UNDERSURFACE ABLATION OF THE CORNEAL FLAP FOR LASIK ENHANCEMENT

Transcription

1 ARCH SOC ESP OFTALMOL 2007; 82: ORIGINAL ARTICLE UNDERSURFACE ABLATION OF THE CORNEAL FLAP FOR LASIK ENHANCEMENT RETRATAMIENTOS EN CARA INTERNA DEL COLGAJO CORNEAL EN LASIK. ESTUDIO COMPARATIVO CON UN GRUPO CONTROL FERNÁNDEZ-VIGO J 1, MACARRO A 1, FERNÁNDEZ SABUGAL J 2 ABSTRACT Objective: To study the efficacy and safety of retreatment in LASIK surgery, comparing under surface ablation of the corneal flap and stromal bed ablation techniques. Methods: The patients studied were those who had LASIK enhancement because of a residual negative refractive error, and were divided in two groups: a Control group of 14 eyes that had ablation of the residual stromal bed and a Study group of 14 eyes that had ablation of the undersurface of the flap. In the Study group we included patients in whom we calculated that the post-enhancement treatment of the residual stromal bed would be less than 300 microns. We compared the refraction, visual acuity and cornea pre and post-retreatment, three months later. Results: Prior to retreatment, both groups were similar and no statistically significant differences were found in age, sex, pachymetry, corneal curvature or residual refraction. Ablation was 21.5 (SD 11) microns in study group and 22.6 (SD 7) microns in control group (p=0.796). Post-retreatment refraction was statistically lower in both groups than before the procedure (p<0.01), however there were no differences between the control and study groups. Visual RESUMEN Objetivo: Estudiar la eficacia y seguridad de los retratamientos en cirugía LASIK comparando el tratamiento sobre el lecho estromal con el tratamiento sobre la cara interna del colgajo corneal. Métodos: Pacientes intervenidos mediante LASIK con defecto residual negativo sometidos a retratamiento divididos en dos grupos. Grupo control: 14 ojos tratados sobre lecho estromal. Grupo estudio: 14 ojos tratados sobre cara interna del colgajo. En el grupo estudio se incluyeron aquellos pacientes en los que se preveía que el grosor del lecho post-retratamiento sería inferior a 300 micras. Se evaluaron la refracción, la agudeza visual y la córnea pre y postretratamiento a los 3 meses del retratamiento. Resultados: Antes del retratamiento no había diferencias entre ambos grupos en lo referido a edad, sexo, paquimetría, curvatura corneal y defecto de refracción. La ablación fue de 21.5 DE 11 en el grupo estudio y 22,6 DE 7 micras en el control (p=0,796). La refracción post-retratamiento fue significativamente menor en ambos grupos que antes (p<0,01). No hubo diferencias entre el grupo control y el estudio. La agudeza visual final mejoró con respecto a la que presentaban antes del retratamien- Received: 31/5/06. Accepted: 20/9/07. Extremadura University. Medicine College. Ophthalmology Area. Badajoz. Spain 1 Ph.D in Medicine. University of Extremadura and International Advanced Ophthalmology Centre. Madrid and Badajoz. 2 Ph.D. in Medicine. University of Extremadura. This paper was partially presented at the LXXI Congress of SEO (Barcelona 2001). Correspondence: José Fernández-Vigo Área de Oftalmología. Facultad de Medicina Avda. de Elvas, s/n Badajoz Spain catoftal@unex.es

2 FERNÁNDEZ-VIGO J, et al. acuity improved after retreatment in both groups (p<0.01) but no statistically significant differences were found between control and study groups. No patient lost two lines of visual acuity. We observed slight alterations in the corneal flap in both groups. Conclusions: In our study, the undersurface retreatment technique produced similar results to those achieved using conventional stromal bed techniques with both being effective and safe in patients with a small residual negative refractive error (Arch Soc Esp Oftalmol 2007; 82: ). Key words: LASIK, refractive surgery, excimer laser, cornea, complications. to en ambos grupos (p<0,01). No hubo diferencias entre ambos grupos. Ningún paciente perdió dos líneas de agudeza visual. Se observaron leves alteraciones en el colgajo corneal en ambos grupos. Conclusiones: En nuestro estudio la técnica de retratamiento sobre la cara interna del colgajo es similar en eficacia y seguridad a la técnica convencional sobre el lecho estromal en defectos negativos con poco defecto residual. Palabras clave: LASIK, cirugía refractiva, laser excimer, córnea, complicaciones. INTRODUCTION Although refractive surgery with excimer laser is an extremely accurate technique aimed at eliminating refraction defects, it is not free of minor faults such as the need to repeat treatment (retreatment) in order to adjust results. Changes in weather conditions, fluctuations in laser power and variability between individuals and per individual require retreatment in LASIK surgery in rates usually ranging from 10 to 20 percent depending on several circumstances (1-4). Currently, patients view retreatment as a small failure. Thus, retreatment should guarantee the final adjustment of patient refraction and be very safe. However, for some time now an increase in the number of ectasias after treatment has been reported (5,6). Maldonado (7) described a technique designed to reduce the risk of ectasias by retreating the inner surface of the disk as to preserve a larger thickness of the residual stromal bed and showed excellent results in a group of patients. The present paper reports the findings of a study comparing a group of patients retreated with the conventional technique and another group retreated with Maldonado s technique. SUBJECTS, MATERIAL AND METHOD The comparative prospective study covered in total 28 eyes corresponding to 28 patients divided into two groups: one control group, where retreatment involved the stromal bed, made up by 14 consecutive eyes in 14 patients, including the eyes whose bed residual thickness amounted to 300 microns or more after retreatment. The study group, where retreatment involved the inner surface of the corneal flap, made up by 14 consecutive eyes in 14 patients, including those eyes whose bed residual thickness after treatment was below 300 microns. Furthermore, all had to fulfill the following criteria: corneal thickness after retreatment exceeding 400 microns, bed thickness over 300 microns, and flap thickness exceeding 100 microns. All eyes had previously undergone LASIK surgery duo to myopia and/or myopic astigmatism at least three months earlier and currently exhibiting a negative refractive defect observed after cyclopegia. Those patients not fulfilling the requirements described in the paragraphs above or suffering from other ocular and systemic disorders where surgery is contraindicated have been excluded. All patients were informed about the technique and warned about the possibility of treating the inner surface prior to signing the informed consent. Surgical Technique The first surgery and retreatment were performed by the same surgeon. The LASIK technique applied during the first surgery was conventional, including pachymetry of the bed (sonogage corneo-gage plus, Sonogage Inc, Cleveland, Ohio, USA), using a Moria ONE LSK microkeratome with 100-micron head (Microtech, Inc., 698 ARCH SOC ESP OFTALMOL 2007; 82:

3 Retreatment with LASIK Moria, France) and applying its nomogram to obtain 9.5 mm disks with a nasal hinge and ablation with excimer laser. Retreatment starts in all cases by elevating the corneal flap and flap-rhexis (8). Subsequently, an everted flap is placed over the tip of a wet absorbing sponge approximately 6 mm in size on the nasal conjunctiva, the vortex aligned with the hinge and the base over the internal canthus, seeking to preserve its concavity (unlike the conventional technique, whereby the flap rests over the nasal conjunctiva) (fig. 1). The pachymetry is repeated. If thickness of the stromal bed was equal to or greater than 300 microns after retreatment, laser was applied on the bed. In the event that, once ablation was suppressed from retreatment, bed thickness remained below 300 microns, the laser would be applied on the disk inner surface. This maneuver is performed following the criteria described by Maldonado (7). Unlike his technique, and since the flap rests over the sponge, it is not required to stretch the flap nor to ask the patient to move the globe towards the temporal side in order to expose the disk, since it remains parallel and approximately at the bed level (fig. 1). Over the gentian violet mark in the epithelium and after slightly moving the stretcher, patient kept looking straight ahead, fixating the globe by means of a clamp applied to the superior temporal conjunctiva. The other hand covers the hinge with an absorbing sponge, placing both hands in parallel and very close to each other. The ablation required an optic area ranging from 5.8 to 6 mm in size and 1-mm transition. Once treatment was completed, the same patterns used in the original procedure were applied. Retreatment Parameters Laser is programmed using the same criteria as with primary LASIK. As Maldonado suggests, one should bear in mind that treatment of the flap inner surface results in a mirroring effect (7). Since spheric ablations are symmetrical, no transformation is required. However, the Beta: 180- alpha formula is needed to estimate new orientations in the axis of treatments in thoric ablations on the flap stroma. Applying this formula is required in all axes except for the 90- and 180-degree axes. Assessed Parameters Patients were monitored 30 minutes, 24 hours, one week, one month and 3 months after surgery. Optometrists and ophthalmologists assessing patients after surgery were unaware of the technique applied in each eye. Exploration consisted in measuring visual acuity (VA) from far and near and without correction, auto-refractometry and biomicroscopy, topography and pachymetry. During the third-month checkup, it was adjusted with cyclopegia after dilation. Under biomicroscopy, the following were analyzed: flap movements, formation of folds, opacification of the cornea, stripes and interphase epithelization. Fig. 1: Surgery detail: this picture shows the treatment performed on the bed or inner surface of the corneal flap resting everted over a sponge. It illustrates the placement, centering and parallelism compared to the stromal bed. Statistical Analysis ARCH SOC ESP OFTALMOL 2007; 82: Quantitative variables were expressed as the mean, range and standard deviation, whereas qualitative variables were expressed as frequency distribution. The mean distribution of variables with asymmetrical distribution was assessed. The traits of both groups were compared using the t-student test for independent samples in quantitative variables, whereas qualitative variables were compared using the c 2 test or Fisher s exact test. Comparative analysis of parameters before and after surgery was performed using the t-student test for matched sam- 699

4 FERNÁNDEZ-VIGO J, et al. ples. Data were collected into an Access database (Microsoft for Windows, Microsoft Co, Portland, USA) and subsequently exported to Excel (Microsoft for Windows, Microsoft Co, Portland, USA) for analysis (9). RESULTS Results pertain to checkups during the third month. The population is described in Table I, while Tables II and III provide individual descriptions of results. 1. Refraction: before retreatment, the spherical equivalent was 1.58 SD 0.69 in the study group and 1.62 SD 0.61 in the control group (p=.853). After retreatment, refraction measured based on the spherical equivalent was SD 0.02 in the control group (range to 0.5) and SD 0.03 (range +0.5 to 0.25) in the study group (p=.814). In both groups, refraction after retreatment was significantly lower than before (p<.01). In the control group, residual defects persisted in three eyes and in the study group in two eyes (range: to -0.5 in both groups). 2. Visual Acuity: before retreatment, manifest VA was 0.58 SD 0.14 and corrected VA was 0.92 SD 0.09 in the control group, while manifest VA was 0.57 SD 0.14 and corrected VA was 0.92 SD 0.1 in the study group (p=.815 and.876). Manifest VA after retreatment in the control group was 0.9 SD 0.11 and 0.9 SD 0.1 (p=.876) in the study group. Corrected VA was 0.93 SD 0.08 in the control group and 0.91 SD 0.1 in the study group (p=.729). In the study group: manifest VA after retreatment was significantly higher than before (p<.01). No differences were observed in terms of corrected visual acuity before and after retreatment nor between corrected visual acuity before retreatment and spontaneous visual acuity after retreatment (p=.08). In the control group: manifest VA after retreatment was significantly higher than before (p<.01). Corrected VA before and after retreatment were identical, and no differences were found in terms of corrected visual acuity before retreatment and manifest visual acuity after retreatment (p=.18). In the control group, final manifest VA was identical to corrected VA before retreatment in 10 out of 14 eyes, one line less in two cases and improved in another two cases. In the study group, 11 eyes achieved similar visual acuity, two eyes recorded one line less and another eye improved by one line. There were no cases of loss of best corrected visual acuity. 3. Pachymetry: the ablation required for retreatment was SD microns in the study group and SD 7.42 microns in the control group (p=.796). 4. Biomicroscopy of the anterior segment: neither group revealed significant alterations, since there were no flap movements nor stripe formation. No corneal opacification was observed. In two eyes in both groups, fine stripes were observed before retreatment. After surgery, stripes emerged in two eyes in the control group (one new, one disappeared and another remained the same) and three in the study group (two new, disappearing in one and persisting in the other). A slight peripheral epithelization was observed in two eyes belonging to the control group and two in the study group. DISCUSSION Retreatments are maneuvers performed in a small but significant percentage of surgeries (1-4). This percentage has likely increased due to patients demands. Nevertheless, caution is advised when retreating those patients whose cornea is already thinner due to prior treatment, thus avoiding significant risks for patients. Based on such circumstances and the unawareness of the exact limits of the bed, the flap, etc., the rate of ectasias has increased (5,6,10). Furthermore, significant changes after Table I. Comparative data before surgery between the control and study group Control Group Study Group Mean Age 37 SD 9.5 years 37.4 SD 8.5 years (p=.910) Gender 6 men (42.8%) 7 men (50%) (p=.775) Pachymetry SD µm SD µm (p=.105) K (corneal curvature) SD 1.40 diopters SD 1.73 diopters SD = Standard Deviation. 700 ARCH SOC ESP OFTALMOL 2007; 82:

5 Retreatment with LASIK Table II. Control Group: Bed Treatment Bed Age Gender SE SVA CVA Pachymetry K TMT SE SVA CVA pre pre pre microns ablation µm post post post 1 37 M F M M F F F F M F M F M M SE: spherical equivalent, SVA pre: spontaneous visual acuity before retreatment, CVA pre: corrected visual acuity before retreatment, SVA post: spontaneous visual acuity after retreatment, CVA post: corrected visual acuity after treatment, TMT ablation: ablation performed during treatment expressed as microns, K: corneal curvature expressed in diopters. Table III. Study Group: Treatment of the Corneal Flap Inner Surface Bed Age Gender SE SVA CVA Pachymetry K TMT SE SVA CVA pre pre pre microns ablation µm post post post 1 45 F µ µ F M F F M M M F M F M F F SE: spherical equivalent, SVA pre: spontaneous visual acuity before retreatment, CVA pre: corrected visual acuity before retreatment, SVA post: spontaneous visual acuity after retreatment, CVA post: corrected visual acuity after treatment, K: corneal curvature expressed in diopters, TMT ablation: ablation performed during treatment expressed as microns. retreatment have been described in the curvature of the cornea posterior side (11) in patients not suffering from ectasia. Thus, in order to avoid complications, it is necessary to adequately assess patients before repeating surgery. One of the most important factors in the pathogenesis of ectasia is a reduced stromal bed (possibly below 250 microns), although it is not always the case, since there are many more reasons (10). This excess thinning of the bed is the result of excessively deep ablations combined with two additional factors: microkeratomes are still inaccurate in ARCH SOC ESP OFTALMOL 2007; 82: terms of cut depth (12,13) and on certain occasions ablations turn out to be different than planned (14). This scenario becomes even more complex during retreatment due to the differences between flap thickness (estimated based on the initial LASIK cut) and thickness once the flap has been lifted for touch-up, most likely by compensatory epithelial hyperplasia (15,16). Maldonado published one paper describing a new retreatment technique in those eyes whose posterior stroma was insufficient to perform the necessary ablation (7) as well as the instruments needed to 701

6 FERNÁNDEZ-VIGO J, et al. facilitate surgery (17). One of the main reasons to back his technique is the fact that the stroma present in the flap does not contribute to the tectonic integrity of both the cornea and the stromal bed, and he thus hypothesized that retreatment by means of ablation of the disk inner surface would help preventing corneal ectasias in the future and preserving Bowman s membrane. Based on the findings reported by the same author, retreatment of the inner surface of the corneal disk by means of this technique is safe and efficient and is a good solution to prevent excessive thinning of the residual stromal bed and thus avoid the emergence of corneal ectasias (7). This author has shown greater changes in the posterior cornea curvature using the conventional technique and not his (18). In this prospective study with control and study groups, we analyzed the efficacy and safety of conventional treatments on residual stromal beds compared with treatment on the disk inner surface. Although the technique is similar to Maldonado s, our treatment concept is partly different. This author recommends not performing treatments that bring corneal disk thickness below 150 microns and increase bed thickness above 250 microns, whereas in our opinion limits should be set at 100 and 300 microns, respectively. We prefer 100-micron disks since they provide a wide margin, without coming close to the 250-micron bed limit, and due to the fact that ablations larger than expected (14) or epithelial hyperplasia (15,16) may result in very significant estimation errors. As we described in the corresponding section, during the first surgery we performed a pachymetry to obtain the estimated flap thickness. We thus know very accurately its value and on this basis we determine the treatment area and estimate the ablation. The results obtained when treating the inner surface are similar to those already published (7,9). Furthermore, its efficacy to correct residual refraction in the two groups making up our study is similar, since no significant differences were found in terms of results. The same applies to visual acuity, which is very similar in both groups with no statistically significant differences being observed. On the other hand, none of the eyes treated with either technique lost two lines, nor were there any alterations in the corneal flap, and therefore it can be said that this surgical technique is extremely safe for both groups. Our study poses several limitations. Since refractive surgery usually results in very few complications, we did not expect to find any significant ones in a study involving just 28 eyes. On the other hand, the residual defects treated herein recorded few diopters. Furthermore, even though the control and study groups were very similar in what concerns their characteristics, they are hardly homogeneous, especially when taking into account the differences found among individual responses. Therefore, for the time being, we are studying a group of patients undergoing bilateral retreatment, one in the bed and the other in the flap inner surface. Finally, the efficacy and safety of the technique described by Maldonado for retreating the inner surface is similar to the conventional technique used on the bed for the target population. REFERENCES 1. Hu DJ, Feder RS, Basti S, Fung BB, Rademaker AW, Stewart P, et al. Predictive formula for calculating the probability of LASIK enhancement. J Cataract Refract Surg 2004; 30: Walter KA, Stevenson AW. Effect of environmental factors on myopic LASIK enhancement rates. J Cataract Refract Surg 2004; 30: Netto MV, Wilson SE. Flap lift for LASIK retreatment in eyes with myopia. Opthalmology 2004; 111: Perlman EM, Reinert SE. Factors influencing the need for enhancement after laser in situ keratomileusis. J Refract Surg 2004; 20: Amoils SP, Deist MB, Gous P, Amoils PM. Iatrogenic keratectasia after laser in situ keratomileusis for less than -4.0 to -7.0 diopters of myopia. J Cataract Refract Surg 2000; 26: Argento C, Consentino MJ, Tytium A, Rapetti G, Zarate J. Corneal ectasia after laser in situ keratomileusis. J Cataract Refract Surg 2001; 27: Maldonado MJ. Undersurface ablation of the flap for laser in situ keratomileusis retreatment. Ophthalmology 2002; 109: Pérez-Santonja JJ, Medrano M, Ruiz-Moreno JM, Cardona-Ausina C, Alió JL. Flaprhexis circular: una técnica minuciosa para el retratamiento tras lasik. Arch Soc Esp Oftalmol 2001; 76: Versace P, Watson SL. Cornea-sparing laser in situ keratomileusis: ablation on the flap. J Cataract Refract Surg 2005; 31: Lifshitz T, Levy J, Klemperer I, Levinger S. Late bilateral keratectasia after LASIK in a low myopic patient. J Refract Surg 2005; 21: Rani A, Murthy BR, Sharma N, Fitiyal JS, Vajpayee RB, Pandey RM, et al. Posterior corneal topographic changes after retreatment LASIK. Opthalmology 2002; 109: ARCH SOC ESP OFTALMOL 2007; 82:

7 Retreatment with LASIK 12. Solomon KD, Donnenfeld E, Sandoval HP, Al Sarraf O, Kasper TJ, Holxer MP, et al. Flap thickness accuracy: comparison of 6 microkeratome models. J Cataract Refract Surg 2004; 30: Hammer T, Hanschke R, Worner I, Wilhelm FW. Evaluation of four microkeratome models: quality and reproducibility of cut edge and cut surface as determined by scanning electron microscopy. J Refract Surg 2005; 21: Flanagan GW, Binder PS. The theoretical vs. measured laser resection for laser in situ keratomileusis. J Refract Surg 2005; 21: Muallem MS, Yoo SH, Romano AC, Marangon FB, Schiffman JC, Culbertson WW. Flap and stromal bed thickness in laser in situ keratomileusis enhancement. J Cataract Refract Surg 2004; 30: 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: Maldonado MJ. Laser-assisted in situ keratomileusis posterior ablation platforum. Arch Ophthalmol 2005; 123: Maldonado MJ, Nieto JC, Díez Cuenca M, Piñero DP. Posterior corneal curvature changes after undersurface ablation of the flap and in-the-bed LASIK retreatment. Ophthalmology 2006; 113: ARCH SOC ESP OFTALMOL 2007; 82: