To evaluate the effect of preoperative keratometry on visual and refractive outcomes after Myopic LASER in Situ Keratomileusis (LASIK) in eyes with preoperative spherical equivalent (SE) of -6.00D or less. Material and Methods: A retrospective study enrolling clinical records of 482 eyes of 275 patients with myopia who underwent LASIK between 2009 and 2016. Subjects were grouped according to the degree of preoperative mean keratometry (Km), into three groups: Group 1 (Flat Cornea): Km ≤ 42.00 diopters (D); Group 2: 42.00 < Km > 46.00D; Group 3 (Steep Cornea): Km ≥ 46.00D. To evaluate the prognostic impact of keratometry in Myopic LASIK, we considered the results measured at 6 months postoperatively, including uncorrected distance visual acuity (UDVA), postoperative sphere, cylinder, SE and its variation. Results: The mean preoperative SE was -3.91 ± 1.54D, ranging between -0.88 and -6.00D. The percentage of eyes achieving a postoperative SE of ±0.50D was 39.5%, 31.8% and 26% in groups 1, 2 and 3 respectively. Moreover, in group 3, 14.3% of the eyes had a residual SE of -2.00D or greater, contrasting with the groups 1 and 2 with only 6% - 7%. These results were found to be statistically significant. Concerning UDVA, eyes achieving 20/25 or more were 81.5%, 81.8% and 71.5%, and 20/50 or less were 6.7%, 6.2% and 11.7% in groups 1, 2 and 3 respectively. Conclusions: Myopic eyes with steeper corneas seem to have greater tendency to undercorrection, also presenting worse visual outcomes.
Laser in situ keratomileuis (LASIK) is, nowadays, the most commonly performed keratorefractive surgery, with well-established surgical indications, with numerous studies validating its long-term efficacy and safety [
LASIK is based on the modification of corneal curvature, inducing biomechanical changes with the purpose of altering refractive power [
The purpose of our study is to evaluate the effect of preoperative mean keratometry on refractive and visual outcomes of LASIK in myopic eyes with preoperative spherical equivalent (SE) of −6.00 or less.
The clinical records of patients who underwent LASIK between 2009 and 2016 in Instituto de Oftalmologia Dr. Gama Pinto (IOGP) were retrospectively analyzed. As inclusion criteria, we considered myopic eyes with a preoperative spherical equivalent of −6.00 D or less, without prior history of ocular procedures or other known ocular pathology, including keratoconus or high-order aberrations, with a minimum follow-up period of 6 months postoperatively. Due to the retrospective nature of this study, the sample size was limited to the number of subjects respecting inclusion criteria.
LASIK was performed with the Lasersight Laserscan Lsx Excimer Laser System (Lasersight Technologies, Inc), using standard protocol, by four different refractive surgeons at IOGP. The preoperative keratometry and pachymetry were considered to plan the surgery. The minimum residual stromal bed was planned to be greater than 250 µm to avoid excessive corneal thinning and possible post-LASIK ectasia. Emmetropia was the final goal in all cases. The superior lamellar flaps were created with the microkeratome MORIA One Use-Plus SBK, creating a 6.5 mm optical zone with 8 mm of blend zone, centered with the center of the pupil. Following ablation, the flap was replaced. Postoperative evaluation and measurements included in this study took place at 6 months post-LASIK.
Subjects were grouped according to the degree of preoperative mean keratometry (Km), into three groups [
The statistical analysis was performed with IBM® SPSS® Statistics version 23 software. Descriptive statistics were performed, including means, standard deviation (SD), minimum (min) and maximum (max) and percentage of qualitative variables. The Kruskal-Wallis H test was performed, a non-parametric test for independent variables, with a Mann-Whitney U post-hoc test, to compare the keratometry groups (independent factor), with the corresponding SE and UCVA as dependent factors. The results were considered statistically significant if p < 0.05. The Spearman’s correlation test was run to analyze correlation between variables.
A total of 482 eyes of 275 patients were enrolled in this study, having met all inclusion criteria. The demographic data and preoperative characteristics are summarized in
In our study, the mean age was of 32 ± 6.3 years old, ranging from 20 to 53 years old. The average of SE was −3.90 ± 1.54D, ranging from −6.00 to −0.88D. The mean pachymetry was 549.5 ± 27.9 µm and mean Km was 43.64 ± 1.40D, ranging from 40.38 to 48.13D.
The preoperative characteristics of each Km group are organized in
The mean ablation depth in group 1 was 57.60 ± 19.55 µm, contrasting with 66.35 ± 19.0 µm in group 3 which was considerably higher. This difference might be due to the slightly lower preoperative sphere and SE values in group 1, comparing to the other groups.
A significant statistical moderate correlation was found between the variation of preoperative and postoperative Km (∆Km) and the variation of SE (∆SE) after LASIK (R = 0.51 p < 0.01).
Global Demographic Characteristics | ||
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M:F | 159 : 323 | |
Age | 32.9 ± 6.3 | (20 to 53) |
Spherical Equivalent (D) | −3.90 ± 1.54 | (−6.00 to −0.88) |
Sphere (D) | −3.30 ± 1.66 | (−6.00 to 0.00) |
Cylinder (D) | −1.20 ± 1.13 | (−5.50 to 0.00) |
K1 (D) | 42.97 ± 1.47 | (39.50 to 47.50) |
K2 (D) | 44.34 ± 1.42 | (40.50 to 49.00) |
Km (D) | 43.64 ± 1.40 | (40.38 to 48.13) |
Pachymetry (µm) | 549.5 ± 27.9 | (480 to 633) |
Ablation Depth (µm) | 63.16 ± 19.6 | (23 to 113) |
Group 1 - Flat Cornea Km ≤ 42.00D (N: 116) | Group 2 42.00D < Km > 46.00D (N: 283) | Group 3 - Steep Cornea Km ≥ 46.00D (N: 77) | ||||
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Parameter | Mean ± SD | Min to max | Mean ± SD | Min to max | Mean ± SD | Min to max |
Spherical Equivalent (D) | −3.47 ± 1.54 | (−6.00 to −1.00) | −4.06 ± 1.55 | (−6.00 to −1.00) | −3.99 ± 1.38 | (−6.00 to −1.75) |
Sphere (D) | −2.71 ± 1.67 | (−6.00 to 0.00) | −3.53 ± 1.60 | (−6.00 to 0.00) | −3.33 ± 1.61 | (−6.00 to −0.50) |
Cylinder (D) | −1.51 ± 1.29 | (−5.00 to 0.00) | −1.06 ± 1.01 | (−5.50 to 0.00) | −1.25 ± 1.21 | (−4.50 to 0.00) |
K1 (D) | 40.15 ± 0.75 | (39.50 to 42.00) | 43.20 ± 0.84 | (40.75 to 45.00) | 44.92 ± 0.99 | (42.50 to 47.50) |
K2 (D) | 42.67 ± 0.77 | (40.50 to 45.00) | 44.45 ± 0.76 | (42.75 to 46.50) | 47.91 ± 0.71 | (46.00 to 49.00) |
Mean K (D) | 40.80 ± 0.47 | (40.38 to 42.00) | 43.83 ± 0.69 | (42.25 to 45.75) | 46.79 ± 0.63 | (46.00 to 48.13) |
Pachymetry (µm) | 552.43 ± 32.27 | (480 to 633) | 547.29 ± 26.22 | (492 to 622) | 553.5 ± 26.5 | (505 to 624) |
Ablation Depth (µm) | 57.60 ± 19.55 | (23 to 112) | 64.61 ± 19.48 | (23 to 113) | 66.35 ± 19.0 | (23 to 101) |
The refractive and keratometric outcomes at 6 months postoperatively are summarized in
However, analyzing the distribution of refractive results in each Km group (
Group 1 Flat Cornea Km ≤ 42.00D | Group 2 42.00D < Km > 46.00D | Group 3 Steep Cornea Km ≥ 46.00D | |
---|---|---|---|
SE postop(D) | −0.80 ± 0.85 | −0.95 ± 0.85 | −1.01 ± 1.00 |
Sphere postop(D) | −0.46 ± 0.92 | −0.64 ± 0.87 | −0.66 ± 0.99 |
Cylinder postop (D) | −0.69 ± 0.60 | −0.63 ± 0.55 | −0.71 ± 0.47 |
Mean K postop (D) | 39.99 ± 1.25 | 41.40 ± 1.54 | 43.16 ± 1.46 |
∆Km (D) | 1.83 ± 1.20 | 2.43 ± 1.37 | 2.63 ± 1.34 |
∆SE (D) | 2.66 ± 1.43 | 3.10 ± 1.50 | 2.98 ± 1.39 |
only 6.7% and 7.7% of eyes respectively, were left with this degree of undercorrection. The percentage of eyes achieving a postoperative SE between −0.50 and +0.50D, therefore achieving surgical success, was of 39.5%, 31.8% and 26% in groups 1, 2 and 3 respectively.
Considering visual outcomes, steep corneas also seemed to have worse UDVA comparing to eyes in the other groups.
The influence of keratometry on refractive outcomes of myopic LASIK has already been debated in various studies, although with contradictory results. Rao et al. [
reported tendency for undercorrection in flat corneas in myopic eyes with preoperative SE ranging from −6.00 to −12.00D. Contrasting with these studies, Christiansen et al. [
Our results seem to be concordant with Christiansen et al. [
In our study, all three groups were homogeneous concerning preoperative SE and pachymetry. The mean ablation depth performed was greater in steeper corneas comparing to the other groups, showing greater variation of Km after LASIK, with an apparent greater surgical effect. However, although steep corneas suffered a greater corneal applanation, this was not reflected on SE variation in the same magnitude. Despite the greater ablation depth and surgical effect in steeper corneas, these showed a greater tendency for undercorrection concerning SE and worse visual outcomes comparing to other degrees of preoperative keratometry. Christiansen et al. proposes more aggressive laser ablation in eyes with Km ≥ 46.00D.
One of our study’s limitations is its retrospective nature, with a short follow-up period especially due to disparity in clinical records. It would be interesting to project a prospective longitudinal study, with a larger population, dividing the sample per myopia grades, so as to categorize the influence of keratometry in different grades of preoperative myopia. This would certainly be helpful in clinical practice, to optimize the procedure and maximize the success of this refractive surgery.
In conclusion, myopic eyes with steeper corneas seem to have greater tendency to undercorrection, also presenting worse visual outcomes.
Frazão, S., Rodrigues, P., Alves, S., Carvalho, A.M., Sêco, J.M. and Nogueira, H. (2018) The Influence of Keratometry on Visual and Refractive Outcomes after Myopic LASER in Situ Keratomileusis. Open Journal of Ophthalmology, 8, 84-90. https://doi.org/10.4236/ojoph.2018.82012