Labree L.: Intraoperative optical refractive biometry for intraocular lens power estimation without axial length and keratometry measurements. J Cataract Refract Surg 2005; 31:1530-1536.
[23]. Mackool R.J., Ko W., Mackool R.: Intraocular lens power calculation after laser in situ keratomileusis: aphakic refraction technique. J Cataract Refract Surg 2006; 32:435-437.
[24]. Borasio E., Stevens J., Smith G.T.: Estimation of true corneal power after keratorefractive surgery in eyes requiring cataract surgery: BESSt formula. J Cataract Refract Surg 2006; 32:2004-2014.
[25]. Wang L., Dai E., Koch D.D., Nathoo A.: Optical aberrations of the human anterior cornea. J Cataract Refract Surg 2003; 29:1514-1521.