Thin-flap (sub-bowman keratomileusis) versus thick-flap laser in situ keratomileusis for moderate to high myopia: Case-control analysis

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1 ARTICLE Thin-flap (sub-bowman keratomileusis) versus thick-flap laser in situ keratomileusis for moderate to high myopia: Case-control analysis Dimitri T. Azar, MD, Ramon C. Ghanem, MD, Jose de la Cruz, MD, Joelle A. Hallak, Takashi Kojima, MD, Faisal M. Al-Tobaigy, MD, Sandeep Jain, MD UROSE: To compare the refractive and visual outcomes of sub-bowman keratomileusis (SBK) and thick-flap laser in situ keratomileusis (LASIK) for moderate to high myopia and evaluate the effect of corneal flap thickness on outcomes. SETTING: Massachusetts Eye and Ear Infirmary, Boston, Massachusetts, USA. METHODS: Two studies were performed. In the first study, the refractive and visual outcomes in 33 eyes that had SBK (flap thickness 82 to 120 and 62 eyes that had thick-flap LASIK (flap thickness R160) were retrospectively analyzed. Inclusion criteria were spherical equivalent 4.0 to 10.0 diopters (D), astigmatism 3.0 D or less, and follow-up 3 months or more. In the second study, the influence of flap thickness was evaluated. A case-control matched study (21 pairs) that controlled for residual stromal bed (RSB) thickness was performed. RESULTS: The mean flap thickness was mm G 9.2 (SD) in the SBK group and G 19.5 mm in the thick-flap LASIK group. There were no significant differences in visual outcomes. In the second study with equivalent RSB thickness, case-control matched comparisons between SBK (mean G 8.0 and thick-flap LASIK (mean G 12.6 showed no differences in preoperative and postoperative refractive and visual outcomes. Comparison of the intended versus achieved correction showed no significant differences between the 2 groups. CONCLUSION: Retrospective analyses showed that the safety, efficacy, and predictability of SBK were similar to those of conventional thick-flap LASIK in corneas with equivalent RSB thickness. J Cataract Refract Surg 2008; 34: Q 2008 ASCRS and ESCRS Sub-Bowman keratomileusis (SBK) is a laser in situ keratomileusis (LASIK) procedure in which the flap is thinner. A major advantage of creating a thin flap during SBK is leaving sufficient stromal tissue to allow safer excimer laser ablation, especially in patients with moderate or high myopia. 1,2 Sufficient residual stromal bed (RSB) thickness (exceeding 250 is important to reduce the likelihood of corneal ectasia, a complication that leads to significant visual loss after LASIK. 1 6 One factor that may affect the RSB thickness is flap thickness. 7 Variability in flap thickness has been well documented. 8 Some reports 9 13 advocate that the ideal flap thickness in LASIK should exceed 130 mm because thin flaps may be associated with a higher frequency of potential complications such as flap folds, striae, epithelial ingrowth, and irregular astigmatism. A more recent report 14 advocates performing SBK with flaps ranging between 90 mm and 110 mm in thickness. The incidence of thin flaps after LASIK has been reported to vary between 0.30% and 0.75%. 10 Recent retrospective studies evaluated the effect of intended thin flaps on the outcomes of LASIK 1, 3, and 6 months after surgery and proposed that intended thin flaps (%100 may be advantageous over thicker flaps for myopic LASIK randi et al. 15 showed that thin flaps were associated with better uncorrected visual acuity (UCVA) at 1 month and better residual spherical equivalent (SE) at 6 months. Eleftheriadis et al. 16 reported faster visual recovery (UCVA at 1 week and 1 month) and lower postoperative myopic SE in eyes with thinner flaps. Cobo-Soriano et al. 17 found that patients with thin flaps achieved better contrast sensitivity and lower retreatment rates. These studies paved the way to SBK, which may combine the advantages of LASIK and surface ablation. 14 The purpose of this study was to compare the visual outcomes in patients with moderate to high myopia Q 2008 ASCRS and ESCRS ublished by Elsevier Inc /08/$dsee front matter 2073 doi: /j.jcrs

2 2074 SUB-BOWMAN KERATOMILEUSIS FOR HIGH MYOIA treated with SBK versus thick-flap LASIK. In addition, we investigated the influence of flap thickness on the final refractive and visual results after myopic SBK and LASIK using a control-matched analysis after controlling for preoperative and intraoperative characteristics as well as RSB thickness. ATIENTS AND METHODS atients Data of 144 patients (177 eyes) who had LASIK for moderate to high myopia ( 4.0 to 10.0 diopters [D]) by the same surgeon (D.T.A.), a follow-up visit of 3 months or more, and preoperative astigmatism of 3.0 D or less were retrospectively analyzed. Two sets of analyses were performed. In the first analysis, 30 patients (33 eyes) who had SBK and a flap thickness between 82 mm and 120 mm were compared with 53 patients (62 eyes) who had thick-flap LASIK with a flap thickness of 160 mm or more. In the second analysis, SBK eyes were matched with eyes that had thick-flap LASIK with a flap thickness greater than 144 mm. Case-control matched analyses was performed for 21 matched pairs. Surgical rocedure A corneal flap was created with the IntraLase FS laser (IntraLase Corp.) or Hansatome microkeratome (Bausch & Lomb). The flap was lifted, and excimer laser ablation of the stromal bed was performed with the Visx (Star S2 or S4, Visx Inc.) or Technolas 217z (Zyoptix or lanoscan, Bausch & Lomb Inc.). The flap was repositioned on the stromal bed. The corneal thickness was measured intraoperatively before and after flap creation with a contact ultrasound pachymeter (RK-5000 achymeter, KMI Surgical roducts). The mean of 3 central measurements was recorded as corneal pachymetry. The flap thickness was estimated by the subtraction method (measurement before flap creation minus stromal bed thickness). Analysis of the Accepted for publication August 12, From the Department of Ophthalmology and Visual Sciences (Azar, Ghanem, de la Cruz, Hallak, Kojima, Jain), Illinois Eye and Ear Infirmary, University of Illinois at Chicago, Chicago, Illinois, and the Department of Ophthalmology (Azar, de la Cruz, Hallak, Kojima, Al-Tobaigy, Jain), Massachusetts Eye and Ear Infirmary and the Schepens Eye Research Institute, Harvard Medical School, Boston, Massachusetts, USA. No author has a financial or proprietary interest in any material or method mentioned. Supported by the Research to revent Blindness Lew Wasserman Award (Dr. Azar), an unrestricted grant from Research to revent Blindness NY, New York, New York, USA (Drs. Azar and Jain), and NIH grants EY and EYR (Dr. Azar). Corresponding author: Dimitri T. Azar, MD, Department of Ophthalmology and Visual Sciences, Illinois Eye and Ear Infirmary, University of Illinois at Chicago, 1855 West Taylor Street, Chicago, Illinois 60612, USA. dazar@uic.edu. visual outcomes of the SBK and thick-flap LASIK groups was performed. Comparative Study Data were abstracted from patient charts in a systematic fashion. reoperative data included age, sex, pachymetry, flap thickness, preoperative best spectacle-corrected visual acuity (BSCVA), manifest refraction, date of surgery, type of laser and microkeratome, and method of laser treatment (wavefront-guided versus noncustomized). data included date of the last follow-up visit, UCVA, BSCVA, manifest refraction, and need for enhancement. Snellen visual acuity data were converted to logmar as described by Holladay. 18 For visual acuity records of patients who did not read all letters on a single line correctly, the conversion was made by interpolating between the values of the logmar acuity using the fraction of the number of letters correctly read on a visual acuity line. 18 Efficacy (percentage of eyes with postoperative UCVA better than or equal to 20/20, 20/25, 20/30, and 20/40), predictability (percentage of eyes with postoperative manifest refractive SE within G0.50 D and within G1.00 D), and safety (percentage of eyes losing 1 line of BSCVA) were calculated. Visual and refractive data from the final visit were analyzed using SSS software (version 15.0, SSS, Inc.); t tests and chi-square tests were applied for analyses. A value less than 0.05 was considered statistically significant. Calculation of RSB thickness was performed by subtracting flap thickness and ablation depth from the central pachymetry. The ablation depth was estimated using the Munnerlyn approximation formula. 19 Influence of Flap Thickness: Control-Matched Analyses Control-matched paired analyses were performed to control for RSB thickness. The preoperative data were matched for eyes with SBK and thick-flap LASIK. Each SBK eye was matched with a single eye with thick-flap LASIK after fulfilling all criteria mentioned below. Due to the matching parameters, the thick-flap LASIK group was expanded to include eyes with a corneal thickness greater than 144 mm. The postoperative data were masked from the researchers throughout the matching process. The criteria used for matching were (1) flap thickness (minimum of 40 mm difference), (2) calculated RSB in the SBK eye within G40 mm of the matched thick-flap eye, (3) preoperative myopic SE ( 4.0 to 6.0 D or more than 6.0 D), (4) flap creation method (laser or mechanical), (5) laser ablation type (wavefront guided or noncustom), (6) laser type (Visx or Bausch & Lomb), and (7) follow-up duration. After all 6 criteria were satisfied, eyes were matched to minimize the difference between patient age of case and control eyes. Twenty-one matched pairs satisfied these criteria. Analysis was performed using paired t tests and the Fischer exact test. RESULTS Comparative Study Table 1 shows the preoperative patient data. In the SBK group (33 eyes of 30 patients), 10 flaps were created with the IntraLase laser and 23 using the mechanical microkeratome. In the thick-flap

3 SUB-BOWMAN KERATOMILEUSIS FOR HIGH MYOIA 2075 Table 1. reoperative data of patients having SBK or thick-flap LASIK for moderate to high myopia ( 4.0 to 10.0 D). arameter SBK ( (33 Eyes, 30 atients) LASIK (R160 (62 Eyes, 53 atients) Value Sex, n (%).757 z Female 16 (53.33) 29 (54.72) Male 14 (46.67) 24 (45.28) Age (y).752 x Mean G SD G G 9.03 Range 24 to to 58 BSCVA.623 x Mean logmar G G 0.03 G SD Range logmar 0.12 to to 0.10 Mean Snellen* 20/20 20/20 Sphere (D).462 x Mean G SD 6.18 G G 1.75 Range 3.50 to to Cylinder (D).906 x Mean G SD 0.80 G G 0.60 Range 0.00 to to 2.75 SE (D).482 x Mean G SD 6.58 G G 1.79 Range 4.00 to to achymetry!.001 x ( Mean G SD G G Range 472 to to 634 Flap thickness!.001 x ( Mean G SD G G Range 82 to to 261 Mean residual bed ( G SD G G 36.26!.001 x equivalent *Converted from logmar Calculated by subtracting ultrasonic pachymetry before and after flap creation z earson chi square x Independent-samples t test LASIK group (62 eyes of 53 patients), 15 flaps were created using the IntraLase and 47 using the mechanical microkeratome. Table 2 shows the postoperative visual acuity in the SBK group and the thick-flap LASIK group at a mean follow-up of 10.2 months and 11.1 months, respectively. No significant differences in the postoperative UCVA, BSCVA, or change in BSCVA were found between the 2 groups. Although retreatment rates were higher in the SBK group (24.2%) than in the thickflap LASIK group (19.4%), the difference was not significant ( Z.58). Table 2. Outcomes of SBK and thick-flap LASIK for moderate to high myopia ( 4.0 to 10.0 D). arameter SBK ( (n Z 33) LASIK (R160 (n Z 62) Value.503 z UCVA* Mean logmar 0.16 G G 0.15 G SD Range logmar 0.12 to to 0.54 Mean Snellen 20/29 20/28 Acuity.947 z BSCVA Mean logmar G G 0.04 G SD Range logmar 0.12 to to 0.10 Mean Snellen 20/20 20/20 Change in.708 z BSCVA Mean G SD G G 0.05 Range 0.12 to to z sphere (D) Mean G SD 0.33 G G 0.62 Range 1.75 to to z cylinder (D) Mean G SD 0.43 G G 0.41 Range 1.25 to to z SE (D) Mean G SD 0.55 G G 0.64 Range 2.00 to to 0.88 Follow-up, m.511 z Mean G SD G G 7.02 Range 3 to 24 3 to 48 Retreatment rate (%) x equivalent; UCVA Z uncorrected visual acuity *Without monovision Converted from logmar z Independent-samples t test x earson chi square Table 2 also shows the postoperative sphere, cylinder, and SE. No significant differences were found between the SBK group and the thick-flap LASIK group. No flap complications were reported. No differences in predictability and efficacy parameters were found between the SBK group and thick-flap LASIK group (Table 3). No eye in either group lost 2 or more lines of BSCVA. However, 27.3% of eyes in the SBK group lost 1 line of BSCVA compared with 8.1% of eyes in the thick-flap LASIK group ( Z.01).

4 2076 SUB-BOWMAN KERATOMILEUSIS FOR HIGH MYOIA Table 3. Safety, efficacy, and predictability of SBK and thickflap LASIK for moderate to high myopia ( 4.0 to 10.0 D). arameter SBK ( (n Z 33) ercentage LASIK (R160 (n Z 62) Value* Safety (BSCVA) Loss of 1 line No loss Gain of 1 line Efficacy R20/ R20/ R20/ redictability G0.50 D G1.00 D keratomileusis; SBK Z sub-bowman keratomileusis *earson chi square Influence of Flap Thickness: Control-Matched Analyses aired analyses were performed for 42 eyes (21 matches) with SBK or thick-flap LASIK that satisfied all matching criteria. The flap was created using the IntraLase laser in 5 matches and the manual microkeratome in 16 matches. Although matching considerably reduced the sample size and statistical power of the data, control matched analysis was considered to be useful in determining whether corneal flap thickness independently affects postoperative outcomes. The preoperative controlled matched data did not show a difference in vision or refractive data between the SBK group and the thick-flap LASIK group (Table 4). There were no significant differences in postoperative refractive and visual outcomes (Table 5). Safety, efficacy, and predictability parameters were better in eyes that had thick-flap LASIK than in those that had SBK; however, no statistically significant differences were found (Table 6). There was no significant difference in the intended versus achieved refractive error between the SBK group (mean 0.33 G 0.78 D) and the thick-flap LASIK group (mean 0.47 G 0.47) ( Z.44), indicating corneal flap thickness does not affect the refractive outcome in patients with adequate RSB thickness. DISCUSSION In this study, we evaluated the safety, efficacy, and predictability of SBK by retrospectively analyzing the pooled data of SBK (82 to 120 and thick-flap Table 4. reoperative visual and refractive results in 21 matched eyes having SBK or thick-flap LASIK for moderate to high myopia ( 4.0 to 10.0 D). reoperative Data SBK ( Mean G SD Thick Flap LASIK ( Value Age (y) G G Sphere (D) 6.19 G G Cylinder (D) 0.69 G G SE (D) 6.53 G G BSCVA.832 LogMAR 0.001G G 0.03 Snellen* 20/20 20/20 Flap thickness G G 12.58!.001 ( Residual bed G G ( Follow-up (mo) G G equivalent *Converted from logmar aired-samples t test LASIK (R160. The visual and refractive outcomes of SBK and thick-flap LASIK were similar; this was true in corneas with equivalent RSB thickness. The importance of a thick RSB after LASIK is widely accepted. 1 6 The RSB thickness depends on the preoperative corneal thickness, thickness of the corneal flap, and amount of tissue ablation by the excimer laser. Thus, a thin flap (as in SBK) may be desirable; it can help maximize the RSB thickness and preserve the biomechanical stability of the cornea. 14 With SBK, it is possible to safely perform LASIK in patients with thin corneas as the RSB thickness is greater than that achieved by conventional LASIK. To our knowledge, this is the first study to evaluate the effect of flap thickness in matched SBK and thick-flap LASIK patients having similar RSB values. Studies evaluating the effect of corneal flap thickness on LASIK outcomes report conflicting results. In a prospective study of 69 eyes, Yi and Joo 20 found slightly better visual outcomes in the thick-flap group (O165 than in the thin-flap group (!135. Yeo and Song 21 observed a higher incidence of central corneal opacity after LASIK with thin flaps (mean thickness G A possible explanation is injury to Bowman layer by the blade or a hidden/ masked buttonhole, camouflaged by intact epithelium, which may have caused central corneal scarring. Using confocal microscopy, other studies found increased keratocyte activation associated with thin

5 SUB-BOWMAN KERATOMILEUSIS FOR HIGH MYOIA 2077 Table 5. visual and refractive results in 21 matched eyes having SBK or thick-flap LASIK for moderate to high myopia ( 4.0 to 10.0 D). Mean G SD Table 6. Safety, efficacy, and predictability in 21 matched eyes having SBK or thick-flap LASIK for moderate to high myopia ( 4.0 to 10.0 D). ercentage Data SBK ( LASIK ( Value arameter SBK ( LASIK ( Value* Sphere (D) 0.39 G G Cylinder (D) 0.54 G G SE (D) 0.66 G G BSCVA.253 LogMAR G G 0.05 Snellen* 20/20 20/21 UCVA.266 LogMAR 0.17 G G 0.07 Snellen* 20/30 20/26 BSCVA change 0.01 G G Retreatment rate (%) z equivalent; UCVA Z uncorrected visual acuity *Converted from LogMAR aired-samples t test z earson chi- square flaps after LASIK. 22,23 In a study by Vesaluoma et al., 23 patients with increased interface reflectivity due to abnormal extracellular matrix or activated keratocytes had significantly thinner flaps than patients with normal interface reflectivity. isella et al. 22 found higher postoperative cellular activation in the posterior stroma in patients with thin flaps. The reason for the better outcomes with thick-flap LASIK in these studies seems to be that the thin flap was an unintended complication; hence, it may have been irregular. Other studies report that thin flaps were associated with better early visual and refractive results than thick flaps These include some of the more recent studies in which a thin flap was intentionally created as part of SBK. 14 randi et al. 15 report that patients with flaps of 100 mm or less had better functional results at 1 month than those with thicker flaps. Eleftheriadis et al. 16 observed better early UCVA after myopic LASIK with thinner flaps than with thicker flaps at 1 week and 1 month but not at 1 day and 3 months. One month after LASIK, sphere and cylinder were not related to flap thickness; however, SE was negatively correlated. In both reports, flap thickness was unrelated to BSCVA. 15,16 Cobo-Soriano et al. 17 report better contrast sensitivity and lower retreatment rates with thin flaps. Although not statistically significant, there was a trend toward a lower retreatment rate in the thin-flap group than in the thick-flap group (13.7% versus 19.7%; Z 0.32). Safety (BSCVA) Loss of 1 line No loss Gain of 1 line Efficacy R20/ R20/ R20/ redictability G0.50 D G1.00 D * keratomileusis; SBK Z sub-bowman keratomileusis *Fisher exact test Kymionis et al. 24 report the long-term refractive results of photorefractive keratectomy (RK) and LASIK in patients with thin corneas (!500. Intraoperative flap thickness ranged between 69 mm and 110 mm. After flap lifting and stromal ablation, the mean RSB thickness was G mm in LASIK and G mm in RK (after epithelial removal and stromal ablation). The authors report that both procedures resulted in safe and predictable results, with no post-refractive corneal ectasia. A recent study 25 of 3009 eyes that had SBK using the femtosecond laser found a low complication rate. Intraoperative complications included flap tear, free cap, bubble escape, and flap folds. complications included diffuse lamellar keratitis and epithelial ingrowth. Flap-related complications, such as an uneven bed, buttonhole, short flap, flap striae, or wrinkles, did not occur. No complications were noted in our study. Data in our series have certain limitations. The retrospective nature of our study and that our data were obtained only at the last follow-up visit are obvious limitations. roblem-free patients with a good outcome tend not to return for longer follow-up visits. This may have biased the results toward some overestimation of the visual loss, which may have a similar effect on all groups. One rationale for performing case-control matched analyses in this study was to overcome limitations in data analyses due to difference in sample size and the presence of laser ablation variables.

6 2078 SUB-BOWMAN KERATOMILEUSIS FOR HIGH MYOIA Mechanical microkeratomes create flaps with a thickness directly dependent on corneal pachymetry. 8,20,22,26,27 Flap thickness varies widely, especially with mechanical microkeratomes, according to microkeratome type, turbine and translational blade velocities, reuse of blades, and nominal labeled head. 20,21,23,26 28 The achieved flap thickness frequently differs from the expected. 5,8,20, The actual flap may be much thicker than planned, making the RSB less than expected, which may increase the risk for corneal ectasia. 1 6 In our study, we estimated the actual flap thickness by performing intraoperative pachymetry before and after flap creation. This practice is especially useful when treating moderate to highly myopic eyes and relatively thin corneas or when performing LASIK retreatments. Future prospective controlled matched studies may be needed to ascertain whether SBK is advantageous relative to thick-flap LASIK in patients with moderate to high myopia. REFERENCES 1. Condon I, O Keefe M, Binder S. Long-term results of laser in situ keratomileusis for high myopia: risk for ectasia. J Cataract Refract Surg 2007; 33: Randleman JB. ost-laser in-situ keratomileusis ectasia: current understanding and future directions. Curr Opin Ophthalmol 2006; 17: Seiler T, Koufala K, Richter G. Iatrogenic keratectasia after laser in situ keratomileusis. J Refract Surg 1998; 14: allikaris IG, Kymionis GD, Astyrakakis NI. Corneal ectasia induced by laser in situ keratomileusis. J Cataract Refract Surg 2001; 27: Binder S. Ectasia after laser in situ keratomileusis. J Cataract Refract Surg 2003; 29: Randleman JB, Russell B, Ward MA, Thompson K, Stulting RD. Risk factors and prognosis for corneal ectasia after LASIK. Ophthalmology 2003; 110: Flanagan G, Binder S. 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Azar, MD Department of Ophthalmology and Visual Sciences, Illinois Eye & Ear Infirmary, University of Illinois at Chicago, Chicago, Illinois