Corneal Refractive Power Estimation and Intraocular Lens Calculation after Hyperopic LASIK

Transcription

1 Corneal Refractive Power Estimation and Intraocular Lens Calculation after Hyperopic LASIK Shady T. Awwad, MD, 1,2 Patrick S. Kelley, MD, 1 R. Wayne Bowman, MD, 1 H. Dwight Cavanagh, MD, PhD, 1 James P. McCulley, MD 1 Purpose: To identify key independent variables in estimating corneal refractive power (KBC) after hyperopic LASIK. Design: Retrospective study. Participants: We included 24 eyes of 16 hyperopic patients who underwent LASIK with subsequent phacoemulsification and posterior chamber intraocular lens (IOL) implantation in the same eye. Methods: Pre-LASIK and post-lasik spherical equivalent (SE) refractions and topographies, axial length, implant type and power, and 3-month postphacoemulsification SE were recorded. Using the double-k Hoffer Q formula, corneal power was backcalculated for every eye (KBC), regression-based formulas derived, and corresponding IOL powers calculated and compared with published methods. Main Outcome Measures: The Pearson correlation coefficient (PCC) and arithmetic and absolute corneal and IOL power errors. Results: Adjusting either the average central corneal power (ACCP 3mm ) or SimK based on the laser-induced spherical equivalent change ( SE) resulted in an estimated corneal power (ACCP adj and SimK adj ) with highest correlation with KBC (PCC and 0.956, respectively) and lowest absolute corneal estimation error ( and diopter [D], respectively). The ACCP adj closely mirrored published SE-based adjustments of central corneal power on different topographers, whereas SE-based SimK adjustments varied across platforms. Using ACCP adj or SimK adj in the double-k Hoffer Q, using ACCP 3mm or SimK in single-k Hoffer Q and adjusting the resultant IOL power based on SE, or applying Masket s formula all yielded accurate and similar IOL powers. The Latkany method consistently underestimated IOL power. The Feiz Mannis and clinical history methods yielded poor IOL correlations and large IOL errors. Conclusion: After hyperopic LASIK, adjusting either corneal power or IOL power based on SE accurately estimates the appropriate IOL power. Financial Disclosure(s): Proprietary or commercial disclosure may be found after the references. Ophthalmology 2009;116: by the American Academy of Ophthalmology. Determination of the refractive corneal power after laser keratorefractive surgery has been the subject of numerous studies over the past decade Fortunately, continuous clinical research that spurred creation and refinement of formulas, along with newer imaging systems, have helped to improve refractive outcomes Most published works, however, have targeted patients with previous myopic keratorefractive surgery; there are few data on corneal power estimation and intraocular lens (IOL) power calculation after hyperopic keratorefractive surgery Using the double-k IOL formula concept, we have used data from patients who have undergone cataract extraction after hyperopic LASIK to backcalculate the ideal refractive corneal power and performed multiple regression analysis to identify key variables with their corresponding formulas that would best predict corneal refractive power (KBC) in an accurate, and most important, reproducible manner. Patients and Methods A retrospective chart review was conducted to locate patients with a history of hyperopic LASIK who subsequently underwent phacoemulsification and posterior chamber IOL implantation in the same eye between January 2001 and June 2006 at the University of Texas Southwestern Medical Center at Dallas. All patients had to have full prephacoemulsification refractive history, comprising pre-lasik keratometry (K pre-lasik ), pre-lasik refraction, post- LASIK refraction before cataract development, and post-lasik topography. Exclusion criteria included vitreoretinal or corneal disease, history of any other ocular surgery, as well as uveitis, trauma, or systemic diseases affecting vision, and intraoperative complications. Data on 25 eyes could be located, with a total of 24 eyes of 16 patients meeting the criteria and being included in the study. All eyes had a corneal topography using the Topography Modeling System (TMS, Tomey Inc, Phoenix, AZ), and axial length measurement with optical coherence interferometry using the IOLMaster (Carl Zeiss GmbH, Jena, Germany). The 3-month 2009 by the American Academy of Ophthalmology ISSN /09/$ see front matter Published by Elsevier Inc. doi: /j.ophtha

2 Ophthalmology Volume 116, Number 3, March 2009 postcataract surgery spherical equivalent refraction (SE PCE ), type and power of the implanted IOL, axial length, and topographic data extracted from the TMS topographer, simulated keratometry (SimK) and average central corneal power over the central 3 mm of the cornea (ACCP 3mm ), were all recorded. The ACCP 3mm is the average of the mean powers of the central placido rings over the central 3 mm area, and is analogous to the effective refractive power of the cornea (EffRP) available on the Holladay Diagnostic Summary on the EyeSys topographer, without the Stiles-Crawford compensation effect, and similar to the annular central power (AnnCP) on the Humphrey topographer. 3,17,18 The types of IOL implanted were AcrySof SA60AT and SN60AT, AcrySof IQ, and AcrySof Restor (Alcon Labs, Fort Worth, TX). Cataract surgeries were performed by RWB, HDC, and JPM, and the IOL constants optimized for the partial coherence interferometery method of axial length measurement. Laser correction surgery was performed using the VISX S2, S3, and S4 systems (VISX, Santa Clara, CA) with an optical zone of 6.0 mm and a transition zone of 8.5 mm. The study was performed with the approval of the University of Texas Southwestern Medical Center Institutional Review Board and in accordance with the Declaration of Helsinki guidelines for human research and the Health Insurance Portability and Accountability Act. Two types of methods are currently available to calculate IOL power in postkeratorefractive eyes. One method adjusts the measured corneal power, and then uses the adjusted value in a double-k modified IOL formula. The second method simply inputs the measured corneal power in a single-k IOL formula, and then adjusts the resultant IOL power according to the laser induced refractive change. Regression Analysis Derivation: Corneal Power Adjustment For each eye, the ideal KBC was backcalculated using the double-k Hoffer Q formula and the 3-month SE, the implanted IOL power and constant, and axial length. The KBC was considered the gold standard, and both corneal power measurements, ACCP 3mm and simulated K (SimK) were adjusted according to KBC using multiple regression analyses of the form Y ax bz C, where Y represents the dependent variable and is KBC, and X and Z are independent variables. X and Z can include either ACCP 3mm or SimK, together with the laser-induced SE change: pre-lasik SE (SE pre-lasik ) post-lasik SE (SE post-lasik ). Four regression analyses were created based on 4 sets of parameters: 1. KBC and ACCP 3mm 2. KBC and SimK 3. KBC, ACCP 3mm, and (SE pre-lasik SE post-lasik ) 4. KBC, SimK, (SE pre-lasik SE post-lasik ) Regression Analysis Derivation: Intraocular Lens Power Adjustment To compare to published formulas that adjust the final IOL power calculated from single-k third-generation formulas, bypassing corneal power adjustment (the Masket and Latkany methods), regression analyses were created based on IOL power adjustment. In this scenario, the implanted IOL power (IOL i ) was considered the gold standard, and the resulting postphacoemulsification spherical equivalent (SE PCE ) was used as the target correction to calculate the IOL power via the single-k Hoffer Q formula. The calculated IOL power was then adjusted by regression analysis according to the laser-induced refractive change. For every eye, 2 values of IOL power were calculated, IOL ACCP3mm and IOL SimK, depending on whether ACCP 3mm or SimK were used in the single-k formula, hence leading to 2 more regression formulas with the following parameters: 5. IOL i, IOL ACCP3mm, and (SE pre-lasik SE post-lasik ) 6. IOL i, IOL SimK, and (SE pre-lasik SE post-lasik ) The first 4 regression formulas were used in every eye to compute the corresponding refractive corneal power, which was compared with KBC. For each formula, the Pearson correlation coefficient was calculated, and the corneal power estimation error was calculated as such: Arithmetic corneal power estimation error estimated corneal power KBC The absolute corneal power estimation error was derived using the absolute value of the arithmetic corneal power estimation error. The proportion of eyes within 1.0 diopters (D) of corneal power error was evaluated. The results were compared with the clinical history method (CHM). For the CHM, the pre-lasik and post- LASIK spherical equivalents (SE pre-lasik and SE post-lasik ) were converted to the corneal plane, and then the difference in SE was subtracted from the pre-lasik keratometric value to obtain the refractive power (K CHM ). 12 The last 2 regression formulas (5 and 6) were also used in every eye to compute the corresponding IOL (IOL c ), which was compared with the IOL i, with the IOL power estimation error being equal to (IOL c IOL i ). For every formula, the Pearson correlation coefficient, the arithmetic and absolute IOL power estimation error, and the proportion of eyes within 1.0 D of IOL power error were calculated. The results were compared with available methods for computing and adjusting IOL power after posthyperopic keratorefractive surgery, and assuming the postoperative refraction target to be equal to the actual postoperative spherical equivalence in each case (SE PCE ). The latter formulas included the Feiz Mannis, 5,16 modified Feiz Mannis, 16 Masket, 7 modified Masket (Hill WE. IOL power calculation after keratorefractive surgery. Paper presented at: the Annual Meeting of the American Society of Cataract and Refractive Surgery, San Francisco, California, March 2006), and Latkany method. 16 These methods all use the single-k IOL formulas and are summarized below. 1. Feiz Mannis method: IOL is computed using K pre-lasik as the corneal power value, then is adjusted using the following correction factor: (SE pre-lasik SE post-lasik )/0.7; hence, IOL IOL K pre-lasik (SE pre-lasik SE post-lasik )/ Modified Feiz Mannis method: Similar to the original method, the modified formula is theoretically more exact, in that the laser-induced change in spherical equivalence is input as the target refraction in the IOL formula. K pre-lasik is still used as the corneal power value. 3. Masket method: IOL IOL SimK (LSE) 0.101, where LSE is the vertex distance corrected laser vision correction SE. 4. Modified Masket method: IOL IOL SimK LSE Latkany method: IOL IOL SimK (0.27 SE 1.53), where SE is the difference between preoperative and postoperative spherical equivalents (SE pre-lasik SE post-lasik ). The Feiz Mannis and modified Feiz Mannis methods bypass the problem of the effective lens position (ELP) estimation error by using the K pre-lasik for the corneal power in the single-k IOL 394

3 Awwad et al IOL Calculations after Hyperopic LASIK formula and adjusting with laser-induced change in refraction. As for the Masket and Latkany methods, both use the single-k IOL formula and account for both the corneal power estimation error and the ELP estimation error by adjusting the IOL power based on the laser-induced refractive change. The double-k adjustment of the original Hoffer Q formula with its updated errata and with its derivation of the backcalculated refraction was carefully programmed in Excel and is detailed in Appendix 1 (available online at 19 The Hoffer Q formula, as programmed in Excel, was tested against the Hoffer Q formula programmed in the Holladay IOL Consultant software (officially licensed to calculate Hoffer Q formula) using SimK as the default corneal power with and without double-k adjustment and results were found to match for all eyes. The Excel version was subsequently used to run simultaneous tandem calculations on all eyes using different parameters each time and to integrate the data efficiently for statistical analysis. The statistical data in Excel was exported to Sigmastat software (Jandel Scientific, San Rafael, CA) for analysis. The paired Student t-test (2-tailed distribution) was used to compare the algebraic and absolute IOL errors, and the McNemar test for nonparametric analysis of related samples was used to compare proportions between related groups. P 0.05 was considered significant. Results Patient Demographics and Characteristics The average patient age was years. The axial length measurements averaged mm, ranging from to mm. The average of the effective refractive correction ( SE SE pre-lasik SE post-lasik ), which takes into consideration the refractive regression effect, was D (range, 0.25 to 3.87 D). Regression Formulas Regression analysis using KBC as the dependent variable and different combinations of independent variables yielded the following equations: and ACCP' ACCP 3mm (r ; P 0.001); [1] SimK' SimK (r ; P 0.001). Both formulas are roughly 90% of the K reading plus 4: ACCP adj ACCP 3mm (SE pre-lasik SE post-lasik ) (r ; P 0.001). Using KBC ACCP 3mm as a dependent variable, we get: ACCP adj ACCP 3mm (SE pre-lasik SE post-lasik ) Hence, the equation: ACCP adj ACCP 3mm (SE pre-lasik SE post-lasik ) [3] SimK adj SimK (SE pre-lasik SE post-lasik ) (r ; P 0.001). Using KBC SimK as a dependent variable, we get: [2] SimK adj SimK (SE pre-lasik SE post-lasik ) Hence, the equation: SimK adj SimK (SE pre-lasik SE post-lasik ) [4] A regression analysis bypassing the double-k method was also devised with IOL i IOL ACCP3mm, taken as the dependent variable, and SE as the independent variable (IOL ACCP3mm being the IOL power derived via the single-k Hoffer Q formula and using ACCP 3mm as the corneal power): IOL i IOL ACCP3mm SE IOL adj_accp IOL ACCP3mm 0.35 SE [5] Another regression analysis bypassing the double-k method, that could be used when K pre-lasik is unknown, was also devised with IOL i IOL SimK, taken as the dependent variable, and SE as the independent variable (IOL SimK being the IOL power derived via the single-k Hoffer Q formula and using SimK as the corneal power): IOL i IOL SimK SE IOL adj_simk IOL SimK SE [6] The Pearson Correlation Coefficient and Arithmetic and Absolute Refractive Corneal Power Estimation Errors The Pearson correlation coefficients for the estimated refractive corneal power derived using the different regression formulas and the arithmetic and absolute errors are summarized in Table 1, together with their corresponding arithmetic and absolute corneal power estimation error. ACCP 3mm approximated the refractive corneal power slightly better than SimK, which tended to underestimate it (arithmetic and absolute corneal power of and , respectively, vs and , respectively, for SimK). On the other hand, SimK had a slightly better Pearson correlation with the refractive corneal power represented by KBC than ACCP 3mm (0.946 vs 0.935, respectively). Regression analyses based on SimK and SE (SE pre-lasik SE post-lasik ), (SimK adj ; equation 4), yielded the highest correlation coefficients (0.956), followed by regression based on SimK alone (SimK=; equation 2; 0.946), and regression based on ACCP 3mm and SE (ACCP adj ; equation 3; 0.940). The CHM yielded the lowest correlation with only The SimK adj improved the correlation and the arithmetic and absolute corneal power error of SimK ( vs [P 0.001] and vs [P 0.09], respectively; Table 1). Similarly, ACCP adj improved on the correlation and the absolute error of ACCP 3mm, although the difference in absolute error was not significant (P 0.170; Table 1). The difference in absolute error between ACCP adj and SimK adj was not significant ( vs ; P 0.929). The CHM yielded the largest absolute corneal power estimation error ( ), which was statistically significant when compared with the error of SimK adj and ACCP adj (P for both). Table 2 summarizes the percentage of eyes within a certain corneal power estimation error for all the formulas. Only 83% and 92% of CHM eyes lay within 1.5 D and 2.0 D of error, as opposed to 96% to 100% and 100% for ACCP 3mm, SimK, or the rest of the regression formulas. The difference, however, was not significant. The adjustments that need to be made on the TMS topographic measurements ACCP 3mm and SimK based on the laser treated 395

4 Table 1. Arithmetic and Absolute Corneal Power Estimation Errors of the Different Derived Regression, as well as the Historical K Method, and the Pearson s Correlation (r) of Their Respective Derived Corneal Power with the Backcalculated Corneal Power for Each Eye (K backcalc ) Regression Formulas and Corneal Power Estimation Methods Ophthalmology Volume 116, Number 3, March 2009 Arithmetic Corneal Power Estimation Error (D) Absolute Corneal Power Estimation Error (D) Pearson s Correlation* ACCP 3mm SimK Regression based on ACCP Regression based on SimK ACCP adj SimK adj Clinical history method ACCP adj Regression based on ACCP 3mm and SE; ACCP 3mm average central corneal power within the central 3 mm; D diopters; SE change in spherical equivalent refractions (SE pre-lasik SE post-lasik ); SimK simulated keratometric value; SimK adj regression based on SimK and SE. *P for all. correction are detailed in Table 3, with the adjustments published by Wang et al for phakic eyes posthyperopic keratorefractive surgery listed in parallel for comparison purposes. 15 As can be seen, the adjustments on the ACCP 3mm are strikingly similar to those of EffRP and AnnCP, which are similar indices of the average central corneal power on the EyeSys and Atlas topographers, respectively. On the other hand, the SimK adjustments seem to be dependent of the topography platform used (Table 3). Comparison with Published Posthyperopic Keratorefractive Surgery Intraocular Lens Power Calculation Methods The Pearson correlation and the arithmetic and absolute IOL power errors of the derived regression formulas and the published IOL power calculation methods for eyes after hyperopic keratorefractive surgery are summarized in Table 4. Performing corneal adjustment based on ACCP 3mm and SE (ACCP adj ; equation 3) or SimK and SE (SimK adj ; equation 4) and using the calculated Table 2. Percentage of Eyes with Corneal Power Estimation Error within a Certain Range, as a Function of Topographic Parameters, Regression Formulas, as well as the Historical K Method, in Eyes with Previous Hyperopic Keratorefractive Surgery Refractive Corneal Power Estimation Methods Percentage of Eyes within a Certain Corneal Power Estimation Error 0.5 D 1.0 D 1.5 D 2.0 D ACCP 3mm SimK Regression based on ACCP Regression based on SimK ACCP adj SimK adj Clinical history method ACCP 3mm average central corneal power within the central 3 mm; ACCP adj regression based on ACCP 3mm and SE; D diopters; SimK simulated keratometric value; SimK adj regression based on SimK and SE; SE change in spherical equivalent refractions (SE pre-lasik SE post-lasik ). value in the double-k Hoffer Q formula resulted in a similar Pearson correlation and arithmetic and absolute errors compared with the Masket method (Table 4), and better Pearson correlation as well as smaller arithmetic and absolute IOL power estimation errors than the CHM (P and P 0.362, respectively, for arithmetic error; P and P for absolute error), Feiz Mannis/double-K Hoffer Q (P for both, in arithmetic and absolute errors), modified Masket method (P for both, in arithmetic and absolute errors), as well as the modified Latkany method (P for both, in arithmetic and absolute errors), all of which, except CHM, were derived from a regression-based adjustment of IOL power using the single-k Hoffer Q formula (Table 4). The Latkany method resulted in a systematic underestimation of the IOL power, as shown by the arithmetic IOL error, which was about equal in value to the absolute IOL error (Fig 1). On the other hand, the modified Masket method resulted in slight overestimation of the IOL power (Fig 1). IOL calculation using ACCP 3mm and the single-k Hoffer Q and subsequent adjustment based on SE (IOL adj_accp ; equation 5) had an arithmetic and absolute IOL power error of and D, essentially similar to and D obtained by the double-k Hoffer Q using adjusted corneal power based on ACCP 3mm and SE (ACCP adj ; equation 3; P and P 0.646, respectively; Table 4; Fig 2). In addition, IOL calculation using SimK and the single-k Hoffer Q and subsequent adjustment based on SE (IOL adj_simk ; equation 6) had an arithmetic and absolute IOL power error of and D, which compares with the and D obtained by the double-k Hoffer Q using adjusted corneal power based on SimK and SE (SimK adj ; equation 4; P and P 0.339, respectively; Fig 3). The percentage of eyes within a certain error range is summarized in Table 5. When using ACCP adj (equation 3) with the double-k Hoffer Q, 92% and 100% of eyes were within 1.5 D and 2.0 D of IOL power error, respectively, compared with 92% and 96%, respectively, with SimK adj (equation 4) with double-k Hoffer Q, 79% and 83%, respectively, for the Feiz Mannis and the CHM (P for both), 92% and 96% for both the Masket and the modified Masket methods (P for both), and 54% and 79% for the Latkany method (P and P 0.063, respectively). The single-k regression-derived IOL adjustment method IOL adj_accp (equation 5) and IOL adj_simk (equation 6) both had 92% and 96% of eyes within 1.5 D and 2.0 D of IOL power error, respectively (P for both; Table 5). 396

5 Table 3. Adjustment of Measurements from Tomey Topographic Modulation System (TMS) Topographer with Comparison with Adjustment Figures Published by Wang et al on the EyeSys and Atlas Topographers in Eyes after Hyperopic LASIK 15 LASIK-Induced Refractive Correction (D) Awwad et al IOL Calculations after Hyperopic LASIK Adjustment for Hyperopic LASIK (D) TMS ACCP 3mm TMS SimK EyeSys EffRP EyeSys SimK Atlas AnnCP Atlas SimK ACCP 3mm average central corneal power within the central 3 mm; AnnCP Annular central power (3 mm area); D diopters; EffRP effective refractive power (3 mm area); SimK simulated keratometric value. Discussion Although numerous publications and studies have tackled the problem of IOL calculation and corneal power prediction in eyes after myopic keratorefractive surgery, very few studies have evaluated the IOL and corneal power calculations after hyperopic laser keratorefractive procedures. Wang et al evaluated phakic eyes after hyperopic LASIK without cataract surgery and IOL insertion, and using the CHM as the gold standard, tried to improve the accuracy of the corneal power estimation by taking into consideration the change in spherical equivalence from the laser procedure. 15 Two topography systems were used and both SimK and average central corneal powers were evaluated (EffRP for EyeSys and AnnCP for the Humphrey system); interestingly, the suggested modifications to the measured values, based on regression analysis, were similar between the 2 topographers for the average central corneal power, but different for SimK (Table 3). Most important, the average corneal power modifications derived are very similar to our regression-derived modifications for ACCP 3mm on the TMS system (Table 3). On the other hand, Latkany et al recently reported their results on posthyperopic LASIK eyes that underwent phacoemulsification and IOL insertion. Using backcalculation, and only average keratometry and SimK values, they determined that the best predictive IOL formula would rely on the concept of Feiz Mannis, which also was not statistically Table 4. Arithmetic and Absolute Intraocular Lens (IOL) Power Estimation Errors of the Regression-Derived Formulas, Compared with Popular Methods to Calculate IOL Power in Eyes after Hyperopic Keratorefractive Surgery, with Their Corresponding IOL Pearson Correlation with the Implanted IOL Power (IOL implanted ) Regression Formulas and IOL Power Estimation Methods Arithmetic IOL Power Estimation Error (D) Absolute IOL Power Estimation Error (D) Pearson s Correlation Double-K methods ACCP adj and double-k Hoffer Q SimK adj and double-k Hoffer Q Historical K and double-k Hoffer Q * Single-K methods Feiz Mannis method * Modified Feiz Mannis method * Masket method Modified Masket method Latkany method * * IOL adjaccp IOL adjsimk ACCP 3mm average central corneal power within the central 3 mm; ACCP adj regression based on ACCP 3mm and SE; D diopters; F(x,y) regression formula with x and y as dependent variables; IOL power error IOL calculated IOL implanted ; SE pre-lasik SE post-lasik ; SimK simulated keratometric value; SimK adj regression based on SimK and SE. *Statistical significance compared to ACCP adj or SimK adj and double-k Hoffer Q. Formula derived by regression analysis. 397

6 Ophthalmology Volume 116, Number 3, March 2009 Figure 1. Scatter plot of implanted intraocular lens power (IOL i ) versus calculated IOL power (IOL c ) using the Masket, Masket modified, and Latkany methods (based on SimK), and the regression-derived single-k IOL adjustment based on average central corneal power (ACCP 3mm ), (IOL ACCP3mm 0.53 SE 0.085) in posthyperopic LASIK eyes. D diopters; SE spherical equivalent. different from a simplistic SimK-based regression formula they determined to be: IOL IOL SimK (0.27 SE 1.53 [the Latkany method]). This formula, together with the SimK-based modified Masket formula, yielded poor results when applied to our patients sample. This finding, together with the discrepancies found by Wang et al among SimKbased regression formulas generated from different topographers, indicate that SimK measurements in posthyperopic LASIK eyes are highly platform dependent. 15 On the other hand, average central corneal power measurements, such as EffRP (EyeSys), ACCP 3mm (TMS), and AnnCP (Humphrey), seem to have little platform dependency, as the adjustments that need to be made to the measured values Figure 2. Scatter plot of implanted intraocular lens (IOL) power versus calculated IOL power using adjusted average central corneal power (ACCP adj ), (ACCP 3mm SE 0.256), and adjusted simulated keratometric values (SimK adj ), (SimK SE 0.105), as the post-lasik K values in the double-k Hoffer Q formula in posthyperopic LASIK eyes. D diopters; SE spherical equivalent. Figure 3. Scatter plot of implanted intraocular lens (IOL) power versus calculated IOL power using regression derived single-k IOL adjustment (IOL ACCP3mm 0.53 SE 0.085) and the double-k Hoffer Q formula with adjusted average central corneal power (ACCP adj ), (ACCP 3mm SE 0.256) as the post-lasik K value in post hyperopic LASIK eyes. The average laser-induced spherical equivalent change was diopters (D; range, 0.25 to 3.87 D). SE spherical equivalent. after posthyperopic LASIK eyes are strikingly similar; the average central corneal power adjustments suggested by Wang et al on the EyeSys and Humphrey systems compare rigidly with those derived by our regression analysis on the TMS system (Table 3). It is interesting to note that although the original Masket formula was developed with the majority of included eyes being postmyopic LASIK and only 7 eyes after hyperopic LASIK, 7 it yielded nearly similar results when compared with the regressions based on average central corneal power in our patients sample. However, because it relies on SimK measurements, the extension of the obtained results to other platforms needs to be verified; the modified Masket formula, for instance, had somehow inferior results in our study sample. As for CHM and Feiz Mannis methods, they yielded suboptimal results. We believe this is due to the fact that these 2 formulas, although theoretically correct and able to be considered as gold standards, fail to perform well in real life. Many errors could be introduced in pre- and postoperative refraction owing to accommodation, regression, or myopic shift of an early and yet clinically occult cataract. In addition, the error produced in any step is carried to the final postphacoemulsification refraction on a 1:1 basis, unlike regression-based formulas where a coefficient is multiplied by the change in spherical equivalence, hence decreasing the final error and imparting more leeway and forgiveness to the formula. For instance, with ACCP adj ACCP 3mm SE 0.256, an error of 1Din SE is multiplied by Ater hyperopic LASIK, unlike after radial keratotomy and myopic LASIK, results with adjusted SimK were similar to those with adjusted average central corneal power. 3,17 However, adjustment based on the latter seems to be universal across all topography platforms, whereas SimK adjustment seems to be machine dependent. Hence, unlike the 398

7 Awwad et al IOL Calculations after Hyperopic LASIK Table 5. Percentage of Eyes within a Certain Intraocular Lens (IOL) Power Error for the Regression- Derived Formulas, Compared with Popular Methods to Calculate IOL Power in Eyes after Hyperopic Keratorefractive Surgery IOL Power Prediction Methods Percentage of Eyes within a Certain IOL Power Estimation Error 0.5 D 1.0 D 1.5 D 2.0 D Double-K methods ACCP adj and double-k Hoffer Q SimK adj and double-k Hoffer Q Historical K and double-k Hoffer Q Single-K methods Feiz Mannis method 33* Modified Feiz Mannis method Masket method Modified Masket method Latkany method 13* 25* 54* 79* IOL adjaccp IOL adjsimk ACCP 3mm average central corneal power within the central 3 mm; ACCP adj ACCP 3mm SE 0.256; D diopters; IOL adjaccp single-k IOL ACCP3mm 0.35 SE 0.085; IOL adjsimk single-k IOL SimK SE 0.092; SE SE pre-lasik SE post-lasik ; SimK simulated keratometric value; SimK adj SimK SE *Statistical significance compared to ACCP adj or SimK adj and double-k Hoffer Q. Formula derived by regression analysis. Latkany and Masket methods, a regression method that bypasses the double-k concept should seemingly rely on the average central corneal power, not SimK, to be universally applicable on all topographers; this has been proven by our results and those of Wang et al on corneal power estimation. 15 Although the original and modified Masket methods use the Hoffer Q for hyperopic eyes with axial length 23.0 mm, the Latkany method was originally described using SRK/T. In our comparison, we used the Hoffer Q for all methods to avoid confounding variables in our comparison and because of its known performance in eyes with short axial length. One limitation of our study is that we did not measure the actual ELP of eyes after phacoemulsification (calculated as measured ACD added to half the IOL thickness). Had the actual true ELP been measured, then the refractive corneal power could have been calculated using the single-k Hoffer Q and it would have been more accurate when compared with the values obtained from using a personalized ACD in the double-k Hoffer Q formula. Another limitation of our study is that our sample size of 24 eyes, although relatively large compared with previous studies on posthyperopic LASIK pseudophakic eyes, is still considered small on a statistical level. In summary, for accurate IOL calculations after hyperopic keratorefractive surgery, we recommend adjusting the average central corneal power according to the induced refractive change SE (ACCP adj ACCP 3mm (SE pre-lasik SE post-lasik ) [equation 3]) or as displayed in Table 3, then using the obtained value in the double-k Hoffer Q formula. This method ensures both accurate results and potentially reproducible outcomes among topographers. In the absence of historical data on the patient s refractive status, the average central corneal power can be adjusted as such: ACCP= ACCP 3mm (equation 1), then used in the double-k Hoffer Q formula with pre-lasik keratometric value assumed to be If the average central corneal power is not available, we recommend adjusting the SimK according to the refractive change: SimK adj SimK (SE pre-lasik SE post-lasik ) (equation 4), and if the refractive change is also missing, then using SimK= SimK (equation 2) would be appropriate. If the clinician does not have access to the double-k Hoffer Q formula, using the average central corneal power or the SimK values as such in the single-k Hoffer Q formula and adjusting the final IOL power based on the SE seems to be equally accurate: IOL adj_accp IOL ACCP3mm 0.35 SE and IOL adj_simk IOL SimK SE (equations 5 and 6, respectively). In this scenario, the original Masket formula seems also to perform very well and can be used as well (IOL IOL SimK [LSE] 0.101). However, it is important to bear in mind that, especially for regression methods using SimK, prior validation on the clinician s topographer is recommended to confirm reproducibility of the suggested formulas. In addition, although the suggested formulas bypassing the double-k method (including Masket s formula) seem simple and accurate, it would be wise to await reproducibility on a large sample of eyes before recommending it across the board, because the accuracy of such method for large values of SE changes has not yet been tested. The advent of slit-beam videokeratography with the ability to measure both the anterior and the posterior surface of the cornea seems promising in improving the accuracy of KBC measurement after keratorefractive surgery. In the meantime, relying on accurate, and most importantly, reproducible, regression-derived formulas on 399

8 Ophthalmology Volume 116, Number 3, March 2009 the placido systems after hyperopic keratorefractive surgery is key to satisfactory patients outcomes. References 1. Kalski RS, Danjoux JP, Fraenkel GE, et al. Intraocular lens power calculation for cataract surgery after photorefractive keratectomy for high myopia. J Refract Surg 1997;13: Holladay JT. Corneal topography using the Holladay Diagnostic Summary. J Cataract Refract Surg 1997;23: Maeda N, Klyce SD, Smolek MK, McDonald MB. Disparity between keratometry-style readings and corneal power within the pupil after refractive surgery for myopia. Cornea 1997;16: Seitz B, Langenbucher A. Intraocular lens calculations status after corneal refractive surgery. Curr Opin Ophthalmol 2000; 11: Feiz V, Mannis MJ, Garcia-Ferrer F, et al. Intraocular lens power calculation after laser in situ keratomileusis for myopia and hyperopia: a standardized approach. Cornea 2001;20: Wang L, Booth MA, Koch DD. Comparison of intraocular lens power calculation methods in eyes that have undergone LASIK. Ophthalmology 2004;111: Masket S, Masket SE. Simple regression formula for intraocular lens power adjustment in eyes requiring cataract surgery after excimer laser photoablation. J Cataract Refract Surg 2006;32: Walter KA, Gagnon MR, Hoopes PC Jr, Dickinson PJ. Accurate intraocular lens power calculation after myopic laser in situ keratomileusis, bypassing corneal power. J Cataract Refract Surg 2006;32: Ianchulev T, Salz J, Hoffer K, et al. Intraoperative optical refractive biometry for intraocular lens power estimation without axial length and keratometry measurements. J Cataract Refract Surg 2005;31: Camellin M, Calossi A. A new formula for intraocular lens power calculation after refractive corneal surgery. J Refract Surg 2006;22: Aramberri J. Intraocular lens power calculation after corneal refractive surgery: double-k method. J Cataract Refract Surg 2003;29: Hamed AM, Wang L, Misra M, Koch DD. A comparative analysis of five methods of determining the corneal refractive power in eyes that have undergone myopic laser in situ keratomileusis. Ophthalmology 2002;109: Borasio E, Stevens J, Smith GT. Estimation of true corneal power after keratorefractive surgery in eyes requiring cataract surgery: BESSt formula. J Cataract Refract Surg 2006;32: Qazi MA, Cua IY, Roberts CJ, Pepose JS. Determining corneal power using Orbscan II videokeratography for intraocular lens calculation after excimer laser surgery for myopia. J Cataract Refract Surg 2007;33: Wang L, Jackson DW, Koch DD. Methods of estimating corneal refractive power after hyperopic laser in situ keratomileusis. J Cataract Refract Surg 2002;28: Chokshi AR, Latkany RA, Speaker MG, Yu G. Intraocular lens calculations after hyperopic refractive surgery. Ophthalmology 2007;114: Awwad ST, Dwarakanathan S, Bowman RW, et al. Intraocular lens power calculation after radial keratotomy: estimating the refractive corneal power. J Cataract Refract Surg 2007;33: Langenbucher A, Haigis W, Seitz B. Difficult lens power calculations. Curr Opin Ophthalmol 2004;15: Hoffer KJ. The Hoffer Q formula: a comparison of theoretic and regression formulas [published correction appears in J Cataract Refract Surg 1994;20:677]. J Cataract Refract Surg 1993;19: Footnotes and Financial Disclosures Originally received: January 8, Final revision: August 31, Accepted: September 25, Manuscript no Department of Ophthalmology, University of Texas Southwestern Medical Center, Dallas, Texas. 2 Department of Ophthalmology, American University of Beirut Medical Center, Beirut, Lebanon. Supported in part by an unrestricted grant from Research to Prevent Blindness, New York, New York. Financial Disclosure(s): J.P.M. is a consultant for Alcon Labs, Fort Worth, Texas. The authors have no proprietary or commercial interest in materials discussed in this article. Correspondence: James P. McCulley, MD, Department of Ophthalmology, University of Texas Southwestern Medical Center at Dallas, 5323 Harry Hines Blvd. Dallas, Texas james.mcculley@utsouthwestern.edu. 400

9 Awwad et al IOL Calculations after Hyperopic LASIK Appendix 1 Double-K modification of the Hoffer Q Formula Recommended constants: Refractive index of cornea Retinal thickness factor 0 Measured and extrapolated values: K pre-lasik average K-reading before LASIK (D) K post-lasik estimated refractive corneal power after LASIK (D) AL measured axial length (mm) Chosen values V vertex distance of pseudophakic spectacles (mm), default 12 mm pacd personalized anterior chamber depth constant pacd A constant Calculated variables P power of the IOL (D) R refractive error at corneal plane (D) Rx target refractive error at spectacle plane (D) Equations Equation 1: Anterior chamber depth ACD pacd 0.3(AL 23.5) tan(k pre-lasik ) 2 (0.1 * M * [23.5 AL] 2 [tan{0.1(g AL) 2 }]) M: ifal 23.00, M 1; if AL 23 mm, M 1 G: ifal 23.00, G mm; if AL 23 mm, G 23.5 mm If AL 31, AL 31.0; if AL 18.5, AL 18.5 Equation 2: Refractive error at corneal plane R Rx ( Rx) Equation 3: Intraocular lens power P (1336 [AL ACD 0.05]) (1.336 [{1.336 (K post-lasik R)} {(ACD 0.05) 1000}]) Equation 4: Refractive error R (1.336 [1.336 {1336 (AL ACD 0.05) P} {ACD 0.05} 1000]) K post-lasik Rx R ( R) 400.e1