Journal of Refractive Surgery

Original Article Supplemental Data

Clinical Outcomes of SMILE With a Triple Centration Technique and Corneal Wavefront-Guided Transepithelial PRK in High Astigmatism

Ikhyun Jun, MD, PhD; David Sung Yong Kang, MD; Dan Z. Reinstein, MD, MA(Cantab), FRCSC; Samuel Arba-Mosquera, PhD; Timothy J. Archer, MA(Oxon), DipCompSci(Cantab); Kyoung Yul Seo, MD, PhD; Tae-im Kim, MD, PhD

Abstract

PURPOSE:

To comparatively investigate the clinical outcomes, vector parameters, and corneal aberrations of small incision lenticule extraction (SMILE) with a triple centration technique and corneal wavefront-guided transepithelial photorefractive keratectomy (PRK) for the correction of high astigmatism.

METHODS:

This retrospective, comparative case series study included 89 eyes (89 patients) that received treatment for myopia with high astigmatism (≥ 2.50 diopters) using SMILE with a triple centration technique (SMILE group; 45 eyes) and corneal wavefront-guided transepithelial PRK (transepithelial PRK group; 44 eyes). Visual acuity measurement, manifest refraction, slit-lamp examination, autokeratometry, corneal topography, and evaluation of corneal wavefront aberration were performed preoperatively and at 1, 3, and 6 months after surgery. The safety, efficacy, vector parameters, and corneal aberrations at 6 months after surgery were compared between the two groups.

RESULTS:

At 6 months after surgery, the transepithelial PRK and SMILE groups exhibited comparable mean uncorrected distance visual acuities (−0.06 ± 0.07 and −0.05 ± 0.07 logMAR, respectively), safety, efficacy, and predictability of refractive and visual outcomes. There was a slight but statistically significant difference in the correction index between the transepithelial PRK and SMILE groups (0.96 ± 0.11 and 0.91 ± 0.10, respectively). Whereas the transepithelial PRK group exhibited increased corneal spherical aberration and significantly reduced corneal coma and trefoil, no changes in aberrometric values were noted in the SMILE group.

CONCLUSIONS:

Both SMILE with a triple centration technique and corneal wavefront-guided transepithelial PRK are effective and provide predictable outcomes for the correction of high myopic astigmatism, although slight undercorrection was observed in the SMILE group. The triple centration technique was helpful in astigmatism correction by SMILE.

[J Refract Surg. 2018;34(3):156–163.]

Abstract

PURPOSE:

To comparatively investigate the clinical outcomes, vector parameters, and corneal aberrations of small incision lenticule extraction (SMILE) with a triple centration technique and corneal wavefront-guided transepithelial photorefractive keratectomy (PRK) for the correction of high astigmatism.

METHODS:

This retrospective, comparative case series study included 89 eyes (89 patients) that received treatment for myopia with high astigmatism (≥ 2.50 diopters) using SMILE with a triple centration technique (SMILE group; 45 eyes) and corneal wavefront-guided transepithelial PRK (transepithelial PRK group; 44 eyes). Visual acuity measurement, manifest refraction, slit-lamp examination, autokeratometry, corneal topography, and evaluation of corneal wavefront aberration were performed preoperatively and at 1, 3, and 6 months after surgery. The safety, efficacy, vector parameters, and corneal aberrations at 6 months after surgery were compared between the two groups.

RESULTS:

At 6 months after surgery, the transepithelial PRK and SMILE groups exhibited comparable mean uncorrected distance visual acuities (−0.06 ± 0.07 and −0.05 ± 0.07 logMAR, respectively), safety, efficacy, and predictability of refractive and visual outcomes. There was a slight but statistically significant difference in the correction index between the transepithelial PRK and SMILE groups (0.96 ± 0.11 and 0.91 ± 0.10, respectively). Whereas the transepithelial PRK group exhibited increased corneal spherical aberration and significantly reduced corneal coma and trefoil, no changes in aberrometric values were noted in the SMILE group.

CONCLUSIONS:

Both SMILE with a triple centration technique and corneal wavefront-guided transepithelial PRK are effective and provide predictable outcomes for the correction of high myopic astigmatism, although slight undercorrection was observed in the SMILE group. The triple centration technique was helpful in astigmatism correction by SMILE.

[J Refract Surg. 2018;34(3):156–163.]

Small incision lenticule extraction (SMILE) is a femtosecond laser–based technique for the correction of myopic errors.1–4 It has become popular in the field of refractive surgery since Shah et al.5 and Sekundo et al.6 first reported the clinical outcomes of SMILE in 2011. SMILE provides faster recovery and a lower risk of haze than photorefractive keratectomy (PRK).7 In contrast, relative to LASIK, corneas treated by SMILE might remain more resistant and exhibit a lesser decrease in corneal sensitivity.7,8

Single-step transepithelial PRK, which involves the removal of epithelium by laser phototherapeutic keratectomy, is an alternative to conventional PRK.9–11 Transepithelial PRK provides visual outcomes comparable to conventional or alcohol-assisted PRK and LASIK. Furthermore, transepithelial PRK has advantages including a shorter operation time and epithelial healing period and lesser postoperative pain and haze formation compared to conventional and alcohol-assisted PRK.9–11

The correction of astigmatism is as important as the correction of spherical equivalent (SE) in refractive surgery. However, few studies have evaluated both treatment modalities in eyes with high astigmatism. Furthermore, no study to date has compared clinical outcomes between SMILE and transepithelial PRK. In this study, we adopted a triple centration technique to minimize decentration in SMILE.

The purpose of the current study was to comparatively evaluate the clinical outcomes (ie, visual acuity, refractive error, astigmatic vector parameters, and corneal aberrometric changes) of SMILE with a triple centration technique and corneal wavefront-guided transepithelial PRK in patients with high astigmatic myopia.

Patients and Methods

Patients

This interventional study was a retrospective, comparative, observational case series approved by the institutional review board of Yonsei University College of Medicine, Seoul, South Korea (Institutional Review Board No. 4-2017-0203). The study followed the tenets of the Declaration of Helsinki. Patients enrolled in this study received treatment by corneal wavefront-guided transepithelial PRK (transepithelial PRK group) or SMILE (SMILE group) from a single experienced surgeon (DSK) at Eyereum Eye Clinic, Seoul, South Korea, between October 2014 and November 2016.

The inclusion criteria for this study were myopia of less than 8.00 diopters (D) with refractive astigmatism of 2.50 D or more, age 20 to 45 years, stable refraction for at least 1 year, and corrected distance visual acuity (CDVA) of 0.8 Snellen or better. The exclusion criteria were presence of severe ocular surface diseases, keratoconus, or cataract, and a history of intraocular or corneal surgery. We retrospectively reviewed the medical records of 89 patients (89 eyes; transepithelial PRK group: 44 eyes; SMILE group: 45 eyes) who met the inclusion and exclusion criteria. Right or left eye data were randomly chosen using randomization tables, regardless of ocular dominance, refraction, or presence of aberrations.

Preoperative and Postoperative Assessment

Before surgery, all patients underwent a detailed ophthalmological examination that included evaluation of the logMAR uncorrected distance visual acuity (UDVA) and CDVA, manifest refraction, slit-lamp examination (Haag-Streit AG, Köniz, Switzerland), keratometry, measurement of central corneal thickness (CCT) (ARK-530A autokeratometry; Nidek Co., Ltd., Gamagori, Japan), and Scheimpflug-based corneal topography (Pentacam HR; Oculus Optikgeräte GmbH, Wetzlar, Germany). Corneal wavefront aberrations were measured using the Keratron Scout topographer (Optikon 2000 spA, Rome, Italy). These examinations were all repeated at 1 week and 1, 3, and 6 months postoperatively.

Surgical Technique

Surgeries were performed as described previously.12,13 For the transepithelial PRK group, ablation profile planning was performed using the integrated Optimized Refractive Keratectomy-Custom Ablation Manager software (version 5.1; SCHWIND eye-tech-solutions, Kleinostheim, Germany). A static cyclotorsion compensation algorithm profile was used for corneal wavefront-guided treatment, and dynamic cyclotorsion control was implemented automatically for all treatments. Centration on the corneal vertex was ensured by input from the topographer. The epithelium and stroma were ablated with the SCHWIND Amaris 1050RS excimer laser platform (SCHWIND eyetech-solutions) using a single continuous profile. The ablation profiles used here were customized full corneal wavefront-guided ablation profiles and were calculated using the ORK-CAM software module (SCHWIND eyetech-solutions).

For the SMILE group, surgery was performed with standardized techniques using the 500-KHz VisuMax system (Carl Zeiss Meditec AG, Jena, Germany). Before surgery, three centration points were marked at the slit lamp. While the patient was in the sitting position and fixated on the center of the slit-lamp beam, which was narrowed as much as possible until all three point markings were completed, the first two markings were made 7 mm apart at the horizontal meridian; these markings were made appropriately by bisecting the first Purkinje reflex or the coaxially sighted corneal light reflex. The third marking was made on the inferior cornea by vertically (90°) rotating the slit-lamp beam and bisecting the first Purkinje reflex or the coaxially sighted corneal light reflex. This triple centration technique helped verify the subjective fixation of the patient during docking by providing an objective view through the surgical screen (Figure A and Video 1, available in the online version of this article). Once the anterior (upper) and posterior (lower) planes of the lenticule were defined, the anterior and posterior interfaces were dissected using a microspatula with a blunt circular tip and extracted with microforceps. The integrity of the lenticule was subsequently assessed.

Triple centration technique. Surgeon's view after docking of the small incision lenticule extraction operating eye. Triple markings were made at the horizontal meridian and inferior cornea by dissecting the coaxially sighted corneal light reflex. Decentration and static cyclotorsion of the eye was noted before docking (blue markings and lines). During the docking procedure, decentration and static cyclotorsion were corrected in aid of triple markings. The eye was centered at the coaxially sighted corneal light reflex and corrected cyclotorsion after docking (red markings and lines). See also Video 1 (available in the online version of this article).

Figure A.

Triple centration technique. Surgeon's view after docking of the small incision lenticule extraction operating eye. Triple markings were made at the horizontal meridian and inferior cornea by dissecting the coaxially sighted corneal light reflex. Decentration and static cyclotorsion of the eye was noted before docking (blue markings and lines). During the docking procedure, decentration and static cyclotorsion were corrected in aid of triple markings. The eye was centered at the coaxially sighted corneal light reflex and corrected cyclotorsion after docking (red markings and lines). See also Video 1 (available in the online version of this article).

Statistical Analysis

Data are expressed as mean ± standard deviation/standard error. The Student's t test was used to determine significant differences between the two groups. Continuous variables were compared by linear regression analysis. Statistical analysis was performed using the SPSS statistics software (version 23; IBM Corporation, Armonk, NY). A P value of .05 or less was considered statistically significantly different.

Results

This study included 44 eyes in the transepithelial PRK group and 45 eyes in the SMILE group. Table 1 presents the baseline characteristics of the two groups of patients. There was no significant difference in mean preoperative SE (−5.19 ± 1.55 vs −4.64 ± 1.52 D) or cylinder (−2.84 ± 0.35 vs −2.90 ± 0.42 D) between the transepithelial PRK and SMILE groups, respectively. There was also no significant difference in preoperative UDVA, CDVA, or CCT between the two groups. Optical zone, which might affect the spherical aberration in refractive surgery, was not different between the two groups. The ablation depth in the transepithelial PRK group was significantly smaller than the lenticule thickness in the SMILE group (P = .011), although the optical zones and SE were comparable.

Characteristics of Eyes That Underwent Corneal Wavefront-Guided Transepithelial PRK and SMILEa

Table 1:

Characteristics of Eyes That Underwent Corneal Wavefront-Guided Transepithelial PRK and SMILE

Visual Acuity,Efficacy, and Safety

At 6 months after surgery, there were significant improvements in mean UDVA in the transepithelial PRK (1.34 ± 0.33 to −0.06 ± 0.07 logMAR) and SMILE (1.25 ± 0.33 to −0.05 ± 0.07 logMAR) groups (both P < .001; Tables 12); 43 eyes each in the transepithelial PRK (98%) and SMILE (96%) groups exhibited UDVA of 20/20 or better (Figure 1). Relative to the preoperative CDVA, the UDVA in treated eyes improved, with 10 (23%) and 8 (18%) eyes in the transepithelial PRK and SMILE groups, respectively, exhibiting a gain of one or more lines in the 6-month postoperative UDVA (Figure 1). At 6 months after surgery, there was no significant difference in the mean efficacy index (ratio of postoperative UDVA to preoperative CDVA) between the transepithelial PRK (1.06 ± 0.15) and SMILE (1.02 ± 0.08) groups (P = .121). There was also no significant difference in mean safety index (ratio of postoperative to preoperative CDVA) between the transepithelial PRK (1.08 ± 0.15) and SMILE (1.03 ± 0.09) groups (P = .057).

Comparison of Postoperative Visual Acuity and Refractive Errors Between Patients Who Underwent Corneal Wavefront-Guided Transepithelial PRK and SMILEa

Table 2:

Comparison of Postoperative Visual Acuity and Refractive Errors Between Patients Who Underwent Corneal Wavefront-Guided Transepithelial PRK and SMILE

Visual outcomes after small incision lenticule extraction (SMILE) and corneal wavefront-guided transepithelial photorefractive keratectomy (TransPRK). (A) Cumulative 6-month postoperative uncorrected distance visual acuity (UDVA) and preoperative corrected distance visual acuity (CDVA). Changes in Snellen lines of (B) postoperative UDVA and (C) CDVA relative to preoperative CDVA. (D) The accuracy of spherical equivalent refraction (SEQ) to the intended target and (E) attempted versus achieved changes in SEQ at 6 months after surgery. (F) Comparative distribution of preoperative and 6-month postoperative cylinder and (G) target induced versus surgically induced astigmatism vectors at 6 months after surgery. (H) Refractive astigmatism angle of error distribution at 6 months after surgery. D = diopters

Figure 1.

Visual outcomes after small incision lenticule extraction (SMILE) and corneal wavefront-guided transepithelial photorefractive keratectomy (TransPRK). (A) Cumulative 6-month postoperative uncorrected distance visual acuity (UDVA) and preoperative corrected distance visual acuity (CDVA). Changes in Snellen lines of (B) postoperative UDVA and (C) CDVA relative to preoperative CDVA. (D) The accuracy of spherical equivalent refraction (SEQ) to the intended target and (E) attempted versus achieved changes in SEQ at 6 months after surgery. (F) Comparative distribution of preoperative and 6-month postoperative cylinder and (G) target induced versus surgically induced astigmatism vectors at 6 months after surgery. (H) Refractive astigmatism angle of error distribution at 6 months after surgery. D = diopters

Refraction

The mean manifest refraction SE significantly improved after transepithelial PRK and SMILE (Tables 12). The accuracy of refractive correction was good (Figure 1). All treated eyes in the transepithelial PRK group and 43 (96%) eyes in the SMILE group achieved SE within ± 1.00 D of the intended value. The linear regression model of attempted versus achieved SE in the transepithelial PRK group had a slope and coefficient (R2) of 0.9438 and 0.9421, respectively; the corresponding values in the SMILE group were 0.9470 and 0.9591, respectively.

Outcomes of Astigmatism Correction and Vector Analysis

In terms of astigmatism, 39 (89%) and 38 (84%) eyes in the transepithelial PRK and SMILE groups, respectively, exhibited a postoperative cylinder of 0.50 D or less (Figure 1). Linear regression models of the target induced astigmatism (TIA) versus surgically induced astigmatism (SIA) vectors showed slopes and coefficients (R2) of 0.8937 and 0.5131 in the transepithelial PRK group and 0.7622 and 0.5820 in the SMILE group, respectively.

Vector analysis of astigmatism was performed using the methods described by Alpins14 and Reinstein et al.15 The results of vector analysis of the 6-month refractive data are shown in Table A (available in the online version of this article). There was no significant difference in TIA, SIA, difference vector, or index of success between the transepithelial PRK and SMILE groups. However, considering the whole sample, the correction index of the transepithelial PRK group was significantly better than that of the SMILE group (P = .030). The two groups also exhibited a significant difference in the arithmetic values of angle of error. The SMILE group exhibited achieved correction vectors that were counterclockwise to the axes of the intended correction vectors, whereas the transepithelial PRK group exhibited the opposite tendency. There was no significant difference in absolute mean values of angle of error between the two groups. The magnitude of error values were slightly negative in both groups, which indicated a small undercorrection; however, the level of undercorrection, which was indicated by the magnitude of error, was lesser in the transepithelial PRK group than in the SMILE group (P = .024). Because there was a mirror symmetric effect in the axes of astigmatism between the right and left eyes,16 we analyzed the data of the right and left eyes separately. Because the findings of all analyses of the right and left eyes separately exhibited similar tendencies to those of the whole sample (Table A), we have illustrated the data of the whole sample in Figure B (available in the online version of this article), which presents the polar plots of TIA, SIA, difference vector, and correction index using the same axes; these outcomes were comparable between the transepithelial PRK and SMILE groups.

Comparison of Vector Parameters for Patients Who Underwent Corneal Wavefront-Guided Transepithelial PRK and SMILEa

Table A:

Comparison of Vector Parameters for Patients Who Underwent Corneal Wavefront-Guided Transepithelial PRK and SMILE

(A) Single-angle polar plots of target induced astigmatism vector, (B) surgically induced astigmatism vector, (C) difference vector, and (D) correction index at 6 months after small incision lenticule extraction (SMILE) and corneal wavefront-guided transepithelial photorefractive keratectomy (TransPRK). D = diopters

Figure B.

(A) Single-angle polar plots of target induced astigmatism vector, (B) surgically induced astigmatism vector, (C) difference vector, and (D) correction index at 6 months after small incision lenticule extraction (SMILE) and corneal wavefront-guided transepithelial photorefractive keratectomy (TransPRK). D = diopters

Refractive Correction According to Preoperative Refractive Error

There was no significant correlation between SE error (attempted minus achieved SE) and attempted SE in either group, although a slight trend toward increased undercorrection in eyes with high preoperative attempted SE was observed in the SMILE group (Figure 2A). A slight but significant negative correlation was observed between TIA and magnitude of error in the SMILE group, but not in the transepithelial PRK group (Figure 2B).

(A) Spherical equivalent (SEQ) error versus attempted SEQ at 6 months after surgery. (B) Magnitude of error versus target induced astigmatism vector at 6 months after surgery. D = diopters; SMILE = small incision lenticule extraction; TransPRK = transepithelial photorefractive keratectomy

Figure 2.

(A) Spherical equivalent (SEQ) error versus attempted SEQ at 6 months after surgery. (B) Magnitude of error versus target induced astigmatism vector at 6 months after surgery. D = diopters; SMILE = small incision lenticule extraction; TransPRK = transepithelial photorefractive keratectomy

Higher Order Aberrations

Table B (available in the online version of this article) and Figure 3 summarize the changes in corneal aberrations after surgery. There were no changes in total corneal root mean square higher order aberrations after surgery in either group. Whereas the transepithelial PRK group exhibited a significant increase in corneal spherical aberration from the preoperative level (P < .001), there was no such change in the SMILE group (P = .635). Absolute and delta values of spherical aberration in the SMILE group were both significantly smaller than those in the transepithelial PRK group. In contrast, corneal coma and trefoil decreased significantly after surgery in the transepithelial PRK group, but remained unchanged in the SMILE group. The delta values of coma and trefoil were also significantly lower in the transepithelial PRK group than in the SMILE group (Table B, Figure 3).

Comparison of Corneal Aberrations (μm) Between Eyes With High Astigmatism Treated by Corneal Wavefront-Guided Transepithelial PRK and SMILEa

Table B:

Comparison of Corneal Aberrations (μm) Between Eyes With High Astigmatism Treated by Corneal Wavefront-Guided Transepithelial PRK and SMILE

Changes in higher order aberrations (HOA) at 6 months after small incision lenticule extraction (SMILE) and corneal wavefront-guided transepithelial photorefractive keratectomy (TransPRK). Data are presented as mean values ± standard error of the mean. RMS = root mean square; SphAb = spherical aberration; ns = not significant; *Significant difference

Figure 3.

Changes in higher order aberrations (HOA) at 6 months after small incision lenticule extraction (SMILE) and corneal wavefront-guided transepithelial photorefractive keratectomy (TransPRK). Data are presented as mean values ± standard error of the mean. RMS = root mean square; SphAb = spherical aberration; ns = not significant; *Significant difference

Discussion

In the current study, we evaluated the surgical outcomes of SMILE with a triple centration technique and corneal wavefront-guided transepithelial PRK in myopic eyes with high astigmatism. Similar to the findings of previous studies,2,3,12,17 the visual outcomes after SMILE and corneal wavefront-guided transepithelial PRK in the current study indicated that these methods are safe and effective. There were no significant differences in postoperative visual acuity or refractive errors between the two groups. The efficacy and safety indices were also comparable between the two groups.

Although some previous studies have reported slight undercorrection of SE after SMILE,8 there was no myopic residual SE in the SMILE group in this study. Transepithelial PRK also elicited excellent SE correction. Additionally, there was no significant relationship between attempted SE and SE errors in either group (Figure 2A), indicating that both profiles provided good refractive correction for SE.

Few studies have evaluated the efficacy of astigmatism correction by transepithelial PRK.18–20 These studies have indicated no undercorrection of astigmatism with transepithelial PRK; rather, they have reported overcorrection, with correction index values ranging from 1.0 to 1.06. In the current study, 39 (88%) eyes in the transepithelial PRK group exhibited postoperative astigmatism of 0.50 D or less, which appears to be an effective and predictable outcome (Figure 1). Although some authors have reported a certain amount of undercorrection of astigmatism after SMILE,8 a recent study described that astigmatism correction in SMILE was effective and predictable, with 70% and 94% of patients exhibiting postoperative cylinder within ±0.50 and ±1.00 D, respectively.21 However, the authors also reported a significant increase in magnitude of error with an increase in TIA after SMILE.21 Zhang et al.16,22 reported correction index values of 0.87 to 0.88 in eyes with high astigmatism; however, the number of patients in their study was relatively small. In the current study, 38 (85%) eyes in the SMILE group exhibited postoperative astigmatism of 0.50 D or less (Figure 1) and the mean correction index was 0.91, which indicates efficient and predictable correction of astigmatism. However, the relationship between TIA and magnitude of error in the SMILE group was significant (Figure 2B). There was no significant relationship between TIA and magnitude of error in the transepithelial PRK group. Taken together, we suggest adjustment of nomograms for SMILE by enhancing the degree of correction of cylinder in high astigmatism.

Because astigmatism is a vector, proper alignment along the intended correction axis is paramount for effective correction. Cyclotorsion of the eye is an important factor for the proper correction of astigmatism, especially in eyes with high astigmatism. Therefore, the importance of an effective cyclotorsion compensation system for the correction of astigmatism cannot be overlooked. For transepithelial PRK, the SCHWIND system allows for both static and dynamic cyclotorsion control during surgery, whereby preoperative errors are compensated for by incorporating topography-guided data into the laser system. For SMILE, the need for a dynamic cyclotorsion control system is negligible because the entire refractive ablation occurs under suction and the eye is subsequently immobilized. However, a system for static cyclotorsion and calibration of axes prior to eye docking is important for proper vector correction in astigmatism. The lack of a pupil-tracking system and the absence of an evaluation system for distortion for the docking of eyes might cause inadequate astigmatism correction after SMILE, especially in eyes with high astigmatism. Theoretically, only 4° of cyclotorsion can result in 14% of astigmatism undercorrection.21,23 Furthermore, in a previous study, the mean cyclotorsion was reported to be approximately 2.6°, and 13% of eyes treated with LASIK had experienced greater than 5° of cyclotorsion.24 In fact, Chan et al.25 performed a comparative study on SMILE and femtosecond laser–assisted LASIK and the SMILE group exhibited a significantly higher absolute angle of error value than the femtosecond laser–assisted LASIK group (22° vs. 12°). Additionally, Zhang et al.16,22 reported absolute angle of error values ranging from 2.24° to 3.42° after SMILE in eyes with high astigmatism. In the current study, static cyclotorsion control in SMILE was achieved by providing a reference in both the horizontal and vertical meridians by means of triple centration. Cyclotorsion correction with two corneal markings would theoretically be sufficient for cyclotorsion control. However, by providing the third reference point, the first Purkinje reflex or the coaxially sighted corneal light reflex that serves to identify the corneal vertex helps achieve better centration. We believe that better centration and horizontal cyclotorsion control together helped achieve better astigmatism correction and fewer corneal higher order aberrations after SMILE. Moreover, there was no significant interaction between angle kappa and refractive outcomes (Figure C, available in the online version of this article), which indicates that centration to the corneal vertex is useful.

(A) Spherical equivalent (SEQ) error versus preoperative offset. (B) Magnitude of error versus preoperative offset. SMILE = small incision lenticule extraction; TransPRK = transepithelial photorefractive keratectomy; D = diopters

Figure C.

(A) Spherical equivalent (SEQ) error versus preoperative offset. (B) Magnitude of error versus preoperative offset. SMILE = small incision lenticule extraction; TransPRK = transepithelial photorefractive keratectomy; D = diopters

In the current study, there was no significant difference in absolute angle of error values between the SMILE and transepithelial PRK groups. Moreover, all treated eyes exhibited angle of error values of 12° or less in both groups, and the mean absolute angle of error values in the transepithelial PRK and SMILE groups were only 1.5° and 1.76°, respectively; these findings appear to indicate safe and predictable outcomes and suggest that the triple centration technique was useful for the management of cyclotorsion.

Aberrometric analysis in the current study produced interesting results. There were no significant differences between preoperative and postoperative corneal root mean square higher order aberrations in either group. However, significant induction of spherical aberration after surgery was observed in the transepithelial PRK group, but not in the SMILE group. Postoperative spherical aberration in the transepithelial PRK group was significantly higher than that in the SMILE group. This is in line with the findings of previous studies that reported that SMILE induces less spherical aberration than LASIK or PRK.8,26,27 The nature of lenticule creation by femtosecond laser–assisted tissue dissection rather than photoablation of corneal tissue has been hypothesized to be one of the reasons for the relatively low induction of spherical aberration in SMILE.

Alternatively, corneal coma and trefoil in the transepithelial PRK group were significantly decreased, whereas no change was observed in the SMILE group. This is also in line with previous findings that have indicated that corneal wavefront-guided transepithelial PRK does not induce postoperative corneal coma or trefoil aberration13 and that SMILE induces greater coma aberrations relative to LASIK, which might be associated with the mild levels of treatment decentration.8 In the current study, we observed no induction of corneal coma aberration after surgery in the SMILE group, which suggests that the triple centration technique was helpful in preventing changes in coma values. In essence, SMILE uses a Munnerlyns equation-based spherical lenticule for the correction of lower order aberrations without consideration of the preoperative corneal higher order aberrations in individual eyes. Alternatively, corneal wavefront-guided ablation is designed to detect and correct preoperative corneal higher order aberrations in a customized ablation pattern for each individual eye. The results presented in the current study are in agreement with this difference in ablation.

The limitations of this study include its relatively small sample size, non-randomized retrospective design, and lack of functional vision evaluation (eg, a contrast sensitivity test). Nonetheless, this study is valuable because it is the first to comparatively demonstrate the clinical outcomes and vector parameters of transepithelial PRK and SMILE in eyes with high astigmatic errors. Furthermore, the current findings provide clues for the appropriate application of treatment profiles in eyes with high astigmatism.

Both SMILE and corneal wavefront-guided transepithelial PRK were effective and safe for the correction of high myopic astigmatism. The triple centration technique was helpful in astigmatism correction by SMILE. Although slight undercorrection of astigmatism was still noted after SMILE, it can be compensated for by the development of proper nomograms for not only spherical errors but also astigmatism correction. Additionally, the patterns of surgically induced aberrations with the two procedures were apparently different. Therefore, future studies that aim to understand the SMILE lenticule and excimer laser ablation profile are necessary to identify the best refractive correction procedure on the basis of the ocular condition of the patient.

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Characteristics of Eyes That Underwent Corneal Wavefront-Guided Transepithelial PRK and SMILEa

CharacteristicsTransepithelial PRK GroupSMILE GroupP
No. of eyes (R/L)44 (19/25)45 (18/27).761
Sex (M/F)16/2825/20.069
Age (y)25.80 ± 3.47 (21 to 35)24.80 ± 4.56 (20 to 44).250
Refractive errors (D)
  Sphere−3.77 ± 1.57 (−6.25 to −0.50)−3.19 ± 1.55 (−6.50 to −0.25).081
  Cylinder−2.84 ± 0.35 (−3.75 to −2.50)−2.90 ± 0.42 (−4.37 to −2.50).502
  Spherical equivalent−5.19 ± 1.55 (−7.63 to −1.75)−4.64 ± 1.52 (−7.75 to −1.75).091
logMAR CDVA−0.04 ± 0.07 (−0.18 to 0.10)−0.05 ± 0.07 (−0.18 to 0.05).788
logMAR UDVA1.34 ± 0.33 (0.52 to 2.00)1.25 ± 0.33 (0.52 to 2.00).221
CCT (μm)555.70 ± 29.01 (509 to 656)560.49 ± 29.20 (506 to 641).440
Optical zone (mm)6.60 ± 0.22 (6.10 to 7.26)6.66 ± 0.25 (6.20 to 7.20).308
Ablation depth or lenticule thickness (μm)111.24 ± 20.87 (69.52 to 164.24)122.47 ± 19.67 (75 to 172).011b

Comparison of Postoperative Visual Acuity and Refractive Errors Between Patients Who Underwent Corneal Wavefront-Guided Transepithelial PRK and SMILEa

Postoperative MeasurementTransepithelial PRK GroupSMILE GroupP
logMAR UDVA−0.06 ± 0.07 (−0.18 to 0.05)−0.05 ± 0.07 (−0.18 to 0.05).501
logMAR CDVA−0.07 ± 0.07 (−0.18 to 0.05)−0.06 ± 0.07 (−0.18 to 0.05).274
Sphere (D)0.20 ± 0.34 (−0.50 to 1.00)0.23 ± 0.30 (−0.62 to 1.25).584
Cylinder (D)−0.31 ± 0.25 (−1.00 to 0.00)−0.37 ± 0.26 (−1.12 to 0.00).299
Spherical equivalent (D)0.04 ± 0.37 (−0.75 to 0.875)0.05 ± 0.31 (−1.18 to 1.125).895

Comparison of Vector Parameters for Patients Who Underwent Corneal Wavefront-Guided Transepithelial PRK and SMILEa

ParameterTotal EyesRight EyesLeft Eyes



TransPRK GroupSMILE GroupPTransPRK GroupSMILE GroupPTransPRK GroupSMILE GroupP
TIA (D)2.84 ± 0.35 (2.50 to 3.75)2.90 ± 0.42 (2.50 to 4.37).5022.89 ± 0.38 (2.50 to 3.62)2.96 ± 0.44 (2.62 to 4.00).6122.81 ± 0.34 (2.50 to 3.75)2.86 ± 0.41 (2.50 to 4.37).630
SIA (D)2.73 ± 0.44 (1.95 to 3.94)2.64 ± 0.42 (1.71 to 3.88).3262.79 ± 0.54 (2.01 to 3.94)2.73 ± 0.43 (2.13 to 3.88).7242.69 ± 0.36 (1.95 to 3.30)2.58 ± 0.41 (1.71 to 3.67).323
Difference vector (D)0.31 ± 0.25 (0.00 to 1.00)0.37 ± 0.26 (0.00 to 1.12).2990.33 ± 0.23 (0.00 to 0.87)0.32 ± 0.26 (0.00 to 1.12).9660.30 ± 0.27 (0.00 to 1.00)0.40 ± 0.26 (0.00 to 1.00).189
Correction index0.96 ± 0.11 (0.72 to 1.20)0.91 ± 0.10 (0.65 to 1.10).030b0.97 ± 0.12 (0.72 to 1.18)0.93 ± 0.08 (0.74 to 1.08).2820.96 ± 0.10 (0.74 to 1.20)0.91 ± 0.10 (0.65 to 1.10).061
Index of success0.11 ± 0.09 (0.00 to 0.40)0.13 ± 0.08 (0.00 to 0.38).3810.11 ± 0.07 (0.00 to 0.29)0.11 ± 0.07 (0.00 to 0.28).8090.11 ± 0.10 (0.00 to 0.40)0.14 ± 0.09 (0.00 to 0.38).239
Angle of error (degrees)−0.86 ± 2.44 (−12.00 to 5.00)1.04 ± 2.26 (−4.00 to 7.00)< .001b−1.00 ± 1.33 (−4.00 to 1.00)0.56 ± 1.98 (−4.00 to 5.00).009b−0.76 ± 3.05 (−12.00 to 5.00)1.37 ± 2.40 (−2.00 to 7.00).007b
|Angle of error| (degrees)1.50 ± 2.10 (0.00 to 12.00)1.76 ± 1.75 0.00 to 7.00).5331.11 ± 1.24 (0.00 to 4.00)1.44 ± 1.42 (0.00 to 5.00).4451.80 ± 2.55 (0.00 to 12.00)1.96 ± 1.93 (0.00 to 7.00).795
Magnitude of error (degrees)−0.11 ± 0.31 (−0.84 to 0.51)−0.26 ± 0.29 (−1.04 to 0.25).024b−0.10 ± 0.35 (−0.84 to 0.51)−0.22 ± 0.28 (−1.08 to 0.23).239−0.12 ± 0.28 (−0.75 to 0.49)−0.28 ± 0.29 (−1.00 to 0.25).056

Comparison of Corneal Aberrations (μm) Between Eyes With High Astigmatism Treated by Corneal Wavefront-Guided Transepithelial PRK and SMILEa

AberrationTransPRK GroupSMILE GroupP
Root mean square higher order aberrations
  Preoperative0.55 ± 0.15 (0.31 to 0.86)0.52 ± 0.11 (0.28 to 0.86).317
  6 months postoperative0.59 ± 0.15 (0.31 to 1.04)0.56 ± 0.14 (0.30 to 0.87).312
   P (preoperative vs 6 months postoperative).073.076
  Δ (preoperative vs 6 months postoperative)0.04 ± 0.15 (−0.36 to 0.45)0.04 ± 0.14 (−0.19 to 0.41).911
Spherical aberration
  Preoperative0.29 ± 0.12 (0.04 to 0.56)0.25 ± 0.10 (−0.05 to 0.43).065
  6 months postoperative0.38 ± 0.17 (0.04 to 0.79)0.24 ± 0.13 (0.00 to 0.55)< .001b
   P (preoperative vs 6 months postoperative)< .001b.635
  Δ (preoperative vs 6 months postoperative)0.09 ± 0.14 (−0.19 to 0.36)−0.01 ± 0.10 (−0.20 to 0.23)< .001b
Coma
  Preoperative0.31 ± 0.14 (0.05 to 0.65)0.33 ± 0.13 (0.12 to 0.60).416
  6 months postoperative0.22 ± 0.11 (0.02 to 0.46)0.31 ± 0.15 (0.04 to 0.63).001b
   P (preoperative vs 6 months postoperative).001b.394
  Δ (preoperative vs 6 months postoperative)−0.09 ± 0.17 (−0.44 to 0.29)0.02 ± 0.15 (−0.29 to 0.41).046b
Trefoil
  Preoperative0.23 ± 0.13 (0.04 to 0.53)0.20 ± 0.11 (0.02 to 0.54).313
  6 months postoperative0.17 ± 0.09 (0.02 to 0.39)0.22 ± 0.13 (0.02 to 0.86).072
   P (preoperative vs 6 months postoperative).014b.459
  Δ (preoperative vs 6 months postoperative)−0.05 ± 0.14 (−0.36 to 0.23)0.02 ± 0.16 (−0.28 to 0.39).026b
Authors

From the Departments of Ophthalmology of the Institute of Vision Research (KYS) and Corneal Dystrophy Research Institute, Yonsei University College of Medicine, Seoul, Korea (IJ, TK); Eyereum Eye Clinic, Seoul, South Korea (DSYK); London Vision Clinic, London, United Kingdom (DZR, TJA); the Department of Ophthalmology, Columbia University Medical Center, New York, New York (DZR); Centre Hospitalier National d'Ophtalmologie, Paris, France (DZR); Biomedical Science Research Institute, University of Ulster, Coleraine, Northern Ireland (DZR, TJA); and Biomedical Engineering Office, Research and Development, SCHWIND eye-tech-solutions, Kleinostheim, Germany (SA-M).

Supported by the Basic Science Research Program through the National Research Foundation of Korea (NRF), funded by the Ministry of Education, Science and Technology (NRF-2016R1A2B4009626).

Dr. Kang is an advisor for SCHWIND eye-tech-solutions (Kleinostheim, Germany). Dr. Reinstein is a consultant for Carl Zeiss Meditec (Carl Zeiss Meditec AG, Jena, Germany) and has a proprietary interest in the Artemis technology (ArcScan, Inc., Golden, Colorado) through patents administered by the Center for Technology Licensing at Cornell University (CTL), Ithaca, New York. Dr. Arba-Mosquera is an employee of SCHWIND eye-tech-solutions. The remaining authors have no proprietary or financial interest in the materials presented herein.

AUTHOR CONTRIBUTIONS

Study concept and design (DSYK, KYS, TK); data collection (DSYK); analysis and interpretation of data (IJ, DSYK, DZR, SA-M, TJA, TK); writing the manuscript (IJ, DZR, TJA); critical revision of the manuscript (DSYK, SA-M, KYS, TK); statistical expertise (DZR, SA-M, TJA); supervision (TK)

Correspondence: Tae-im Kim, MD, PhD, Department of Ophthalmology, Yonsei University College of Medicine, 50-1 Yonsei-ro, Seodaemungu, Seoul 03722, Korea. E-mail: tikim@yuhs.ac

Received: August 01, 2017
Accepted: December 20, 2017

10.3928/1081597X-20180104-03

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