Simple regression formula for intraocular lens power adjustment in eyes requiring cataract surgery after excimer laser photoablation

Transcription

1 J CATARACT REFRACT SURG - VOL 32, MARCH 26 Simple regression formula for intraocular lens power adjustment in eyes requiring cataract surgery after excimer laser photoablation Samuel Masket, MD, Seth Everett Masket, PhD PURPOSE: To develop a simple and accurate method for determining appropriate intraocular lens (IOL) power in cataract patients who had prior excimer laser photoablation for myopia or hyperopia, because laser vision corrective surgery interferes with traditional keratometry and corneal topography, rendering IOL power calculations inaccurate. SETTING: Private Practice in Century City (Los Angeles), California, and free-standing outpatient surgery centers with institutional review boards. METHODS: Based on the empiric experience of the senior author, an IOL power correction factor that was proportional to the prior laser photoablation was determined and applied to the IOL power calculated by the IOLMaster (Zeiss). It was necessary to add to the predicted IOL power in eyes with prior myopic laser ablation, whereas eyes having prior hyperopic laser vision correction required a reduction in the IOL power. The correction factor was applied to 3 eyes that required cataract surgery at some time after laser refractive surgery; 23 eyes had prior treatment for myopia, and the remaining 7 eyes had prior hyperopic laser ablation. A regression formula was generated from the IOL power correction factor that was used in the 3 eyes. RESULTS: Using the correction factor for 3 eyes, the mean deviation from the desired postcataract refractive outcome was ÿ.15 diopter (D) G.29 (SD); 28 of 3 eyes were within G.5 D of the intended goal; the remaining 2 eyes were both ÿ.75 D from the desired optical result of cataract surgery. Fourteen of the 3 eyes were emmetropic. CONCLUSIONS: A simple IOL power corrective adjustment regression formula allowed accurate determination of IOL power after laser refractive photoablation surgery. The weakness of the current method is that knowledge of the amount of prior laser vision correction is necessary. J Cataract Refract Surg 26; 32: Q 26 ASCRS and ESCRS Incorrect optical power remains a significant cause of intraocular lens (IOL) removal and replacement. 1 Although IOL power calculation formulas have evolved progressively and equipment for measuring axial length of the eye has become more sophisticated and accurate, it is not yet possible Accepted for publication October 24, 24. From the Department of Ophthalmology, Geffen School of Medicine, University of California Los Angeles, Los Angeles, California, USA. Neither author has a financial or proprietary interest in any material or method mentioned. Correspondence to Samuel Masket, MD, Advanced Vision Care, 28 Century Park East, Suite 911, Century City, California, USA. avcmasket@aol.com. to be certain of accurate IOL power calculation in all cases before surgery. All formulas must estimate the final IOL distance from the back surface of the cornea; this effective lens position (ELP) cannot, under any circumstances, be predicted with strict assurance before crystalline lens removal and IOL placement. Furthermore, estimation of corneal power, a key factor in determining correct IOL power, may be difficult if the corneal surface is irregular, or even more troublesome if it has been altered by keratorefractive surgery. Traditional methods that measure the optical power of the cornea, such as keratometry or automated corneal topography, assume that the anterior and posterior corneal surfaces are nearly parallel (the posterior corneal surface is typically modestly steeper than the anterior surface) and assign a value of approximately ÿ6. diopters (D) as the Q 26 ASCRS and ESCRS Published by Elsevier Inc /6/$-see front matter doi:1.116/j.jcrs

2 back corneal power, in accordance with Gullstrand s original concept for the schematic eye. 2 That number is automatically factored when reading corneal power with standard devices. As an example, when routine keratometry readings indicate a power of 42. D, the reality is that the anterior corneal optical power is approximately 48. D and the ÿ6. D (approximate) back surface value is added algebraically, resulting in 42. D reading with the device. However, when the anterior corneal curvature is altered by excimer laser ablation, the relationship between the anterior and posterior corneal curves is altered and the assumed power of the back corneal surface is no longer valid. It can be inferred, as suggested by the senior author in the 1998 Binkhorst Medal Lecture ( Challenges to Small Incision Cataract Surgery, presented at the Annual Meeting of the American Academy of Ophthalmology, New Orleans, Louisiana, USA, November 1998) and has been reported initially by Seitz et al., 3 that the effect of laser ablation on IOL power determination varies with the amount of prior laser photoablation. 3,4 This is likely true for both hyperopia or myopia. Because of the altered anterior/posterior corneal curvature relationships after excimer laser vision correction, standard topographers and keratometers will overestimate corneal power in previously myopic eyes and underestimate corneal power in previously hyperopic eyes. Figure 1 shows the schematic conceptual relationship between the anterior and posterior corneal curvatures in the unoperated eye and in eyes that have been subjected to keratorefractive surgery. Figure 1, B simulates the change after incisional (radial) keratotomy; note that both the anterior and posterior surfaces have been flattened by the surgery, which is distinctly different from the effects of excimer laser photoablation. As a result, IOL power calculation formulas after radial keratotomy (RK) differ from those after laser ablation; the former are not considered in the current investigation because the mechanisms of IOL power inaccuracy vary between incisional and laser ablative refractive surgery. As noted in Figure 1, C, the effect of excimer laser ablation for the correction of myopia induces a flattening of the anterior corneal surface; as a result, the posterior and anterior corneal surfaces are no longer near parallel, and the back surface power value relative to the anterior corneal curvature increases. It is this phenomenon that likely causes keratometers and topographers to misread the postexcimer-ablated cornea because they read only the anterior corneal surface and assume the approximately preset ÿ6. D power for the back corneal surface. Additionally, it has been reported that posterior corneal curvature may increase (because of variable limited ectasia) after myopic laser photoablation. 5 Although this phenomenon may also effect the accuracy of corneal power determination with standard devices, there remains no consensus on this subject matter because the method for reading postoperative posterior corneal curvature with the Orbscan (Bausch & Lomb) has not been validated. 6 In contrast to the effect of myopic photoablation, and as shown in Figure 1, D, after hyperopic excimer laser treatment, the anterior corneal power is increased independent of the posterior surface; this will also result in a misinterpretation of corneal power by standard instruments because the back surface will represent a lower value than ÿ6. D. Figure 1. A: Schematic conceptual relationship between the anterior and posterior corneal curvatures in the unoperated eye. Note that the 2 curves are approximately parallel. B: Schematic conceptual corneal shape change after RK. Note that the 2 surfaces are near parallel, but they are significantly flatter than the unoperated cornea in A. C: Schematic conceptualized drawing of corneal shape after excimer photoablation for myopia. Note that the anterior surface is considerably flatter than the posterior surface. D: Schematic conceptualized drawing of corneal shape after hyperopic excimer photoablation. Note that the anterior surface is steeper than the posterior surface. J CATARACT REFRACT SURG - VOL 32, MARCH

3 As a result, standard devices for reading corneal power will underestimate the power of an eye that has been subjected to excimer laser treatment of hyperopia. Given that corneal refractive power is 1 of 2 key factors for IOL power determination, it can be surmised from the above that after excimer laser photoablation, it is difficult to determine appropriate IOL power as traditional methods for estimating corneal power will be in error. In early experience with a few cases after myopic laser in situ keratomileusis, we recognized that the IOL was underpowered by roughly 1 D for each 3 D of prior laser vision correction. As a result, in 1 case it was necessary to perform an IOL exchange, whereas in another, a secondary piggyback IOL was implanted. Given this experience, IOL power was adjusted proportionally rather than attempting to determine true corneal power after laser vision correction because the latter is elusive with current equipment. The current investigation reports the optical outcomes in 3 eyes in which final IOL power selection was determined by adjusting the routinely measured IOL power by approximately 1 D for each 3 D of prior laser treatment. The data were then used to formulate a simple regression formula, as the optical outcomes after cataract surgery were very acceptable clinically. PATIENTS AND METHODS Institutional Review Board (at free-standing surgical centers) approval was obtained to evaluate IOL power data for 3 consecutive eyes of 2 patients in conjunction with indicated cataract surgery after previous excimer laser vision correction surgery; 23 eyes had prior myopic ablation, and 7 had earlier hyperopic ablation. Table 1 shows the range of prior excimer laser vision correction. Biometry was performed in all cases with the IOLMaster (Zeiss). In patients with prior myopia, baseline IOL power was determined with the SRK/T formula, whereas in cases of prior hyperopia, in keeping with the senior author s custom, the Hoffer Q formula was used for baseline readings. In all cases, the IOLMaster readings and IOL power determination were obtained before cataract surgery as though no prior corneal refractive surgery had been performed, although in reality excimer laser photoablation had been carried out in all eyes. As the next step in determining the final IOL power for implantation, IOL power was corrected by approximately 1 D for each 3 D of the intended spherical equivalent (SE) of the prior laser correction, as adjusted for vertex distance by the laser software; optical power was added to the IOL for prior myopia and subtracted from IOL power in cases of Table 1. Range and mean of laser refractive surgical correction (spherical equivalent) before cataract surgery. Correction (D) Group Mean Range Myopic eyes (n Z 23) ÿ6.23 ÿ2. to ÿ27. Hyperopic eyes (n Z 7) C3.36 C1.25 to C5. hyperopic ablation. In all patients, it was both necessary and possible to obtain prior records of their laser treatment(s); in cases in which enhancements or multiple treatments were performed, the total amount of treatment was used to determine the amount of IOL power adjustment. Given that biometry was obtained as though no prior surgery had been performed, the patient s current refraction was of no significance in determining final IOL power. Figure 2 shows the applied adjustment in each case. At 1 extreme, 9 D of IOL power were added to that determined by the IOLMaster in an eye that had 27 D of myopic laser correction (multizone treatment with a small central zone). In the majority of cases, emmetropia was the desired endpoint, although a few eyes were calculated for near monovision; irrespective of the desired outcome, the above IOL power adjustment was performed identically. The Microsoft Excel spreadsheet program was used to store the data and to retrospectively calculate a regression formula of the graph (Figure 2) for the IOL power adjustment for the 3 eyes in the investigation. The equation was determined to be as follows: IOL Power Adjustment = LSE 3 ðl:326þ +:11 where LSE is the total prior laser treatment, adjusted for vertex distance, in SE. Clinical examples are as follows: Previously myopic eye: SRK/T formula suggests 16. D for emmetropia after cataract surgery Prior laser correction (SE) Z ÿ6. D Adjustment calculation: ÿ6. D (ÿ.326) C.11 Z C2.57 D IOL power adjusted by adding C2 D to the original C16 D Z C18 D for emmetropia Adjustment Hyperopia -5 Myopia Laser SE Figure 2. Graphic representation of adjustments to final IOL power determination. The spherical equivalent of the prior laser photoablation is noted on the x-axis, and the amount of IOL power adjustment from the IOLMaster calculation is noted on the y-axis J CATARACT REFRACT SURG - VOL 32, MARCH 26

4 Previously hyperopic eye: Hoffer Q formula suggests C22 D IOL for emmetropia after cataract surgery Prior laser vision correction (SE) Z C3. D Adjustment calculation: C3. D (ÿ.326) C.11 Z ÿ.877 D IOL power adjusted by subtracting 1 D from the original C22 D Z C21 D for emmetropia All 3 eyes had routine phacoemulsification that was performed through temporal clear corneal tunnel incisions under topical anesthesia; in all cases a foldable IOL was used, although a variety of lens styles was implanted. Specific A-constants and surgeon factors were used for the particular IOL. The optical outcomes of surgery were determined by objective and subjective refractions between 2 weeks and 6 weeks after surgery in all cases given that optical outcomes after temporally oriented, clear corneal cataract surgery with foldable IOLs are typically stable and clinically and statistically similar (S. Masket, MD, Wound Construcion, presented at the ASCRS 3rd American International Congress on Cataract, IOL and Refractive Surgery, Seattle, Washington, USA, May 1993) between 2 weeks and 6 weeks after cataract surgery. RESULTS The optical outcomes with adjusted IOL power in all 3 eyes after surgery are shown in Figure 3. Note that data are represented for 23 myopic eyes (dark bars) and for 7 hyperopic eyes (light bars). All eyes were within G.75 D of the desired optical outcome. Fourteen of the 3 eyes were emmetropic (or optically corrected as desired by the patient), and 28 of 3 were within G.5 D of Number of Cases Hyperopia Myopia Outcome SE Figure 3. Bar graph shows the final optical outcome after cataract extraction and IOL implantation with the empirically adjusted power. The optical results in spherical equivalent are noted on the x axis and the number of cases on the y axis. Note that 28 of the 3 eyes were within G.5 D, and that the other 2 eyes were within.75 D of emmetropia. The eyes with prior myopic ablation are noted in purple, and the eyes with hyperopic treatment are represented in white. the desired optical result. The mean error was ÿ.15 D.29 D (SD). The resultant regression formula that was determined from the adjustments to IOL power is noted above and can be applied to individual eyes requiring cataract surgery after excimer laser photoablation. DISCUSSION Ophthalmologists are significantly challenged by the expectations of patients associated with any form of refractive surgery; the challenge becomes even greater when former refractive surgery patients require cataract surgery. The paradox is that, as a group, postrefractive surgery patients may have unrealistic goals for the exactness of the optical results after cataract surgery, although their outcomes have been less predictable than in routine eyes. Fortunately, there is an understanding of the changes in corneal shape after excimer laser photoablation and the proportional relationship between the amount of laser vision correction and the resultant effect on requisite IOL power. In our view, corneal power measurements are flawed after laser photoablation, primarily because of an alteration in the geometric relationship between anterior and posterior corneal curvatures, as noted in Figure 1, and traditional keratometry and topography are based on nearly parallel anterior and posterior corneal surfaces. Laser ablation is designed to change only the anterior surface of the cornea, although it remains possible and controversial that the posterior corneal power may increase modestly after myopic ablation. 5,6 Another explanation for the inaccuracy of corneal power measurements after laser ablation may be an altered postoperative corneal refractive index, as suggested by Seitz et al. 3 As noted in Figure 3, the results in the current study suggest that appropriate IOL power adjustments can be made after excimer laser alteration of corneal shape. In simplistic terms, and in accord with the current investigation, 3 D of laser correction will change the necessary IOL power by approximately 1 D, and the empiric data of the study has allowed for a simple regression formula that may be applied to individual eyes, as noted in the clinical examples (see Patients and Methods section). In cases of previous myopic laser correction, power is added to the IOL calculation because standard instruments that measure corneal power overestimate the true power of the cornea. The opposite is true in eyes that have had hyperopic laser ablation. Other factors that could induce errors in IOL power calculation after excimer laser photoablation include technical difficulty in determining the flattest axial corneal curvature (although this is generally more problematic after incisional refractive surgery) and erroneous assumption of the ELP by IOL calculation formulas as the flattened postlaser cornea fools the formulas (except for the Haigis J CATARACT REFRACT SURG - VOL 32, MARCH

5 formula, which does not use corneal power readings to predict postoperative ELP 7 ) with respect to the relationship between corneal curvature and postoperative anterior chamber depth. In fact, the Aramberri double-k method was established specifically to correct for this problem. 8 How does the method in the current investigation compare with those previously published? A recent and comprehensive report of Wang et al. 9 reviewed and considered 5 previously published or established methods of post excimer laser IOL power determination; the authors tested all 5 formulas in 11 eyes that had cataract surgery after prior laser ablation for myopia. Additionally, each method was then fine tuned by retesting with the double-k method. 9 Interestingly, the outcomes of the method of the current investigation exhibit a lower mean prediction error and tighter range of errors than those noted for the 5 formulas reported in the Wang et al. study. The results in the current investigation suggest that a simple regression formula can be applied to IOL power calculations to accurately predict the correct IOL power for eyes that have had prior myopic or hyperopic excimer laser photoablation. In a previous retrospective investigation, Latkany et al. 1 established a regression formula for IOL power calculation in 21 eyes that had cataract surgery after previous myopic photoablation. Similar to the present study, they found that the amount of preablative myopia was the chief corrective factor in determining the most accurate IOL power calculation. The chief weaknesses of the current method is that the amount of prior laser vision correction must be known and that the outcomes are reported in only 7 previously hyperopic eyes. However, the outcomes of the current method appear to be quite accurate. In the future, it would be useful if a central data bank could be established for all eyes that had excimer laser vision correction surgery so that details and records of previous laser treatment would be available. Alternatively, it would be most beneficial to have clinical devices that can accurately and directly determine the optical power of the cornea without relying on the mathematical assumptions associated with traditional keratometry and topography, irrespective of prior laser vision corrective surgery. It must be emphasized that the regression formula suggested here should be applied only in eyes that have had laser photoablation, not those that have had prior RK. In the latter, the relationship between anterior and posterior corneal curvature remains essentially unchanged, but the cornea is flattened centrally as a result of peripheral weakening. The problems of IOL power calculation after RK are the result of difficulty obtaining accurate direct central corneal power readings and that the cornea may flatten further, either transiently or permanently, after cataract surgery. REFERENCES 1. Mamalis N. Complications of foldable intraocular lenses requiring explantation or secondary interventiond1998 survey. J Cataract Refract Surg 2; 26: Speicher L. Intra-ocular lens calculation status after corneal refractive surgery. Curr Opin Ophthalmol 21; 12: Seitz B, Langenbucher A, Nguyen NX, et al. Underestimation of intraocular lens power for cataract surgery after myopic photorefractive keratectomy. Ophthalmolgy 1999; 16: Koch DD, Wang L. Calculating IOL power in eyes that have had refractive surgery [editorial]. J Cataract Refract Surg 23; 29: Wang Z, Chen J, Yang B. Posterior corneal surface topographic changes after laser in situ keratomileusis are related to residual corneal bed thickness. Ophthalmology 1999; 16:46 49; discussion by RK Maloney, Seitz B, Torres F, Langenbucher A, et al. Posterior corneal curvature changes after myopic laser in situ keratomileusis. Ophthalmology 21; 18: ; discussion by ED Donnenfeld, Haigis W, Lege B, Miller N, Schneider B. Comparison of immersion ultrasound biometry and partial coherence interferometry for intraocular lens calculation according to Haigis. Graefes Arch Clin Exp Ophthalmol 2; 238: Aramberri J. Intraocular lens power calculation after corneal refractive surgery: double-k method. J Cataract Refract Surg 23; 29: Wang L, Booth MA, Koch DD. Comparison of intraocular lens power calculation methods in eyes that have undergone LASIK. Ophthalmology 24; 111: Latkany RA, Chokshi AR, Speaker MG, et al. Intraocular lens calculations after refractive surgery. J Cataract Refract Surg 25; 31: J CATARACT REFRACT SURG - VOL 32, MARCH 26