|Year : 2021 | Volume
| Issue : 1 | Page : 33-38
Small incision lenticule extraction versus photorefractive keratectomy: A comparative study
Reshmi Sreekumari, Hariprasad Vokuda, Krishnaprasad Kudlu
Department of Cornea and Refractive Surgery, Prasad Netralaya Super Specialty Eye Hospital, Udupi, Karnataka, India
|Date of Submission||01-Jun-2020|
|Date of Decision||23-Jun-2020|
|Date of Acceptance||26-Jun-2020|
|Date of Web Publication||19-Apr-2021|
Dr. Reshmi Sreekumari
Sreepuri, Balussery P.O., Kozhikode - 673 612, Kerala
Source of Support: None, Conflict of Interest: None
Purpose: The purpose of the study is to compare the objective and subjective quality of vision following small incision lenticule extraction (SMILE) and photorefractive keratectomy (PRK). We compared the postoperative safety, efficacy, predictability, refractive stability, contrast sensitivity, and spherical aberration changes in two groups over a 1-year follow-up period. Materials and Methods: This was a retrospective comparative study of 1-year duration. Fifty eyes of patients who underwent SMILE and 50 eyes of patients who underwent PRK were selected based on the inclusion and exclusion criteria. Methodology: Standard preoperative evaluation for all patients included corneal topography, refraction, anterior segment, retina evaluation, and intraocular pressure. Uncorrected visual acuity, best-corrected visual acuity, spherical aberration, and contrast sensitivity were measured preoperatively followed by 1, 3, 6, 9, and 12 months postoperatively. Results: Ninety-six percent of the eyes of patients who underwent SMILE and 92% of the eyes of patients who underwent PRK achieved postoperative refraction within ±0.5 D of the intended target refraction. No eyes in both the groups had any loss of visual acuity. Both procedures induced statistically significant spherical aberrations. However, the difference between the two groups was not statistically significant (P > 0.05). One hundred percent of eyes in SMILE and PRK group were noted to have a stable postoperative refraction till the last follow-up. Contrast sensitivity was better in eyes of patients who underwent SMILE comparing to PRK. Conclusion: Overall, both SMILE and PRK have shown excellent safety, efficacy, and predictability for correction of myopia and myopic astigmatism. SMILE is better in terms of refractive accuracy and quality of vision.
Keywords: Contrast sensitivity, photorefractive keratectomy, small incision lenticule extraction, spherical aberration, visual outcomes
|How to cite this article:|
Sreekumari R, Vokuda H, Kudlu K. Small incision lenticule extraction versus photorefractive keratectomy: A comparative study. Kerala J Ophthalmol 2021;33:33-8
|How to cite this URL:|
Sreekumari R, Vokuda H, Kudlu K. Small incision lenticule extraction versus photorefractive keratectomy: A comparative study. Kerala J Ophthalmol [serial online] 2021 [cited 2021 Jun 18];33:33-8. Available from: http://www.kjophthal.com/text.asp?2021/33/1/33/314105
| Introduction|| |
Small incision lenticule extraction (SMILE) and photorefractive keratectomy (PRK) are two flapless procedures for the correction of myopia. PRK is a surface ablation procedure that induces significant postoperative pain, relatively slow visual recovery, and corneal haze.,, SMILE is a novel form of “flapless” surgery, where the lenticule is extracted through a smaller corneal incision. It is useful for the correction of myopia up to −10 D SEQ and astigmatism up to 5 D. In SMILE, the visual outcome is generally good, and the time to return to normal life after surgery is short.
This study has been undertaken with the aim of comparing the spherical aberrations, predictability, safety, stability, efficacy, and contrast sensitivity changes of SMILE and PRK as both of them are flapless procedures for the correction of myopia and of studying their long-term results.
| Materials and Methods|| |
This was a retrospective study done in patients who underwent SMILE and PRK. Fifty eyes of patients who underwent SMILE and 50 eyes of patients who underwent PRK were selected and were followed up for 1 year. Patients in the 18–40-year age group with myopia and myopic astigmatism, stable refraction for more than 1 year, manifest myopic refraction spherical equivalent (SE) up to 10.0 D, and astigmatism up to 4 D were included in the study. Patients with residual stromal bed <300 μm, suspicion of keratoconus on corneal topography, ocular inflammation, trauma, and infection were excluded from the study. Patients with severe dry eyes and the use of any systemic or ocular medication were also excluded from the study.
This study was accepted by IRB and was in accordance with the ethical standards of the Institutional and/or National Research Committee and with the 1964 Helsinki declaration and its later amendments or comparable ethical standards.
All patients received the same preoperative examinations: uncorrected distance visual acuity (UDVA) and corrected distance visual acuity (CDVA), cycloplegic refraction, detailed fundus evaluation, contrast sensitivity, intraocular pressure, topography, and dry eye evaluation. Spherical aberration was calculated from the topography data over a 6 mm diameter central corneal zone. All aberrometry measurements were made in the same room with the same dark lighting conditions for all patients. Manifest refraction, UDVA, CDVA, corneal topography data, and spherical aberration were recorded at 1, 3, 6, 9, and 12 months postoperatively. Visual acuities were converted to logMAR before the statistical analysis.
SMILE was performed using the 500 kHz VisuMax femtosecond laser (Carl Zeiss Meditec, Jena, Germany) using a standard surgical technique. The following parameters were used: cap thickness, 120 μm; cap diameter, 7.5 mm; lenticule diameter, 6.5 mm with a transition zone of 0.1 mm; cut energy, 1.4 μJ; and spot and tracking distance, 2.0–3.0 μm. The back of the intrastromal lenticule was created from the periphery to center of cornea. The anterior lamellar cut was subsequently created from center to periphery of cornea, which extended toward the surface to create a 2–3 mm incision, from which the stromal lenticule was extracted. A thin blunt spatula was used to separate the lenticule, which was then grasped with a pair of forceps and removed. The corneal interface was flushed with balanced salt solution.
Alcohol-assisted corneal epithelial removal was performed over a 9.0-mm optical zone centered over the pupil. 20% alcohol in distilled water was dropped into a 9.0-mm well and kept in contact with the epithelium for 50 s and then absorbed with a dry cellulose sponge. The eye was then washed with balanced salt solution. A blunt spatula was then used to remove the loosened epithelium. Stromal ablation was performed with MEL 80 Excimer Laser using a 6.5-mm optical zone. After the stromal ablation, a circular cellulose sponge soaked with mitomycin C 0.02% (0.2 mg/mL) was placed on the cornea for 20 s. The eye was then copiously irrigated with a balanced salt solution to remove the residual mitomycin C. A bandage contact lens was placed over the cornea at the end of the surgery.
The patients on whom PRK was performed were instructed to apply one drop of 0.5% moxifloxacin (4 times/day) and artificial tears until the epithelium was healed. When the epithelial defect was completely closed, the therapeutic contact lens was removed, and 1% prednisolone (4 times/day) was added to the regimen for 10 days. This was followed by less potent steroids (Lotepred) which was then tapered over 2 months. The patients on whom SMILE was performed were instructed to use 0.5% moxifloxacin (4 times/day), 1% prednisolone (4 times/day), and artificial tears starting 3 h after surgery. The dosage of eye drops decreased over time, and the eye drops were continued for 1 month.
Full refractive and visual assessments were performed at the end of 1 week and at 1-, 3-, 6-, 9-, and 12-month intervals. All adverse events were also documented. Uncorrected visual acuity (UCVA), BCVA, spherical aberration, and contrast sensitivity were measured during the follow-up period and at the end of 12 months. Contrast sensitivity was measured with Pelli–Robson contrast sensitivity chart, and the mean spherical aberration on a 6-mm pupil zone was measured using VISANTE AS-OCT.
The various outcome measures were postoperative safety, efficacy, predictability, refractive stability, contrast sensitivity, and spherical aberration changes in the two groups over the follow-up period. Safety was defined as the number and percentage of eyes losing two or more lines of best spectacle-corrected visual acuity. Efficacy was defined as the percentage of eyes with an UCVA of 20/20 or 20/40 or better. Predictability was the proportion of eyes achieving a postoperative SE within ±0.50 D of the target at 6 months of follow-up.
Statistical analysis was performed using IBM SPSS Statistics 19.0 (SPSS Inc., Chicago, IL, USA). Visual and refractive outcomes, efficacy, and safety were evaluated at different time points. Paired t-test and independent samples t-test were used. P < 0.05 was considered statistically significant.
| Results|| |
The average age of patients who underwent SMILE was 25.8 years and that of PRK was 22.7 years. Among the PRK group, 40 (80%) patients were female and 10 (20%) patients were male [Figure 1]. In the SMILE group, 32 (64%) patients were female and 18 (36%) patients were male [Figure 2].
|Figure 2: Gender distribution in in small incision lenticule extraction group|
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The other preoperative and 12-month postoperative parameters are listed in [Table 1].
|Table 1: Comparison of pre-and post-operative parameters in small incision lenticule extraction and photorefractive keratectomy groups|
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All surgeries were uneventful without any intraoperative complications. No postoperative complications, such as wound dehiscence, inflammation, and infection, were observed in any patient.
Using paired t-test, it was found that both procedures induced statistically significant spherical aberrations as compared to the preoperative spherical aberrations (P < 0.05).
Using independent samples t-test, it was found that even though, postoperatively, spherical aberration was found to be more in the PRK group (0.4918) as compared to the SMILE group (0.4020), the difference between the two groups was not statistically significant (P = 0.35). Spherical aberration changes over time in both groups are shown in [Figure 3].
|Figure 3: Spherical aberration changes over time in in small incision lenticule extraction and photorefractive keratectomy|
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Forty-eight (96%) eyes of patients who underwent SMILE and 46 (92%) eyes of patients who underwent PRK achieved postoperative refraction within 0.5 D of the intended target refraction.
No eyes in both the groups were noted to have any complications intra- or postoperatively causing any loss of visual acuity. Ten (20%) eyes of patients who underwent SMILE had an improvement in visual acuity (unaided) as compared to preoperative BCVA by half to one line. Five (10%) eyes of patients who underwent PRK noted this improvement. Comparison of safety in both groups is shown in [Figure 4].
|Figure 4: Comparison of safety in in small incision lenticule extraction and photorefractive keratectomy|
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All fifty (100%) eyes in the SMILE and PRK group were noted to have postoperative refraction which was stable from 1-month postoperative till the last follow-up.
Forty-eight (96%) eyes of patients who underwent SMILE achieved a postoperative UCVA of 20/20. Forty-five (90%) eyes of patients who underwent PRK achieved a postoperative UCVA of 20/20.
There is no significant difference in postoperative UDVA in SMILE and PRK groups (P = 0.52). Comparison of efficacy of small incision lenticule extraction and photorefractive keratectomy is shown in [Figure 5].
|Figure 5: Comparison of efficacy in in small incision lenticule extraction and photorefractive keratectomy|
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Contrast sensitivity was better postoperatively in SMILE versus PRK group.
Using independent samples t-test, the difference between the two groups was not statistically significant (P = 0.720).
Contrast sensitivity changes in SMILE and PRK are shown in [Figure 6].
|Figure 6: Contrast sensitivity changes in in small incision lenticule extraction and photorefractive keratectomy|
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| Discussion|| |
The current study showed that SMILE was more effective in low-to-moderate myopic correction compared to PRK at 12 months. Although both procedures were found to be similar in safety, visual rehabilitation was quicker after SMILE compared to PRK.
Ninety-six percent of eyes had postoperative refraction within ±0.5 D in the SMILE group, whereas 92% had within ±0.5 D in the PRK group. This difference in the postoperative refraction can be attributed to the differential healing response between the two procedures. PRK utilizes ultraviolet light from excimer laser to reshape the corneal curvature through direct photoablation. SMILE uses femtosecond laser in near-infrared spectrum to achieve the cutting effect within the cornea through photodisruption. The main advantage of femtosecond laser compared to excimer laser is its high cutting precision with minimal collateral damage and associated cellular and structural changes.
Variation in hydration of the corneal stroma is the most likely cause for underablation or overablation of stromal tissue.,, In PRK, after the removal of epithelium, the stroma is exposed to hydration changes before refractive correction. On the other hand, in SMILE, the refractive lenticule is cut by a femtosecond laser before any disturbance of the stroma. It is likely that this difference contributed to greater predictability.
Adjunctive mitomycin C has been demonstrated to prevent post-PRK corneal haze, especially for high myopic correction. Mitomycin C could delay healing and induce apoptosis of anterior stromal cells, modifying the healing response.,
Similar study on SMILE versus PRK conducted by Ganesh et al. showed similar predictability, HOA, and contrast sensitivity changes. Efficacy and safety were found to be higher in SMILE group, while stability was comparable in both groups. Yildrim et al. reported that predictability of SMILE group was 92% and PRK group was 89%. Both groups had same efficacy and safety. Stability was found to be more in SMILE group. In this study, HOA was found to be higher in the SMILE group probably because they used aberration-free ablation platform in PRK.
Shah et al. demonstrated that 79% of all eyes had a UDVA of 20/25 or better after SMILE. The efficacy that we achieved in our study with SMILE was slightly better than that of previous studies. This finding may be caused by the slightly lower myopic correction in our study.
In our study, no significant haze developed after PRK. The mean attempted refractive correction (−3.24 D) was relatively low and likely below the typical threshold of haze formation reported in other studies.
A study conducted by Diakonis et al. on PRK with MMC demonstrated 0.21 D changes in manifest refraction from the 3-month to the 12-month follow-up. Furthermore, these authors found that 6% of eyes regressed more than 0.50 D. Diakonis et al. employed a different excimer laser platform, and this may attribute for this difference.
Aslanides et al. reported that 85.7% of the eyes that underwent PRK to correct high myopia were within ±0.50 D changes. This discrepancy may be attributed to the difference in the intended refractive correction between the two reports (−8.25 vs. −3.24 D).
Comparison of our study with similar studies on SMILE and PRK is shown in [Table 2].
Our study results did not include the impact of SMILE and PRK on corneal biomechanics. We did not evaluate changes in epithelial thickness and other higher-order aberrations. We measured preoperative and postoperative SEs only and did not separately record the astigmatic component. We had very few patients with higher refractive error, and most of them underwent SMILE procedure. In lower refractive errors, the SMILE lenticule will be very thin, and the removal may be difficult. The higher refractive error provides an adequate thickness of lenticule for easy and safe removal. We also acknowledge the relatively small sample size and short follow-up duration, which precludes analysis of the long-term stability of our results.
| Conclusion|| |
Overall, both SMILE and PRK have shown excellent safety, effectiveness, and predictability for correction of myopia and myopic astigmatism. Keeping in view better quality of vision and faster postoperative recovery time, SMILE is a safe option for higher refractive corrections. Further studies with a larger number of eyes are needed.
Financial support and sponsorship
Conflicts of interest
There are no conflicts of interest.
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[Figure 1], [Figure 2], [Figure 3], [Figure 4], [Figure 5], [Figure 6]
[Table 1], [Table 2]