|Year : 2019 | Volume
| Issue : 1 | Page : 25-29
Visual and keratometric outcomes in eyes undergoing small incision lenticule extraction for compound myopic astigmatism
Ashraf H Soliman
Lecturer of Ophthalmology, Faculty of Medicine, Ain Shams University, Egypt
|Date of Submission||19-Dec-2018|
|Date of Acceptance||24-Feb-2019|
|Date of Web Publication||26-Apr-2019|
Ashraf H Soliman
1 Mostafa El Nahass St., 4th floor, Nasr City, Cairo 11566
Source of Support: None, Conflict of Interest: None
Context The use of small incision lenticule extraction (SMILE) for compound myopic astigmatism has been subject to experimentation in the recent years. Furthermore, examination of corneal alterations has revealed conflicting conclusions, which could be attributable to demographic variation.
Aims The aim of this study was to measure visual and keratometric efficacy of SMILE and correlate spherical equivalent (SE) decrements to keratometric alterations.
Settings and design This was a prospective nonrandomized study. Thirty eyes of 15 patients were included in the study. Preoperative assessment included oculus Pentacam analysis. Patients were followed-up for up to 3 months, after which tomography was repeated.
Statistical analysis used Descriptive statistics, t test, and linear regression analysis were carried out.
Results There was a statistically significant improvement in postoperative uncorrected distance visual acuity and SE and a statistically significant change in Q value (P<0.001). The mean residual SE postoperatively was −0.14 D. A highly statistically significant negative correlation existed between SE difference and mean K difference with r=−0.980 and P value less than 0.001, and a highly statistically significant positive correlation existed between SE difference and Q value difference, with r=0.787 and P value less than 0.001.
Conclusion SMILE is a safe and effective technique to correct cases with myopic astigmatism with an SE of up to −10 D, and a predictable positive correlation could be established between decrements in SE and decrements in anterior corneal steepening.
Keywords: keratometry, Pentacam, refractive correction, small incision lenticule extraction
|How to cite this article:|
Soliman AH. Visual and keratometric outcomes in eyes undergoing small incision lenticule extraction for compound myopic astigmatism. J Egypt Ophthalmol Soc 2019;112:25-9
|How to cite this URL:|
Soliman AH. Visual and keratometric outcomes in eyes undergoing small incision lenticule extraction for compound myopic astigmatism. J Egypt Ophthalmol Soc [serial online] 2019 [cited 2019 May 23];112:25-9. Available from: http://www.jeos.eg.net/text.asp?2019/112/1/25/257217
| Introduction|| |
Small incision lenticule extraction (SMILE) is a relatively new surgical method for refractive correction of myopia and compound myopic astigmatism. It involves flapless extraction of a femtosecond-laser-created stromal lenticule through a peripheral incision. Overall efficacy and safety are comparable to laser-in-situ keratomileusis (LASIK), but reported advantages over the preceding refractive corneal techniques include superior biomechanical strength, more stability of the ocular surface with less dry eye symptoms, better contrast sensitivity, and less surgically induced aberrations ,.
The US Food and Drug Administration approved the use of SMILE for the correction of myopia ranging between −1 and −8 D, with astigmatism not exceeding −0.5 D in patients aged 22 and older . Later studies have demonstrated the technique’s success, predictability, and safety in correcting high degrees of astigmatism (up to −10 D and even more). The Food and Drug Administration is thus currently reviewing the results of SMILE when it comes to myopic astigmatism, and approval is expected in the first half of 2019. The Conformité Européene has already approved the procedure, but only for milder degrees of astigmatism. Yet, the existing literature is compelling enough for many surgeons to adopt SMILE for both myopia and compound myopic astigmatism with high cylindrical errors ,.
Contrary to excimer laser-assisted techniques, which are associated with corneal weakening and affect posterior corneal surface, SMILE leaves a nearly intact anterior lamellae and Bowman layer. This would be expected to result in different postoperative modelling patterns of anterior and posterior corneal surfaces. Previous reports on this subject have not been conclusive, with some studies reporting remodelling of the anterior corneal surface with a nearly unchanged posterior corneal surface, and others reporting an added significant steepening of the posterior corneal surface in high error groups. The differences resulting from population stratification may be responsible for such discrepancy ,,.
Consequently, in this work, our aim was to assess visual outcome, corneal asphericity, and relationship between spherical equivalent (SE) decrements and keratometric alterations in eyes undergoing SMILE for compound myopic astigmatism.
| Patients and methods|| |
This prospective study involved 30 eyes of 15 patients who underwent SMILE over a period of 8 months. The number was equally split between left and right to avoid any of the potential laterality discrepancies described in the literature . All surgeries were conducted by the same surgeon (A.H.S) at Almashreq Eye Centre, a private eye centre in Cairo, Egypt. The study abided by the declaration of Helsinki, and an informed consent was obtained from all patients to include their clinical nonidentifying information in our study.
We included patients aged 18 years or more with compound myopic astigmatism, and with manifest refraction SE of less than or equal to −10 D, stable refraction for at least 1 year, and a target of emmetropia. Exclusion criteria included unstable refractive error, ectatic corneal disorders, ocular surface abnormalities, pregnancy and lactation, one-eyed patients, and an active inflammatory eye condition.
All patients underwent routine preparatory clinical ophthalmic examination, including uncorrected distance visual acuity (UDVA) and corrected distance visual acuity recorded using the decimal point system, slit-lamp biomicroscopy, dilated fundus examination, and intraocular pressure assessment using Goldmann aplanation tonometry. A preoperative corneal tomography map was obtained for each eye using the Pentacam (Oculus GmbH, Wetzlar, Germany), including K readings and Q value.
The 500 kHz femtosecond laser (software version 3.0; Carl Zeiss Meditech AG, Jena, Germany) was used to create an intrastromal refractive lenticule after application of topical anaesthesia (benoxinate hydrochloride eye drops 0.4%; EPICO, 10th of Ramadan City, Egypt). Manufacturer’s protocol steps described before  were adhered to. The energy was set to 150 nJ, and the cap thickness was 130 mm with a diameter of ∼7.4 mm and a depth of 120 μm. Lenticule diameter ranged from 6.4 to 6.8 mm depending on the degree of astigmatism. A 90° single side cut with a circumferential length of 2 mm was made. A blunt spatula was then inserted for dissection, followed by the removal of the lenticule through the side cut using forceps. No intraoperative complications occurred in any of the patients.
Postoperatively, the patients were prescribed lubricant eye drops to be used once every hour for the first 3 days postoperatively, followed by gradual decrements in dosing schedule. The patients were also prescribed antibiotic eye drops (Ofloxacin 0.3%; Allergan Pharmaceuticals Ltd, Dublin, Ireland) to be used four times daily for 5 days postoperatively. In addition, presdnisolone acetate 1% eye drops were also prescribed five times per day for 1 week for all patients. Follow-up visits were at day 1, day 5, and month 3; the latter was considered the study’s endpoint. The patients were assessed for any visual or ocular complaints, and the UDVA and corrected distance visual acuity were recorded, together with a slit-lamp examination. Postoperative corneal tomography was obtained at month 3 and was used as means for the comparative primary outcome assessment.
Statistical analysis was carried out using SPSS version 25 (SPSS ver. 25, IBM Corp., Armonk, NY, USA). Where appropriate, the t test was used for comparative analysis of statistical significance. A value of 0.05 or less was considered the cut off for statistical significance. Formulation of correlation coefficient values into equation forms was carried out using linear regression analysis to allow estimates of per unit-associated alterations in keratometric measurements.
| Results|| |
Our study included 15 left and 15 right eyes. The age of the patients in our sample ranged from 18 to 39 years, with a mean of 26.5 years (SD, 6.5 years). The percentage of male patients and female patients was 40 and 60, respectively (n=12, n=18, respectively).
The detailed preoperative and postoperative data of the patients are presented in [Table 1], with a comparative analysis for statistical significance where appropriate. Note that there were statistically significant improvements in UDVA, spherical and cylindrical errors, and SE, and a statistically significant change of K readings, and Q value with a mean difference of 3.78 D and 0.58, respectively. The mean residual spherical and cylindrical errors were negligible, with an achieved final mean SE of −0.14 D.
|Table 1 Preoperative and postoperative data for our sample (n=30) with a comparative t test analysis where appropriate|
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The previous table ([Table 2]) shows that there was a highly statistically significant negative correlation between SE difference and mean K difference with r=−0.980 and P value less than 0.001 and also a highly statistically significant positive correlation between SE difference and Q value difference with r=0.787 and P value less than 0.001. This is shown using scatter plots in [Figure 1] and [Figure 2].
|Table 2 Correlation of spherical equivalent difference with mean K difference and Q value difference|
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|Figure 1 Correlation between SE difference and Q value difference. SE, spherical equivalent.|
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|Figure 2 Correlation between SE difference and mean K difference. SE, spherical equivalent.|
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Linear regression analysis was then used to formulate the correlation between difference in K reading values and SE values, and the following equation was obtained:
| Discussion|| |
In this work, we have attempted to corroborate the efficacy of SMILE procedure in correcting compound myopic astigmatism cases, showing significant improvement in visual outcomes and a significant change in corneal asphericity and keratometric readings. We have also attempted to numerically correlate decrements in SE values to K reading alterations.
In the sample at hand, having astigmatism values of up to −3.5 D did not alter the final visual outcome as long as the intended SE did not pass −10 D. This is in line with the work of Ağca et al.  that showed safety and efficacy of SMILE within the −10 D SE range. Taneri et al.  have even demonstrated safety and efficacy of SMILE in cases up to −12 D of SE. Our mean residual SE value was, however, superior to that reported in both groups. This cements the ability of SMILE to successfully tackle high cylindrical, high SE cases.
The study also shows significant flattening of anterior cornea at 3 months postoperatively. This is in line with work done by Ganesh et al. , Sideroudi et al. , and Gyldenkerne et al. , which showed marked flattening of the anterior cornea. We did not examine the posterior corneal surface in our work. We did, however, use Q value to measure changes in anterior corneal asphericity and demonstrated statistically significant change in the postoperative values, with a mean difference of 3.78 D towards flattening. Changes in the corneal curvature may be related to the degree of myopia , and future larger studies should examine different cohorts with different myopic degrees to examine changes among each cohort; our sample was too small for this. We further plan to correlate both anterior and posterior Q value alterations to SE decrements in a numerical expectancy formula as was carried out with keratometric alterations. This may aid the debate about whether significant change in posterior corneal surface curvature is a real thing or not ,,.Conflicting data still exist about the noninferiority of SMILE when compared with femtosecond laser-assisted LASIK in cases of compound myopic astigmatism ,. A limitation of our study is that it was single armed. We hope to pursue future work comparing matched cohorts undergoing femtosecond LASIK and SMILE to obtain comparative data. Another limitation is the small sample size, but the expenses of the technique is limiting at the time being. A final limitation is the short follow-up interval. However, SMILE has been shown to exhibit better biomechanical stability properties over the years .
In conclusion, SMILE is a safe and effective technique to correct cases with myopic astigmatism with an SE of up to −10 D at least, and a predictable positive correlation could be established between decrements in SE and decrements in anterior corneal steepening.
Financial support and sponsorship
Conflicts of interest
There are no conflicts of interest.
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[Figure 1], [Figure 2]
[Table 1], [Table 2]