Clinical and Experimental Vision and Eye Research

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Effect of treatment zone diameter on the clinicalresults of femtosecond laser-assisted in situ keratomileusis andtrans-photorefractive keratectomy for the correction of myopia
  CLEVER
ORIGINAL ARTICLE
Effect of treatment zone diameter on the clinical
results of femtosecond laser-assisted in situ keratomileusis and
trans-photorefractive keratectomy for the correction of myopia
Assaf Gershoni1,2,3, Sabaa Knaneh1,2, Michael Mimouni4, Eitan Livny1,2,3, Irit Bahar1,2,3, Yoav Nahum1,2
1Department of Ophthalmology, Rabin Medical Center, Petach Tikva, Israel
2Sackler Faculty of Medicine, Tel Aviv University, Tel Aviv, Israel
3Assuta Optic, Assuta Medical Center, Tel Aviv, Israel
4Department of Ophthalmology, Rambam Health Care Campus, Haifa, Israel
Address for correspondence: Dr. Yoav Nahum MD, Department of Ophthalmology, Rabin Medical Center, Beilinson Campus, Jabotinsky 39, Petach Tikva, 49100 Israel.
E-mail: yoav.nahum@gmail.com
Received: 09-03-2018;
Accepted: 21-04-2018
doi: 10.15713/ins.clever.2
 
ABSTRACT
Aim: The aim of this study is to examine and compare the effect of treatment zonediameter on the results of femtosecond laser-assisted in situ keratomileusis (FS-LASIK)and trans- photorefractive keratectomy (PRK) procedures performed for the treatmentof myopia.
Materials and Methods: This was a retrospective cohort study. The study reviewedmedical files of patients who underwent trans-PRK (2630 eyes) and FS-LASIK (879eyes) in which different treatment area diameters were used. For each type of surgery,the eyes were divided into three groups, based on the treatment zone diameter (6 mm,6.5 mm. and 7 mm).
Results: In the FS-LASIK group, there was no difference in both the safety and efficacyindices or in the distance from the intended result between the groups (P = 0.79,P = 0.57, and P = 0.09, respectively). In myopic trans-PRK, a treatment area of 7 mm wasassociated with worse outcomes in terms of safety (P = 0.01) and efficacy (P < 0.01) incomparison with the other groups. In addition, a treatment zone of 7 mm was associatedwith a significantly larger distance from the refractive target (P < 0.001). There were nosignificant differences between the 6 mm and 6.5 mm groups in any outcome measure.These results recurred in a multivariate analysis, after correcting them for age, gender,pre-operative refractive error, and pachymetry.
Conclusions: Different treatment zone sizes gave similar results in FS-LASIK, whilein trans-PRK, a 7 mm zone was associated with inferior outcomes in comparison tosmaller treatment zones. Hence, in trans-PRK, we recommend choosing a treatmentzone smaller than 7 mm while taking pupillometry into account and opting FS-LASIKwhenever a very large treatment zone is required.
Keywords: Laser-assisted in situ keratomileusis, opticzone, photorefractive keratectomy, refractivesurgery, treatment zone
How to cite this article: Gershoni A, Knaneh S, Mimouni M,Livny E, Bahar I, Nahum Y. Effect of treatment zone diameteron the clinical results of femtosecond laser-assisted in situkeratomileusis and trans-photorefractive keratectomy for thecorrection of myopia. Cli Exp Vis Eye Res J 2018;1(1): 3-8.
 
 

Introduction

The world of refractive surgery has seen many changing trendsin the past three decades. The introduction of laser ablationfor the correction of myopia has significantly increased boththe safety and the efficacy of procedures compared to manualapproaches.[1] Nowadays, laser refractive procedures can bedivided into two main groups: Laser surface ablation proceduresand laser-assisted in situ keratomileusis (LASIK).

 
Transepithelial photorefractive keratectomy (trans-PRK)uses an excimer laser to ablate the epithelium and then reshapethe cornea to correct the refractive error. This platform obviatesthe need of alcohol epithelial debridement or mechanicalremoval of the epithelium during PRK.

Currently, LASIK is the most popular procedure for thesurgical correction of refractive error.[2] The technologicalevolution of flap creation enabled the creation of a more preciseand reproducible flap with the femtosecond laser.[3]

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Gershoni, et al. Effect of treatment zone diameter on results of FS-LASIK and trans-PRK for myopia

The anatomical area of the cornea which is ablated duringthe procedure is called the treatment zone, which is composedof the optical and transition zones. The transition zone is thepassageway between the treated and untreated zones. Accordingto Munnerlyn's formula,[4] as the size of the treatment zoneincreases, so does the volume of cornea tissue removed, andto avoid corneal ectasia, there might be a necessity to limit thetreatment zone size for each individual patient.

In the 1st year of LASIK, small treatment zones of up to 5 mmwere used; however, these resulted in a high frequency of regressionand vision disturbances within scotopic conditions. Hence, theminimal treatment zone increased to 6 mm and more. Night visiondisorders were reported even while using larger treatment areas,and it was recommended that the treatment area, including thetransition zone, will be 0.5-1 mm larger than the size of the pupilat low illumination conditions.[5] Schallhorn et al.[6] demonstratedthat, for a given treatment zone, there is an inverse correlationbetween pupil size and vision quality in the early post-operativeperiod, but no such correlation was established after 6 months ofsurgery. Some patients with a mesopic pupil size larger than thetreatment zone were asymptomatic, while others with a mesopicpupil size smaller than the treatment zone suffered from halos.The researchers concluded that there are other factors influencingpatients' symptoms such as cortical adaptation mechanisms. In twostudies, Buhren and Kohnen[7,8] demonstrated that, for patientswith large pupils, there is a correlation between optical aberrationsand the size of the treatment zone and that a correlation existsbetween the optical zone-to-pupil ratio and optical aberrations.

Literature about the treatment area's diameter and its effecton PRK results is scant in comparison to LASIK. Endl et al.[9]demonstrated an advantage in using an optical area of 5.5 mm witha transition area of 7 mm compared to a treatment area of 5 mmwithout a transition area. Rajan et al.[10] concluded that a 6.0 mmablation zone in PRK was superior to ablation zones of 4.0 mm and5.00 mm, with regard to refractive predictability, early hyperopicshift, regression, corneal transparency, and night haloes. In anotherstudy, Mohammadi et al.[11] concluded that an optical zone smallerthan 6.00 mm leads to a higher prevalence of undercorrectionand regression. We have found no studies comparing the effect oftreatment zones diameter in LASIK versus PRK.

The purpose of the current study was to compare the effect oftreatment zone diameter on the results of femtosecond LASIK(FS-LASIK) and trans-PRK procedures performed for thetreatment of myopia.

Materials and Methods

A retrospective cohort study design was used. The study followedthe tenets of the Declaration of Helsinki and was approved bythe Institutional Review Board of Assuta Medical Center.

Study cohort

The study group consisted of consecutive patients treated withFS-LASIK or trans-PRK for myopia of various severities at theoptical outpatient clinic of the largest private medical servicein Israel from January 2013 to December 2014. Results of thetrans-PRK and FS-LASIK groups were analyzed separately. Ineach group, patients were divided into subgroups according tothe treatment zone diameter utilized during the surgery.

 
Inclusion criteria for the procedure were the age of 18 yearsor higher and a myopic spherical equivalent (SE). Exclusioncriteria were the age lower than 18 years, change of more than0.5D in refraction during the year before the initial consultation,abnormal or keratoconus topography, coexisting ocularpathology or previous surgery, inflammatory or infectiouscorneal disease, relevant systemic dermatologic or connectivetissue disorders, hyperopia, mixed astigmatism, a follow-upperiod of under 3 months, pregnancy, intended monovision, andincomplete medical records.

Study procedure

The medical files of the patients were reviewed for demographics,operative data, length of follow-up, manifest refraction,uncorrected and best corrected visual acuity (UCVA andBCVA), corneal thickness, efficacy and safety indexes, refractiondistance from intended target, and post-operative complications.Efficacy was calculated as the ratio of mean post-operativeUCVA to mean pre-operative BCVA (efficacy index). Safety wascalculated as the ratio of mean post-operative BCVA to meanpre-operative BCVA (safety index). Findings were comparedbetween groups of patients treated with different treatment zonediameters in both the FS-LASIK and trans-PRK groups.

Pre-operative evaluation

The pre-operative evaluation included manifest and cycloplegicrefraction, autorefraction, slit-lamp biomicroscopy, dilatedfundoscopy, Goldmann tonometry, and mesopic pupil diametermeasurement. Slit-scan corneal Scheimpflug tomography(Sirius, SCHWIND eye-tech-solutions GmbH, Kleinostheim,Germany) and total ocular wavefront measurement (Hartmann-Shack Aberrometer/ORK-Wavefront Analyzer; SCHWINDeye-tech-solutions) were carried out as well.

Surgical technique

Decision to perform FS-LASIK or Trans-PRK was left to thediscretion of the operating physician. The common practice inour institution is not to perform LASIK when the central cornealthickness is < 500 µm. The procedures were performed by one ofseven experienced surgeons.

In the trans-PRK group, all treatments were asphericaberration-neutral non-wave front-guided profiles, and excimerlaser application was preceded by standardized wet spongeapplication. Single-step laser delivery with the Schwind Amaris500E excimer laser (SCHWIND eye-tech-solutions GmbH,Kleinostheim, Germany) was carried out immediately afterwardwith a 6.0-7.0 mm treatment zone, and mitomycin C (MMC0.02%) was immediately applied for up to 50 s (depending on theamount of ablation) using a damp Merocel sponge, then copiouslyirrigated with balanced saline solution, and dried. One drop ofofloxacin (0.3%) was subsequently instilled, and a bandage contactlens (Purevision, Bausch and Lomb) was inserted. After surgery,all eyes received topical ofloxacin (0.3%) qid until removal of thecontact lens, dexamethasone (0.1%) drop qid with a slow taperingdown over 12 weeks, and artificial tear drop qid for 3 months.
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Effect of treatment zone diameter on results of FS-LASIK and trans-PRK for myopia Gershoni, et al.

In the FS-LASIK group, a minimum residual stromal bedof 300 microns was mandatory for the procedure. The cornealflaps were created under topical anesthesia using the ZiemerLDV Z6 femtosecond laser (Ziemer Ophthalmic Systems,Allmendstrasse, Switzerland). Nominal flap thickness was setat 110 µm and flap diameter, to 9.5 mm, with a 0.4 mm hingeplaced superiorly. After the flap was lifted, ablations wereperformed using the Schwind Amaris 500E excimer laser witha 6.0-7.0 mm treatment zone. The corneal flap and stromalsurface were irrigated with balanced salt solution, and the flapwas repositioned. After surgery, patients were instructed to instilltopical moxifloxacin qid for 1 week, dexamethasone (0.1%) dropqid for 2 weeks, and artificial tear qid for 3 months.

Patients were examined immediately after surgery andinvited for follow-up visits at 1 day, 1 week, 1 month, 3 months,6 months, and 1 year after surgery.

Statistical analysis

Data were analyzed with the Minitab Software, version 16(Minitab Inc., State College, PA). For the analysis of categoricalvariables, Chi-square test was used. Comparisons betweennormal distribution variables were made using the ANOVA testwith post hoc Tukey's test for multiple comparisons. A P < 0.05was considered statistically significant. We also performed astepwise multiple regression analysis when needed. Due tothe lack of relevant results from past studies and since weanalyzed data of thousands of patients, a power analysis wasnot completed. We expected to find statistical significant resultsfor each difference found and to examine the importance of theresults within their clinical significance.

 
Results

The FS-LASIK group was comprised of 879 eyes of 441 patientswith a female predominance of 54.28% and a mean age of 29.10± 7.44 years [Table 1]. The trans-PRK group included 2630 eyesof 1315 patients, with a clear male predominance of 60.75%, anda younger mean age of 25.66 ± 6.92 years [Table 2]. For eachtype of surgery, the patients were divided into three subgroups,based on the treatment zone diameter (6 mm, 6.5 mm, and7 mm). The pre-operative SE for FS-LASIK and trans-PRK was-3.7 ± 1.9 and -4.6 ± 2.3, respectively (P < 0.0001).

In the FS-LASIK group, no difference was found regardingthe safety and efficacy indices or in the distance from theintended refractive result between all subgroups (P = 0.79,P = 0.57, and P = 0.09, respectively) [Table 1]. In myopictrans-PRK, a treatment area of 7 mm was associated with worseoutcomes in terms of safety (P = 0.01) and efficacy (P < 0.01)in comparison with the other groups [Table 2]. Furthermore,a treatment zone of 7 mm was associated with a significantlylarger distance from the refractive target in comparison to theother areas (P < 0.001). There were no significant differencesbetween the 6 mm and 6.5 mm groups in any of the outcomemeasures. These results recurred in a multivariate analysis, aftercorrecting them for age, gender, preoperative refractive error,and pachymetry [Tables 3-5].

Discussion

The adequate treatment zone selection for optimal outcomesand minimal adverse effects has been a topic of controversyfor many years in the field of refractive surgery. While severalstudies tried to examine this subject with regard to the LASIKprocedure,[5-8,12-16] literature about the treatment area diameterand its effect on PRK results is scant,[9-11] and as this procedureis gaining its popularity back,[2] this issue is of great importance.In a meticulous search through the relevant literature, we havefound no papers comparing the effect of treatment zone diameterin LASIK versus PRK. In this study, we aimed to examine andcompare the effect of treatment zone diameter on the resultsof FS-LASIK and trans-PRK procedures performed for thetreatment of myopia.

Table 1: Treatment outcomes for different treatment zone sizes in FS-LASIK
Effect of treatment zone diameter on the clinicalresults of femtosecond laser-assisted in situ keratomileusis andtrans-photorefractive keratectomy for the correction of myopia
*Values that do not share a letter are significantly different. SE: Spherical equivalent, FS-LASIK: Femtosecond laser-assisted in situ keratomileusis

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Gershoni, et al. Effect of treatment zone diameter on results of FS-LASIK and trans-PRK for myopia

Table 2: Treatment outcomes for different treatment zone sizes in FS-LASIK
Effect of treatment zone diameter on the clinicalresults of femtosecond laser-assisted in situ keratomileusis andtrans-photorefractive keratectomy for the correction of myopia
*Values that do not share a letter are significantly different. SE: Spherical equivalent, FS-LASIK: Femtosecond laser-assisted in situ keratomileusis

Table 3: Safety index - multivariant analysis for different treatment zone sizes in trans-PRK
Effect of treatment zone diameter on the clinicalresults of femtosecond laser-assisted in situ keratomileusis andtrans-photorefractive keratectomy for the correction of myopia
PRK: Photorefractive keratectomy

Table 4: Efficacy index - multivariant analysis for different treatment zone sizes in trans-PRK
Effect of treatment zone diameter on the clinicalresults of femtosecond laser-assisted in situ keratomileusis andtrans-photorefractive keratectomy for the correction of myopia
PRK: Photorefractive keratectomy

Table 5: Distance from target - multivariant analysis for different treatment zone sizes in trans-PRK
Effect of treatment zone diameter on the clinicalresults of femtosecond laser-assisted in situ keratomileusis andtrans-photorefractive keratectomy for the correction of myopia
PRK: Photorefractive keratectomy

In the 1st year of LASIK, small treatment zones were usedwhich resulted in a high frequency of regression and visiondisturbances within scotopic conditions, when the pupil islarger than the ablation zone.[7,17-19] Pop and Payette[20] showed a2.5 times increase in night vision complaints for an optical zoneof 6.00 mm or lower. Night vision disorders were reported evenwhile using larger treatment areas, and it was recommendedthat the treatment area, including the transition zone, will be0.5-1 mm larger than the size of the pupil at low illuminationconditions.[5] In a recent study, Milivojevic et al.[12] concludedthat diameter enlargement of the treated optical zone from6.5 mm to 7.00 mm does not threaten the stability of the corneastructure and significantly improves outcomes for corneas inwhich larger ablation (resulting in deeper ablation and increasedrisk for ectasia)[21,22] can be safely done.

 
As mentioned before, the treatment area diameter and its effecton PRK results were explored to a lower magnitude. One study[9]demonstrated an advantage in using an optical area of 5.5 mm witha transition area of 7 mm compared to a treatment area of 5 mmwithout a transition area. Two other studies[10,11] concluded thata 6.0 mm ablation zone in PRK was superior to smaller ablationzones with regard to outcomes and adverse effects.

When discussing elective refractive procedures, oneshould be aware that the most critical factor to our patients iseliminating their dependency on spectacles. This factor can beassessed most accurately with the efficacy index. In this study,we found no significant differences between the treatment zonediameters (6 mm, 6.5 mm, and 7 mm) in FS-LASIK with regardto the efficacy index, the safety index, and the distance from therefractive target [Table 1]. It is worthwhile to point out that the7 mm group consisted of only 32 eyes. However, in trans-PRK, a7 mm zone was associated with inferior outcomes in comparisonto smaller treatment zones even though the pre-operative SE inthis group was significantly lower than in the other two groups[Table 2]. This variance can stem from the fact that ablationsof larger zones can lead to more high order aberrations. Whilethe source of these aberrations in PRK is on the corneal surface,the area which most influences the refraction, in LASIK, theseaberrations may be deducted to some degree by the flap or maybe less influential as they lie deep within the stroma and not onthe surface.

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Effect of treatment zone diameter on results of FS-LASIK and trans-PRK for myopia Gershoni, et al.

As described earlier, the pre-operative SE of the trans-PRKgroup was higher than that of the FS-LASIK group, althougheyes with a high degree of myopia were rarely operated with theFS-LASIK approach. Even though the degree of myopia wastaken into account in the multivariant analysis [Tables 3-5],this could have altered the results to some degrees. For instance,perhaps, some patients who were treated with trans-PRK for veryhigh myopia and needed a larger treatment zone due to a largepupil received a suboptimal correction because of the restraintsof the ablation depth which is proportional to the square of thediameter.

There are several limitations to this study. First, although thesample was large, we used a retrospective study design with alimited follow-up time of 12 months. Second, a bias exists sincesome patients with a very good UCVA in the early post-operativeexaminations tended not to adhere to the full 12-month followup,whereas those with worse early outcomes were motivatedto appear for reexamination. Third, there was also a potentialnegative bias in terms of the safety index because we do notroutinely examine BCVA in patients with a good post-operativeUCVA; instead, we use the post-operative UCVA value for bothparameters. This may have lowered the expected safety indexpostoperatively, in both procedures. Fourth, due to technicalconstraints, we did not adjust the results according to themesopic pupil size, which is the main drawback of this study.

Conclusion

In this large-scale study, we found that different treatment zonesizes gave similar results in FS-LASIK, while in trans-PRK, a7 mm zone was associated with inferior outcomes in comparisonto smaller treatment zones. Hence, in PRK, we recommend usinga treatment zone smaller than 7 mm when possible while takingpupillometry into account and opting FS-LASIK whenever avery large treatment zone is required.

 
Clinical Significance

This study shed some more light on a topic of much controversyin the field of refractive surgery and may help the ophthalmicsurgeon to select the adequate treatment zone when correctingmyopia with trans-PRK or FS-LASIK, to gain optimal outcomesand minimal adverse effects.

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