To improve functional outcomes and reduce the induced higher order aberrations (HOAs) after laser vision correction, different ablation profiles are available. For example, the wavefront-optimized (WFO) profile uses an aspheric ablation to avoid a significant increase of the postoperative spherical aberration.1,2 Topography-guided ablation (TCAT) focuses on preserving the aspheric corneal profile and reducing HOAs.3,4 Previous studies reported the visual and refractive outcomes of this treatment modality in the management of irregular corneal astigmatism secondary to corneal disease or previous surgery.4–7 Interestingly, the same type of custom ablation revealed excellent efficacy and safety levels in primary eyes that underwent laser vision correction for myopia or myopic astigmatism.8–11
Recent prospective and contralateral studies compared the results between different treatment modalities of laser vision correction. Although TCAT and WFO ablations had essentially similar visual and refractive outcomes after myopic LASIK, the former induced fewer HOAs in the postoperative period.10–12 The relative absence of contralateral studies that have compared these ablation approaches in patients who underwent photorefractive keratectomy (PRK) prompted this analysis. The goal of this study was to compare the visual and refractive outcomes of TCAT and WFO profiles in patients undergoing PRK ablation with the EX500 excimer laser platform (Alcon Laboratories, Inc) for the treatment of myopia with and without astigmatism.
Patients and Methods
This prospective and randomized clinical study received approval from the ethics committee of Fernando Faria Correia, Lda, and adhered to the tenets of the Declaration of Helsinki. Informed consent was provided and documented in writing from each patient prior to the time of the intervention. The study was conducted on patients in our clinical practice by two surgeons (FF-C, SR) during scheduled preoperative and postoperative procedure visits and unscheduled visits when necessary (between May 2017 and December 2018).
The inclusion criteria for the study were: age between 18 and 40 years, no previous ocular surgery, documented refractive stability for at least 1 year (a change of 0.50 diopters [D] or less), a corrected distance visual acuity (CDVA) of 20/25 or better, a spherical equivalent refraction less than 6.00 D, and refractive astigmatism less than 3.00 D.
All patients were instructed to stop wearing soft and rigid gas permeable contact lenses for 2 and 3 weeks prior to the clinical visit, respectively. A complete preoperative ophthalmologic evaluation was performed in every patient to exclude ocular pathology other than a refractive error. Exclusion criteria for the laser vision correction procedure comprised a history of herpetic ocular disease, corneal scarring, severe dry eye, diabetes mellitis, pregnancy, breastfeeding, collagen vascular disease, and keratoconus or ectasia susceptibility findings in the Scheimpflug-based tomography (Oculyzer II; Alcon Laboratories, Inc).13,14
The Alcon WaveLight EX500 excimer laser was employed for all procedures. One eye of each patient was randomly assigned (coin flip) to the TCAT group and the fellow eye was then assigned to the WFO group. Each patient was not aware which eye received TCAT or WFO treatment until the completion of the study.
Placido-based corneal topography images obtained by the Allegro Topolyzer Vario system (Alcon Laboratories, Inc) were used to plan the TCAT ablation. The data derived from eight scans were transferred to the laser platform software. The selected topography images were based on the proper coverage (accurate mire recognition, no interference of dry spots or eyelashes/nose shadows, and precise identification of the limbus and pupil entrance by the software) and consistency among the eight scans. The TCAT treatment was then planned by the protocol described in a previous study (Figure A, available in the online version of this article).15
(A) Data derived from the Topolyzer Vario system are uploaded to the Wavelight EX500 platform (Alcon Laboratories, Inc., Fort Worth, TX). The reproducibility of the maps and the measurements of keratometry and axes can be checked. In this clinical case, 7 scans were uploaded and only 5 were selected to plan the custom ablations (scans 1 and 5 were excluded). Note that the mean average deviation (MAD) was less than 0.05 diopters. (B) After the Topolyzer scans are processed, the patient's manifest refraction (“Method Subjective”), pachymetry values, and pupil size are entered in the software. (C) On this display, the surgeon can evaluate the discrepancy between manifest versus measured astigmatic axis. In the figure, the difference between the refractive and topographic astigmatism axis was less than 15°. The higher order aberrations ablation profile is displayed when the refractive error is set by the user to zero sphere and cylinder (“Modified”). This image shows the ablation that would be attempted to regularize the cornea regarding the corneal vertex. In the current case, a tetrafoil-like pattern and a maximum ablation of less than 10 microns are shown. (D) The subjective refraction is introduced again (“Modified”) to visualize the final ablation profile (optical zone of 6.5 mm). “Clinical” refraction should contain manifest refraction without nomogram adjustment. “Measured” refraction is the one measured by the Topolyzer Vario device. The “Measured” spherical component has no clinical significance. It is the best fit sphere and should not be used for surgery planning. The “Measured” cylinder and axis components are required for planning and are usually different from the ones of the manifest refraction. They are derived from the analysis of the anterior corneal elevation and are separated from the corneal higher order aberrations (coma, trefoil, etc.). “Modified” refraction is the treatment that the excimer laser will use and must be determined by the surgeon. Reprinted with permission from Faria-Correia F, Ribeiro S, Monteiro T, Lopes BT, Salomão MQ, Ambrósio R Jr. Topography-guided custom photorefractive keratectomy for myopia in primary eyes with the WaveLight EX500 platform. J Refract Surg. 2018;34(8):541–546.
All PRK procedures were performed by the same surgeon (FF-C). The epithelium removal was assisted by a 20% alcohol solution (20 seconds of exposure) in the 9-mm central zone. After this step, the laser ablation was applied (6.5-mm optical zone and 1.25-mm transition zone for both groups), followed by application of topical mitomycin C 0.05% (15 seconds of exposure). A therapeutic contact lens was placed at the end of the procedure, and it was removed at postoperative day 5 or 6, after confirmation of complete epithelial healing.
Postoperative treatment included ofloxacin drops five times a day for 2 weeks and ocular lubricants (0.15% sodium hyaluronate) at least five times daily. Bromfenac drops were also applied two times a day until contact lens removal. Then, the patients were instructed to use dexamethasone drops (0.1%) five times a day gradually tapered for 1 month.
Postoperative examinations were performed at 1 day, 1 week, and 1, 3, 6, and 12 months. The primary outcome measures included manifest refraction, uncorrected distance visual acuity (UDVA), CDVA, change in lines of vision, actual versus targeted change in spherical equivalent, and complete ophthalmologic examinations at 1, 3, 6, and 12 postoperative months. The corneal root mean square (RMS) HOAs from the 3rd to the 5th order Zernike polynomials (6-mm zone) and spherical aberration of the preoperative and last postoperative visits were also registered from the Scheimpflug-based tomography (Oculyzer II; Alcon Laboratories, Inc). Visual and refractive outcome graphs were plotted as defined by Waring et al.16 At the last follow-up visit, patients were asked to report their comfort during the postoperative period and presence of visual symptoms.
Statistical analyses were performed using MedCalc software (version 17.9.7). The Kolmogorov–Smirnov test was used to check the normality of data distribution. For intragroup (preoperative vs postoperative data) and intergroup (TCAT vs WFO groups) comparisons, paired t tests were used for normally distributed variables and the Wilcoxon signed-rank test was used for those that were not normally distributed. A P value of less than .05 was considered statistically significant.
Twenty-three patients (13 women and 10 men) were included in the study. The mean age of the patients was 27.32 ± 5.6 years (range: 23 to 36 years). No losses were reported during the follow-up. As presented in Table 1, the preoperative CDVA, spherical and cylindrical refractive error, and manifest refraction spherical equivalent (MRSE) were similar in both groups. One year after the surgery, there also was no significant difference in the variables mentioned above between the two groups.
Mean ± SD Preoperative Parameters for WFO and TCAT Profiles
In both groups, 96% of eyes achieved a UDVA of 20/20 or better at 12 months (Figure 1). The CDVA remained stable in 46.2% and 53.9% of the eyes in the TCAT and WFO groups, respectively (P = .42, Figure 2). Compared to the preoperative CDVA, 19.2% of eyes gained one or more lines of postoperative UDVA at 12 months in both groups (Figure 3).
Cumulative postoperative uncorrected distance visual acuity (UDVA) showing the percentage of eyes that achieved UDVA of 20/20 in the (A) topography-guided customized ablation treatment (TCAT) and (B) wavefront-optimized (WFO) groups. CDVA = corrected distance visual acuity
Comparison between the preoperative and postoperative corrected distance visual acuity (CDVA) (change in Snellen lines) in the (A) topography-guided customized ablation treatment (TCAT) and (B) wavefront-optimized (WFO) groups.
Postoperative uncorrected distance visual acuity (UDVA) compared with preoperative corrected distance visual acuity (CDVA) (change in Snellen lines) in the (A) topography-guided customized ablation treatment (TCAT) and (B) wavefront-optimized (WFO) groups.
As shown in Figure 4, the coefficient of determination (R2) between the attempted (0.969) and achieved (0.979) MRSE was also similar between the WFO and TCAT groups (P = .72). Figures 5–6 demonstrate the accuracy of the surgical procedure in terms of refractive correction. Regarding the spherical equivalent refractive accuracy (Figure 5), the percentage of eyes within ±0.50 D was 81% in the WFO group and 85% in the TCAT group (P = .81). Both types of ablation presented a similar stability during the 12-month follow-up (Figure 7).
Attempted versus achieved change in manifest refraction spherical equivalent at 12 months in the (A) topography-guided customized ablation treatment (TCAT) and (B) wavefront-optimized (WFO) groups. D = diopters
Accuracy of manifest refraction spherical equivalent at 12 months in the (A) topography-guided customized ablation treatment (TCAT) and (B) wavefront-optimized (WFO) groups. D = diopters
Accuracy of refractive astigmatism correction at 12 months in the (A) topography-guided customized ablation treatment (TCAT) and (B) wavefront-optimized (WFO) groups. D = diopters
Change in manifest refraction spherical equivalent during the follow-up period in the (A) topography-guided customized ablation treatment (TCAT) and (B) wavefront-optimized (WFO) groups. D = diopters; SD = standard deviation
Table 2 shows the magnitudes of the corneal HOAs between the preoperative and 12-month postoperative visits for both groups. The total RMS of the anterior surface of the cornea (RMS HOAs) and the spherical aberration did not present a statistically significant difference between the WFO and TCAT groups (P = .51 and .32, respectively).
HOAs and Spherical Aberration Derived From Oculyzer II in the WFO and TCAT Profiles
Only one patient had delayed epithelial healing during the first month after surgery (for this case, complete epithelial healing was achieved in the third postoperative week). During the follow-up period, 20 patients (86.96%) reported transient dry eye syndrome (foreign body sensation and light sensitivity) in both eyes (17 patients during the first month and 3 patients during the first 2 months after the surgery). Twenty patients (86.96%) described vision fluctuation during the first month after the surgery.
No patient reported any symptoms related to glare, halos, or starbursts. During the 12-month follow-up, other postoperative complications (eg, infection, corneal infiltrates, haze, or persistent epithelial defects) were not described.
We found excellent outcomes using both TCAT and WFO platforms for PRK, and found no differences between groups in any outcome. To the best of our knowledge, no previous report has described and compared the 12-month postoperative results between the WFO and TCAT profiles in eyes that underwent PRK surgery. Falavarjani et al17 also compared these profiles in patients who underwent surface ablation for myopia correction with the laser excimer platform. At 3 and 6 months postoperatively, CDVA and contrast sensitivity results were statistically similar between the TCAT and WFO groups in eyes with low to moderate myopia. These authors used a different excimer laser platform (ALLEGRETTO WAVE Eye-Q 400; Alcon Laboratories, Inc.) and another corneal imaging device (Orbscan; Bausch & Lomb).17 In the current study, we only analyzed visual acuity, refraction, and corneal wavefront between the two groups.
Other studies analyzed the results between the WFO and TCAT ablation treatment in patients who had laser in situ keratomileusis (LASIK) with a 6-month follow-up, describing similar or slightly better outcomes for the latter approach.10–12,18 It should be highlighted that some of these studies do not describe the protocol used to plan TCAT ablations. For example, Zhang and Chen11 described a similar UDVA and CDVA between the TCAT and WFO groups, but with better accuracy for the cylinder correction for the latter group. The authors mentioned that the topography-modified refraction protocol was applied to the TCAT design. In the current study, custom ablations were planned based on the findings described in a previous report.15
At the 12-month postoperative visit, the refractive and visual outcomes did not present statistically significant differences between both groups. Our results also demonstrated a good profile in terms of efficacy and safety for both profiles in eyes that had PRK. We obtained an improvement in CDVA in 50% and 42.3% of eyes in the TCAT and WFO groups, respectively, and a same or better postoperative UDVA than the preoperative CDVA in 96.2% and 95.4% of eyes, respectively. The procedure was accurate, with both groups presenting 80% or more of eyes within ±0.50 D of spherical equivalent.
As shown in the results section, there was a tendency for vision improvement for both profiles during the follow-up. However, we described a reduction of the UDVA compared to the preoperative CDVA of 6 (26.9%) of the included eyes at 1 month postoperatively for both groups. We attribute these findings to the epithelial healing/remodeling process that occurs in this type of surgery. Concerning the refractive stability, our outcomes are consistent with those presented in the scientific literature.15,19,20
Regarding the corneal wavefront analysis, the RMS HOAs and spherical aberration was lower in eyes that had undergone TCAT, although not statistically significant (P = .51 and .32, respectively). Similar results were described by Shetty et al10 at 6 months after LASIK surgery. Contrariwise, Zhang and Chen11 reported that the corneal RMS HOAs were significantly lower in eyes in the TCAT group at 6 months postoperatively. Compared to this study, several factors might explain the presented outcomes. As mentioned previously, the laser vision correction procedures and the protocol used to design TCAT ablations was distinct among the reports. Second, the wavefront data were also derived from different corneal tomography devices. In addition to the different population size, the preoperative data were also not equivalent, and our study revealed the lowest mean for the myopia, cylinder, and MRSE variables in the TCAT and WFO groups (Table 1). The refractive error ablation, along with the customized component, can affect the postoperative outcome, especially for wavefront findings.15,21
Salah-Mabed et al22 compared the epithelium (air–tear film) and Bowman layer's curvature findings in patients with low to moderate myopia corrected by PRK, before and after epithelial removal. Based on Placido disc topography findings, the authors found that the epithelial layer tended to decrease the magnitude of astigmatism and prolateness of the Bowman layer, which was significantly steeper than the surface of the epithelium. An individual analysis of the Bowman's membrane or anterior stromal surface topography may enable new scientific and clinical findings to be reached or even assist in the selection of the ablation type, to optimize and personalize vision correction. Our data suggest that the Bowman's membrane topography of the eyes included in this study probably presented mild irregularity magnitude and interocular symmetry. Future studies with new corneal imaging technology, such as segmental tomography, might provide new insights to explain these results.23
Both profiles presented an excellent safety profile during the study follow-up. Only one patient presented with delayed epithelial healing during the first month after surgery. Other postoperative complications, such as infection, corneal infiltrates, or haze, were not described during the 12-month follow-up.
Our study has some limitations. The number of eyes included in this report is small and some of the results described could also be specific to the sample used. The relative absence of previous studies with a surface ablation technique comparing the TCAT and WFO profiles makes it challenging to extrapolate the current data. Other parameters related to visual quality, such as contrast sensitivity function, should be studied in future reports.
The current study describes the 12-month postoperative visual and refractive outcomes of the WFO and TCAT profiles in primary eyes that had PRK for myopia correction. Both profiles are similarly accurate, safe, and effective in correcting myopia and myopic astigmatism. Our results did not discern the superiority of a profile in terms of postoperative corneal wavefront. There is a need to improve the available ocular imaging methods to understand the results and to improve available ablation profiles.
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Mean ± SD Preoperative Parameters for WFO and TCAT Profiles
|Sphere (D)||−2.34 ± 0.98||−2.23 ± 0.97||.96||−0.40 ± 0.77||−0.28 ± 0.37||.87|
|Cylinder (D)||−0.52 ± 0.59||−0.51 ± 0.62||.81||−0.14 ± 0.40||−0.28 ± 0.37||.92|
|MRSE (D)||−2.60 ± 0.93||−2.52 ± −0.98||.79||−0.29 ± 0.37||−0.13 ± 0.40||.82|
|Preoperative CDVA (logMAR)||−0.008 ± 0.03||−0.02 ± 0.04||.14||−0.06 ± 0.09||−0.07 ± 0.08||.97|
HOAs and Spherical Aberration Derived From Oculyzer II in the WFO and TCAT Profilesa
|RMS HOAs (µm)||0.277 (0.242 to 0.297)||0.279 (0.245 to 0.304)||.98||0.304 (0.269 to 0.337)||0.299 (0.263 to 0.339)||.51|
|Spherical aberration (µm)||0.224 (0.215 to 0.234)||0.240 (0.226 to 0.324)||.72||0.360 (0.333 to 0.385)||0.340 (0.329 to 0.370)||.32|