Journal of Refractive Surgery

Original Article 

Predictors for the Outcome of Small-incision Lenticule Extraction for Myopia

Jesper Ø. Hjortdal, MD, DrMedSci, PhD; Anders H. Vestergaard, MD; Anders Ivarsen, MD, PhD; Suganiah Ragunathan, MD; Sven Asp, MD, DrMedSci

Abstract

Click here to read the Letter to the Editor about this article

PURPOSE:

To study the influence of patient- and surgery-related parameters on the predictability, efficacy, and safety of small-incision lenticule extraction (SMILE) for treatment of myopia.

METHODS:

This prospective, clinical quality, control study comprised 670 eyes from 335 patients with myopia up to −10.00 diopters (D) (spherical equivalent refraction) and astigmatism up to 2.00 D treated with SMILE in both eyes and followed for 3 months.

RESULTS:

Preoperative mean spherical equivalent refraction was −7.19±1.30 D. In eyes with emmetropia as the target refraction, 84% obtained uncorrected distance visual acuity ⩽0.10 logMAR (20/25 or better Snellen equivalent) at 3 months. Mean corrected distance visual acuity (CDVA) improved from −0.03 to −0.05 logMAR (P<.01). Two or more lines of CDVA were lost in 2.4% (16 eyes). The achieved refraction was 0.25±0.44 D less than attempted after 3 months, and 80% (537 eyes) and 94% (631 eyes) were within ±0.50 and ±1.00 D of attempted correction, respectively. Multiple linear regression analyses revealed that spherical equivalent refraction undercorrection was predicted by increasing patient age (0.10 D per decade; P<.01) and steeper corneal curvature (0.04 D per D; P<.01). Safety and efficacy of the procedure were minimally affected by age, gender, and simultaneous cylinder correction.

CONCLUSIONS:

Undercorrection of 0.25 D and small effects of patient age and corneal curvature suggest that the standard nomogram for SMILE need only minor adjustments. This study suggests that safety and efficacy are not influenced to any clinically significant degree by easily discernible patient factors.

From the Department of Ophthalmology, Aarhus University Hospital, Aarhus, Denmark.

Drs Hjortdal and Asp receive travel reimbursement from Carl Zeiss Meditec, Jena, Germany. The remaining authors have no financial interest in the materials presented herein.

Preliminary results were presented at the Carl Zeiss Meditec European user meeting, Cyprus, Greece, and the Asian-Pacific user meeting, Hangzhou, China, June 2012. Presented as a free paper at the Refractive Surgery Subspecialty Day Meeting of the International Society of Refractive Surgery, Chicago, Illinois, November 2012.

AUTHOR CONTRIBUTIONS

Study concept and design (J.Ø.H.); data collection (J.Ø.H., A.I., S.R., S.A.); analysis and interpretation of data (J.Ø.H., A.H.V.); drafting of the manuscript (J.Ø.H., S.R.); critical revision of the manuscript (J.Ø.H., A.H.V., A.I., S.A.)

Correspondence: Jesper Ø. Hjortdal, MD, DrMedSci, PhD, Dept of Ophthalmology, Aarhus University Hospital, Nørrebrogade 44, 8000 Aarhus C, Denmark. Tel: 45 78463221; Fax: 45 86121653; E-mail: jesper.hjortdal@dadlnet.dk

Received: August 11, 2012
Accepted: October 23, 2012

Abstract

Click here to read the Letter to the Editor about this article

PURPOSE:

To study the influence of patient- and surgery-related parameters on the predictability, efficacy, and safety of small-incision lenticule extraction (SMILE) for treatment of myopia.

METHODS:

This prospective, clinical quality, control study comprised 670 eyes from 335 patients with myopia up to −10.00 diopters (D) (spherical equivalent refraction) and astigmatism up to 2.00 D treated with SMILE in both eyes and followed for 3 months.

RESULTS:

Preoperative mean spherical equivalent refraction was −7.19±1.30 D. In eyes with emmetropia as the target refraction, 84% obtained uncorrected distance visual acuity ⩽0.10 logMAR (20/25 or better Snellen equivalent) at 3 months. Mean corrected distance visual acuity (CDVA) improved from −0.03 to −0.05 logMAR (P<.01). Two or more lines of CDVA were lost in 2.4% (16 eyes). The achieved refraction was 0.25±0.44 D less than attempted after 3 months, and 80% (537 eyes) and 94% (631 eyes) were within ±0.50 and ±1.00 D of attempted correction, respectively. Multiple linear regression analyses revealed that spherical equivalent refraction undercorrection was predicted by increasing patient age (0.10 D per decade; P<.01) and steeper corneal curvature (0.04 D per D; P<.01). Safety and efficacy of the procedure were minimally affected by age, gender, and simultaneous cylinder correction.

CONCLUSIONS:

Undercorrection of 0.25 D and small effects of patient age and corneal curvature suggest that the standard nomogram for SMILE need only minor adjustments. This study suggests that safety and efficacy are not influenced to any clinically significant degree by easily discernible patient factors.

From the Department of Ophthalmology, Aarhus University Hospital, Aarhus, Denmark.

Drs Hjortdal and Asp receive travel reimbursement from Carl Zeiss Meditec, Jena, Germany. The remaining authors have no financial interest in the materials presented herein.

Preliminary results were presented at the Carl Zeiss Meditec European user meeting, Cyprus, Greece, and the Asian-Pacific user meeting, Hangzhou, China, June 2012. Presented as a free paper at the Refractive Surgery Subspecialty Day Meeting of the International Society of Refractive Surgery, Chicago, Illinois, November 2012.

AUTHOR CONTRIBUTIONS

Study concept and design (J.Ø.H.); data collection (J.Ø.H., A.I., S.R., S.A.); analysis and interpretation of data (J.Ø.H., A.H.V.); drafting of the manuscript (J.Ø.H., S.R.); critical revision of the manuscript (J.Ø.H., A.H.V., A.I., S.A.)

Correspondence: Jesper Ø. Hjortdal, MD, DrMedSci, PhD, Dept of Ophthalmology, Aarhus University Hospital, Nørrebrogade 44, 8000 Aarhus C, Denmark. Tel: 45 78463221; Fax: 45 86121653; E-mail: jesper.hjortdal@dadlnet.dk

Received: August 11, 2012
Accepted: October 23, 2012

 

Recently, small-incision lenticule extraction (SMILE) has been developed as a corneal refractive flap-free procedure in which an intrastromal lenticule is cut by a femtosecond laser and manually extracted through a peripheral corneal tunnel incision.1,2 It is anticipated that the procedure could reduce some of the potential side effects of LASIK, such as eye dryness, epithelial ingrowth at the flap edge,3 long-term risk for traumatic flap dislocation,4 and corneal ectasia, which may develop during the years after surgery.5 In the published studies to date, the refractive predictability, safety, and patient satisfaction of SMILE are high and comparable to femtosecond laser–assisted LASIK.1,2,6–9

The purpose of the present study was to investigate whether patient-specific and surgery-related factors influence the predictability, efficacy, and safety of the procedure.

Patients and Methods

Small-incision lenticule extraction has been performed at the Department of Ophthalmology, Aarhus University Hospital, Denmark, since January 2011.

All patients referred to the department for corneal refractive surgery underwent a preoperative examination including autokeratometry and autorefractometry, tonometry, and pupillometry (TONOREF II; NIDEK Co Ltd, Gamagori, Japan), Pentacam HR corneal tomography (Oculus Optikgeräte GmbH, Wetzlar, Germany), central corneal thickness measurement by laser interferometry (OLCR; Haag Streit, Koeniz, Switzerland), measurement of uncorrected (UDVA) and corrected (CDVA) distance visual acuity, measurement of manifest and cycloplegic clinical refraction, and slit-lamp examination and funduscopy in mydriasis. Patients with normal corneal topography (based on evaluation of Pentacam HR tomographs) and central corneal thickness >480 μm were informed about the outcome of laser refractive procedures, including side effects and complications. After informed consent, patients were offered SMILE treatment. The study was conducted in agreement with the tenets of the Declaration of Helsinki.

For the present study, only patients with preoperative spherical equivalent (SE) myopia up to −10.00 diopters (D) and refractive astigmatism up to 2.00 D treated by uneventful SMILE surgery in both eyes were included.

Surgical Procedure

A VisuMax 500-kHz femtosecond laser (Carl Zeiss Meditec) was used for the SMILE treatment. Surgery was performed bilaterally and under topical anesthesia using two drops of 0.8% oxybuprocaine tetrachloride applied 5 and 1 minute before surgery. The patient was positioned under the curved contact glass of the VisuMax femtosecond laser and asked to fixate on a blinking target. When appropriate centration (center of pupil) was observed, suction was applied to the contact glass. Two laser energy settings were used. In setting 1, a laser cut energy index of 25 (corresponding to approximately 125 mJ) and a spot spacing of 2.5 μm was used. In setting 2, a cut energy index of 34 (equivalent to an energy of approximately 170 mJ) and a spot spacing of 4.5 μm was used. Setting 1 was used in 458 eyes and setting 2 in 212 eyes. The lenticule front cut was extended towards the surface to create a 40° to 60° incision located at the 12-o’clock position. Lenticule diameter (optical zone) was 6.0 to 6.5 mm, and the cap diameter was 7.3 mm. Intended cap thickness was 110 to 120 μm. After laser treatment, a thin blunt spatula was used to break the remaining tissue bridges and loosen the stromal lenticule, which afterwards was grasped with a pair of forceps and removed. The corneal stromal pocket was flushed with saline. All patients received one drop of chloramphenicol and one drop of diclofenac (Voltaren Ophtha; Novartis Healthcare, Copenhagen, Denmark) at the end of the procedure.

Postoperative Treatment and Follow-up

The postoperative regimen included fluorometholone (Flurolon; Allergan Pharmaceuticals, County Mayo, Ireland) and chloramphenicol drops four times a day for 1 week, followed by two times a day for 1 week. Patients were encouraged to use lubricating drops.

On the first day after surgery, UDVA was measured and slit-lamp examination was performed. Patients were recommended to visit their private ophthalmologist 1 week after surgery and to contact the department by phone or e-mail if needed. At 3 months after surgery, patients returned to the department for a thorough follow-up examination with measurement of the same parameters as before surgery.

Outcome Parameters and Predictors

Predictability was evaluated by calculation of the error in SE correction: the attempted change subtracted by the achieved change in SE refraction.

Safety was evaluated by calculation of the difference in CDVA before surgery and 3 months after surgery (logMAR) and by a safety index: CDVA 3 months after surgery/CDVA before surgery (decimal notation).

Efficacy was evaluated by the change in CDVA before surgery to UDVA at 1 day and 3 months after surgery (logMAR) and by an efficacy index: UDVA 3 months after surgery/CDVA before surgery (decimal notation). Efficacy was only evaluated for patients targeted to be within ±0.25 D of emmetropia.

Statistical Analysis

Unpaired t tests (dichotomous variables), bivariate correlation analysis (continuous variables), and multiple linear regression analyses were used for evaluation of the effect of possible predictors for the outcome parameters predictability, efficacy, and safety, which were assumed to follow a normal distribution.

The effect of patient age and sex, eye (right/left), corneal curvature and central thickness, intraocular pressure, attempted change in spherical equivalent refraction, and laser correction for astigmatism (yes/no) were analyzed as independent variables. In addition, the two femtosecond laser settings and the difference in millimeters between pupil center and lenticule center were studied to determine whether they influenced the outcome parameters. This difference was measured manually in GIMP ( http://www.gimp.org) from calibrated video capture images included in the exported standard operative report PDF files of the VisuMax laser.

Statistical analyses were performed in SPSS version 11 (SPSS Inc, Chicago, Illinois). Only variables with a significant (P<.05) bivariate correlation (continuous variables) or significant unpaired t test results (dichotomous variables) were included in the multiple linear regression analysis (stepwise model with inclusion criteria: P<.05, and exclusion criteria: P>.10).

Results

From January 2011 to March 2012, 790 eyes from 395 patients fulfilled these criteria. All patients were seen on the first day after surgery, but only 335 patients (670 eyes) attended 3-month follow-up. The 60 patients who did not attend 3-month follow-up were slightly, but significantly, younger (36.3 vs 38.3 years of age, P<.05 [unpaired t test]) and less myopic (−6.87 vs −7.17 D spherical equivalent refraction, P<.05 [unpaired t test]), but similar with respect to cylinder correction, corneal power, intraocular pressure, central corneal thickness, and UDVA on the first day after surgery. Demographic, refractive, and biometric characteristics for the study group are shown in Table 1. In addition, 135 of the treated patients were men and 200 were women. In 393 eyes, the target refraction was emmetropia (±0.25 D). In patients older than 40 years, a small residual myopic correction was aimed for to avoid overcorrection. Cylinder correction up to 2.00 D was attempted in 521 eyes. None of the eyes underwent retreatment.

Characteristics of Patients Undergoing SMILE

Table 1: Characteristics of Patients Undergoing SMILE

Preoperative demographic, refractive, and biometric characteristics of the 670 eyes are summarized in Table 1. No significant difference was noted between any of the parameters for right and left eyes.

Figure 1 shows the relationship between attempted and achieved change in spherical equivalent refraction as well as the error in spherical equivalent refractive correction versus the attempted correction. The relationship between attempted and achieved correction is high with a correlation coefficient of 0.95 (see Fig 1A). The dependency between the error in treatment and the attempted refractive change was not significant (R2=0.004, P=.08) (see Fig 1B).

A) Achieved (3 months after surgery) versus attempted change in spherical equivalent refraction in 670 eyes from 335 patients treated with small-incision lenticule extraction (SMILE). B) Error in spherical equivalent refraction (attempted subtracted by achieved change). Correlation coefficient R2 and linear regression results are shown.

Figure 1. A) Achieved (3 months after surgery) versus attempted change in spherical equivalent refraction in 670 eyes from 335 patients treated with small-incision lenticule extraction (SMILE). B) Error in spherical equivalent refraction (attempted subtracted by achieved change). Correlation coefficient R2 and linear regression results are shown.

The predictability of all surgeries is illustrated in Figure 2. Of all eyes, 80.1% (537 eyes) were within ±0.50 D and 94.2% (631 eyes) were within ±1.00 D of the attempted refraction 3 months after surgery. The mean error in spherical equivalent refraction 3 months after surgery was −0.25±0.44 D (range: −2.13 to 1.38 D).

Distribution of the error in spherical equivalent refraction (attempted subtracted by achieved change) in 670 eyes from 335 patients treated with SMILE.

Figure 2. Distribution of the error in spherical equivalent refraction (attempted subtracted by achieved change) in 670 eyes from 335 patients treated with SMILE.

Safety is illustrated in Figure 3. In 83.1% (557 eyes), CDVA was the same or had improved after surgery; 2.4% (16 eyes) lost ⩾2 lines of CDVA whereas 3.7% (25 eyes) gained ⩾2 lines. On average, mean CDVA (logMAR) improved significantly from −0.028±0.087 before surgery to −0.049±0.097 3 months after surgery (paired t test, P<.01). The safety index was 1.07±0.22.

Distribution of the change in Snellen lines of corrected distance visual acuity (CDVA) in 670 eyes from 335 patients treated with SMILE.

Figure 3. Distribution of the change in Snellen lines of corrected distance visual acuity (CDVA) in 670 eyes from 335 patients treated with SMILE.

The frequency distributions of UDVA 1 day and 3 months after surgery in comparison with CDVA before surgery in eyes targeted for emmetropia (393 eyes) are shown in Figure 4. In 62.3% (245 eyes), UDVA was 20/25 or better on the first day, improving to 84% (330 eyes) 3 months after surgery. Uncorrected distance visual acuity was 20/40 or better in 92.8% (365 eyes) on the first day and in 97.2% (382 eyes) 3 months after surgery.

A) Cumulated Snellen corrected distance visual acuity (CDVA) before surgery and uncorrected distance visual acuity (UDVA) after surgery in 393 eyes from 197 patients treated with SMILE targeted for emmetropia. B) Distribution of the change in Snellen lines of CDVA before surgery to UDVA at 3 months.

Figure 4. A) Cumulated Snellen corrected distance visual acuity (CDVA) before surgery and uncorrected distance visual acuity (UDVA) after surgery in 393 eyes from 197 patients treated with SMILE targeted for emmetropia. B) Distribution of the change in Snellen lines of CDVA before surgery to UDVA at 3 months.

The efficacy index was calculated to be 0.90±0.25 at 3 months after surgery. The results of the bivariate analyses on the effect of predictors for predictability, safety, and efficacy are summarized in Table 2. The error in spherical equivalent refraction at 3 months after surgery was significantly influenced by patient age and gender and corneal power. The results of the multiple linear regression analysis are shown in Table 3 and the model could explain 9.0% of the variation in the error in spherical equivalent refraction at 3 months. Patient age was the most important predictor resulting in an undercorrection of 0.012 D per increasing year of age. Increasing preoperative corneal power influenced the refractive outcome by an undercorrection of 0.041 D per diopter of corneal steepening. Female gender was significantly predictive for an undercorrection of 0.085 D.

Bivariate Correlation Coefficients (Pearson) for Continuous Variables and Mean Values for Dichotomous Predictors in Patients Undergoing SMILE

Table 2: Bivariate Correlation Coefficients (Pearson) for Continuous Variables and Mean Values for Dichotomous Predictors in Patients Undergoing SMILE

Multiple Linear Regression Analyses in Patients Undergoing SMILE

Table 3: Multiple Linear Regression Analyses in Patients Undergoing SMILE

The change in CDVA correlated with eye side and presence of astigmatic correction (Table 2). In the multiple linear regression analysis, presence of cylinder correction was associated with a small worsening in CDVA in addition to surgery in left eyes compared to right eyes (Table 3). The two variables could explain 2.2% of the variation in CDVA change.

The change in preoperative CDVA to UDVA at 3 months after surgery was significantly affected by gender and age (Table 2), and the two variables continued to be significant in the multiple linear regression model, explaining 6.4% of the variation (Table 3): female gender was associated with poorer UDVA at 3 months compared with CDVA before surgery and increasing age was associated with inferior outcome.

Uncorrected distance visual acuity 1 day after surgery (characterized as change in preoperative CDVA to UDVA at 1 day) correlated with eye side, age, and energy setting of the laser (Table 2). In the multiple linear regression analysis, only eye side and laser settings were significant, explaining 10.1% of the variation (Table 3).

The dependency between predictability, safety, and efficacy in right and left eyes of the patient was investigated. The correlation coefficients were slightly higher than observed for the outcome predictors, amounting to 0.16 (predictability), 0.32 (safety), and 0.28 (efficacy), and all were significantly different from zero (P<.01). This means that 2.7%, 10.0%, and 8.0% of the variation in spherical equivalent treatment error, change in CDVA, and change from CDVA before surgery to UDVA after surgery, respectively, in one eye could be explained by the outcome in the other eye.

Discussion

Small-incision lenticule extraction is a fairly new technique for corneal refractive surgery. Recent publications,1,2,6–9 including up to 150 patients, suggest this femtosecond laser flap-free refractive procedure has predictability, safety, and efficacy similar to femtosecond laser–assisted LASIK. The present study involving treatment of moderate and high myopia in 670 eyes from 335 patients confirms that SMILE is predictable, effective, and safe.

During the past 20 years, studies have documented that the efficiency of excimer laser ablation is affected by several factors such as the ablation parallax whereby the effective treatment depends on the curvature of the cornea and corneal hydration.10 Nomogram adjustments based on empirical data analysis are often implemented11 and additional adjustments based on wavefront refractions have also been shown to be useful.12

In the present study, the relation between attempted and achieved correction was high and the error in spherical equivalent correction did not depend on the degree of myopic correction or the presence of simultaneous astigmatic correction. The average difference between achieved and attempted spherical equivalent correction was 0.25 D of undercorrection, which may be added when planning SMILE treatments. Patient age and corneal curvature significantly influenced the error in spherical equivalent correction. The effect of patient age amounted to approximately 0.10 D of undercorrection per decade of increasing age, which contrasts with excimer laser–based procedures where overcorrection may be seen with increasing age.10 It can be speculated whether subtle, age-related changes in corneal biomechanical stiffness affect the actual shape of the intrastromal refractive lenticule that is removed. Alternatively, age-dependent differential effects on corneal wound healing and stromal or epithelial thickening may explain this small effect. Steeper corneas were also associated with a small undercorrection corresponding to approximately 0.04 D per diopter of increasing curvature. Gender and eye side had slight statistically significant effects, but these seem clinically insignificant and should be confirmed in other, even larger, studies. Overall, the present lenticule excision nomogram for SMILE spherical equivalent refraction corrections does not need to be adjusted for higher myopic corrections or for the presence of simultaneous astigmatic corrections up to 2.00 D. Although the effect of corneal shape is small, it may be considered to adjust the planned treatment by 0.25 D in very steep (increase) or very flat (decrease) corneas.

Retreatments were not performed during the study period, and it remains to be established how these are best performed. Photorefractive keratectomy, LASIK, laser-based opening of the cap with subsequent laser ablation, or a new SMILE procedure located deeper in the cornea may be possible retreatment options.

The safety was overall high with a safety index of 1.07, indicating that CDVA on average was higher after SMILE than before surgery. This is to be expected due to the image magnification effect of corneal refractive procedures. Of the treated eyes, 2.4% (16 eyes) lost ⩾2 lines of CDVA, of which 4 (0.6%) eyes lost 3 lines (1 eye in each of 4 patients; in every case, CDVA reduced from 0.0 to 0.3 logMAR [20/20 to 20/40 Snellen equivalent]). Multiple linear regression analyses revealed no clinically significant influence of any predictors, as the effect of the two statistically significant predictors each equaled approximately 1 letter on an ETDRS chart. Interestingly, the amount of attempted spherical correction and patient age did not influence the safety of the SMILE procedure.

The value of the efficacy index suggests that patients undergoing SMILE on average can expect an uncorrected visual acuity of 90% of their preoperative corrected visual acuity. Only patient age and gender correlated statistically significantly with efficacy, but the influence was clinically insignificant. On the other hand, the efficacy of the procedure did not depend on the amount of attempted spherical correction.

In contrast to studies of the dependency between two eyes of the same patient undergoing bilateral LASIK,13 the correlation between the error in spherical refractive correction in left and right eyes was low and at the same magnitude as the preoperative predictors (age, corneal power). Changes in laser settings from a low energy, close spot, slow (45 seconds) setting to a faster (28 seconds), slightly higher energy, wider spot spacing laser setting had, however, no significant influence on the predictability of the procedure. Similarly, the magnitude of decentration of the lenticule with respect to the pupil center did not influence predictability, possibly because the variation in decentration was small. Further standardization of the manual dissection technique during the operative procedure may further reduce the variation in outcomes. Femtosecond laser settings had an effect on UDVA on the first day after surgery. Visual acuity was better with the faster setting having slightly wider (4.5 μm) spot spacing (setting 2). The laser settings did not influence the safety or efficacy measured 3 months after surgery.

Multiple linear regression analysis has been widely used for nomogram optimization of excimer lasers. Two eyes of the same patient cannot be assumed to be independent and usually only one eye should be included for statistical analysis. In this study, the statistical dependency between eyes was found to be low. Additional correlation analyses using only one randomly chosen eye from each patient confirmed the main findings of this study. However, results from both eyes were presented, as this gave a more precise estimate of the coefficients in linear regression analysis.

Small-incision lenticule extraction was found to be acceptably predictable, safe, and efficient in this study of 670 surgeries for moderate and high myopia in 335 patients. The present treatment nomogram may need only minor corrections based on the findings of a 0.25-D undercorrection and small confounding effects of patient age and corneal curvature. Safety and efficacy were, from a clinical perspective, found to be independent of spherical treatment attempts up to 10.00 D of myopia and were not influenced by patient age (range: 19 to 58 years). Further studies are needed to investigate surgical or individual biological sources of variations in the outcome after SMILE.

References

  1. Sekundo W, Kunert KS, Blum M. Small incision corneal refractive surgery using the small incision lenticule extraction (SMILE) procedure for the correction of myopia and myopic astigmatism: results of a 6 month prospective study. Br J Ophthalmol. 2011;95(3):335–339. doi:10.1136/bjo.2009.174284 [CrossRef]
  2. Shah R, Shah S, Sengupta S. Results of small incision lenticule extraction: all-in-one femtosecond laser refractive surgery. J Cataract Refract Surg. 2011;37(1):127–137. doi:10.1016/j.jcrs.2010.07.033 [CrossRef]
  3. Toda I. LASIK and the ocular surface. Cornea. 2008;27(Suppl 1):S70–S76. doi:10.1097/ICO.0b013e31817f42c0 [CrossRef]
  4. Kim HJ, Silverman CM. Traumatic dislocation of LASIK flaps 4 and 9 years after surgery. J Refract Surg. 2010;26(6):447–452. doi:10.3928/1081597X-20090710-03 [CrossRef]
  5. Randleman JB, Woodward M, Lynn MJ, Stulting RD. Risk assessment for ectasia after corneal refractive surgery. Ophthalmology. 2008;115(1):37–50. doi:10.1016/j.ophtha.2007.03.073 [CrossRef]
  6. Sekundo W, Kunert K, Russmann C, et al. First efficacy and safety study of femtosecond lenticule extraction for the correction of myopia: six-month results. J Cataract Refract Surg. 2008;34(9):1513–1520. doi:10.1016/j.jcrs.2008.05.033 [CrossRef]
  7. Blum M, Kunert K, Schröder M, Sekundo W. Femtosecond lenticule extraction for the correction of myopia: preliminary 6-month results. Graefes Arch Clin Exp Ophthalmol. 2010;248(7):1019–1027. doi:10.1007/s00417-009-1293-1 [CrossRef]
  8. Vestergaard A, Ivarsen A, Asp S, Hjortdal J. Femtosecond (FS) laser vision correction procedure for moderate to high myopia: a prospective study of ReLEx flex, and comparison with a retrospective study of FS-laser in situ keratomileusis [published online ahead of print April 18, 2012]. Acta Ophthalmol. doi:
  9. Vestergaard A, Ivarsen A, Asp S, Hjortdal J. ReLEx smile for moderate to high myopia: a prospective study of predictability, safety and patient satisfaction. J Cataract Refract Surg. 2012;38(11):2003–2010. doi:10.1016/j.jcrs.2012.07.021 [CrossRef]
  10. Kim WS, Jo JM. Corneal hydration affects ablation during laser in situ keratomileusis surgery. Cornea. 2001;20(4):394–397. doi:10.1097/00003226-200105000-00011 [CrossRef]
  11. Huang D, Stulting RD, Carr JD, Thompson KP, Waring GO III, . Multiple regression and vector analyses of laser in situ keratomileusis for myopia and astigmatism. J Refract Surg. 1999;15(5):538–549.
  12. Liyanage SE, Allan BD. Multiple regression analysis in myopic wavefront laser in situ keratomileusis nomogram development. J Cataract Refract Surg. 2012;38(7):1232–1239. doi:10.1016/j.jcrs.2012.02.043 [CrossRef]
  13. Chiang PK, Hersh PS. Comparing predictability between eyes after bilateral laser in situ keratomileusis: a theoretical analysis of simultaneous versus sequential procedures. Ophthalmology. 1999;106(9):1684–1691. doi:10.1016/S0161-6420(99)90390-1 [CrossRef]

Characteristics of Patients Undergoing SMILE

Demographic Mean±SD (Range)
Age (y) 38.3±8.3 (19.8 to 59.0)
Preop sphere (D) −6.89±1.29 (−9.50 to −1.50)
Preop cylinder (D) −0.60±0.46 (−1.75 to 0.00)
Preop spherical equivalent refraction (D) −7.19±1.30 (−9.88 to −1.63)
Preop corneal power (D) 43.91±1.51 (38.03 to 48.60)
Preop central corneal thickness (μm) 535±28 (480 to 606)
Spherical treatment attempt (D) −6.79±1.32 (−9.50 to −0.75)
Cylinder treatment attempt (D) −0.57±0.47 (−1.75 to 0.00)
Spherical equivalent treatment attempt (D) −7.05±1.33 (−9.75 to −0.75)
Lenticule–pupil deviation in treatment (mm) 0.129±0.088 (0 to 0.737)

Bivariate Correlation Coefficients (Pearson) for Continuous Variables and Mean Values for Dichotomous Predictors in Patients Undergoing SMILE

Variable Error in SE Correction (D) Change in CDVA (logMAR) Change From CDVAPreop to UDVA1 Day (logMAR) Change From CDVAPreop to UDVA3 Months (logMAR)
Gender (M/F) −0.17/−0.29* 0.02/0.02 −0.14/−0.16 −0.04/−0.08*
Eye (right/left) −0.28/−0.21 0.03/0.01 −0.13/−0.17 −0.05/−0.07
Age (y) −0.230* −0.040 −0.113 −0.201*
Central corneal thickness (μm) 0.009 −0.019 −0.027 −0.002
Corneal power (D) 0.168* 0.026 −0.068 −0.040
Intraocular pressure (mmHg) −0.019 0.037 −0.062 0.056
Spherical equivalent treatment (D) −0.068 0.065 0.069 0.037
Laser settings (setting 1/setting 2) −0.24/−0.26 0.02/0.03 −0.18/−0.09* −0.07/−0.05
Astigmatic treatment (no/yes) −0.24/−0.25 0.04/0.02* −0.15/−0.15 −0.05/−0.06
Lenticule–pupil deviation (mm) −0.01 −0.017 0.103 0.014

Multiple Linear Regression Analyses in Patients Undergoing SMILE

Coefficients
Error in spherical equivalent correction (difference between attempted and achieved change) (D)
  (Constant) (D) 2.052*
  Age (D/y) −0.012*
  Corneal power (D/D) −0.041*
  Gender (females) −0.085
  Eye (Left) 0.067
Change in CDVA (difference in CDVA before surgery and 3 months after surgery)
  (Constant) (logMAR) 0.049*
  Cylinder treatment (yes) −0.024*
  Eye (Left eye) −0.019*
Change from CDVA before surgery to UDVA 3 months after surgery (only eyes aimed for emmetropia)
  (Constant) (logMAR) 0.080*
  Age (units/y) −0.003*
  Gender (females) −0.039*
Change from CDVA before surgery to UDVA 1 day after surgery (only eyes aimed for emmetropia)
  (Constant) (logMAR) −0.255*
  Laser setting (setting 2) 0.091*
  Eye (Left) −0.031
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