Intraindividual comparison of changes in corneal biomechanical parameters after femtosecond lenticule extraction and small-incision lenticule extraction

Clinical Evaluation of Corneal Biomechanics following Laser Refractive Surgery in Myopic Eyes: A Review of the Literature

The role of corneal biomechanics in laser vision correction (LVC) is currently being raised in the assessment of postoperative corneal ectasia risk. The aim of the paper was to evaluate the changes in corneal biomechanics after LVC procedures based on a systematic review of current studies. The results of a search of the literature in the PubMed, Science Direct, Google Scholar, and Web of Science databases were selected for final consideration according to the PRISMA 2020 flow diagram. Included in our review were 17 prospective clinical studies, with at least 6 months of follow-up time. Corneal biomechanical properties were assessed by Ocular Response Analyzer (ORA), or Corvis ST. The results of the study revealed the highest corneal biomechanics reduction after laser in situ keratomileusis (LASIK) followed by small incision lenticule extraction (SMILE) and surface procedures, such as photorefractive keratectomy (PRK) or laser-assisted sub-epithelial keratectomy (LASEK). In SMILE procedure treatment planning, the use of thicker caps preserves the corneal biomechanics. Similarly, reduction of flap thickness in LASIK surgery maintains the corneal biomechanical strength. Future prospective clinical trials with standardization of the study groups and surgical parameters are needed to confirm the results of the current review.

The best optical zone for small-incision lenticule extraction in high myopic patients

Small-incision lenticule extraction (SMILE) is an effective and safe procedure for the correction of myopia due to minimally invasive and noncorneal flap surgery. However, the SMILE procedure has certain requirements for corneal cap thickness, attempted refractive correction, residual stromal bed thickness, and optical zone diameter, which sometimes make surgeons hesitant to choose SMILE or other refractive surgeries. The requirements limit its use in patients with high myopia. The purpose of this review was to find the optimal parameters of SMILE through discussing the best optical zone for high myopic patients, the visual quality of different optical zones, the choice of corneal cap thickness, and their effects on corneal biomechanical parameters, so surgeons can provide reference recommendations for patients with high myopia in choosing a reasonable and safe procedure.

Corneal biomechanical properties after SMILE versus FLEX, LASIK, LASEK, or PRK: a systematic review and meta-analysis

BACKGROUND

The aim of this study was to compare the postoperative corneal biomechanical properties between small incision lenticule extraction (SMILE) and other corneal refractive surgeries.

METHODS

A systematic review and meta-analysis were conducted. Articles from January 2005, to April 2019, were identified searching PubMed, EMBASE, Web of Science, and International Clinical Trials Registry Platform. Studies that compared SMILE with other corneal refractive surgeries on adult myopia patients and evaluated corneal biomechanics were included. Multiple effect sizes in each study were combined. Random-effects model was conducted in the meta-analysis.

RESULTS

Twenty-two studies were included: 5 randomized controlled trials (RCTs), 9 prospective and 6 retrospective cohort studies, and 2 cross-sectional studies. Using the combined effect of corneal hysteresis (CH) and corneal resistance factor (CRF), which were obtained from ocular response analyzer (ORA), the pooled Hedges' g of SMILE versus femtosecond laser-assisted in situ keratomileusis (FS-LASIK) was 0.41 (95% CI, 0.00 to 0.81; p = 0.049; I² = 78%), versus LASIK was 1.31 (95% CI, 0.54 to 2.08; p < 0.001; I² = 77%), versus femtosecond lenticule extraction (FLEX) was - 0.01 (95% CI, - 0.31 to 0.30; p = 0.972; I² = 20%), and versus the group of photorefractive keratectomy (PRK) and laser-assisted sub-epithelial keratectomy (LASEK) was - 0.26 (95% CI, - 0.67 to 0.16; p = 0.230; I² = 54%). The summary score of Corvis ST (CST) after SMILE was comparable to FS-LASIK/LASIK with the pooled Hedges' g = - 0.05 (95% CI, - 0.24 to 0.14; p = 0.612, I² = 55%).

CONCLUSIONS

In terms of preserving corneal biomechanical strength after surgeries, SMILE was superior to either FS-LASIK or LASIK, while comparable to FLEX or PRK/LASEK group based on the results from ORA. More studies are needed to apply CST on evaluating corneal biomechanics after refractive surgeries.

Comparison of corneal biomechanical changes after refractive surgery by noncontact tonometry: small-incision lenticule extraction versus flap-based refractive surgery - a systematic review

Corneal refractive surgery disrupts corneal integrity and reduces biomechanical stability in consequence of the beneficial refractive alteration. The minimal invasive cap-based refractive procedure, small-incision lenticule extraction (SMILE), has been proposed to affect corneal integrity less than flap-based procedures, due to the fibre-sparing incision of the strong anterior corneal lamellae. Flap-based procedures include laser-assisted in situ keratomileusis (LASIK), femtosecond laser-assisted in situ keratomileusis (FS-LASIK) and femtosecond lenticule extraction (FLEx). The purpose of this systematic review was to evaluate corneal biomechanical changes by noncontact air pulse tonometry after treatment of myopia/myopic astigmatism with SMILE compared to flap-based refractive surgery. A total of 220 publications were identified through a systematic search in PubMed and Embase. Two levels of screening identified nine studies (three randomised controlled trials (RCT) and six nonrandomised clinical trials) eligible for the review. All the nonrandomised clinical trials were graded to have an overall serious risk of bias. Measurements with the Corvis ST were not included in any of the eligible studies. The RCTs found no statistical significant differences between SMILE or flap-based procedures concerning corneal hysteresis (CH) or corneal resistance factor (CRF), as measured with the Ocular Response Analyzer. However, a greater reduction in CRF and CH was found in the flap-based group in five and two of the nonrandomised studies, respectively. The findings in this review illustrate that the presumed biomechanical advantages of a cap-based small incision could not be demonstrated in existing studies and by commercially available technology. However, studies with lower levels of evidence suggest less affection of corneal viscoelastic properties after SMILE when evaluating corneal stability by noncontact tonometry.

Comparison Between Q-Adjusted LASIK and Small-Incision Lenticule Extraction for Correction of Myopia and Myopic Astigmatism

PURPOSE

To compare Q-adjusted femtosecond-assisted laser in situ keratomileusis (FS-LASIK) and small-incision lenticule extraction (SMILE) in terms of safety, efficacy, and predictability, as well as in terms of changes in dry eye parameters, corneal biomechanics, higher-order aberrations (HOAs), and corneal asphericity.

METHODS

A total of 60 eyes were enrolled in this prospective comparative study: 30 underwent SMILE and 30 underwent FS-LASIK. Changes in manifest refraction, uncorrected distance visual acuity and corrected distance visual acuity, corneal topography, corneal hysteresis (CH), and corneal resistance factor (CRF) (Ocular Response Analyzer, ORA), and dry eye parameters were evaluated during a 3-month follow-up.

RESULTS

Mean manifest refraction spherical equivalent (MRSE) was -4.17 (±1.86) preoperatively in FS-SMILE group and -3.97 (±2.02) in FS-LASIK group (P=0.69). Mean postoperative MRSE in FS-SMILE group was -0.25±0.38 diopter (D) and -0.183±0.33 D in FS-SMILE group (P=0.50). A total of 93% of treated eyes in both groups had a MRSE within ±0.5 D. The mean percentage change of CH was 14.23±17.59 and 18.89±7.2 mm Hg (P=0.1871) in FS-SMILE and FS-LASIK groups, respectively. The mean percentage change of CRF was 27.43±16.8 and 21.32±17.1 mm Hg (P=0.1682) in FS-SMILE and FS-LASIK groups, respectively. A statistically significant difference between techniques were found in the values of Schirmer test (P=0.0002) and tear breakup time (P=0.0035). Regarding corneal HOAs, no statistically significant differences between groups were found in the change in the root mean square of HOAs (P=0.2), coma aberration (P=0.0589), and spherical aberration (P=0.0543).

CONCLUSION

SMILE is as safe and predictable as FS-LASIK. According to the ORA system, SMILE is not better than FS-LASIK in terms of biomechanical changes. However, SMILE causes less effect on dry eye parameters during the first 3 postoperative months.

Biomechanical Properties of Human Cornea Tested by Two-Dimensional Extensiometry Ex Vivo in Fellow Eyes: Femtosecond Laser-Assisted LASIK Versus SMILE

PURPOSE

To investigate the biomechanical properties of the ex vivo human cornea after flap-based versus cap-based laser refractive surgery in the same donor.

METHODS

In this experimental study, 11 pairs of human corneas unsuitable for transplantation were equally divided into two groups. Corneas from the right eye were treated with femtosecond laser-assisted LASIK (FSLASIK) and corneas from the left eye with small incision lenticule extraction (SMILE). Pachymetry was measured in each eye directly before laser refractive surgery. All corneas were subjected to a refractive correction of -10.00 diopters (D) sphere and -0.75 D cylinder at 0° with a 7-mm zone, using either a 110-μm flap (FS-LASIK) or 130-μm cap (SMILE). For two-dimensional biomechanical measurements, corneoscleral buttons underwent two testing cycles (preconditioning stress-strain curve from 0.03 to 9.0 N and stress-relaxation at 9.0 N during 120 sec) to analyze the elastic and viscoelastic material properties. The effective elastic modulus was calculated. Statistical analysis was performed with a confidence interval of 95%.

RESULTS

In stress-strain measurements, the effective elastic modulus was 1.47 times higher (P = .003) after SMILE (median = 8.22 [interquartile range = 4.76] MPa) compared to FS-LASIK (median = 5.59 [inter-quartile range = 2.77] MPa). The effect size was large (r = 0.83). No significant differences (P = .658) were observed among stress-relaxation measurements, with a mean remaining stress of 181 ± 31 kPa after SMILE and 177 ± 26 kPa after FS-LASIK after relaxation.

CONCLUSIONS

Compared to a flap-based procedure such as FS-LASIK, the SMILE technique can be considered superior in terms of biomechanical stability, when measured experimentally in ex vivo human fellow eye corneas. [J Refract Surg. 2018;34(6):419-423.].

Evaluation of femtosecond laser in flap and cap creation in corneal refractive surgery for myopia: a 3-year follow-up

Purpose

To evaluate femtosecond laser in flap and cap creation, detect some corneal biomechanical changes, and evaluate dry eye after laser in situ keratomileusis (LASIK), Femto-LASIK, and small incision lenticule extraction (SMILE) with 3-year follow-up.

Patients and methods

Preoperative evaluation taken: full ophthalmic examination, Pentacam, ocular response analyzer, ocular surface disease index (OSDI), and tear breakup time (TBUT). LASIK flap was created using Moria microkeratome in 30 eyes (LASIK group) and using VisuMax femtosecond laser in 38 eyes (FS-LASIK group) and SMILE was done by VisuMax in 35 eyes (SMILE group). Postoperative evaluation: anterior segment optical coherence tomography to measure flap and cap thickness, ocular response analyzer to measure corneal hysteresis (CH) and corneal resistance factor (CRF), OSDI, and TBUT at 1, 3, 6, 12, 24, and 36 months after surgery.

Results

This study included 103 eyes of 103 patients. The mean deviation of central cap or flap thickness from intended was statistically higher in the LASIK group (P<0.001). Both CH and CRF showed significant reduction postoperatively but were significantly higher in the SMILE group during follow up (P<0.05). The mean OSDI scores were significantly elevated in all groups postoperatively (P<0.01) but were significantly lower in the SMILE group 3 months postoperatively (P<0.05). The mean TBUT was significantly decreased in all groups postoperatively (P<0.01) but was significantly higher in the SMILE group 6 months postoperatively (P<0.05).

Conclusion

Femtosecond laser is more accurate than microkeratomes. CH and CRF changes were least after SMILE. The three procedures led to significant dryness but for shorter duration with SMILE.

Unilateral corneal ectasia after small-incision lenticule extraction in a 43-year-old patient

Unilateral corneal ectasia developed after small-incision lenticule extraction for mild myopia in a 43-year-old man with preoperative asymmetric astigmatism. The ectasia was diagnosed 4 years postoperatively. Preoperative data showed asymmetric astigmatism with no signs of forme fruste keratoconus. Inferior anterior curvature steepening exceeded 2.00 diopters without bulging of the posterior curvature, and pachymetric thickness exceeded 515 μm. Corneal ectasia can occur after small-incision lenticule extraction in patients older than 40 years with preoperative asymmetric astigmatism.

Refractive lenticule extraction small incision lenticule extraction: A new refractive surgery paradigm

Small incision lenticule extraction (SMILE), a variant of refractive lenticule extraction technology is becoming increasingly popular, as a flapless and minimally invasive form of laser vision correction (LVC) for the treatment of myopia and myopic astigmatism. This review aims at summarizing the principles, surgical technique, and clinical outcomes in terms of visual and refractive results, safety, efficacy, postoperative dry eye, aberrations, and biomechanics of SMILE and its comparison with other conventional techniques of LVC, such as laser in situ keratomileusis (LASIK) and photorefractive keratectomy (PRK). Recent advancements in the laser frequency and energy delivery patterns, instrumentation, and surgical techniques have shown significant improvement in the visual recovery and outcomes after SMILE, compared to the initial results published by Sekundo and Shah et al. Most of the recently published literature on long-term outcomes of SMILE shows excellent stability of the procedure, especially for higher myopia. In terms of the postoperative dry eye, SMILE shows a clear advantage over LASIK as numerous studies have shown significant differences about the Schirmer's, Tear film break up time, corneal sensitivity, and corneal nerve regeneration to be better following SMILE compared to LASIK. There is some evidence that since the Bowman's membrane (BM) and the anterior lamellae remain intact after SMILE, this may be a potential advantage for corneal biomechanics over LASIK and PRK where the BM is either severed or ablated, respectively, however, the data on biomechanics are inconclusive at present. Overall, this procedure has proved to be promising, delivering equivalent, or better visual and refractive results to LASIK and providing clear advantage in terms of being a flapless, minimally invasive procedure with minimal pain and postoperative discomfort thus offering high patient satisfaction.

Evaluation of Human Corneal Lenticule Quality After SMILE With Different Cap Thicknesses Using Scanning Electron Microscopy

PURPOSE

To evaluate the surface characteristics of lenticules created by small-incision lenticule extraction (SMILE) with different cap thicknesses.

METHODS

This prospective study included 20 consecutive patients who underwent bilateral SMILE. Surface regularity of the extracted corneal lenticule was analyzed using scanning electron microscopy (SEM) combined with 2 methods: qualitative and quantitative regularity. Qualitative regularity of SEM images was graded by masked observers using an established scoring system. Quantitative regularity of SEM images was assessed by counting the total number and areas of tissue bridges using Image-Pro Plus software. Four different cap thickness of 120, 130, 140, and 150 μm were compared. Refractive outcomes of patients were measured at baseline and 1 month after surgery.

RESULTS

As 10 specimens were not analyzable, only 30 eyes were included. Postoperatively, all eyes had postoperative uncorrected distance visual acuity of 20/20 or better; 43% had an unchanged corrected distance visual acuity; 43% gained 1 line; 10% lost 1 line. Ultrastructurally, surface irregularity was primarily caused by tissue bridges. The average surface regularity score obtained was 10.87 ± 2.40 for 120 μm, 10.78 ± 2.60 for 130 μm, 8.76 ± 2.16 for 140 μm, and 8.70 ± 2.66 for 150 μm (P < 0.001). The total number and areas of tissue bridges of 120 to 130 μm were significantly less than 140 to 150 μm (P < 0.05). Surface regularity decreased as cap thickness increased (P < 0.05).

CONCLUSIONS

There is smoother appearance of the lenticular surface as seen through SEM when a thin cap is created compared with a thick cap qualitatively and quantitatively.

Corneal biomechanics after laser refractive surgery: Unmasking differences between techniques

The hypothesis that small-incision lenticule extraction provides better preservation of corneal biomechanics than previous laser refractive techniques has led to a growth in the interest in clinical and experimental research in this field. This hypothesis is based on the fact that corneal layers with greater stiffness are preserved with this new technique. However, this hypothesis is controversial because clinical research has shown a great disparity in the outcomes. In this review, we performed an in-depth analysis of the factors that might affect corneal biomechanics in laser refractive surgery procedures from a macrostructural to a microstructural viewpoint. New advances in algorithms with current devices or the introduction of new devices might help unmask the possible advantages of small-incision lenticule extraction in corneal biomechanics.

Influence of corneal power on circumpapillary retinal nerve fiber layer and optic nerve head measurements by spectral-domain optical coherence tomography

AIM

To evaluate the influence of corneal power on circumpapillary retinal nerve fiber layer (cpRNFL) and optic nerve head (ONH) measurements by spectral-domain optical coherence tomography (SD-OCT).

METHODS

Twenty-five eyes of 25 healthy participants (mean age 23.6±3.6y) were imaged by SD-OCT using horizontal raster scans. Disposable soft contact lenses of different powers (from -11 to +5 diopters including 0 diopter) were worn to induce 2-diopter changes in corneal power. Differences in the cpRNFL and ONH measurements per diopter of change in corneal power were analyzed.

RESULTS

As corneal power increased by 1 diopter, total and quadrant cpRNFL thicknesses, except for the nasal sector, decreased by --0.19 to -0.32 µm (P<0.01). Furthermore, the disc, cup, and rim areas decreased by -0.017, -0.007, and -0.015 mm², respectively (P<0.001); the cup and rim volumes decreased by -0.0013 and -0.006 mm³, respectively (P<0.01); and the vertical and horizontal disc diameters decreased by -0.006 and -0.007 mm, respectively (P<0.001).

CONCLUSION

For more precise OCT imaging, the ocular magnification should be corrected by considering both the axial length and corneal power. However, the effect of corneal power changes on cpRNFL thickness and ONH topography are small when compare with those of the axial length.

[Refractive lenticule extraction - The ReLEx/SMILE technique : Video article]

Refractive lenticule extraction (ReLEx/SMILE) is a refractive surgical method developed by the authors in stages primarily for correction of myopia and myopic astigmatism, in which a femtosecond laser is exclusively used. In the (ReLEx/SMILE) method a refractive lenticule is generated by the femtosecond laser, which is subsequently extracted through a small incision. Opening of the corneal surface with a flap and use of an excimer laser is no longer necessary. In 2013 the method was newly assessed and classified by the Joint Commisssion for Refractive Surgery (KRC) of the Professional Association of Ophthalmologists (BVA) and the German Ophthalmological Society (DOG). This article presents the individual stages of the technique in a video and possible errors and their avoidance are explained.

Small-incision lenticule extraction versus femtosecond lenticule extraction for myopic: a systematic review and Meta-analysis

AIM

To examine differences in efficacy, accuracy, safety, aberrations and corneal biomechanical between Small incision lenticule extraction (SMILE) and femtosecond lenticule extraction (FLEx) for myopia.

METHODS

Comprehensive studies were conducted on the PubMed, MEDLINE, EMBASE, and Cochrane Controlled Trials Register before 31 July, 2015. Meta-analyses were performed on the primary outcomes [loss of ≥2 lines of corrected distance visual acuity (CDVA), uncorrected distance visual acuity (UDVA) ≥20/20, spherical equivalent (SE) within ±0.50 diopters (D), final refractive SE], secondary outcomes were high-order aberrations (HOAs) and corneal biomechanical [central corneal thickness (CCT), corneal hysteresis (CH) and corneal resistance factor (CRF)].

RESULTS

Seven trials describing a total of 320 eyes with myopia were included in this Meta-analysis. No significant differences were found in the efficacy [UDVA weighted mean difference (WMD) -0.01; 95%CI: -0.04 to 0.01; P=0.37, UDVA ≥20/20, OR 1.49; 95%CI: 0.78 to 2.86; P=0.23], accuracy (SE WMD -0.03; 95%CI: -0.12 to 0.07; P=0.58, SE within ±0.5 D OR 1.25; 95%CI: 0.34 to 4.65; P=0.74), HOAs (WMD -0.04; 95%CI: -0.09 to 0.01; P=0.14) and CCT WMD 1.83; 95%CI: -7.07 to 10.72; P=0.69, CH WMD -0.01; 95%CI: -0.42 to 0.40; P=0.97, CRF WMD 0.17; 95%CI: -0.33 to 0.67; P=0.50) in the last fellow-up. But for safety, FLEx may achieve fewer CDVA lost two or more two lines (OR 11.11; 95%CI: 1.27 to 96.86; P=0.03) than SMILE, however CDVA (WMD 0.00; 95%CI: -0.03 to 0.02; P=0.77) is similar.

CONCLUSION

SMILE and FLEx are comparable in terms of both efficacy, accuracy, aberrations and corneal biomechanical measures in the follow-up, but FLEx seems to be better in safety measures. The results should be interpreted cautiously since relevant evidence is still limited, although it is accumulating. Further large-scale, well-designed randomized controlled trials are urgently needed.

Comparison of biomechanical effects of small incision lenticule extraction and laser-assisted subepithelial keratomileusis

PURPOSE

To compare the corneal biomechanical properties after small incision lenticule extraction (SMILE) group and laser-assisted subepithelial keratomileusis (LASEK) group.

METHODS

The SMILE group comprised 39 patients (75 eyes) while it was 38 patients (76 eyes) in the LASEK group. Corneal hysteresis (CH) and the corneal resistance factor (CRF) parameters were determined using the Ocular Response Analyzer preoperatively and at 1 and 3 months postoperatively.

RESULTS

There were significant differences in the preoperative manifest refraction spherical equivalent (MRSE), central corneal thickness (CCT), the planned ablation depth (AD) or CRF values between the two groups, except for the CH value (p = 0.07). The CH and CRF values at 1 and 3 months postoperatively were significantly lower than the preoperative values in both groups (p < 0.01). The postoperative CH values were significantly lower in the LASEK group than in the SMILE group at both follow-up visits after adjusting for age, sex, preoperative CCT, preoperative SE, planned AD and the use of both eyes in the same subject (p < 0.01). However, no significant differences were observed in postoperative CRF between groups (p = 0.22 at 1 month and p = 0.41 at 3 months). No significant correlation was found between the planned AD and the changes in CH or CRF at any follow-up visit in the SMILE group. The preoperative magnitude of CH and CRF may be predictors of postoperative changes in CH and CRF in the both groups.

CONCLUSIONS

CH and CRF decreased after SMILE and LASEK. However, the changes in the CH values were less after SMILE than after LASEK.

Biomechanical Properties of the Cornea Measured With the Dynamic Scheimpflug Analyzer in Young Healthy Adults

PURPOSE

To investigate the biomechanical properties of the cornea using the dynamic Scheimpflug analyzer in young healthy adults.

METHODS

This prospective cross-sectional population study included 944 eyes of 472 participants aged 20 to 40 years. Participants underwent ophthalmic investigations, including evaluation of biomechanical properties of the cornea using the dynamic Scheimpflug analyzer, manifest refraction, and measurements of keratometric values by autokeratometry, intraocular pressure (IOP) by noncontact tonometer, central corneal thickness (CCT) by ultrasound, and white-to-white distance by Scheimpflug tomography. Statistical analyses included determination of the reference interval with a bootstrapping method, linear quantile mixed-effects model, and Spearman correlation analysis between the corneal biomechanical parameters and other variables (age, manifest refraction spherical equivalent, CCT, IOP, white-to-white, and keratometric values).

RESULTS

The 90% CIs of all corneal biomechanical parameters demonstrated that the ranges of the 90% CIs for the reference data were almost identical with and without bootstrapping. Quantile regression to determine the fifth, 50th, and 95th percentiles of each corneal biomechanical parameter supported the findings from the nonparametric method with the 90% CIs. Correlation analysis showed significant correlations between the parameters and variables, but there was a relatively high Spearman correlation coefficient in the case of the correlations with the CCT and IOP.

CONCLUSIONS

Using data from a large population of young healthy adults, we developed a database of normal values for multiple corneal biomechanical parameters obtained from the dynamic Scheimpflug analyzer. We conclude that the biomechanical properties of the cornea are influenced by the CCT and IOP.

Unilateral corneal ectasia following small-incision lenticule extraction

We describe a case of unilateral corneal ectasia in a 26-year-old man following small-incision lenticule extraction. The preoperative corneal topography was normal, with a minimum corneal thickness of 511 μm and 513 μm in the right eye and left eye, respectively. Lenticules of 85 μm and 82 μm were fashioned to offer a refractive correction of -3.75 -1.50 × 180 and -3.50 -1.50 × 165 in the right eye and left eye, respectively. Twelve months after small-incision lenticule extraction, the patient presented with early signs of ectasia in the left eye on corneal topography, which had worsened at the 18-month examination. Intrastromal corneal ring segment implantation with corneal collagen crosslinking was performed to arrest further progression and to improve uncorrected distance visual acuity. On the last examination, the corrected distance visual acuity was 20/20(-2).

FINANCIAL DISCLOSURE

Dr. Mahipal S. Sachdev receives travel grants from Carl Zeiss Meditec AG. No author has a financial or proprietary interest in any material or method mentioned.

Efficacy, safety, predictability, aberrations and corneal biomechnical parameters after SMILE and FLEx: Meta-analysis

AIM

To identify possible differences of efficacy, safety, predictability, higher-order aberrations and corneal biomechnical parameters after small-incision lenticule extraction (SMILE) and femtosecond lenticule extraction (FLEx).

METHODS

A systematic literature retrieval was conducted in Medline, Embase and the Cochrane Library, up to October, 2015. The included studies were subject to a Meta-analysis. Comparison between SMILE and FLEx was measured as pooled odds ratio (OR) or weighted mean differences (WMD). Of 95% confidence intervals (CI) were used to analyze data.

RESULTS

A total of seven studies were included. Firstly, there were no differences in uncorrected distance visual acuity (UDVA) 20/20 or better (OR, 1.37; 95% CI, 0.69 to 2.69; P=0.37) and logMAR UDVA (WMD, -0.02; 95% CI, -0.05 to 0.01; P=0.17) after SMILE versus FLEx. We found no differences in corrected distance visual acuity (CDVA) unchanged (OR, 0.98; 95% CI, 0.46 to 2.11; P=0.97) and logMAR CDVA (WMD, -0.00; 95% CI, -0.01 to 0.01; P=0.90) either. Secondly, we found no differences in refraction within ±1.00 D (OR, 0.98; 95% CI, 0.13 to 7.28; P=0.99) and ±0.50 D (OR, 1.62; 95% CI, 0.62 to 4.28; P=0.33) of target postoperatively. Thirdly, for higher-order aberrations, we found no differences in the total higher-order aberrations (WMD, -0.04; 95% CI, -0.09 to 0.01; P=0.14), coma (WMD, -0.04; 95% CI, -0.09 to 0.01; P=0.11), spherical (WMD, 0.01; 95% CI, -0.02 to 0.03; P=0.60) and trefoil (WMD, -0.00; 95% CI, -0.04 to 0.03; P=0.76). Furthermore, for corneal biomechanical parameters, we also found no differences (WMD, 0.08; 95% CI, -0.17 to 0.33; P=0.54) after SMILE versus FLEx.

CONCLUSION

There are no statistically differences in efficacy, safety, predictability, higher-order aberrations and corneal biomechnical parameters postoperative between SMILE and FLEx.

Evaluation of Changes in Human Corneas After Femtosecond Laser-Assisted LASIK and Small-Incision Lenticule Extraction (SMILE) Using Non-Contact Tonometry and Ultra-High-Speed Camera (Corvis ST)

PURPOSE

To evaluate theoretical biomechanical advantages of human corneas treated with small incision lenticule extraction (SMILE) compared with femtosecond laser-assisted LASIK (FS-LASIK) Patients and methods: In a prospective, comparative, non-randomized, consecutive case series patients with moderate to high myopia and/or astigmatism underwent corneal refractive surgery. Patients either received standard FS-LASIK or SMILE. Preoperatively and up to 3 months postoperatively data were analyzed including tomography with Pentacam HR, Goldmann tonometry, non-contact tonometry, and deformability of the cornea using an ultra-high-speed camera (Corvis ST). Data were analyzed and p < 0.05 was considered as statistically significant (t-test).

RESULTS

Seventy-three patients (128 eyes) were treated. Forty-eight eyes of 29 patients underwent FS-LASIK and 80 eyes of 44 patients underwent SMILE. Preoperative spherical equivalent (SE) was -3.23 ± 1.64 D (FS-LASIK group) and -4.83 ± 1.63 D (SMILE group) (p < 0.0001). Almost all parameters obtained by Corvis ST between preoperative and postoperative measurements showed significant changes after refractive surgery. Significant changes were seen between both groups postoperatively. However, matched pair subgroup analysis (n = 69; 26 eyes FS-LASIK; 43 eyes SMILE) of eyes with initially equal pachymetry, intraocular pressure, SE, and difference of pre- to postoperative pachymetry (p>0.05), showed no significant changes in parameters measured with Corvis ST (p>0.05).

CONCLUSIONS

Corneal biomechanical parameters measured preoperatively with Corvis ST showed significant differences postoperatively in total and in both groups. In subgroup analysis with homogenous groups, FS-LASIK showed no significant changes in biomechanical data measured with Corvis ST compared with SMILE.

Small incision lenticule extraction (SMILE) history, fundamentals of a new refractive surgery technique and clinical outcomes

This review summarizes the current status of the small incision lenticule extraction (SMILE) procedure. Following the early work by Sekundo et al. and Shah et al., SMILE has become increasingly popular. The accuracy of the creation of the lenticule with the VisuMax femtosecond laser (Carl Zeiss Meditec) has been verified using very high-frequency (VHF) digital ultrasound and optical coherence tomography (OCT). Visual and refractive outcomes have been shown to be similar to those achieved with laser in situ keratomileusis (LASIK), notably in a large population reported by Hjortdal, Vestergaard et al. Safety in terms of the change in corrected distance visual acuity (CDVA) has also been shown to be similar to LASIK. It was expected that there would be less postoperative dry eye after SMILE compared to LASIK because the anterior stroma is disturbed only by the small incision, meaning that the anterior corneal nerves should be less affected. A number of studies have demonstrated a lower reduction and faster recovery of corneal sensation after SMILE than LASIK. Some studies have also used confocal microscopy to demonstrate a lower decrease in subbasal nerve fiber density after SMILE than LASIK. The potential biomechanical advantages of SMILE have been modeled by Reinstein et al. based on the non-linearity of tensile strength through the stroma. Studies have reported a similar change in Ocular Response Analyzer (Reichert) parameters after SMILE and LASIK, however, these have previously been shown to be unreliable as a representation of corneal biomechanics. Retreatment options after SMILE are discussed. Tissue addition applications of the SMILE procedure are also discussed including the potential for cryo-preservation of the lenticule for later reimplantation (Mohamed-Noriega, Angunawela, Lim et al.), and a new procedure referred to as endokeratophakia in which a myopic SMILE lenticule is implanted into a hyperopic patient (Pradhan et al.). Finally, studies reporting microdistortions in Bowman's layer and corneal wound healing responses are also described.