Comparison of Outcomes after Wavefront-optimized and Topography-guided Transepithelial Photorefractive Keratectomy
Article information
Abstract
Purpose
To evaluate the outcomes of wavefront-optimized (WFO) and topography-guided (TG) transepithelial photorefractive keratectomy (transPRK) in the treatment of myopia and myopic astigmatism.
Methods
Patients who underwent transPRK using the WaveLight EX500 excimer laser for the correction of myopia and myopic astigmatism between January 2022 and March 2023 were divided into groups of WFO transPRK (77 eyes of 36 patients) or TG transPRK (63 eyes of 31 patients) in this retrospective, observational cohort study. The preoperative and postoperative 3-month refractive and visual outcomes of the two groups were analyzed.
Results
In both groups, the uncorrected distance visual acuity was 0.0 logarithm of the minimum angle of resolution or better in 95% of eyes 3 months postoperatively, and the mean manifest refraction spherical equivalent was within ±1.0 diopter in 90% of eyes. No significant differences were observed between the groups in terms of the uncorrected distance visual acuity or astigmatism. A significant induction of higher order aberrations (HOAs) was observed in both groups. However, the induction of total corneal HOAs (p = 0.014) and spherical aberrations (p < 0.001) was significantly lower in the TG group than that in the WFO group.
Conclusions
WFO and TG transPRK effectively improved the visual and refractive outcomes; however, the induction of total corneal HOAs and spherical aberration was lesser following the TG ablation.
Keratorefractive surgeries effectively improve visual acuity by correcting lower order aberrations; however, preexisting and surgically induced higher order aberrations (HOAs) often lead to deterioration of vision. The inadvertent creation of HOAs following corneal ablation may result in the incidence of glare, halos, haze, or starbursts, which affect visual quality [1]. Consequently, HOAs are a significant source of concern in the field of refractive surgery.
New treatment profiles and topography-guided (TG) surgeries have been used to minimize the postoperative induction of HOAs. Several studies have investigated the clinical outcomes of wavefront-optimized (WFO) and TG laser in situ keratomileusis (LASIK) in the treatment of myopia and astigmatism [2–4]. However, studies comparing the clinical outcomes of WFO and TG transepithelial photorefractive keratectomy (transPRK) in patients with myopia and astigmatism are lacking. Therefore, this study aimed to compare the clinical outcomes, including visual and refractive outcomes, as well as corneal HOAs, after transPRK following WFO and TG algorithms.
Materials and Methods
Ethics statement
This study was approved by the Institutional Review Board of First Samsung Eye Clinic (No. FSEC-202310-HR008-02). The informed consent requirement was waived due to the retrospective design of the study, and the analysis used deidentified patient data. This research adhered to the tenets of the Declaration of Helsinki.
Study design and setting
Patients who had undergone WFO and TG transPRK performed by two experienced surgeons (SHC and SN) for the correction of myopia and myopic astigmatism between January 2022 and March 2023 were included in this retrospective observational cohort study. Surgical treatment-naive eyes with preoperative refractive error ranging between −0.50 and −9.50 diopters (D) of spherical myopia and between 0.00 and 4.50 D of astigmatism and distance visual acuity correctable to 0.1 logarithm of the minimum angle of resolution (logMAR) or better were included in this study. Patients with a history of systemic conditions, such as autoimmune disease and pregnancy; dystrophy or herpetic eye disease; recurrent corneal erosion; keratoconus; severe dry eye; cataracts; corneal scarring; and retinal and optic nerve disease were excluded. The type of surgery was selected based on the preference of the patient. We retrospectively reviewed the medical records of 140 eyes (67 patients) satisfying the inclusion criteria.
Preoperative assessment
Each patient underwent a comprehensive ophthalmological assessment, including measurement of the uncorrected distance visual acuity (UDVA) and corrected distance visual acuity, manifest refraction, slit-lamp examination of the anterior segment of the eye, fundus examination using widefield digital imaging (Optomap, Optos Inc), measurement of the intraocular pressure, keratometry, and corneal topography (Pentacam, Oculus), preoperatively. Anterior segment optical coherence tomography (Cirrus HD-OCT, Zeiss) was performed to measure the corneal epithelial thickness. Ultrasonic pachymeter (US-500, Nidek) was used to measure the central corneal thickness.
Surgical procedure
Corneal elevation data were obtained using Topolyzer Vario (Alcon) and transmitted to EX500 via an intranet for TG ablation. The target refraction was determined using the protocol by the US Food and Drug Administration (FDA). The optical diameter was set as 6.5 mm for all eyes, and the residual corneal stroma thickness was maintained at ≥300 μm. The thickest epithelial measurement within the central 7 mm region of the cornea obtained via optical coherence tomography was used to determine the corneal epithelial t hickness. A value of <5 μm beyond t his measurement was used in the transPRK program.
The surgical procedure was performed under topical anesthesia (Alcaine eye drops 0.5%, Alcon). Two-step ablation was performed in the TG group. Removal of the epithelium and correction of −0.25 D of the cylinder were performed using the transPRK mode (StreamLight, Wave-Light) as the first step. The TG custom ablation treatment (TCAT) mode was used to eliminate the remaining errors and corneal HOAs. WFO transPRK was performed only in the transPRK mode.
Mitomycin C (0.02%) was applied for 40 seconds in all cases, and copious irrigation with cold balanced salt solution was performed subsequently. A bandage contact lens (Acuvue 1 week, Johnson & Johnson) was applied for 4 to 5 days. The postoperative medication regimen comprised the instillation of moxifloxacin and fluorometholone eye drops four times daily for 2 weeks and 3 months, respectively. In addition, sodium hyaluronate eye drops were instilled every 2 hours for 3 months.
Vector analysis of refractive astigmatism
Alpine vector analysis was performed to determine the changes in astigmatism [5]. The values calculated included target-induced astigmatism, defined as the change in astigmatism that the surgery was expected to induce; surgically induced astigmatism, defined as the actual change in astigmatism achieved postoperatively; difference vector, defined as the disparity between the attained astigmatism and the desired astigmatism; correction index, calculated by dividing surgically induced astigmatism by target-induced astigmatism; and index of success, determined by dividing difference vector by target-induced astigmatism. Microsoft Excel (Microsoft Corp) was used to perform all calculations.
Statistical analysis
Continuous data are presented as standard deviation. The normality of the data was assessed using the Kolmogorov-Smirnov test. An independent t-test was performed to determine the intergroup differences. A paired t-test was performed to compare the preoperative and postoperative values within the group that were normally distributed. Statistical significance was set at p < 0.05. IBM SPSS ver. 20.0 (IBM Corp) was used to perform all statistical analyses.
Results
Overall, 77 eyes of 36 patients and 63 eyes of 31 patients were included in the TG and WFO groups, respectively. Table 1 presents the results of the comparison between the preoperative characteristics and parameters of the WFO and TG groups. No statistically significant differences were observed between the two groups in terms of the preoperative characteristics.
Preoperative parameters (n = 140)
Refraction and visual acuity
A UDVA of 0.0 logMAR or better was achieved by 133 eyes (95%) in both groups at 3 months postoperatively (Fig. 1A). A significant improvement in the mean manifest refractive spherical equivalent (MRSE) was observed in both groups after transPRK (Fig. 1B). The accuracy of the refractive correction was excellent (Fig. 1C, 1D). The MRSE was 0.13 ± 0.53 D in the WFO group and 0.42 ± 0.65 D in the TG group (p = 0.004). The MRSE was within ±1.0 D of emmetropia in 126 cases (90%) in both cases.
Refractive and visual outcomes after wavefront-optimized (WFO) and topography-guided (TG) transepithelial photorefractive keratectomy. (A) Cumulative distribution of Snellen uncorrected distance visual acuity (UDVA) compared with that of the preoperative corrected distance visual acuity (CDVA). (B) Accuracy of manifest refractive spherical equivalent (MRSE) compared with that of the intended target. (C) Scatter plots of attempted versus achieved MRSE. (D) Cumulative distribution of refractive astigmatism compared with the preoperative value. VA = visual acuity; D = diopters.
Vector analysis of astigmatism
All parameters were comparable between the two groups (Table 2). The index of success was <1 in both groups, indicating a reduction in astigmatism compared with that preoperatively. However, the correction index was <1 and magnitude of error was <0 in both groups, indicating a slight undercorrection of astigmatism.
Comparison of the change in astigmatism based on the Alpins vector analysis method (n = 140)
Higher order aberrations
No significant differences were observed between the two groups in terms of the preoperative corneal HOAs with 6.0-mm pupil diameter (Table 3). The postoperative total corneal HOAs and spherical aberration were significantly higher than the preoperative values in both groups ( p < 0.001) (Table 4 and Fig. 2). Compared with WFO transPRK, TG transPRK resulted in significantly lower induction of total HOAs (p = 0.014) and spherical aberration (p < 0.001) at 3 months postoperatively (Table 5).
Comparison of the preoperative corneal higher order aberrations with 6.0-mm diameter (n = 140)
Comparison of the preoperative and 3-month postoperative corneal HOAs with 6.0-mm diameter (n = 140)
Changes in higher order aberrations (HOAs) 3 months after wavefront-optimized (WFO) and topography-guided (TG) transepithelial photorefractive keratectomy. NS = not significant. *Statistically signficiant (p < 0.05).
Comparison of the magnitude of surgically induced corneal aberrations with 6.0-mm diameter (n = 140)
Discussion
The requirement for consistent, repeatable, and secure postoperative outcomes has led to the development of numerous advanced laser platforms. The classical two-step transPRK, which comprises phototherapeutic keratectomy followed by PRK, was first documented in 1999 [6]. An excimer laser is used instead of mechanical or chemical debridement techniques to ablate the corneal epithelium and stroma in transPRK, which can mitigate the complications associated with epithelial debridement in conventional PRK and corneal flap-related complications in LASIK. However, the integration of transPRK into mainstream refractive surgery is limited as a standardized procedure remains to be established. The planning required for the division into the phototherapeutic keratectomy and PRK modes results in loss of time and potential corneal dehydration [7]. Subsequently, with a refined plan, improvements have been applied to the existing transPRK laser platform resulting in unified and advanced surgical techniques that reduce corneal dehydration due to shortened surgical times, while also enhancing both lower order aberrations and HOAs.
The postoperative outcomes of two ablation profiles following myopic transPRK were evaluated in the present study. The WFO ablation profile was developed as a straightforward approach to precompensate for the anticipated HOAs, particularly the spherical aberration in the typical eye, which is often caused by the commonly used laser platforms [8]. The WFO ablation profile incorporates a nonadjustable aspheric target, along with addressing the refractive error [9]. In contrast, TG treatments overlook the HOAs of the internal structures of the eye and rely solely on information acquired from the corneal front surface topographic height maps as a reference [10]. The ablation profiles can be determined by matching the current corneal height map with an ideal rotationally symmetric shape and adjusting for existing refractive spherocylindrical errors [11]. Conceivably, addressing corneal irregularities using corneal surface mapping may enhance visual function in cases of highly irregular corneas (such as those with severe decentration), a very small optical zone due to prior refractive surgery, or corneal scars [10].
TG transPRK induced fewer total corneal HOAs and spherical aberrations in the present study. However, the refractive and visual outcomes of the TG transPRK group were comparable to those of the WFO group. No significant differences in terms of the proportion of patients who achieved a postoperative UDVA of −0.1 and 0.0 logMAR were observed between the WFO group (31.2% and 96.1%, respectively) and TG group (39.7% and 98.4%, respectively). Furthermore, no significant discrepancy was observed between the groups in terms of the proportion of patients who achieved postoperative refractive astigmatism of <1.0 D (83.1% vs. 74.6%). The comparable visual and refractive outcomes may be attributed to the same correction target based on the manifest sphere and cylinder being used for both groups.
The visual outcomes observed in the present study are consistent with those observed in the study by Falavarjani et al. [10], wherein a contralateral eye comparison of 40 eyes was performed using the WaveLight excimer laser platform. No statistically significant differences were observed between the treatment methods in terms of the UDVA and contrast sensitivity outcomes at 3 and 6 months postoperatively in their study (p = 0.4 and p = 0.3, respectively). However, a previous version of the EX500 excimer laser system (Allegretto Wave Eye-Q 400, Alcon Laboratories Inc) was used in their study. Moreover, they did not clarify the method used to remove the epithelium or the protocol used to plan the ablation. Faria-Correia et al. [12] analyzed the results from the fellow eyes of myopic patients undergoing WFO and TG alcohol-assisted PRK using the EX500 excimer laser. No notable differences were observed between the two groups in terms of the UDVA, MRSE, sphere, and cylinder 1 year postoperatively (p > 0.05). These findings suggest that visual acuity outcomes cannot be attributed solely to the ablation profile and that both profiles demonstrate similar efficacy and safety. The results of the present study are consistent with those of other studies comparing different ablation profiles in LASIK [13–16]. However, visual acuity measurements were conducted using a Snellen chart with 100% contrast and in well-lit conditions in the present study. Therefore, the results of the two surgeries may differ in everyday environments, such as indoor settings where low-contrast images are viewed or during nighttime driving. This discrepancy can be attributed to the differences in corneal HOAs [17–19].
The induction of total corneal HOAs and spherical aberrations was significantly lower after TG ablation in the present study. HOAs degrade the quality of images on the retina and distort retinal images more than lower order aberrations [20,21]. Spherical aberration and coma are the main HOAs that increase after laser refractive surgery, and this increase is attributed to the changes in the nonsphericity of the corneal anterior surface due to laser ablation [22,23]. An increase in the spherical aberration leads to a decrease in night and indoor vision in large pupils. Furthermore, given the close relationship between coma and astigmatism [2], it is anticipated that coma would increase concomitantly with the postoperative increase in astigmatism. Faria-Correia et al. [12] also reported lower postoperative HOAs and spherical aberration in the TG group; however, this difference was not statistically significant (p = 0.51 and p = 0.32, respectively). The markedly lower total corneal HOAs and spherical aberration observed in the present study compared with those of previous studies can be attributed to various factors. The data originated from different corneal topography devices. In addition, apart from the difference in population size, the preoperative data were not equivalent, and the present study targeted moderate to high myopia unlike previous studies, which focused on low to moderate myopia. Moreover, distinct excimer laser correction procedures and protocols were used to design the ablation in these studies. TG treatment was performed using the TCAT mode of EX500 in the present study, and the correction target was based on the manifest refraction adjusted using the FDA and Alcon’s nomograms for TG transPRK and WFO transPRK, respectively. TG-LASIK facilitated a significant reduction in the incidence of HOAs in previous studies that evaluated HOAs after LASIK using the same TCAT mode of EX 500 and the FDA and Alcon’s nomograms [2,3].
Differences between the epithelial removal methods may also have led to the significantly lower induction of total HOAs and spherical aberrations in the present study. An excimer laser, rather than mechanical or chemical debridement methods, is used to remove the corneal epithelium and stroma in transPRK. Compared with other modalities using brushes or alcohol-assisted surgeries, transPRK facilitates a reduction in postoperative pain and the incidence of haze, dry eye, and quicker postoperative re-epithelialization [24]. The formation of a corneal flap, consistent delivery of laser energy during surgery, minimal intraoperative eye movements, tear film quality, and corneal healing may have attributed to the increase in HOAs [25].
Significant residual astigmatism was observed in the TG-LASIK group in our previous comparative study of TG-LASIK and WFO-LASIK [2,3]. However, in the present study, no significant differences were observed between the two groups regarding the postoperative astigmatism. The difference in the correction effect of astigmatism observed in the present study may be attributed to the discrepancy in astigmatism between the epithelial and Bowman layers. The differences in epithelial thickness contribute to astigmatism. Thus, removal of the epithelium results in changes in the extent and direction of astigmatism. The differences in postoperative astigmatism depend on the epithelial removal method. The epithelial layer is removed in alcohol-assisted PRK, leading to the creation of different astigmatism patterns by the Bowman layer. In contrast, sequential removal of the epithelium and stroma is performed during TG transPRK; thus, the stromal bed maintains the shape of the corneal epithelial surface during ablation [26]. TG transPRK yields a better HOAs correction effect than mechanical removal of the epithelium in terms of preserving the anterior corneal shape [27–29].
Analysis of the refractive astigmatic changes using vector analysis revealed a notable reduction in astigmatism in the WFO and TG groups in the present study. A slight undercorrection of preexisting astigmatism was observed in both groups, indicating the requirement for collecting supplementary data to develop a more individualized and reliable nomogram for the WaveLight EX500 excimer laser.
We assert that our study represents a pioneering endeavor utilizing the WaveLight EX500 excimer laser in conjunction with the Contoura system to compare surgical outcomes between WFO and TG transPRK. Previous investigations contrasting outcomes between WFO and TG PRK relied on outdated laser platforms or surgical method. By adopting this methodology, we aimed to mitigate potential confounding variables, including biomechanical characteristics and corneal dehydration. Other parameters related to visual quality (such as contrast sensitivity) and symptoms that impact vision quality (such as glare, haze, and halo) are necessary for a more comprehensive evaluation of vision quality. Several previous studies have shown that correcting HOAs has improved contrast sensitivity [30,31]. Although contrast sensitivity was not measured in this paper, we examined corneal HOAs, and considering the results of previous studies [30,31], it can be speculated that TG transPRK result in improvements in the high contrast sensitivity performance. Subsequent studies are warranted to conduct a more thorough assessment of visual quality.
Thus, although no significant differences were observed between the TG and WFO ablation profiles in terms of refractive and visual results, the induction of HOAs in patients who underwent TG transPRK performed using the TCAT mode of the WaveLight EX500 excimer laser was lesser. Further studies with longer follow-up periods and larger sample sizes must be conducted in the future to analyze the corneal responses to customized ablations and identify the superior ablation profile to develop a more robust customized transPRK technique. This could contribute to enhancing the results of TG transPRK.
Acknowledgements
None.
Notes
Conflicts of Interest: None.
Funding: None.