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Review/Update

Clinical outcomes of small-incision lenticule extraction and femtosecond laser–assisted wavefront-guided laser in situ keratomileusis

Piñero, David P. PhD*; Teus, Miguel A. MD, PhD

Author Information
Journal of Cataract & Refractive Surgery: July 2016 - Volume 42 - Issue 7 - p 1078-1093
doi: 10.1016/j.jcrs.2016.05.004
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Abstract

Recently, a new femtosecond laser–based technique for the correction of refractive errors, small-incision lenticule extraction (SMILE, Carl Zeiss Meditec AG), was developed and has increased in popularity.1 Instead of creating a corneal flap, the technique creates an intrastromal lenticule that corresponds to the desired refractive correction with a femtosecond laser and then removes the lenticule through a small incision.2 The femtosecond laser applies pulses at a predetermined depth within the corneal tissue. The size and shape of the lenticule are based on the patient’s refractive error.3 The all-in-one femtosecond laser procedure may reduce the complications associated with flap cutting.4 As only a small incision is required, fewer corneal nerves are severed and this may lead to fewer side effects after surgery (eg, a lower incidence of postoperative dry eye).5

To date, the clinical results of the small-incision lenticule extraction technique have been good.3,4,6–17 However, the advantages of the small-incision lenticule extraction technique compared with previous techniques for refractive correction, such as the femtosecond laser–assisted laser in situ keratomileusis (LASIK), have not been demonstrated conclusively. Although there is consistent scientific evidence supporting less corneal nerve damage, less decrease in corneal sensitivity, and, consequently, a lower incidence of dry eye with small-incision lenticule extraction than with femtosecond laser–assisted LASIK in the immediate postoperative period,5,18–27 the superiority of small-incision lenticule extraction over femtosecond laser–assisted LASIK in visual, refractive, and aberrometric outcomes,13,14,28 as well as in corneal biomechanical behavior, is not clear.29–32 Furthermore, complications of the small-incision lenticule extraction technique have been reported, and some are equivalent to those reported for femtosecond laser–assisted LASIK.33–41

There is consistent scientific evidence of the efficacy and safety of femtosecond laser–assisted LASIK for the correction of myopia and astigmatism.42–44 The use of wavefront-guided ablation profiles with the technique has been shown to provide a benefit over conventional aspheric ablation profiles in the control of higher-order aberrations (HOAs) and visual quality.45,46 Excellent visual, refractive, and aberrometric outcomes have been reported for wavefront-guided femtosecond laser–assisted LASIK with the latest excimer laser platforms.47–56

The aim of the current analysis was to review the scientific literature published to date on the clinical outcomes of small-incision lenticule extraction and wavefront-guided femtosecond laser–assisted LASIK techniques for the correction of myopia. For this purpose, all studies of small-incision lenticule extraction and wavefront-guided femtosecond laser–assisted LASIK published from January 2012 to September 2015 were reviewed and the results obtained were analyzed following a sequential scheme: visual and refractive outcomes, ocular and corneal aberrometric outcomes, contrast sensitivity outcomes, corneal sensation and dry-eye outcomes, corneal backscattering changes, corneal biomechanical changes, and complications.

To our knowledge, this is the first review to compile all the scientific peer-reviewed literature on small-incision lenticule extraction and compare it with that on wavefront-guided femtosecond laser–assisted LASIK. In addition, the review includes an updated discussion of the impact of small-incision lenticule extraction on corneal biomechanics, including findings in the latest studies.

Criteria for the bibliographic search

A bibliographic search was performed in PubMed and Google Scholar to find all articles reporting outcomes of small-incision lenticule extraction and wavefront-guided femtosecond laser–assisted LASIK for the correction of myopia. For the wavefront-guided femtosecond laser–assisted LASIK technique, the search was limited to the period between 2012 and 2015 to include the results of the latest laser platforms commercially released and therefore the most advanced and optimized systems. Some reviews of small-incision lenticule extraction have been published in peer-reviewed journals.1,57 The objective of our review was to compile all the scientific evidence on small-incision lenticule extraction and wavefront-guided femtosecond laser–assisted LASIK to detect and define the advantages and disadvantages of 1 technique over the other. A study by Shaheen et al.53 reporting the outcomes of wavefront-guided LASIK using a mechanical microkeratome was also included in the analysis because it has the longest follow-up of wavefront-guided LASIK outcomes.

The review excluded letters to the editor and articles about refractive lenticule extraction (ReLEx, Carl Zeiss Meditec AG) and femtosecond lenticule extraction (FLEx, Carl Zeiss Meditec AG) (previous techniques of small-incision lenticule extraction).

Literature review

Visual Outcomes

Table 1 is a summary of the mean results of uncorrected (UDVA) and corrected (CDVA) distance visual acuity reported in the reviewed studies.6,7,9–17,26,28–30,38,48–52,55,58,59 The mean logMAR UDVA ranged from 0.0229 to −0.1712 for small-incision lenticule extraction and from −0.0447,49,50 to −0.1755 for wavefront-guided femtosecond laser–assisted LASIK. The mean logMAR CDVA ranged from 0.0129 to −0.2012 and from −0.0447 to −0.19,48 respectively. All the mean values were obtained in studies with a relatively short follow-up, ranging from 1 to 12 months. The study by Shaheen et al.53 reported visual outcomes 4 years after wavefront-guided LASIK using a mechanical microkeratome. The mean UDVA and CDVA were 0.01 logMAR ± 0.07 (SD) and −0.05 ± 0.04 logMAR, respectively.

Table 1
Table 1:
Visual and refractive outcomes in small-incision lenticule extraction and wavefront-guided femtosecond laser–assisted LASIK studies.

Figure 1 summarizes the distribution of postoperative UDVA. There was a significant variability between studies. The percentage of eyes with a postoperative UDVA of 0.10 logMAR or better ranged from 83%9 to 100%22 for small-incision lenticule extraction and from 95%48 to 97%48 for wavefront-guided femtosecond laser–assisted LASIK. This percentage was reported in only 4 studies evaluating the outcomes of wavefront-guided femtosecond laser–assisted LASIK.47,48,50,51 The percentage of eyes with a postoperative UDVA of 0.00 logMAR or better ranged from 60%9 to 100%10,16 and from 83.80%49 to 99.40%,56 respectively. In the Shaheen et al. study,53 the logMAR UDVA was 0.1 or better (20/25 Snellen) at 4 years in all eyes evaluated. Of the 15 small-incision lenticule extraction studies reporting the percentage of eyes with a postoperative UDVA of 0.00 logMAR or better, 10 studies (66.7%) reported values of 80% or better and 6 studies (40.0%) reported values of 90% or better. In contrast, all 8 studies of wavefront-guided femtosecond laser–assisted LASIK reported the percentage of eyes achieving a UDVA of 20/20 or better was 80% or better and 6 of the studies (75%) reported 90% or better (Figure 2).

Figure 1
Figure 1:
LogMAR UDVA reported in small-incision lenticule extraction (top) and wavefront-guided femtosecond laser–assisted LASIK (bottom) studies (UDVA = uncorrected distance visual acuity).
Figure 2
Figure 2:
Percentages of UDVA of 20/20 or better or 80% or better (blue bars) and 90% or better (red bars) in small-incision lenticule extraction (SMILE) and wavefront-guided femtosecond laser–assisted LASIK (WFG FS-LASIK) studies (UDVA = uncorrected distance visual acuity).

Figure 3 summarizes the distribution of changes in lines of CDVA after surgery. For small-incision lenticule extraction, the percentage of eyes that lost 1 line of CDVA ranged from 0%13,29,58 to 15%12,39 and the percentage that lost 2 lines ranged from 0%6,8–10,12,13,15,29,58 to 2.0%.39 For wavefront-guided femtosecond laser–assisted LASIK, the percentage of eyes that lost 1 line of CDVA ranged from 0%47,48,50 to 23.0%48 and the percentage that lost 2 lines ranged from 0%47–52 to 2.0%.48 The percentage of eyes that gained 1 line of CDVA ranged from 0%14 to 57.10%8 for small-incision lenticule extraction and from 14.0%51 to 51.0%52 for wavefront-guided femtosecond laser–assisted LASIK. The percentage of eyes gaining 2 lines of CDVA ranged from 0%8,9,12–14,29,58 to 11%39 and from 0%47,52 to 32%,50 respectively. Wang et al.16 did not find a decrease in CDVA in 88 eyes having small-incision lenticule extraction surgery, and Kunert et al.7 reported only 1 eye losing more than 2 lines of CDVA in 91 eyes having small-incision lenticule extraction. Lin et al.14 report mean safety indexes (mean postoperative CDVA/mean preoperative CDVA) of 1.00 ± 0.06 and 1.01 ± 0.05 at 1 month and 3 months, respectively, for small-incision lenticule extraction. Ivarsen et al.38 reported that CDVA was within 1 line of the preoperative level at 18 months in all 1800 eyes having small-incision lenticule extraction. Shaheen et al.53 reported that 98.8% of eyes having wavefront-guided LASIK achieved a CDVA of 0.0 logMAR (approximately 20/20 Snellen) or better at 4 years.

Figure 3
Figure 3:
Changes in lines of CDVA after surgery in small-incision lenticule extraction (top) and wavefront-guided femtosecond laser–assisted LASIK (bottom) studies.

The percentage of loss of lines of CDVA reported in the reviewed studies was less than 30% (Figure 4). The percentage of gain of lines of CDVA was 15.0% or more in wavefront-guided femtosecond laser–assisted LASIK studies (Figure 5) and ranged from 0% to 59.60% in small-incision lenticule extraction studies (Figure 4).

Figure 4
Figure 4:
Percentages of eyes losing or gaining lines of CDVA of ≤5%, 10%, 15%, 20%, 30%, 40%, and 50% in small-incision lenticule extraction (SMILE) and wavefront-guided femtosecond laser–assisted LASIK (FS-LASIK) studies.
Figure 5
Figure 5:
Postoperative SE in small-incision lenticule extraction (top) and wavefront-guided femtosecond laser–assisted LASIK (bottom) studies (SE = spherical equivalent).

According to these outcomes, both small-incision lenticule extraction and wavefront-guided femtosecond laser–assisted LASIK have the ability to provide good visual outcomes. The UDVA outcome seems to be slightly better after wavefront-guided femtosecond laser–assisted LASIK as higher percentages of eyes achieving a UDVA of 0.00 logMAR or better have been reported. This may correlate with the trend toward a slightly higher magnitude of residual myopia after small-incision lenticule extraction that has been detected and will be discussed in the next section. Both procedures show a similar outcome with no clear differences in loss of lines of CDVA. The trend toward higher percentages of eyes gaining lines of CDVA after wavefront-guided femtosecond laser–assisted LASIK may be related to the different aberrometric behavior of each technique or that less corneal backscattering is present in the early postoperative period after femtosecond laser–assisted LASIK. It should be noted that small-incision lenticule extraction was not a wavefront-guided procedure at the time this article was prepared.

Refractive Outcomes

Table 1 shows the mean values of spherical equivalent (SE) in the reviewed studies.6,7,9–17,26,28–30,38,48–52,55,58,59 A slight trend toward a more myopic residual SE was found in small-incision lenticule extraction studies; that is, the mean postoperative SE ranged from −0.0117 to −0.33 diopter (D)29 in small-incision lenticule extraction and from −0.0250 to −0.17 D48 in wavefront-guided femtosecond laser–assisted LASIK. The percentage of eyes with a postoperative SE within ±1.00 D of the targeted SE ranged from 95%29 to 100%12,15 and from 96%52 to 100%,50 respectively. The percentage of eyes with a postoperative SE within ±0.50 D ranged from 67.60%58 to 100%12 and from 80%52 to 100%, respectively (Figure 5). All studies reporting refractive outcomes had follow-up periods between 1 month and 12 months. Only Shaheen et al.53 reported refractive results of wavefront-guided LASIK at 4 years; the mean SE was within ±0.50 D in 97.3% of eyes.

In addition to reporting the predictability of SE correction, some studies evaluated the predictability of the cylinder correction. In 98 eyes having small-incision lenticule extraction, Zhang et al.8 reported that the mean postoperative astigmatism in vector form was −0.10 D × 28.63 at 12 months, with a correction index of 1.00 ± 0.32. Reinstein et al.10 reported that 95.0% and 100.0% of 122 eyes having small-incision lenticule extraction had a cylinder of 0.50 D or less and 1.00 D or less, respectively, at 12 months. Good outcomes in astigmatic correction after small-incision lenticule extraction were also reported by Sekundo et al.,15 with 92% of eyes having a cylinder of 0.50 D or less at 12 months. In contrast, Yao et al.58 reported that 79.1% of eyes had a cylinder of 0.50 D or less; Gyldenkerne et al.28 reported a mean cylinder of −0.40 ± 0.32 D in 368 eyes, and Qian et al.60 reported a mean cylinder of −0.36 ± 0.38 D in 122 eyes.

In wavefront-guided femtosecond laser–assisted LASIK studies, the astigmatic results were excellent, with mean postoperative cylinder values of −0.14 ± 0.20 (1 month), −0.37 ± 0.38 (3 months), −0.28 ± 0.30 (12 months), −0.17 ± 0.12 (1 month), −0.09 ± 0.13 (12 months), and −0.05 ± 0.20 D (3 months) reported by Schallhorn et al.,51 Schallhorn et al.,49 He et al.,52 Yu and Manche,48 Prakash et al.,50 and Smadja et al.,47 respectively. In their study of 243 eyes, Schallhorn et al.51 found no statistically significant differences between the magnitude of surgically induced and intended astigmatism vectors at 1 month (P = .27). They also observed that an angle between the orientation of surgically and intended astigmatism vectors of 5 degrees or less was present in most eyes (90.1%, 219 eyes). In another study,49 the same authors reported a slight trend toward undercorrection of refractive cylinder in 611 eyes, with a correction ratio of 0.92 ± 0.14 at 3 months. In this sample, the percentages of eyes with an angle between the orientation of surgically and intended astigmatism vectors within 5 degrees and 10 degrees were 89.2% (545 eyes) and 98.4% (601 eyes), respectively.49 The percentage of eyes with a manifest cylinder of 0.50 D or less was 87% at 12 months in a study by He et al.52 and 95% at 1 month in a study by Yu and Manche.48 In a study by Prakash et al.,50 94% of eyes had a postoperative astigmatism of 0.25 D or less. There was a slight trend toward undercorrection after the relationship between surgically induced (SIA) and target induced (TIA) astigmatisms was analyzed (SIA = 0.91 × TIA − 0.01; R2 = 0.95; P < .001).

These outcomes suggest a slight trend toward more myopic residual SE in small-incision lenticule extraction, which would explain the trend toward the lower UDVA values that have been mentioned. This trend was particularly observed in the first studies conducted to evaluate the results of small-incision lenticule extraction. Therefore, minimal inaccuracies in the algorithm used to estimate the depth of the extracted lenticule from the intended refractive correction may be related to the lower UDVA finding. Likewise, the potential effect of the different ranges of correction between small-incision lenticule extraction and wavefront-guided femtosecond laser–assisted LASIK may account for this trend, but this effect is expected to be minimal as similar ranges of myopia have been treated in the reviewed studies (Table 1). No clear differences in the correction of astigmatism have been reported between the 2 techniques; some authors report some level of undercorrection of refractive cylinder after small-incision lenticule extraction procedure, and others report no undercorrection.

Ocular and Corneal Aberrometric Outcomes

Although ocular and corneal aberrations were analyzed in several studies that evaluated the clinical outcomes of the small-incision lenticule extraction and wavefront-guided femtosecond laser–assisted LASIK techniques, the studies analyzed aberrations for different pupil apertures and therefore comparisons should be made carefully (Figure 6). Only corneal aberrations were evaluated in the small-incision lenticule extraction studies. Specifically, the mean HOA root mean square (RMS) was reported as 0.34 ± 0.11 μm (5.0 mm pupil) by Vestergaard et al.9 and as 0.52 ± 0.13 μm (6.0 mm pupil) by Agca et al.29 The latter authors also reported a mean corneal coma RMS and spherical aberration of 0.28 ± 0.12 μm and 0.30 ± 0.13 μm, respectively, for a 6.0 mm pupil after small-incision lenticule extraction. Most studies of small-incision lenticule extraction reported the ocular aberrometric outcomes for a 5.0 mm pupil, whereas all studies of wavefront-guided femtosecond laser–assisted LASIK reported the outcomes for a 6.0 mm pupil. In small-incision lenticule extraction studies, the mean HOA RMS ranged from 0.267 μm13 (5.0 mm pupil) to 0.427 μm14 (6.0 mm pupil), whereas in wavefront-guided femtosecond laser–assisted LASIK studies, the mean HOA RMS ranged from 0.29 μm56 to 0.46 μm52 (all 6.0 mm pupil). The mean coma RMS ranged from 0.24 μm58 (5.0 mm pupil) to 0.706 μm (6.0 mm pupil)14 and from 0.15 μm56 to 0.28 μm52 in small-incision lenticule extraction studies and femtosecond laser–assisted LASIK studies, respectively. Therefore, there was a trend toward more coma aberration in patients having small-incision lenticule extraction. This can be related to the presence of mild levels of treatment decentration, as suggested and analyzed by Li et al.39 (Figure 6), who found that vertical coma, horizontal coma, spherical aberration, and HOA RMS increased at the sixth postoperative month, with the greatest increase in vertical coma. Using means of multivariate association analysis, Li et al. also noted a relationship between the magnitude of the horizontal decentration and the induced horizontal coma.

Figure 6
Figure 6:
Mean values of ocular aberrometric outcomes in small-incision lenticule extraction (top) and wavefront-guided femtosecond laser–assisted LASIK (bottom) studies (HOA RMS = higher-order aberrations root mean square; RMS = root mean square; Sph aberr = spherical aberrations).

Some comparative studies of small-incision lenticule extraction versus LASIK have reported different trends.13,14,28,29 However, these studies did not include eyes having wavefront-guided LASIK and some did not include eyes having femtosecond laser–assisted LASIK. Lin et al.14 compared eyes operated on with small-incision lenticule extraction and non-wavefront-guided femtosecond laser–assisted LASIK and found significantly higher values of HOA RMS (P = .006) and spherical aberration (P < .001) after femtosecond laser–assisted LASIK, but the level of coma was significantly higher after small-incision lenticule extraction than after femtosecond laser–assisted LASIK (P = .039). Yao et al.58 also found that small-incision lenticule extraction eyes had more coma postoperatively and femtosecond laser–assisted LASIK eyes had more spherical aberration.

These studies suggest a trend toward a higher level of coma aberration after small-incision lenticule extraction than after wavefront-guided femtosecond laser–assisted LASIK. This seems to be related to the presence of mild levels of treatment decentration. A correlation between horizontal decentration and coma aberration has been reported in a study evaluating the outcomes of the small-incision lenticule extraction.39 No clear differences in primary spherical aberration between small-incision lenticule extraction and wavefront-guided femtosecond laser–assisted LASIK can be determined from the reviewed studies.

Contrast Sensitivity Outcomes

Heterogeneity in the mode, procedure, and analysis of contrast sensitivity outcomes was found between the studies. Consequently, different trends have been reported, some of them apparently contradictory. Vestergaard et al.9 did not find significant differences in contrast sensitivity from baseline to 6 months after small-incision lenticule extraction using the Freiburg Acuity and Contrast Test (P = .12). In contrast, Reinstein et al.10 reported a significant increase in contrast sensitivity 6 months after small-incision lenticule extraction for the spatial frequencies of 3, 6, 12, and 18 cycles per degree (P < .05). Sekundo et al.15 found no significant differences between the preoperative and postoperative contrast sensitivity values for either photopic or scotopic ambient illumination at any time during the 12-month follow-up of small-incision lenticule extraction (P > .05). Ganesh and Gupta13 compared the postoperative contrast sensitivity obtained after small-incision lenticule extraction and non-wavefront-guided femtosecond laser–assisted LASIK and found that it was reduced in both groups at all spatial frequencies. These authors also found that contrast sensitivity was better in the LASIK group than in the small-incision lenticule extraction group the day after surgery, with no significant between-group differences at 15 days (P = .15).13

Corneal Sensation and Dry-Eye Outcomes

Several studies evaluated and compared corneal sensitivity after small-incision lenticule extraction and LASIK with the Cochet-Bonnet esthesiometer18–27 but did not specifically evaluate the corneal sensitivity after wavefront-guided femtosecond laser–assisted LASIK. Because corneal sensitivity and the subbasal nerve plexus are affected by the surgical technique itself, not by the optical calculations used to define the ablation profile, the comparative studies of small-incision lenticule extraction and femtosecond laser–assisted LASIK that are available to date on this issue have been analyzed. The decrease in corneal sensitivity in the early postoperative period after small-incision lenticule extraction and femtosecond laser–assisted LASIK has been confirmed by several authors,21,22,24–27,61 although the decrease was significantly lower after small-incision lenticule extraction (P < .05).22,25,26,61 Demirok et al.61 reported that corneal sensation was significantly lower after femtosecond laser–assisted LASIK than after small-incision lenticule extraction at 1 week, 1 month, and 3 months (P < .01 at all timepoints). In the comparative study by Li et al.,26 the mean central corneal sensitivity was greater after small-incision lenticule extraction than after femtosecond laser–assisted LASIK during a 6-month follow-up (P ≤ .03). A similar trend was found by Gao et al.21during a 3-month follow-up of small-incision lenticule extraction and femtosecond laser–assisted LASIK. Li et al.22 also evaluated changes in corneal sensitivity after both techniques, but they studied different quadrants of the cornea, not the central area only. They reported that corneal sensation was less compromised in the central, inferior, nasal, and temporal areas in small-incision lenticule extraction eyes than in femtosecond laser–assisted LASIK eyes at the 1-week and 1-month visits. They confirmed that the inferior, nasal, and temporal quadrants recovered faster than other areas after small-incision lenticule extraction.22 In the femtosecond laser–assisted LASIK group, the sensation over the flap did not recover to preoperative levels until 6 months postoperatively.22 Wei and Wang27 also performed a corneal sensitivity analysis by quadrants after small-incision lenticule extraction and femtosecond laser–assisted LASIK and reported higher corneal sensitivity values in every quadrant 1 week and 1 and 3 months after small-incision lenticule extraction than after femtosecond laser–assisted LASIK (P < .01).

The primary reason for the between-group difference in postoperative corneal sensitivity is the effect of each surgical technique on the corneal nerve plexus. Li et al.25 reported that the decrease in subbasal nerve density was less severe in small-incision lenticule extraction eyes than in femtosecond laser–assisted LASIK eyes at 1 week (P = .0147), 1 month (P = .0243), and 3 months (P = .0498), with no significant between-group difference at 6 months (P = .5277). The subbasal nerve density has been found to be correlated positively with central corneal sensitivity after small-incision lenticule extraction and femtosecond laser–assisted LASIK.25 Agca et al.18 also reported that eyes treated with small-incision lenticule extraction had a higher density of subbasal nerve fibers than eyes treated with femtosecond laser–assisted LASIK in the early postoperative period (1 week to 3 months), but the density was equivalent by 6 months. Factors other than the technique may have an effect on the differences in corneal sensitivity between small-incision lenticule extraction and femtosecond laser–assisted LASIK in the initial postoperative period. In an animal model, less keratocyte apoptosis, proliferation, and inflammation were reported with small-incision lenticule extraction than with femtosecond laser–assisted LASIK.23 Milder ocular surface changes have been reported in the early postoperative period with small-incision lenticule extraction than with femtosecond laser–assisted LASIK, with tear inflammatory mediators interleukin (IL)-6 and nerve growth factor playing crucial roles in the ocular surface healing process following the 2 surgical techniques.21 Specifically, lower and faster recovery of IL-6 and nerve growth factor levels in tears were observed after small-incision lenticule extraction than after femtosecond laser–assisted LASIK (P < .05).21

As might be expected, the changes in corneal sensitivity after small-incision lenticule extraction and femtosecond laser–assisted LASIK had a relevant effect on clinical parameters commonly used to characterize the dry-eye syndrome.13,19,20,21,24–26,61 Vestergaard et al.24 reported that osmolarity, reflex tearing, and tear meniscus height measurements were unchanged from preoperatively to 6 months after surgery (P > .05), whereas the tear breakup time (TBUT) decreased significantly at 6 months (P = .01). Demirok et al.61 also found that dry-eye parameters such as TBUT, Schirmer test, and tear-film osmolarity did not change significantly after small-incision lenticule extraction (P > .05). Other authors have reported significant changes in the Schirmer test outcomes and tear osmolarity after small-incision lenticule extraction (P < .05).13,20 Ganesh and Gupta13 reported a reduction in the Schirmer 1 and 2 tests and TBUT after small-incision lenticule extraction and after femtosecond laser–assisted LASIK at 3 months, but the postoperative values were significantly lower after femtosecond laser–assisted LASIK than after small-incision lenticule extraction (P < .0001). Li et al.26 also found a longer TBUT 1 month (P = .029) and 3 months (P = .045) after small-incision lenticule extraction than after femtosecond laser–assisted LASIK. Xu and Yang20 did not find a significant decrease in the results from the Schirmer secretion test without anesthesia after small-incision lenticule extraction (P = .97); however, at 3 months (P = .04) and 6 months (P = .03), the values were significantly higher than after femtosecond laser–assisted LASIK. The same authors found a significant decrease in TBUT after both techniques that did not return to preoperative levels within 6 months (P < .01).20 In a comparative contralateral-eye study, Vestergaard et al.24 found that more patients reported pronounced dryness and postoperative foreign-body sensation during the first week after femtosecond lenticule extraction than after small-incision lenticule extraction. Other studies characterizing symptomatology with validated questionnaires (McMonnies20 and Ocular Surface Disease Index26) after the 2 techniques have reported significantly higher scores in eyes that had femtosecond laser–assisted LASIK (P < .05).20,26

Corneal Backscattering Changes

To date, only 1 study has evaluated and compared corneal backscattering data after small-incision lenticule extraction and femtosecond laser–assisted LASIK.62 In this study, 60 eyes of 30 patients were randomized to receive small-incision lenticule extraction in 1 eye and femtosecond laser–assisted LASIK in the fellow eye. At 1 week and 1, 3, and 6 months, in vivo confocal microscopy analysis was performed and the following parameters were measured: maximum backscattered intensity and the depth from which it was measured, backscattered light intensity 30 μm below Bowman membrane at the flap interface and 150 μm below the superficial epithelium, and the number of refractive particles at the flap interface. The mean backscattered light intensity at all measured depths and the maximum backscattered light intensity were statistically significantly higher after small-incision lenticule extraction than after femtosecond laser–assisted LASIK at 1 week and 1 and 3 months (P < .05). Differences in light intensity between small-incision lenticule extraction and femtosecond laser–assisted LASIK at 6 months were not statistically significant, nor were differences in the number of refractive particles at the flap interface at any postoperative visit (P < .01).62

Several factors may contribute to the increased backscattered light intensity after small-incision lenticule extraction, such as the laser energy used or the increased number of microdistortions of Bowman membrane in the initial postoperative period.58 In a comparative study, Yao et al.58 found that on the first postoperative day, microdistortions in Bowman layer were visible in 88.5% of eyes after small-incision lenticule extraction and 42.1% of eyes after femtosecond laser–assisted LASIK. From 4 to 14 months, the total number of microdistortions was significantly higher in small-incision lenticule extraction eyes than in femtosecond laser–assisted LASIK eyes (3.0 ± 2.3 versus 0.8 ± 1.1) (P < .001).

Corneal Biomechanical Changes

Various studies have evaluated corneal biomechanical changes after small-incision lenticule extraction with the 2 devices currently available to measure these changes in clinical practice: a biomechanical waveform analyzer (Ocular Response Analyzer, Reichert Technologies) and a dynamic Scheimpflug analyzer (Corvis ST, Oculus Optikgeräte GmbH).29–32,63,64 In a comparative study, Agca et al.29 found no statistically significant differences in corneal hysteresis (CH) or corneal resistance factor (CRF) values obtained with the biomechanical waveform analyzer system between small-incision lenticule extraction eyes and femtosecond laser–assisted LASIK eyes in any preoperative or postoperative measurements during a 6-month follow-up (P > .05). The mean 6-month postoperative CH values were 8.95 ± 1.47 mm Hg and 9.02 ± 1.27 mm Hg in small-incision lenticule extraction eyes and femtosecond laser–assisted LASIK eyes, respectively, whereas the mean CRF values were 7.77 ± 1.37 mm Hg and 8.07 ± 1.26 mm Hg, respectively.29

Wang et al.31 evaluated corneal biomechanical changes after small-incision lenticule extraction and femtosecond laser–assisted LASIK using the biomechanical waveform analyzer system and found a significant decrease in CH, CRF, and other parameters derived from analysis of the response curve to the air puff (p1 area, p2 area) after both techniques (P < .05). No significant between-group differences in CH and CRF were reported by Pedersen et al. (P ≥ .62).32 Therefore, the scientific evidence indicates that both techniques generate relevant biomechanical changes when measured with the biomechanical waveform analyzer system. Wang et al.31 confirmed that between-group differences in the biomechanical parameters measured by the biomechanical waveform analyzer were not significant in eyes with myopia of −6.0 D or less (P > .05), whereas CH, CRF, p1 area, and p2 area decreased significantly more in femtosecond laser–assisted LASIK eyes than in small-incision lenticule extraction eyes with myopia of more than −6.0 D (P < .05). Nevertheless, a closer look at the Wang et al. article shows that the corneas in the high myopia femtosecond laser–assisted LASIK group were thinner than those in the small-incision lenticule extraction group (CCT of 539.4 versus 556 μm, respectively), and although this difference was not statistically significant (P = .06), it might have biased the results.

Using the dynamic Scheimpflug analyzer, Mastropasqua et al.63 detected significant differences in the deformation amplitude and applanation times (time until first applanations [AT1] and time until second applanation [AT2]) between 7 days and 30 to 90 days in a group of eyes having small-incision lenticule extraction (P < .05), with no significant changes in these parameters in the period from 30 to 90 days (P ≥ .263). The authors also observed a pattern of less structural integrity with progressively higher corrections at 7 days but not at the later follow-up.63 Shen et al.59 also detected changes in biomechanical parameters after small-incision lenticule extraction with the dynamic Scheimpflug analyzer. Specifically, they found statistically significant differences in AT1 (P < .001), AT2 (P = .001), and deformation amplitude (P < .001) between presurgery, post-lenticule creation, and post-lenticule extraction. The significant changes in these parameters occurred following subsequent corneal lenticule extraction compared with values obtained preoperatively.59 When changes in the dynamic Scheimpflug analyzer parameters after small-incision lenticule extraction and femtosecond laser–assisted LASIK were compared, no clear differences were observed.30,32 Pedersen et al.32 did not find significant differences between small-incision lenticule extraction and femtosecond laser–assisted LASIK in radius at highest concavity, AT1, AT2, and deflection length at highest concavity (highest concavity deflection length) (P < .05). The only difference they found was that femtosecond laser–assisted LASIK eyes had statistically significantly shorter times to highest concavity than small-incision lenticule extraction eyes (P = .01).32

The clinical scientific evidence appears to provide little support to the mathematical model determining in vitro corneal stromal tensile strength as a function of depth after small-incision lenticule extraction and LASIK that was proposed by Reinstein et al.64 using previously published data. This mathematical model predicted that total stromal tensile strength postoperatively was considerably higher after small-incision lenticule extraction than after photorefractive keratectomy and LASIK; however, the main assumption that the strongest anterior corneal lamellae remains intact after small-incision lenticule extraction is probably flawed by the fact (not considered by the authors) that the anterior corneal stroma does not remain at the same position as preoperatively but has to modify its radius of curvature. In other words, the anterior “uncut” stroma (the cap) undergoes backward displacement after the stromal lenticule is resected, becoming looser and thus offering less resistance to a possible forward movement of the stromal bed. This may explain the case of ectasia reported after a small-incision lenticule extraction procedure performed to correct a case of low myopia.65 More studies are necessary to develop a consistent model predicting the real effect of small-incision lenticule extraction on corneal biomechanics.

Complications

Complications after small-incision lenticule extraction and wavefront-guided femtosecond laser–assisted LASIK are generally uncommon, and most are the same for both techniques. Table 2 summarizes the complications reported in the reviewed studies.8–10,12,13,16,33,38,50,56,58 The following complications have been reported after small-incision lenticule extraction: epithelial defects,10,38 epithelial ingrowth,10 interface haze,10,12 microstriae,12 diffuse lamellar keratitis (DLK),33 suction loss,10,13,16,38 corneal cap tear or perforation,38 interface infiltrates,38 microdistortions in Bowman layer,58 corneal ectasia,33–37 and irregular corneal topography.38 Ivarsen et al.38 reported 18 cases of DLK in 1112 eyes having small-incision lenticule extraction, with 13 eyes (72.2%) having stage 1 DLK, 4 eyes (22.2%) having stage 2, and 1 (5.6%) having stage 3. All cases resolved after intensive treatment with topical corticosteroids, within 3 weeks for stage 1 and stage 2 cases and within 1 month for the stage 3 case. The authors confirmed there was a statistically significant increase in the incidence of DLK with thinner lenticules (P = .02) and larger diameter lenticules (P = .01).38 To date, 4 case reports of corneal ectasia have been reported after small-incision lenticule extraction.34,36,37,65 El-Naggar37 and Remy and Kohnen34 reported cases suggesting that patients with preoperative forme fruste keratoconus or early keratoconus might develop significant progression of corneal ectasia after small-incision lenticule extraction. Wang et al.36 presented the case of a 19-year-old patient with forme fruste keratoconus who developed corneal ectasia 6.5 months after surgery. The patient had a preoperative minimum central corneal thickness (CCT) of 546 μm in the right eye and 542 μm in the left eye, and a manifest refraction of −6.75 −1.00 × 45 and −6.75 −0.75 × 140, respectively. No corneal endothelial alterations have been reported after small-incision lenticule extraction.66 Epithelial ingrowths, microstriae, and DLK have also been reported as complications after wavefront-guided femtosecond laser–assisted LASIK, which confirms that small-incision lenticule extraction and femtosecond laser–assisted LASIK share the same type of complications,50,56 except those that are flap-related.67 However, other cap-related complications have been described after small-incision lenticule extraction.10,38

Table 2
Table 2:
Complications reported in small-incision lenticule extraction and wavefront-guided femtosecond laser–assisted LASIK studies.
Table 2
Table 2:
Continued.

Successful retereatments have been reported using wavefront-guided ablations after the primary flap was lifted in eyes with previous LASIK.68,69 However, to date, scientific evidence showing the feasibility of successful retreatments with the small-incision lenticule extraction technique is very limited.70 Retreatments using topography-guided surface ablation in small-incision lenticule extraction–treated eyes have been shown to provide poor results, with relevant postoperative haze problems.71

Discussion

According to the scientific articles reviewed and analyzed, the following conclusions can be drawn:

  • Both small-incision lenticule extraction and wavefront-guided femtosecond laser–assisted LASIK provide good visual outcomes and effective correction of the refractive error, with the mean postoperative UDVA ranging from 0.01 to −0.20 logMAR. However, higher percentages of eyes achieving a postoperative UDVA of 0.00 logMAR or better have been reported in studies evaluating the outcomes of wavefront-guided femtosecond laser–assisted LASIK. Therefore, a slightly better UDVA outcome seems to be present after wavefront-guided femtosecond laser–assisted LASIK than after small-incision lenticule extraction
  • The safety of both procedures (small-incision lenticule extraction and wavefront-guided femtosecond laser–assisted LASIK) is good, with no clear differences in loss of lines of CDVA between techniques. However, a trend toward a higher percentage of eyes gaining lines of CDVA after wavefront-guided femtosecond laser–assisted LASIK was observed. All studies evaluating the visual outcomes after wavefront-guided femtosecond laser–assisted LASIK reported a percentage of gain of lines of CDVA of 15.00% or more, whereas in small-incision lenticule extraction studies, the percentage of gain of lines of CDVA ranged from 0% to 59.60%.
  • A slight trend toward a more myopic residual SE was found in small-incision lenticule extraction studies, which would explain the trend toward slightly more limited UDVA outcomes. The mean postoperative SE ranged from −0.01 to −0.33 D in small-incision lenticule extraction studies and from −0.02 to −0.22 D in wavefront-guided femtosecond laser–assisted LASIK studies. Furthermore, the percentage of eyes with a postoperative SE within ±0.50 D of the target ranged from 67.60% to 100% and from 80% to 100%, respectively. This may result from minimal inaccuracies in the algorithm used to estimate the depth of the lenticule extracted according to the intended refractive correction, which may have improved or will improve in the near future.
  • Some authors have reported a level of undercorrection of refractive cylinder after the small-incision lenticule extraction procedure, but others have reported no undercorrection. In contrast, the predictability of refractive cylinder was excellent in studies evaluating the outcomes of wavefront-guided femtosecond laser–assisted LASIK. More studies are necessary to determine whether the predictability of the cylinder correction is more limited after small-incision lenticule extraction than after wavefront-guided femtosecond laser–assisted LASIK.
  • There was a trend toward a higher level of coma aberration in patients having small-incision lenticule extraction than in those having wavefront-guided femtosecond laser–assisted LASIK. This could be related to the presence of mild levels of treatment decentration. No clear differences in spherical aberration between small-incision lenticule extraction and wavefront-guided femtosecond laser–assisted LASIK were detected, with no consistent evidence of the superiority of 1 technique over the other in this area.
  • A more limited outcome in contrast sensitivity seems to be present during the first 15 postoperative days in eyes having small-incision lenticule extraction. This may be related to the increased intrastromal light backscattering and number of Bowman layer microdistortions observed in the initial postoperative period. More studies are needed to determine the difference in contrast sensitivity between small-incision lenticule extraction and wavefront-guided femtosecond laser–assisted LASIK.
  • Various studies have confirmed the decrease in corneal sensitivity in the early period after small-incision lenticule extraction, but this decrease has been found to be less than the decrease after femtosecond laser–assisted LASIK. This is primarily due to a smaller decrease in subbasal nerve density in the early postoperative period in eyes having small-incision lenticule extraction than in those having femtosecond laser–assisted LASIK. At 6 months, no differences in subbasal corneal nerve density were present between small-incision lenticule extraction and femtosecond laser–assisted LASIK. More studies are needed to confirm potential differences at the inflammatory level between the techniques in the early postoperative period.
  • The TBUT decreases after both small-incision lenticule extraction and femtosecond laser–assisted LASIK, with apparently less reduction after small-incision lenticule extraction in the early postoperative period. A similar trend has been reported for the outcomes of the Schirmer test. This may be related to the generation of more dry-eye symptomatology after femtosecond laser–assisted LASIK than after small-incision lenticule extraction, as suggested by studies evaluating this issue by means of validated questionnaires. More research is needed on this issue to extract consistent conclusions.
  • There is increased backscattered light intensity at the intrastromal level after small-incision lenticule extraction compared with after femtosecond laser–assisted LASIK, which disappears 6 months after surgery. This may be related to a more limited visual recovery in the early postoperative period. The significantly higher number of microdistortions of Bowman layer after small-incision lenticule extraction in the early postoperative period may be another reason for this potential limitation in the visual recovery. Future studies should confirm whether there is a direct relationship between the levels of laser energy used and the level of intrastromal backscattering in the initial postoperative period of small-incision lenticule extraction.
  • Corneal biomechanical changes occur after both small-incision lenticule extraction and femtosecond laser–assisted LASIK, with no scientific evidence supporting the superiority of 1 technique over the other in this area. There may be a potential benefit of small-incision lenticule extraction over femtosecond laser–assisted LASIK in eyes with high myopia, but this has to be confirmed in future studies.
  • Small-incision lenticule extraction and femtosecond laser–assisted LASIK procedures share certain types of complications, including epithelial defects, epithelial ingrowth, and corneal ectasia, but not flap-related complications. However, other cap-related complications have been described after small-incision lenticule extraction.
  • Scientific evidence supporting the stability of results exists for wavefront-guided LASIK in the long term (4 years) but not for small-incision lenticule extraction (12 months only).
  • There is limited evidence of the outcomes of small-incision lenticule extraction retreatments, and the results of surface ablation retreatments in small-incision lenticule extraction eyes are poor.

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