No-history method of intraocular lens power calculation for cataract surgery after myopic laser in situ keratomileusis

Transcription

1 J CATARACT REFRACT SURG - VOL 33, JANUARY 2007 No-history method of intraocular lens power calculation for cataract surgery after myopic laser in situ keratomileusis H. John Shammas, MD, Maya C. Shammas, MD PURPOSE: To prospectively evaluate the no-history method for intraocular lens (IOL) power calculation in 15 cataractous eyes that had previous myopic laser in situ keratomileusis (LASIK) and for which the pre-lasik K-readings were not available. SETTING: Private practice, Lynwood, California, USA. METHODS: The predicted IOL power was calculated in each case. Also calculated were the mean arithmetic and absolute IOL predictor errors, range of the prediction errors, and number of eyes in which the error was within G1.00 diopter (D). RESULTS: The mean arithmetic IOL prediction error was D G 0.63 (SD), and the mean absolute IOL prediction error was 0.55 G 0.31 D (range 0.89 to C1.05 D). Fourteen eyes (93.3%) were within G1.00 D. The results of the Shammas post-lasik formula compared favorably to the results obtained with the optimized Holladay 1 (P Z.42), Hoffer Q (P Z.25), Haigis (P Z.30), and Holladay 2 (P Z.19) formulas and were better than the results obtained with the optimized SRK/T formula (P Z.0005). CONCLUSION: The no-history method is a viable alternative for IOL power calculation after myopic LASIK when the refractive surgery data are not available. J Cataract Refract Surg 2007; 33:31 36 Q 2007 ASCRS and ESCRS Routine intraocular lens (IOL) power calculations in cataractous eyes that have had previous myopic refractive surgery underestimate the IOL power for emmetropia, 1 5 resulting in a high incidence of postoperative unintentional hyperopia. After laser in situ keratomileusis (LASIK), 1 or 2 errors are introduced into IOL power calculation formulas. The first error occurs in all formulas and involves the evaluation of the correct post-lasik K-values needed for accurate IOL power calculations. The second error Accepted for publication August 29, From the Department of Ophthalmology, University of Southern California, The Keck School of Medicine (H.J. Shammas), Los Angeles, California, and the Joan and Sanford I. Weill Medical College of Cornell University (M.C. Shammas), New York, New York, USA. Neither author has a financial or proprietary interest in any material or method mentioned. Corresponding author: H. John Shammas, MD, 3510 M.L. King, Jr. Boulevard, Lynwood, California 90262, USA. hshammas@ aol.com. is in the calculation of the estimated lens position (ELP) by the commonly used third-generation IOL power formulas. When the refractive surgery parameters (pre-lasik K-readings, amount of myopia corrected, or both) are known, many methods are available to correct the K-values and calculate a more accurate ELP However, when the refractive surgery parameters are not known, these corrections are not possible. We recently described a corneal power correction method that uses only data available at the time of cataract surgery, the post-lasik K-readings. 14 This corrected K-value is used in the Shammas post-lasik (Shammas-PL) formula, in which the ELP does not vary with the corneal curvature that has been altered by the LASIK procedure. We named this the no-history method. In this study, we prospectively evaluated the no-history method in 15 eyes scheduled for cataract surgery and for which the pre-lasik K-readings were not available. The results of the Shammas-PL formula and its corneal power correction equation were also compared with results obtained with other optimized formulas and published equations. Q 2007 ASCRS and ESCRS Published by Elsevier Inc /07/$-see front matter doi: /j.jcrs

2 PATIENTS AND METHODS Prospective Evaluation of the No-History Method The no-history method uses the Shammas-PL formula (Figure 1), a post-lasik modification of a previously described formula, 15 in which the average corneal power, K, is replaced by the corrected mean corneal power, Kc, and where Kc Z 1.14 Kpost 6.8, with Kpost being the post-lasik K-readings in diopters (D). 14 No other modification was made to the original formula. The estimated postoperative anterior chamber depth (pacd) is expressed as the C-value in the formula. The conversion equation from the A-constant of a specific IOL to the C-value used in the Shammas-PL formula reads as follows: C Z pacd Z ( A) (Figure 1). This conversion equation is different than the one used to calculate the pacd values in other formulas. Fifteen eyes of 15 consecutive patients who had previous myopic LASIK had uneventful phacoemulsification cataract surgery between June 2004 and December If both eyes had cataract surgery, only the first operated eye was included in the study to avoid data duplication. In each case, the pre-lasik K-values and the amount of myopic correction were not available at the time of the cataract surgery. Axial length measurement was performed with an immersion biometry technique. The K-readings were obtained from an autokeratometer. The IOL power calculations were performed using the no-history method. The aim was a final refraction ranging from plano to 3.00 D depending on the lens status in the fellow eye (clear or cataractous lens) and its refractive error. All surgeries were performed by the same surgeon (H.J.S.). An acrylic IOL (SA60AT, Alcon Surgical Inc.) was placed in the capsular bag in all cases. The final refraction was obtained 10 to 12 weeks after cataract surgery. Calculating Predicted Intraocular Lens Power Using the same formula, the predicted IOL power that would give the actual postoperative refraction in each operated eye was calculated. The IOL prediction error was obtained by subtracting the predicted IOL power from the implanted IOL power. A positive value indicates the formula predicted a lower power IOL than the implanted power and would have left the eye more hyperopic than desired. A negative value indicates that the formula predicted a higher power IOL than the implanted power and would have left the eye more myopic than desired. For each operated eye, the following were evaluated: (1) mean arithmetic IOL prediction error with its standard deviation; (2) mean absolute IOL prediction error with its standard deviation; (3) range of the prediction errors; (4) number of eyes in which the IOL prediction error was within G1.00 D. Comparing Intraocular Lens Power Calculation Formulas Using the same methodology, the IOL prediction error that would have been obtained using optimized versions for post- LASIK calculations of the SRK/T, 16 Holladay 1, 17 Hoffer Q, 18 Haigis, 19 and Holladay 2 20 IOL power formulas was calculated. To be optimized, all formulas required a correction of the corneal power. In this study, the K-readings were corrected using the previously described equation Kc Z 1.14 Kpost 6.8, where Kc is the corrected power and Kpost is the post-lasik K-readings in diopters. 14 In addition to the corneal correction, the popular and commonly used third-generation formulas (SRK/T, 16 Holladay 1, 17 and Hoffer Q 18 ) required ELP recalculation using the Aramberri double-k method. 11 Because the pre-lasik K-values were not known, Aramberri recommended an arbitrary Figure 1. The Shammas-PL formula. 32 J CATARACT REFRACT SURG - VOL 33, JANUARY 2007

3 value of D (personal communication, September 7, 2005), which represents the average K-value of his myopic population database. In our study, the arbitrary pre-lasik K- value of D was used in all cases in which Kpost was flatter than D. In cases 5, 10, and 13, in which the mean Kc value was D, D, and D, respectively, an arbitrary post-lasik K-value of D was used because some myopia had been presumably corrected by the LASIK procedure. The Haigis formula 19 required only a corneal power correction. The ELP is not altered because the corneal power is not used to calculate the estimated position of the IOL. The Holladay 2 formula 20 required the use of the Holladay IOL Consultant Program. Comparing Corneal Power Correction Methods The corneal power correction equation used in this study was also compared to the following published equations: (1) Maloney equation, 9 in which Kc Z Kpost 4.9; (2) Koch-Maloney equation, 9 in which Kc Z Kpost 6.1; (3) Haigis equivalent power formula, 21 in which Kc Z Kpost 5.78; (4) Rosa et al. equation, 22 in which Kc Z Kpost O (0.0276TL C ) and TL is the total eye length. Statistical Analysis Although the number of cases in the study was relatively small, the results were compared using the paired t test. A probability less than 5% (P!.05) was considered statistically significant. RESULTS The patients ranged in age from 28 to 67 years. There were 7 men and 8 women. Table 1 shows the IOL power calculation data in all 15 cases. The average axial length was G 2.52 mm (range to mm). The average post-lasik K-reading (Kpost) was G 3.43 D (range to D). The average corrected K-reading (Kc) was G 3.91 D (range to D). Using the Shammas-PL formula, the average arithmetic IOL prediction error was G 0.63 D and the mean absolute IOL prediction error was 0.55 G 0.31 D (range 0.89 to C1.05 D). Fourteen eyes (93.3%) were within G1.00 D. Table 2 compares the results of the post-lasik corneal power correction equations. The Maloney and Haigis equations yielded, respectively, an average of 0.88 D (P Z.40) and 0.20 D (P Z.84) steeper K-values than our equation, while the Koch-Maloney equation yielded an average of 0.32 D flatter K than our equation (P Z.75). The Rosa et al. equation yielded an average 2.39 D flatter K than our equation (P Z.08). However, contrary to the other methods, the Rosa et al. method is designed to be used with a single-k SRK/T formula and does not necessitate recalculation of ELP. Table 3 compares the results of the Shammas-PL formula with the optimized SRK/T, Holladay 1, Hoffer Q, Haigis, and Holladay 2 formulas. The average arithmetic IOL prediction error obtained with the Shammas-PL formula was compared to the average error obtained with the SRK/T (P Z.0005), Holladay 1 (P Z.42), Hoffer-Q (P Z.25), Haigis (P Z.30), and the Holladay 2 (P Z.19) formulas. The difference between the absolute IOL prediction power errors obtained by all the formulas was not statistically significant; the P value was 0.19, 0.52, 0.19, 0.35, and 0.23, respectively. DISCUSSION Most IOL power calculation formulas are based on standard vergence equations, and the K-readings inserted into these formulas represent the corneal power measurement obtained by keratometry or by topography. Most keratometers and topography units use a conventional index of refraction to convert the measured radius of curvature of the anterior corneal surface to a total corneal dioptric power. This index is based on an assumed fixed Table 1. Intraocular lens power calculation data. Case Parameter Axial length (mm) Average K-value (D) Kc values (D) IOL power for emmetropia (D) IOL used (D) Expected postop refraction (D) Actual postop SE refraction (D) Kc Z corrected K-value; IOL Z intraocular lens; SE Z spherical equivalent J CATARACT REFRACT SURG - VOL 33, JANUARY

4 Table 2. Average K-values in the series with the different corneal power correction methods. Corneal Power Correction Method Average Kpost (D) G SD Average Kc (D) G SD Difference (D) from the Results with the Shammas Equation Shammas G G 3.91 Maloney G G 3.73 C0.88 G 0.09 Koch-Maloney G G G 0.09 Haigis G G 3.84 C0.20 G 0.07 Rosa et al G G G 1.43 Kc Z corrected K-reading; Kpost Z post-lasik K-reading ratio between the front and back curvatures of the cornea. After LASIK, this ratio is altered, introducing an error in the measurement of the corneal power. 11 In other words, the measured K-readings (Kpost) do not represent the correct K-values needed for accurate IOL power calculation in these cases. Different methods to correct the corneal power measurements after LASIK have been advocated. The contact lens overrefraction method remeasures the corneal power instead of recalculating its value. The limitations of this method in our clinical setting revolved around the difficulty in getting accurate refractions in 13 cases (86.67%) with poor vision and induced lenticular myopia. A more appropriate way to calculate the K-value after LASIK is the clinical history method described by Holladay, 10 in which the corrected K-value is obtained by algebraically adding the myopic correction at the corneal plane to the preoperative K-value. A second method is a refractionderived method in which the measured post-lasik keratometry readings are reduced by 0.23 to 0.24 D for each diopter of myopia corrected by the refractive surgery. 6,14 Several newer methods to estimate the corneal power after LASIK have been described. 7 9 However, when the refractive surgical data (pre-lasik K-readings, amount of myopia corrected, or both) are not available, many of these corneal estimation methods cannot be used. In 2003, we described a clinically derived method 14 to calculate the corrected corneal power after LASIK in which Kc Z 1.14 Kpost 6.8. The equation uses the post-lasik K readings (Kpost) taken before cataract surgery and modifies them to determine the corrected K-value (Kc) to be used in the IOL power calculation formulas. Similar corneal power correction methods have been described in the literature. The Maloney and the Koch- Maloney correction equations 9 convert the post-lasik K- readings taken from corneal topography to the exact power present at the anterior corneal surface and then add an average negative power value of the posterior corneal surface. The Maloney equation is best used with the SRK/T formula, and the Koch-Maloney equation is recommended for use with the double-k Holladay 2 and Hoffer Q formulas. 9 Haigis 21 derived his equation theoretically by performing model calculations on a myopic Gullstrand eye using custom computer programs based on thick lens optics and commercial ray-tracing software. The Haigis equivalent power formula corrects the K-values taken from the IOLMaster (Carl Zeiss Meditec) and is best used with the Haigis formula. A new comprehensive eye scanner, the Pentacam (Oculus), images the anterior segment of the eye by a rotating Scheimpflug camera measurement. This rotating process supplies pictures in 3 dimensions, provides a topographic analysis of the corneal front and back surfaces, and generates a TrueNetPower map of the cornea. Such measurements should be valuable in the evaluation of post-lasik corneas. The corneal power correction is the only modification needed to accurately calculate the IOL power with the Table 3. Average IOL power prediction error using the Shammas corneal power correction method with the available IOL power calculation formulas. Mean G SD Formula Arithmetic Error (D) Absolute Error (D) Range of Errors (D) Number of Eyes Within G1.00 D (%) Shammas-PL G G to C (93.3) SRK/T* 0.65 G G to C (80.0) Holladay 1* 0.02 G G to C (80.0) Hoffer Q* 0.23 G G to C (86.7) Haigis 0.22 G G to C (86.7) Holladay 2* 0.25 G G to C (93.3) *Using Aramberri double-k method for ELP calculation J CATARACT REFRACT SURG - VOL 33, JANUARY 2007

5 formula used in this study. The same holds true for the Haigis formula. 19 Neither formula depends on corneal curvature to estimate ELP. However, an additional error is introduced when using a third-generation IOL power formula such as the SRK/T, 16 Holladay 1, 17 or Hoffer Q. 18 The error is caused by the manner in which these formulas internally calculate the ELP, which is the estimated postoperative distance between the anterior corneal surface and the principle plane of a thin IOL. In the SRK/T and the Holladay 1 formulas, the ELP varies with the measurement of the anatomical ACD, which is the distance between the corneal apex and the anterior surface of the iris in an aphakic eye. The anatomical ACD is calculated with the Fyodorov corneal height equation 16,17 Anatomical ACD Z 0:56 þ R R 2 AG 2 =4Þ 2 where R is the anterior corneal radius in millimeters (R Z 337.5/K) and AG is the ACD from angle to angle in millimeters. In a normal eye with a flat cornea, the Fyodorov equation correctly calculates a shallower ACD than in an eye with average K-readings. In an eye that had LASIK, the correct ACD value should be calculated by entering the pre-lasik K-values into the equation. By entering the flatter post-lasik K-values, the formula will underestimate the ACD. This, in turn, will translate into an underestimation of the ELP value used in these third-generation formulas. The error produced by the ELP underestimation and the error produced by the corneal power s overestimation are cumulative, yielding the high hyperopic surprises reported in the literature. The Hoffer Q formula 18 corrects the ELP with a complex formula that is affected by the axial length and the tangent to the K-readings (personal communication, August 4, 2006). In our study, the SRK/T, Holladay 1, and Hoffer Q formulas were modified by replacing the post-refractive K-readings (Kpost), with the corrected value of Kc and by recalculating the ELP using the Aramberri double-k method. 11 Excellent results with the Holladay 2 formula have also been reported in the literature. 26,27 In our series, 1 eye (case 7) had a very long axial length (32.26 mm) and very flat post-lasik K-readings (30.48 D). In this case, the double-k SRK/T formula yielded the most accurate result, with an IOL prediction error of 0.18 D; this error exceeded 1.00 D with all other formulas. In the rest of the cases, the lowest average arithmetic IOL power prediction errors were recorded with the optimized Holladay 1, Hoffer Q, Haigis, Holladay 2, and Shammas-PL formulas. All the formulas yielded an average error of 0.25 D or less. The Rosa et al. corneal power correction method 22 adjusts the corneal radius of curvature with a regression equation based on axial length. This method is best used with the single-k SRK/T formula and does not require ELP correction. In our study, the Rosa et al. method calculated a much flatter Kc value (Table 2) and advocated the use of stronger power IOLs in very long eyes (ie, axial length O27.0 mm). A new technique bypassing corneal power and axial length measurements was recently described. After cataract removal, the aphakic eye is refracted in the operating room 28 or 30 minutes later 29 ; both groups of authors use nomograms to calculate the IOL power needed for emmetropia, and the proper IOL is then inserted in the capsular bag. The disadvantages of such a technique include the interruption of the surgical procedure and the accuracy of the nomograms in these long eyes. Our prospective study shows that the no-history method is a viable alternative for IOL power calculation after myopic LASIK, especially when the data of the refractive surgery are not available. Further evaluation of this method in a larger series is needed to better gauge its validity. REFERENCES 1. Koch DD, Liu JF, Hyde LL, et al. Refractive complications of cataract surgery after radial keratotomy. Am J Ophthalmol 1989; 108: Holladay JT. Comment in: Consultations in refractive surgery. Refract Corneal Surg 1989; 5: Hoffer KJ. Intraocular lens power calculation for eyes after refractive keratotomy. J Refract Surg 1995; 11: Seitz B, Langenbucher A, Nguyen NX, et al. Underestimation of intraocular lens power for cataract surgery after myopic photorefractive keratectomy. Ophthalmology 1999; 106: Odenthal MTP, Eggink CA, Melles G, et al. Clinical and theoretical results of intraocular lens power calculation for cataract surgery after photorefractive keratectomy for myopia. Arch Ophthalmol 2002; 120: Hamed AM, Wang L, Misra M, Koch DD. A comparative analysis of five methods of determining corneal refractive power in eyes that have undergone myopic laser in situ keratomileusis. Ophthalmology 2002; 109: 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: Koch DD, Wang L. Calculating IOL power in eyes that have had refractive surgery [editorial]. J Cataract Refract Surg 2003; 29: Wang L, Booth MA, Koch DD. Comparison of intraocular lens power calculation methods in eyes that have undergone LASIK. Ophthalmology 2004; 111: Holladay JT. IOL calculations following radial keratotomy surgery. Questions and answers. Refract Corneal Surg 1989; 5: Aramberri J. Intraocular lens power calculation after corneal refractive surgery: double-k method. J Cataract Refract Surg 2003; 29: 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: J CATARACT REFRACT SURG - VOL 33, JANUARY

6 13. 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: Shammas HJ, Shammas MC, Garabet A, et al. Correcting the corneal power measurements for intraocular lens power calculations after myopic laser in situ keratomileusis. Am J Ophthalmol 2003; 136: Shammas HJF. The fudged formula for intraocular lens power calculations. Am Intra-Ocular Implant Soc J 1982; 8: Retzlaff JA, Sanders DR, Kraff MC. Development of the SRK/T intraocular lens implant power calculation formula. J Cataract Refract Surg 1990; 16: ; correction, Holladay JT, Praeger TC, Chandler TY, et al. A three-part system for refining intraocular lens power calculations. J Cataract Refract Surg 1988; 14: Hoffer KJ. The Hoffer Q formula: a comparison of theoretic and regression formulas. J Cataract Refract Surg 1993; 19: ; errata, 1994; 20: Haigis W. The Haigis Formula. In: Shammas HJ, ed, Intraocular Lens Power Calculations. Thorofare, NJ, Slack, 2003; Holladay JT. Holladay IOL Consultant User s Guide and Reference Manual. Houston, TX, Holladay Lasik Institute, Haigis W. Corneal power after refractive surgery for myopia: Contact lens method. J Cataract Refract Surg 2003; 29: ; erratum, Rosa N, Capasso L, Lanza M, et al. Reliability of a new correcting factor in calculating intraocular lens power after refractive corneal surgery. J Cataract Refract Surg 2005; 31: Ridley F. Development in contact lens theory and practicedmoulding, computation, and veiling. Trans Ophthalmol Soc UK 1948; 68: Soper JW, Goffman J. Contact lens fitting by retinoscopy. In: Soper JW, ed, Contact Lenses; Advances In Design, Fitting, Application. Miami, FL, Symposium Specialists, 1974; Zeh WG, Koch DD. Comparison of contact lens overrefraction and standard keratometry for measuring corneal curvature in eyes with lenticular opacity. J Cataract Refract Surg 1999; 25: Argento C, Cosentino MJ, Badoza D. Intraocular lens power calculation after refractive surgery. J Cataract Refract Surg 2003; 29: Packer M, Brown LK, Hoffman RS, Fine IH. Intraocular lens power calculation after incisional and thermal keratorefractive surgery. J Cataract Refract Surg 2004; 30: 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: Mackool RJ, Ko W, Mackool R. Intraocular lens power calculation after laser in situ keratomileusis; aphakic refraction technique. J Cataract Refract Surg 2006; 32: J CATARACT REFRACT SURG - VOL 33, JANUARY 2007