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 Table of Contents  
ORIGINAL ARTICLE
Year : 2021  |  Volume : 22  |  Issue : 1  |  Page : 10-18

Wavefront-guided photorefractive keratectomy combined with prophylactic corneal collagen cross-linking in myopic patients with borderline corneal tomography


Department of Ophthalmology, Faculty of Medicine, Sohag University, Sohag, Egypt

Date of Submission03-Jun-2020
Date of Decision03-Jul-2020
Date of Acceptance16-Aug-2020
Date of Web Publication24-Mar-2021

Correspondence Address:
MD Mortada A Abozaid
Department of Ophthalmology, Faculty of Medicine, Sohag University, Sohag 82524
Egypt
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Source of Support: None, Conflict of Interest: None


DOI: 10.4103/DJO.DJO_41_20

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  Abstract 

Purpose The aim of this study was to assess the safety, efficacy, and predictability of wavefront-guided photorefractive keratectomy with simultaneous half-fluence accelerated collagen cross-linking (WFG-PRK Xtra) in myopic eyes with suspicious or borderline corneal tomography.
Patients and methods A total of 67 eyes of 40 patients with myopia or myopic astigmatism (sphere <−5.0 D and cylinder <−3.0 D), with abnormal corneal tomography not amounting to forme fruste keratoconus, were included in this retrospective comparative study. Eyes were divided into two groups: 37 eyes (group 1) had WFG-PRK Xtra, whereas 30 eyes (group 2) had conventional PRK with accelerated collagen cross-linking (PRK Xtra). All cases completed at least 1 year of follow-up.
Results In group 1 (WFG-PRK Xtra), the mean uncorrected visual acuity improved from 0.96±0.3 to 0.05±0.06 (logMAR) and the mean spherical equivalent decreased from −5.22±2.03 to −0.14±0.32 D. The spherical aberration showed statistically significant increase from 0.05 μm preoperatively to 0.09 μm postoperatively. Three patients developed significant haze and two patients showed overcorrection. However, in group 2 (conventional PRK Xtra), the mean uncorrected visual acuity improved from 0.97±0.35 to 0.04±0.05 (logMAR) and the mean spherical equivalent decreased from −4.98±1.88 to −0.15±0.29 D, whereas the spherical aberration increased from 0.04 to 0.05 μm. Two patients developed significant haze and one patient showed regression. No cases of postoperative ectasia were detected in either group.
Conclusion The results of this study suggest that both WFG and conventional PRK with simultaneous half-fluence corneal collagen cross-linking were safe, effective, and predictable in refractive candidates with abnormal or borderline corneal tomography.

Keywords: abnormal Pentacam, customized surface ablation, photorefractive keratectomy with accelerated cross-linking


How to cite this article:
Abozaid MA. Wavefront-guided photorefractive keratectomy combined with prophylactic corneal collagen cross-linking in myopic patients with borderline corneal tomography. Delta J Ophthalmol 2021;22:10-8

How to cite this URL:
Abozaid MA. Wavefront-guided photorefractive keratectomy combined with prophylactic corneal collagen cross-linking in myopic patients with borderline corneal tomography. Delta J Ophthalmol [serial online] 2021 [cited 2022 Aug 18];22:10-8. Available from: http://www.djo.eg.net/text.asp?2021/22/1/10/311888


  Introduction Top


Although more and more patients are seeking laser refractive surgery, the complications that may happen during or after surgery make many patients afraid of surgery. Iatrogenic or postrefractive ectasia is one of the most feared complications that may follow corneal refractive surgery owing to excessive weakening of the corneal biomechanical properties, leading to progressive corneal steepening and increasing myopia and astigmatism [1],[2]. Photorefractive keratectomy (PRK) theoretically induces less structural changes and is thought to have a very low risk of postrefractive ectasia.

Corneal collagen cross-linking (CXL) is a procedure that induces a photochemical reaction between riboflavin (vitamin B2) and ultraviolet A light within the corneal stroma with formation of chemical bonds between collagen fibrils, thus increasing the corneal rigidity and halting or slowing progression of different types of corneal ectasia [3],[4],[5]. CXL can, in addition, be used as a prophylactic measure in patients seeking refractive surgery and are at risk of developing postoperative ectasia because of thin cornea, high refractive error, or abnormal corneal tomography [6],[7],[8].

Higher order aberrations (HOAs), referred to as irregular astigmatism, constitute ∼15% of the total aberrations in the general population. They usually cause haloes, glare, ghost images, starburst patterns, and monocular diplopia, especially in low lighting conditions and during night driving and in people with larger pupil size. They result from any disturbance, scar, haziness, opacity, or irregularity in one or more of the refractive components of the ocular optical system including the tear film, cornea, aqueous humor, crystalline lens, and vitreous humor. They cannot be corrected with glasses or conventional laser refractive surgery [9],[10],[11],[12].

The wavefront guided (WFG) ablation allows for not only sphero-cylindrical corrections like conventional ablations but also allows additional correction of HOAs of the total eye. The main disadvantage of WFG ablations is the higher possibility of inducing postrefractive ectasia owing to excessive removal of corneal tissue than conventional ablations [13].

Performing WFG-PRK in patients with borderline corneas may theoretically increase the risk of iatrogenic ectasia. Hence, the aim of this retrospective study was to detect the safety, efficacy, and predictability of wavefront-guided photorefractive keratectomy with simultaneous half-fluence collagen cross-linking (WFG-PRK Xtra) in eyes with suspicious corneal tomography compared with conventional PRK Xtra.


  Patients and methods Top


This retrospective comparative study was done as a collaboration between Sohag University Hospital and The FUTURE Private Center in Egypt. Patients’ charts in the period from May 2018 to February 2020 were reviewed after obtaining approval from the Ethics Committee of Sohag Faculty of Medicine. The study followed the tenets of the Declaration of Helsinki, and a written informed consent was obtained from all candidates after explaining the benefits and complications of surgery.

A total of 67 eyes of 40 refractive candidates were included in this study. Patients were divided into two groups: group 1 included 22 patients (37 eyes) who underwent WFG-PRK Xtra and group 2 included 18 patients (30 eyes) who underwent conventional PRK Xtra. At the time of surgery, all candidates were more than 21 years old, with myopia less than −5.0 D and with less than −3.0 D of cylinder, with stable refraction over 1 year (change in manifest sphere or cylinder <0.5 D within 1 year), and with suspicious or borderline corneal tomography (not amounting to subclinical or forme fruste keratoconus) in the form of mildly skewed bow tie, high posterior elevation, inferior–superior asymmetry, overall D red, depressed percentage thickness increase (PTI) curve, and/or too thin cornea (450–500 μm). In addition, patients in group 1 (WFG-PRK Xtra) had a root mean square (RMS) of total HOAs of more than 0.25 μm in aberrometry and can afford the cost of customized ablation.

The exclusion criteria, at the time of surgery, were frank keratoconus, amblyopia, previous refractive surgery or herpetic eye disease, and patients with very high expectations. Postablation corneal thickness of less than 400 μm was another cause of exclusion. In addition, patients with contraindications to refractive surgery, in general, such as pregnancy or autoimmune disease, were excluded. For patients in group 1, the exclusion criteria included also candidates with too small pupil or highly aberrated wavefront.

Preoperative assessment of the patients included detailed history and accurate examination with stress on evaluation of uncorrected (UCVA) and best-corrected visual acuity (BCVA), manifest and cycloplegic refraction, slit-lamp and fundus examination, and tonometry. Corneal tomography was done for all cases using the Pentacam HR Scheimpflug camera (Oculus Optikgeräte GmbH, Wetzlar, Germany), and ocular aberrometry was done by the iDesign ((Abbott Medical Optics, Santa Ana, California, USA) under mesopic conditions without pupillary dilatation with a minimum pupil diameter of 6 mm. Soft contact lenses were discontinued 2 weeks before the preoperative examination. Contrast sensitivity was measured with the Nidek SC-1600 chart (Nidek, Tokyo, Japan) at three levels below the normal threshold, namely, 25, 12.5, and 6%.

On the day of surgery, the wavefront refraction of group 1 cases was adjusted according to the manifest and cycloplegic refractions, and the ablation zone was adjusted according to the mesopic pupil size measured with the aberrometry.

Surgical procedure

Under topical anesthesia (Benox, Epico, Cairo, Egypt), the corneal epithelium was removed using 20% ethanol applied to the cornea in an 8.50mm alcohol fixation well for 15 s and then the loosened epithelium was removed with a microsponge. After ensuring iris registration (in group 1 only), the excimer laser ablation was done using VISX Star S4 IR (Abbott Medical Optics). Then mitomycin-C 0.02% (Biochem, Mumbai, Maharashtra, India) was applied for 30 s followed by immediate flushing with 10 ml of balanced salt solution (Bausch+Lomb, Rochester, New York, USA). The half-fluence CXL followed immediately starting with dripping of 0.25 riboflavin solution (VibeX Xtra; Avedro Inc., Waltham, Massachusetts, USA) every 30 s for 90 s followed by irrigation of excess riboflavin with balanced salt solution. The accelerated CXL device (Avedro Inc.) was then used to deliver an energy dose of 2.7 J/cm2 (30 mW/cm2 irradiation for 90 s). A contact lens was then applied and left till complete epithelial healing.

Postoperative treatment included topical gatifloxacin (Tymer; Jamjoom Pharma, Jeddah, KSA) every hour for the first day, and then decreased to five times daily for 2 weeks, prednisolone eye drops (Orchapred; Orchidia, Cairo, Egypt) five times daily for 1 week and then decreased gradually over 4 weeks, and artificial tears eye drops (Systane UD; Alcon Laboratories, Fort Worth, Texas, USA) every hour during the first day, and five times daily for 1 month. A systemic analgesic was prescribed to manage pain.

Patients were followed after 1 day, 1 week, and 1 month for a complete ophthalmological assessment, including slit-lamp examination, visual acuity, and manifest refraction. In addition, Pentacam and iDesign imaging were done at 1, 3, 6, and 12 months, postoperatively ([Figure 1]). Anterior segment optical coherence tomography-OCT (Topcon, Tokyo, Japan) was also done at 1 month to detect the depth and position of the corneal demarcation line of cross-linking.
Figure 1 Preoperative corneal tomography of a refractive candidate of group 1 (−0.75 D sphere and −2.5 D cylinder) demonstrating decreased pachymetry with depressed percentage thickness increase curve.

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Fantes et al. [14] classification was used to grade the postoperative haze:
  1. +0.5: refers to trace haze.
  2. +1: more prominent haze but without interfering with visibility of the fine iris details.
  3. +2: mild obscuration of iris details.
  4. +3: moderate obscuration of iris and lens.
  5. +4: complete opacification in the area of ablation.


Statistical analysis

Testing for normality of data was done using Shapiro–Wilk test. Data were expressed as means±SD and range. The independent samples t test and paired samples t test were used for normally distributed data (e.g. manifest refraction), whereas the nonparametric Wilcoxon test and Mann–Whitney test were used for non-normally distributed data (e.g. UCVA and BCVA). A P value of 0.05 was considered as the level of significance in all statistical tests. SPSS computer program (SPSS Inc., Chicago, Illinois, USA), version 22.0. was used for data analysis.


  Results Top


A total of 67 eyes of 40 refractive candidates were included in this retrospective comparative study. There were 21 males and 19 females. The age ranged from 21 to 35 years (mean=27.3±1.9 years), with no statistically significant difference between the two groups. Overall, 27 cases had bilateral surgery (15 in group 1 and 12 in group 2), whereas the other 13 candidates had a unilateral treatment (seven in group 1 and six in group 2).

On comparing the preoperative parameters of the two groups, no statistically significant difference was noted except for manifest cylinder (−1.64±0.73 D in group 1 and −1.2±0.55 D in group 2, P=0.03) and RMS of total HOA (0.29 in group 1 and 0.21 in group 2, P=0.02).

Postoperatively, in group 1, the mean UCVA showed marked improvement from 0.96±0.3 to 0.05±0.06 (logMAR), whereas the BCVA showed a little change from 0.03±0.05 to 0.04±0.07. Two eyes lost one line of their preoperative BCVA and three eyes gained one line. The mean manifest sphere was reduced from −3.88±1.05 to −0.09±0.02 D, whereas the manifest cylinder decreased from −1.64±0.73 to −0.3±0.19 D. Similarly, the mean spherical equivalent decreased from −5.22±2.03 to −0.14±0.32 D ([Table 1]).
Table 1 Preoperative and 1-year postoperative parameters of wavefront-guided photorefractive keratectomy with simultaneous half-fluence accelerated collagen cross-linking group

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The mean preoperative central corneal thickness was 468.3±17.3 μm, whereas the mean postoperative one was 441.9±12.2 μm. The mean preoperative keratometric value was 44.39±2.1 D, whereas the mean postoperative value was 40.56±1.8 D ([Table 1] and [Figure 2]).
Figure 2 Postoperative corneal tomography of the same patient after 1 year of wavefront-guided photorefractive keratectomy with simultaneous half-fluence accelerated corneal collagen cross-linking.

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Regarding HOAs, the RMS of the coma and trefoil showed statistically insignificant reduction from 0.23 and 0.13, respectively, preoperatively to 0.21 and 0.11, respectively, postoperatively. The mean total HOAs showed also more reduction from 0.29 to 0.25 μm, which was not statistically significant (P=0.19). On the contrary, the spherical aberration showed a statistically significant increase from 0.05 to 0.09 μm (P=0.02, n=28, [Figure 3]).
Figure 3 Preoperative (upper) and postoperative (lower) aberrometry of the same patient in [Figure 1] and [Figure 2] showing reduction in total higher order aberrations, coma and trefoil with rise in spherical aberration.{Figure 1}{Figure 2}

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In group 2, the mean UCVA showed marked improvement postoperatively from 0.97±0.35 to 0.04±0.05 (logMAR), whereas the BCVA changed from 0.04±0.03 to 0.03±0.08. Three eyes lost one line of their preoperative BCVA and two eyes gained one line. The mean manifest sphere was reduced from −3.91±1.2 to −0.1±0.03 D, whereas the manifest cylinder decreased from −1.2±0.55 to −0.2±0.13 D. Similarly, the mean spherical equivalent decreased from −4.98±1.88 to −0.15±0.29 D ([Table 2]).
Table 2 Preoperative and 1-year postoperative parameters of conventional photorefractive keratectomy with accelerated cross-linking group

Click here to view


The mean preoperative central corneal thickness was 477.5±12.9 μm, whereas the mean postoperative one was 445.3±14.3 μm. The mean preoperative keratometric value was 44.52±2.6 D, whereas the mean postoperative value was 40.92±2.2 D ([Table 2]).

Regarding HOAs, in group 2, the mean total HOAs increased insignificantly from 0.21 to 0.22, whereas the RMS of the coma and trefoil showed statistically insignificant reduction from 0.18 and 0.09, respectively preoperatively to 0.17 and 0.08, respectively, postoperatively. The spherical aberration showed statistically insignificant increase from 0.04 to 0.05 μm ([Table 2]).

Comparing the postoperative values of the two groups revealed significant differences only in the aberrometric parameters, mostly owing to differences in the preoperative values of the two groups, with group 1 having higher degrees of HOAs.

Regarding log contrast sensitivity, insignificant changes were detected at 25, 12.5, and 6% in both groups, with preoperative values of 1.6±0.3, 1.9±0.2, and 1.2±0.2 in group 1 and 1.5±0.2, 1.8±0.4, and 1.4±0.2 in group 2, respectively, whereas the postoperative values were 1.5±0.2, 1.9±0.3, and 1.3±0.1 in group 1 and 1.6±0.5, 1.6±0.3 and 1.1±0.3 in group 2, respectively.

Anterior segment OCT after 1 month revealed a patchy demarcation line ([Figure 4]) in all cases, with a mean depth of 164.5±18.3 μm in group 1 and 171.3±15.9 μm in group 2.
Figure 4 Postoperative anterior segment optical coherence tomography of a patient in group 1 showing patchy superficial demarcation line.

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No intraoperative complications were reported in either group. Postoperatively, five patients developed significant (+1 or + 2) corneal haze (three in group 1 and 2 in group 2), which was managed with steroids. Two patients in group 1 showed overcorrection, which improved gradually over 6 months, and one patient in group 2 showed regression of myopia and is waiting for a redo surgery when the refraction becomes stable. No cases of postoperative ectasia were detected in either group.


  Discussion Top


Since its first use in 1999, the WFG ablation has increasingly become the ‘gold standard’ for initial laser refractive treatment with many laser platforms. In this procedure, information obtained from a wavefront-sensing aberrometer is electronically transferred to the treatment laser to program the ablation. The difference between the desired and the actual wavefront is used to generate a three-dimensional map of the planned ablation. This is different from conventional and wavefront-optimized ablations, in which the subjective refraction is used to program the laser ablation [13],[15].

The aim of this study was to assess the safety, efficacy, and predictability of WFG-PRK Xtra compared with conventional PRK Xtra in refractive surgery candidates with abnormal or borderline corneal tomography. The significance of this study comes from the fact that performing WFG ablation in eyes with already abnormal corneal tomography may theoretically increase the risk of postoperative ectasia because WFG ablation removes more corneal tissue than conventional ablation [13].

After 1 year of surgery, no difference was noted between the two groups regarding the safety, efficacy, or predictability of the procedures. Although the WFG-PRK Xtra group has the advantage of decreasing the RMS of total HOAs from 0.29 to 0.25 compared with a rise in this value in the conventional PRK Xtra group from 0.21 to 0.22, the former group was associated with a significant rise in the spherical equivalent value, which may be attributed to a difference in the inclusion criteria between the two groups, with group 1 including patients with a high RMS value of total HOAs more than 0.25 μm.

In their comparative study, Sachdev et al. [16] compared conventional PRK alone (118 eyes) versus conventional PRK Xtra (109 eyes) and followed them for 1 year. They found almost no difference between the two groups in terms of BCVA and reported that PRK Xtra was not associated with increased postoperative risk of hyperopic shift nor corneal haze.

Lee et al. [17] retrospectively compared PRK alone with PRK+CXL regarding changes in anterior and posterior corneal elevations that were calculated by fitting to the corneal shape a best-fit sphere and best-fit toric ellipsoid with 8.0-mm diameter and fixed eccentricity of 0.4. They reported that prophylactic CXL combined with PRK significantly decreased the degree of displacement of peripheral and central posterior corneal surfaces.

Moshirfar et al. [18] compared PRK using WFG VISX Customvue versus wavefront optimized (WFO) Wavelight Allegretto in a fellow-eye clinical trial that included 46 eyes from 23 patients. They concluded that both platforms performed equally regarding efficacy, safety, and 3-month predictability. However, the WFG arm showed better postoperative contrast sensitivity.

In their meta-analysis, Kobashi et al. [19] compared WFG-PRK with conventional PRK covering five randomized controlled clinical trials that included 298 eyes. There were no differences between the two groups regarding safety, efficacy, or predictability. However, the WFG-PRK group induced less postoperative trefoil aberrations.

Jun et al. [20] retrospectively compared WFO-PRK (101 eyes) with corneal WFG-PRK (95 eyes) in myopic patients with moderate to high astigmatism. They reported similar efficacy and safety in the two groups. However, the vector analysis of astigmatism showed better predictability of astigmatism correction axis in the corneal WFG-PRK group. Moreover, the corneal coma aberration showed a significant postoperative increase only in the WFO-PRK group.

Aslanides et al. [21] retrospectively studied the long-term efficacy (5 year) of all surface laser ablation combined with accelerated cross-linking for the treatment of myopia without the use of mitomycin-C in 202 eyes of 118 myopic patients and found that this technique was encouraging in terms of preventing corneal haze formation while maintaining good efficacy and stability.

Compared with the previous studies, the current study was retrospective, comparative, and tested two risk factors of post-PRK ectasia, namely, WFG ablation and abnormal corneal tomography, and found that it was safe and effective to perform WFG-PRK Xtra in patients with abnormal or borderline corneal tomography. Future studies with a larger number of cases and longer duration of follow-up are needed to confirm the results.

Acknowledgements

The author acknowledges Dr Nesreen Hammad for her statistical advices.

Financial support and sponsorship

Nil.

Conflicts of interest

There are no conflicts of interest.



 
  References Top

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    Figures

  [Figure 1], [Figure 2], [Figure 3], [Figure 4]
 
 
    Tables

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