Comparison of residual stromal bed and flap thickness in primary and repeat laser in situ keratomileusis in myopic patients

J CATARACT REFRACT SURG - VOL 32, DECEMBER 2006

Comparison of residual stromal bed and flap thickness in primary and repeat laser in situ keratomileusis in myopic patients Sujata Das, MS, FRCS (Glasg), Laurence J. Sullivan, MBBS, FRANZCO

PURPOSE: To compare the change in residual stromal thickness and flap thickness between primary laser in situ keratomileusis (LASIK) and repeat LASIK in myopic patients. SETTING: Melbourne Excimer Laser Group, East Melbourne, Australia. METHODS: This retrospective nonrandomized comparative trial comprised 46 eyes of 34 patients who had repeat LASIK. The thickness of the residual stromal bed was calculated by subtracting the calculated stromal ablation from pachymetry of the stromal bed after cutting the flap in primary treatment and directly measuring during retreatment. The thickness of the LASIK flap in primary and repeat LASIK was calculated by subtracting the central pachymetry of the stromal bed after creating the flap from pachymetry before cutting and lifting the flap, respectively. The main outcome measures were comparison of the residual stromal bed and flap thickness between the primary treatment and the retreatment. RESULTS: The mean thickness of the calculated residual stromal bed after primary treatment was 329.8 mm G 40.8 (SD), and the mean measured residual stromal bed at retreatment was 317.3 G 42.8 mm. The mean difference in residual stromal bed thickness was 12.5 G 13.0 mm (P<.001). Sixteen eyes (34.7%) had a decrease in bed thickness between 11 mm and 20 mm. The mean flap thickness during primary LASIK and repeat LASIK was 145.2 G 17.1 mm and 169 G 18.3 mm, respectively. The mean interval between primary treatment and retreatment was 7.4 G 4.1 months. The mean change in flap thickness was 23.8 G 15.2 mm (P<.001). Fifteen eyes (32%) had an increase in flap thickness between 11 mm and 20 mm. There was a negative correlation between refractive error before primary treatment and the difference in flap thickness. No correlation was found between the difference in flap thickness and the interval between the primary treatment and the repeat treatment. CONCLUSIONS: Intraoperative pachymetry of the stromal bed during retreatment is strongly recommended as the residual stromal bed and flap thickness changes between primary and retreatment. There is a tendency for the measured stromal bed at retreatment to be thinner than the calculated stromal bed and for the flap to be thicker than previously measured. J Cataract Refract Surg 2006; 32:2080–2084 Q 2006 ASCRS and ESCRS

Laser in situ keratomileusis (LASIK) is a safe and effective method for the correction of refractive errors.1–6 Among the complications associated with LASIK, the most common and easily treated are undercorrection and overcorrection. Retreatment after LASIK can be done by relifting the primary flap or creating a new flap before reablating the corneal stromal bed with the excimer laser,7–13 although the former has become the preferred technique. Retreatment can safely and effectively improve uncorrected visual acuity. However, repeat LASIK further reduces residual stromal bed thickness and thereby carries a risk for inducing corneal ectasia.14,15 Leaving sufficient remaining Q 2006 ASCRS and ESCRS Published by Elsevier Inc.

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posterior corneal stroma postoperatively is an important factor to consider before embarking on retreatment. Therefore, predicting flap thickness and residual stromal bed thickness is critical in planning retreatment. The purpose of this study was to compare residual stromal bed thickness and flap thickness in primary and repeat LASIK.

PATIENTS AND METHODS The charts of all patients who had an enhancement after primary LASIK for myopia were identified. Medical records were retrospectively reviewed. 0886-3350/06/$-see front matter doi:10.1016/j.jcrs.2006.08.031

RESIDUAL STROMAL BED AND FLAP THICKNESS IN PRIMARY AND REPEAT MYOPIC LASIK

Terry Couper, Caroline Gibbs, and the staff of Melbourne Excimer Laser Group assisted in data collection. Corresponding author: Dr. Sujata Das, Flat #C-6/8, Block-C, Chandrama Complex, Kharavel Nagar, Unit-3, Bhubaneswar 751001, Orissa, India. E-mail: [email protected].

Measured Residual Stromal Bed Thickness (in µm)

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Figure 1. Correlation between estimated and measured residual stromal bed thickness. Estimated residual stromal bed Z bed thickness after lifting flap at primary LASIK ablation depth (r Z 0.95).

thickness during repeat LASIK was 169.0 G 18.3 mm (range 128 to 207 mm) (Figure 3). The mean change in thickness was 23.8 G 15.2 mm (range 17 to 53 mm) (P!.001). In 3 eyes, the flap was thinner during retreatment than at the primary treatment. The most common change was an increase in flap thickness at retreatment; the increase ranged from 11 to 20 mm (Figure 4). A strong correlation was found between flap thickness during primary LASIK and flap thickness during repeat LASIK (r Z 0.63, P!.001). The mean interval between primary and repeat LASIK was 7.4 G 4.1 months (range 2.1 to 17.5 months). There was no correlation between the difference in flap thickness and interval between treatments (Figure 5). A negative correlation existed between the refractive error before primary treatment and the difference in flap thickness (r Z 0.43, P Z .003) (Figure 6). No correlation was found between flap thickness and refractive error in repeat LASIK (Figure 7). 18

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Neither author has a financial or proprietary interest in any material or method mentioned.

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From the Royal Victorian Eye and Ear Hospital (Das, Sullivan) and the Melbourne Excimer Laser Group (Sullivan), East Melbourne, Australia.

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Forty-six eyes of 34 patients who had retreatment were identified. The mean patient age was 42 years G 10 (SD) (range 25 to 62 years). Fourteen patients (41%) were men. The spherical equivalent of refractive error before primary treatment ranged from 8.37 diopters (D) to 0.62 D. The mean estimated residual stromal bed during primary treatment was 329.8 G 40.8 mm (range 275 to 468 mm). The mean measured residual stromal bed during retreatment was 317.3 G 42.8 mm (range 264 to 449 mm) (Figure 1). The mean difference in bed thickness was 12.5 G 13.0 mm (range 15 to 47 mm) (P!.001). The estimated residual stromal bed usually was thicker than the measured thickness (Figure 2). A strong correlation was found between the estimated and measured residual stromal beds (r Z 0.95, P!.001). The mean flap thickness during primary LASIK was 145.2 G 17.1 mm (range 110 to 190 mm). The mean flap

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All procedures were performed by a single surgeon (L.J.S.) using identical ultrasonic pachymeters (Sonogage, CorneoGage-Plus), microkeratomes (Amadeus I, AMO), and excimer lasers (Technolas 217 Z, Bausch & Lomb). During primary LASIK, the central corneal thickness was measured 3 times by ultrasonic pachymetry before the flap was created. All the flaps were hinged nasally. After the flap was lifted, the central corneal bed thickness was measured 3 times with the ultrasonic pachymeter. Flap thickness was calculated by subtracting the thinnest pachymetry after the flap lifting from the thinnest initial central pachymetry before flap cutting. The residual stromal bed was calculated by subtracting the thickness of stroma to be ablated from the initial thinnest pachymetry. For LASIK retreatment all the flaps were lifted. The central corneal pachymetry was measured 3 times before the flap was lifted. The central corneal stromal bed thickness was measured 3 times with the ultrasonic pachymeter before the laser ablation was performed. Analysis included patient age, sex, initial manifest refraction, central corneal pachymetry at primary LASIK, initial flap thickness, interval between primary treatment and retreatment, calculated residual stromal bed, refraction before retreatment, central corneal pachymetry, measured residual stromal bed, and flap thickness at the time of retreatment.

Difference in Bed Thickness (in µm) (Estimated RSB Thickness – Measured RSB Thickness)

Figure 2. Histogram showing distribution of difference between the estimated residual stromal bed thickness and measured residual stromal bed thickness.

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RESIDUAL STROMAL BED AND FLAP THICKNESS IN PRIMARY AND REPEAT MYOPIC LASIK

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DISCUSSION

Corneal pachymetry is mandatory for patients having laser refractive procedures, both primary and retreatment. Ultrasound pachymetry has been the gold standard for measuring corneal thickness. It is more accurate than the Orbscan pachymetry measurement, especially in eyes that have had LASIK.16 However, there is significant variability in the measurement of corneal thickness, flap thickness, and ablation depth.17–22 The thickness of the corneal epithelium also increases after LASIK without a change in stromal thickness.23,24 Thus, pachymetry measurement before repeat LASIK may overestimate the residual stromal bed thickness compared with the estimated residual stromal bed thickness measurement obtained during primary LASIK. Laser in situ keratomileusis decreases the tensile strength of the cornea by reducing the load-bearing portion of the cornea through creation of the flap and stromal ablation. Repeat LASIK further reduces the residual stromal bed. The absolute minimum residual stromal bed thickness

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Figure 5. Difference in flap thickness versus time to enhancement.

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Figure 4. Histogram showing the distribution of the difference in flap thickness between primary LASIK and repeat LASIK.

to prevent ectasia remains undetermined, and an ‘‘adequate’’ residual stromal bed does not absolutely protect against the development of ectasia.25 Nevertheless, the general recommendation is that at least 250 mm of residual stromal bed be left to prevent corneal ectasia after myopic LASIK.3,14,15 Residual stromal bed can be calculated with ultrasound pachymetry by various methods before and after LASIK.26,27 In our study, the measured residual stromal bed at retreatment was thinner than the estimated residual stromal bed at primary LASIK in most eyes. Muallem et al.28 found a mean difference of 3 G 29 mm; the residual stromal bed thickness at enhancement was thicker than the estimated thickness after primary LASIK, a finding that is opposite to the finding in our study. Variables that determine the accuracy of residual stromal bed after initial LASIK include corneal pachymetry measurements, flap thickness estimations, and ablation depth estimations. Sources of error in pachymetry measurements include intersession, intrasession, and interobserver variability.19,20 Difference in Flap Thickness (in µm)

Figure 3. Correlation between flap thickness in primary and repeat LASIK (r Z 0.63).

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Spherical Equivalent (in Diopters) of Refractive Error in Primary LASIK

Figure 6. Difference in flap thickness versus SE of refractive error in primary LASIK.

J CATARACT REFRACT SURG - VOL 32, DECEMBER 2006

Difference in Flap Thickness (in µm)

RESIDUAL STROMAL BED AND FLAP THICKNESS IN PRIMARY AND REPEAT MYOPIC LASIK

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Figure 7. Difference in flap thickness versus SE of refractive error in repeat LASIK.

Current ultrasound pachymetry is associated with intersession errors on the same cornea of G5.0 to 10.0 mm and interobserver errors of G5.0 mm.19,20 Dougherty et al.21 found that corneal hydration affects stromal ablation depth, with excessive dehydration of the stromal bed leading to increased ablation. Treatment diameters and attempted corrections also affect actual ablation depth.22,29 Studies show actual ablation depth is greater than estimated ablation depth.18,22 This might explain why the measured residual stromal bed can be thinner than the estimated residual stromal bed. As flap thickness must be calculated indirectly by subtraction pachymetry, it can be affected by the abovementioned variables, which can have an impact on the measurement of the residual stromal bed and initial pachymetry before flap creation or lifting. The stromal bed can become hydrated during creation of the flap by the mechanical trauma of the suction ring, resulting in an acute increase in intraocular pressure, which causes fluid to shift into the stroma during primary LASIK. In contrast, retreatment is a dry procedure. In primary LASIK, the lubricating fluid used before the microkeratome pass might be drawn beneath the flap, causing stromal hydration. During enhancement, a change in stromal hydration change is less likely to affect the measurement of the residual stromal bed. Despite preservation of the corneal epithelium and Bowman’s layer, epithelial remodeling can occur after LASIK.30 Changing epithelial thickness appears to be a factor in refractive regression and in the variable stability in some patients after LASIK.24,31 Pe´rez-Santonja et al.32 found a significant increase in corneal thickness of 11.8 mm between 1 month and 3 months postoperatively. A confocal microscopy study showed a 22% increase in corneal epithelium thickness 1 month after myopic LASIK, and the epithelium remained thicker at 1 year.23 Lohmann and Gu¨ell31 report that compensatory epithelial hyperplasia is a significant contributor to regression after LASIK

for myopia. In eyes within G1.00 D of the refractive target, there was an increase in epithelial thickness of less than 5 mm postoperatively. In eyes with severe regression, there was a significant increase in the epithelial thickness after LASIK. Spadea et al.24 report an increase in epithelial thickness from 1 week postoperatively that peaked at the third month and remained stable through 12 months. In our study, the increase in flap thickness during repeat treatment can be explained by epithelial hyperplasia. However, it did not correlate with the interval between primary LASIK and enhancement. It had negative correlation with preoperative refractive error. In conclusion, intraoperative pachymetry of the stromal bed during retreatment is strongly recommended as the residual stromal bed thickness and flap thickness change between primary treatment and retreatment. There is a tendency toward the measured stromal bed at retreatment to be thinner than the calculated stromal bed and for the flap to be thicker than at the previous measurement.

REFERENCES 1. Chayet AS, Magallanes R, Montes M, et al. Laser in situ keratomileusis for simple myopic, mixed, and simple hyperopic astigmatism. J Refract Surg 1998; 14:S175–S176 2. Lindstrom RL, Hardten DR, Chu YR. Laser in situ keratomileusis (LASIK) for the treatment of low, moderate, and high myopia. Trans Am Ophthalmol Soc 1997; 95:285–296; discussion, 296–306 3. El-Maghraby A, Salah T, Waring GO III, et al. Randomized bilateral comparison of excimer laser in situ keratomileusis and photorefractive keratectomy for 2.50 to 8.00 diopters of myopia. Ophthalmology 1999; 106:447–457 4. Hersh PS, Brint SF, Maloney RK, et al. Photorefractive keratectomy versus laser in situ keratomileusis for moderate to high myopia; a randomized prospective study. Ophthalmology 1998; 105:1512–1522; discussion by JH Talamo, 1522–1523 5. Pallikaris IG, Siganos DS. Laser in situ keratomileusis to treat myopia: early experience. J Cataract Refract Surg 1997; 23:39–49 6. Knorz MC, Wiesinger B, Liermann A, et al. Laser in situ keratomileusis for moderate and high myopia and myopic astigmatism. Ophthalmology 1998; 105:932–940 7. Pe´rez-Santonja JJ, Ayala MJ, Sakla HF, et al. Retreatment after laser in situ keratomileusis. Ophthalmology 1999; 106:21–28; discussion by ME Whitten, 28 8. Brahma A, McGhee CN, Craig JP, et al. Safety and predictability of laser in situ keratomileusis enhancement by flap reelevation in high myopia. J Cataract Refract Surg 2001; 27:593–603 9. Durrie DS, Aziz AA. Lift-flap retreatment after laser in situ keratomileusis. J Refract Surg 1999; 15:150–153 10. O¨zdamar A, Aras C, Bahc¸eciog˘lu H, xS ener B. Secondary laser in situ keratomileusis 1 year after primary LASIK for high myopia. J Cataract Refract Surg 1999; 25:383–388 11. Lyle WA, Jin GJC. Retreatment after initial laser in situ keratomileusis. J Cataract Refract Surg 2000; 26:650–659 12. Zadok D, Maskaleris G, Garcia V, et al. Outcomes of retreatment after laser in situ keratomileusis. Ophthalmology 1999; 106:2391–2394

J CATARACT REFRACT SURG - VOL 32, DECEMBER 2006

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RESIDUAL STROMAL BED AND FLAP THICKNESS IN PRIMARY AND REPEAT MYOPIC LASIK

13. Martines E, John ME. The Martines enhancement technique for correcting residual myopia following laser assisted in situ keratomileusis. Ophthalmic Surg Lasers 1996; 27:S512–S516 14. Seiler T, Koufala K, Richter G. Iatrogenic keratectasia after laser in situ keratomileusis. J Refract Surg 1998; 14:312–317 15. Randleman JB, Russell B, Ward MA, et al. Risk factors and prognosis for corneal ectasia after LASIK. Ophthalmology 2003; 110:267–275 16. Iskander NG, Anderson Penno E, Peters NT, et al. Accuracy of Orbscan pachymetry measurements and DHG ultrasound pachymetry in primary laser in situ keratomileusis and LASIK enhancement procedures. J Cataract Refract Surg 2001; 27:681–685 17. Jacobs BJ, Deutsch TA, Rubenstein JB. Reproducibility of corneal flap thickness in LASIK. Ophthalmic Surg Lasers 1999; 30:350–353 18. Durairaj VD, Balentine J, Kouyoumdjian G, et al. The predictability of corneal flap thickness and tissue laser ablation in laser in situ keratomileusis. Ophthalmology 2000; 107:2140–2143 19. Salz JJ, Azen SP, Bernstein J, et al. Evaluation and comparison of sources of variability in the measurement of corneal thickness with ultrasonic and optical pachymeters. Ophthalmic Surg 1983; 14:750–754 20. Marsich MM, Bullimore MA. The repeatability of corneal thickness measures. Cornea 2000; 19:792–795 21. Dougherty PJ, Wellish KL, Maloney RK. Excimer laser ablation rate and corneal hydration. Am J Ophthalmol 1994; 118:169–176 22. Chang AW, Tsang AC, Contreras JE, et al. Corneal tissue ablation depth and the Munnerlyn formula. J Cataract Refract Surg 2003; 29:1204– 1210 23. Erie JC, Patel SV, McLaren JW, et al. Effect of myopic laser in situ keratomileusis on epithelial and stromal thickness; a confocal microscopy study. Ophthalmology 2002; 109:1447–1452

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24. Spadea L, Fasciani R, Necozione S, Balestrazzi E. Role of the corneal epithelium in refractive changes following laser in situ keratomileusis for high myopia. J Refract Surg 2000; 16:133–139 25. Ou RJ, Shaw EL, Glasgow BJ. Keratectasia after laser in situ keratomileusis (LASIK): evaluation of the calculated residual stromal bed thickness. Am J Ophthalmol 2002; 134:771–773 26. Flanagan G, Binder PS. Estimating residual stromal thickness before and after laser in situ keratomileusis. J Cataract Refract Surg 2003; 29:1674–1683 27. Randleman JB, Hewitt SM, Lynn MJ, Stulting RD. A comparison of 2 methods for estimating residual stromal bed thickness before repeat LASIK. Ophthalmology 2005; 112:98–103 28. Muallem MS, Yoo SH, Romano AC, et al. Flap and stromal bed thickness in laser in situ keratomileusis enhancement. J Cataract Refract Surg 2004; 30:2295–2302 29. Gatinel D, Hoang-Xuan T, Azar DT. Volume estimation of excimer laser tissue ablation for correction of spherical myopia and hyperopia. Invest Ophthalmol Vis Sci 2002; 43:1445–1449 30. Reinstein DZ, Silverman RH, Sutton HFS, Coleman DJ. Very highfrequency ultrasound corneal analysis identifies anatomic correlates of optical complications of lamellar refractive surgery; anatomic diagnosis in lamellar surgery. Ophthalmology 1999; 106: 474–482 31. Lohmann CP, Gu¨ell JL. Regression after LASIK for the treatment of myopia: the role of the corneal epithelium. Semin Ophthalmol 1998; 13:79–82 32. Pe´rez-Santonja JJ, Bellot J, Claramonte P, et al. Laser in situ keratomileusis to correct high myopia. J Cataract Refract Surg 1997; 23:372– 385

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