Response of the posterior corneal surface to myopic laser in situ keratomileusis with different ablation depths

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1 ARTICLE Response of the posterior corneal surface to myopic laser in situ keratomileusis with different ablation depths David Smadja, MD, Marcony R. Santhiago, MD, Glauco R. Mello, MD, Cynthia J. Roberts, PhD, William J. Dupps Jr, MD, PhD, Ronald R. Krueger, MD, MSE PURPOSE: To evaluate the posterior corneal surface response at a very early stage after myopic laser in situ keratomileusis (LASIK) with different ablation depths. SETTING: Cole Eye Institute, Cleveland Clinic, Cleveland, Ohio, USA. DESIGN: Cohort study. METHODS: Healthy myopic eyes were divided based on the achieved ablation depth as follows: Group 1, more than 100 mm; Group 2, between 50 mm and 99 mm; Group 3, less than 50 mm. Posterior eccentricity and central (0.0 to 4.0 mm), paracentral (4.0 to 7.0 mm), and peripheral (7.0 to 10.0 mm) posterior corneal curvatures were measured with the Galilei system preoperatively and postoperatively after 1 day, 1 week, and 1 and 3 months. RESULTS: Eighty eyes were evaluated. Posterior surface steepening and a shift toward prolateness occurred in all groups, with a peak within the first week before returning toward the original level after 1 month. The maximum change in the central posterior cornea occurred after 1 day in Group 1 and reached diopter (D). This change was statistically significant (PZ.03) and statistically greater than the change in Group 2 (mean D; PZ0.02) and Group 3 (mean D; P<.01). This change was not significant after 3 months (PZ.5). CONCLUSIONS: Posterior steepening and a shift toward prolateness of the posterior surface were observed very early after myopic LASIK, with a tendency to return toward the preoperative level between 1 month and 3 months. The degree of change was related to the amount of anterior tissue severed. Financial Disclosure: No author has a financial or proprietary interest in any material or method mentioned. J Cataract Refract Surg 2012; 38: Q 2012 ASCRS and ESCRS Iatrogenic corneal ectasia after laser in situ keratomileusis (LASIK) is the most feared complication for refractive surgeons. Although its prevalence is low, estimated at between 0.04% 1 and 0.60%, 2 when it develops, early recognition is essential. Early treatment, such as corneal collagen crosslinking or intraocular pressure (IOP) reduction, may reverse the course of ectasia, thereby preventing its progression and positively influencing the visual prognosis. 3 Histopathologic and ultrastructural studies of corneas that developed ectasia after keratorefractive surgery have shown that the primary effect of the biomechanical failure process occurs in the posterior corneal stroma. 4 A good understanding of how the biomechanical response affects the posterior corneal surface in keratorefractive procedures is thus becoming important for early recognition of induced ectatic disorders. Several studies report a forward shift of the posterior cornea after refractive surgery, 5 7 and it has been suggested that this might represent a subclinical form of post-lasik ectasia because both have some similar characteristics. 8 However, most studies report posterior corneal changes with a topography-based measurement (Orbscan II, Bausch & Lomb), a method whose accuracy in posterior corneal surface assessment after LASIK has been questioned. 9,A Although the most assumed risk factors for post- LASIK ectasia include a thin cornea, a residual stromal bed (RSB) less than 250 mm, form fruste keratoconus, young age (!30 years), and high myopia, 10 the 1222 Q 2012 ASCRS and ESCRS Published by Elsevier Inc /$ - see front matter doi: /j.jcrs

2 POSTERIOR CORNEAL RESPONSE TO MYOPIC LASIK 1223 maximum ablation depth has also been proposed as a key variable in the biomechanical response to refractive surgery because of its relationship to the number of severed lamellae. 11 To our knowledge, this study is the first to evaluate changes in the posterior corneal surface in response to myopic LASIK with a rotating dual Scheimpflug imaging based device, although there have been studies using a single Scheimpflug device. 12,13 Our first objective was to analyze early changes in the posterior corneal curvature 1 day, 1 week, and 1 and 3 months postoperatively for different amounts of tissue ablated and to follow the progression over the various time points. Our second objective was to evaluate the relevance of the attempted percentage of anterior tissue severed during surgery as a new, useful variable for screening for the risk for post-lasik ectasia in patients before refractive surgery. PATIENTS AND METHODS This prospective comparative study was performed at Cole Eye Institute, Cleveland, Ohio, USA. It was approved by an institutional review board and performed in accordance with the tenets of the Declaration of Helsinki. Healthy eyes with myopia or myopia with astigmatism were prospectively enrolled in this study when they met previously published standard criteria for LASIK after a screening evaluation. 14 Patients with an ocular history or ocular surgery or who were candidates for hyperopic treatment were not included in this study. After surgery, eyes were divided into 3 groups based on the achieved ablation depth. Group 1 included patients with more than 100 mm of tissue ablated. Group 2 included patients with between 50 mm and 99 mm of tissue ablated. Group 3 included patients with less than 50 mm of tissue ablated. All eyes were imaged with a dual Scheimpflug analyzer preoperatively and 1 day, 1 week, and 1 and 3 months postoperatively. In addition, eyes in which the mean postoperative RSB was similar were divided into 2 subsets based on the amount of anterior tissue severed after the surgery (flap thickness C achieved ablation depth). The objective was to evaluate whether for a similar amount of RSB, well known as a main risk factor for post-lasik ectasia, changes in Submitted: November 21, Final revision submitted: January 30, Accepted: February 3, From the Refractive Surgery Department (Smadja, Santhiago, Mello, Dupps, Krueger), Cole Eye Institute, Cleveland Clinic Foundation, Cleveland, and the Department of Ophthalmology and Biomedical Engineering Center (Roberts), Ohio State University, Columbus, Ohio, USA; the Anterior Segment and Refractive Surgery Unit (Smadja), University Center Hospital of Bordeaux, Bordeaux, France. Corresponding author: David Smadja, MD, Refractive Surgery Unit, Ophthalmology Department, H^opital Pellegrin, Place Amelie Raba Leon, Bordeaux, France. davidsmadj@hotmail.fr. posterior corneal surface in response to surgery could be a function of another variable, such as the amount of anterior tissue severed. This variable takes into account how deep the anterior cornea is affected compared with its preoperative thickness and regardless of the amount of RSB. Surgical Technique The same experienced surgeon (R.R.K.) performed all LASIK procedures using a Wavelight Allegretto Wave Eye-Q 400 Hz excimer laser (Alcon Laboratories, Inc.) with an optical zone of 6.50 mm and a transition zone of 1.25 mm. The Wavelight FS200 femtosecond laser (Alcon Laboratories, Inc.) was used to create a 100 mm thickness corneal flap with a superior hinge. Postoperatively, moxifloxacin ophthalmic solution (Vigamox) and prednisolone acetate 1.0% ophthalmic solution were applied 4 times daily for 1 week. Dual Scheimpflug Imaging Measurements were performed with the Galilei dual Scheimpflug analyzer system (software version 5.2.1, Ziemer Ophthalmic Systems AG) according to the manufacturer's guidelines. This rotating Scheimpflug tomography based device combines dual-channel Scheimpflug cameras and a Placido disk and was designed to improve the accuracy of the anterior corneal curvature calculation. 15 Height data from the Scheimpflug images and slope data, converted into height data from the Placido disk, are merged to provide a surface fitted to the anterior corneal data. Posterior corneal surface data are measured using edge detection in images provided by the dual-scheimpflug system. In addition, the software has enhanced the posterior edge location and uses ray tracing with the Snell law of refraction through the anterior surface to locate and help in reliably reconstructing the posterior surface. The ray-tracing algorithm takes into account the 45- degree camera-viewing angle relative to the anterior surface and calculates the pathway through the cornea using the relative indices of refraction, for the cornea and for the aqueous. Only measurements that satisfied the minimum quality required by the system were included in this study. Analyzed Parameters and Description Posterior eccentricity and posterior corneal curvature along 3 optical zones (central [0.0 to 4.0 mm], paracentral [4.0 to 7.0 mm], and peripheral [7.0 to 10.0 mm]) were analyzed to study changes in the posterior corneal surface in response to the myopic ablation. In addition, correlations between the changes in the posterior corneal curvature (diopters [D]) and posterior corneal eccentricity with the achieved ablation depth, the amount of anterior tissue severed, the postoperative RSB, the preoperative central corneal thickness (CCT), and the IOP were tested. Posterior Corneal Curvature Keratometric (K) values (mean K, steep K, flat K) in the posterior surface were calculated with the indices of refraction of the cornea (1.376) and the aqueous humor (1.336). These values were derived from the axial curvature map for the central cornea (0.0 to 4.0 mm). In addition, values were recorded from the posterior instantaneous curvature map for the paracentral and peripheral cornea (4.0 to 10.0 mm). This decision was made to minimize bias in the measurement of the peripheral corneal curvature because axial curvature is highly dependent

3 1224 POSTERIOR CORNEAL RESPONSE TO MYOPIC LASIK Table 1. Preoperative patient data. Group 1 (n Z 18) Group 2 (n Z 38) Group 3 (n Z 24) P Value Parameter Mean G SD Range Mean G SD Range Mean G SD Range G1 Vs G2 G1 Vs G3 G2 Vs G3 Age , , , 59 d d d SE 8.00 G , G , G , 6.50 d d d Cylinder 0.85 G , G , G , 2.25 d d d ATS (mm) 125 G , G , G , 49 d d d % ATS 40.5 G , G , G , 27.7 d d d RSB (mm) 318 G , G , G , 462 d d d % RSB 57.0 G , G , G , 84.0 d d d CCT (mm) G , G , G , !.01* PCC (D) mm 6.27 G , G , G , mm 5.86 G , G , G , mm 5.24 G , G , G , ε G , G , G , ATS Z anterior tissue severed; CCT Z central corneal thickness; ε 2 Z eccentricity; PCC Z posterior corneal curvature; RSB Z residual stromal bed; SE Z spherical equivalent *Statistically significant on the position of the reference axis and could lead to underestimation in areas of relatively higher curvature. Eccentricity Eccentricity is 1 of 4 parameters by which the shape of a conic section can be described. The eccentricity (ε 2 ) of the posterior surface is calculated within a central diameter of 8.0 mm averaged over all meridians. A positive value refers to a prolate posterior surface, whereas a negative value refers to an oblate surface. Its relationship with the Q factor (asphericity) is such that ε 2 Z Q. Table 2. Changes in the posterior corneal surface over the first 3 months postoperatively. Group 1 (O100 mm) Group 2 (50 99 mm) Parameter D SD Range P Value D SD Range P Value PCC (D) mm 1d , * , wk , * , mo , , mo , , mm 1d , * , wk , , mo , , mo , , mm 1d , 1.69!.001* , 1.48!.001* 1wk , 1.35!.01* , 1.18!.01* 1mo , , 0.82!.01* 3mo , * , 1.03!.01* ε 2 1 d , * , wk , * , mo , , mo , , D Z magnitude of the change; ε 2 Z eccentricity; PCC Z posterior corneal curvature *Statistically significant

4 POSTERIOR CORNEAL RESPONSE TO MYOPIC LASIK 1225 Percentage of Anterior Tissue Severed and Residual Stromal Bed It has been shown that the flap created in LASIK does not play a significant role in the corneal tensile strength postoperatively. 16 Therefore, the anterior tissue severed is the amount of tissue with severed lamellae that does not contribute to the corneal tensile strength after surgery. This tissue includes the flap thickness and the maximum achieved ablation depth. The percentage of anterior tissue severed is calculated as follow: (flap thickness C maximum ablation depth) 100/preoperative CCT, with the maximum ablation depth determined using ultrasound pachymetry during surgery by subtracting the residual stromal thickness underneath the flap before and after ablation. The RSB was calculated based on ultrasound pachymetry readings of the central cornea immediately after the excimer laser ablation was completed. The Spearman coefficient was used to test correlations between the central posterior corneal curvature changes and the eccentricity with the achieved ablation depth, the percentage of anterior tissue severed, the RSB, the percentage of RSB over the total preoperative corneal thickness, the IOP, and the preoperative CCT 1 day and 3 months after surgery. Statistical Analysis Statistical analyses were performed using JMP software (version 8.0, SAS Institute, Inc.). Normality of data was evaluated with the Kolmogorov-Smirnov test. Differences between data were evaluated using the Wilcoxon test or analysis of variance, whereas correlation coefficients were established by Spearman rank correlations. Data were expressed as the mean G standard deviation (SD). The level of significance for each parameter was set at P value of less than RESULTS Eighty healthy eyes of 46 patients were prospectively enrolled in this study. Group 1 comprised 18 eyes of 11 patients; Group 2, 38 eyes of 21 patients; and Group 3, 24 eyes of 14 patients. Table 1 shows the preoperative patient data in each group. Preoperatively, there was no statistically significant difference in any parameter between the 3 groups except the CCT between Group 2 and Group 3 (significantly thicker in Group 2; PZ.008). Changes in Posterior Corneal Curvature and Eccentricity Table 2 shows the postoperative changes in each parameter over the first 3 months. When the entire population of eyes was included, no statistically significant steepening of the central posterior cornea (0.0 to 4.0 mm) was observed at any time point (PZ.14). However, when each group was analyzed separately, there was statistically significant greater steepening of the central posterior curvature and an increase in eccentricity (shift toward prolateness) in Group 1 after Table 2. (Cont.) Group 3 (!50 mm) Total D SD Range P Value D SD Range P Value PCC (D) mm , , , , , , , , mm , , 0.66!.01* , , , , , , mm , , 1.69!.01* , * (0.46, 1.35)!.01* , , 0.88!.01* , , 1.25!.01* ε , , * , , 1.84!.01* , , , ,

5 1226 POSTERIOR CORNEAL RESPONSE TO MYOPIC LASIK Table 3. Statistical significance of the difference between groups for each parameter over the first 3 months postoperatively. P Value PCC Change ( mm) PCC Change ( mm) PCC Change ( mm) Eccentricity Change Comparison 1 D 1 Wk 1 Mo 3 Mo 1 D 1 Wk 1 Mo 3 Mo 1 D 1 Wk 1 Mo 3 Mo 1 D 1 Wk 1 Mo 3 Mo Group 1 vs Group 2.02*.01*.2.92!.01* !.01*!.01* Group 1 vs Group 3!.01*!.01*!.01*.19!.01*!.01*.03*.01*!.01*!.01*.05!.01*.01*.02* Group 2 vs Group !.01*.23.16!.01*!.01*.01*.19!.01* PCC Z posterior corneal curvature *Statistically significant 1 day (PZ.03 and PZ.03, respectively) and after 1week(PZ.02 and PZ.005, respectively), but not after 1 month or 3 months. There were no statistically significant differences in any parameter postoperatively in Group 2 or Group 3. Table 3 shows the differences in the posterior corneal surface response to surgery. Statistically significant differences were found between the 3 groups. Figure 1 and Figure 2 show the progression of the change in the central posterior cornea (0 to 4 mm) and the eccentricity, respectively, 3 months postoperatively. The maximum magnitude of change occurred within the first week after surgery and started to decrease toward the preoperative level after the first month. Correlations with Other Parameters Table 4 shows the results of the multiple regression analysis. Figure 3 shows the relationship between the variable and the posterior central steepening after 1 day. After 3 months, none of the tested correlations was statistically significant. Comparison of Posterior Corneal Surface Changes in Corneas with Similar Residual Stromal Bed but Different Anterior Tissue Severed Twenty eyes with an RSB ranging from 315 mm to 358 mm were divided into 2 subgroups (each with 10 eyes of 5 patients) of homogeneous RSB (PZ.09) but different anterior tissue severed (P!.01). Table 5 shows the results. Although there was no statistically significant difference in the RSB between the 2 groups, there was statistically greater posterior central steepening (P!.01) and a greater shift toward prolateness (P!.01) after surgery in the group with the higher amount of anterior tissue severed. DISCUSSION Many studies report the sensitivity of posterior corneal surface changes as a key variable in discriminating subclinical keratoconus from normal corneas. Rao et al. 20 suggest that an increase in posterior elevation might be the earliest sign of subclinical keratoconus. However, this finding is derived from Placido disk based videokeratography (Orbscan II), whose accuracy of posterior corneal surface assessment, especially after refractive surgery, has been largely contested. 9,A Roberts et al. A evaluated the performance of the posterior edge extraction algorithm of the Orbscan topographer after LASIK and found that the posterior edge tracker consistently failed in post- LASIK corneas due to higher scatter conditions. This failure resulted in generating an artificially low posterior radius of curvature and thus a steeper posterior surface and higher central posterior elevation, which might be misinterpreted as ectasia. Although the Figure 1. Progression of the central posterior corneal curvature (PCC) change (D) in the different groups over the first 3 months.

6 POSTERIOR CORNEAL RESPONSE TO MYOPIC LASIK 1227 Figure 2. Progression of the posterior eccentricity (ε 2 ) change in the different groups over the first 3 months. reliability of the posterior corneal surface assessment with actual topographers remains controversial, Perez-Escudero et al. 13 recently used a hybrid porcine plastic eye model to validate the accuracy of the posterior corneal surface geometry measurement with a Scheimpflug imaging based device (Pentacam, Oculus, GmbH). Scheimpflug technology uses an algorithm to correct for optical distortions and compensate for the camera-viewing angle in the images by taking into account the Snell law of refraction at the anterior surface. The authors report that posterior corneal radius of curvature was minimally affected by the geometry of the anterior surface and could be reliably used to assess changes in the posterior corneal surface after LASIK. The Galilei system used in this study, although not yet validated for the accuracy of posterior corneal measurements, uses software to enhance posterior edge detection as described above and has shown high repeatability of the posterior corneal measurement in eyes after refractive surgery. 21 To our knowledge, this study is the first to assess early changes in the posterior corneal curvature and eccentricity in response to myopic LASIK using the Galilei system. Although elevation data can lead to a better and more intuitive understanding of the posterior corneal shape, there are many concerns about the correct alignment when comparing the changes in height against a best-fit sphere at each follow-up visit. Therefore, we decided to choose parameters, such as curvature, which are less dependent on alignment accuracy. We found that early changes in the posterior corneal surface after surgery are a function of time; steepening of the posterior cornea and a shift toward prolateness occur after the first day. These changes are maintained during the first week and start to decrease toward the preoperative level after 1 month. This progression over time was found in each group and over the entire surface of the posterior cornea independently of the maximum ablation depth. Our results are similar to those reported in recent studies using the Pentacam system. Perez-Escudero et al. 13 detected steepening of the posterior corneal surface and a shift toward prolateness after the first day that disappeared after 1 week. Zhang and Wang 12 also found time-dependent Table 4. Correlations between changes in central posterior curvature and eccentricity with different parameters. ATS AD RSB % RSB IOP Preop CCT Time r Value P Value r Value P Value r Value P Value r Value P Value r Value P Value r Value P Value 1 day D in central PCC 0.32!.01* 0.39!.01* 0.32!0.01* 0.28!.01* D in ε !.01* 0.38!.01* 0.34!0.01* * months D in central PCC D in ε D Z magnitude of the change; AD Z ablation depth achieved; ATS Z anterior tissue severed; CCT Z central corneal thickness; ε 2 Z eccentricity; IOP Z intraocular pressure; PCC Z posterior corneal curvature; r Z Spearman coefficient; RSB Z residual stromal bed *Statistically significant

7 1228 POSTERIOR CORNEAL RESPONSE TO MYOPIC LASIK Figure 3. Correlation between the magnitude of the central posterior corneal curvature (PCC) change after the first day. A: As a function of the percentage of anterior tissue severed. B: As a function of ablation depth. C: As a function of RSB. D: As a function of the percentage of RSB. changes in the posterior corneal surface by measuring the posterior elevation changes in response to myopic LASIK and interpreted that as forward protrusion 1 month after surgery that returned to the original level after 6 months. In a long-term followup study, Ciolino et al. 22 found that the posterior cornea in post-lasik myopic eyes was stable and found no found significant forward protrusion of the posterior surface after a mean of 14 months. In another study analyzing the response of the posterior surface to LASIK after 6 months in a large sample (2380 eyes), Grzybowski et al. 23 found similar results. They found a significant central increase in posterior elevation but a significant peripheral decrease. Therefore, the authors suggest that this increase in central posterior elevation would be more likely related to backward peripheral corneal swelling into the anterior chamber rather than to forward bulging of the central posterior cornea. This hypothesis might explain why some studies found paradoxical findings; that is, an increase in central posterior elevation, interpreted in most studies as a forward bulging of the posterior cornea, and a decrease in anterior chamber depth. 24 Regarding the clinical relevance of the posterior surface changes after surgery, we found that the maximum change in the central posterior cornea occurred in Group 1 after 1 day and reached D, corresponding to a decrease in radius of curvature of 96 mm. This change is too small to have a clinical impact on refraction. Although small, a change of D in the posterior surface is equivalent in proportion to a change of 1.00 D in the anterior surface because the refractive indices are different by a factor 9.4 (anterior Z 0.376; posterior Z 0.04). 25 Our results are higher than those reported in a recent study by Perez-Escudero et al., 13 which found a maximum decrease in the radius of posterior curvature of 40 mm after 1 day. The difference between our results can be explained by several factors. In our study, we divided patients based on the ablation depth and found a statistically significant difference in the magnitude of changes between groups, with a greater change in eyes with a greater ablation depth. Thus, it is likely that the globalization of the results without distinction between the patients' ablation depth as well as the small sample size (n Z 13) could have lowered the mean value of change. When we included the entire population of eyes in our study, the mean change in the central posterior cornea was 0.05 D (45 mm in radius of curvature) and thus appears to be similar to the 40 mm reported by Perez-Escudero et al. 13 Considering that a postoperative RSB of less than 250 mm is one of the most accepted risk factors for ectasia, Seitz et al. 5 measured the change in posterior corneal curvature after LASIK in eyes with less than 250 mm RSB. They found a mean change of 0.20 D, which was significantly greater than the change in eyes with an RSB higher than 250 mm (0.08 D) and almost twice what we observed (0.106 D). However, this cutoff value in eyes with a higher risk for developing ectasia because of a thin RSB has to be interpreted with caution because it was reported using the Orbscan system in post-refractive surgery eyes. The mechanism of this time-dependent change at an early stage after surgery is still unclear; however,

8 POSTERIOR CORNEAL RESPONSE TO MYOPIC LASIK 1229 Table 5. Comparison of the posterior corneal response to surgery between 2 groups with homogeneous RSB and different percentage of anterior tissue severed. Group 1 (n Z 10) Group 2 (n Z 10) Parameter Mean G SD Range Mean G SD Range P Value Mean preop CCT (mm) G , G , * Mean ATS G , G , 95!.01* Mean % ATS 39.0 G , G , 34.9!.01* Mean RSB 332 G , G , Mean % RSB 59.3 G , G , 67.7!.01* Mean D central PCC ( mm) 0.08 G , G , 0.06!.01* Mean D ε G , G , 0.17!.01* D Z change; ATS Z anterior tissue severed; CCT Z central corneal thickness; ε 2 Z eccentricity; PCC Z posterior corneal curvature; RSB Z residual stromal bed *Statistically significant it is likely related to the timing of the wound-healing process after LASIK. It has been shown that in the active wound-healing stage after surgery, a hypocellular primitive stromal scar occurs in the flap interface, resulting in a biomechanically weaker central and paracentral cornea with no evidence of stromal remodeling over time. In contrast, a hypercellular fibrotic stromal scar occurs at the wound margin with a gradual increase in tensile wound strength up to 3.5 years after LASIK. 26,27 These findings are consistent with the observed shift toward prolateness of the posterior cornea after surgery. With the central cornea being weaker than the periphery, greater deformation occurs in the center, leading to greater steepening of the central posterior cornea. Subsequently, the fibrotic wound-remodeling stage likely plays a role in the moderate biomechanical strength recovery of the cornea, as seen with the return toward the original level of the posterior corneal curvature after 3 months. An ultrastructural and histopathologic study 26 showed that newly created collagen lamellae in the stromal scar never reconnected end-to-end with old cut fibril ends. In addition, interweaving of collagen lamellae, which was considered the most important structural corneal characteristic for its tensile strength, is not regenerated after surgery; this might explain why when healed, corneal stromal wounds after LASIK are weaker than normal. In addition to this biomechanical theory to explain such time-dependent changes, it cannot be ruled out that higher scatter conditions in post-lasik corneas might lead to a relative higher failure of the posterior edge detection, as previously suggested by Roberts et al. A Netto et al. 28 found a relationship between the amount of achieved correction and the level of stromal surface irregularity. Along with this finding, it is likely that higher ablation depths result in a greater postoperative scatter condition in these corneas and thus greater artifacts are induced. However, it is unlikely that the higher ablation depth magnitude induced relative failure of the posterior edge tracker because the quality of the measurement and the position of the edge tracker were checked after every scan and accepted only when they met the required quality. This hypothesis should be further evaluated in a prospective study of the progression of posterior corneal changes after LASIK as a function of postoperative light scattering. In our study, we found positive correlations between the amount of anterior tissue severed and the steepening of the posterior central cornea as well as an increase in eccentricity. The entire surface of the posterior cornea postoperatively was significantly steeper and more prolate in the first week after surgery in Group 1 only; no significant changes in the central posterior corneal curvature occurred in Group 2 (PZ.5), Group 3 (PZ.56), or when all eyes were included (PZ.14). This finding is consistent with the theory of Dupps and Roberts, 11 who consider the maximum ablation depth and the number of disrupted lamellae as key variables in the biomechanical response to refractive surgery. Eyes in Group 1 had a greater ablation depth (O100 mm); more important, the mean percentage of anterior tissue severed was 40.5% (range 35.8% to 44.8%) compared with 31.4% (range 25.7% to 35.5%) in Group 2 and 24.2% (range 19.4% to 27.7%) in Group 3. In other words, the percentage of tissue that did not contribute to the tensile strength of the cornea after surgery was significantly higher in Group 1 than in Group 2 and Group 3 (both P!.001). According to the findings of Randleman et al, 29 the cohesive

9 1230 POSTERIOR CORNEAL RESPONSE TO MYOPIC LASIK tensile strength of the cornea is not uniform throughout the corneal stroma and is inversely correlated with stromal depth. They found that the anterior 40% of the corneal stroma was the strongest region of the cornea and had a significantly greater cohesive tensile strength than the posterior 60%, which was 50% weaker. The authors point out the importance of leaving the maximum amount of anterior stroma to preserve the postoperative integrity of the cornea and improve the safety of the procedure. In LASIK, by combining flap creation and stromal ablation, it is not rare to extend into the posterior 60% of the cornea. To further support the value of anterior tissue severed as a key variable of the biomechanical response to surgery, we analyzed a small subset of eyes with an RSB of 315 to 358 mm (PZ.09); a different amount of anterior tissue severed was the only variable (P!.01). We found significantly greater central posterior steepening (P!.01) in the first group, with a mean anterior tissue severed of 39%, than in the second group, with a mean of 31.3%. Although our sample was small in this evaluation, this finding might have a direct clinical implication when considering this index in the preoperative surgical plan. This variable might improve the safety of the procedure by allowing the surgeon to anticipate the risk for losing tectonic integrity of the cornea, which can lead to postoperative corneal ectasia. The most widely accepted risk factors for post-lasik ectasia include an RSB less than 250 mm, a preoperative CCT less than 500 mm, age younger than 30 years, a suspect topography map, and high myopia (O8.00 D). In addition, we propose further study of the role of anterior tissue severed as another risk factor for ectasia. Longer follow-up of patients may help determine whether the amount of anterior tissue severed has an influence in the delay in posterior shape recovery after surgery. Although the Galilei system is not yet validated for the accuracy of the posterior corneal surface assessment, our results concerning the magnitude and progression of curvature changes after LASIK were consistent with those in previously published studies using the Pentacam system. Some drawbacks of this study include the small sample of patients analyzed and the absence of statistical validation of the anterior tissue severed index as a risk factor for ectasia. Ideally, a retrospective analysis of a large number of post- LASIK ectasia cases would be performed to test the relevance of this index. Along with our findings, further experimental studies will help us better understand the biomechanical response of the cornea after myopic LASIK and thus further improve the safety of the procedure. WHAT WAS KNOWN A forward shift of the posterior cornea after myopic LASIK has been frequently reported with scanning-slit topography and Scheimpflug imaging technology. WHAT THIS PAPER ADDS Posterior steepening and a shift toward prolateness of the posterior surface after myopic LASIK were also found with a new imaging technology comprising a dual Scheimpflug analyzer combined with Placido-disk technology. This change was time dependent, with a return toward the preoperative level after 3 months. The magnitude of the change in the posterior surface was correlated with the magnitude of the myopic ablation and to the percentage of anterior tissue severed. REFERENCES 1. Randleman JB, Russell B, Ward MA, Thompson KP, Stulting RD. Risk factors and prognosis for corneal ectasia after LASIK. Ophthalmology 2003; 110: Pallikaris IG, Kymionis GD, Astyrakakis NI. Corneal ectasia induced by laser in situ keratomileusis. J Cataract Refract Surg 2001; 27: Randleman JB. 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