Contralateral Eye Comparison Between Femtosecond Small Incision Intrastromal Lenticule Extraction at Depths of 100 and 160 μm

Investigation of the optimal cutting depth in small incision lenticule extraction based on a collagen fibril crimping constitutive model of the cornea

To investigate the optimal cutting depth (Cap) in small incision lenticule extraction from the perspective of corneal biomechanics, a three-dimensional finite element model of the cornea was established using a stromal sub-regional material model to simulate small incision lenticule extraction. The displacement difference PΔ at the central point of the posterior corneal surface before and after lenticule extraction, as well as the von Mises stress at four points of different thicknesses in the center of the cornea, were analyzed using the finite element model considering the hyperelastic property and the difference in stiffness between the anterior and posterior of the cornea. The numerical curves of PΔ-Cap and von Mises Stress-Cap relations at different diopters show that the displacement difference PΔ has a smallest value at the same diopter. In this case, the von Mises stress at four points with different thicknesses in the center of the cornea was also minimal. Which means that the optimal cutting depth exsisting in the cornea. Moreover, PΔ-Cap curves for different depth of stromal stiffness boundaries show that the optimal cap thickness would change with the depth of the stromal stiffness boundary. These results are of guiding significance for accurately formulating small incision lenticule extraction surgery plans and contribute to the advancement of research on the biomechanical properties of the cornea.

Corneal subbasal nerve plexus reinnervation and stromal cell morphology with different cap thicknesses in small incision lenticule extraction

PURPOSE

The corneal cap thickness is a vital parameter designed in small incision lenticule extraction (SMILE). The purpose was to investigate the changes in corneal subbasal nerve plexus (SNP) and stromal cells with different cap thicknesses and evaluate the optimized design for the surgery.

METHODS

In this prospective, comparative, non-randomized study, a total of 108 eyes of 54 patients who underwent SMILE were allocated into three groups with different corneal cap thicknesses (110 μm, 120 μm or 130 μm group). The SNP and stromal cell morphological changes obtained from in vivo corneal confocal microscopy (IVCCM) along with their refractive outcomes were collected at 1 week, 1 month, 3 months and 6 months postoperatively. One-way analysis of variance (ANOVA) was used to compare the parameters among the three groups.

RESULTS

The SNPs in the three groups all decreased after surgery and revealed a gradual increasing trend during the 6-month follow-up. The values of the quantitative nerve metrics were significantly lower in the 110 μm group than in the 120 μm and 130 μm groups, especially at 1 week postoperatively. No difference was detected between the 120 μm and 130 μm groups at any time point. Both Langerhans cells and keratocytes were activated after surgery, and the activation was alleviated during the follow-up.

CONCLUSIONS

The SMILE surgeries with 110 μm, 120 μm or 130 μm cap thickness design achieved good efficacy, safety, accuracy and stability for moderate to high myopic correction while the thicker corneal cap was more beneficial for corneal nerve regeneration.

Comparison of 110- and 145-µm Small-Incision Lenticule Extraction Cap Thickness: A Randomized Contralateral Eye Study

PURPOSE

To investigate the visual, refractive, and biomechanical outcomes of small incision lenticule extraction (SMILE) with two different cap thicknesses.

METHODS

Thirty-four patients were included in this prospective, randomized, contralateral eye study. Subjects were randomized to receive SMILE surgery with a 110-m cap thickness in one eye and 145-µm cap thickness in the fellow eye. Uncorrected and corrected distance visual acuity, contrast sensitivity (CS), total higherorder aberrations (THOAs) and corneal biomechanical properties were compared 3 months after surgery.

RESULTS

Postoperative refractive and visual outcomes, CS and THOAs were similar between the two groups ( P > 0.05 for all parameters). At 3 months postoperatively, there was a significant difference in Corvis ST Biomechanical Index (CBI); stiffness parameter at first applanation (SP A1), and Integrated Radius between the two groups (all P < 0.05).

CONCLUSIONS

Eyes with thicker SMILE corneal caps showed no advantage regarding visual acuity, CS and THOAs over eyes with thinner caps. However, higher cap thickness may result in better corneal biomechanical properties postoperatively.

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.

Corneal Ectasia after Laser-Assisted Small-Incision Lenticule Extraction: The Case for an Enhanced Ectasia Risk Assessment

Purpose

To present a case of asymmetric progressive corneal ectasia following femtosecond laser-assisted small-incision lenticule extraction.

Methods

After obtaining a patient's consent, preoperative and postoperative findings were represented in this case report.

Results

A 29-year-old woman presented with normal preoperative Placido disk-based corneal topography and tomographic findings. The corrected refractive error was -4.00 and -4.50 -1.00 × 177 in the right and left eye, respectively, with a maximal lenticule thickness of 87 and 115 μm OD/OS. Twenty months postoperatively, the patient presented with decreased vision in the left eye and mild ectatic changes in corneal shape in both eyes. The retrospective evaluation of the integrated rotating Scheimpflug tomography (Pentacam; Oculus, Wetzlar, Germany) and corneal biomechanical (Corvis ST) assessment revealed moderate susceptibility for corneal ectasia in the right eye and a significant corneal ectasia in the left eye.

Conclusion

This case corroborates the need for an enhanced multimodal approach to characterize the risk for postoperative corneal ectasia after laser vision correction.

Comparison of visual outcomes between 120-µm and 140-µm cap thicknesses 12 months after small incision lenticule extraction

To compare visual and refractive outcomes as well as changes in high-order aberrations in patients with 120- versus 140-µm cap thicknesses 12 months after small incision lenticule extraction. Ninety-four patients were randomized to receive small incision lenticule extraction with either a 120-µm cap thickness (n = 47) or a 140-µm cap thickness (n = 47) to treat myopia or myopic astigmatism, if not both. In an analysis of right eyes only during the 12-month follow-up period, uncorrected distance visual acuity (UDVA), corrected distance visual acuity (CDVA), refractive outcomes, and high-order aberrations were evaluated. The distribution of patients by age and sex between the groups did not differ significantly (P = .803 and P = .680, respectively). CDVA, spherical and cylindrical refraction, and changes in total high-order aberration, spherical aberration, coma, and trefoil were similar between the groups at 6 and 12 months, postoperatively. However, UDVA was statistically significantly higher in patients with 140-μm cap thicknesses than with 120-μm cap thicknesses at 6 and 12 months postoperatively (P < .001 and P < .001, respectively). Patients with 140-µm cap thickness showed greater improvement in UDVA than ones with 120-µm cap thickness at 12-month follow-up (P = .005). Both 120- and 140-μm cap thicknesses in small incision lenticule extraction were safe and effective thicknesses for correcting myopia or myopic astigmatism. The patients with 140-μm cap thickness had better improvement in UDVA after 12-month follow-up compared to patients with 120-μm cap thickness.

Research Progress on Morphological Changes and Surgery-related Parameters of Corneal Cap in Small Incision Lenticule Extraction

Small incision lenticule extraction (SMILE) is an "all-in-one" surgical method for refractive correction. An advantage of the SMILE over traditional surgery is that it depends on the corneal cap's design. This review discusses the morphological evaluation of the corneal cap, selection of the corneal cap with different thickness and diameters, influence of the corneal cap design on retreatment, and management of corneal cap-related complications. The following points should be recognized to define the correct morphology and design of the operation-related parameters of the corneal cap during SMILE: (1) the thickness and diameter of the corneal cap are predictable and influence postoperative visual quality, (2) the change in anterior surface curvature of the corneal cap should be considered in the design of nomogram value, (3) for patients with moderate myopic correction, early visual quality is better with a 6.9-mm than with a 7.5-mm diameter corneal cap, (4) there is no significant difference in visual quality or biomechanics among corneal caps with different thickness; (5) primary corneal cap thickness plays an important role in the SMILE retreatment, (6) a 7.78-mm diameter corneal cap has a greater risk of suction loss than a 7.60-mm diameter corneal cap, (6) if suction loss occurs when lenticular scanning exceeds 10%, then SMILE can be continued by changing corneal cap thickness, (7) preventive collagen cross-linking with SMILE caps are 90-120 μm thick and 7-7.8 mm in diameter, and (8) properly treating SMILE-related complications ensures better postoperative results. The data presented herein shall deepen the understanding of the importance of the corneal cap during SMILE and provide diversified analysis for personalized operational design of corneal cap parameters.

Five-year results of refractive outcomes and vision-related quality of life after SMILE for the correction of high myopia

AIM

To evaluate the long-term visual, refractive outcomes and vision-related quality of life after small incision lenticule extraction (SMILE) for the correction of high myopia.

METHODS

Thirty patients (60 eyes) with high myopia who underwent SMILE more than 5y were selected as the SMILE group. Another 30 high myopia patients (60 eyes) who had worn corrective spectacles for more than 5y were selected as the control group. In SMILE group, the postoperative follow-up time were 3, 6mo, 1 and 5y. Uncorrected distance visual acuity (UDVA), corrected distance visual acuity (CDVA), spherical equivalent (SE), and ocular axial length (AL) were analyzed. The Chinese version of the National Eye Institute Visual Function Questionnaire-25 (CHI-NEI-VFQ-25) was used to evaluate the vision-related quality of life in the SMILE group and the control group.

RESULTS

In SMILE group, the mean preoperative SE was -7.29±0.87 D (range -6.00 to -9.125 D). At 5-year follow up, the efficacy index and safety index of SMILE were 1.09±0.18 and 1.19±0.12, respectively. Five years postoperatively, 44 eyes (73%) obtained a visual acuity of 20/20 or better. There were no eyes with CDVA loss of one or more Snellen lines. Forty-nine eyes (82%) and 57 eyes (95%) were within ±0.50 and ±1.00 D of attempted correction at 5-year follow-up, respectively. Forty-eight eyes (80%) had astigmatism <0.50 D at 5-year follow-up. The postoperative mean SE values at 3, 6mo, 1, and 5y were 0.11±0.44, 0.07±0.45, -0.02±0.41, and -0.15±0.46 D, respectively. No significant change was observed in the ocular AL from before operation to 5y postoperatively (26.08±0.96 mm vs 26.01±0.94 mm, P=0.068). Compared to the control group, the SMILE group showed a significantly higher total score on the CHI-NEI-VFQ-25 (90.14 vs 81.43, P<0.001).

CONCLUSION

In the present study, in a long-term follow-up we demonstrate that correcting high myopia with SMILE is safe, effective, and predictable. Vision-related quality of life after SMILE is better in the SMILE group than in the control group who wore corrective spectacles.

Comparison of Clinical and Biomechanical Outcomes of Small Incision Lenticule Extraction With 120- and 140-µm Cap Thickness

Purpose

The purpose of this study was to compare the clinical outcomes and corneal biomechanical changes between 120-µm and 140-µm cap thickness after small incision lenticule extraction (SMILE).

Methods

This prospective study included 150 eyes (150 patients: 91 eyes in the 120-µm group, and 59 eyes in the 140-µm group) who underwent SMILE. Enhanced correction nomograms were applied for patients according to cap thickness. Clinical outcomes, including visual acuity, refraction, and corneal wavefront aberrations, were compared between the two groups. Corneal biomechanics were evaluated using the Corvis ST (Oculus, Wetzlar, Germany).

Results

The mean uncorrected-distance visual acuity, safety and efficacy indices, and refractive predictability were comparable in the 120-µm and 140-µm groups after SMILE. The postoperative total corneal root mean square higher-order aberrations (HOAs) and spherical aberrations was 0.48 ± 0.31 and 0.26 ± 0.10 in the 120-µm group, and 0.53 ± 0.16 and 0.34 ± 0.13 in the 140-µm group, which showed significant differences between the two groups (P = 0.027, and <0.001, respectively). Although corneal stiffness decreased after SMILE in both groups, the changes in the deformation amplitude ratio were significantly higher in the 140-µm group than in the 120-µm group (P = 0.022).

Conclusions

SMILE with 120-µm and 140-µm cap thickness provided excellent predictable outcomes according to our enhanced correction nomogram. The amount of tissue removal required to achieve the same amount of refractive correction was greater in the thicker cap group. The induction of corneal HOAs and weakening of corneal biomechanics were less pronounced in the thin-cap group, which may be associated with the thinner cap, lesser lenticule thickness, or thicker residual stromal bed.

Translational Relevance

Although SMILE with different cap thickness was effective, thicker lenticule thickness in the thick-cap group may be associated with induction of HOAs, and corneal stiffness changes.

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.

Adjustment of Spherical Equivalent Correction According to Cap Thickness for Myopic Small Incision Lenticule Extraction

PURPOSE

To evaluate the amount of spherical equivalent correction for three different cap thicknesses (120, 130, and 140 µm) during myopic small incision lenticule extraction (SMILE) and determine the association between the amount of spherical equivalent correction and several variables in each cap thickness group.

METHODS

In this retrospective, comparative, observational case series study, the authors compared refractive errors, keratometric values, laser setting (sphere correction, cylinder correction, spherical equivalent correction, optical zone, and cap diameter), and spherical aberration measured preoperatively and at 3 months postoperatively between three different cap thickness groups: 120 µm (n = 554), 130 µm (n = 377), and 140 µm (n = 90). Multiple linear regression analyses were used to determine the associations between the amount of spherical equivalent correction and several variables, including age, preoperative spherical equivalent, optical zone diameter, central corneal thickness, preoperative mean keratometric values, and preoperative corneal asphericity.

RESULTS

According to cap thickness, attempted correction is adjusted to achieve the same refractive outcomes for different cap thicknesses. There were significant differences in the amount of sphere correction and spherical equivalent correction, as well as lenticule thickness, among subgroups. Changes in keratometric values, corneal asphericity, and spherical aberration were also significantly different among subgroups (all P < .001). Changes in keratometric values, corneal asphericity, and spherical aberration significantly increased as cap thickness increased. Preoperative spherical equivalent mainly influenced the amount of spherical equivalent correction in each group.

CONCLUSIONS

Dioptric adjustment of spherical equivalent correction according to cap thickness is essential to obtain similar refractive outcomes in myopic SMILE procedures. [J Refract Surg. 2019;35(3):153-160.].

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.

[SMILE (Small Incision Lenticule Extraction) among the corneal refractive surgeries in 2018 (French translation of the article)]

Refractive surgery is a field in constant evolution. In recent years, a new procedure has appeared under the name SMILE (Small Incision Lenticule Extraction). This technique, carried out solely with a femtosecond laser, should make it possible to better preserve corneal innervation and biomechanics. After a detailed review of the technique itself, we then focus on the scientific evidence for the safety and efficacy of SMILE and its current indications. Advantages of SMILE will be discussed in comparison to disadvantages of the conventional techniques, particularly concerning dry eye and the risk of corneal ectasia with LASIK. Lastly, the current limitations of SMILE (indications, retreatment) are discussed, and future applications are considered regarding improvements in the technique.

Corneal Cap Thickness and Its Effect on Visual Acuity and Corneal Biomechanics in Eyes Undergoing Small Incision Lenticule Extraction

Purpose

To evaluate the effect of corneal cap thickness on visual acuity and corneal biomechanics in small incision lenticule extraction (SMILE) for the treatment of myopia.

Methods

Forty eyes of 20 patients undergoing SMILE for the treatment of myopia were enrolled in this prospective controlled study. The patients with 510 μm–560 μm central corneal thickness (CCT) and a refractive spherical equivalent of −3.00 D to −8.00 D were included. It was designed randomly to undergo SMILE with a 110 μm cap thickness in one eye and 150 μm cap thickness in the other. Ophthalmic examinations included best-corrected and uncorrected visual acuity (UCVA); refractive status, contrast sensitivity, and objective visual quality were evaluated at 2 h, 4 h, and 24 h postoperatively; while at 3 months after the procedure, corrected intraocular pressure (IOP), higher order aberrations (HOAs), and morphologic modifications of corneal architecture of both eyes were assessed.

Results

Compared with the 150 μm group, the incidence of OBL was significantly higher in the 110 μm cap thickness group (P=0.004), and UCVA, Strehl ratio (SR), objective scatter index (OSI), modulation transfer function (MTF) cutoff frequency, and photopic and scotopic contrast sensitivity at medium spatial frequency were all significantly better in 110 μm group at 2 h and 24 h postoperatively (P < 0.05). Corneal spherical aberration and corrected IOP by Corvis ST were significantly higher in the 110 μm group at 3 months postoperatively (P < 0.05). No statistically significant differences were found in manifest refraction, UCVA, SR, OSI, MTF cutoff, and mesopic and photopic contrast sensitivity at low frequency, photopic contrast sensitivity at high frequency, endothelial density, corneal coma, and total HOAs at 3 months after the procedure. No visual decline was found in the patients in this study.

Conclusions

Both 110 μm and 150 μm cap thickness in SMILE were safe and effective for treatment of myopia. A 110 μm cap thickness demonstrated better visual outcomes during early and late postoperative periods with higher corneal spherical aberration and corrected IOP at 3 months postoperatively. This trial is registered with ChiCTR-IOR-17013369.

SMILE (Small Incision Lenticule Extraction) among the corneal refractive surgeries in 2018

Refractive surgery is a field in constant evolution. In recent years, a new procedure has appeared under the name SMILE (SMall Incision Lenticule Extraction). This technique, carried out solely with a femtosecond laser, should make it possible to better preserve corneal innervation and biomechanics. After a detailed review of the technique itself, we then focus on the scientific evidence for the safety and efficacy of SMILE and its current indications. Advantages of SMILE will be discussed in comparison to the conventional techniques, particularly concerning dry eye and the risk of corneal ectasia related to LASIK. Lastly, the current limitations of SMILE (indications, retreatment) are discussed, and future applications are considered regarding new improvements in the technique.

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.

Comparison of 120- and 140-μm SMILE Cap Thickness Results in Eyes With Thick Corneas

PURPOSE

To evaluate clinical outcomes after small incision lenticule extraction (SMILE) with different cap thicknesses in thick corneas.

METHODS

Forty patients with central corneal thickness of more than 560 μm were recruited in this prospective, randomized, masked, paired-eye study. Patients were randomized to receive SMILE with a 120-μm cap thickness in 1 eye and 140-μm cap thickness in the other. Uncorrected and corrected distance visual acuity (CDVA), contrast sensitivity (CS), higher-order aberrations (HOAs), and morphologic modifications of corneal architecture were measured during the 3-month follow-up period.

RESULTS

Postoperative refractive outcomes, visual outcomes, CS, and the changes in HOAs were similar between both groups. The persistence of brightly reflective particles in the corneal interface layer was 1388.6 ± 219.5/mm in eyes with 120-μm cap thickness and 54.7 ± 8.6/mm in eyes with 140-μm cap thickness (P < 0.001). The hyperreflectivity line at the interface layer almost disappeared in all eyes with 140-μm cap thickness, and it still persisted in 43% of the fellow eyes at 3 months postoperatively. The anterior surfaces of lenticules in the 140-μm cap thickness group exhibited more smoothness than in the 120-μm cap thickness group.

CONCLUSIONS

There was a lower level corneal wound-healing response after SMILE with a 140-μm cap thickness than with a 120-μm cap thickness, although the thickness of cap creation did not affect visual outcomes by 3 months postoperatively.

Comparison of Two Cap Thickness in Small Incision Lenticule Extraction: 100μm versus 160μm

PURPOSE

To compare the changes of biomechanical properties, endothelial cell density (ECD), and posterior corneal elevation (PCE) after femtosecond small incision lenticule extractions (SMILEs) with 100μm versus 160μm cap thicknesses.

METHODS

A total of 12 rabbits were randomly assigned into two groups of 6 each. SMILE was performed at a depth of either 160μm (160-cap group) or 100μm (100-cap group). Corneal biomechanics, PCE, ECD were evaluated pre-operatively, 1week, 1 month, 2 months, 3 months, and 4 months post-operatively by Pentacam, Corvis ST, in vivo confocal microscopy (IVCM) respectively. The Young's modulus was obtained by strip-extensometry test 4 months after surgery.

RESULTS

At each time point, the second applanation time (A2T) was similar between the groups with the exception of 4 months after surgery (22.66±0.16 ms in the 160-cap group versus 21.75±0.29 ms in the 100-cap group, p = 0.004). Neither deformation amplitude (DA) nor the first applanationtime (A1T) were significantly different between the two groups. The postoperative posterior surface did not shift forward, the changes of PCE and ECD were not significantly different between the two groups at any observation time. Young's modulus was higher in the 160-cap group than that in the 100-cap group with no statistical significance (P>0.05). Regression analyses showed that the PCE changes and Young's modulus were not affected by cap thickness (CT) or residual stromal bed thickness (RBT) (All P>0.05).

CONCLUSIONS

The differences of corneal biomechanics, posterior surface elevation, or ECD changes were quite small when using 100μm or 160μm cap thicknesses in SMILE.