Photorefractive keratectomy (PRK) is the oldest excimer surface ablation procedure, having been introduced in the late 1980s.1,2 The introduction of new surgical techniques and technological advances has led to a reduction of the complications related to PRK such as patient discomfort, regression, and haze and to improvement of the outcome predictability.3–5 Nevertheless, advanced surface ablation techniques are still classified as safe for the treatment of low myopia, especially to avoid flap-related complications and iatrogenic corneal ectasia.5–10
Different laser ablation profiles have been designed to enhance functional outcomes and to reduce the higher order aberrations (HOAs) induced after laser vision correction. For example, the wavefront-optimized (WFO) ablation uses an aspherical ablation profile to avoid a significant increase of the postoperative spherical aberration.11,12 Topography-guided ablation treatment (T-CAT) aims to maintain the aspheric corneal shape and to eliminate corneal irregularities.13,14 Previous studies have demonstrated the efficacy and safety of this approach in the treatment of irregular corneal astigmatism secondary to disease or previous surgery.14–17 Interestingly, the same type of custom ablation has revealed excellent refractive and visual outcomes in primary eyes that underwent LASIK for myopia or myopic astigmatism correction with the WaveLight EX500 platform (Alcon Laboratories, Inc., Fort Worth, TX).18,19 In a recent clinical trial from the U.S. Food and Drug Administration (FDA), Stulting et al.18 described the efficacy and safety of T-CAT custom LASIK for the correction of myopia and myopic astigmatism in primary eyes. The current study was designed to describe the visual and refractive results of the same type of treatment profile applied to patients undergoing PRK with the WaveLight EX500 platform for the treatment of myopia with or without astigmatism.
Patients and Methods
The study was conducted on a retrospective, non-comparative case series according to the tenets of the Declaration of Helsinki. Written informed consent was obtained from all patients before the surgical procedure. The study was conducted as part of the authors' clinical practice (FF-C and SR) during scheduled preoperative and postoperative visits between May 2016 and December 2017.
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), corrected distance visual acuity (CDVA) of 20/25 or better, spherical equivalent refraction of less than 6.00 D, and refractive astigmatism of less than 3.00 D.
All patients were instructed to stop wearing soft or rigid gas permeable contact lenses for 2 and 3 weeks prior to the clinical visit, respectively. Each patient had a complete preoperative ophthalmologic evaluation to exclude ocular pathology other than a refractive error. Exclusion criteria for the laser vision correction procedure were: a history of herpetic ocular disease, corneal scarring, severe dry eye, diabetes mellitus, pregnancy, breastfeeding, collagen vascular disease, and keratoconus or ectasia susceptibility findings in the Scheimpflug-based tomography (Oculyzer II; Alcon Laboratories, Inc.).20
All patients were treated with the same laser platform (WaveLight EX500). Corneal topographies obtained with the Allegro Topolyzer Vario system (Alcon Laboratories, Inc.) were used to plan the ablation. The data derived from the scans were transferred to the laser platform software after being analyzed by the same reviewer (FF-C). The selected topography images were based on 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 data consistency (±0.75 D) among four to eight scans.
To proceed with the custom treatment, the platform analysis software must display a mean average deviation of less than 0.05 D relative to the scans selected for planning (Figure AA, available in the online version of this article). After processing the data from the Topolyzer scans, the subjective clinical refraction was entered into the topography-guided software (Figure AB). To advance with the T-CAT PRK procedure, the surgeon confirmed two checkpoints. First, the difference between the refractive and topographic astigmatism axis should not exceed 15° (Figure AC). Then, the HOAs ablation profile provided by the laser platform was also analyzed. To obtain this profile in the laser platform layout, the modified refraction was set to 0. Consequently, the topography-guided software revealed the ablation that would be attempted to regularize the cornea (HOA correction) regarding the corneal vertex. By the analysis of the ablation profile map, the presentation of trefoil or tetrafoil-like patterns and a maximum ablation of less than 10 microns were required to proceed with the customized surgical procedure (Figure AC). After these steps, the subjective refraction was introduced again to visualize the final ablation profile (Figure AD). The optical treatment zone was set for 6.5 mm in all ablations.
(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.
Regarding the target Q-value, the surgeon did not perform any adjustment and the mean value of asphericity obtained from topography scans was used as the final target.
All PRK procedures were performed by the same surgeon (FF-C). Each patient received three drops of topical 0.4% oxybuprocaine hydrochloride (Anestocil; Edol, Linda-a-Velha, Portugal) in the conjunctival fornix before the insertion of the eyelid speculum. For epithelium removal, 20% ethanol was placed on the cornea in a 9-mm well for 20 seconds. The cornea was rinsed with a balanced salt solution, and a blunt spatula was used to peel off the epithelium. After this step, the laser ablation treatment was applied. A sponge soaked with 0.02% mitomycin C was placed on the stroma for 15 seconds immediately after excimer ablation. After rinsing to remove mitomycin C, a therapeutic contact lens was placed.
Postoperative treatment included moxifloxacin drops (Vigamox; Alcon Laboratories, Inc.) five times a day for 2 weeks, and bromfenac drops (Yellox; Croma Pharma GmbH, Leobendorf, Austria) twice a day until contact lens removal. The patients were advised to use ocular lubricants containing 0.15% sodium hyaluronate (Hyabak; Thea Laboratories, Clermont-Ferrand, France) at least five times daily. The contact lenses were removed on the fifth and sixth postoperative day after confirmation of complete epithelial healing. The patients were then instructed to use tobramycin/dexamethasone drops (Tobradex; Alcon Laboratories, Inc.) in a tapered regimen for 1 month.
Postoperative examinations were performed at 1 day, 1 week, and 1, 3, and 6 months. The primary out-come measures included manifest refraction, uncorrected distance visual acuity (UDVA), corrected distance visual acuity (CDVA), change in lines of vision, actual versus targeted change in spherical equivalent, and complete ophthalmologic examinations at 1, 3, and 6 postoperative months. The corneal root mean square (RMS) HOAs from the 3rd to the 5th order Zernike polynomials (6.5-mm zone) of the preoperative and last postoperative visits were also recorded. Visual and refractive outcome graphs were plotted as defined by Waring et al.21 The Kolmogorov–Smirnov test was used to check the normality of data distribution. The manifest refractive spherical equivalent (MRSE), sphere, cylinder, and corneal HOA changes observed during follow-up were compared to the preoperative data. A P value of less than .05 was considered statistically significant.
The study enrolled 40 eyes of 25 patients (12 men and 13 women) who had T-CAT custom PRK. Mean age was 28.34 years (range: 18 to 35 years). All eyes were evaluated during the 6-month follow-up. Topography-guided ablation resulted in a significant reduction of MRSE, sphere, and cylinder between the preoperative and all postoperative assessments (P < .05; paired t test). Table 1 shows the refractive data regarding the preoperative and 6-month postoperative visits. Mean MRSE was −2.61 ± 1.03 D preoperatively, −0.23 ± 0.58 D at 1 month, −0.11 ± 0.32 D at 3 months, and −0.03 ± 0.20 D at 6 months (Figure 1).
Change in manifest refraction spherical equivalent during the follow-up period. SD = standard deviation; D = diopters
Figure 2 shows the cumulative postoperative UDVA throughout the follow-up period. At the first month, the UDVA was 20/16 or better in 6 (15%) of 40 eyes and 20/20 or better in 35 (87.5%) of 40 eyes. Three months postoperatively, the UDVA was 20/12.5 or better in 2 (5%) of 40 eyes, 20/16 or better in 11 (27.5%) of 40 eyes and 20/20 or better in 38 (95%) of 40 eyes. At 6 months, the UDVA was 20/12.5 or better in 3 (7.5%) of 40 eyes, 20/16 or better in 10 (32.5%) of 40 eyes and 20/20 or better in 39 (97.5%) of 40 eyes.
Cumulative postoperative uncorrected distance visual acuity (UDVA).
Compared to the preoperative CDVA, 8 (20%) and 10 (25%) of 40 eyes gained one or more lines of postoperative UDVA at 3 and 6 months, respectively (Figure 3). Regarding the safety of the procedure (Figure 4), only 1 eye presented loss of one line of CDVA at 6 months postoperatively. As shown in Figure 4, vision tended to increase after the procedure throughout the follow-up period. Figures 5–7 demonstrate the accuracy of the surgical procedure in terms of refractive correction. Regarding the spherical equivalent refractive accuracy, 38 eyes (95%) were within ±0.50 D (Figure 6).
Postoperative uncorrected distance visual acuity compared with preoperative corrected distance visual acuity (CDVA, change in Snellen lines) (UDVA = uncorrected distance visual acuity).
Comparison between the preoperative and postoperative corrected distance visual acuity (CDVA, change in Snellen lines).
Attempted versus achieved change in manifest refraction spherical equivalent at 6 months. D = diopters
Accuracy of manifest refraction spherical equivalent at 6 months. D = diopters
Accuracy of refractive astigmatism correction at 6 months. D = diopters
Only one episode of delayed epithelial healing (3 weeks for complete wound healing) was reported during the first month after surgery or later. No cases of infection were reported during the follow-up period. Ten patients (15 eyes) showed transient dry eye, foreign body sensation, and light sensitivity lasting from weeks 1 to 3 postoperatively. Slit-lamp examination revealed signs of superficial punctate keratitis demonstrated with fluorescein dye staining of the cornea during the first month in this group of patients. Fluctuation in vision was also reported by 21 patients (17 eyes) during the first month after surgery. During the follow-up period, the patients did not report symptoms related to visual quality, such as glare, halos, or starbursts. No haze findings were described during the follow-up period.
The magnitudes of RMS for the corneal HOAs between the preoperative and 6-month postoperative visits were 0.398 ± 0.009 and 0.427 ± 0.016 μm, respectively. Despite the lack of statistical significance (P = .347; Wilcoxon test), there was a mean increase of 7.3% of the corneal RMS HOAs.
Previous studies have reported the efficacy of T-CAT ablation as a therapeutic procedure to correct corneal irregularities in patients with keratoconus and other types of corneal ectasia, trauma, or complications related to laser vision correction.14–17 Stulting et al.18 reported the safety and effectiveness of T-CAT custom LASIK for the correction of myopia and myopic astigmatism in primary eyes. Our results also demonstrate good outcomes for customized ablation in eyes with PRK. Interestingly, we also obtained an improvement in CDVA in 40% of eyes and the same or better postoperative UDVA than the preoperative CDVA in 97.5% of eyes. The procedure was accurate, with 95% of eyes being within ±0.50 D of the preoperative spherical equivalent. Vision improved after T-CAT PRK during follow-up. Nevertheless, we registered a reduction of the UDVA compared to the preoperative CDVA of 5 (12.5%) of the included eyes during the first postoperative month. 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 reported in the scientific literature.5,22 Compared to the results reported by Stulting et al.,18 the stability of the postoperative spherical equivalent occurred later in our case series.
At 6 months postoperatively, only one eye developed a decrease of UDVA and CDVA compared to the preoperative data. During the follow-up time, this eye presented delayed epithelial healing compared to the remaining eyes included in this study. We attributed this complication to a potential combination of factors, such as preoperative mild dry eye and epithelial healing delay related to bromfenac use after surgery. Interestingly, no haze or scar findings were reported during follow-up.23,24 Compared to PRK, one advantage of LASIK is related to corneal epithelium integrity, which remains practically intact after surgery. This difference in surgical technique increases patient comfort during the early postoperative period, allowing a faster visual recovery and a reduction of the wound healing response.6,23,25,26 This fact explains the superiority of the outcomes reported in the scientific literature regarding the speed of visual recovery compared to our results.
In primary eyes, T-CAT custom ablations are based on corneal topography data, but also on the refractive error present preoperatively (Figure 1). In addition to the refractive error correction, the ablation profile is also designed to provide an optimized corneal curvature shape. Due to the technology used, the centration of the treatments is optimized, being less susceptible to pupil decentration and cyclotorsional errors. This algorithm enhances the correction of peripheral corneal HOAs (eg, trefoil or tetrafoil), from which most of the HOAs of the ocular system derive.18,19,27 For this study, we did not compensate for the spherical error induced by HOA correction because the magnitude of the corneal irregularities was low preoperatively.28–30 In the T-CAT LASIK study, there is no reference regarding this point and the manifest refraction was only used for the laser treatment.18 Previous studies using the T-CAT custom LASIK approach have used different algorithms. Kanellopoulos et al.27 described the “topography modified refraction” LASIK technique, with topographic adjustment of the amount and axis of astigmatism treated by the ablation. The “layer yolked reduction of astigmatism” (LYRA) protocol was also described. With this approach, the authors reported that T-CAT custom LASIK can be used to create laser corrections that are efficient in astigmatism correction and to design uniformly shaped corneas.31–33 Similar to the LYRA protocol, in the method described in our study we did not perform any adjustment of the target Q value.33
Shetty et al.34 compared the refractive and visual outcomes of the WFO and T-CAT ablation profiles in 60 eyes that underwent LASIK with the WaveLight EX500 platform. A postoperative target Q value of −0.4 was planned for the T-CAT ablation. The outcomes revealed that both ablation profiles provided essentially equivalent outcomes after myopic LASIK, with induction of fewer HOAs after the T-CAT approach.
Our results showed a mild increase of the corneal RMS HOAs after the procedure. Stulting et al.18 also reported similar findings, although their study group had higher preoperative corneal RMS HOAs compared to ours. Future studies with new imaging technology, such as segmental tomography, might provide new insights based on epithelium healing response and the stromal surface topography to explain the results.35
Our study has some limitations. The number of eyes included in this report is small, follow-up was short (particularly for surface ablation procedures), and there was no control group of eyes treated with a WFO profile to compare safety and effectiveness.
Significant developments have been achieved with laser vision correction surgery in the past decade. With the introduction of T-CAT custom LASIK for primary refractive correction purposes, patients can also benefit from the effectiveness and safety of this customized treatment algorithm.18 In this study, we used a T-CAT PRK approach for myopic correction in primary eyes. Although we did not perform contrast sensitivity and did not apply quality of life questionnaires postoperatively, our results suggest that T-CAT PRK provides good outcomes and improves the visual performance. We recommend caution with this approach because distinct algorithms have been described in the literature that may result in complications related to incorrect planning by the surgeon with no experience with customized ablations.
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|Sphere (D)||−2.32 ± 1.04 (−1.00 to −4.50)||+0.05 ± 0.17 (−0.25 to 0.75)||< .05|
|Cylinder (D)||−0.54 ± 0.44 (0.00 to −2.00)||−0.01 ± 0.25 (−0.75 to 0.50)||< .05|
|MRSE (D)||−2.63 ± 0.21 (−1.00 to −4.50)||−0.03 ± −0.20 (−0.88 to 0.25)||< .05|