Small incision lenticule extraction (SMILE) is a relatively new technique for the correction of myopia and myopic astigmatism,1–5 and has recently received approval from the U.S. Food and Drug Administration. In SMILE, a femtosecond laser is used to create an intrastromal lenticule that is mechanically removed through a small corneal incision.6
SMILE has been suggested as an alternative to LASIK and numerous recent studies reported comparable safety, efficacy, and predictability for both procedures.7,8 Compared to LASIK, SMILE has the advantage of being an all-in-one femtosecond laser procedure, hence avoiding flap-associated complications.9 However, for the same reason accurate centration of treatment in SMILE may be subject to a surgeon's operative experience because no active eye tracker such as in LASIK is used during the SMILE procedure. This may increase the risk of decentered treatments, potentially resulting in undesirable side effects such as halos, glare, monocular diplopia, and decreased visual acuity.10
A crucial step to achieve correct centration during SMILE is docking the treatment applanation interface (disposable curved contact glass) to the cornea while the patient fixates on a blinking green target light.6 Minor movements of the eye occasionally can be observed during docking as the cornea approaches the curved contact glass. These minor movements may occur when the location of the maximum corneal curvature (corneal apex11) deviates from the center of the cornea (corneal vertex = corneal intercept with the patient's line of sight11,12) and hence the applanation interface is not centered on the intended point.
Thus, we divided eyes following SMILE in two groups according to preoperative distance between corneal apex and vertex (A-V distance) to investigate the relationship between a displaced corneal apex and postoperative visual outcome.
Patients and Methods
SMILE has been performed at the Department of Ophthalmology, Medical University of Graz, Austria, since April 2015. The first consecutive eyes treated uneventfully between April 2014 and September 2016 with SMILE surgery were retrospectively reviewed for this investigation. The study was conducted in accordance with the tenets of the Declaration of Helsinki and approved by the local ethics committee of the Medical University of Graz.
Patients presenting for refractive surgery were examined with automated refraction, Pentacam HR corneal tomography (Oculus Optikgeräte GmbH, Wetzlar, Germany), measurement of uncorrected (UDVA) and corrected (CDVA) distance visual acuity, determination of manifest and cycloplegic refraction, tonometry, and slit-lamp examination including funduscopy. Contact lens wear was discontinued prior to examination for at least 2 weeks. Patients who were found suitable for keratorefractive surgery and gave informed consent after a thorough explanation were offered SMILE surgery. Examinations at the 3-month postoperative visit included manifest refraction, slit-lamp microscopy, tonometry, and corneal tomography. All preoperative and postoperative examinations were performed by ophthalmologists during scheduled visits.
A VisuMax femtosecond laser (Carl Zeiss Meditec, Jena, Germany) was used for SMILE. All treatments were performed under topical anesthesia with two drops of 0.8% oxybuprocaine tetrachloride 5 to 10 minutes before surgery. The patient was positioned under the laser and asked to fixate on the blinking green target light. While the contact glass was approaching the eye, the reflection of the circularly illuminated border of the contact glass was placed concentric with the green fixation light. As soon as contact between the cornea and the contact glass occurred, the green blinking fixation light was focused sharply for the patient because the focus of the light was automatically adjusted according to the patient's refraction. The patient was told to look directly at the green blinking light while the coaxially fixating green Purkinje image was brought into the intended position (patient's line of sight) by small joystick correction movements. The concentric circles of the right ocular reticule in the surgical microscope were used for centering the contact glass on the corneal reflex. After checking, appropriate centration suction was applied. The patient was told to continue focusing on the target light even during suction. The creation of the posterior lenticule surface from periphery to center was followed by the creation of the anterior lenticule surface from center to periphery. At the 110° position, a 4-mm–wide incision was created for lenticule extraction. In all eyes, lenticule diameter was 6.5 mm, cap diameter was 7.5 mm, lenticule side cut was 15 μm, and cap thickness was 120 μm. Laser configuration parameters were repetition rate of 500 kHz, spot distance of 4.5 μm for the lenticule and 2 μm for its border, and pulse energy level of 170 nJ. After laser treatment, delineation of the anterior and posterior lenticule surface was performed by moving a blunt circular tip back and forth to break any remaining tissue bridges, followed by lenticule extraction with forceps. After lenticule extraction, the stromal pocket was flushed with a 1:1 mixture of saline and dexamethasone sodium phosphate 0.1%, and patients received one drop of ofloxacin 0.3%.
Postoperative follow-up examinations were conducted at 1 day, 1 week, 1 month, and 3 months. All patients were postoperatively treated with a combination of dexamethasone 0.1% and tobramycin 0.3% administered topically four times a day for 1 week, and then tapered to once daily for another week. Patients were also instructed to use preservative-free artificial tears at least every 2 hours after the surgery and at least four times a day for 1 month.
Retrospectively analyzed tomographic parameters included preoperative and 3-month postoperative values for central corneal thickness, maximum keratometry (Kmax), distance between corneal vertex and apex (location of Kmax), distance between vertex and pupil center, and mesopic pupil diameter. Corneal wavefront aberrations were analyzed using root mean square (RMS) of higher order aberrations (HOAs); coefficients of the Optical Society of America were calculated for a standardized diameter of 6 mm. The ambient light condition in the examination room was kept stable at a mesopic level of 3 candelas [cd]/m2.
Decentration was assessed with a method recently described by Kanellopoulos and Asimellis13 (digitized method) and later by Reinstein et al.14 (manual technique using a grid paper). For each eye, a difference map of the tangential curvature was generated using the preoperative and 3-month postoperative Pentacam HR scans. To locate the center of the optical zone, a tracing paper with concentric circles and a central cross was superimposed on the difference map shown on the computer screen. Thereby, the central zone up to the mid-peripheral power inflection point on the tangential difference map was defined as the optical zone. The best-fitting circle was aligned with the optical zone and the center of the optical zone with reference to the corneal vertex was determined by adjusting the computer mouse curser at the central cross of the tracing paper. While holding the computer mouse button, the x and y coordinates of centration offset between the center of the optical zone and the corneal vertex were displayed. Then the distance between the center of the optical zone and the corneal vertex was calculated by using the Pythagorean theorem.
For analysis of the impact of a displaced corneal apex in SMILE, treated eyes were divided in two groups based on their preoperative A-V distance. An A-V distance of 1 mm or greater was defined as “large” and less than 1 mm as “small.” The limit of 1 mm was chosen because it was close to the median A-V distance and therefore divided our patients into groups of almost the same size. Analyzed refractive parameters included 3-month postoperative UDVA, preoperative and 3-month postoperative manifest refractive spherical equivalent (MRSE), refractive astigmatism, and CDVA.
Statistical analysis was performed using SPSS for Windows software (version 23; SPSS Inc., Chicago, IL). The two-sample t test was used for analyzing normally distributed data and the non-parametric Mann–Whitney U test was used for non-normally distributed data. A P value less than .05 was considered statistically significant.
Ninety-four eyes of 48 consecutive patients were included (24 women, 24 men). Mean patient age was 34.8 ± 8.8 years (range: 19 to 60 years). Preoperative mean MRSE was −5.78 ± 1.76 diopters (D) (range: −2.75 to −10.00 D), and mean refractive astigmatism was 0.59 ± 0.59 D (range: 0 to 3.50 D). Preoperative CDVA was −0.02 ± 0.06 logMAR (range: −0.10 to 0.20 logMAR).
All SMILE surgeries were conducted by two surgeons (NA = 26 eyes, BV = 68 eyes). There were no significant differences between surgeons in preoperative MRSE (P = .819), refractive astigmatism (P = .353), CDVA (P = .167), A-V distance (P = .652), and RMS HOAs (P = .951) and postoperative UDVA (P = .525), MRSE (P = .088), refractive astigmatism (P = .211), CDVA (P = .732), and RMS HOAs (P = .203).
Postoperative refractive outcomes of the two groups are displayed in Figure 1. In both groups, 70 of the treated eyes (74%) had a postoperative UDVA of 0.00 logMAR or better, and 85 eyes (90%) had 0.10 logMAR or better. Regarding safety, 59 eyes (63%) had an unchanged CDVA, 18 eyes (19%) gained one line, and 17 eyes (18%) lost one line. In the group with a large A-V distance, 9 eyes lost one line of Snellen acuity (20%) and 7 eyes (16%) gained one line, whereas in the group with a small A-V distance 8 eyes (16%) lost one line and 11 eyes (22%) gained one line. No eye in either group lost two or more lines of Snellen acuity.
Refractive outcomes after small incision lenticule extraction. (A) Uncorrected distance visual acuity (UDVA), (B) change in corrected distance visual acuity (CDVA), (C and D) spherical equivalent attempted versus achieved, (E) spherical equivalent refractive accuracy, and (F) refractive astigmatism. group 1 = ≥ 1 mm A-V distance; group 2 = < 1 mm A-V distance; D = diopters
The distribution and range of the A-V-distances for the entire population are displayed in Figure 2.
Preoperative distance between corneal apex and corneal vertex (A-V distance). group 1 = ≥ 1 mm A-V distance; group 2 = < 1 mm A-V distance
Preoperative and postoperative tomographic and visual characteristics of the two groups with eyes with large or small A-V distance are displayed in Table A (available in the online version of this article). Regarding preoperative parameters, apart from HOAs, which were significantly higher in eyes with large A-V distance, no significant differences were found between groups. Also, postoperative HOAs were significantly higher in eyes with large A-V distance; however, the mean increase in HOAs from preoperatively to postoperatively did not differ significantly between groups (P = .214). Comparison of angle kappa, determined by calculating the offset of the corneal intercept with the Pentacam's optical axis and the geometric pupil center, revealed no significant difference between the two groups (P = .279).
Characteristics of Eyes With a Large (≥ 1 mm) or Small (< 1 mm) Distance Between Corneal Apex and Corneal Vertex Before and After SMILE
Postoperative CDVA was significantly decreased in the group with a large A-V distance compared to the group with a small A-V distance (−0.01 ± 0.06 and −0.04 ± 0.06 logMAR, respectively, P = .014). Other postoperative parameters including UDVA, MRSE, refractive astigmatism, pachymetry, mean keratometry value, and optical zone decentration did not differ significantly between groups.
SMILE has gained widespread acceptance in recent years due to its reduced surgical time and flap-related side effects,9,15 but it is a technically challenging procedure associated with a significant learning curve that precedes surgical competence.9,16,17 This has to be considered when interpreting our reported visual outcomes because all evaluated procedures were performed by two surgeons in the beginning phase after the introduction of the technique in our department. Seventy-four percent of the treated eyes achieved a UDVA of 0.00 logMAR or better. Better visual results than ours were reported by Lin et al.18 (60 eyes) and Ganesh and Gupta19 (50 eyes), who observed a UDVA of 0.00 logMAR or better in 85% and 84% of eyes, respectively. However, in a recent review, Moshirfar et al.6 analyzed 1,759 eyes treated with SMILE and reported a cumulative visual acuity of 0.00 logMAR or better in 62%. Their findings were in accordance with those of Hjortdal et al.,15 who observed a UDVA of 0.00 logMAR or better in 61% of 670 included eyes.
One potential limitation of the SMILE technique is that the centration is conducted by the surgeon during the docking process, which may increase the risk of decentered treatments. Liu et al.20 evaluated SMILE treatment decentration from pupil center and corneal vertex against its effect on visual results, and observed better refractive outcomes in eyes with the treatment center closer to the corneal vertex. Li et al.21 investigated eyes after SMILE in the early learning curve and measured the postoperative decentration from the corneal vertex. Although they detected mild decentration, good visual outcomes could be achieved. Wong et al.22 showed that a decentration greater than 0.6 mm from the kappa intercept may result in compromised visual outcomes after SMILE. However, Reinstein et al.14 and Lazaridis et al.23 found no significant differences in centration accuracy between eyes treated with SMILE or LASIK.
In our study, we defined the corneal intercept with the patient's line of sight as the corneal vertex. However, as reference for the analysis of Scheimpflug images, we actually used the corneal intercept with the imaging device's optical axis, which is the center of the coordinate system in the Pentacam software. These two intercepts are not exactly the same, but the latter is a good approximation of the first when the patient looks directly at the fixation light. Many refractive surgeons use the coaxially sighted corneal light reflex as the intended point of centration for the same reason.24 Although we were aware of this deviation, we decided to use the term “corneal vertex” for both intercepts for better readability.
Our aim was to evaluate the impact of a displaced corneal apex in SMILE. Unlike LASIK, where the alignment of the treatment can be objectively adjusted and is controlled by eye trackers, the centration in SMILE is more subjective and relies on the fixation of the patient while the surgeon controls the patient's fixation during the docking process. In this procedure, it is important that the patient bed moves up slowly, especially at the moment when the curved contact glass touches the cornea, to allow the corneal vertex to automatically fit the vertex of the contact glass. Because the steepest point of the cornea touches the contact surface first, in eyes with a larger distance between that point (corneal apex) and the corneal vertex, this may lead to a mild shift of the cornea and hence to consecutive decentration.
Visual results were good for both groups, although postoperative CDVA was significantly worse in eyes with a large A-V distance. Nonetheless, the mean decentration of the optical zone did not differ significantly between the two groups in our study, indicating that a displaced corneal apex does not predispose to a decentered SMILE treatment. The worse postoperative CDVA in eyes with a larger A-V distance may rather be a consequence of differences in preoperative corneal wavefront aberrations between the groups.
Corneal wavefront aberrations were significantly higher in eyes with a larger A-V distance both preoperatively and postoperatively, reflecting a higher topographical irregularity in those eyes with a displaced apex. HOAs increased from preoperatively to postoperatively in both groups of eyes, but the change in HOAs did not differ significantly between the groups. Previous studies reported a comparable increase in HOAs after SMILE treatment.18,25,26
Angle kappa, defined as the angle formed by the pupillary and visual axis, was approximatively determined by calculating the offset of the corneal intercept with the imaging device's optical axis and the geometrical pupil center. It did not significantly affect the refractive outcome in our eyes. This may be due to the technique we used with centration of the treatment on the corneal intercept with the line of sight as described above and previously suggested by other studies.14,26 Wong et al.,22 who used a different approach and attempted centration of photodissection on the pupil center, observed compromised visual outcomes in eyes with large angle kappa.
A limitation of the current study is the retrospective study design with its inherent weaknesses. Further prospective studies are recommended, particularly to investigate whether the observed impact of a displaced apex on visual outcomes is the same in more experienced surgeons.
The results of our study suggest that SMILE can achieve good visual results in the early phase of the learning curve. We observed an impaired postoperative CDVA in eyes with a larger A-V distance than in eyes with a smaller A-V distance. However, despite a displaced corneal apex, postoperative visual results were good in those eyes.
- 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]
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Characteristics of Eyes With a Large (≥ 1 mm) or Small (< 1 mm) Distance Between Corneal Apex and Corneal Vertex Before and After SMILEa
|Characteristic||≥ 1.0 mm||< 1.0 mm||P|
|Age (y)||35.9 ± 7.9 (21.0 to 54.0)||34.1 ± 9.7 (19.0 to 60.0)||.335|
| MRSE (D)||−6.13 ± 1.92 (−10.00 to −2.75)||−5.48 ± 1.58 (−9.88 to −3.00)||.073|
| Refractive astigmatism (D)||0.65 ± 0.75 (0.00 to 3.50)||0.53 ± 0.41 (0.00 to 1.75)||.921|
| CDVA (logMAR)||−0.01 ± 0.06 (−0.10 to 0.20)||−0.03 ± 0.05 (−0.10 to 0.10)||.132|
| Pachymetry (μm)||555 ± 34 (515 to 646)||553 ± 30 (491 to 620)||.802|
| Mean keratometry value (D)||43.07 ± 3.35 (40.40 to 46.35)||43.77 ± 1.26 (40.90 to 46.60)||.420|
| Corneal RMS HOAs 6 mm (μm)||0.391 ± 0.094 (0.251 to 0.701)||0.338 ± 0.070 (0.219 to 0.559)||.002b|
| Distance vertex to location of maximum keratometry value (mm)||2.5 ± 1.1 (1.2 to 4.7)||0.5 ± 0.2 (0.1 to 0.9)|
| Distance vertex to pupil center (mm)||0.2 ± 0.1 (0 to 0.3)||0.2 ± 0.1 (0 to 0.60)||.279|
| Pupil diameter (mm)||3.3 ± 0.6 (2.1 to 4.4)||3.3 ± 0.6 (2.2 to 5.0)||.842|
| UDVA (logMAR)||0.02 ± 0.11 (−0.10 to 0.40)||0.00 ± 0.12 (−0.10 to 0.40)||.191|
| MRSE (D)||−0.01 ± 0.26 (−1.38 to 0.75)||0.02 ± 0.34 (−1.00 to 1.00)||.087|
| Refractive astigmatism (D)||0.09 ± 0.27 (0.00 to 1.25)||0.11 ± 0.31 (0.00 to 1.50)||.815|
| CDVA (logMAR)||−0.01 ± 0.06 (−0.10 to 0.20)||−0.04 ± 0.06 (−0.10 to 0.10)||.014b|
| Pachymetry (μm)||457 ± 46 (381 to 547)||463 ± 35 (384 to 522)||.478|
| Mean keratometry value (D)||38.95 ± 2.07 (35.20 to 41.95)||39.49 ± 1.75 (35.50 to 42.35)||.328|
| Corneal RMS HOAs 6 mm (μm)||0.846 ± 0.360 (0.310 to 1.696)||0.664 ± 0.227 (0.279 to 1.294)||.008b|
| Optical zone decentration (mm)||0.31 ± 0.17 (0.06 to 0.75)||0.29 ± 0.18 (0.03 to 0.74)||.608|