CLINICAL SCIENCES. A Direct Method to Measure the Power of the Central Cornea After Myopic Laser In Situ Keratomileusis

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1 CLINICAL SCIENCES A Direct Method to Measure the Power of the Central Cornea After Myopic Laser In Situ Keratomileusis Sergio Sónego-Krone, MD; Gerson López-Moreno, MD; Oscar V. Beaujon-Balbi, MD; Carlos G. Arce, MD; Paulo Schor, MD; Mauro Campos, MD Objective: To measure the corneal power after myopic laser in situ keratomileusis (LASIK). Methods: Six central areas in 6 corneal power maps were studied using the Orbscan II statistical analysis device in 26 eyes that underwent myopic LASIK. Refractive and corneal power changes were compared. Factors related to wrong corneal power measurement were evaluated. Main Outcome Measures: Cycloplegic refraction, refractive change at the corneal plane, and Orbscan II corneal power maps. Results: Preoperatively, only posterior-mean power (P.001) and anterior-posterior power ratio (P.001) varied according to the size of the analyzed area. Postoperatively, total-optical (P=.03), keratometric-mean (P=.04), total-mean (P.001), anterior-mean (P=.03), and posterior-mean (P.001) powers; and anteriorposterior power ratio (P.001) varied according to the area. Postoperatively, the difference between keratometricmean and total-mean powers became larger (P.001), and the anterior-posterior power ratio was reduced (P.001). A posterior-mean power change occurred (P=.04). Refractive change after myopic LASIK was best estimated by 2-mm total-mean power (mean±sd difference, 0.07±0.62 diopters [D]; P=.55) and 4-mm totaloptical power (mean±sd difference, 0.08±0.53 D; P=.37). Conclusions: Total corneal power is more positive and refractive change is underestimated when deduced from the anterior surface radius and keratometric refractive index. The anterior-posterior power ratio is not a fixed value. The best area to estimate the refractive change depends on the method used to obtain the power in diopters. Refractive change tended to be underestimated in larger areas and higher preoperative myopia. Orbscan II total-mean and total-optical power maps accurately assess the corneal power after myopic LASIK independent of preoperative data or correcting factors, and should improve intraocular lens calculation. Arch Ophthalmol. 2004;122: From the Department of Ophthalmology, Paulista School of Medicine, Federal University of São Paulo, São Paulo, Brazil (Drs Sónego-Krone, López-Moreno, Beaujon-Balbi, Arce, Schor, and Campos); and the Luis Razetti Clinic and Metropolitan Center of Ophthalmology, Francisco Risquez Hospital, Caracas, Venezuela (Dr Beaujon-Balbi). Dr Sónego-Krone is now with the Eye Service, Hospital of Clinics, Faculty of Medical Sciences, National University of Asunción, Asunción, Paraguay. The authors have no relevant financial interest in this article. AS PATIENTS WHO UNDERGO refractive surgery become older, their probabilities of development of cataract are increased. It has been observed that residual refractive errors can occur after cataract surgery in these cases. Thus, a residual hyperopia was found in patients undergoing previous myopic laser in situ keratomileusis (LASIK) or photorefractive keratectomy, and the reason for this error is not clear. It has been suggested that formulas for intraocular lens (IOL) power calculation may not be accurate, 1-3 and/or that the corneal power may be incorrectly measured Myopic LASIK produces deliberate modifications in the anterior surface of the cornea and its thickness to correct a refractive defect. The normal prolate (convexity steeper in the center) anterior surface is converted to an oblate (convexity flatter in the center) surface. Therefore, it may not be correct to apply conventional variables assumed for normal corneas to surgically modified corneas. Most IOL calculation formulas 1,13,14 assume that the cornea is spherical and commonly use a corneal power determined by means of manual or computerized keratometry or by means of corneal videokeratography (CVK) using Placido disks. These methods deduce the total power of the cornea by measuring the radius of the corneal anterior surface curvature from a central area with a diameter of approximately 3 mm. Conversion of millimeters of radius to diopters (D) is performed using a theoretical effective (keratometric) refractive index of ,15,16 To improve accuracy, attempts to correct this index have been made, 17 resulting in other adapted theoretical indexes that have been applied in automatic keratometers or IOL formulas. 7,18 159

2 Table 1. General Data* Characteristic Finding Eye, No. right/left 11/15 Sex, No. F/M 14/4 Age, y ± 8.29 (25 to 56) Preoperative spherical equivalent, D 4.09 ± 1.59 ( 8.01 to 1.34) Preoperative sphere, D 3.39 ± 1.83 ( 6.88 to 0.25) Preoperative cylinder, D 1.41 ± 1.17 ( 4.22 to 0.00) Postoperative spherical 0.50 ± 0.56 ( 1.47 to +0.89) equivalent, D Refractive change, D 3.59 ± 1.46 ( 6.86 to 0.97) Refractive change, D 3.77 ± 1.70 ( 7.50 to 1.00) Programmed ablation, µm ± (30.50 to 96.70) Diameter of the treatment area, mm 5.60 ± 0.25 (5.5 to 6.5) Postoperative follow-up, mo 2.54 ± 1.66 (1 to 6) Abbreviation: D, diopter. *Laser in situ keratomileusis performed in 26 eyes with the Ladar Vision excimer laser system (Alcon-Summit Autonomous, Orlando, Fla). Microkeratomes used included the following: Moria M2 and Moria CB (Moria, Antony, France); Hansatome (Bausch & Lomb Surgical, Miami, Fla); and Automated Corneal Sharper (Chiron Vision Inc, Irvine, Calif). Unless otherwise indicated, data are expressed as mean ± SD (range). Indicates corneal plane. Indicates spectacle plane. On the other hand, combined slit-scanning and Placido-disk CVK (Orbscan II; Bausch & Lomb Orbtek Inc, Salt Lake City, Utah) is a relatively new technology able to localize 9000 points of the cornea and anterior chamber and transform them to topographic maps. This equipment calculates the power of the cornea by diverse mathematical methods, using the keratometric refractive index (keratometric power maps) or the physiologic refractive indexes (anterior, posterior, or total power maps). Ray tracing (optical power maps), spherical equivalent (mean power maps), astigmatism (astigmatic power maps), differential with best-fit sphere (elevation maps), and thickness (power and pachymetry maps) may be represented. 19,20 In addition, this system may statistically analyze areas as small as a central point with a 40-µm diameter, 21,22 and as large as the peripheral limit of data achievement around a 9-mm diameter. The definition and explanation of Orbscan II principles and maps may be obtained elsewhere Manufacturer nomenclature has been criticized. 24 In scientific reports, the terms total and mean, used to designate the type of some Orbscan II power maps, may be confused with terms representing values achieved from data analysis. Thus, unless otherwise indicated, this article hyphenates compound words that identify Orbscan II maps. Using this equipment, our group recently reported that the power of normal corneas deduced from the measurement of only the anterior surface using the keratometric refractive index is approximately 1.5 D more positive than the power calculated from all of its optical components and using the physiologic refractive indexes. It has been shown that the size of an analyzed area of less than a 5-mm diameter has no effect in the total power of normal corneas or in the power of their anterior surface. It also has been shown that the power of their posterior surface becomes less negative and that their thickness increases when larger areas are analyzed. Furthermore, the 10:1 ratio traditionally accepted for anterior and posterior corneal powers was not confirmed. 22 This study evaluates the variability of corneal power measurements obtained by means of the Orbscan II topography system before and after myopic LASIK. Changes in the spherical equivalent (calculated using the keratometric or the physiologic refractive indexes) and ray tracing (calculated using the physiologic refractive indexes) corneal powers are compared with the refractive change at the corneal plane. Among factors that have been related to wrong corneal power measurement after refractive surgery, 4-12 this study analyzes the influence of keratometric and physiologic refractive indexes, the anterior-posterior corneal power ratio, the corneal power change according to the size of the analyzed area, and the contribution of thickness and posterior surface on total power change, with the goal to determine the best variables that should be applied for an accurate and direct assessment of corneal power after myopic LASIK. METHODS This study was designed as an observational case series. A retrospective review of preoperative and postoperative combined slitscanning and Placido-disk CVK with the Orbscan II corneal topography system was made in 26 eyes of 18 patients at the Department of Ophthalmology, Paulista School of Medicine, Federal University of São Paulo (UNIFESP/EPM), São Paulo, Brazil. Postoperative examination was made at least 1 month after eyes underwent myopic LASIK using the Ladar Vision excimer laser system (Alcon-Summit Autonomous, Orlando, Fla). Written informed consent and cycloplegic refraction were used routinely. Patients had no other abnormality except myopia (mean±sd sphere, 3.39±1.83 D) or myopic astigmatism (mean±sd cylinder, 1.41±1.17 D), and no postoperative complication other than some residual ametropia (mean±sd spherical equivalent, 0.50±0.56 D). Cases were consecutively selected from the Orbscan II hard disk (Table 1). Average values of all points of totaloptical (representing the ray tracing of anterior and posterior surfaces using physiologic refractive indexes), keratometric-mean (representing the spherical equivalent of anterior surface using the keratometric refractive index), total-mean (representing the spherical equivalent of both corneal surfaces plus thicknessmean power using physiologic refractive indexes), anteriormean and posterior-mean (representing the spherical equivalent of each corneal surface using physiologic refractive indexes), and thickness-mean (representing the contribution of thickness to the total power of the cornea) power maps were assessed using the Orbscan II statistical analysis device (software version 3.00D) for 6 central areas with 0.04-, 1.0-, 2.0-, 3.0-, 4.0-, and 5.0-mm diameters (±0.02 mm) as described elsewhere. 21,22,24 Spherical-cylindrical spectacle refraction was converted to a spherical equivalent corneal plane value using a vertex distance of 12 mm. The refractive change (spherical equivalent at the corneal plane) induced by LASIK was calculated by subtracting the postoperative residual refractive defect from the preoperative ametropia, and it was compared with the corneal power change determined by each Orbscan II power map in every analyzed area. The influence of keratometric and physiologic refractive indexes on total corneal power calculation was studied by determining the difference between keratometric-mean and total-mean powers for each eye and then the average for each area. The anteriorposterior corneal power ratio was studied by analyzing the relationship between anterior-mean and posterior-mean powers. A posterior surface change was studied by comparison of the posterior-mean power before and after surgery. The 2-tailed paired t test, analysis of variance with 1 factor, and linear correlation (R 2 ) were calculated using Excel

3 Table 2. Orbscan II Corneal Powers Before Myopic LASIK* P Value Total-optical power ± ± ± ± ± ± Keratometric-mean power ± ± ± ± ± ± Total-mean power ± ± ± ± ± ± Thickness-mean power ± ± ± ± ± ± Anterior-mean power ± ± ± ± ± ± Posterior-mean power 6.74 ± ± ± ± ± ± Pearson Anterior-posterior power ratio 7.27 ± ± ± ± ± ± Abbreviation: LASIK, laser in situ keratomileusis. *Orbscan II indicates combined slit-scanning and Placido-disk corneal videokeratography (Bausch & Lomb Orbtek Inc, Salt Lake City, Utah). Unless otherwise indicated, data are expressed as mean ± SD in diopters (n = 26). Calculated by analysis of variance between diameters. Indicates 2-tailed Pearson correlation between anterior-mean and posterior-mean powers with P.008. Table 3. Orbscan II Corneal Powers After Myopic LASIK* P Value Total-optical power ± ± ± ± ± ± Keratometric-mean ± ± ± ± ± ± power Total-mean power ± ± ± ± ± ± Thickness-mean power ± ± ± ± ± ± Anterior-mean power ± ± ± ± ± ± Posterior-mean power 7.27 ± ± ± ± ± ± Pearson Anterior-posterior power ratio 6.25 ± ± ± ± ± ± Abbreviation: LASIK, laser in situ keratomileusis. *Orbscan II indicates combined slit-scanning and Placido-disk corneal videokeratography (Bausch & Lomb Orbtek Inc, Salt Lake City, Utah). Unless otherwise indicated, data are expressed as mean ± SD in diopters (n = 26). Calculated by analysis of variance between diameters. Indicates 2-tailed Pearson correlation between anterior-mean and posterior-mean powers with P.017. (Microsoft Corporation, Redmond, Wash). Nonparametric Wilcoxon signed rank test and Pearson correlation factor (multiple R) were performed using SPSS 7.5.2S (SPSS Inc, Chicago, Ill). An risk of.05 was established. Unless otherwise indicated, data are expressed as mean±sd. This study was reviewed and approved by the UNIFESP/EPM Ethics Committee in Research. RESULTS Mean preoperative spherical equivalent was 4.09±1.59 D. Mean refractive change at the corneal plane (gold standard) was 3.59±1.46 D. Mean follow-up for postoperative refraction and topography was 2.54±1.66 months. Information about cases and LASIK variables are summarized in Table 1. CENTRAL CORNEAL POWER ACCORDING TO THE SIZE OF THE ANALYZED AREA Preoperative (Table 2) total-optical, keratometricmean, total-mean, anterior-mean, and thickness-mean powers were not statistically different in all analyzed areas. The average of the posterior-mean power varied from 6.74 D when measured in the smallest central area to 6.39 D when assessed from an area with a 5-mm diameter. The standard deviation of posterior-mean power in all areas was proportionally similar (3%-5% of average values) to the standard deviation found in all other maps, except thickness-mean power maps (around 10%). Postoperative (Table 3) total-optical, keratometricmean, total-mean, and anterior-mean powers became larger (more positive), and posterior-mean power became smaller (less negative) when larger areas were analyzed. Thickness-mean power was not statistically different in all analyzed areas. ANTERIOR-POSTERIOR CORNEAL POWER RELATIONSHIP Preoperative (Table 2) and postoperative (Table 3) better Pearson correlations between anterior-mean and posterior-mean powers were found when larger analyzed areas were measured. Before LASIK, the average of the anteriorposterior corneal power ratio varied from 7.27 in the smallest analyzed area to 7.63 in the 5-mm-diameter area. After the myopic LASIK, this ratio was smaller (2-tailed paired t test, P.001) and varied from 6.25 to 7.28, re- 161

4 Table 4. Difference Between Keratometric-Mean and Total-Mean Power Maps* Preoperative ± ± ± ± ± ± 0.11 Postoperative ± ± ± ± ± ± 0.16 *Unless otherwise indicated, data are expressed as mean ± SD in diopters (n = 26). Preoperative keratometric-mean power was more positive than total-mean power in all 6 analyzed areas (2-tailed paired t test; P.001). Postoperative keratometric-mean power was more positive than total-mean power in all 6 analyzed areas (2-tailed paired t test; P.001). Postoperative difference was larger than preoperative in all 6 analyzed areas (2-tailed paired t test; P.001). Table 5. Total Corneal Power Changes After Myopic LASIK* P Value Total-optical power Postoperative power 4.26 ± ± ± ± ± ± Difference with refractive 0.66 ± ± ± ± ± ± 0.62 No. of cases with difference greater than ± 1.0 D No. of cases with power refractive P value Keratometric-mean power Postoperative power 3.21 ± ± ± ± ± ± Difference with refractive 0.39 ± ± ± ± ± ± 0.92 No. of cases with difference greater than ± 1.0 D No. of cases with power refractive P value Total-mean power Postoperative power 4.10 ± ± ± ± ± ± Difference with refractive 0.50 ± ± ± ± ± ± 0.86 No. of cases with difference greater than ± 1.0 D No. of cases with power refractive P value Abbreviations:, corneal or refractive change after LASIK; D, diopter; LASIK, laser in situ keratomileusis. *Refractive at the corneal plane (gold standard) was 3.59 ± Unless otherwise indicated, data are expressed as mean ± SD in diopters (n = 26). Calculated by analysis of variance between diameters. Calculated by the 2-tailed paired t test between refractive and respective corneal power. spectively. Preoperatively and postoperatively, this variability was extremely significant, meaning that the anterior-posterior power ratio was not a fixed value. INFLUENCE OF KERATOMETRIC AND PHYSIOLOGIC REFRACTIVE INDEXES Preoperative keratometric-mean power was more positive than total-mean power by 1.61 D in the smaller analyzed area, and by 1.31 D in the 5-mm-diameter area (Table 4). Postoperatively, this difference was larger and ranged from 2.50 to 1.61 D, respectively. TOTAL CORNEAL POWER CHANGES AFTER MYOPIC LASIK Changes in keratometric-mean and total-mean powers after myopic LASIK were smaller when assessed in larger areas (Table 5). Changes in total-optical power had this tendency, but they were not statistically different in all 6 analyzed areas. The refractive change at the corneal plane had very good Pearson correlation (P.001) with the total-optical (R 0.80), total-mean (R 0.87), and keratometric-mean (R 0.89) power changes. Linear correlation showed that measurement of power on larger areas with total-optical (Figure 1) and total-mean (Figure 2) power maps tended to underestimate the LASIK outcome, especially for higher ametropias. Total-optical power changes in a 3-mm- (Wilcoxon, P=.07) and a 4-mm- (Wilcoxon, P=.37) diameter area were not different from the refractive change. Total-optical power changes measured in other area sizes were different. Keratometric-mean power changes were best assessed in the smallest analyzed area (Wilcoxon, P=.009), but they were different from the refractive change in all 6 areas. Total-mean power changes were best assessed in a 2-mm-diameter area (Wilcoxon, P=.60). Total-mean power changes in other area sizes were different from the refractive change. Symmetry of values above and below confirmed the more representative areas. A difference of larger than 1 162

5 9 8 Ø: 0.04 mm y = x R 2 = Ø: 1.0 mm y = x R 2 = Ø: 2.0 mm y = x R 2 = Total-Optical Power Change, D Ø: 3.0 mm y = x R 2 = Ø: 4.0 mm y = x R 2 = Ø: 5.0 mm y = x R 2 = Refractive Change at the Corneal Plane, D Figure 1. Linear correlation (in diopters [D]) between refractive change at the corneal plane (x-axis) and total-optical power changes (y-axis) assessed in 6 central areas. indicates diameter. D was found in only 2 eyes (1.15 and 1.02 D) with the 4-mm total-optical power, and in 1 case (1.40 D) with the 2-mm total-mean power. CONTRIBUTION OF ANTERIOR SURFACE, POSTERIOR SURFACE, AND THICKNESS- DERIVED POWERS ON TOTAL CORNEAL POWER CHANGE Changes in anterior-mean power after myopic LASIK were smaller when assessed in larger areas (Table 6). The refractive change at the corneal plane correlated (R 0.89; P.001) with the anterior-mean power change in all 6 analyzed areas. Anterior-mean power changes in the central smallest area (Wilcoxon, P=.85) and in the 1-mmdiameter area (Wilcoxon, P=.27) were not different from the refractive change. Anterior-mean power changes in other area sizes were different from the refractive change. The posterior-mean power increased (steepening) almost 0.50 D in the central point and the 1-mmdiameter area (12%-13% of total-mean power change), and some more than 0.25 D in the 2-mm- and 3-mmdiameter areas (8%-10%). The larger the analyzed area, the smaller its contribution to the total-mean power change. Although it had statistical significance, posteriormean power change was without clinical importance and hardly noticeable when a 5-mm-diameter area was analyzed. The refractive change had no correlation with the change found in the posterior-mean power (R 0.28; P.17). Preoperative thickness-mean power ranged from 0.11 to 0.17 D (mean, 0.14 D [Table 2]). Changes of thicknessmean power were too small (mean, 0.01 D; never larger than 0.03 D) to have clinical importance, so we waived statistical tests for them. COMMENT The cornea is an optic system composed of anterior and posterior surfaces, a distance between them (thickness), and basically 3 optical media (air, corneal tissue, and aqueous humor), each one with its own refractive index. Presently, it is known that all these components must be considered when calculating the real power of the cornea. 7,15,22 Traditional methods (eg, keratometry, simulated keratometry [sim-k]) assume the total power of the cornea from the measurement of the radius of curvature of its anterior surface. Only recently, when slit-scanning CVK (Orbscan I) measurement of the posterior surface power of the cornea became available, such an assumption began to be challenged. Accuracy questioning 12,25 and verification 26 of this technology have been reported. Although its reliability may be controversial, it seems improved after it was combined with Placido-disk based CVK (Orbscan II). A great variability on corneal power may be achieved with the Orbscan II by measuring different area sizes using different mathematical processes. 22 Essentially, the basis of calculation of all maps is the measurement of the distance between each point localized by the system and the sphere that best fits these points. This distance is called 163

6 9 8 Ø: 0.04 mm y = x R 2 = Ø: 1.0 mm y = x R 2 = Ø: 2.0 mm y = 0.808x R 2 = Total-Mean Power Change, D Ø: 3.0 mm y = x R 2 = Ø: 4.0 mm y = x R 2 = Ø: 5.0 mm y = x R 2 = Refractive Change at the Corneal Plane, D Figure 2. Linear correlation (in diopters [D]) between refractive change at the corneal plane (x-axis) and total-mean power changes (y-axis) assessed in 6 central areas. indicates diameter. Table 6. Anterior Surface, Posterior Surface, and Thickness-Derived Corneal Power Changes After Myopic LASIK* P Value Anterior-mean power Postoperative power 3.64 ± ± ± ± ± ± P value Posterior-mean power Postoperative power 0.52 ± ± ± ± ± ± P value Thickness-mean power Postoperative power 0.01 ± ± ± ± ± ± Abbreviations:, corneal or refractive change after LASIK; D, diopter, LASIK, laser in situ keratomileusis. *Refractive at the corneal plane (gold standard) was 3.59 ± Unless otherwise indicated, data are expressed as mean ± SD in diopters (n = 26). Calculated by analysis of variance between diameters. Calculated by the 2-tailed paired t test between refractive and anterior-mean power. Calculated by the 2-tailed paired t test between preoperative and postoperative posterior-mean power. elevation and may be negative (down to the best-fit sphere and represented by cold colors) or positive (up to the bestfit sphere and represented by warm colors) Optical, mean, keratometric, total, and other power maps may be obtained. 21,22 However, a few values automatically shown are a potential source of confusion if applied in IOL calculation. The power of the posterior best-fit sphere shown on top of default quad maps, for example, is calculated using the keratometric refractive index of the cornea (1.3375), despite the fact that components of this optical interface are the cornea (refractive index of 1.376) and the aqueous humor (refractive index of 1.336). On the other hand, diopters automatically repeated at right of several individual maps, for standard or default statistical zones, also seem to be obtained using the keratometric refractive index, are not the same values found by the Orbscan II statistical analysis device, 22 and should be interpreted only as keratometric data. The Orbscan II statistical analysis device is able to give the average measurement of all identified points in a selected area of any of multiple available maps, its lowest and highest value, and its standard deviation. It is a simple (using the control-a keyboard short- 164

7 cut) but time-consuming process, and it seems to be the best way to obtain information from this equipment. A suggestion has been made to the manufacturer to simplify the achievement of data with it.variability of this method is on the third decimal in power maps. 22 Several methods were suggested to measure the power of corneas subjected to refractive surgery. Keratometry has been replaced by Placido-disk based CVK, by sim-k, 7,27-29 or by mean central corneal power. 4 However, these methods evaluate areas of approximately 3 mm in diameter that might not be appropriate to calculate the power of corneas that have undergone refractive surgery when the keratometric refractive index is used. The tendency these keratometric powers have to indicate a more positive total corneal power and to underestimate the refractive change after myopic LASIK 7,15,30 or photorefractive keratectomy 7,29,31,32 seems to be compatible with the undercorrection usually found after cataract surgery in patients with myopic refractive surgery. 7,15,29-34 Presently, the criterion standard and the more accurate method for corneal power estimation after refractive surgery is the clinical history method, also called refraction-derived keratometry. The refractive change is subtracted from the power of the cornea before the refractive surgery to determine the final corneal power to be applied in IOL calculation. 5,7,11,12,15,32-37 Since this information is not always available, it was suggested that an overrefraction with a rigid contact lens with a known base curve should be made, 7,12,15,35,38 a higher fictitious refractive index for the cornea should be adopted, 10,12,17 regression formulas to adjust the smallest ring on CVK should be applied, 30 corneal radius correcting factors should be calculated, 39 or computer programs should be used. 40 Although these empirical methods may be effective, they assume an expected value and do not assess a realistic corneal power. A more pragmatic approach would be to understand why traditional methods result in errors and then, instead of adapting these errors, look for how to avoid them. The capability of the Orbscan II corneal topography system to evaluate the central portion of the cornea seems to give us such an opportunity. The keratometric refractive index of assumes that the power of the anterior surface of the cornea is almost 10 times the power of the posterior surface. However, our results show a smaller anterior-posterior corneal power ratio, confirming our earlier report. 22 This seems to be because the Gullstrand model eye from which the keratometric refractive index is deduced assumes a lower theoretical posterior surface power than what was actually assessed by the Orbscan I 7,41 and II. 22 Furthermore, this ratio is not fixed in all of the extension of both corneal surfaces, but it changes according to the size of the measured area. That correlation between powers of both surfaces improves when larger areas are assessed. This study also confirms that the total corneal power is more positive when deduced from the anterior surface radius using such a keratometric refractive index. As a consequence of changes on corneal surfaces, these findings are more remarkable after myopic LASIK, suggesting that traditional methods to calculate the corneal power may be inappropriate in these cases. Preoperatively, the total power of the normal cornea and the power of its anterior surface do not vary and are independent of the size of the analyzed area if it has a diameter of less than 5 mm. Nevertheless, the power of the posterior surface becomes less negative when measured in larger areas. 22 Myopic LASIK iatrogenically modifies the normal geometry of the cornea by promoting higher power changes in the central portion of the cornea. The expected flattening (less positive power) of the anterior surface, reflected by the change in anteriormean power maps, tended to diminish progressively when larger areas were analyzed and, consequently, the total power of the cornea became more positive. The changes on the anterior surface were responsible for most but not all of the myopic correction induced by the LASIK. As this study shows, a change in the power of the posterior surface of the cornea contributes around 10% of the total power change inside a central area 3 mm in diameter. Theoretically, the refractive change (gold standard) induced by the LASIK should be the same as the power change found in the cornea. However, this was not observed in all the Orbscan II maps or in all of the analyzed areas. The refractive change at the spectacle plane might be used, but we decided to compare this change at the corneal plane because the Orbscan II assesses corneal power changes and not spectacle power changes. Although changes in anterior and total power maps in all 6 analyzed areas had good correlation, correlation tests did not confirm which map or which area reflected better the refractive change after myopic LASIK. Thus, refractive and Orbscan II corneal power changes were also compared using the paired t and Wilcoxon tests. Both tests had equivalent results. To simplify, the P values shown in Table 5 correspond to paired t test results. Areas larger than 4 mm in diameter in most maps tended to underestimate the refractive change, particularly for eyes with higher preoperative myopia. The larger underestimation occurred with the keratometricmean power (Table 5), suggesting that traditional keratometric methods used to obtain the corneal power may be more sensitive to the oblate shape of the anterior surface secondary to the higher energy applied to the central point. Although the effective optical zone of treatment is related to the amount of treatment, 42 we do not yet know whether the size of the effective optical zone may also influence the selection of the best representative central area to compute the refractive change. From this study, we know that the size of this area modifies the result according to the method used to obtain the value in diopters. The best totaloptical power map to reflect the refractive change was found using a 4-mm-diameter area (Table 5 and Figure 1, bottom and center). Keratometric-mean power maps had their best measurement in the smallest, most central point, but always underestimated the refractive change. The best totalmean power map was found using a 2-mm-diameter area (Table 5 and Figure 2, top and right). Anterior-mean power maps also accurately resemble the refractive change when assessed in areas smaller than 1 mm in diameter; however, they do not represent the total corneal power (Table 6). Despite some apparent differences in our methods to obtain the data and the upgraded equipment we used, our more accurate results are in agreement with those of an earlier report, suggesting the use of the 4-mm total-optical power map. 43 We were unable to find any other previous report of this in the literature. 165

8 Although we studied a limited number of patients, from our results we find it reasonable to recommend the use of the 2-mm total-mean power and/or the 4-mm totaloptical power, assessed by the Orbscan II statistical analysis device, as accurate values to be applied for IOL calculation in patients who underwent myopic LASIK. Until more information is collected, it might be prudent to use the smaller of both values when it is known that the corrected refractive defect was higher than 5 D. Caution must be taken to extrapolate our results to other refractive errors. Further prospective studies with patients who will undergo cataract surgery might demonstrate the validity of these recommendations. More research is needed to understand all factors that may induce the variability observed in corneal power measurement and to find the best variables that should be used to directly assess the most representative corneal power after other corneal surgeries like photorefractive keratectomy, hyperopic LASIK, radial keratotomy, and penetrating keratoplasty, and even in corneal abnormalities like keratoconus. Submitted for publication February 19, 2003; final revision received September 1, 2003; accepted October 6, This study was presented at the XXVII Annual Symposium of the Paraná Association of Ophthalmology; June 13, 2003; Curitiba, Brazil; at the XXXII Brazilian Congress of Ophthalmology; September 11, 2003; Salvador, Brazil; and at the 23rd Biennial Cornea Research Conference, the Schepens Eye Research Institute and Massachusetts Eye and Ear Infirmary; Harvard Medical School, October 4, 2003; Boston, Mass. This study is the winner of the Paraná Association of Ophthalmology 2003 Prize and of the 23rd Biennial Cornea Conference Research Award. We thank Peter J. Polack, MD, for editorial assistance. Corresponding author and reprints: Carlos G. Arce, MD, Ocular Bioengineer Laboratory, Institute of Vision, Department of Ophthalmology, UNIFESP/EPM, Rua Borges Lagoa 368, São Paulo, SP , Brazil ( cgarce@mpc.com.br). REFERENCES 1. Holladay JT. Intraocular lens power calculations for the refractive surgeon. Oper Tech Cataract Refract Surg. 1996;1: Ishikawa T, Hirano A, Inoue J, et al. Trial for new intraocular lens power calculation following phototherapeutic keratectomy. Jpn J Ophthalmol. 2000;44: Aramberri J. Intraocular lens power calculation after corneal refractive surgery: double-k method. J Cataract Refract Surg. 2003;29: Maeda N, Klyce SD, Smolek MK, MacDonald MB. Disparity between keratometrystyle readings and corneal power within the pupil after refractive surgery for myopia. 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