Journal of Refractive Surgery

Original Article Supplemental Data

Energy Setting and Visual Outcomes in SMILE: A Retrospective Cohort Study

Liuyang Li, MD; Julie M. Schallhorn, MD; Jiaonan Ma, MD; Tong Cui, MD; Yan Wang, MD, PhD

Abstract

PURPOSE:

To assess the independent effect of energy setting on postoperative uncorrected distance visual acuity (UDVA) in small incision lenticule extraction (SMILE) and further investigate an optimal energy setting for the 4.5-μm spot-track-distance, which is in wide clinical use.

METHODS:

A total of 1,130 eyes were included in a retrospective cohort study from Tianjin Eye Hospital, Tianjin Medical University from April 2015 to July 2016. Energy settings and baseline characteristics were recorded and 3-month UDVA was tested by a nurse blinded to the energy settings used. Multiple regression analysis and generalized estimating equations were used to take into account the correlation between the measurements from two eyes.

RESULTS:

The 3-month UDVA (mean ± standard deviation) of 125 to 160 nJ (by 5-nJ increments) was 1.39 ± 0.19, 1.40 ± 0.32, 1.33 ± 0.27, 1.36 ± 0.27, 1.34 ± 0.25, 1.29 ± 0.19, 1.36 ± 0.27, and 1.19 ± 0.22, respectively. Energy was significantly associated with postoperative logMAR UDVA in different models and the regression coefficient (β) was robust (β = 0.01, 95% confidence interval = 0.00 to 0.01). The regression coefficient β (0.01, 95% confidence interval = 0.00 to 0.02, P = .0029) of energy (125 to 150 nJ, by 5-nJ increments) on 4.5-μm spot-track-distance was still associated with the logMAR UDVA when adjusted for sex, age, myopia, astigmatism, mean keratometry, central corneal thickness, preoperative logMAR CDVA, and side spot-track-distance.

CONCLUSIONS:

The lower end of the energy studied was associated with a better postoperative UDVA in this population. The spot-track-distance of 4.5 μm with 125 nJ energy was the optimal combination within this range.

[J Refract Surg. 2018;34(1):11–16.]

Abstract

PURPOSE:

To assess the independent effect of energy setting on postoperative uncorrected distance visual acuity (UDVA) in small incision lenticule extraction (SMILE) and further investigate an optimal energy setting for the 4.5-μm spot-track-distance, which is in wide clinical use.

METHODS:

A total of 1,130 eyes were included in a retrospective cohort study from Tianjin Eye Hospital, Tianjin Medical University from April 2015 to July 2016. Energy settings and baseline characteristics were recorded and 3-month UDVA was tested by a nurse blinded to the energy settings used. Multiple regression analysis and generalized estimating equations were used to take into account the correlation between the measurements from two eyes.

RESULTS:

The 3-month UDVA (mean ± standard deviation) of 125 to 160 nJ (by 5-nJ increments) was 1.39 ± 0.19, 1.40 ± 0.32, 1.33 ± 0.27, 1.36 ± 0.27, 1.34 ± 0.25, 1.29 ± 0.19, 1.36 ± 0.27, and 1.19 ± 0.22, respectively. Energy was significantly associated with postoperative logMAR UDVA in different models and the regression coefficient (β) was robust (β = 0.01, 95% confidence interval = 0.00 to 0.01). The regression coefficient β (0.01, 95% confidence interval = 0.00 to 0.02, P = .0029) of energy (125 to 150 nJ, by 5-nJ increments) on 4.5-μm spot-track-distance was still associated with the logMAR UDVA when adjusted for sex, age, myopia, astigmatism, mean keratometry, central corneal thickness, preoperative logMAR CDVA, and side spot-track-distance.

CONCLUSIONS:

The lower end of the energy studied was associated with a better postoperative UDVA in this population. The spot-track-distance of 4.5 μm with 125 nJ energy was the optimal combination within this range.

[J Refract Surg. 2018;34(1):11–16.]

Since their introduction, femtosecond lasers have played a large role in the field of refractive corneal surgery, with the newest application being small incision lenticule extraction (SMILE).1,2 The energy needed to cause this reaction has been the focus of significant study.3–6 The functional parameters of femtosecond lasers, such as the repetition rate of the laser system, energy, and spot-track-distance, significantly affect the interaction of the laser with the corneal tissue. De Medeiros et al.7 reported that femtosecond laser energy level had an effect on corneal stromal cell death and inflammation in a rabbit model. Serrao et al.8 reported that the femtosecond laser stromal interface quality was improved with pulse energy lower and spot separations narrower than those currently used in the clinical setting in femtosecond laser–assisted LASIK, and there is evidence that laser energy has a significant impact on the quality of the stromal interface between the cut lamella in femtosecond laser lenticule extraction.9 Lombardo et al.10 reported that femtosecond laser energy pulse also has a significant impact on the surface quality of posterior stromal lenticules in Descemet stripping automated endothelial keratoplasty.

SMILE is performed using the VisuMax femtosecond laser (Carl Zeiss Meditec, Oberkochen, Germany), which has a 500-kHz repetition rate. A higher repetition rate may enable lower energy per pulse and a tighter line separation.11 Some previous studies12 have indicated that the differences in laser setting did not significantly affect the optical quality in SMILE. However, other studies13,14 have suggested that lower energy resulted in improved healing after SMILE. More studies are needed about the independent relationship between energy setting and the postoperative visual outcome in SMILE.

This study evaluated the independent effect of energy on postoperative UDVA in SMILE with a large sample size. We explored the effect of energy setting in the 125 to 160 nJ range on postoperative visual outcome and discerned an optimal energy setting for a 4.5-μm spot distance.

Patients and Methods

Study Design and Patients

We performed a retrospective cohort study from April 2015 to July 2016 in the Refractive Surgery Center in Tianjin Eye Hospital, Tianjin Medical University, Tianjin, China. The ethics committee of Tianjin Eye Hospital approved the study protocol. The study adhered to the tenets of the Declaration of Helsinki. All patients signed an informed consent before the surgery.

A total of 3,004 eyes had undergone SMILE for myopia and astigmatism correction within this period, and 1,130 eyes were included in this study. We analyzed the independent association of energy and 3-month logMAR uncorrected distance visual acuity (UDVA) in 1,130 eyes after adjusting for spot-track-distance and other possible confounding factors. We further analyzed the relationship between energy and postoperative UDVA in 991 of 1,130 eyes whose spot-track-distance was 4.5 μm.

Inclusion criteria were as follows: age 18 years or older, corrected distance visual acuity (CDVA) of 0.8 (20/25 Snellen) or better and a stable refraction in the past 2 years, intraocular pressure between 10 and 21 mm Hg, myopic spherical correction of less than −10.00 diopters (D), and myopic astigmatism correction of less than −4.00 D. Patients with rigid and soft contact lenses were required to cease wear for at least 4 and 2 weeks, respectively, before the surgery and to have completed postoperative 3-month follow-up records. Exclusion criteria included: active ocular disease, history of ocular surgery or trauma, and keratoconus on corneal topography.

Surgical Technique

All surgeries were performed by the same surgeon (YW). Preoperative medications included 0.5% levofloxacin eye drops (Cravit; Santen, Osaka, Japan) four times a day for 3 days. Intraoperatively, 0.4% oxybu-procaine hydrochloride eye drops (Benoxil; Santen) were used for topical anesthesia. The VisuMax femtosecond laser platform (500-kHz) was used to create the refractive lenticule. Treatment parameters were: cap diameter of 7.6 to 7.9 mm, cap thickness of 110 to 120 μm, side cut angle of 90°, incision position of 90°, incision angle of 45°, incision width of 3 to 3.12 mm, and optical zone of 6.5 to 7 mm. Energy was 125 to 160 nJ, cap and lenticule spot-track-distance was 4.3 to 4.5 μm, and cap side and lenticule side spot-track-distance was 1.8 to 2 μm. The posterior stromal lenticule was scanned from the periphery to the central cornea. The anterior plane of the lenticule was subsequently created from the center to the periphery, which extended toward the corneal surface to create a small incision located at the 12-o'clock position, from which the lenticule was extracted. A blunt spatula was used to separate the stromal lenticule, which the surgeon would then grasp with a pair of forceps and remove. Postoperative medications included 0.5% levofloxacin eye drops four times a day for three days and 0.1% fluorometholone eye drops (Santen) four times a day. The fluorometholone eye drops were administered in tapering doses over a period of 2 months.

Statistical Analyses

All analyses were performed using Empower (R) ( http://www.empowerstats.com, X&Y solutions, Inc., Boston MA) and R ( http://www.Rproject.org) software. Descriptive statistics were used to summarize baseline characteristics and postoperative visual acuity. Smooth curve fitting and the identity regression analyses were conducted after adjusting the confounding factors for analyzing the independent relationship between energy and postoperative UDVA in SMILE. Multivariate-adjusted regression coefficient β (95% confidence interval [CI]) for the associations between independent and dependent variables were assessed using generalized estimating equations, which take into account the correlation between the measurements from two eyes. Changes in UDVA over time were modeled using a mixed model of repeated data. A P value of less than .05 was considered statistically significant.

Results

A total of 1,130 eyes from 565 patients met the criteria for this study, including 591 (52.3%) eyes from males and 539 (47.7%) from females. The patient characteristics are listed in Table 1. The 3-month UDVA (mean ± standard deviation [SD]) of 125 to 160 nJ (by 5-nJ increments) was 1.39 ± 0.19, 1.40 ± 0.32, 1.33 ± 0.27, 1.36 ± 0.27, 1.34 ± 0.25, 1.29 ± 0.19, 1.36 ± 0.27, and 1.19 ± 0.22, respectively. For 3,004 eyes, the percentage that did not conform to this cohort for each record (125 to 160 nJ, by 5 nJ increments) was 66.3%, 56.9%, 71.2%, 60.0%, 55.5%, 72.2%, 66.3%, and 68.9%, respectively, except for 84 eyes for which the data could not be found.

Characteristics of the Study Eyes (N = 1,130)

Table 1:

Characteristics of the Study Eyes (N = 1,130)

Smooth curve fitting (Figure 1) was performed after the adjustment of possible confounding factors, which were sex, age, myopia, astigmatism, central corneal thickness, mean keratometry, preoperative CDVA, spot-track-distance, and side spot-track-distance. The 3-month logMAR UDVA linearly worsened, whereas energy increased from 125 to 160 nJ. Although all of the mean 3-month UDVA was better than 1.0 (20/20 Snellen) for each energy increment, there was still a statistically significant linear reduction in UDVA with increasing energy from 125 to 160 nJ.

The smooth curve fitting showed the association between energy (nJ) and the 3-month uncorrected distance visual acuity (UDVA) after adjusting the relative confounding factors, which were sex, age, myopia, astigmatism, central corneal thickness, mean keratometry, preoperative corrected distance visual acuity, spot-track-distance, and side spot-track-distance. The red lines represented the upper and lower 95% confidence intervals (CIs).

Figure 1.

The smooth curve fitting showed the association between energy (nJ) and the 3-month uncorrected distance visual acuity (UDVA) after adjusting the relative confounding factors, which were sex, age, myopia, astigmatism, central corneal thickness, mean keratometry, preoperative corrected distance visual acuity, spot-track-distance, and side spot-track-distance. The red lines represented the upper and lower 95% confidence intervals (CIs).

Multivariate regression analysis was used to study the relationship between energy and the postoperative UDVA in SMILE. The results suggested that energy was significantly associated with the 3-month UDVA in different models and the regression coefficient β was robust (Table 2). The model was adjusted for sex, age, myopia, astigmatism, mean keratometry, central corneal thickness, preoperative CDVA, spot-track-distance, and side spot-track-distance. Although the regression coefficient β values in the non-adjusted and adjusted models were slightly different when the energy processed as a categorical variable, it was 0.01 (95% CI = 0.00 to 0.01) in two models when the energy processed as a continuous variable.

Multivariate Regression Analysis of Energy and 3-Month Postoperative logMAR UDVA for Spot-Track-Distance of 4.3 to 4.5 μm (N = 1,130)

Table 2:

Multivariate Regression Analysis of Energy and 3-Month Postoperative logMAR UDVA for Spot-Track-Distance of 4.3 to 4.5 μm (N = 1,130)

Multivariate regression analysis was performed further in patients whose spot-track-distance was 4.5 μm, which was the most common clinical setting (Table A, available in the online version of this article). The results suggested that energy was still significantly associated with 3-month UDVA in different models and the correlation was robust (regression coefficient β = 0.01). The model was adjusted for sex, age, myopia, astigmatism, mean keratometry, central corneal thickness, preoperative CDVA, and side spot-track-distance.

Multivariate Regression Analysis of Energy and 3-Month Postoperative logMAR UDVA for Spot-Track-Distance of 4.5 μm (N = 991)

Table A:

Multivariate Regression Analysis of Energy and 3-Month Postoperative logMAR UDVA for Spot-Track-Distance of 4.5 μm (N = 991)

We also analyzed the changes in UDVA from preoperatively to 3 months postoperatively by mixed models (Figure 2). Data were plotted separately for the low energy (preoperative energy setting was 125 to 140 nJ) and high energy (preoperative energy setting was 145 to 160 nJ) groups. The difference of the change at 1 day of the two groups was not statistically significant (difference = −0.03; standard error = 0.02; P = .09). However, the changes at 1 week, 1 month, and 3 months were statistically different between the two groups (1 week: difference = −0.05; standard error = 0.02; P = .01; 1 month: difference = −0.06; standard error = 0.02; P < .01; 3 months: difference = −0.08; standard error = 0.02; P < .01).

Overall changes in uncorrected visual acuity from preoperatively to 3 months postoperatively by mixed models. Data were plotted separately for the low energy (preoperative energy setting was 125 to 140 nJ) and high energy (preoperative energy setting was 145 to 160 nJ) groups. The difference of the change at 1 day of the two groups was not statistically significant (difference = −0.03; standard error [SE] = 0.02; P = .09). However, the changes at 1 week, 1 month, and 3 months were statistically different between the two groups (1 week: difference = −0.05, SE = 0.02, P = .01; 1 month: difference = −0.06, SE = 0.02, P < .01; 3 months: difference = −0.08, SE = 0.02, P < .01). Vertical bars represent standard deviation.

Figure 2.

Overall changes in uncorrected visual acuity from preoperatively to 3 months postoperatively by mixed models. Data were plotted separately for the low energy (preoperative energy setting was 125 to 140 nJ) and high energy (preoperative energy setting was 145 to 160 nJ) groups. The difference of the change at 1 day of the two groups was not statistically significant (difference = −0.03; standard error [SE] = 0.02; P = .09). However, the changes at 1 week, 1 month, and 3 months were statistically different between the two groups (1 week: difference = −0.05, SE = 0.02, P = .01; 1 month: difference = −0.06, SE = 0.02, P < .01; 3 months: difference = −0.08, SE = 0.02, P < .01). Vertical bars represent standard deviation.

Discussion

In this large cohort analysis, we demonstrated that decreasing energy is significantly correlated with improved 3-month UDVA within the range of 125 to 160 nJ. For the entire cohort, the adjusted mean 3-month UDVA for each energy setting was better than 1.0 (20/20 Snellen), which indicates the energy range of 125 to 160 nJ is safe and effective for SMILE. This is consistent with previous studies.15–17

However, smooth curve fitting (Figure 1) demonstrated that the 3-month UDVA was linearly inversely correlated with energy in this range. This correlation remained true in a multivariate regression analysis (Table 2). The regression coefficient in the final model indicated that the 3-month logMAR UDVA would worsen 0.01 with each incremental increase in energy of 5 nJ, increasing after the adjustment of spot distance and other potential confounding factors. Because this study had a large sample size, we further analyzed the energy setting as a categorical variable. The correlation coefficients of 130 to 160 nJ (by 5-nJ increments) were 0.02, 0.03, 0.04, 0.04, 0.05, 0.05, and 0.09, respectively, when 125 nJ was used as the reference. In other words, for every 5 nJ increase in energy, the 3-month UDVA was worsened by 0.02, 0.03, 0.04, 0.04, 0.05, 0.05, and 0.09 logMAR, respectively, compared to the 125 nJ energy setting. The results of our study suggest that the lowest energy used in our study (125 nJ) was the optimal setting for achieving maximum 3-month UDVA within our study population. Ziebarth et al.18 also found that low energy (130 nJ) produced smooth cuts in SMILE with the environmental scanning electron microscopy.

Based on previous studies, we can assume reducing the spot distance too much may generate interference between subsequent shots,19 yielding similar results to using too much energy. During our own observation, high energy may result in an easier lenticular dissection. However, low energy in the range of our study did not cause more difficulties than high energy. Although some studies also showed that increased energy leads to easier dissection of the lenticule,14 others examined the disadvantages of using high energy. Netto et al.19 reported that higher energy in rabbits could induce an inflammatory response and may be responsible for the early blurred vision in femtosecond laser–assisted LASIK. Ziebarth et al.20 and Lombardo et al.10 showed that high energy was associated with more irregular posterior corneal lenticules than low energy using atomic force microscopy. Also, using higher energy or narrower spot distance would lead to a less regular lenticule cutting due to interference from a large gas bubble cavitation and the following laser shots,3,21 which may result in a transitory irregular corneal shape that provides worse vision outcomes until the natural smoothing of the epithelium compensates for the partial regularity of the cornea.8,21 Kunert et al.9 demonstrated a decrease in the surface regularity index in femtosecond lenticule extraction accompanying an increase in pulse energy (150, 180, and 195 nJ), which also supported this conclusion. Moreover, the temporary accumulation of gas bubbles in the intrastromal interface creates transient opacity, or opaque bubble layer, which sometimes diffuses into the corneal stroma, subconjunctival space, and anterior chamber,4,22 and further interferes with the subsequent procedures in SMILE, such as femtosecond laser photo-disruption pulses and lenticule separation.

These reasons may help to explain the better results of low energy compared to high energy or narrow spot-track-distance. However, we can assume that too low energy may also lead to poor visual acuity because it may bring about more tissue bridges, and a mechanical dissection would also increase the surface irregularity. Previous studies3,14 have reported that the plasma threshold of the VisuMax femtosecond laser was from 95 to 105 nJ (data from the manufacturer), which was lower than the lowest energy setting in our study (125 nJ). This may also explain why we believe that low energy in the range of our study did not result in more difficulties than the high. A recent study by Ji et al.13 examining low energy in SMILE (100 to 110 nJ) demonstrated more rapid visual recovery compared to conventional SMILE (150 to 150 nJ). This supports our findings of lower energy settings yielding improved visual outcomes, and may suggest that low energy of 100 to 110 nJ could have a role in SMILE.

Besides demonstrating the independent association between energy setting and postoperative visual acuity after adjustment of spot-track-distance and other potential confounding factors, we further explored an optimal energy setting for 4.5-μm spot-track-distance, which is widely used clinically. The results of multivariate regression analysis (Table A) revealed that energy was still significantly associated with 3-month UDVA in different models and the correlation coefficient was still robust, similar to the coefficient for the model with all spot-track distances (Table 2). The results of analyzing as a categorical variable also indicate that an energy setting of 125 nJ may be optimal within the range of 125 to 150 nJ for a 4.5-μm spot-track-distance in SMILE.

As a retrospective cohort study, we also collected UDVA at 1 day, 1 week, and 1 month postoperatively to analyze the trends between different energy settings by mixed models. We divided all of the energy settings into two groups (low energy and high energy) and could clearly see that the low energy group had a better recovery at 1 week, 1 month, and 3 months (1 day results were not statistically significant). We also analyzed each energy setting and found that the difference was larger between 125 and 160 nJ. Ji et al.13 reported in their study that UDVA 1 day and 1 week postoperatively was significantly better in the lower energy group (100 to 110 nJ) than in the higher energy group (115 to 150 nJ), which indicates that lower laser energy may lead to better visual recovery after SMILE. The better recovery at 1 and 3 months in the low energy group in our study may be because of the different range of energy grouping or the different sample characteristics. However, the consistent conclusion was that low energy may lead to a better UDVA after SMILE.

There are limitations in our study. Because this was a retrospective study, we did not have every patient's follow-up data. If patients who had low energy with poor visual outcomes or those with high energy with good visual outcomes were more likely not to follow up, our results may have been affected. However, there is no evidence that this would happen. The percentage that did not conform to this cohort among 3,004 eyes for each energy level was from 55.5% to 72.2%, which may also indicate that the missing follow-up was relatively random. Additionally, there was a relatively lower proportion of patients with higher energy settings (155 to 160 nJ) and these patients were treated earlier in the course of the study, before our group realized that energy settings could be safely lowered without affecting visual outcomes. However, Dr. Wang was the first surgeon to perform SMILE in China in 2011 and she had performed more than 10,000 SMILE procedures before this study. Thus, we think the confounding factor of learning curve may not affect this study to some extent. In addition, we found that a 4.5-μm spot-track-distance with 125 to 150 nJ energy was relatively dispersive and the results in Table A still showed that low energy was associated with a better visual acuity. Finally, we can only study the association between energy and visual outcome within the range of 125 to 160 nJ. There is a suggestion that even lower energy may have improved visual recovery,13 which deserves further attention.

Energy settings of 125 to 160 nJ can provide good visual outcomes in SMILE, and the low energy setting within this range was associated with a better postoperative UDVA. The spot-track-distance of 4.5 μm with 125 nJ energy setting was the optimal combination within this energy setting range and may provide a good clinical reference for surgeons in SMILE. Whether energy lower than 125 nJ is more effective for visual outcomes is an area for further study.

References

  1. Kymionis GD, Kankariya VP, Plaka AD, Reinstein DZ. Femto-second laser technology in corneal refractive surgery: a review. J Refract Surg. 2012;28:912–920. doi:10.3928/1081597X-20121116-01 [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. Donate D, Albert O, Colliac JP, et al. Femtosecond laser: a micromachining system for corneal surgery [article in French]. J Fr Ophtalmol. 2004;27:783–789. doi:10.1016/S0181-5512(04)96214-6 [CrossRef]
  4. Courtin R, Saad A, Guilbert E, Grise-Dulac A, Gatinel D. Opaque bubble layer risk factors in femtosecond laser-assisted LASIK. J Refract Surg. 2015;31:608–612. doi:10.3928/1081597X-20150820-06 [CrossRef]
  5. Issa A, Al Hassany U. Femtosecond laser flap parameters and visual outcomes in laser in situ keratomileusis. J Cataract Refract Surg. 2011;37:665–674. doi:10.1016/j.jcrs.2010.10.049 [CrossRef]
  6. Calhoun WR 3rd, llev lK. Effect of therapeutic femtosecond laser pulse energy, repetition rate, and numerical aperture on laser-induced second and third harmonic generation in corneal tissue. Lasers Med Sci. 2015;30:1341–1346. doi:10.1007/s10103-015-1726-5 [CrossRef]
  7. de Medeiros FW, Kaur H, Agrawal V, et al. Effect of femtosecond laser energy level on corneal stromal cell death and inflammation. J Refract Surg. 2009;25:869–874. doi:10.3928/1081597X-20090917-08 [CrossRef]
  8. 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]
  9. Kunert KS, Blum M, Duncker GI, Sietmann R, Heichel J. Surface quality of human corneal lenticules after femtosecond laser surgery for myopia comparing different laser parameters. Graefes Arch Clin Exp Ophthalmol. 2011;249:1417–1424. doi:10.1007/s00417-010-1578-4 [CrossRef]
  10. Lombardo M, De Santo MP, Lombardo G, et al. Surface quality of femtosecond dissected posterior human corneal stroma investigated with atomic force microscopy. Cornea. 2012;31:1369–1375. doi:10.1097/ICO.0b013e31823f774c [CrossRef]
  11. Monterosso C, Galan A, Böhm E, Zampini A, Parekh M, Caretti L. Effect of 60-kHz and 150-kHz femtosecond lasers on corneal stromal bed surfaces: a comparative study. ISRN Ophthalmol. 2013;2013:971451. doi:10.1155/2013/971451 [CrossRef]
  12. 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]
  13. Ji YW, Kim M, Kang DSY, et al. Lower laser energy levels lead to better visual recovery after small-incision lenticule extraction: prospective randomized clinical trial. Am J Ophthalmol. 2017;179:159–170. doi:10.1016/j.ajo.2017.05.005 [CrossRef]
  14. Donate D, Thaëron R. Lower energy levels improve visual recovery in small incision lenticule extraction (SMILE). J Refract Surg. 2016;32:636–642. doi:10.3928/1081597X-20160602-01 [CrossRef]
  15. Vestergaard A, Ivarsen AR, Asp S, Hjortdal JØ. Small-incision lenticule extraction for moderate to high myopia: predictability, safety, and patient satisfaction. J Cataract Refract Surg. 2012;38:2003–2010. doi:10.1016/j.jcrs.2012.07.021 [CrossRef]
  16. Blum M, Täubig K, Gruhn C, Sekundo W, Kunert KS. Five-year results of small incision lenticule extraction (ReLEx SMILE). Br J Ophthalmol. 2016;100:1192–1195. doi:10.1136/bjophthalmol-2015-306822 [CrossRef]
  17. Fernández J, Valero A, Martínez J, Piñero DP, Rodríguez-Vallejo M. Short-term outcomes of small-incision lenticule extraction (SMILE) for low, medium, and high myopia. Eur J Ophthalmol. 2017;27:153–159. doi:10.5301/ejo.5000849 [CrossRef]
  18. Ziebarth NM, Lorenzo MA, Chow J, et al. Surface quality of human corneal lenticules after SMILE assessed using environmental scanning electron microscopy. J Refract Surg. 2014;30:388–393. doi:10.3928/1081597X-20140513-01 [CrossRef]
  19. 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.
  20. Ziebarth NM, Dias J, Hürmeriç V, et al. Quality of corneal lamellar cuts quantified using atomic force microscopy. J Cataract Refract Surg. 2013;39:110–117. doi:10.1016/j.jcrs.2012.07.040 [CrossRef]
  21. 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]
  22. Lifshitz T, Levy J, Klemperer I, Levinger S. Anterior chamber gas bubbles after corneal flap creation with a femtosecond laser. J Cataract Refract Surg. 2005;31:2227–2229. doi:10.1016/j.jcrs.2004.12.069 [CrossRef]

Characteristics of the Study Eyes (N = 1,130)

CharacteristicValue
Males, no. (%)591 (52.3%)
Age, mean ± SD (y)23.7 ± 5.6
Myopia, mean ± SD (D)−4.95 ± 1.65
Astigmatism, median (range) (D)−0.50 (−3.75 to 0.00)
Flat keratometry, mean ± SD (D)42.60 ± 1.27
Steep keratometry, mean ± SD (D)43.85 ± 1.39
Mean keratometry, mean ± SD (D)43.22 ± 1.29
CCT, mean ± SD (μm)553.69 ± 28.90
Preoperative logMAR UDVA, mean ± SD1.09 ± 0.36
Preoperative logMAR CDVA, median (range)0.00 (0.00 to 0.10)
Spot-track-distance, no. (%), (μm)
  4.374 (6.6%)
  4.464 (5.7%)
  4.5992 (87.7%)
Side spot-track-distance, no. (%) (μm)
  1.8198 (17.5%)
  2.0932 (82.5%)
Energy, mean ± SD (nJ)137.41 ± 7.70
Energy, no. (%), (nJ)
  12531 (2.7%)
  130386 (34.2%)
  135137 (12.1%)
  140334 (29.7%)
  145161 (14.2%)
  15020 (1.8%)
  15528 (2.4%)
  16033 (2.9%)
3-month logMAR UDVA, median (range)−0.10 (−0.30 to 0.20)
3-month logMAR CDVA, median (range)−0.10 (−0.20 to 0.15)

Multivariate Regression Analysis of Energy and 3-Month Postoperative logMAR UDVA for Spot-Track-Distance of 4.3 to 4.5 μm (N = 1,130)

Energy (nJ)Non-adjustedAdjusteda


β (95% CI)bPβ (95% CI)bP
50.01 (0.00 to 0.01).0040.01 (0.00 to 0.01).002
125referencereference
1300.01 (−0.02 to 0.03).490.02 (−0.01 to 0.05).23
1350.03 (0.00 to 0.06).060.03 (0.00 to 0.07).04
1400.02 (−0.01 to 0.04).140.04 (0.00 to 0.07).03
1450.02 (0.00 to 0.05).100.04 (0.01 to 0.08).02
1500.03 (−0.01 to 0.08).120.05 (0.01 to 0.10).02
1550.02 (−0.02 to 0.06).370.05 (−0.02 to 0.11).20
1600.08 (0.03 to 0.13).0010.09 (0.02 to 0.17).02

Multivariate Regression Analysis of Energy and 3-Month Postoperative logMAR UDVA for Spot-Track-Distance of 4.5 μm (N = 991)

Energy (nJ)Non-adjustedAdjusteda


β (95% CI)bPβ (95% CI)bP
50.01 (0.00 to 0.01).060.01 (0.00 to 0.02).003
125referencereference
1300.01 (−0.02 to 0.03).610.02 (−0.01 to 0.06).22
1350.03 (0.00 to 0.06).070.03 (0.00 to 0.07).04
1400.02 (−0.01 to 0.04).190.04 (0.00 to 0.08).03
1450.02 (0.00 to 0.05).080.05 (0.01 to 0.08).02
1500.03 (−0.02 to 0.08).260.06 (0.01 to 0.10).02
Authors

From the Clinical College of Ophthalmology, Tianjin Medical University, Tianjin, China (LL, JM, TC, YW); the Department of Ophthalmology, University of California, San Francisco, California (JMS); and Tianjin Eye Hospital and Eye Institute, Tianjin Key Lab of Ophthalmology and Visual Science, Tianjin, China (YW).

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

Supported in part by the National Natural Science Foundation of China (NSFC) (No. 81670884).

AUTHOR CONTRIBUTIONS

Study concept and design (LL, YW); data collection (JM, TC); analysis and interpretation of data (LL, JMS, YW); writing the manuscript (LL); critical revision of the manuscript (JMS, JM, TC, YW)

Correspondence: Yan Wang, MD, PhD, Tianjin Eye Hospital and Eye Institute, Tianjin Key Lab of Ophthalmology and Visual Science, Clinical College of Ophthalmology, Tianjin Medical University, No. 4. Gansu Road, He-ping District, Tianjin 300020, China. E-mail: wangyan7143@vip.sina.com

Received: June 24, 2017
Accepted: November 09, 2017

10.3928/1081597X-20171115-01

Advertisement

Sign up to receive

Journal E-contents
Advertisement