Laser-assisted subepithelial keratectomy (LASEK) has become a popular technique in refractive surgery because of the absence of the stromal flap–related complications associated with laser in situ keratomileusis (LASIK) and because it allows treatment of thin corneas while achieving good safety, efficacy, and predictability outcomes.1–6 Despite the disadvantages of LASEK over LASIK (eg, slower visual recovery and higher postoperative discomfort), surface ablation has become the technique of choice in patients with thin corneal pachymetry, those at risk for trauma, and those with corneal surface problems such as dry eye, recurrent erosion syndrome, or epithelial basement membrane disease.7,8
Even though excimer laser corneal refractive surgery (LASIK and surface ablation) has become highly precise, the predictability of the technique remains an issue, especially in eyes with high degrees of ametropia (either myopia9,10 or hyperopia11,12). With the aim of improving the nomograms used with different excimer lasers to increase the predictability of the procedures,13 several studies have focused on the possible preoperative14,15 and environmental16 factors that could influence the final outcomes. The predictability and efficacy of LASIK and surface ablation for myopia have been associated with age,15,17,18 the optical zone,19,20 and the degree of ametropia.14,15,18 Regarding the influence of preoperative keratometry on postoperative results, some authors14,21 report a possible trend toward undercorrection when using LASIK on flatter corneas to treat high myopia. We found only one paper22 on the effect of preoperative corneal curvature on photorefractive keratectomy (PRK) for myopia; the authors did not find a relationship between preoperative corneal curvature and postoperative predictability. To our knowledge, no study in the literature focuses on this relationship in LASEK.
Because a relationship between preoperative keratometry and postoperative refraction could have an important influence on excimer laser nomograms, we decided to evaluate whether there was such a relationship based on our LASEK results.
PATIENTS AND METHODS
This retrospective study comprised 1149 consecutive eyes that had LASEK to correct myopia with or without astigmatism. When evaluated for surgery, patients with 1 or more of the following were excluded: unstable refraction, previous ocular surgery (refractive or other surgical procedures), suspicion of keratoconus, ocular disease, or systemic disease that could alter the wound-healing process such as diabetes and connective tissue disorders.
The preoperative keratometry and 3-month postoperative residual refraction (before enhancement) were recorded. The relationship between the mean preoperative keratometric reading and the postoperative spherical equivalent (SE) was analyzed using linear regression. We also identified two groups: the first decile (percentile 10) with the flattest keratometry readings and the decile with the steepest keratometry readings; the regression analysis was repeated in each subgroup. The 2 quartiles with the lowest and the highest preoperative refractive error were also identified, and the analysis was repeated in each quartile.
All patients had a full preoperative ophthalmologic examination including measurements of uncorrected visual acuity (UCVA) and best spectacle-corrected visual acuity (BSCVA) using the Snellen chart (Autochart Projector CP 670, Nidek Co. Ltd.), including manifest and cycloplegic refractions; slitlamp biomicroscopy; tonometry (CT-80, Topcon); corneal pachymetry (DGH 5100 contact pachymeter, DGH Technology, Inc.); keratometry and corneal topography (Costruzione Strumenti Oftalmici); mesopic pupil measurement (Colvard pupillometer, Oasis); and fundoscopy. Patients wearing soft contact lenses were instructed to stop their use 2 weeks before the preoperative examination and patients wearing hard contact lenses, 1 month before the examination.
The procedure was performed using topical anesthesia (lidocaine 2%). A povidone–iodine solution was applied to the eye, and a sterile surgical drape and a rigid eyelid speculum were positioned.
A 7.0 mm corneal semisharp marker (ASICO) was placed on the cornea and centered on the pupil. A 20% alcohol solution diluted in balanced salt solution (BSS) was instilled inside the trephine and left for 40 seconds. The alcohol was removed with a cellulose sponge, and the BSS was copiously instilled to rinse the ocular surface. The edges of the flap were dried with a sponge, and the epithelial flap was peeled back with a crescent blade (Alcon Surgical), leaving a hinge at the 12 o'clock position. After the stromal bed was dried with a sponge, the eye tracker was set in the center of the pupil, and the ablation was performed with the Technolas 217C excimer laser (Bausch & Lomb Surgical) using a PRK nomogram, programming the full spherocylindrical correction. The optical zone was determined by the mesopic pupil measurement. When the ablation depth was greater than 50 μm, a 7.0 mm round cellulose sponge soaked in mitomycin-C (MMC) 0.02% was applied over the ablated stroma for 30 seconds, with care taken to avoid leakage of the MMC onto the epithelial flap and limbus. When the ablation depth was 50 μm or less, no MMC was applied. The stroma was then copiously rinsed with BSS, after which the epithelial flap was repositioned using the same cannula. A therapeutic soft contact lens (Acuvue, Johnson & Johnson Vision Care, Inc.) was carefully placed on the eye, and ciprofloxacin 3 mg/mL and ketorolac trometamol 5 mg/mL were applied.
The postoperative medications consisted of ciprofloxacin 3 mg/mL and dexamethasone alcohol 1 mg/mL drops 4 times daily during the first postoperative week. The dexamethasone alcohol drops were tapered over the subsequent 2 months as follows: 3 times daily the first month, twice daily for the following 15 days, once daily for another 15 days, and then stopped. The therapeutic contact lens was removed 1 week after surgery.
Examinations were scheduled 1 day, 1 week, and 1 and 3 months postoperatively. At each examination, 2 optometrists recorded the UCVA. At the last examination, they performed refraction and recorded the UCVA and the BSCVA.
StatView Graphics software (Abacus Concept, Inc.) was used for data analysis. Linear regression analysis and the unpaired 2-tailed Student t test were performed. A P value of 0.05 or less was considered statistically significant. Data are expressed as the mean ± standard deviation.
Preoperatively, the mean spherical refraction was −3.90 diopters (D) ± 2.80 (SD) (range 0.00 to −12.00 D), the mean cylinder was −1.20 ± 1.30 D (range 0.00 to −6.50 D), the mean SE was −4.59 ± 2.80 D (range −0.25 to −13.00 D), and the mean keratometry was 44.2 ± 1.6 D (range 39.00 to 49.00 D). The mean preoperative pachymetry was 508 ± 31 μm (range 428 to 685 μm).
Three months postoperatively, the mean spherical refraction was +0.20 ± 0.50 D and the mean SE was +0.05 ± 0.50 D. Linear regression analysis showed a statistically significant correlation between the mean preoperative keratometry and the residual SE; however, the correlation was weak (y = 0.019, x = −0.769, r2 = 0.003, P = .04). Figure 1 shows a scatterplot of the correlation analysis.
Table 1 shows the data in the 2 deciles and the regression analysis in each decile. Linear regression analysis in the decile with the flattest corneas found a very weak correlation between preoperative corneal curvature and residual SE, whereas it showed a stronger positive correlation in the decile with the steepest corneas. In the latter subgroup, the steeper the cornea, the more overcorrected the refractive result (ie, correction of myopia in steep corneas resulted in more flattening than that predicted by the laser nomogram).
Correlation between the preoperative spherical refraction and the postoperative residual spherical refraction was significant (P = .001) (ie, the higher the preoperative refraction, the more tendency to overcorrection); however, it was weak (r2 = 0.04) and therefore not clinically relevant. Nevertheless, to avoid a possible confounding factor, the regression analysis was repeated in 2 quartiles (lowest preoperative myopia and highest preoperative myopia). Table 2 shows the results. Linear regression analysis in the quartile with the lowest preoperative SE did not find a significant correlation between preoperative corneal curvature and residual SE. The regression analysis in the quartile with the highest preoperative refraction found a statistically significant correlation between preoperative keratometry and residual SE; however, the correlation was weak. In fact, the difference in postoperative refraction between the 2 quartiles was not significant (ie, corneas with a higher preoperative myopia were more influenced by the preoperative corneal curvature, but that influence was not clinically relevant); however, the influence was not clinically relevant. To exclude a possible correlation between preoperative myopia and keratometry, a regression analysis between preoperative mean keratometric value and preoperative SE was performed and showed no statistically significant correlation (r2 = 0.01, P = .3).
In the current study, we found a weak positive correlation between preoperative keratometry and postoperative SE, mostly in the subgroup with steeper corneas and when the preoperative refractive error was higher.
To our knowledge, no other studies have focused on the influence of preoperative keratometry on refractive results after LASEK. Only one study, by Blaker and Hersh,22 analyzed the results in the phase III U.S. Food and Drug Administration clinical trials of PRK and found no relationship between preoperative corneal curvature and predictability after PRK.
A few studies have evaluated the influence of preoperative keratometry on LASIK results. Rao et al.21 found a trend toward undercorrection in flatter corneas, although the only statistically significant difference was in the subgroup with preoperative myopia from −10.00 to −12.00 D. The linear regression between preoperative keratometry and postoperative refraction that included all eyes showed a weak correlation (r2 = 0.03). Pérez-Santonja et al.14 also found a tendency toward undercorrection in flatter corneas treated with LASIK to correct high myopia (≥−8.00 D). Nevertheless, other studies found no relationship between preoperative corneal curvature and postoperative cylinder23 or postoperative need for enhancement after myopic ablation.18 Regarding hyperopic LASIK, we found few studies with contradictory results. Cobo-Soriano et al.12 obtained lower predictability in flatter corneas with a high hyperopic ablation. They found no difference in the low and moderate hyperopic groups. Ditzen et al.24 found a tendency toward undercorrection in high hyperopes (>+6.00 D) with flatter corneas (keratometry <42.00 D). In contrast, Esquenazi and Mendoza25 obtained undercorrection in steeper corneas (keratometry >45.00 D) and high hyperopia.
Although no study has found a definite correlation between preoperative corneal curvature and refractive outcomes, it seems that the influence of preoperative keratometry is greater when higher refractive corrections are intended.12,14,21,24,25 We also found a correlation between the degree of preoperative myopia and the amount of overcorrection, although the correlation was very weak. When eyes were separated by preoperative refraction, we found that the effect of corneal curvature was stronger in the group with a higher preoperative myopia but that the correlation was very weak (r2 = 0.05) and therefore not clinically relevant.
That the regression analysis including all eyes in our study showed a statistically significant correlation means that preoperative keratometry has an effect on refractive outcomes. Nevertheless, because of the high number of eyes in this study, the analysis found a statistically significant correlation, even though the correlation was very weak (r2 = 0.003). However, when the steepest corneas were analyzed separately, the correlation was slightly stronger, meaning that steeper corneas, when treated to correct myopia, show a trend toward overcorrection (ie, the cornea flattened more than what was expected by the excimer laser nomogram). We did not find a tendency toward undercorrection in flatter corneas, as some studies of LASIK have.14,21 Rao et al.21 suggest that because the change in corneal curvature is responsible for correcting myopia, more ablation might be required in a flatter cornea than a steeper cornea to produce a similar amount of effective change. As others also suggest,26 the laser beam incises less perpendicularly to the surface in the midperiphery of steep corneas than of flatter corneas, which may cause a loss of ablative efficiency away from the corneal apex in steep corneas. This would result in a deeper central ablation and a shallower paracentral ablation and thus lead to a slightly greater myopic correction in steep corneas than in flat corneas. Although we do not find, as Rao et al.21 suggest, that flatter corneas are more difficult to flatten, we do find that steeper corneas are slightly easier to flatten, which would agree with their suggestion. Maybe differences in the excimer laser or nomogram used is why Rao et al. found undercorrection in flatter corneas and we found overcorrection in steeper corneas.
Nevertheless, the differences between the results of LASEK in the current study and of PRK by Blaker and Hersh22 and the results in studies of LASIK may be due to the fact that creating a stromal flap could be a confounding factor in LASIK. Cutting the peripheral stroma produces by itself a flattening of the central cornea due to interlamellar forces.27,28 Different depths of the stromal cut have different effects on the resulting central flattening and therefore may affect the refractive outcome differently.27,28 In LASIK, the microkeratome produces a meniscus-shaped flap that is thinner in the center and thicker in the periphery.29,30 The preoperative corneal curvature seems to affect the profile of the stromal flap. In flatter corneas, the flap tends to be smaller in diameter,31,32 is sometimes thicker (depending on the microkeratome),33,34 and carries a higher risk for free caps.29,32,35 In steeper corneas, the flap tends to be larger in diameter31,32 and thinner32,35 and seems to increase the risk for buttonholes.35,36 This different profile of the stromal cut based on preoperative corneal curvature could affect the refractive outcomes of the surgery; therefore, it could be a confounding factor. On the contrary, surface laser ablation alters only the corneal curvature, thus avoiding this potential confounding factor.
On the other hand, the cornea also responds with central flattening when surface ablation is performed, even if no refractive ablation profile is programmed, as in phototherapeutic keratectomy. This causes a hyperopic shift. This central flattening is secondary to the thickening of the peripheral cornea that follows the interruption in the superficial lamellae. The deeper the ablation, the greater the hyperopic shift, provided the ablation is not deep enough to cause corneal ectasia.27,28 When surface ablation is performed to correct myopia, the hyperopic shift increases the effect of the laser profile. This could explain, in part, the slight tendency toward overcorrection found in eyes with high myopia in our study. In addition, our results suggest that with deep ablations, the flattening of the central cornea is more pronounced in steep corneas than in flat or standard corneas.
We used intraoperative MMC 0.02% in those in which the ablation depth exceeded 50 μm. This could have altered the refractive results and introduced a bias in our study if the 2 groups had a different number of eyes treated with MMC. As the 2 groups (flattest and steepest corneas) had the same percentage of eyes receiving MMC, we believe the use of MMC did not influence our results. In fact, when the quartile with the highest preoperative myopia (all eyes treated with MMC) and the quartile with the lowest preoperative myopia (no eye treated with MMC) were compared, no significant differences in the refractive results were found.
According to our results, preoperative corneal curvature does not seem to influence the refractive outcomes after LASEK. However, when treating higher preoperative myopia in steeper corneas, there is a weak tendency toward overcorrection. In conclusion, our study suggests that the ability to flatten the cornea using LASEK to correct myopia does not depend on the preoperative corneal curvature.
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