Excimer laser photorefractive keratectomy

Transcription

1 Intraocular Lens Power Calculation for Cataract Surgery after Photorefractive Keratectomy for High Myopia Richard S. Kalski, MD; Jean-Pierre Danjoux, FRCOphth; Graham E. Fraenkel, BM, BS; Michael A. Lawless, FRACO, FRACS; Christopher Rogers, FRACO, FRACS From the Sydney Refractive Surgery Centre, St. Leonards, New South Wales, Australia. The authors have no proprietary interest in the materials presented in this article. Correspondence: Christopher Rogers, FRACO, FRACS, 66 Pacific Highway, St Leonards, New South Wales 2065, Australia. Tel: ; Fax: ; srsc@acay.com.au Received: December 28, 1995 Accepted: April 3, 1997 ABSTRACT OBJECTIVE: To assess intraocular lens (IOL) power calculations in patients undergoing cataract surgery after excimer laser photorefractive keratectomy (PRK) for myopia. METHODS: Four eyes of two patients underwent phacoemulsification with IOL implantation after PRK for myopia. The estimated refractive error that would have been induced had the IOL predicted for emmetropia been implanted was calculated using SRK-II, SRK/T, Holladay, and Binkhorst formulas. Manual keratometry and videokeratography-simulated keratometry values measured before surgery were used. Keratometry values calculated by subtracting the refractive change induced by the excimer laser PRK from the manual keratometry or videokeratography-simulated keratometry values measured before PRK were also used. Both spectacle and corneal plane calculations were performed. RESULTS: Manual keratometry and videokeratography-simulated keratometry values underpredicted the IOL power. Corneal plane manual or videokeratography refraction-derived keratometry calculations were most accurate using the SRK/T formula, while spectacle plane calculations were most accurate using the SRK-II formula. In both methods the calculated refractive error was within 0.52 diopters (D) for the emmetropic lens power predicted. Statistical analysis was not performed. CONCLUSIONS: Refraction-derived keratometric values provided the most accuracy in calculating IOL powers. Our results suggest the SRK/T formula was the most accurate for corneal plane calculations, while the SRK-II formula was the most accurate for spectacle plane calculations. [J Refract Surg 1997;13: ] Excimer laser photorefractive keratectomy (PRK) is the standard international procedure for the correction of myopia. Expectations are that the number of PRK procedures will surpass the number of radial keratotomy procedures in the United States now that the excimer laser has been granted Federal Drug Administration approval for treating myopia. Since PRK is a relatively new procedure performed mainly on younger patients, data is limited regarding individuals who have undergone PRK and cataract surgery. 1 This paper retrospectively analyzes four eyes of two patients who underwent cataract surgery after PRK for myopia. Assessment is made of various formulas used to calculate intraocular lens (IOL) power together with methods of calculating keratometric power for use with these formulas. MATERIALS AND METHODS Two patients (four eyes) underwent correction of myopia using a Summit Technology ExciMed UV200 LA excimer laser (Summit Technology, Waltham, Mass). Patient number one subsequently developed bilateral nuclear cataracts over a 3 month period. Myopic shift during this period was from-1.25 to D in the right eye, and to D in the left eye, with stable keratometry. Patient number two developed bilateral posterior subcapsular cataracts. Both patients experienced a 362 Journal of Refractive Surgery Volume 13 July/August 1997

2 reduction in spectacle-corrected visual acuity and underwent bilateral small-incision cataract extraction with IOL insertion. IOL powers were calculated using the SRK-II formula and manual keratometry readings. In patient two s second eye, the IOL power used was increased by D over the calculated value in light of the undercorrections in the previous three eyes. The emmetropic IOL power for each eye was calculated retrospectively using the SRK-II, SRK/T, Binkhorst, and Holladay formulas. For each eye, manual keratometry values were used and when available, videokeratography (EyeSys Laboratories, Houston, TX) simulated keratometry (Sim-K) values (three of four eyes) were also used. Keratometry readings measured before cataract surgery and a refraction-derived keratometry value attributed to Jack Holladay, MD, by Koch et al 2 was used. Videokeratography refraction-derived keratometry values were not calculated on both eyes of patient number two because a videokeratography unit was not available before the excimer laser PRK was performed. The refraction-derived keratometry value was calculated by subtracting the refractive change induced by the PRK from the keratometric measurement before PRK. The spherical equivalent refraction measured before each patient presented with visual complaints associated with the cataract was used to calculate the refractive change induced by the PRK. These calculations were done at both the spectacle and corneal plane. For example, patient number one had a left eye spherical equivalent refraction before PRK of D at the spectacle plane and D at the corneal plane. The spherical equivalent refraction was D at routine follow-up 10 months after PRK, before the patient s visual complaints and observed cataract. The refractive change by the PRK was therefore D at the spectacle plane and D at the corneal plane. Considering that the mean manual keratometry reading before PRK was D, then the refraction-derived manual keratometric value at the spectacle plane was =32.25 D. At the corneal plane this value was = D. These calculated values are in contrast to the measured manual keratometry reading of D after PRK. The manufacturer s stated anterior chamber depth constant was used. This may require adjustment for high myopes where the anterior chamber depth is increased. The surgeon s personalized A-constant was included. The estimated refractive error that would have been induced had the IOL predicted for emmetropia been implanted was calculated by using each eye s final postoperative refractive error. RESULTS Refractive and keratometric measurements before and after PRK and cataract surgery are summarized in Table 1. Mean corneal flattening 1 week after cataract surgery was 0.44 D (range, 0 to 1.00 D). The change in the absolute mean of manual keratometric values at the final postoperative visit (mean, 5.1 months, range 3.5 to 7 months) was minimal (0.44 D, range to D). In all instances the refraction-derived keratometry values were flatter than manual keratometry or Sim-K videokeratography measurements (Table 2). Table 2 gives the mean values and Table 3 the absolute mean values for the calculated refractive error for emmetropic lens power predicted by calculation formulas. There is a trend for the SRK-II formula utilizing refraction-derived manual keratometry or refraction-derived Sim-K videokeratography at the spectacle plane to be the most accurate in predicting IOL power. In both cases, the absolute mean was within 0.37 D for the emmetropic lens power predicted. Using refraction-derived manual keratometry or refraction-derived Sim-K videokeratography values at the corneal plane with the SRK/T formula also provided a good IOL power prediction with absolute mean values within 0.52 D. The only other combination to provide an absolute mean prediction within 1.00 D was the Holladay formula in conjunction with spectacle plane refraction-derived manual keratometry or refraction-derived Sim-K videokeratography. Both manual keratometry and Sim-K videokeratography values appeared to be the least accurate in calculating IOL power, except with the Binkhorst formula. DISCUSSION PRK is a procedure that has gained world wide acceptance as a safe, efficacious, stable, and predictable method of treating low to moderate levels of myopia The number of PRK procedures performed each year is increasing, yet information about patients who have had cataract surgery after PRK is limited. Refraction and keratometry data after excimer laser PRK typically remains stable with minimal variation after 3 months in low myopia and 6 months in high myopia. 3,5,6,8,10 In the four eyes assessed, corneal flattening was minimal and keratometric values remained consistent after cataract surgery and during the observed postoperative Journal of Refractive Surgery Volume 13 July/August

3 Table 1 Keratometric and Refractive Changes in Four Eyes of Two Patients Time between Before 1 Week Before 12 Months PRK and Cataract Cataract after Cataract Follow-up Final PRK (D) after PRK (D) Surgery (mos) Surgery (D) Surgery (D) Period (mos) Results (D) Patient 1 SE SE SE SE SE Right eye* MK MK MK MK MK VK VK n/a VK AL mm IOL Patient 1 SE SE SE SE SE Left eye MK MK MK MK MK VK VK VK AL mm IOL Patient 2 SE SE SE 0.00 SE SE Right eye MK MK MK MK MK VK n/a VK VK AL mm IOL Patient 2 SE SE SE SE SE Left eye MK MK MK MK MK VK n/a VK VK AL mm IOL *Data were recorded 10 months after PRK in Patient 1, right eye. SE = spherical equivalent; MK = mean manual keratometric measurement; VK = mean videokeratography keratometric measurement; n/a = data not available; AL = axial length Table 2 Calculated Mean Refractive Error (D) (SD) for Emmetropic IOL Power Predicted by Calculation Formulas Keratometric Measurement SRK/T SRK-II Holladay Binkhorst Manual keratometry (1.21) (0.93) (0.76) (1.49) Videokeratography (0.73) (0.64) (0.64) (0.77) Refraction-derived manual (0.49) (0.34) (0.66) (0.76) keratometry spectacle plane Refraction-derived videokeratography (0.43) (0.45) (0.46) (0.40) spectacle plane Refraction-derived manual (0.68) (0.67) (1.05) (1.02) keratometry corneal plane Refraction-derived videokeratography (0.59) (0.60) (0.62) (0.66) corneal plane period. Keratometric and corneal stability 6 months after phacoemulsification following PRK has been previously reported. 1 This is in contrast to Koch and colleagues report of a hyperopic shift and keratometric flattening observed during the first week after cataract surgery in patients who had previously undergone radial keratotomy. 2 As the severity of these findings decreased with an increased time interval between radial keratotomy and cataract surgery, they hypothesized that after cataract surgery the cornea underwent to a lesser extent the acute keratometric changes that occur after radial keratotomy surgery itself. Refraction-derived keratometric values calculated using either manual keratometry or Sim-K videokeratography measurements appear to provide the most accuracy in calculating IOL powers after PRK, except with the Binkhorst formula. Following excimer laser PRK, the central 2 to 3 mm of the cornea is flatter than the adjacent cornea and 364 Journal of Refractive Surgery Volume 13 July/August 1997

4 Table 3 Calculated Absolute Mean Refractive Error (D) (SD) for Emmetropic IOL Power Predicted by Calculation Formulas Keratometric Measurement SRK/T SRK-II Holladay Binkhorst Manual keratometry 2.28 (1.21) 3.44 (0.93) 3.49 (0.76) 1.22 (1.00) Videokeratography 4.58 (0.73) 5.31 (0.64) 5.65 (0.64) 3.74 (0.77) Refraction-derived manual 2.04 (0.49) 0.33 (0.34) 0.83 (0.66) 4.95 (0.76) keratometry spectacle plane Refraction-derived videokeratography 2.43 (0.43) 0.37 (0.45) 0.58 (0.46) 5.44 (0.40) spectacle plane Refraction-derived manual 0.52 (0.50) 1.19 (0.67) 1.20 (0.44) 2.69 (1.02) keratometry corneal plane Refraction-derived videokeratography 0.45 (0.59) 1.33 (0.60) 1.36 (0.62) 2.76 (0.66) corneal plane this change is more marked in high corrections. This discrepancy may explain the calculated underpredictions of IOL power with all the formulas when the refractive change was not taken into consideration. The refraction-derived keratometry values possibly compensated for this underprediction by accounting for the change in the refractive power of the cornea. 2 This presupposes no change in the axial length or the lens power. For excimer laser PRK corrections of D or more of myopia, treatment algorithms adjust for correction of a corneal plane value. Theoretically, refraction-derived keratometry values calculated at the corneal plane should be more accurate at determining the true corneal power than spectacle plane values. 12,13 Refraction-derived keratometry values calculated at the corneal plane were most accurate using the SRK/T formula, with calculated absolute mean refractive errors within 0.52 D for the emmetropic lens power predicted. In eyes that have a long axial length, such as the eyes in this study, the SRK/T formula is more accurate than other theoretical and regression formulas. 14,15 Refraction-derived keratometry values calculated at the spectacle plane using the SRK-II and Holladay formulas were the only other calculations with absolute mean refractive errors within 1.00 D for the emmetropic lens power predicted. We and other investigators have reported a discrepancy between manual keratometry or videokeratography change and refractive change after PRK, with eyes having a greater refractive effect than is indicated by keratometry. 9,10,16,17 Manual and videokeratography measurements are based on several assumptions. The index of refraction is adjusted to represent the total refractive contribution of the cornea, including the anterior and posterior surfaces. The overall power of the cornea depends on both surfaces, with a far greater convergence occurring at the anterior surface than at the posterior surface. A relatively small change in the anterior surface of the cornea can create large changes in refractive effect. The discrepancy between manual keratometry or videokeratography change and refractive change is larger after PRK in higher myopes. 9,17 The four eyes in this series had D or more of myopia. Lesher and colleagues 1 reported an error in IOL power calculation of 1.60 D using the SRK/T formula with automated keratometry measurements in their patient with D of myopia. Using a videokeratography system that provides information on the front and back curvatures of a PRK-treated cornea may allow for more accurate calculations of the total corneal power, decreasing the likelihood of underpredicting the IOL power in higher myopes. Regardless of the formula used, manual keratometry and Sim-K videokeratography values underpredicted the IOL power with videokeratography calculations giving a greater underprediction. There is evidence that keratometry measurements after PRK are steeper than expected. 9,10,16,17 This could account for the hyperopic errors seen with manual keratometry and Sim-K videokeratography calculations using all formulas. It also gives further credence to the use of refraction-derived keratometry measurements which are calculated from keratometry readings taken before PRK. The Sim-K videokeratography values after PRK and after cataract surgery in these four eyes are consistently less steep than the manual keratometry values. The differences noted between these measurements may reflect the differences between the principles of manual keratometry and videokeratography. Manual keratometry Journal of Refractive Surgery Volume 13 July/August

5 measures two points approximately 3.00 mm apart. Since these points are measured outside the flattest central area, the manual keratometer reads values that are steeper. However, the Sim-K calculations on videokeratography are also performed at a diameter approximately 3.00 mm apart. There may be idiosyncrasies in the reconstruction algorithms of videokeratography units and these can lead to errors in quantitatively analyzing corneas after PRK. Averaging the keratometry values around the central second and third ring of the videokeratograph in a Placido-based system should provide readings in the flattest part of the central cornea. These keratometry values may provide more accurate measurements than the Sim- K values of the central flatter corneal curvature. 16 Although this paper reports on only four eyes of two patients, these results provide further information in an area of ophthalmology that has been minimally addressed. Statistical analysis on this small cohort was considered inappropriate. Refractionderived keratometry seems to be a more appropriate method of calculating the required IOL power in high myopes requiring phacoemulsification after myopic PRK than manual keratometry or videokeratography. In this small group, the SRK/T formula provided the most accurate calculation at the corneal plane while the SRK-II formula provided the most accuracy at the spectacle plane. It must be emphasized, however, that there is no statistically proven evidence that any one method of calculation, nor any one formula, is superior to another. It is possible that some of the differences in these formulas result from the fact that these eyes have increased axial lengths and increased anterior chamber depths. Furthermore, lower myopes who require cataract surgery after PRK may not follow the same trend as higher myopes and may benefit more from using standard manual keratometry or videokeratography to determine IOL power. Lesher and colleagues 1 suggest that this is not the case. Further studies on these patients are required, incorporating larger numbers and control groups of similar myopes who have not undergone PRK prior to phacoemulsification. Such studies will help to elucidate guidelines for treating cataracts in patients after PRK, and determine the most appropriate method for calculating IOL power. REFERENCES 1. Lesher MP, Schumer DJ, Hunkeler JD, Durrie DS, McKee FE. Phacoemulsification with intraocular lens implantation after excimer photorefractive keratectomy: A case report. J Cataract Refract Surg 1994; 20(suppl):S265-S Koch DD, Liu JF, Hyde LL, Rock RL, Emery JM. Refractive complications of cataract surgery after radial keratotomy. Am J Ophthalmol 1989;108: Gartry DS, Kerr Muir MG, Marshall J. Excimer laser photorefractive keratectomy. Eighteen month follow-up. Ophthalmology 1992;99: Tengroth B, Epstein D, Fagerholm P, Hamberg-Nystrom H, Fitzsimmons TD. Excimer laser photorefractive keratectomy for myopia. Clinical results in sighted eyes. Ophthalmology 1993;100: Dutt S, Steinert RF, Raizman MB, Puliafito CA. One-year results of excimer laser photorefractive keratectomy for low to moderate myopia. Arch Ophthalmol 1994;112: Maguen E, Salz JJ, Nesburn AB, Warren C, Macy JI, Papaioannou T, Hofbauer J, Berlin MS. Results of excimer laser photorefractive keratectomy for the correction of myopia. Ophthalmology 1994;101: Epstein D, Fagerholm P, Hamberg-Nystrom H, Tengroth B. Twenty-four month follow-up of excimer laser photorefractive keratectomy for myopia. Refractive and visual acuity results. Ophthalmology 1994;101: Salz JJ, Maguen E, Nesburn AB, Warren C, Macy JI, Hofbauer JD, Papaioannou T, Berlin M. A two year experience with excimer laser photorefractive keratectomy for myopia. Ophthalmology 1993;100: Lawless MA, Rogers C, Cohen P. Excimer laser photorefractive keratectomy: 12 months follow-up. Med J Aust 1993;159: Seiler T, Wollensak J. Myopic photorefractive keratectomy with the excimer laser. One-year follow-up. Ophthalmology 1991;98: McDonald MB, Liu JC, Byrd TJ, Abdelmegeed M, Angotti Andrade H, Klyce SD, Varnell R, Munnerlyn CR, Clapham TN, Kaufman HE. Central photorefractive keratectomy for myopia: partially sighted and normally sighted eyes. Ophthalmology 1991;98: Guyton DL. Consultations in refractive surgery. J Refract Corneal Surg 1989;5: Holladay JT. Consultations in refractive surgery. J Refract Corneal Surg 1989;5: Sanders DR, Retzlaff JA, Kraff MC, Gimbel HV, Raanan MG. Comparison of the SRK/T formula and other theoretical and regression formulas. J Cataract Refract Surg 1990;16: Retzlaff JA, Sanders DR, Kraff M. Lens Implant Power Calculation. 3rd ed. Thorofare, NJ: SLACK Incorporated; Hersh PS, Schwartz-Goldstein BH, The Summit Photorefractive Keratectomy Topography Study Group. Corneal topography of phase III excimer laser photorefractive keratectomy. Characterization and clinical effects. Ophthalmology 1995;102: Rogers CM, Lawless MA, Cohen PR. Photorefractive keratectomy for myopia of more than -10 diopters. J Refract Corneal Surg 1994;10(suppl):S171-S Journal of Refractive Surgery Volume 13 July/August 1997