Journal of Refractive Surgery

Original Article Supplemental Data

Early Corneal and Epithelial Remodeling Differences Identified by OCT Imaging and Artificial Intelligence Between Two Transepithelial PRK Platforms

Rohit Shetty, MD, PhD; Raghav Narasimhan, MTech; Zelda Dadachanji, MD; Pavitra Patel, MD; Sonia Maheshwari, MD; Aishwarya Chabra, MD; Abhijit Sinha Roy, PhD

Abstract

PURPOSE:

To analyze corneal and epithelial remodeling differences between SmartSurfACE reverse transepithelial PRK (SCHWIND eye-tech-solutions) and Streamlight (Alcon Laboratories, Inc) transepithelial PRK procedure using optical coherence tomography (OCT) and artificial intelligence (AI).

METHODS:

This was a prospective, interventional, and longitudinal study. A contralateral eye study was conducted in which one eye was assigned to the SmartSurfACE group and the fellow eye was assigned to the Streamlight group. OCT was performed preoperatively and 1, 3, and 6 months after surgery. Uncorrected (UDVA) and corrected (CDVA) distance visual acuity and residual refractive error was measured only preoperatively and at 3 and 6 months. From OCT, curvature and aberrations of the air–epithelium (A–E) interface, epithelium–Bowman's layer (E-B) interface, and epithelium Zernike indices (EZI) were derived. Pain was evaluated at 1 day postoperatively using the Wong-Baker scale.

RESULTS:

Both groups had similar UDVA, CDVA, residual refractive error, and changes in A–E and E-B curvatures at 3 and 6 months postoperatively (P > .05). However, many parameters indicated that the Streamlight group underwent a greater change in A–E aberrations, E-B aberrations, and EZI than the SmartSurfACE group postoperatively (P < .05). The EZI indicated a greater level of epithelial thickness distortion in the Streamlight group than in the SmartSurfACE group (P < .05). Using AI, the EZI were most indicative of remodeling differences between the two groups. Further, the pain was significantly greater at 1 day in the Streamlight group (P < .05).

CONCLUSIONS:

Early remodeling differences existed because the Streamlight procedure removed a greater amount of epithelium than the SmartSurfACE procedure. However, the visual and refractive outcomes were comparable.

[J Refract Surg. 2020;36(10):678–686.]

Abstract

PURPOSE:

To analyze corneal and epithelial remodeling differences between SmartSurfACE reverse transepithelial PRK (SCHWIND eye-tech-solutions) and Streamlight (Alcon Laboratories, Inc) transepithelial PRK procedure using optical coherence tomography (OCT) and artificial intelligence (AI).

METHODS:

This was a prospective, interventional, and longitudinal study. A contralateral eye study was conducted in which one eye was assigned to the SmartSurfACE group and the fellow eye was assigned to the Streamlight group. OCT was performed preoperatively and 1, 3, and 6 months after surgery. Uncorrected (UDVA) and corrected (CDVA) distance visual acuity and residual refractive error was measured only preoperatively and at 3 and 6 months. From OCT, curvature and aberrations of the air–epithelium (A–E) interface, epithelium–Bowman's layer (E-B) interface, and epithelium Zernike indices (EZI) were derived. Pain was evaluated at 1 day postoperatively using the Wong-Baker scale.

RESULTS:

Both groups had similar UDVA, CDVA, residual refractive error, and changes in A–E and E-B curvatures at 3 and 6 months postoperatively (P > .05). However, many parameters indicated that the Streamlight group underwent a greater change in A–E aberrations, E-B aberrations, and EZI than the SmartSurfACE group postoperatively (P < .05). The EZI indicated a greater level of epithelial thickness distortion in the Streamlight group than in the SmartSurfACE group (P < .05). Using AI, the EZI were most indicative of remodeling differences between the two groups. Further, the pain was significantly greater at 1 day in the Streamlight group (P < .05).

CONCLUSIONS:

Early remodeling differences existed because the Streamlight procedure removed a greater amount of epithelium than the SmartSurfACE procedure. However, the visual and refractive outcomes were comparable.

[J Refract Surg. 2020;36(10):678–686.]

Photorefractive keratectomy (PRK) is one of the primary surgical techniques for correction of refractive error. Typically, manual removal of epithelium is performed with a blade, alcohol, or Epi-Bowman keratome (Orca Surgical). However, an alternate method of using a laser to remove both the epithelium and stroma was introduced in 1990.1 Subsequently, two methods of removal of epithelium were tested clinically: (1) phototherapeutic keratectomy (PTK) followed by PRK for refractive correction and (2) a single-step laser ablation where both the epithelium and stroma were ablated without the need for a split into PTK and PRK.2 This single-step ablation was called transepithelial PRK. Clinical outcomes showed that the single-step procedure may have several advantages over the former (eg, less dehydration, no unwanted hyperopic shift due to PTK, less pain, lower grade of haze, and faster reepithelialization).2

A key modification to the single-step method was the reverse single-step transepithelial PRK procedure (Amaris 1050RS; SCHWIND eye-tech-solutions).3 In this procedure, the ablation to eliminate the refractive error was performed first, followed by another defined ablation to simulate the epithelium thickness profile of a normal population.3 Smart-Pulse technology, also known as SmartSurfACE (SCHWIND eye-tech-solutions), was added to the transepithelial PRK procedure in 2017 with the claim that it left fewer surface irregularities on the residual stromal bed.4

The new Streamlight method (WaveLight Allegretto Wave Eye-Q Laser; Alcon Laboratories, Inc) is also a single-step transepithelial PRK procedure. The Stream-light transepithelial PRK performs ablation of the epithelium first followed by stromal ablation to correct the refractive error of the eye. It assumes a uniform epithelial thickness for ablation. No clinical data regarding the outcomes of the Streamlight platform have been reported yet. Recently, the anterior corneal surface, Bowman's curvature and aberrations, and epithelium Zernike indices (EZI) were used to study the temporal healing of the cornea after refractive procedures and for the detection of ectasia.5–9 Therefore, the primary objective of this study was to assess the early corneal and epithelial remodeling differences between the two platforms using the methods mentioned above. The study design was contralateral, where one eye of the patient underwent reverse transepithelial PRK with the Smart-SurfACE technology and the other underwent Stream-light transepithelial PRK. The visual and refractive outcomes were also assessed between the two procedures.

Patients and Methods

This was a prospective, interventional, and longitudinal study. The study was approved by the ethics committee of Narayana Nethralaya Eye Hospital, Bangalore, India. The study adhered to the tenets of the Declaration of Helsinki. Inclusion criteria were stable refraction (less than −9.00 diopters [D] spherical equivalent refraction with astigmatism of not more than −4.00 D) for a period of 1 year (change less than 0.25 D). Patients with central corneal thickness of less than 400 µm or a history of keratoconus, diabetes, collagen vascular disease, pregnancy, breastfeeding, and any prior ocular surgery or trauma were excluded from the surgery. In all eyes, the calculated residual stromal thickness relative to the preoperative central corneal thickness was greater than 250 µm. All patients underwent subjective refractive error assessment (sphere, cylinder, and axis) preoperatively for planning of treatment. No nomogram adjustments to the subjective spherocylindrical error were made.

One eye of each patient (chosen at random by a coin toss) underwent SmartSurfACE transepithelial PRK4 (SmartSurfACE group) and the fellow eye underwent Streamlight transepithelial PRK (Streamlight group). The optical diameter (6 mm) was the same for both procedures. However, only tissue within the ablation zone was removed from the SmartSurfACE group. The volume of tissue removed was determined by the laser ablation software. According to the manufacturer, SmartSurfACE transepithelial PRK removed 50 µm of tissue at the center and 60 µm of tissue at the 6-mm location according to the measurements of epithelial thicknesses in healthy corneas and taking into account the greater angle of incidence of the laser beam in the mid to peripheral cornea.10,11 In contrast, the epithelium from the central 8-mm zone was removed from the Streamlight group first. Then, a volume of the corneal stroma, determined by the laser software, was ablated. This volume did not encompass the entire 8-mm central zone in the Streamlight group. A Weck-Cel sponge (BVI) soaked in 0.02% mitomycin C was placed on the stroma for 10 seconds per diopter of spherical equivalent treated immediately after the excimer ablation.12 Both the SmartSurfACE and Stream-light procedures did not use wavefront-guided ablation profiles. Instead, the proprietary ablation profiles aimed to minimize the induction of higher order aberrations.

Following both surgeries, a bandage contact lens soaked in 0.45% ketorolac for 20 minutes was applied.13 The subjective pain was assessed at 1 day postoperatively with the Wong-Baker FACES pain scale.13 The postoperative regimen of topical lubricants and steroids was the same as described in our recent studies.12,13

All eyes underwent uncorrected (UDVA) and corrected (CDVA) distance visual acuity assessment before and 3 and 6 months after the surgery. Further, the patients underwent optical coherence tomography (OCT) imaging (RTVue; Optovue, Inc) before and at 1, 3, and 6 months after surgery. From the OCT imaging, the curvature and wavefront aberrations (ray-tracing) of the air–epithelium (A–E) interface, epithelium–Bowman's layer (E-B) interface, and Zernike analyses of three-dimensional epithelium thickness were computed.5–7 We intended to use the same metrics from OCT image analyses to evaluate whether corneal and epithelium remodeling differences between the two transepithelial PRK platforms existed.

We briefly summarize the Zernike analyses of the epithelium thickness distribution. The epithelial thicknesses were sampled with Zernike polynomials up to the 6th order and as a function of normalized radius and meridian.9,14 Henceforth, the Zernike coefficients (Z) calculated from the epithelial thickness distribution (E) will be referred to as the EZI. The double indexed notation of Zernike coefficients was also used to represent the EZI and are listed below:

  • Z02 (defocus)
  • Root mean square (RMS) of Z−22 and Z+22 (astigmatism)
  • RMS of lower order coefficients (LOC) Z−22, Z02, and Z+22
  • RMS of Z−13 and Z+13 (coma)
  • Z04 (spherical aberration)
  • RMS or higher order coefficients (HOC) from 3rd to 6th order
  • Epithelium distortion index (EDI) = square root of sum of squares of RMS of LOC and HOC

Statistical Analyses

For each index, the normality of distribution was assessed with the Kolmogorov-Smirnov test. Repeated measures analyses of variance with Bonferroni adjustment (for comparing preoperative with 1-, 3-, and 6-month results) and paired t test (for comparing preoperative data between the two groups) were used for statistical comparisons. The postoperative UDVA, CDVA, and refractive outcomes were assessed at 3 and 6 months postoperatively. However, the postoperative OCT anterior, Bowman's curvature and aberrations, and EZI were assessed at 1, 3, and 6 months postoperatively in addition to the preoperative visit. Proportions were compared with the chi-square test. Further, Pearson's correlation coefficients were assessed between the RMS of lower order aberrations (LOAs) and higher order aberrations of the A–E and E-B interfaces, with EDI for the 3- and 6-month follow-up visits. Finally, the postoperative changes in all indices were included in an artificial intelligence (AI) model to identify the indices that were most indicative of postoperative remodeling differences between the two procedures. The AI models implemented were support vector machine, random forest, and logistic regression. The goal of the AI analyses was to identify the indices that were specific to platform-dependent changes in epithelium thickness remodeling.

A P value less than .05 was considered statistically significant. MedCalc v19.1.3 (MedCalc, Inc) and Orange data mining toolbox9 software were used for all analyses.

Results

Table 1 shows that both groups were similar preoperatively with reference to visual acuity, refractive error, and central corneal thickness (P > .05). Table A (available in the online version of this article) shows the preoperative and 1, 3, and 6 months postoperative measurement of mean A–E interface curvature and aberrations. The postoperative (3 and 6 months) maximum, flat, and steep curvature were significantly different from their preoperative magnitudes (P < .05). Only defocus and spherical aberration at 6 months postoperatively differed from the preoperative measurement (P < .05) such that the Streamlight group had a greater magnitude of RMS of LOA than the SmartSurfACE group. However, there were clear differences in aberrations between the two groups postoperatively (Table A). In general, the aberrations increased from preoperative values to 1 month postoperatively and then from 1 to 3 months postoperatively, but decreased from 3 to 6 months postoperatively in the SmartSurfACE group (eg, RMS of LOA and HOA) (Table A). In the Stream-light group, the aberrations tended to increase after surgery and remained greater than the preoperative values (eg, RMS of LOA was 1 ± 0.64 µm preoperatively and 1.42 ± 0.62 µm at 6 months postoperatively) (Table A). Interestingly, the SmartSurfACE group had a smaller RMS of LOA than the Streamlight group at 6 months postoperatively (P = .006).

Preoperative Parameters of the Two Groups of Eyes

Table 1:

Preoperative Parameters of the Two Groups of Eyes

Mean ± SD Values Preoperatively and at 1, 3, and 6 Months Postoperatively

Table A:

Mean ± SD Values Preoperatively and at 1, 3, and 6 Months Postoperatively

Table A shows the preoperative and 1, 3, and 6 months postoperative measurement of mean E-B interface curvature and aberrations. The postoperative flat and steep curvature along with the maximum curvature were significantly different from their preoperative magnitudes (P < .05). In the SmartSurfACE group, only defocus and spherical aberration at 1 and 3 months postoperatively differed from the preoperative values (P < .05) and the 6-month postoperative values were statistically similar to the preoperative values (P > .05). In the Streamlight group, none of the aberrations differed statistically between the preoperative and follow-up time points (P > .05), although the 6-month postoperative magnitude was still greater than the pre-operative magnitude (eg, RMS of LOA was 0.85 ± 0.26 µm preoperatively and 1.30 ± 0.58 µm at 6 months postoperatively). Interestingly (Table A), the SmartSurfACE group also had a smaller RMS of LOA than the Streamlight group at 6 months postoperatively (P = .001).

Table 2 shows the preoperative and 1, 3, and 6 months postoperative measurement of mean epithelium thicknesses and EZI. The mean central thickness and maximum and minimum thickness changed significantly with follow-up (P < .05), although the changes were similar between the two groups postoperatively (P > .05). However, there were significant postoperative changes in the Zernike indices, which differed between the groups. For example (Table 2), Z02 did not alter postoperatively in the SmartSurfACE group (P > .05) but changed significantly in the Streamlight group (P < .05). Z04 differed between all time points (“All” in Table 2) in both groups, indicating continued remodeling of the epithelium at all follow-up time points (P < .05). However, the Streamlight group showed a change in RMS of Z−13 and Z+13 and RMS of HOC postoperatively (P < .05), although the SmartSurfACE group showed no such change (Table 2). In the SmartSurfACE group (Table 2), the EDI remained statistically similar between the preoperative and follow-up time points (P > .05). However, all follow-up time points had significantly greater EDI than the preoperative state in the Streamlight group (P < .05). The assessment of Pearson's correlation led to interesting results (Table B, available in the online version of this article). Most correlations of the RMS of LOA and HOA of A–E and E-B with EDI were statistically insignificant (P > .05).

Epithelium Zernike Values (Mean ± SD) Preoperatively and at 1, 3, and 6 Months Postoperatively

Table 2:

Epithelium Zernike Values (Mean ± SD) Preoperatively and at 1, 3, and 6 Months Postoperatively

Pearson's Correlation at 3 and 6 Months Postoperatively

Table B:

Pearson's Correlation at 3 and 6 Months Postoperatively

Figures 12 show the refractive and visual acuity results of the two groups of eyes at 6 months postoperatively. Because the 3- and 6-month refractive and visual acuity results were identical, only the 6-month results were plotted in Figures 12. Overall, both groups of eyes had excellent postoperative results with no significant differences in proportions of eyes achieving CDVA 20/20 or better (P > .05). The refractive accuracy of spherical equivalent (Figure 1C and Figure 2C) was similar between the two groups (P > .05). Interestingly, 100% and 85% of the eyes were within ±0.50 D cylinder in the SmartSurfACE and Streamlight groups, respectively (P = .07). The mean safety and efficacy were 1.27 ± 0.12 and 1.14 ± 0.21, respectively, for the SmartSurfACE group and 1.26 ± 0.13 (P > .05) and 1.13 ± 0.23 (P > .05), respectively, for the Streamlight group. At 1 day postoperatively, the mean pain score was 2.80 ± 2.22 and 3.54 ± 2.38 in the SmartSurfACE and Streamlight groups, respectively (P = .05). At 1, 3, and 6 months postoperatively, the mean pain scores were low and negligibly different between the two groups (P > .05).

Visual and refractive outcomes of SmartSurfACE group eyes (SCHWIND eye-tech-solutions): (A) uncorrected distance visual acuity (UDVA); (B) change in corrected distance visual acuity (CDVA); (C) attempted verus achieved spherical equivalent; (D) spherical equivalent refractive accuracy. D = diopters

Figure 1.

Visual and refractive outcomes of SmartSurfACE group eyes (SCHWIND eye-tech-solutions): (A) uncorrected distance visual acuity (UDVA); (B) change in corrected distance visual acuity (CDVA); (C) attempted verus achieved spherical equivalent; (D) spherical equivalent refractive accuracy. D = diopters

Visual and refractive outcomes of Streamlight group eyes (Alcon Laboratories, Inc): (A) uncorrected distance visual acuity (UDVA); (B) change in corrected distance visual acuity (CDVA); (C) attempted verus achieved spherical equivalent; (D) spherical equivalent refractive accuracy. D = diopters

Figure 2.

Visual and refractive outcomes of Streamlight group eyes (Alcon Laboratories, Inc): (A) uncorrected distance visual acuity (UDVA); (B) change in corrected distance visual acuity (CDVA); (C) attempted verus achieved spherical equivalent; (D) spherical equivalent refractive accuracy. D = diopters

Figure 3 shows an interesting result from a study patient. The Streamlight eye had a preoperative sphere, cylinder, and central corneal thickness of −3.25 D, −1.00 D, and 524 µm, respectively. The SmartSurfACE eye had a preoperative sphere, cylinder, and central corneal thickness of −3.00 D, −1.00 D, and 530 µm, respectively. Preoperatively, the EZI and EDI were similar between the eyes of the patient. However, significant differences postoperatively were evident because the EDI was 2.01 and 0.98 µm for the Streamlight and SmartSurfACE eyes, respectively. Thus, the epithelial remodeling was greater in the Streamlight group than in the SmartSurfACE group.

A comparison of epithelium thickness remodeling between two eyes of a patient (before and after surgery). A summary of the epithelium Zernike indices is listed in the table next to the contour plots. Streamlight is manufactured by Alcon Laboratories, Inc. RMS = root mean square; LOC = lower order coefficients; HOC = higher order coefficients; EDI = Epithelium Distortion Index; TransPRK = transepithelial photorefractive keratectomy

Figure 3.

A comparison of epithelium thickness remodeling between two eyes of a patient (before and after surgery). A summary of the epithelium Zernike indices is listed in the table next to the contour plots. Streamlight is manufactured by Alcon Laboratories, Inc. RMS = root mean square; LOC = lower order coefficients; HOC = higher order coefficients; EDI = Epithelium Distortion Index; TransPRK = transepithelial photorefractive keratectomy

Among the tested AI models (support vector machine, random forest, and logistic regression), the support vector machine AI with radial basis kernel provided the best segregation between the SmartSurfACE and Streamlight groups with an area under the curve of 0.87 ± 0.05. The classification accuracy, F1 score, precision, and recall were 0.85, 0.86, 0.87, and 0.87, respectively. The top five indices used by the AI model (in the order of 1 to 5) were the postoperative changes in EZI Z02, EDI, and RMS LOC of EZI at 3 months, Z02 of the A–E interface at 6 months, and EZI SA at 3 months. Thus, the remodeling differences between the SmartSurfACE and Streamlight groups were best indicated by changes in the EZI because these were the top three indices.

Discussion

The use of a laser to remove the epithelium is an attractive proposition because it can remove the tissue with more precision.2 However, the laser is programmed with a nearly uniform epithelial thickness that is an approximation.9 At the same time, small amounts of the epithelium may be left unintentionally with manual methods. We investigated the early corneal and epithelial changes in a contralateral eye study between SmartSurfACE reverse transepithelial PRK and Streamlight transepithelial PRK. The postoperative UDVA, spherical equivalent refractive accuracy, and residual astigmatism were mostly similar between the two groups. However, distinct corneal and epithelial thickness changes were observed. Particularly, a greater change in corneal and epithelial indices was observed in the Streamlight group than the SmartSurfACE group. This difference was mostly because of removal of epithelium from a large zone in the former, which delayed the reepithelialization and wound healing in these eyes. Further, AI showed that the indices most representative of remodeling differences between the two groups of eyes were the EZI and EDI. This indicated that temporal monitoring of the spatial features of the epithelium thickness with AI could be used effectively for assessment of visual recovery after transepithelial PRK.9 Also, the fact that pain was lower in the SmartSurfACE group at 1 day postoperatively illustrated the benefit of removing as little of the epithelium as possible.

The Smart-Pulse technology led to rapid visual recovery and was comparable to a few laser in situ keratomileusis (LASIK) study outcomes, but its relative superiority to single-step transepithelial PRK is unknown because only 78% of the eyes achieved monocular UDVA of 20/20 or better.15 The single-step transepithelial PRK has advantages over alcohol-assisted PRK: less tissue removed, less pain, less haze, faster visual recovery, faster reepithelialization, and shorter surgery time.16,17 In fact, the single-step transepithelial PRK with and without Smart-Pulse technology achieved the least amount of postoperative haze among all transepithelial PRK procedures.2 Nearly 90% of the eyes achieved CDVA of 20/20 or better at 3 months of follow-up in the transepithelial PRK group.16 This result was similar to our study (Figures 12). However, the proportion of eyes within ±0.50 D of spherical equivalent and 0.50 D of cylinder postoperatively was better in the earlier studies. This could be due to a larger sample size and use of wavefront-guided ablation in the earlier studies compared to ours.16–19 Nonetheless, there are also studies with outcomes inferior to ours. For example, in a multi-group comparison of transepithelial PRK versus alcohol-assisted PRK versus LASIK, 77.1%, 60.7%, and 48.3% of the eyes achieved UDVA of 20/20 or better at 12 months.20 Further, 91.4%, 85.7%, and 83.9% of the eyes, respectively, were within ±0.50 D postoperatively.20

The SmartSurfACE single-step transepithelial PRK used a specific ablation profile for removing the epithelium and considered some degree of non-uniformity of epithelial thickness.3,19 This was also confirmed in our earlier study that showed that the assumption of a uniform epithelial thickness is invalid for patients.9 This may explain the lower residual refractive error in the single-step transepithelial PRK eyes than the alcohol-assisted PRK eyes.19 The Streamlight procedure assumes a uniform thickness. Corneal aberrations may also be affected by this assumption. For example, there was a greater increase in RMS of HOA and spherical aberration of the A–E interface in the wavefront-optimized transepithelial PRK eyes than the corneal wavefront-guided transepithelial PRK eyes.18 We observed an increase in A–E aberrations (Table A) similar to this earlier study. However, the use of a larger optical zone (7 mm) in alcohol-assisted PRK led to stable A–E aberrations relative to preoperative in eyes with low to moderate myopia.21 Our earlier study showed stable A–E and E-B aberrations in eyes with manual PRK relative to the preoperative magnitudes, where the epithelium was removed by mechanical scraping.8

The Pearson's correlations (Table B) clearly showed that the RMS of A–E and E-B aberrations did not correlate with the EDI in both groups of eyes at 3 and 6 months postoperatively, when reepithelization was completed or near completion in most eyes. Thus, an algorithm that optimizes the selection of the optical zone based on preoperative EZI and A–E and E-B aberrations to minimize the postoperative induction of aberrations after single-step transepithelial PRK could be an interesting area of future study. This algorithm should restrict the removal of epithelium only specific to the ablation zone instead of removing a much larger zone of epithelium in the central cornea. A recent study hypothesized that the rate of change of curvature drove the compensatory remodeling of epithelium after refractive surgery.22 In Table A, the aberrations of the A–E and E-B interface of the Streamlight group always tended to be greater than the SmartSurfACE group and also achieved statistical significance (RMS of LOA) at 6 months postoperatively. Similarly, the EDI of the Streamlight group was always greater than the SmartSurfACE group. Technically, the greater the aberrations, the greater would be the rate of change of curvature. However, the aberrations at the A–E and E-B interface were not predictive of the EDI at 3 and 6 months postoperatively (Table B). Thus, it is possible that the hypothesis was true. Nonetheless, further study is required to confirm the hypothesis.

The limitations of our study include the small sample size and short duration of follow-up. Longer follow-up is required to evaluate the long-term differences between the two procedures. Another limitation of this study was that the corneal healing and nerve regeneration of one eye may influence the healing in the fellow eye. Thus, unilateral eye studies are also required in the future. Additionally, a greater surface area of the Streamlight group was exposed to mitomycin C and the effect of this on healing of the cornea was unknown to us. Another limitation was that the reepithelialization time was not measured in the two groups. This would also require further study.

The two procedures had similar visual acuity and refractive outcomes postoperatively. However, the change in the A–E and E-B aberrations and EZI indicated a greater degree of remodeling in the Streamlight group. The Streamlight group was still undergoing significant remodeling due to a much greater change in the A–E and E-B aberrations and EZI after surgery. This was mostly due to a much greater amount of epithelium removed in the Streamlight group, which delayed the corneal wound healing relative to the SmartSurfACE group.

References

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Preoperative Parameters of the Two Groups of Eyes

ParameterSmartSurfACE Group (n = 20)Streamlight Group (n = 20)P
Sphere (D)−3.84 ± 1.74−3.90 ± 1.81.90
Cylinder (D)−0.74 ± 0.54−0.74 ± 0.431.00
Spherical equivalent (D)−4.21 ± 1.83−4.27 ± 1.87.90
Central corneal thickness (µm)524.5 ± 30.2523.4 ± 32.0.90
UDVA (logMAR)0.66 ± 0.570.66 ± 0.551.00
CDVA (logMAR)0 ± 00 ± 01.00

Epithelium Zernike Values (Mean ± SD) Preoperatively and at 1, 3, and 6 Months Postoperatively

ParameterPreoperative1 Month Postop3 Months Postop6 Months PostopPa





SmartSurfACEStreamlightSmartSurfACEStreamlightSmartSurfACEStreamlightSmartSurfACEStreamlightSmartSurfACEStreamlight
Mean thickness (μm)54.27 ± 2.9154.54 ± 3.1251.34 ± 4.4153.21 ± 4.1657.88 ± 4.2461.66 ± 5.2161.49 ± 5.3263.21 ± 5.55a,ba,b
Maximum thickness (μm)56.3 ± 2.8756.18 ± 3.0254.68 ± 4.6555.72 ± 4.0460.65 ± 4.6263.64 ± 5.4964.35 ± 5.7365.39 ± 5.84a,ba,b
Minimum thickness (μm)48.06 ± 3.0647.32 ± 3.1642.75 ± 3.1641.01 ± 2.3950.44 ± 3.5047.9 ± 4.3452.96 ± 4.6250.48 ± 3.02b; Allca,b
Z02 (μm)−1.13 ± 0.57−1.37 ± 0.64−1.24 ± 0.82−2.53 ± 1.05−1.02 ± 0.69−2.8 ± 0.94−1.25 ± 0.98−2.42 ± 1.13NSAllc
RMS of Z−22 and Z+22 (μm)0.33 ± 0.230.40 ± 0.160.78 ± 0.440.78 ± 0.530.51 ± 0.390.53 ± 0.310.54 ± 0.270.37 ± 0.27NSNS
RMS of Z−13 and Z+13 (μm)0.22 ± 0.110.23 ± 0.140.48 ± 0.290.57 ± 0.370.41 ± 0.250.52 ± 0.390.43 ± 0.240.60 ± 0.45NSb
Z04 (μm)0.29 ± 0.190.35 ± 0.240.01 ± 0.250.04 ± 0.48−0.18 ± 0.25−0.34 ± 0.45−0.24 ± 0.32−0.35 ± 0.50AllcAllc
RMS LOC (μm)1.21 ± 0.541.44 ± 0.641.57 ± 0.732.69 ± 1.071.23 ± 0.652.86 ± 0.961.44 ± 0.902.47 ± 1.10NSAllc
RMS HOC (μm)0.63 ± 0.180.69 ± 0.281.03 ± 0.391.26 ± 0.780.91 ± 0.371.10 ± 0.640.93 ± 0.301.10 ± 0.40NSb
Epithelial Distortion Index (μm)1.39 ± 0.511.61 ± 0.671.92 ± 0.73.02 ± 1.191.59 ± 0.593.10 ± 1.041.78 ± 0.82.75 ± 1.07NSAllc

Mean ± SD Values Preoperatively and at 1, 3, and 6 Months Postoperatively

ParameterPreoperative1 Month Postop3 Months Postop6 Months PostopPa





SmartSurfACEStreamlightSmartSurfACEStreamlightSmartSurfACEStreamlightSmartSurfACEStreamlightSmartSurfACEStreamlight
Air–epithelium interface
  Steep curvature (D)44.57 ± 1.5844.67 ± 1.4740.87 ± 2.5141.49 ± 2.4940.87 ± 2.1541.42 ± 2.2040.85 ± 1.9841.06 ± 2.12bb
  Flat curvature (D)42.61 ± 1.7943.53 ± 1.4939.11 ± 2.5039.95 ± 2.4239.21 ± 2.0939.31 ± 2.1938.89 ± 2.2939.30 ± 2.24bb
  Maximum curvature (D)45.02 ± 1.6645.35 ± 1.4441.53 ± 2.5342.41 ± 2.4841.61 ± 2.0341.96 ± 2.1141.21 ± 1.9142.15 ± 1.86bb
  Defocus (μm)−0.23 ± 0.45−0.59 ± 0.63−0.1 ± 0.820.14 ± 0.70.34 ± 0.700.65 ± 0.840.60 ± 0.690.96 ± 0.81cb
  RMS astigmatism (μm)0.47 ± 0.210.66 ± 0.490.66 ± 0.461.08 ± 0.740.9 ± 0.560.68 ± 0.610.61 ± 0.330.81 ± 0.47NSNS
  RMS coma (μm)0.25 ± 0.120.25 ± 0.120.38 ± 0.180.42 ± 0.250.3 ± 0.120.35 ± 0.340.35 ± 0.190.38 ± 0.21NSNS
  SA (μm)−0.06 ± 0.12−0.16 ± 0.17−0.04 ± 0.210.03 ± 0.20.08 ± 0.180.16 ± 0.220.17 ± 0.190.25 ± 0.20cb
  RMS LOA (μm)0.66 ± 0.291.00 ± 0.641.02 ± 0.491.29 ± 0.731.18 ± 0.561.17 ± 0.760.96 ± 0.631.42 ± 0.62NSNS
  RMS HOA (μm)0.97 ± 0.271.01 ± 0.381.07 ± 0.321.50 ± 0.581.21 ± 0.611.27 ± 0.831.10 ± 0.411.28 ± 0.49NSNS
Epithelium–Bowman's layer
  Steep curvature (D)44.25 ± 1.5744.14 ± 1.4440.48 ± 2.4940.83 ± 2.5140.79 ± 2.2640.54 ± 2.1240.39 ± 2.0140.46 ± 1.90dd
  Flat curvature (D)42.17 ± 1.3842.43 ± 1.1138.69 ± 2.3639.13 ± 2.7838.89 ± 2.3638.57 ± 2.0138.53 ± 1.9539.01 ± 2.00dd
  Maximum curvature (D)44.43 ± 1.4844.49 ± 1.4041.18 ± 2.8341.89 ± 1.9741.48 ± 2.1341.14 ± 2.3140.88 ± 1.9141.03 ± 1.75dd
  Defocus (μm)0.55 ± 0.380.35 ± 0.51−0.02 ± 0.650.40 ± 1.40−0.06 ± 0.57−0.05 ± 1.080.15 ± 0.380.24 ± 0.88eNS
  RMS astigmatism (μm)0.39 ± 0.260.61 ± 0.240.90 ± 0.600.93 ± 0.640.92 ± 0.510.81 ± 0.960.53 ± 0.300.92 ± 0.65NSNS
  RMS coma (μm)0.22 ± 0.110.22 ± 0.120.29 ± 0.110.42 ± 0.220.31 ± 0.140.32 ± 0.390.27 ± 0.140.32 ± 0.17NSNS
  SA (μm)0.13 ± 0.100.08 ± 0.13−0.01 ± 0.180.11 ± 0.37−0.03 ± 0.15−0.04 ± 0.340.07 ± 0.110.06 ± 0.23eNS
  RMS LOA (μm)0.75 ± 0.320.85 ± 0.261.09 ± 0.621.46 ± 1.111.05 ± 0.561.20 ± 1.120.67 ± 0.271.30 ± 0.58NSNS
  RMS HOA (μm)0.88 ± 0.271.03 ± 0.351.26 ± 0.441.63 ± 0.741.37 ± 0.551.43 ± 1.581.15 ± 0.381.24 ± 0.45NSNS

Pearson's Correlation at 3 and 6 Months Postoperatively

GroupSmartSurfACEEDI (µm) at 3 MonthsEDI (µm) at 6 Months
SmartSurfACERMS LOA (µm)a0.06 (NS)−0.21 (NS)
RMS HOA (µm)a−0.10 (NS)0.33 (NS)
RMS LOA (µm)b−0.23 (NS)−0.39 (NS)
RMS HOA (µm)b0.01 (NS)0.47 (NS)
StreamlightRMS LOA (µm)a0.02 (NS)−0.48 (NS)
RMS HOA (µm)a0.39 (NS)0.30 (NS)
RMS LOA (µm)b0.38 (NS)0.54 (P < .05)
RMS HOA (µm)b0.30 (NS)0.19 (NS)
Authors

From the Department of Cornea and Refractive Services, Narayana Nethralaya Eye Hospital, Bangalore, India (RS, ZD, PP, SM, AC); and Imaging, Biomechanics and Mathematical Modelling Solutions, Narayana Nethralaya Foundation, Bangalore, India (RN, ASR).

Drs. Shetty and Sinha Roy have a pending patent application on imaging of Bowman's layer through Narayana Nethralaya Foundation, Bangalore, India. The remaining authors have no financial or proprietary interest in the materials presented herein.

AUTHOR CONTRIBUTIONS

Study concept and design (RS, ASR); data collection (ZD, PP, AC); analysis and interpretation of data (RN, SM); writing the manuscript (RN, ZD, PP, SM, AC); critical revision of the manuscript (RS, ASR)

Correspondence: Abhijit Sinha Roy, PhD, Narayana Nethralaya, #258/A Hosur Road, Bommasandra, Bangalore-560099, India. Email: asroy27@yahoo.com

Received: March 26, 2020
Accepted: July 30, 2020

10.3928/1081597X-20200730-03

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