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 Table of Contents  
ORIGINAL ARTICLE
Year : 2019  |  Volume : 32  |  Issue : 2  |  Page : 683-689

Corneal wavefront-guided versus aberration-free transepithelial photorefractive keratectomy in patients with myopia with high pre-existing corneal higher order aberrations


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

Date of Submission27-Jan-2018
Date of Acceptance03-Mar-2018
Date of Web Publication25-Jun-2019

Correspondence Address:
Rana M Ashour Gebril
Department of Ophthalmology, Menoufia Faculty of Medicine, Menoufia University, Shebin Elkom, Menoufia
Egypt
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Source of Support: None, Conflict of Interest: None


DOI: 10.4103/mmj.mmj_49_18

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  Abstract 


Objective
The aim was to compare the efficacy, safety, and predictability of corneal wavefront-guided (WFG) and aberration-free ablation in single-step transepithelial photorefractive keratectomy (TransPRK) in patients with myopia with high pre-existing corneal higher order aberrations (HOAs).
Background
Corneal WFG and aberration-free treatments have been proposed as methods to achieve better visual, refractive, and optical outcomes.
Patients and methods
A prospective case series was conducted in El-Hekma Eye-LASIK Center, Menoufia Governorate, Egypt, during the period from January 2017 to December 2017. TransPRK was performed for eligible patients with myopia with or without astigmatism with corneal HOAs greater than or equal to 0.35 μm using either aberration-free or corneal WFG patterns provided by ORK-CAM software. Uncorrected distance visual acuity, manifest and cycloplegic refractions, best spectacle corrected distance visual acuity, thorough slit lamp examination, and corneal topography were assessed and repeated 6 months postoperatively.
Results
Six months postoperatively, 97.6 and 100% of patients achieved manifest refraction spherical equivalent within ±1 D and 76.2 and 66.67% of patients achieved uncorrected distance visual acuity of greater than or equal to 20/20 in aberration-free and corneal WFG groups, respectively. Both groups showed increase in spherical and total corneal HOAs, with no significant difference in corneal WFG (P = 0.08 and 0.28, respectively), though significantly higher postoperatively in aberration-free group (P < 0.001 and 0.001, respectively). Coma and trefoil aberrations decreased after corneal WFG technique, yet increased after aberration-free one.
Conclusion
Both aberration-free and corneal WFG TransPRK were safe, effective, and predictable in treatment of myopia in patients with high pre-existing corneal HOAs with better aberrometric outcomes in corneal WFG group.

Keywords: aberration free, corneal wavefront guided, higher order aberrations, photorefractive keratectomy


How to cite this article:
Ashour Gebril RM, El-lakwa AF, Zaky MA. Corneal wavefront-guided versus aberration-free transepithelial photorefractive keratectomy in patients with myopia with high pre-existing corneal higher order aberrations. Menoufia Med J 2019;32:683-9

How to cite this URL:
Ashour Gebril RM, El-lakwa AF, Zaky MA. Corneal wavefront-guided versus aberration-free transepithelial photorefractive keratectomy in patients with myopia with high pre-existing corneal higher order aberrations. Menoufia Med J [serial online] 2019 [cited 2019 Sep 21];32:683-9. Available from: http://www.mmj.eg.net/text.asp?2019/32/2/683/260894




  Introduction Top


Single-step transepithelial photorefractive keratectomy (TransPRK) has been proven as a good method to treat compound myopic astigmatism, achieving comparable results to other surgical techniques and minimizing known drawbacks of photorefractive keratectomy (PRK) such as postoperative pain, corneal haze, irregular epithelial healing, long recovery time, and avoiding flap-related complications[1],[2].

Conventional ablation profiles previously used in PRK are believed to increase the optical aberrations postoperatively significantly[3],[4]. Impaired visual performance (impaired low-contrast visual acuity and glare visual acuity) after PRK was correlated with the increase in the ocular aberrations[5],[6].

This has urged the need to develop new ablation techniques to improve the optical outcome and then subsequently the visual outcome. Corneal wavefront-guided ablation (aberration free) is a nonindividualized pattern aiming at prevention of induction of new higher order aberrations (HOAs) mainly spherical aberrations[7], was first proposed, followed by the Q-value customized aspherical treatment, that targets desired Q-value to preserve the corneal asphericity[8]. Ocular wavefront-guided (ocular WFG) technique aims not only at minimizing the induction of new HOAs but also correction of the pre-existing HOAs[9]. Another method was topography-guided technique, which aims at reshaping the cornea into ideal shape based on the data obtained from topography, taking into consideration the refractive error[10]. Finally, corneal WFG method was proposed, which converts information from the topographer into a corneal wavefront map using Zernike polynomials, expressing them as root main square values[11],[12].

The cornea is responsible for 80% of the aberrations of the eye, and the aberrometers can only measure aberrations at the pupil size. Moreover, internal ocular aberrations change with age, accommodation, and pupil size. Nonetheless, there is a lack of correlation between corneal and total ocular HOAs; thereby, this raised the query if we can depend on corneal HOAs obtained from corneal wavefront map only to plan for refractive surgery[12],[13],[14],[15].

The aim of this study was to compare the safety, efficacy, and predictability of corneal WFG and aberration-free ablation in TransPRK for patients with myopia with or without astigmatism with high pre-existing corneal HOAs.


  Patients and Methods Top


This was a prospective case series conducted in El-Hekma Eye-LASIK Center, Menoufia Governorate, Egypt. The study methods adhered to the tenets of the Declaration of Helsinki for use of humans in biomedical research and were approved by ethical committee of Menoufia Medical College.

Patients with myopia with or without astigmatism who met the eligibility criteria were enrolled in the study after well-informed consent. Enrollment in the study was restricted to individuals 21 years or older with stable refraction for more than 1 year, off soft contact lens for a minimum period of 14 days before preoperative examination, preoperative central corneal thickness at least 480 μm, estimated residual stromal bed thickness at least 350 μm, and total corneal HOAs greater than or equal to 0.35 μm.

Exclusion included any patient with ocular diseases such as severe dry eye, blepharitis, corneal disease, contact lens warpage, cataract, uveitis, and posterior segment anomalies involving the macula or optic nerve, as well as patients with systemic conditions such as diabetes mellitus, connective tissue disease, pregnancy, or nursing. Moreover, any patient who had previous ocular surgery including keratorefractive surgery was excluded.

Preoperative assessment included uncorrected distance visual acuity (UDVA) and best spectacle corrected distance visual acuity (CDVA) using Landolt C chart expressed in 20/20 and decimal notations for statistical analysis, manifest refraction, and cycloplegic refractions using Topcon autorefractometer 5500 (Topcon Inc., Itabashi City, Tokyo, Japan), corneal topography, thorough slit lamp examination, and intraocular pressure measurement using Goldmann applanation tonometer done after manifest refraction, CDVA, and corneal topography to avoid bias. Corneal topography was carried out using corneal wavefront topographer Schwind Sirius (SCHWIND eye-tech solutions, Kleinhostheim, Germany) which combines a rotating scheimpflug camera with a placido disc and involves integrated pupillometry. Corneal wavefront map of the anterior corneal surface was obtained from corneal wavefront topographer Schwind Sirius up to the seventh order of Zernike polynomial at 6-mm diameter and expressed as root mean square values, measured in micrometer (μm) mainly total, coma, spherical, and trefoil aberrations. These measures were repeated 6 months postoperatively.

In this study, a procedure was labeled successful if it achieved the desired outcomes in terms of efficacy and safety. Efficacy index was defined as the ratio between the postoperative UDVA to the preoperative CDVA, whereas safety index was defined as the ratio between the postoperative UDVA and the preoperative UDVA. Efficacy index around 1 and safety index more than 1 were desired. Failure was considered when the aforementioned outcomes failed to be achieved along with persistence or development of any of the following complications at 6-month postoperative follow–up: corneal haze, persistent epithelial defects, ectasia, sterile infiltrate, infectious keratitis, central toxic keratopathy, and steroid-induced complications.

Planning for surgery

Organized Refractive Keratectomy Custom Ablation Manager (ORK-CAM) software (SCHWIND eye-tech solutions) was used to plan corneal wavefront customized treatments and aberration-free aspheric treatments. Aberration-free aspheric treatment involved ablation with optimized aspheric profile centered on the pupil center and the software compensated for the peripheral loss of energy, whereas corneal wavefront mode involved ablation with corneal topography centered on the corneal vertex. Emmetropia was targeted in all patients.

Surgical technique

All patients underwent TransPRK using ESIRIS flying spot excimer laser system (SCHWIND AMARIS 500E; SCHWIND eye-tech solutions), with 500 Hz repetition rate for faster treatments and scanning spots. Eye tracking during laser ablation was achieved using a 1050-Hz infrared eye tracker centered on the pupil. Static cyclotorsion control compensation was obtained for all patients in corneal WFG group and patients with astigmatism more than 1 D in aberration-free group. Dynamic cyclotorsion control was implemented automatically for all treatment types. The optical zone was chosen by the surgeon.

After the patient was prepared, topical Benoxinate hydrochloride (Benox 0.4% Sterile Ophthalmic Solution; Eipico, El Asher Men Ramadan, Egypt) anesthetic drops were placed in the eye. Lid speculum was placed in the operative eye, and a patch was placed over the fellow eye to avoid cross-fixation. The amount of desired correction, accounting for the vertex distance, was entered into the laser, and the patient was asked to fixate on the laser centration light, then the excimer laser was used to remove the epithelium estimating that the central epithelial thickness of a normal cornea is 55 μm at the center and 65 μm at 4 mm from the center, and then continued to ablate the corneal stroma in nontouch technique. Topical mitomycin-C (Mitomycin-C; Kyowa Inc., Japan) 0.2 mg/ml was employed in all cases according to diopteric correction, with minimum 20 s and maximum 60 s at the end of the laser exposure followed by generous irrigation of the eye with room temperature balanced salt solution. After the surgery, a bandage contact lens was applied.

Postoperative treatment included broad-spectrum antibiotics eye drops, moxifloxacin 0.5% (Vigamox; Sterile Ophthalmic Solution, Alcon, Irvine, CA, USA) and lubricant eye drops, hyaluronic acid (Hyfresh; Jamjoom Inc., KSA). Topical steroid drops, prednisolone acetate 1% (PredForte, ophthalmic suspension; Allergan Inc., Irvine, California, USA), were added on the second day postoperative starting with high frequency for 1 week, and then tapered gradually over 2 weeks. The bandage soft contact lens was removed 3–5 days postoperative.

Statistical analysis

IBM SPSS for Windows (version 22.0; IBM Corp., Armonk, New York, USA) was used. Categorical data were expressed as number and percentage, whereas numerical variables were expressed as mean ± SD. Normality of data variables were checked. For normally distributed data, preoperative and postoperative values were compared using paired t-test, whereas Student's t-test was used to compare the mean between both study groups. Corresponding nonparametric tests (Wilcoxon's rank test and Mann–Whitney test) were used if data did not follow a normal distribution. Correlation analysis was conducted between intended and achieved outcomes of interest. For all tests, P value of 0.05 or less was considered statistically significant.


  Results Top


In this study, we included 33 (66 eyes) patients with myopia with or without astigmatism who had performed TransPRK from January 2017 to December 2017: 42 eyes in aberration-free group and 24 eyes in corneal WFG group. In our study, the mean age was 32.04 ± 9 and 25 ± 3.9 years for aberration-free and corneal WFG group, respectively. Female to male ratio was 5: 2 and 2: 1 in aberration-free and corneal WFG group, respectively. Demographic characteristics as well as preoperative and operative characteristics showed no statistically significant difference between the two groups except for astigmatism being higher in aberration-free group (P = 0.04), which was not clinically significant, and total corneal HOAs, though being higher in corneal WFG group (P = 0.02; [Table 1]). Six months postoperatively, there was no statistically significant difference between the two groups in refractive and visual outcomes [Table 2]. Both groups showed significant improvement in UDVA (P = 0.001 and 0.003 in aberration-free and corneal WFG groups, respectively), with 32 eyes of 42 eyes (76.2%) and 16 eyes of 24 eyes (66.67%) having achieved postoperative UDVA of greater than or equal to 20/20 in aberration-free and corneal WFG groups, respectively, and 36 eyes of 42 eyes (85.72%) and 22 eyes of 24 eyes (91.67%), having achieved postoperative CDVA of greater than or equal to 20/20 in aberration-free and corneal WFG groups, respectively [Figure 1]. In terms of efficacy, the ratio between postoperative UDVA and preoperative CDVA was 1.08 ± 0.09 in aberration-free group and 1.04 ± 0.08 in corneal WFG group, with no statistically significant difference between both the groups (P = 0.6), whereas mean safety index (the ratio between postoperative CDVA and preoperative CDVA) was 1.16 ± 0.12 in aberration-free group and 1.17 ± 0.1 in corneal WFG group, with no statistically significant difference between the two groups (P = 0.9), and no eye in either group showed loss of 1 or more line from the preoperative CDVA. In addition, no postoperative complications were encountered, and no patient required retreatment in both groups. Concerning refractive outcome, both groups showed significant change in manifest refraction spherical equivalent (MRSE) (P = 0.001 in both groups). Overall, 97.62% (41 eyes of 42 eyes) and 100% of patients achieved postoperative spherical equivalent (SE) within ±1 D in aberration-free group and corneal WFG group, respectively [Figure 2]. Concerning corneal HOAs, there was an increase in the postoperative value of total HOAs and spherical aberrations in both groups, which was statistically significant in aberration-free group in contrast with corneal WFG, whereas there was a decrease in coma and trefoil aberrations in corneal WFG group in contrast with aberration-free group, which showed increase in both parameters [Table 3] and [Figure 3]. Moreover, there was a significant change in Q-value in both groups (P = 0.001 in both groups) toward more positive values [Table 2], which was positively correlated with the induction of spherical aberrations (R = 0.98, P = 0.001). On the contrary, the induction of spherical aberrations was positively correlated with amount of refractive correction (R = 0.86, P = 0.001), and negatively correlated with optical zone diameter (R=−0.56, P = 0.001). The postoperative central corneal thickness was significantly correlated with expected value preoperatively in both groups (R = 0.92, P = 0.001 in aberration-free group, and R = 0.94, P = 0.03 in corneal WFG group). Moreover, the postoperative average K was significantly correlated with expected value preoperatively in both groups (R = 0.96, P = 0.001 in aberration-free group, and R = 0.9, P = 0.01 in corneal WFG group). The change in central corneal thickness per 1 D treatment applied was lower in aberration-free group (17.6 ± 4.6 μ/D) than corneal WFG group, (20.9 ± 5.4 μ/D), but not statistically significantly different (P = 0.28).
Table 1: Preoperative and operative characteristics of each group

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Table 2: Six-month postoperative outcomes

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Figure 1: Six months postoperative uncorrected distance visual acuity and corrected distance visual acuity in each group.

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Figure 2: Six months postoperative manifest refraction spherical equivalent in each group.

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Table 3: Preoperative and 6 months postoperative corneal higher order aberrations

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Figure 3: Figure 3 Mean change in corneal higher order aberrations in each group.

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  Discussion Top


Visual and refractive outcomes of aberration-free and corneal WFG treatments were compared after laser in-situ keratomileusis (LASIK)[16],[17],[18]. Moreover, many other studies have compared WFG PRK and wavefront optimized PRK based on ocular wavefront analysis, not corneal wavefront analysis. Different laser platforms were compared, adding more heterogeneity to the outcomes[19]. All of this made the comparison difficult, and till now, no method has shown superiority over the other. In our study, TransPRK was performed for all patients according to the previously described techniques. With both techniques, satisfactory visual, refractive, and optical outcomes were obtained 6 months postoperatively. In the study by Kaluzny et al.[20], TransPRK achieved good visual results (postoperative UDVA was 20/20 or better in 97% compared with 94% in the alcohol-assisted PRK), as well as good refractive outcomes with high correlation between attempted and achieved MRSE in both groups[20]. In another retrospective case series, TransPRK was reported as effective and safe as femtosecond-assisted LASIK at 1-year postoperatively[1]. In our study, 76.2 and 66.67% achieved UDVA of greater than or equal to 20/20 in aberration-free group and corneal WFG group, respectively, 6 months postoperatively, whereas in the study by Jun et al.[21], 94 and 99% achieved UDVA of greater than or equal to 20/20 in aberration-free group and corneal WFG group, respectively. Both techniques have also achieved good refractive outcome, with 97.62% of patients and 100% of patients having achieved postoperative SE within ±1 D in aberration-free group and corneal WFG group, respectively. These results are comparable to the result of Jun et al.[21] in which all treated eyes were within ±1 D of the intended SE. Aslanides et al.[22] reported 2 years refractive and visual outcomes using aberration-free TransPRK, with 91% of eyes were within 0.50 D of the intended MRSE 2 years postoperatively with postoperative UDVA 20/20 or better in 70% of patients. Concerning optical outcomes, all the patients in our study had preoperative total corneal HOA more than 0.35 μm, and both groups showed increase in total corneal HOAs and spherical aberrations 6 months postoperatively, with no significant difference in corneal WFG. In contrast, corneal HOAs and spherical aberrations were significantly higher postoperatively in the aberration-free group. Change in corneal HOAs and spherical aberrations value showed no significant difference between both groups. Coma and trefoil aberrations declined after corneal WFG technique, yet increased after aberration-free one. Jun et al.[21] reported similar findings that confirmed the advantage of corneal WFG over aberration-free technique regarding the optical outcome, irrespective of the preoperative corneal HOAs, as corneal WFG did not induce trefoil or coma aberrations in contrast with aberration free, and the increases in spherical and total HOAs were less in corneal WFG. Moreover, Aslanides et al.[22] showed that 2 years after aberration-free TransPRK, the corneal trefoil and corneal total HOAs increased, but they did not significantly change postoperatively in contrast with increase in postoperative coma and spherical aberrations. Other studies have highlighted the role of corneal WFG PRK in cases of highly aberrated cornea as after previous refractive surgery and in keratoconus eyes after cross linking in terms of visual, optical, and refractive outcomes with reduction in the pre-existing corneal HOAs[11],[23],[24],[25]. In general, comparison of WFG PRK against wavefront optimized PRK, using different laser platforms, has been debatable, but all agreed that both techniques are safe and effective with subtle differences between the two techniques. In the study by Moshirfar et al.[26] that used VISX CustomVue and WaveLight Allegretto platforms, they reported the same efficacy, safety, and predictability for the two methods, with comparable significant increase of spherical aberrations with increase in the other HOAs as well. However, better contrast sensitivity was reported in the WFG group[26]. Moreover, in comparing visual outcomes following Visx Star S4 CustomVue WFG and Allegretto Wave Eye-Q 400 Hz wavefront optimized PRK, both techniques were similarly effective and safe, with better contrast visual acuity obtained with the WFG technique[27]. Another study detected an increase in HOA values postoperatively after both technique, with significantly lower coma and trefoil aberrations and astigmatism in WFG group[28]. Contrast sensitivity was always a concern after any refractive procedures. In military settings, contrast sensitivity is a central point to consider. Both ablation procedures were tested in military personnel who were evaluated for contrast sensitivity and target hitting after the procedure. Fortunately, satisfactory visual performance and contrast sensitivity was achieved after both techniques[29],[30]. Nonetheless, other studies showed superiority of aberration-free over WFG ablation in PRK. In the study by Zarei-Ghanavati et al.[19], they reported improved contrast sensitivity after both procedures with significantly lower postoperative HOAs in aberration-free group, particularly the spherical aberrations. In our study, there was significant positive correlation between induction of spherical aberrations and the change in SE, and significant negative correlation with the change in optical zone diameter. However, no significant correlation was detected with the coma aberrations. This correlation was proved by other studies after LASIK and LASEK[31],[32]. This indicates that larger optical and transitional zones are recommended to minimize the induction of spherical aberrations or combining the surgical technique with Q-value customized treatments especially with higher refractive errors. To our knowledge, this was the first comparison between corneal WFG and aberration-free ablation techniques in TransPRK in patients with myopia with high pre-existing corneal HOAs. However, limitations exist in the study. Absence of data about contrast sensitivity testing and other subjective assessment of visual quality detract from the comparison. Moreover, lack of cross matching between the two groups may allow confounders to exist. So, a larger well-designed study with matched groups that combines objective and subjective outcomes is strongly recommended.


  Conclusion Top


Both aberration-free and corneal WFG ablation profiles in TransPRK were safe, effective, and predictable techniques in treatment of myopia with or without astigmatism in patients with high pre-existing corneal HOAs. Comparable visual and refractive outcomes were obtained. However, corneal WFG treatment has the advantage of induction of fewer total corneal HOAs and spherical aberrations with reduction of postoperative coma and trefoil aberrations in contrast with aberration-free treatment.

Financial support and sponsorship

Nil.

Conflicts of interest

There are no conflicts of interest.



 
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