Corneal Epithelial Remodeling and Its Effect on Corneal Asphericity after Transepithelial Photorefractive Keratectomy for Myopia

The effects of fixation stability, corneal density, and epithelial hyperplasia on the efficacy of astigmatism correction by transepithelial photorefractive keratectomy

BACKGROUND

Transepithelial photorefractive keratectomy (transPRK) can be safely and predictably performed to correct low-to-high astigmatism. This study explored the effects of fixation stability, corneal density (CD), ocular residual astigmatism (ORA), and the surgically-induced change in the epithelial thickness (ΔET) on the efficacy of astigmatism correction by transPRK.

METHODS

Eighty-three consecutive patients who underwent transPRK to correct myopia and myopic astigmatism were divided into two groups according to refractive astigmatism [high refractive astigmatism (RA) group: ≥ 2.0 D, n = 31; low RA group: < 2.0 D, n = 52]. Fixation stability was evaluated by measuring the lateral movement of the pupil center on the eye tracker images. The CD was measured using a Pentacam Scheimpflug imaging system, epithelial thickness mapping was performed using optical coherence tomography, and the ORA was determined using vector analysis. Multiple linear regression analyses were performed to identify factors associated with the correction index (CI) and angle of error (AOE).

RESULTS

At 6 months postoperatively, the RA was higher in the high RA group (- 0.66 ± 0.44 D) than in the low RA group (- 0.29 ± 0.29 D, P < 0.001), whereas no significant differences were found in CI or AOE between two groups. Multiple linear regression analyses showed that for the low RA group, preoperative anterior CD of the central 2 mm (CD0-2A, β =  - 0.482, P = 0.011) and ΔET (β = 0.295, P = 0.041), were associated with CI, whereas the vector length of the pupil center shift (PCVL, β =  - 0.404, P = 0.005) and ΔET (β =  - 0.293, P = 0.036) were associated with AOE. For the high RA group, ΔET (β = 0.519, P = 0.038) was associated with CI, whereas static cyclotorsion (β =  - 0.493, P = 0.040) was associated with AOE. No significant associations were found between ORA and CI or AOE.

CONCLUSIONS

Postoperative changes in epithelial thickness were associated with the efficacy of transPRK in both the low and high RA groups, whereas the pupil center shift and anterior CD were associated with the efficacy of transPRK in the low RA group.

Comparative outcomes of mechanical and transepithelial PRK on the same excimer laser: A contralateral eye study

PurposeCompare refractive results between mechanical PRK (mPRK) and transepithelial PRK (tPRK) with WaveLight Allegretto EX500 excimer laser system (Alcon Laboratories, Fort Worth, TX, USA).SettingDepartment of Ophthalmology of the Federal University of Sao Paulo, Brazil.DesignProspective and randomized study.MethodsIn 151 eyes of 73 patients with astigmatism and myopia, both eyes had similar refraction before surgery, with a maximum of 15-µm difference in ablation who underwent mPRK in one eye and tPRK in the contralateral eye. The mean age of the patients in this study was 31.45 ± 6.97 years (range, 22 to 54 years).ResultsA comparison was made with all variables between the two groups, and we found that UDVA and SE were worse in the tPRK group at six months than in the mPRK group. In the mPRK group, there was a higher frequency in the +/- 0.50 range and a lower frequency in the +/- 1.50 range. In the tPRK group, however, there was a lower frequency in the +/- 0.50 range and a higher frequency in the +/- 1.50 range. Concerning gain or loss of lines of sight, there was no association between the two groups (chi-square test, p = 0.887).ConclusionBoth mPRK and tPRK appear to have similar safety. However, mPRK was associated with significantly better UDVA and SE six months post-operatively.

The Correlation Analysis of Factors Affecting the Effective Optical Zone After SMILE

OBJECTIVES

The purpose of this study was to analyze the size of the effective optical zone (EOZ) and its related influencing factors in small incision lenticule extraction.

METHODS

Calculation of the horizontal EOZ (Y1: H-EOZ), vertical EOZ (Y2: V-EOZ), and average EOZ (Y3: A-EOZ) using corneal topographic difference maps. Multivariate linear regression analysis was used to establish a multivariate linear equation for the dependent variable EOZ. Receiver operating characteristic (ROC) curves were constructed based on the experimental cohort and were validated in the validation cohort.

RESULTS

The EOZ was smaller than the programmed optical zone (POZ) (p<0.001), and the spherical (Sp) and astigmatism (As) were positively correlated with H-EOZ (p Sp <0.001, p As <0.001), whereas average K-value (Km) and horizontal corneal diameter (WTW) were negatively correlated with H-EOZ (p Km <0.001, p WTW <0.002). The corresponding multivariate linear regression equation was Y1=5.246+0.168X1-0.278X2+0.027X3-0.131X4. Spherical, As, the asphericity of cornea (Q-value), and POZ were positively correlated with V-EOZ (p As <0.000, p Q <0.001, and p POZ <0.05, respectively) and Y2=3.032+0.102X1+0.175X2+0.642X5+0.415X6. Spherical, astigmatism, and POZ were positively correlated with A-EOZ (p Sp <0.000, p As <0.05, p POZ <0.05, respectively), whereas WTW was negatively correlated with A-EOZ (p WTW <0.004) as follows: Y3=4.384+0.132X1+0.023X3-0.117X4+0.294X6. The H-EOZ model had the largest area under the ROC curve, 0.853 (95% confidence interval: 0.807-0.767).

CONCLUSIONS

The preoperative intended correction of spherical lesions may be the main factor affecting the size of the postoperative EOZ, and this study has strong predictability for the horizontal EOZ.

Impact of corneal sub-basal nerve plexus on epithelial thickness after small incision lenticule extraction (SMILE): a quantitative assessment using in vivo confocal microscopy and optical coherence tomography

Background

Quantitative assessment of corneal epithelial thickness (CET) has attracted a great attention for its wide application in refractive surgeries. The corneal nerves are crucial for epithelial homeostasis, and nerve injury due to corneal refractive surgery may affect the epithelia; however, few clinical studies have investigated this relationship. Our study aimed to observe changes in epithelial thickness and sub-basal nerve plexus (SNP) profile after small incision lenticule extraction (SMILE) for low-to-moderate myopia [spherical refraction of -6.0 to 0 diopters (D)], and investigate the relationship between them.

Methods

This prospective observational study included 52 eyes treated with SMILE from March to May 2023 at Tianjin Medical University Eye Hospital. The epithelial thickness was measured across the central and concentric (paracentral, mid-peripheral, and peripheral) regions using spectral-domain optical coherence tomography (SD-OCT). SNPs were observed in the central and peripheral (temporal, superior, nasal, and inferior) areas using in vivo confocal microscopy (IVCM), and seven nerve parameters were assessed. All eyes were examined preoperatively and 1 week, 1 month, and 6 months postoperatively. Pearson's correlation was employed to investigate the association between anterior Q-value (corneal asphericity) and epithelial thickness. Partial correlation was adopted to examine the relationship between epithelial thickness and corresponding nerve parameters.

Results

Both epithelial thickness and SNP exhibited changes after SMILE. Six months postoperatively, epithelial thickness in the central and paracentral regions increased (all P<0.05), with thickening particularly pronounced in the inferotemporal, temporal, and inferior sections of the paracentral region, and the anterior Q-value was positively correlated with epithelial thickness in the inferonasal, inferior, and inferotemporal sections (inferonasal section: r=0.293, P=0.035; inferior section: r=0.396, P=0.004; inferotemporal section: r=0.374, P=0.006). Furthermore, most central, superior, and nasal nerve parameters had still not reached preoperative levels, while most temporal and inferior nerve parameters had reached or exceeded preoperative levels, and epithelial thickness was positively correlated with corresponding nerve parameters [corneal nerve fiber density (CNFD): r=0.171, P=0.006; corneal nerve branch density (CNBD): r=0.137, P=0.028; corneal nerve fiber length (CNFL): r=0.172, P=0.006; corneal nerve fiber total branch density (CNTB): r=0.141, P=0.024; corneal nerve fiber area (CNFA): r=0.164, P=0.008].

Conclusions

Uneven epithelial thickness changes were observed after SMILE, regional epithelial thickening increased corneal oblateness. Non-uniform SNP regeneration was also observed, positive correlation between epithelial thickness and nerve parameters indicated the impact of nerves on epithelia, which may enhance the clinical value of epithelial thickness measurement.

Effects of corneal epithelial remodeling on corneal asphericity after FS-LASIK and Trans-PRK: A prospective study

PURPOSE

To observe the changes in corneal epithelial thickness after FS-LASIK and Trans-PRK surgery and to investigate the impact of corneal epithelial remodeling on Q-value and HOA.

METHODS

In this prospective cohort study, 50 patients (100 eyes) underwent FS-LASIK and 45 patients (90 eyes) underwent Trans-PRK. Anterior segment OCT was used to measure the corneal epithelial thickness in different corneal zones (central zone: 0-2 mm; paracentral zone: 2-5 mm; and mid-peripheral zone: 5-6 mm) preoperatively and postoperatively at 1 week, 1 month, 3 months, and 6 months. The correlation between △CET in the superior, nasal, inferior, and temporal region at 6 months postoperatively and △Q and △HOA was analyzed.

RESULTS

At 6 months postoperatively, the epithelial thickness increased in the central, paracentral, and mid-peripheral zones in FS-LASIK and Trans-PRK. Central epithelial thickness and different regions of the paracentral zone and mid-peripheral exhibited significant thickening ( P < 0.001). In the para-central zone and mid-peripheral zone, the △CET in different regions after LASIK and Trans-PRK was positively correlated with △Q ( P < 0.05) and △HOA ( P < 0.05).

CONCLUSION

After FS-LASIK and Trans-PRK, significant epithelial thickening was observed. Epithelial changes in different regions lead to different Q-values in different regions and have different effects on HOA. This has a certain guiding significance for the design of refractive surgery, and minimizing the increase of Q-value may improve the postoperative visual quality.

The visual outcomes and corneal biomechanical properties after PRK and SMILE in low to moderate myopia

PURPOSE

To compare the corneal biomechanical parameters, visual outcome, and epithelial remodeling after small-incision lenticule extraction (SMILE) and photorefractive keratectomy (PRK) for low to moderate myopia.

DESIGN

Prospective, interventional, randomized, comparative study.

METHODS

Eighty eyes of 40 patients undergoing bilateral SMILE or PRK for low to moderate myopia (<-5 D SE) were included. They were divided into two groups based on the planned refractive surgery. Visual acuity, corneal biomechanics using Corvis-ST, epithelial mapping via anterior segment optical coherence tomography, and higher-order aberrations were recorded both preoperatively and postoperatively at 6 months and compared.

RESULTS

At 6 months follow-up, the corrected distance visual acuity as well as the postoperative spherical equivalent of both the groups were comparable ( P = 0.13 and P = 0.32, respectively). The biomechanical parameters like deformation amplitude ratio ( P < 0.01), inverse concave radius ( P = 0.006), integrated ratio ( P < 0.01), stress-strain index ( P < 0.01), Ambrosio's relational thickness (ArTH) ( P < 0.01), and corneal biomechanical index-laser vision correction ( P = 0.02) were altered more in the SMILE group than in the PRK group, while the biomechanically corrected intraocular pressure ( P = 0.16) was comparable. Epithelial remodeling ( P = 0.59), higher-order aberrations ( P = 0.53), and stromal keratocyte loss ( P = 0.16) in both the groups were comparable.

CONCLUSION

Both SMILE and PRK are comparably effective procedures for correction of low to moderate myopia in terms of visual outcomes; however, the corneal biomechanical stability in PRK is superior than that in SMILE.

Corneal Epithelial Hyperplasia Related to Chronic Eye Rubbing Mimicking Keratoconus

ABSTRACT

We report a case of corneal epithelial hyperplasia associated with chronic eye rubbing mimicking keratoconus. A 32-year-old man was presented with a 3-year history of suboptimal vision and astigmatism in his left eye. His history was significant for chronic left eye rubbing. The anterior corneal curvature map showed inferior steepening in the left eye; however, other features of ectasia were absent. Corneal epithelium thickness mapping with optical coherence tomography was significant for corresponding epithelial thickening. His corneal imaging remained stable at a 6-month follow-up examination. At month 15-and after cessation of eye rubbing behavior-the vision symptoms, refraction, and corneal imaging had normalized. In conclusion, chronic eye rubbing may cause reversible corneal epithelial hypertrophy. Eye rubbing should be considered in the evaluation of patients presenting with unexplained vision symptoms and changes in astigmatism that are not consistent with ectasia on corneal imaging.

To Investigate the Changes in Corneal Curvature and Its Correlation with Corneal Epithelial Remodeling After Trans-PRK and FS-LASIK

PURPOSE

To evaluate curvature changes in different regions and their correlation with corneal epithelial remodeling in myopic patients undergoing femtosecond laser-assisted in situ keratomileusis (FS-LASIK) and transepithelial refractive keratectomy (Trans-PRK) after surgery.

METHODS

One hundred and sixty-three patients (163 right eyes) undergoing Trans-PRK and LASIK were evaluated for up to 6 months using anterior segment optical coherence tomography (OCT) to measure the epithelial thickness and corneal topography to measure corneal curvature in different areas (2 mm, 5 mm, and 7 mm). We calculated the curvature ΔK (ΔK = preoperative - postoperative), ΔK5-2 (ΔK5-2 = K5mm - K2mm), ΔK7-5 (ΔK7-5 = K7mm - K5mm), and the epithelial thickness ΔET5-2 (ΔET5-2 = ET5mm - ET2mm) and ΔET7-5 (ΔET7-5= ET7mm - ET5mm).

RESULTS

Corneal curvature flattened in each region of the two groups (all p < 0.001) and gradually steepened during the follow-up period. The Trans-PRK group flattened more significantly within 2 mm and 5 mm, while the FS-LASIK group at 7 mm. Both groups of ΔK decreased over time. Both groups of ΔK5-2 and ΔK7-5 gradually decreased during the follow-up period (P5-2=0.025 and P7-5 < 0.001 for Trans-PRK, P5-2 = 0.011 and P7-5 < 0.001 for FS-LASIK). The corneal epithelium of the two groups gradually thickened during the follow-up period, with Trans-PRK significantly thickening in the central and peripheral regions and FS-LASIK in the central and paracentral regions. There is a significant correlation between the ΔK5-2 and ΔET5-2, ΔK7-5 and ΔET7-5 (all r > 0.37, p < 0.001).

CONCLUSIONS

All groups showed significant curvature flattening after surgery and gradually steepening during the follow-up period. The corneal epithelium thickness in both groups of 17 regions became thicker,. In contrast, Trans-PRK group showed more significant thickening to the periphery and the central 5 mm area of the FS-LASIK. This study indicates a significant positive correlation between differences in epithelial thickening in different regions and differences in curvature changes in the corresponding areas PRK, FS-LASIK, curvature, corneal epithelial thickness.

Corneal Epithelial Thickness Mapping: A Major Review

The corneal epithelium (CE) is the outermost layer of the cornea with constant turnover, relative stability, remarkable plasticity, and compensatory properties to mask alterations in the underlying stroma. The advent of quantitative imaging modalities capable of producing epithelial thickness mapping (ETM) has made it possible to characterize better the different patterns of epithelial remodeling. In this comprehensive synthesis, we reviewed all available data on ETM with different methods, including very high-frequency ultrasound (VHF-US) and spectral-domain optical coherence tomography (SD-OCT) in normal individuals, corneal or systemic diseases, and corneal surgical scenarios. We excluded OCT studies that manually measured the corneal epithelial thickness (CET) (e.g., by digital calipers) or the CE (e.g., by confocal scanning or handheld pachymeters). A comparison of different CET measuring technologies and devices capable of producing thickness maps is provided. Normative data on CET and the possible effects of gender, aging, diurnal changes, refraction, and intraocular pressure are discussed. We also reviewed ETM data in several corneal disorders, including keratoconus, corneal dystrophies, recurrent epithelial erosion, herpes keratitis, keratoplasty, bullous keratopathy, carcinoma in situ, pterygium, and limbal stem cell deficiency. The available data on the potential role of ETM in indicating refractive surgeries, planning the procedure, and assessing postoperative changes are reviewed. Alterations in ETM in systemic and ocular conditions such as eyelid abnormalities and dry eye disease and the effects of contact lenses, topical medications, and cataract surgery on the ETM profile are discussed.

Evaluation of the effect of artificial tears on corneal epithelial thickness changes after photorefractive keratectomy

PURPOSE

This study aimed to evaluate the corneal epithelial thickness changes after photorefractive keratectomy (PRK) and the impact of long-term artificial tear usage on epithelial thickness changes in these patients.

METHODS

This study was performed on 71 patients (142 eyes) without dry eye disease who received PRK for myopic refractive correction. The corneal epithelial thickness profile was obtained before, one, three, and six months after surgery using anterior segment optical coherence tomography. Patients were randomly divided into two groups: group A, who received preservative-free artificial tears post-surgery, and group B, who did not receive artificial tears.

RESULTS

The epithelial thickness decreased universally in the first month and then increased in the 3- and 6-month follow-ups. Group A had a significantly thicker epithelium in central, paracentral, and midperipheral zones compared with group B in the 3-month follow-up. In the 6-month follow-up, no significant differences were detected between groups. At the last follow-up, the central, paracentral, and midperipheral zone epithelial thicknesses in all patients were significantly higher than preoperative values, but peripheral zone thickness only increased to preoperative values.

CONCLUSIONS

Patients using artificial tears showed a faster thickening, especially in the central and paracentral zones, but there were no significant differences between the two groups in the final follow-up. Artificial tear usage may increase the rate of the epithelial remodeling process in post-PRK patients without significantly altering the final epithelial thickness profile. Further studies are warranted to evaluate the influence of different factors on epithelial remodeling.

Corneal Epithelial Remodeling in a 6-Month Follow-up Period in Myopic Corneal Refractive Surgeries

PURPOSE

To investigate corneal epithelial thickness changes during a 6-month follow-up period after transepithelial photorefractive keratectomy (tPRK), femtosecond laser-assisted laser in situ keratomileusis (FS-LASIK), and small incision lenticule extraction (SMILE).

METHODS

This prospective study included 76 eyes of 76 participants who underwent myopic refractive surgery (23 FS-LASIK, 22 SMILE, and 31 tPRK). Epithelial thickness and anterior curvature were averaged over 4 regions (subdivided into 25 areas) and measured by spectral-domain optical coherence tomography and Scheimpflug tomography before the operation (pre) and at 1 or 3 days (pos1-3d), 1 week (pos1w), and 1 month (pos1m), 3 months (pos3m), and 6 months (pos6m) postoperatively.

RESULTS

The epithelial thickness of the three groups was similar in both the pre and pos6m (all P > .05), but the tPRK group fluctuated the most during the follow-up period. The largest increase was in the inferior-temporal paracentral area (7.25 ± 2.58 μm for FS-LASIK; 5.79 ± 2.41 μm for SMILE; 4.88 ± 5.84 μm for tPRK; all P < .001). Only the epithelial thickness of tPRK increased from pos3m to pos6m (P < .05), whereas all changes for FS-LASIK and SMILE were not significant (P > .05). A positive correlation of thickness changes with curvature gradient in the paracentral region of tPRK was found (r = 0.549, P = .018), but not in other regions in all groups.

CONCLUSIONS

Epithelial remodeling followed different trends after different surgeries from the early postoperative stage onward, but exhibited similar values at pos6m. Although remodeling after FS-LASIK and SMILE stabilized by pos3m, it remained unstable at pos6m after tPRK. These changes may affect corneal profile and lead to deviation from the intended surgical outcome. [J Refract Surg. 2023;39(3):187-196.].

Comparative study of functional optical zone: small incision lenticule extraction versus femtosecond laser assisted excimer laser keratomileusis

AIM

To investigate the size of functional optical zone (FOZ) after small incision lenticule extraction (SMILE) versus femtosecond laser assisted excimer laser keratomileusis (FS-LASIK) for myopia correction and potential associated factors for FOZ.

METHODS

A total of 133 patients who received corneal refractive surgery in our hospital between November 2018 and July 2021 were retrospectively enrolled. There were 63 patients (123 eyes) in SMILE group and 70 patients (139 eyes) in FS-LASIK group. The size of FOZ was measured using Pentacam 3-dementional anterior segment analyzer before and 3mo after surgery, so as to analyze postoperative achieved functional optical zone (AFOZ) and its contributing parameters.

RESULTS

When planned functional optical zone (PFOZ) was 6.5 mm for both groups, AFOZ was 1.45±0.27 and 1.67±0.25 mm smaller than preoperative FOZ in SMILE group and FS-LASIK group 3mo after surgery. AFOZ in SMILE group was significantly larger than that in FS-LASIK group (P<0.001). Variation of FOZ was negatively correlated with preoperative spherical equivalent (SE) and positively correlated with variation of mean keratometry value (ΔKm), variation of spherical aberration (ΔSA), and variation of Q-value (ΔQ, all P<0.001) in both groups. Multiple variable linear regression equations were ΔFOZ=1.354-0.1×pre-SE+0.336×ΔQ+1.462×ΔSA in SMILE group and ΔFOZ=1.512+0.137×ΔQ+0.468×ΔSA in FS-LASIK group.

CONCLUSION

AFOZ is significantly smaller than preoperative FOZ in both SMILE and FS-LASIK groups. With the same PFOZ, larger AFOZ is achieved in SMILE group than in FS-LASIK group.

Comparison of Vision Correction and Corneal Thickness at 180-Day Follow-Up After Femtosecond Laser-Assisted In-Situ Keratomileusis (FS-LASIK), Photorefractive Keratectomy (PRK), and Small Incision Lenticule Extraction (SMILE): A Study from a Single Center in Poland of 120 Patients with Myopia

BACKGROUND This study from a single center in Poland included 120 patients with myopia, and the aim was to compare vision correction and corneal thickness at the 180-day follow-up after femtosecond laser-assisted in-situ keratomileusis (FS-LASIK), photorefractive keratectomy (PRK), or small incision lenticule extraction (SMILE). MATERIAL AND METHODS The effectiveness and safety of laser vision correction (LVC) procedures were evaluated by determining pre- and post-procedure uncorrected distance visual acuity (UDVA) and corrected distance visual acuity (CDVA) values on the Snell chart. Twenty patients with diagnosed mild myopia (sphere maximum -3.0 diopters D; cylinder maximum 0.5 D) were qualified for PRK surgery. Fifty patients with diagnosed intolerance (sphere maximum -6.0 D; cylinder maximum 5.0 D) were eligible for the FS-LASIK procedure. Fifty patients with diagnosed myopia (sphere maximum -6.0 D cylinder 3.5 D) were qualified for the SMILE procedure. RESULTS Regardless of which procedure was performed, both UDVA and CDVA improved significantly postoperatively (P<0.05). In addition, the UDVA and CDVA values were similar in the postoperative period (P>0.05). For each procedure, the EI was no less than 0.94. Regardless of which type of LVC procedure was performed, CET at the center and 1.5 mm from the center in 4 meridians thickened, and this change was not statistically significant over the observation period (P>0.05). CONCLUSIONS Our analysis demonstrated similar effectiveness of the 3 methods - PRK, FS-LASIK, and SMILE - in patients with mild and moderate myopia.

The Impact of Corneal Epithelial Thickening and Inhomogeneity on Corneal Aberrations After Small Incision Lenticule Extraction

PURPOSE

To investigate the corneal epithelial remodeling profile after small incision lenticule extraction (SMILE), the correlated explanatory variables, and its potential impact on corneal higher order aberrations (HOAs).

METHODS

This single-center study prospectively evaluated 75 right eyes of 75 patients scheduled for SMILE. An anterior segment optical coherence tomography device was used to automatically obtain central 6-mm corneal epithelial thickness (ET), total corneal HOAs, and individual Zernike components before and after surgery. The ET inhomogeneity over the central 3- and 6-mm cornea was quantified with coefficient of variance (CV).

RESULTS

Both ET and CV significantly increased 1 month postoperatively (all P < .05). The stepwise multiple regression analysis showed that ET and CV were significantly correlated with preoperative ET and CV, respectively (all P < .01). The corrected spherical equivalent also significantly influenced ET and CV (all P < .01). Over the central 6-mm zone, the alterations of total corneal HOAs and individual Zernike components such as vertical coma (Z7) and spherical aberration (Z12, Z24) were significantly correlated with ET and CV (all P < .05).

CONCLUSIONS

The SMILE-induced epithelial remodeling involved both ET and ET inhomogeneity. The modulation was associated with preoperative and treatment parameters, and exerted a significant impact on corneal HOA alterations especially over the central 6-mm cornea. Together with the amount of correction and corneal curvature gradient change, preoperative assessment of ET and ET inhomogeneity might help predict postoperative epithelial remodeling. [J Refract Surg. 2023;39(1):23-32.].

Changes of the effective optical zone after small-incision lenticule extraction and a correlation analysis

The objective of the study is to observe the changes in the effective optical zone (EOZ) after small incision lenticule extraction (SMILE) and explore possible correlations with some influencing factors. In total, 133 eyes after SMILE were divided into the mild to moderate myopia group (- 1.75 D to - 5.75 D, 70 eyes) and the high myopia group (- 6.00 D to - 9.50 D, 63 eyes). The postoperative EOZ was calculated by utilizing the corneal tangential curvature map. Changes in EOZ (△-OZ) were monitored and compared between the two groups. Pearson correlation analysis was conducted to determine the correlation between △-OZ and corneal high-order wavefront aberrations. Multicollinearity analysis and ridge regression analysis were performed to assess the correlation between △-OZ and some corneal parameters. After SMILE, the horizontal EOZ (H-EOZ), vertical EOZ (V-EOZ), and average EOZ (A-EOZ) were significantly smaller than the programmed optical zone (POZ) in both groups (p < 0.05). The difference between V-EOZ and POZ (△V-OZ) and the difference between A-EOZ and POZ (△A-OZ) showed more significant changes in the high myopia group than in the mild to moderate myopia group, and △V-OZ was significantly larger than the difference between H-EOZ and POZ (△H-OZ) in the high myopia group. In both groups, the total high-order aberration (T-HOA) and spherical aberration (SA) both increased after SMILE, and they had a similar significant negative correlation with A-EOZ. Moreover, there was a significant negative correlation between △-OZ and Km (X1), Q-value (X2), spherical equivalent (SE, X3), ablating depth (AD, X4) and △e (X6), and a significant positive correlation between △-OZ and △Q (X5). △H-OZ was expressed as Y1, △V-OZ as Y2, and △A-OZ as Y3. The multiple linear regression equations were as follows: Y1 = 3.683 - 0.065X1, Y2 = 1.549 - 0.469X2 - 0.059X3, Y3 = 4.015 - 0.07X1 - 0.03X3, Y1 = 1.337 - 0.005X4 + 0.413X5, Y2 = 1.265 + 0.469X5, and Y3 = 0.852 - 0.002X4 - 0.398X6. The correlation degree with △A-OZ was ranked as Km > △Q > Q-value > AD > e-value > △e > SE > △Km, as represented by the ridge regression analysis. The EOZ was irregularly reduced after SMILE, which should be taken into consideration in the design of POZ, especially for high myopia. Consideration of the refractive diopter and corneal topography is advised for the design of POZ, the latter of which has greater reference significance.

Understanding Corneal Epithelial Thickness Mapping

Corneal epithelial thickness (CET) and the regional variations in response to changes in corneal architecture and biomechanics have recently drawn the interest of corneal surgeons. Corneal epithelium possesses the tremendous capability of remodeling and changing its thickness. This remodeling of corneal epithelium takes place in response to underlying stromal irregularities which can result from a variety of corneal disorders including corneal ectasia. Measurement of CET can reveal the underlying stromal abnormalities and supplement in early diagnosis of corneal disorders especially corneal ectasia which has been one of the leading challenges in planning corneal refractive surgery. A significant number of patients ends up in ectasia after refractive surgery and the most common cause of this complication is the presence of preoperative subclinical keratoconus. Furthermore, postoperative complications of corneal refractive surgery are partly masked by epithelial remodeling and make the diagnosis and management difficult and extremely challenging. This leads not only to unpredictable visual and refractive outcome but also the need of multiple interventions to treat these complications. Although corneal tomography is considered as gold standard in the detection and diagnosis of corneal ectasia, a small number of subclinical cases may still go undetected. In this review, we have highlighted the underlying mechanism of epithelial remodeling, the devices and imaging modalities used to measure CET, and application of epithelial mapping in the diagnosis and management of various corneal disorders.

Corneal Asphericity and Higher-Order Aberrations after FS-LASIK and Trans-PRK for Myopia

Objective

To compare the corneal asphericity and higher-order aberrations (HOAs) of femtosecond laser-assisted in situ keratomileusis (FS-LASIK) with Smart Pulse Technology (SPT) assisted transepithelial photorefractive keratectomy (Trans-PRK) for myopia and myopic astigmatism correction.

Methods

This prospective study analyzed 88 eyes of 44 patients treated with FS-LASIK and 64 eyes of 32 patients treated with Trans-PRK. All eyes had low to moderate myopia with or without astigmatism (spherical equivalent (SE) <−6.00 diopters). The uncorrected distance visual acuity (UDVA), corrected distance visual acuity (CDVA), SE, asphericity (Q value) of the anterior corneal surface, index of surface variance (ISV), corneal higher-order aberrations (HOAs), vertical coma (Z₃⁻¹), horizontal coma (Z₃¹), and spherical aberration (Z₄⁰) over a 6 mm diameter central corneal zone diameter were evaluated preoperatively and 1, 3, and 6 months postoperatively.

Results

At 6 months, the UDVA and SE were −0.14 ± 0.06 and 0.33 ± 0.33D in FS-LASIK and −0.15 ± 0.06 and 0.35 ± 0.37D in Trans-PRK. There was no difference between the two groups in the postoperative UDVA and SE (P > 0.05). After FS-LASIK and Trans-PRK, the Q values in the 6, 7, 8, and 9 mm zones and ISV of the anterior corneal surface significantly increased (P < 0.001). At 1, 3, and 6 months after surgery, corneal HOA, Z₃⁻¹, Z₃¹, and Z₄⁰ in both groups were significantly increased compared with those before surgery, with statistically significant differences (P < 0.001). At 1, 3, and 6 months after surgery, the Z₃⁻¹ of the Trans-PRK group was significantly lower than that of the FS-LASIK group (P < 0.001). ΔHOA and ΔZ₄⁰ were dramatically correlated with the ΔQ value for both FS-LASIK and Trans-PRK procedures. The ΔQ was significantly correlated with the preoperative SE, AD, and AD/CCT after both two procedures (all P < 0.001).

Conclusions

Both FS-LASIK and Trans-PRK caused the anterior corneal surface to become flatter, and the morphology of the corneal surface was irregular. Corneal HOAs were significantly increased after the two procedures. Trans-PRK using SPT introduced less corneal vertical coma than FS-LASIK. Corneal asphericity changes contributed to the corneal aberrations changes following FS-LASIK and Trans-PRK.

Comparison between the change in total corneal astigmatism and actual change in refractive astigmatism in transepithelial photorefractive keratectomy (tPRK), laser in situ keratomileusis (LASIK) and femtosecond laser assisted laser in situ keratomileusis (FsLASIK)

PURPOSE

To compare changes in astigmatism by refraction and total corneal astigmatism after tPRK, LASIK and FsLASIK.

SETTING

Specialty Eye Hospital Svjetlost, Zagreb, Croatia.

DESIGN

Partially masked, semi-randomized, prospective, case-by-case, interventional, clinical study.

METHODS

Patients with a stable refraction (-0.75DS to -8.00DS, astigmatism ≤1.00DC) underwent tPRK, LASIK or FsLASIK without complication. Astigmatism was measured at both corneal surfaces over the central 3.2 mm zone (approximately using Pentacam HR^TM) preoperatively and 3 months postoperatively. Pentacam and refraction data were subjected to vector analysis to calculate the surgically induced changes in i) total corneal astigmatism (SIA_TCA) ii) any astigmatism by refraction (SIA_R) and the vectorial difference (DV) between SIA_TCA and SIA_R.

RESULTS

Reporting key findings (p < .01), there was a significant difference between mean SIA_TCA and SIA_R powers after tPRK (75eyes) but not after LASIK (100eyes) or FsLASIK (100eyes). Mean (±sd,95% CIs) values for DV powers were, tPRK -1.13DC(±0.71, -1.29 to -0.97), LASIK -0.39DC(±0.23,-0.44 to -0.34), FsLASIK -0.55DC(±0.38,-0.62 to -0.47). The differences were significant. For the tPRK and FsLASIK cases, linear regression revealed significant associations between I) SIA_TCA (x) &DV (z) powers (tPRK z = 1.586x-0.179, r  =  0.767, p < .01; FsLASIK z  =  0.442x-0.303, r  =  .484,p < .01), II) sines of SIA_TCA (x₁) &DV (z₁) axes (tPRK, z₁ = 0.523 × ₁ + 0.394, r = .650,p < .01; FsLASIK z₁ = 0.460 × ₁-0.308, r = .465,p < .01).

CONCLUSIONS

tPRK is more prone to unintended changes in astigmatism. The difference between SIA_TCA & SIA_R after tPRK or FsLASIK is mediated by SIA_TCA. Photoablating deeper regions of the cornea reduces the gap between SIA_TCA & SIA_R.

Pain response and symptoms in photorefractive keratectomy: mechanical de-epithelization compared with transepithelial ablation

PURPOSE

To compare postoperative pain and discomfort between mechanical and transepithelial photorefractive keratectomies.

METHODS

This prospective comparative study included 190 eyes of 95 patients with hyperopia (up to +4.00 D), astigmatism (up to -5.00 D), and myopia (up to -8.00 D) who underwent mechanical photorefractive keratectomy in one eye and transepithelial photorefractive keratectomy in the contralateral eye using Wavelight Allegretto EX500 excimer laser. The patients were unaware of the side treated with each technique. The interval between operations in the same patient was 15-30 days. Both eyes had similar refraction before surgery, with a maximum of 15-µm difference in ablation. Postoperative questionnaires were administered on days 1 and 7 to assess the patients' level of discomfort (0=no discomfort to 5=extreme discomfort) with the following symptoms: pain, burning sensation, itchiness, tearing, photophobia, eye redness, foreign body sensation, and eyelid swelling. Patients were also asked about which method they preferred.

RESULTS

The sample consisted of 61 women (64.21%) and 34 men (35.79%). The mean (SD) patient age was 31.66 (6.69) years (range, 22-54 years). On postoperative day 1, the patients reported significantly less discomfort in terms of pain (1.9 ± 1.74 vs 2.5 ± 1.83; p=0.017), burning sensation (1.8 ± 1.56 vs 2.5 ± 1.68; p=0.004), tearing (2.3 ± 1.71 vs 3.1 ± 1.69; p=0.001), and foreign body sensation (1.9 ± 1.77 vs 2.5 ± 1.86; p=0.024) in the eye that received mechanical photorefractive keratectomy than in the eye that received transepithelial photorefractive keratectomy. No significant differences were found between the mechanical and transepithelial photorefractive keratectomies on postoperative day 7. Fifty-nine patients (62.10%) preferred mechanical photorefractive keratectomy, while 32 (33.68%) preferred transepithelial photorefractive keratectomy. Four patients (4.22%) expressed no preference.

CONCLUSIONS

Our results showed that pain scores were significantly lower in the mechanical photorefractive keratectomy-treated eyes than in the transepithelial photorefractive keratectomy-treated eyes on postoperative day 1, which may have provided greater patient comfort after surgery and led patients to prefer the mechanical photorefractive keratectomy technique.

Effect of intraoperative mitomycin-C application on epithelial regeneration after transepithelial photorefractive keratectomy

PURPOSE

To investigate the effect of mitomycin-C (MMC) intraoperative application on postoperative corneal epithelial remodeling, haze incidence, and refractive correction in transepithelial photorefractive keratectomy (transPRK).

SETTING

Eye Hospital of Wenzhou Medical University, Wenzhou, China.

DESIGN

Prospective randomized controlled study.

METHODS

Eyes that completed all follow-ups were included. Epithelial thickness (ET) map measurement was conducted preoperatively and at 1 week, and at 1 month, 3 months, and 6 months postoperatively, generating ET in central, paracentral, and midperipheral zones. The difference between postoperative and preoperative ET (ΔET) was computed for each zone. During follow-up, haze incidence and visual acuity were assessed and compared between groups.

RESULTS

A total of 100 eyes completed all follow-up, including 40 eyes treated with MMC in MMC group and 60 eyes in control group without MMC treatment. For ΔET, between-group difference was found in midperipheral (P = .011) zone at 1 week postoperatively and in central (P = .036) and paracentral zones (P = .039) at 1 month postoperatively. Haze incidence was lower in MMC group at 1 week and 1 month postoperatively (P = .035 and .018, respectively). Safety index (postoperative uncorrected distance visual acuity/preoperative corrected distance visual acuity [CDVA]) and efficacy index (postoperative CDVA/preoperative CDVA) were higher in MMC group (P = .012 and P = .036, respectively) at 1 month postoperatively. No difference was found after 3 months postoperatively.

CONCLUSIONS

With a temporary impact on corneal epithelial regeneration and refractive correction, intraoperative MMC application in transPRK decreased the haze incidence but had no effect on epithelial remodeling.

Comparison of Corneal Wavefront-optimized and Wavefront-guided Alcohol-assisted Photorefractive Keratectomy Using Schwind Amaris 750S Laser for Myopia

Purpose

To compare the visual outcomes and corneal aberrations between wavefront-optimized (WFO) and corneal wavefront-guided (WFG) photorefractive keratectomy (PRK) in low to moderate myopia.

Methods

Twenty-seven eyes treated with WFO and 29 eyes treated with WFG PRK using a Schwind Amaris 750S Excimer laser were included after 6 months of postoperative follow-up. Uncorrected distance visual acuity, corrected distance visual acuity, refractive errors, corneal higher-order aberrations (HOA) and corneal thickness obtained using a Scheimpflug system, and central ablation depth and volume were evaluated during the preoperative period and again at the postoperative 6-month visits.

Results

Postoperatively, uncorrected distance visual acuity, corrected distance visual acuity, manifest spherical equivalent, and refractive astigmatism were improved in both groups, and there was no statistically significant difference between the two groups. There was no significant difference in safety, efficacy, or predictability of the refractive outcome. Postoperative total corneal HOA root mean square (RMS), coma RMS, and spherical aberration were significantly increased in both groups. Among these, only spherical aberration showed a significant difference between the two groups, with greater increase in the WFO group at 6 months postoperatively. The changes in corneal HOA RMS and spherical aberration were smaller in the WFG group, and this benefit was marked in eyes with high HOA RMS (≥0.4 µm) and spherical aberration (≥0.2 µm). Even though ablation volume in the WFG group was much larger than that of the WFO group, there was no significant difference in postoperative central and peripheral corneal thickness between the two groups.

Conclusions

Both WFO and WFG PRK using a Schwind Amaris 750S laser for low to moderate myopia were safe and effective at improving visual and refractive outcomes. However, WFG PRK induced fewer spherical aberrations than WFO PRK and may be more advantageous for eyes with high HOA root mean square or spherical aberration.

Corneal curvature, asphericity, and aberrations after transepithelial photorefractive keratectomy and femtosecond laser-assisted in situ keratomileusis for myopia: A prospective comparative study

Purpose:

We aimed to compare transepithelial photorefractive keratectomy (TPRK) and femtosecond laser-assisted in situ keratomileusis (FS-LASIK) for myopia treatment by analyzing corneal curvature, asphericity (Q-value), and corneal aberration.

Methods:

Corneal topography was measured before and 6 months after the TPRK or FS-LASIK surgery. We measured and compared corneal curvature (sagittal curvature in the 1- to 7-mm zones), change in keratometric measurements (Kmpost – Kmpre, ΔK), Q-values (from the vertex of the 6-, 7-, 8-, and 9-mm zones), higher-order aberration (HOA), vertical and horizontal trefoil (Z3^-3 and Z3³), vertical and horizontal coma (Z3^-1 and Z3¹), and spherical aberration (Z4⁰) between the two surgery groups.

Results:

The sagittal curvature Δ K in the 1-mm zone after TPRK was significantly higher than after FS-LASIK. The Δ K/ΔSE (ΔSE [spherical equivalent] = SEpre – SEpost) ratio in the 1- to 4-mm diameter zones was significantly higher after TPRK than after FS-LASIK. The preoperative Q-values of the 6- and 7-mm zones did not differ between the treatment groups, but postoperative values were significantly higher following FS-LASIK than following TPRK. HOA, Z4⁰, and Z3^-1 were all significantly higher after surgery in both groups. Postoperative Z3^-3 was significantly higher following TPRK but not following FS-LASIK. There were no postoperative differences in aberrations in either group; however, the change in HOA and Z3^-1 was significantly greater following FS-LASIK.

Conclusion:

TPRK changes the corneal curvature to a greater extent and the visual quality (Q-value, aberrations) to a lesser extent than FS-LASIK.

Accelerated Epithelium-off Corneal Cross-linking With High Ultraviolet Energy Dose (7.2 J/cm2) for Progressive Keratoconus: 2-Year Results in a Chinese Population

PURPOSE

To evaluate the results of epithelium-off continuous light accelerated corneal cross-linking (CXL) with a total dose of 7.2 J/cm² for treating progressive keratoconus in a Chinese population during 24 months of follow-up.

METHODS

In this retrospective, interventional case series, 45 eyes of 31 consecutive patients with progressive keratoconus were evaluated. All patients underwent accelerated CXL with settings of 30 mW/cm² for 4 minutes, corresponding to a total dose of 7.2 J/cm². Visual acuity, manifest refraction, epithelial thickness, topography, tomography, aberrometry, endothelial cell count, and intraocular pressure were evaluated at baseline and at 1, 3, 6, 12, 18, and 24 months postoperatively.

RESULTS

Progressive keratoconus was stabilized in 91.11% and 93.33% of the patients at 12 and 24 months, respectively. The improvement in corrected distance visual acuity was significant throughout the postoperative follow-up period (P < .05), excluding month 1. A significant decrease in the maximum keratometric values (0.67 ± 1.68, 0.92 ± 1.78, and 0.97 ± 1.73 D) was observed at months 12, 18, and 24, respectively (P < .05 for all). Corneal irregularity improved, particularly total root mean square and higher order aberrations at 12 to 24 months after CXL. In bilateral CXL, the progression of the first eye was highly predictive of the outcome of the second eye.

CONCLUSIONS

CXL with a total dose of 7.2 J/cm² maintains long-term results in halting the progression of keratoconus, with significant improvement in the corrected distance visual acuity and stability of keratometric values. Further clinical studies with longer follow-up periods and larger samples are necessary to confirm these results. [J Refract Surg. 2020;36(11):731-739.].

Evaluation of corneal structures in myopic eyes more than twenty-two years after photorefractive keratectomy

The aim of this study is to evaluate corneal epithelial thickness (CET), corneal densitometry (CD) in 84 myopic eyes (57 patients) more than 22 years after photorefractive keratectomy, using anterior segment-optical coherence tomography (AS-OCT) and Scheimpflug imaging system. The CET was significantly higher in all operated eyes than in unoperated eyes in central sector. A statistically significant increase in CD in corneal anterior layer of central sector was shown in groups of operated eyes with greater ablation depth respect to unoperated eyes. While there was no significant difference in CD between the operated eyes groups with lower ablation depth and unoperated eyes. A significant trend toward higher values in anterior CD with deeper ablations in central sector was found. These noninvasive imaging techniques allow to better understand the corneal remodeling process after photoablation and to monitor the patients over time.

Changes in the Higher Order Ocular Aberrations and Central Corneal Thickness After T-PRK and Fs-LASIK

Introduction

Refractive surgery procedures, transepithelial photorefractive keratectomy (T-PRK), and femtosecond laser in situ keratomileusis (Fs-LASIK) are regarded as safe and efficacious methods for correcting myopia and myopic astigmatism. These two methods do not have many differences in results when treating spherical myopia while differences exist in the treatment of astigmatism correction. Vector analysis presents a powerful tool to show the real differences between these two methods regarding high ocular aberrations and central corneal thickness of treated eyes.

Aim

The aim of the study is to investigate changes in higher order ocular aberrations (HOAs) and central corneal thickness (CCT) following treatment of myopia and myopic astigmatism above -5.00DS and up to -2.00DC after either T-PRK or Fs-LASIK.

Methods

Patients (30 eyes per group) underwent T-PRK (group I) or Fs-LASIK (group II) procedure using the Schwind Amaris 750S laser. HOAs (3mm&5mm pupil) and CCT were measured objectively at pre-, 1,3 & 6 months postop in each case.

Results

Key results at 6 months were: i) mean values of trefoil (5mm pupil) were 0.092μm (sd,0.055,95% CI 0.072 to 0.112) & 0.126μm (sd,0.078,95% CI 0.098 to 0.154) in group I, and 0.088μm (sd,0.058,95% CI 0.067 to 0.109) & 0.064μm (sd,0.034,95% CI 0.052 to 0.076) in group II (P=0.001 at 6 months); ii) Changes in CTT (ΔCTT) and best spherical equivalent correction (ΔBSE) was significant in group II (ΔCCT=-26.55[ΔBSE]-14.06,R=0.486,P=0.006) but not in group I (p=0.034).

Conclusions

After T-PRK trefoil is worse than Fs-LASIK. The predictability of corneal changes is better following Fs-LASIK.

Vector Analysis of Changes in the Higher Order Ocular Aberrations and Central Corneal Thickness After T-PRK and Fs-LASIK

Introduction:

Refractive surgery procedures, transepithelial photorefractive keratectomy (T-PRK) and femtosecond laser in situ keratomileusis (Fs-LASIK) are regarded as safe and efficacious methods for correcting myopia and myopic astigmatism. These two methods do not have much differences in results when treating spherical myopia, while some differences does exist in treatment of myopic astigmatism. Vector analysis presents powerful tool to show the real differences between these two methods regarding higher order ocular aberrations and central corneal thickness of treated eyes.

Aim:

The aim of the study is to investigate changes in higher order ocular aberrations (HOAs) and central corneal thickness (CCT) following treatment of myopia and myopic astigmatism above -5.00DS and up to -2.00DC after either T-PRK or Fs-LASIK.

Methods:

Patients (30 eyes per group) underwent T-PRK (group I) or Fs-LASIK (group II) procedure using Schwind Amaris 750S laser. HOAs (3mm&5mm pupil) and CCT were measured objectively at pre-, 1,3 & 6 months postop in each case.

Results:

Key results at 6 months were: i) mean values of trefoil (5mm pupil) were 0.092μm (sd,0.055,95% CI 0.072 to 0.112) & 0.126μm (sd,0.078,95% CI 0.098 to 0.154) in group I, and 0.088μm (sd,0.058,95% CI 0.067 to 0.109) & 0.064μm (sd,0.034,95% CI 0.052 to 0.076) in group II (P=0.001 at 6 months); ii) Changes in CCT (ΔCCT) and best spherical equivalent correction (ΔBSE) was significant in group II (ΔCCT=-26.55[ΔBSE]-14.06,R=0.486,P=0.006) but not in group I (p=0.034).

Conclusions:

After T-PRK trefoil is worse than Fs-LASIK. The predictability of corneal changes is better following Fs-LASIK. .

Wavefront properties of the anterior and posterior corneal surface after transepithelial photorefractive keratectomy in myopia

The present study aimed to evaluate the changes of corneal higher-order aberrations (HOAs) after transepithelial photorefractive keratectomy (TransPRK) in patients with myopia. A total of 80 eyes of 80 patients with myopia with or without astigmatism were evaluated. Prior to and 1 year after the surgery, uncorrected and corrected distance visual acuity in logMAR were assessed. Wavefront aberrations on the anterior surface of the cornea, as well as the posterior and total cornea, were measured via Pentacam. The HOA of the anterior surface and total values of spherical and coma aberrations increased significantly (P<0.001). However, HOAs of the anterior surface, and posterior and total values of the trefoil were not significantly changed (P=0.442, 0.805 and 0.936, respectively). A significant correlation of the pre-operative mean spherical equivalent refraction (MSER), astigmatism and central corneal ablation depth (CCAD) with the changes in coma aberration (r=0.268, P=0.016; r=0.260, P=0.020; r=0.323, P=0.004, respectively) and HOA (r=0.554, P<0.001; r=0.312, P=0.005; r=0.583, P<0.001, respectively) of the anterior surface of the cornea was determined. Furthermore, a significant correlation of the pre-operative MSER and CCAD with the changes in spherical aberration of the anterior corneal surface (r=0.462, P<0.001; r=0.510, P<0.001, respectively) was obtained. In conclusion, TransPRK offers an effective option for improving visual function in patients with myopia. The anterior corneal HOA, as well as spherical and coma aberrations, were significantly increased post-operatively. The HOA of the posterior cornea was not significantly changed. The change in the trefoil aberration was significantly correlated with pre-operative astigmatism, but the pre- and post-operative values were not significantly different.

Effect of Corneal Cross-linking on Epithelial Hyperplasia and Myopia Regression After Transepithelial Photorefractive Keratectomy

PURPOSE

To investigate the effect of accelerated corneal cross-linking (CXL) on epithelial thickness change and refractive outcome after myopic transepithelial photorefractive keratectomy (TPRK).

METHODS

This study compared epithelial thickness changes in 49 patients undergoing TPRK-CXL with 49 patients undergoing TPRK who were matched for age and refractive error. Corneal epithelial thickness, obtained using spectral-domain optical coherence tomography preoperatively and 12 months postoperatively, was compared between the groups. Regression analysis was performed to investigate the association between changes in epithelial thickness and keratometric power. Factors affecting myopic regression (> 0.50 diopters] were evaluated using logistic regression analysis.

RESULTS

For TPRK, the mean epithelial thickness of the center (2-mm diameter), paracenter (2- to 5-mm diameter), and pericenter (5- to 6-mm diameter) increased by 6.5 ± 3.1, 7.0 ± 2.9, and 4.9 ± 2.9 µm, respectively; increases of 4.8 ± 3.0, 5.9 ± 2.8, and 4.8 ± 2.7 µm were observed following TPRK-CXL, indicating a significant difference in the center (P = .013). Epithelial thickness increased linearly to the magnitude of myopic correction and was negatively correlated with the optical zone diameter of ablation. Change in epithelial thickness showed a linear correlation with the change in keratometric power between 1 and 12 months postoperatively, indicating regression in eyes following TPRK. Corneal epithelial thickening was significantly associated with myopic regression and simultaneous CXL tended to reduce the risk of regression.

CONCLUSIONS

TPRK-CXL induces less epithelial hyperplasia than does TPRK, presumably owing to the effect of CXL, and the magnitude of epithelial thickening seemed to be associated with myopic regression. [J Refract Surg. 2019;35(6):354-361.].

Multiple Linear Regression Analysis of the Impact of Corneal Epithelial Thickness on Refractive Error Post Corneal Refractive Surgery

PURPOSE

To evaluate the influence of corneal epithelial thickness (CET) on the deviation from the targeted refraction (refractive error [RE]) post corneal refractive surgery.

DESIGN

Retrospective, single-center, cross-sectional study.

METHODS

This study involved 211 eyes of 211 patients who previously underwent laser-assisted in situ keratomileusis (LASIK) or epipolis LASIK for the correction of myopia and myopic astigmatism from August 2000 to May 2014 at the Baptist Eye Institute, Kyoto, Japan, and who subsequently underwent examination of CET via optical coherence tomography imaging of 17 zones (central: n = 1 zone, paracentral: n = 8 zones, and midperipheral: n = 8 zones) within a 6.0-mm-diameter area of the central cornea from April 2014 to February 2015. The relationship between CET and RE was evaluated using Spearman coefficient in 5 parameters: CET in the (1) central, (2) paracentral, and (3) midperipheral zones, and the difference between the (4) central and paracentral zones and the (5) central and midperipheral zones. The influence of CET on RE was evaluated in the CET parameter that showed a significant correlation with RE using multiple liner regression analysis.

RESULTS

The correlation coefficient with RE was (1) -0.238 (P < .01), (2) -0.172 (P < .05), (3) -0.002 (P = .98), (4) -0.186 (P < .01), and (5) -0.266 (P < .01), respectively. Multiple liner regression analysis revealed that the difference of mean CET between the central and midperipheral zones had a significant influence on RE (β coefficient = -0.028, P < .01).

CONCLUSIONS

The difference between CET at the central and midperipheral zones may play a role in the final RE post corneal refractive surgery.

Efficacy and safety of transepithelial photorefractive keratectomy

Transepithelial photorefractive keratectomy (PRK) was introduced to prevent complications from conventional PRK and femtosecond laser-assisted laser in situ keratomileusis (LASIK). In the 2-step platform, phototherapeutic keratectomy is followed by PRK. It did not show notable safety or efficacy superiorities over conventional PRK. In the conventional single-step transepithelial PRK, ablation of epithelium and stroma occurs in a single continuous session by an Amaris laser. It showed better comparative safety results. Reverse single-step transepithelial PRK and the platform using smart-pulse technology were recent improvements in the single-step Amaris laser. They provide a smoother postablative stromal bed counter. In the refined single-step platform, a modified nomogram is used for determination of ablation parameters, along with modifications in postablative measures. It yielded better comparative results in hyperopia. Controlled trials comparing reverse, smart-pulse technology-equipped, or refined platforms of single-step transepithelial PRK with other modern laser-assisted methods could provide more robust evidence on the topic. Some key elements with significant roles in post-transepithelial PRK outcomes are discussed.

Mechanisms of Optical Regression Following Corneal Laser Refractive Surgery: Epithelial and Stromal Responses

Laser vision correction is a safe and effective method of reducing spectacle dependence. Photorefractive Keratectomy (PRK), Laser In Situ Keratomileusis (LASIK), and Small-Incision Lenticule Extraction (SMILE) can accurately correct myopia, hyperopia, and astigmatism. Although these procedures are nearing optimization in terms of their ability to produce a desired refractive target, the long term cellular responses of the cornea to these procedures can cause patients to regress from the their ideal postoperative refraction. In many cases, refractive regression requires follow up enhancement surgeries, presenting additional risks to patients. Although some risk factors underlying refractive regression have been identified, the exact mechanisms have not been elucidated. It is clear that cellular proliferation events are important mediators of optical regression. This review focused specifically on cellular changes to the corneal epithelium and stroma, which may influence postoperative visual regression following LASIK, PRK, and SMILE procedures.