Journal of Refractive Surgery

Biomechanics Supplemental Data

Lower Energy Levels Improve Visual Recovery in Small Incision Lenticule Extraction (SMILE)

David Donate, MD; Rozenn Thaëron, MSc

Abstract

PURPOSE:

To compare the visual outcomes with energy close to the plasma threshold and energy at the standard set-up in small incision lenticule extraction (SMILE).

METHODS:

This retrospective, non-randomized, consecutive clinical case series included 486 eyes of 243 patients who underwent SMILE and were subdivided into two groups depending on the laser energy settings: the standard energy group (164 eyes) using a laser cut energy index of 36 (180 nJ), and the plasma threshold group (322 eyes) using a cut energy index of 20 (100 nJ, close to plasma threshold). A spot spacing of 4.5 µm was used in both groups. Safety, efficacy, predictability, and ocular optical quality outcomes were evaluated and compared among groups during a 3-month postoperative follow-up.

RESULTS:

Significantly better uncorrected (UDVA) and corrected (CDVA) distance visual acuity was found in the plasma threshold group throughout the follow-up (P ≤ .01). A higher percentage of eyes with 20/20 or better UDVA was found in the plasma threshold group at 1 day, 1 month, and 3 months postoperatively. At 1 and 3 months after surgery, no losses of two or more lines of CDVA were found in the plasma threshold group, whereas in the standard energy group these losses were observed in 3.8% and 2.7% of eyes, respectively. Significantly better postoperative modulation transfer function (P ≤ .02) and a lower level of higher order aberrations were found in the plasma threshold group compared to the standard energy group (P ≤ .025).

CONCLUSIONS:

An energy level close to the plasma threshold during SMILE provides a faster and better visual recovery.

[J Refract Surg. 2016;32(9):636–642.]

Abstract

PURPOSE:

To compare the visual outcomes with energy close to the plasma threshold and energy at the standard set-up in small incision lenticule extraction (SMILE).

METHODS:

This retrospective, non-randomized, consecutive clinical case series included 486 eyes of 243 patients who underwent SMILE and were subdivided into two groups depending on the laser energy settings: the standard energy group (164 eyes) using a laser cut energy index of 36 (180 nJ), and the plasma threshold group (322 eyes) using a cut energy index of 20 (100 nJ, close to plasma threshold). A spot spacing of 4.5 µm was used in both groups. Safety, efficacy, predictability, and ocular optical quality outcomes were evaluated and compared among groups during a 3-month postoperative follow-up.

RESULTS:

Significantly better uncorrected (UDVA) and corrected (CDVA) distance visual acuity was found in the plasma threshold group throughout the follow-up (P ≤ .01). A higher percentage of eyes with 20/20 or better UDVA was found in the plasma threshold group at 1 day, 1 month, and 3 months postoperatively. At 1 and 3 months after surgery, no losses of two or more lines of CDVA were found in the plasma threshold group, whereas in the standard energy group these losses were observed in 3.8% and 2.7% of eyes, respectively. Significantly better postoperative modulation transfer function (P ≤ .02) and a lower level of higher order aberrations were found in the plasma threshold group compared to the standard energy group (P ≤ .025).

CONCLUSIONS:

An energy level close to the plasma threshold during SMILE provides a faster and better visual recovery.

[J Refract Surg. 2016;32(9):636–642.]

Since 2008, small incision lenticule extraction (SMILE) has been developed as a flapless surgical approach for the correction of refractive errors. Several studies have reported promising efficacy, predictability, and safety outcomes after SMILE.1–7 However, many surgeons or authors have reported cases with slow visual recovery and some cases with loss of corrected distance visual acuity (CDVA).3,4,8–10

The SMILE technique is performed with a femtosecond laser and is based on material photodisruption. When a femtosecond laser is focused and the energy high enough, it produces a photodisruption of the material. At the focal point, the material is transformed into a hot gas called plasma taking the shape of a cavitation bubble.11 The plasma threshold is the minimal energy necessary to induce plasma. The plasma threshold is the same for all materials.11 With the VisuMax femtosecond laser (Carl Zeiss Meditec AG, Jena, Germany), the plasma threshold is determined during calibration by minimum energy inducing a cut on a glass sample. This energy ranges from 19 to 21 (95 to 105 nJ) for every device (data from the manufacturer). The standard energy used for SMILE in different studies is 28 to 34 (140 to 170 nJ).3,4,9 According to the results of a previous study conducted by our research group, the use of a laser energy near to the plasma threshold increases the regularity of the cut compared to the use of higher energies.11

The purpose of this study was to compare the visual outcomes with energy close to the plasma threshold and energy at the standard set-up.

Patients and Methods

This retrospective comparative study included 486 eyes of 243 patients who underwent bilateral SMILE surgery for the correction of myopia and myopic astigmatism using the VisuMax femtosecond laser system with a 500-kHz repetition rate. The patients were recruited in a continuous cohort and treated consecutively between May 2013 and January 2014 for the standard energy group and between March 2014 and March 2015 for the plasma threshold group. According to the tenets of the Declaration of Helsinki, all patients were informed about the study and signed a written informed consent form. Likewise, approval was received from the local ethics committee.

Inclusion criteria for the study were: age between 18 to 45 years, stable myopia for at least 1 year, CDVA of 20/20 or better, preoperative manifest spherical equivalent refraction from −0.50 to −10.00 diopters (D), manifest refractive cylinder between 0.00 and −4.00 D, and sufficient corneal thickness to perform the surgery (estimated total postoperative corneal thickness > 400 µm and a minimum residual stromal bed of 250 µm). Exclusion criteria were: history of ocular surgery, severe dry eye, progressive corneal degeneration, keratoconus, cataract, uveitis, pregnancy, and breastfeeding. Patients were told to discontinue using soft contact lenses for 4 days prior to the preoperative examination.

Preoperative and Postoperative Examinations

All patients were evaluated preoperatively and 1 day, 1 month, and 3 months after surgery. The examinations included autorefractometry, intraocular pressure measurement, keratometry (Tonoref II; Nidek Co. Ltd., Gamagori, Japan), corneal pachymetry, anterior segment analysis by optical coherence tomography (Optovue Inc., Fremont, CA), topography and ocular wavefront aberrometry (OPD-Scan II; Nidek Co. Ltd.), uncorrected distance visual acuity (UDVA) and CDVA testing, manifest refraction, slit-lamp examination, and funduscopy. All measurements were performed in the same examination room with carefully controlled lighting. Three specially trained optometrists performed manifest refractions and visual acuity tests. At all postoperative visits, side effects and patient symptoms were recorded. Data on adverse events and complications were also collected.

Surgical Technique

This study began after the initial learning process, which included 60 cases not included in this study. The surgeries were performed by the same surgeon (DD). The operating room temperature was between 19°C and 21°C. The rate of humidity was 45% to 55%. Surgery was performed bilaterally under topical anesthesia with one drop of 0.8% oxybuprocaine tetrachloride just before the suction for 20 seconds and rinsed with 20 mL of balanced salt solution. After this, the surgeon instructed the patient to look directly at the green fixation light and the corneal suction ports were activated to fixate the eye in this position. Thus, the patient essentially auto-centered the visual axis and consequently the corneal vertex to the vertex of the contact glass, which was centered to the laser system and the center of the lenticule created. This centration was confirmed by the surgeon by comparing the relative positions of the corneal reflex and pupil center to the Placido disk image projected onto the cornea and obtained by the OPD-Scan II system. A small curved interface cone was used in all cases. Once appropriate centration was achieved, suction was applied to the contact glass. After this, the main refractive and nonrefractive femtosecond incisions were performed in the following automated sequence: the posterior surface of the lenticule (spiral-in pattern), the anterior surface of the lenticule (spiral-out pattern), and finally the side cut of the cap.

In our study, two different laser energy settings were used for the creation of the lenticule: a standard laser cut energy index of 36 (corresponding to approximately 180 nJ) for the standard energy group and 20 (equivalent to approximately 100 nJ), which is an energy level close to the plasma threshold, for the plasma threshold group. The laser was calibrated by the manufacturer. The spot spacing was 4.5 µm in both groups. Likewise, the following femtosecond laser parameters were used in all cases: 130 µm of cap thickness, 7.5 mm of cap diameter, 6.5 mm of lenticule diameter, a spot distance of 2 µm for a side cut, and a 2-mm side cut to access the lenticule with an angle of 90°.

After the creation of the lenticule, a spatula was inserted in all cases through the side cut over the top of the refractive lenticule with the aim of dissecting this plane followed by the bottom of the lenticule. The lenticule was subsequently grasped and removed. After the removal of the lenticule, the intrastromal space was flushed with balanced salt solution using a standard cannula. Our standard postoperative treatment consisted of two drops of antibiotic and steroid drops (Tobradex; Alcon Laboratories, Inc., Fort Worth, TX) three times a day for 2 weeks and, if necessary, artificial tears three to six times a day for up to 1 month.

Statistical Analysis

All measured data were collected and entered into the patient charts and also anonymously into a standardized study spreadsheet in Excel 2010 (Microsoft Corporation, Redmond, WA). Statistical analyses were performed using this software. All visual acuity measurements were transformed into logMAR units for statistical purposes. All values are given as the mean ± standard deviation. The unpaired Student's t test was used for comparing the outcomes among groups, with a P value of less than .05 considered statistically significant. The efficacy index was calculated as the ratio of decimal postoperative UDVA to preoperative CDVA, and the safety index as the ratio of decimal postoperative CDVA to preoperative CDVA.

Results

Study Population

The two groups were differentiated according to the energy settings used: the standard energy group consisted of 164 eyes using a standard energy and the plasma threshold group consisted of 322 eyes with energy near to plasma threshold. Table 1 summarizes the preoperative patient characteristics in each group. As shown, there were no significant differences preoperatively between groups in age, visual acuity, refraction, and keratometry. Only a small but statistically significant difference among groups was found in preoperative central corneal thickness. A suction loss occurred in 1 eye in the standard energy group (0.61%) and 2 eyes (0.62%) in the plasma threshold group. These eyes were excluded from the study. No significant intraoperative complications were observed.


Preoperative Demographics of Eyes Undergoing SMILE

Table 1:

Preoperative Demographics of Eyes Undergoing SMILE

Visual Acuity and Refraction

Table A (available in the online version of this article) summarizes the visual and refractive outcomes of SMILE in both groups evaluated during all postoperative follow-up visits. As shown, statistically significantly better UDVA and CDVA was found for the plasma threshold group throughout the follow-up (P ≤ .01). A higher percentage of eyes with 20/20 or better UDVA at 1 day, 1 month, and 3 months postoperatively was found in the plasma threshold group (100 nJ).


Visual and Refractive Changes After SMILE in Both Groups

Table A:

Visual and Refractive Changes After SMILE in Both Groups

Figure 1 shows the efficacy of the procedure at 1 month after surgery in the two groups analyzed. At 1 day after surgery, 35 eyes (23%) in the standard energy group and 161 eyes (66%) in the plasma threshold group with a preoperative CDVA of 20/20 or better achieved a postoperative UDVA of 20/20 or better. The difference in the percentages between groups was statistically significant (P < .05). At 1 month postoperatively, the efficacy index was 0.86 and 0.95 in the standard energy and plasma threshold groups, respectively. At 3 months postoperatively, the efficacy index was 0.92 and 0.99 in the standard energy and plasma threshold groups, respectively.


Efficacy outcomes at 1 month postoperatively in the standard energy (180 nJ) and plasma threshold (100 nJ) groups showing the distribution of postoperative uncorrected distance visual acuity (UDVA) in each group. CDVA = corrected distance visual acuity

Figure 1.

Efficacy outcomes at 1 month postoperatively in the standard energy (180 nJ) and plasma threshold (100 nJ) groups showing the distribution of postoperative uncorrected distance visual acuity (UDVA) in each group. CDVA = corrected distance visual acuity

The safety index at 1 month postoperatively was 0.94 and 1.04 in the standard energy and plasma threshold groups, respectively. At 3 months after surgery, the safety index was 1.04 and 1.06 in the standard energy and plasma threshold groups, respectively. At 1 and 3 months postoperatively, no losses of two or more lines of CDVA were found in the plasma threshold group, whereas in the standard energy group losses of two or more lines of CDVA were observed in 3.8% (8 eyes) and 2.7% (2 eyes) of eyes at 1 month after surgery, respectively (Figure 2).


Safety outcomes at 1 month after surgery in the standard energy (180 nJ) and plasma threshold (100 nJ) groups showing the distribution of changes in postoperative corrected distance visual acuity (CDVA) in each group.

Figure 2.

Safety outcomes at 1 month after surgery in the standard energy (180 nJ) and plasma threshold (100 nJ) groups showing the distribution of changes in postoperative corrected distance visual acuity (CDVA) in each group.

The scatterplots in Figure 3 show the regression line plot of the achieved versus the attempted refractive correction at 1 month postoperatively in both groups. No differences in predictability between groups were found at 1 month (standard energy group, R2 = 0.96; plasma threshold group, R2 = 0.98) or 3 months (standard energy group, R2 = 0.97; plasma threshold group, R2 = 0.98) after surgery. At 1 month postoperatively, 79.9% and 91.8% of eyes treated in the standard energy and plasma threshold groups had a spherical equivalent within ±0.50 D and 94.8% and 98.9% within ±1.00 D, respectively (Figure 4). At 3 months after surgery, 86.5% and 92.8% of eyes treated in the standard energy and plasma threshold groups had a spherical equivalent within ±0.50 D, respectively, and 100% of eyes in both groups were within ±1.00 D.


Scatterplot showing the relationship among attempted and achieved spherical equivalent at 1 month after surgery in the standard energy (180 nJ) and plasma threshold (100 nJ) groups. The adjusting line to all points obtained by means of least square method is shown with its corresponding R2 value. D = diopters

Figure 3.

Scatterplot showing the relationship among attempted and achieved spherical equivalent at 1 month after surgery in the standard energy (180 nJ) and plasma threshold (100 nJ) groups. The adjusting line to all points obtained by means of least square method is shown with its corresponding R2 value. D = diopters


Predictability outcomes at 1 month after surgery in the standard energy (180 nJ) and plasma threshold (100 nJ) groups showing the distribution of postoperative spherical equivalent (SE) in each group. D = diopters

Figure 4.

Predictability outcomes at 1 month after surgery in the standard energy (180 nJ) and plasma threshold (100 nJ) groups showing the distribution of postoperative spherical equivalent (SE) in each group. D = diopters

Optical Quality Outcomes

Figure 5 displays the modulation transfer function (MTF) at 1 month after surgery derived from the ocular optical aberrations measured with the OPD-Scan II system. Statistically significant differences among the standard energy and plasma threshold groups were found in the MTF value corresponding to the spatial frequencies of 5 and 10 cycles/degree at 1 and 3 months postoperatively (P ≤ .02) (Table B, available in the online version of this article). Likewise, significantly higher 1-month postoperative MTF values were found in the plasma threshold group compared to the standard energy group for the spatial frequencies of 15 and 20 cycles/degree (P < .001) (Table B). The higher order aberrations root mean square was significantly higher in the standard energy group compared to the plasma threshold group during all postoperative follow-up visits (P ≤ .025) (Table B).


Modulation transfer function (MTF) derived from ocular aberrometric data obtained with the OPD-Scan II system (Nidek Co. Ltd., Gamagori, Japan) at 1 month after surgery in the standard energy (180 nJ) and plasma threshold (100 nJ) groups.

Figure 5.

Modulation transfer function (MTF) derived from ocular aberrometric data obtained with the OPD-Scan II system (Nidek Co. Ltd., Gamagori, Japan) at 1 month after surgery in the standard energy (180 nJ) and plasma threshold (100 nJ) groups.


Ocular Aberrometric and Optical Quality Outcomes After SMILE in Both Groups

Table B:

Ocular Aberrometric and Optical Quality Outcomes After SMILE in Both Groups

Discussion

To our knowledge, this is the first study demonstrating the improvement of clinical results of SMILE by using energy close to the plasma threshold. Our results showed a better mean UDVA and efficacy index in the plasma threshold group (close to plasma threshold) than in the standard energy group, from day 1 to 3 months postoperatively. This is a significant confirmation of a faster visual recovery in the plasma threshold group. Moreover, the safety index was better in the plasma threshold group. There were no patients with loss of CDVA in this group, unlike the standard energy group, where we observed 3.8% and 2.7% with two lines or more of loss of CDVA at 1 and 3 months postoperatively, respectively. Furthermore, optical quality was better in the plasma threshold group, where the MTF values and the higher order aberrations root mean square were significantly better. Results of the standard energy group are similar to previous published results of SMILE,4,5,12–14 meaning the superiority of results of the plasma threshold group are not linked to poor results of the standard energy group. In this study, we noted predictability was not completely similar between the two groups, with 10% fewer eyes within ±0.50 D in postoperative refraction at 1 month and 5% at 3 months. We do not know whether these differences were due to the difference in energy used or whether they were the consequence of a modification in the accuracy of the laser during the study. Even if these results could explain partial UDVA differences between the two groups, they could not constitute a real study bias because they do not explain the high UDVA differences found between the groups. In all cases, they were also not linked to the differences found for CDVA, loss of CDVA, and higher order aberrations, meaning that the use of the standard energy induces poor vision quality.

The reasons for these results are not well understood. We assume that using higher energy could induce an inflammatory response and be responsible for the early blurred vision usually described, as published with the first femtosecond laser-assisted LASIK surgeries.15 Concerning the longer slow vision recovery also often described and found in the standard energy group, we suppose the reason may be a less regular lenticule cutting due to interference from a large gas bubble cavitation and following laser shots when using high energy.11,16 The irregular lenticule cutting resulting in a transitory irregular corneal shape provides worse vision outcomes until the natural smoothing of the epithelium improves the regularity of the cornea.16,17 Depending on the efficacy of this epithelium smoothing the vision could be improved slowly or incompletely, with a loss of CDVA. This theory could explain the optical aberration results found in this study.

Other studies have compared visual outcomes in function of the laser setting (Table 2).2,4,5,8,18,19 Hjortdal et al.5 showed a better UDVA at 1 day only using 170 nJ pulse energy and 4.5 µm spot spacing (plasma threshold group: 212 eyes) than using 125 nJ pulse energy and 2.5 µm spot spacing (standard energy group: 458 eyes). These results come from a multiple linear regression analysis about change from CDVA before surgery to UDVA. Results are difficult to analyze because results of each group are not described and the study included amblyopic eyes and patients not treated for emmetropia. Kamiya et al.18 found no statistical difference between postoperative outcomes with the standard energy group energy setting of 140 nJ and spot spacing of 3 µm and the plasma threshold group energy setting of 170 nJ and spot spacing of 4.5 µm in a study with 44 eyes.


Results From Previous Studies Comparing Visual Outcomes of SMILE Laser Settings

Table 2:

Results From Previous Studies Comparing Visual Outcomes of SMILE Laser Settings

Based on the results of the current study, we recommend using energy close to the plasma threshold. However, the exact settings have to be adapted to each laser, because the same setting on different lasers does not induce the same effects on the cornea. Indeed, the energy can be set for the intensity but the effects on the cornea are linked to the power of the shot, which is linked to the duration of the laser pulse and is not exactly the same for each laser. We also warn that using energy close to the plasma threshold requires taking care of the epithelium of the cornea to keep its transparency. Every shot induces plasma, leading to an easy dissection of the lenticule (Video, available in the online version of this article). Based on previous scientific evidence, we can assume reducing the spot spacing too much generates interference between subsequent shots and/or inflammatory response, yielding poor results similar to using too much energy.

There are two limitations of the current study that must be acknowledged. First, including both eyes of the same patient is a questionable choice because of the statistical lack of independence leading to an underestimation of the P values and the widths of confidence intervals, increasing the statistical power for detecting treatment differences. Second, the two groups were not treated during the same period. However, we considered it unethical to continue with the initial parameters due to the radical improvement of the results when the energy setting was changed. Because no other parameter had been changed during the study and the study started after the learning curve of the surgeon, the role of the laser energy used seems to have a strong implication on our results.

References

  1. Sekundo W, Kunert K, Russmann C, et al. First efficacy and safety study of femtosecond lenticule extraction for the correction of myopia: six months results. J Cataract Refract Surg. 2008;34:1513–1520. Erratum in: J Cataract Refract Surg. 2008;34:1819. doi:10.1016/j.jcrs.2008.05.033 [CrossRef]
  2. 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;95:335–339. doi:10.1136/bjo.2009.174284 [CrossRef]
  3. Shah R, Shah S, Sengupta S. Results of small incision lenticule extraction: all-in-one femtosecond laser refractive surgery. J Cataract Refract Surg. 2011;37:127–137. doi:10.1016/j.jcrs.2010.07.033 [CrossRef]
  4. Verstergaard A, Ivarsen AR, Asp S, Hjortdal JØ. Small-incision lenticule extraction for moderate or high myopia: predictability, safety, and patient satisfaction. J Cataract Refract Surg. 2012;38:2003–2010. doi:10.1016/j.jcrs.2012.07.021 [CrossRef]
  5. Hjortdal JO, Vestergaard AH, Ivarsen A, Ragunathan S, Asp S. Predictors for the outcome of small-incision lenticule extraction for myopia. J Refract Surg. 2012;28:865–871. doi:10.3928/1081597X-20121115-01 [CrossRef]
  6. Sekundo W, Gertnere J, Bertelmann T, Solomatin I. One-year refractive results, contrast sensitivity, high-order aberrations and complications after myopic small-incision lenticule extraction (ReLEx SMILE). Graefes Arch Clin Exp Ophthalmol. 2014;252:837–843. doi:10.1007/s00417-014-2608-4 [CrossRef]
  7. Reinstein DZ, Carp GI, Archer TJ, Gobbe M. Outcomes of small incision lenticule extraction (SMILE) in low myopia. J Refract Surg. 2014;30:812–818. Erratum in: J Refract Surg. 2015;31:60. doi:10.3928/1081597X-20141113-07 [CrossRef]
  8. Shah R, Shah S. Effect of scanning patterns on the results of femtosecond laser lenticule extraction refractive surgery. J Cataract Refract Surg. 2011;37:1636–1647. doi:10.1016/j.jcrs.2011.03.056 [CrossRef]
  9. Kamiya K, Shimizu K, Igarashi A, Kobashi H. Visual and refractive outcomes of femtosecond lenticule extraction and small-incision lenticule extraction for myopia. Am J Ophthalmol. 2014;157:128–134. doi:10.1016/j.ajo.2013.08.011 [CrossRef]
  10. Demirok A, Agca A, Ozgurhan EB, et al. Femtosecond lenticule extraction for correction of myopia: a 6 month follow-up study. Clin Ophthalmol. 2013;7:1041–1047.
  11. Donate D, Albert O, Colliac JP, et al. Femtosecond laser: a micromachining system for corneal surgery [article in French]. J Fr Ophtalmol. 2004;25:783–789. doi:10.1016/S0181-5512(04)96214-6 [CrossRef]
  12. Lin F, Xu Y, Yang Y. Comparison of the visual results after SMILE and femtosecond laser-assisted LASIK for myopia. J Refract Surg. 2014;30:248–254. Erratum in: J Refractive Surg. 2014;30:582. doi:10.3928/1081597X-20140320-03 [CrossRef]
  13. Moshirfar M, McCaughey MV, Reinstein DZ, Shah R, Santiago-Caban L, Fenzl CR. Small-incision lenticule extraction. J Cataract Refract Surg. 2015;41:652–665. doi:10.1016/j.jcrs.2015.02.006 [CrossRef]
  14. Pedersen IB, Ivarsen A, Hjortdal J. Three-year results of small incision lenticule extraction for high myopia: refractive outcomes and aberrations. J Refract Surg. 2015;31:719–724. doi:10.3928/1081597X-20150923-11 [CrossRef]
  15. Netto MV, Mohan RR, Medeiros FW, et al. Femtosecond laser and microkeratome corneal flaps: comparison of stromal wound healing and inflammation. J Refract Surg. 2007;23:667–676.
  16. Juhasz T, Kastis GA, Suárez C, Bor Z, Bron WE. Time-resolved observations of shock waves and cavitation bubbles generated by femtosecond laser pulses in corneal tissue and water. Lasers Surg Med. 1996;19:23–31. doi:10.1002/(SICI)1096-9101(1996)19:1<23::AID-LSM4>3.0.CO;2-S [CrossRef]
  17. Serrao S, Buratto L, Lombardo G, De Santo MP, Ducoli P, Lombardo M. Optimal parameters to improve the interface quality of the flap bed in femtosecond laser-assisted laser in situ keratomileusis. J Cataract Refract Surg. 2012;38:1453–1459. doi:10.1016/j.jcrs.2012.05.021 [CrossRef]
  18. Kamiya K, Shimizu K, Igarashi A, Kobashi H. Effect of femtosecond laser setting on visual performance after small-incision lenticule extraction for myopia. Br J Ophthalmol. 2015;99:1381–1387. doi:10.1136/bjophthalmol-2015-306717 [CrossRef]
  19. Kim JR, Hwang HB, Mun SJ, Chung YT, Kim HS. Efficacy, predictability, and safety of small incision lenticule extraction: 6-months prospective cohort study. BMC Ophthalmol. 2014;14:117. doi:10.1186/1471-2415-14-117 [CrossRef]

Preoperative Demographics of Eyes Undergoing SMILE

ParameterStandard Energy (180 nJ)Plasma Threshold (100 nJ)P
Eyes (n)164322
Gender (male/female)45/3794/67
Age (y)29.74 ± 6.18 (18 to 45)29.78 ± 5.50 (18 to 43).96
Manifest spherical equivalent (D)−3.99 ± 2.01 (−0.75 to −10.00)−3.92 ± 2.11 (−0.50 to −9.88).69
Manifest sphere (D)−3.71 ± 2.00 (−0.50 to −9.75)−3.61 ± 2.08 (0.00 to −9.75).61
Manifest cylinder (D)−0.56 ± 0.54 (plano to −2.75)−0.62 ± 0.68 (plano to −4.00).33
logMAR UDVA1.16 ± 0.40 (0.10 to 1.52)1.12 ± 0.40 (0.10 to 1.52).46
logMAR CDVA−0.03 ± 0.04 (−0.10 to 0.00)−0.04 ± 0.05 (−0.10 to 0.00).06
Mean keratometric reading (D)43.79 ± 1.40 (39.40 to 47.51)43.75 ± 1.50 (39.12 to 47.64).78
Central corneal thickness (µm)555.80 ± 34.55 (475 to 649)549.00 ± 30.86 (480 to 670).03

Results From Previous Studies Comparing Visual Outcomes of SMILE Laser Settings

StudyYearRepetition Rate (kHz)Cut Energy (nJ)Spot Spacing (µm)No. of EyesFollow-up (mo)Preop SEQ (D)Postop SEQ (D)±0.50 D20/20 CDVA PreopUDVA 20/20 or Better Postop
Shah et al.820112001602 to 3516−4.87 ± 2.16 (−1.75 to −10.00)+0.03 ± 0.3 (−0.75 to +0.75)91%67%62%
Sekundo et al.220112003003 to 5916−4.75 ± 1.56−0.01 ± 0.4980%84%
Verstergaard et al.42012500120 to 1502.52793−7.18 ± 1.57 (−1.63 to −11.50)−0.09 ± 0.45 (−1.63 to +1.38) (−1.63 to +1.38)77%37%
Hjortdal et al.52012500125; 1702.5; 4.56703−7.19 ± 1.30 (−1.63 to −9.88)−0.25 ± 0.44 (−2.13 to 1.38)80%88%61%
Kim et al.1920145001804.52936−6.75 ± 1.65 (−2.25 to −10.00)−0.21 ± 0.3786.1%79.8%
Kamiya et al.182015500140; 1703; 4.5446−5.38 (−5.94 to −4.31); −5.06 (−6.19 to −4.06)0.00 (0.00; 0.00); 0.00 (−0.25; 0.00)95%; 95%

Visual and Refractive Changes After SMILE in Both Groups

ParameterPreoperativePostoperative

1 Day1 Month3 Months
logMAR UDVA
  Standard energy (180 nJ)1.16 ± 0.40 (0.10 to 1.52)0.13 ± 0.15 (−0.10 to 0.60)0.04 ± 0.11 (−0.10 to 0.49)0.002 ± 0.10 (−0.20 to 0.40)
  Plasma threshold (100 nJ)1.12 ± 0.40 (0.10 to 1.52)0.03 ± 0.10 (−0.20 to 0.40)−0.03 ± 0.08 (−0.20 to 0.40)−0.03 ± 0.07 (−0.20 to 0.40)
   P.46< .001< .001.001
logMAR CDVA
  Standard energy (180 nJ)−0.03 ± 0.04 (−0.10 to 0.00)−0.01 ± 0.06 (−0.10 to 0.40)−0.05 ± 0.07 (−0.20 to 0.40)
  Plasma threshold (100 nJ)−0.04 ± 0.05 (−0.10 to 0.00)−0.06 ± 0.05 (−0.20 to 0.40)−0.07 ± 0.06 (−0.20 to 0.40)
   P.06< .001.01
UDVA 20/20 or more (%)
  Standard energy (180 nJ)23.6556.8270.27
  Plasma threshold (100 nJ)64.6687.1191.56
   P< .001< .001< .001
Manifest spherical equivalent (D)
  Standard energy (180 nJ)−3.99 ± 2.01 (−0.75 to −10.00)−0.08 ± 0.41 (−1.50 to 1.00)0.11 ± 0.36 (−0.75 to 0.88)
  Plasma threshold (100 nJ)−3.92 ± 2.11 (−0.50 to −9.88)−0.03 ± 0.29 (−1.12 to 0.63)−0.01 ± 0.29 (−0.88 to 0.75)
   P.69.21.02
Manifest sphere (D)
  Standard energy (180 nJ)−3.71 ± 2.00 (−0.50 to −9.75)0.11 ± 0.39 (−1.25 to 1.25)0.31 ± 0.33 (−1.50 to 0.00)
  Plasma threshold (100 nJ)−3.61 ± 2.08 (0.00 to −9.75)0.14± 0.28 (−1.00 to 1.00)0.16 ± 0.28 (−0.50 to 1.00)
   P.61.19.002
Manifest cylinder (D)
  Standard energy (180 nJ)−0.56 ± 0.54 (0.00 to −2.75)−0.38 ± 0.29 (−1.13 to 0.63)−0.40 ± 0.33 (−1.50 to 0.00)
  Plasma threshold (100 nJ)−0.62 ± 0.68 (0.00 to −4.00)−0.33 ± 0.30 (−1.25 to 0.00)−0.35 ± 0.30 (−1.25 to 0.00)
   P.33.48.13

Ocular Aberrometric and Optical Quality Outcomes After SMILE in Both Groups

ParameterPreoperativePostoperative 1 MonthPostoperative 3 Months
MTF 5 CPD
  Standard energy (180 nJ)0.70 ± 0.110.57 ± 0.140.59 ± 0.13
  Plasma threshold (100 nJ)0.69 ± 0.130.65 ± 0.130.64 ± 0.11
   P.25< .001.03
MTF 10 CPD
  Standard energy (180 nJ)0.40 ± 0.120.28 ± 0.120.29 ± 0.14
  Plasma threshold (100 nJ)0.39 ± 0.140.36 ± 0.130.34 ± 0.10
   P.43< .001.02
MTF 15 CPD
  Standard energy (180 nJ)0.22 ± 0.100.16 ± 0.080.18 ± 0.11
  Plasma threshold (100 nJ)0.23 ± 0.120.21 ± 0.100.19 ± 0.08
   P.71< .001.07
MTF 20 CPD
  Standard energy (180 nJ)0.17 ± 0.110.11 ± 0.050.13 ± 0.08
  Plasma threshold (100 nJ)0.17 ± 0.090.15 ± 0.070.13 ± 0.04
   P.97< .001.12
HOA RMS
  Standard energy (180 nJ)0.52 ± 0.240.53 ± 0.42
  Plasma threshold (100 nJ)0.46 ± 0.210.42 ± 0.14
   P.013.025
Authors

From Ophteo, Lyon, France.

The authors have no financial or proprietary interest in the materials presented herein.

AUTHOR CONTRIBUTIONS

Study concept and design (DD, RT); data collection (DD, RT); analysis and interpretation of data (DD, RT); writing the manuscript (DD, RT); critical revision of the manuscript (DD, RT); statistical expertise (DD, RT); administrative, technical, or material support (DD, RT); supervision (DD, RT)

Correspondence: David Donate, MD, Ophteo, 52 avenue Marechal de Saxe, 69006 Lyon, France. E-mail: david.donate@yahoo.fr

Received: December 14, 2015
Accepted: May 11, 2016

10.3928/1081597X-20160602-01

Sign up to receive

Journal E-contents