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 Table of Contents  
ORIGINAL ARTICLE
Year : 2019  |  Volume : 112  |  Issue : 2  |  Page : 43-51

Does photorefractive keratectomy with mitomycin results in less weakening effect on corneal biomechanics compared to thin-flap Lasik procedure in mild to high myopia? An Ocular Response Analyzer study


Department of Ophthalmology, Research Institute of Ophthalmology, Giza, Egypt

Date of Submission20-Jan-2019
Date of Acceptance28-Jan-2019
Date of Web Publication19-Jul-2019

Correspondence Address:
Tamer A Refai
168 Nile Street, Agouza, Giza 12211
Egypt
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Source of Support: None, Conflict of Interest: None


DOI: 10.4103/ejos.ejos_5_19

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  Abstract 

Purpose To understand the changes in corneal biomechanics studied by Ocular Response Analyzer after thin-flap Lasik technique compared with photorefractive keratectomy (PRK) with mitomycin C in mild to high myopia to decrease the risk of ectasia.
Patients and methods Forty eyes of 24 myopic patients with mild to severe myopia were divided into two groups; group A (included 20 eyes treated by thin-flap Lasik technique and group B (included 20 eyes treated by PRK with application of mitomycin C 0.02%). Patients were examined preoperatively and postoperatively. For the Ocular Response Analyzer, corneal hysteresis (CH), corneal resistance factor (CRF), and the keratoconus suspect (KS) were studied. Collected data were subjected to statistical analyses.
Results In this study, postoperatively, a highly significant decrease (P<0.01) occurred in both groups regarding CH (mean reduction from 10.26±1.41 to 7.65±1.35 in group A and from 10.70±1.26 to 7.77±1.63 in group B) as well as CRF (mean reduction from 10.37±1.32 to 7.53±1.29 in group A and from 10.71±1.58 to 7.70±1.55 in group B). Similarly a highly significant increase (P<0.01) occurred in both groups regarding KS% (mean increase from 6±7.27 to 38.25±19.87% in group A and from 7.8±10.44 to 31.8±23.61% in group B). There was no statistically significant difference (P>0.05) between both groups regarding postoperative changes in CH, CRF, and KS%. A highly significant correlation (P<0.01) existed between total corrected spherocylindrical refraction and the postablative decrease in CH as well as in CRF. A statistically significant correlation (P<0.05) existed between the total corrected spherocylinderical refraction and the postablative increase in the KS% in both groups.
Conclusion Both thin-flap Lasik and PRK with mitomycin for mild to high myopia resulted in significant comparable reduction in the corneal biomechanical properties, the magnitude of which is highly dependent on the extent of correction.

Keywords: corneal hysteresis, corneal resistance factor, keratoconus suspect percentage, mild to high myopia, photorefractive keratectomy mitomycin, thin-flap Lasik


How to cite this article:
Refai TA. Does photorefractive keratectomy with mitomycin results in less weakening effect on corneal biomechanics compared to thin-flap Lasik procedure in mild to high myopia? An Ocular Response Analyzer study. J Egypt Ophthalmol Soc 2019;112:43-51

How to cite this URL:
Refai TA. Does photorefractive keratectomy with mitomycin results in less weakening effect on corneal biomechanics compared to thin-flap Lasik procedure in mild to high myopia? An Ocular Response Analyzer study. J Egypt Ophthalmol Soc [serial online] 2019 [cited 2019 Aug 18];112:43-51. Available from: http://www.jeos.eg.net/text.asp?2019/112/2/43/263008


  Introduction Top


Background and statement of the problem

Thin-flap Lasik treatment for myopia is considered a very successful surgical procedure with a success rate of approximately 95% regarding patient satisfaction [1]. However, progressive ectasia caused by biomechanical corneal failure probably owing to the lamellar corneal cut in addition to the excimer laser ablation had been reported in some cases [2]. Surface ablation in which the lamellar corneal cut is abolished has been considered as an alternative procedure that could decrease the risk of ectasia but with increased possibility of postoperative haze and slower visual recovery as principal disadvantages [3]. Mitomycin C, derived from Streptomyces caespitosus, is an alkylating agent for DNA, capable of inhibiting DNA/RNA replication mainly in rapidly dividing cells, like fibroblasts, thus suppressing wound healing [4], thus decreasing the risk of corneal haze in both human [5] and animal model [6]. This has led to increasing use of photorefractive keratectomy (PRK) in conjunction with mitomycin C application for correction of high myopic refractive errors, in contrary to the previous concept of limiting PRK usage too low to moderate myopia [7] because it is generally believed that Lasik flap creations at 100–140 μm can sever the stronger anterior cornea collagen fibers, thus reducing the corneal biomechanical stability [8].

Objective

The aim of this work is to compare the reduction of corneal biomechanical properties after thin-flap Lasik with that occurring after PRK with mitomycin C application by Ocular Response Analyzer (ORA) and to find out if surface ablation is truely safer for mild to high myopia.


  Patients and methods Top


The study included 40 eyes of 24 Egyptian patients who were candidates for ablative refractive surgery for low to high myopia with and without astigmatism at the Research Institute of Ophthalmology, Giza, Egypt, up to November 2018. Approval of the ethical committee was obtained as well as a written informed consent. The patients were divided into two groups: group A (20 eyes) included eyes of patients in whom lasik using thin-flap technique (using Moria 90 microkeratome) was performed, and group B (20 eyes) included eyes of patients in whom PRK with mitomycin C application was performed. Eyes with previous ocular or corneal surgery, ocular or systemic diseases associated with corneal weakness or affection of wound healing, pregnant, and lactating women, aged below 18 years, eyes with abnormal tomography as well as eyes with history of eye trauma were excluded from the study to abolish false effects on results. Eyes with thin corneas (<500 μm, for Lasik and <475 μm for PRK with mitomycin) were abandoned from ablative surgery. Similarly, eyes with extremely abnormal ORA preoperative values [i.e. corneal hysteresis (CH) and corneal resistance factor (CRF) less than 8 and keratoconus suspect (KS%) >10%] were abandoned from surgery. In either groups, at least 300-μm stromal bed was left unablated. In either groups, optimized excimer laser ablation was performed by Allegretto Wave Light Eye-Q 1010 (400 Hz) with active eye tracker (frequency 400 Hz). Patients were examined preoperatively and again 1, 7, and 14 days, as well as one and a half months after surgery postoperatively. Visual acuity (uncorrected and best corrected), Scheimpflug imaging, ORA, and slit-lamp examination were performed. Postoperatively, visual acuity, slit-lamp examination were performed in each visit, in addition to ORA performed at approximately one and half months (after stabilization of healig process and refractive status). The three main items that were considered in ORA were CH, which reflects the corneal viscoelastic properties; CRF, which measures the whole corneal rigidity [9]; and the KS% (of the keratoconus match probabilities), which reflects the degree of weakness of corneal biomechanics.

The LASIK procedure performed for the cases under study

  1. Proparacaine hydrochloride 0.4% (Benox eye drops) is applied 5 min just before the operation for topical anesthesia.
  2. The eyelids were sterilized with povidone–iodine 10% (Betadine).
  3. Speculum application.
  4. Corneal marks were made by a surgical marker, with marks created at lower quadrants.
  5. The conjunctival sac was dried with sponge to allow vaccum created by microkeratome ring to build up.
  6. The microkeratome handle was placed over the cornea creating a corneal flap with a superior hinge for Moria M2 90 microkeratome keeping the cornea wet with balanced salt solution (BSS) during flap creating to achieve smooth cut.
  7. Ablation was done by the Allegretto Wave Light Eye-Q 1010 (400 Hz) with active eye tracker (frequency 400 Hz) using ablation zone of 6.5 mm.
  8. BSS was used to irrigate the stromal bed.
  9. The flap was repositioned and painted aided by few air jets.
  10. The flap alignment was checked by preoperative corneal marks.
  11. Postoperatively, antibiotics (norfloxacin 0.5%), corticosteroid eye (prednisolone acetate 1%) drops, and preservatives free artificial tears eye drops were prescribed.


The photorefractive keratectomy with mitomycin C procedure performed for the cases under study

  1. Proparacaine hydrochloride 0.4% (Benox eye drops) was administered 5 min just before the operation for topical anesthesia.
  2. The eyelids were sterilized with povidone–iodine 10% (betadine).
  3. Speculum application.
  4. After corneal marking of the debridement area by a corneal marker, mechanical debridement of the epithelium was done by a blunt spatula keeping the cornea wet by continued BSS application to allow for smooh debridement.
  5. Ablation was done by the Allegretto Wave Light Eye-Q 1010 (400 Hz) with active eye tracker (frequency 400 Hz) using ablation zone of 6.5 mm.
  6. Mitomycin C 0.02% was applied for 1.5 min to the ablation site.
  7. The cornea, the conjunctival sac, the puncti areas as well as the lid margins were washed thoroughly with cold BSS (to avoid endothelial and conjunctival toxicity as well as trying to decrease local pain by the coldness of the BSS).
  8. Bandage contact lenses was applied for 4 days (to counteract infection and pain) and then removed.
  9. Postoperatively, antibiotics (norfloxacin 0.5%), corticosteroid eye (prednisolone acetate 1%) drops, topical NSAIDs (diclofenac sodium 0.1%) and preservative-free artificial tear eye drops were prescribed in addition to GABA analog (gabapentin 300-mg tablets) twice daily for 2 days as a pain killer.


Collected data were subjected to statistical analyses with determination of mean and SDs, as well as minimum and maximum values, in addition to comparison (Student’s t test) and correlation (Pearson’s) tests.


  Results Top


General findings

Results

In group A (thin-flap Lasik group), the age range was 19–42 years (mean, 30.35±7.47 years), whereas in group B (PRK with mitomycin group), it was 19–36 years (mean, 25.75±5.34 years). The preoperative flattest keratometric readings (K1) in group A had a range from 39 to 45.00 D (mean, 42.82±1.71 D), whereas in group B, it ranged from 40.30 to 46.2 D (mean, 43.50±1.58 D). The preoperative steepest keratometric readings (K2) in group A had a range from 41.1 to 47.2 D (mean, 43.36±1.65 D), whereas in group B, it ranged from 42.3 to 46.8 D (mean, 44.80±1.21 D). The preoperative mean keratometric readings in group A had a range from 40.1 to 45.85 D (mean, 43.59±1.62 D), whereas in group B, it ranged from 41.65 to 46.5 D (mean, 44.15±1.26 D) ([Table 1]). The preoperative central pachymetry in group A had a range of 507 to 585 μm (mean, 549.1±24.53 μm), whereas in group B, it ranged from 493 to 624 μm (mean, 528.85 μm), with t test (t=1.97) showing a nonsignificant difference (0.06, i.e. >0.05) between both groups ([Table 2] and [Figure 1]). The preoperative total spherocylinderical value in group A had a range of −1.75 to −11.0 D (mean, −5.38±2.58 D) and in group B, it had a range of −2.00 to −9.75 D (mean, −5.19±2.56 D), with t test (t=0.59) showing a nonsignificant difference (0.06; i.e. >0.05) between both groups ([Table 2] and [Figure 2]).
Table 1 Demographic data

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Table 2 The mean value and standard deviations for the preoperative central pachymetry (μm) and total corrected spherocylindrical refraction (D) for both the thin-flap Lasik (A) group and photorefractive keratectomy with mitomycin (B) group and their comparison by t test and significance

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Figure 1 The mean value for preoperative central pachymetry (μm) for the thin-flap Lasik (A) group and PRK with mitomycin (B). PRK, photorefractive keratectomy.

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Figure 2 The mean value for the total spherocylindrical value (D) for both thin-flap Lasik(A) group and PRK with mitomycin (B). PRK, photorefractive keratectomy.

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In this study, the CH decreased from a mean of 10.26±1.41 preoperatively to 7.65±1.35 postoperatively in thin-flap Lasik group, with a highly significant decrease (t test=4.95, P<0.01), and similarly, it decreased from a mean of 10.70±1.26 preoperatively to 7.77±1.63 postoperatively in PRK with mitomycin group, with a highly significant decrease (t test=6.96, P<0.01). The CRF decreased from a mean of 10.37±1.32 preoperatively to 7.53±1.29 postoperatively in thin-flap Lasik group, with a highly significant decrease (t test=6.24, P<0.01), and similarly, it decreased from a mean of 10.71±1.58 preoperatively to 7.70±1.55 postoperatively in PRK with mitomycin group, with a highly significant decrease (t test=7.70, P<0.01) ([Table 2] and [Figure 3]). KS% increased from a mean of 6±7.27 preoperatively to 38.25±19.87 postoperatively in thin-flap Lasik group, with a highly significant increase (t test=5.57, P<0.01), and similarly, it increased from a mean of 7.8±10.44 preoperatively to 31.8±23.61 postoperatively in PRK with mitomycin group, with a highly significant increase (t test=4.11, P<0.01) ([Table 2] and [Figure 4]).
Figure 3 The mean value for corneal hysteresis (CH) and corneal resistance factor (CRF) by ORA, both preoperative and postoperative, for the thin-flap Lasik (A) group and PRK with mitomycin (B) group. ORA, Ocular Response Analyzer; PRK, photorefractive keratectomy.

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Figure 4 The mean value for keratoconus suspect (KS) by ORA, both preoperatively and postoperatively, for the thin-flap Lasik (A) group and PRK with mitomycin (B) group. ORA, Ocular Response Analyzer; PRK, photorefractive keratectomy.

Click here to view


The postablative decrease in CH had a mean value of 2.61±2.01 in thin-flap Lasik group and 0.8±1.44 in PRK with mitomycin group with a nonsignificant difference (t test=0.57, P>0.05). The postablative decrease in CRF had a mean value of 2.85±1.63 in thin-flap Lasik group and 3.01±1.28 in PRK with mitomycin group with a nonsignificant difference (t test=0.34, P>0.05) ([Table 3] and [Figure 5]). The postablative increase in the KS% had a mean value of 32.25±21.49 in thin-flap Lasik group and 24.0±23.33 in PRK with mitomycin group, with a nonsignificant difference (t test=1.12, P>0.05) ([Table 3] and [Figure 6]).
Table 3 The mean value and standard deviations for various Ocular Response Analyzer parameters, preoperative and postoperative parameters, for the thin-flap Lasik (A) group and photorefractive keratectomy with mitomycin (B) group and their comparison by t test and significance

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Figure 5 The mean value for the postoperative change (decrease) for corneal hysteresis (CH) and corneal resistance factor (CRF) for both the thin-flap Lasik (a) group and PRK with mitomycin (b) group. PRK, photorefractive keratectomy.

Click here to view
Figure 6 The mean value for the postoperative change (increase) for keratoconus suspect (KS) percentage for both the thin-flap Lasik (A) group and PRK with mitomycin (B) group. PRK, photorefractive keratectomy.

Click here to view


It was found from preablation and postablation pachymetry comparison that every diopter of myopic or astigmatic correction results in removal of approximately 15 µm of corneal stromal tissue by optimized ablation (by Allegretto Wave Light Eye-Q 1010) (400 Hz) using 6.5 mm optical zone.

In this study, a highly significant correlation existed between the postablative decrease in the CH and the total corrected spherocylinderical refraction (which reflects the extent of pachymetry reduction) for both thin-flap Lasik group (r=0.66, P<0.01) and PRK with mitomycin group (r=0.66, P<0.01). A highly significant correlation existed between the postablative decrease in the CRF and the total corrected spherocylindrical refraction for both thin-flap Lasik group (r=0.57, P<0.01) and PRK with mitomycin group (r=0.79, P<0.01). A statistically significant correlation existed between the postablative increase in the KS% and the total corrected spherocylindrical refraction for both thin-flap Lasik group (r=0.27, P<0.05) and PRK with mitomycin group (r=0.21, P<0.05) ([Tables 4] and [Table 5]).
Table 4 The mean value and standard deviations for the postoperative change for various studied parameters for both the thin-flap Lasik (A) group and photorefractive keratectomy with mitomycin (B) group and their comparison by t test and significance

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Table 5 Correlations between different studied items and total corrected spherocylindrical refraction (D)

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The age had no significant effect on the results, but the female sex showed a statistically significant effect (Pearson correlation r=0.25, <0.05) on the postablative reduction of the CRF in particular.


  Discussion Top


Refraction of the eye is dependent on two refractive media, namely, the cornea (main refractive medium) and the lens [10]. Two commonly performed procedures nowadays are the Lasik procedure using thin-flap for mild to large refractive errors [11] and the PRK, which is usually reserved for low errors [12]. The Lasik has the advantage of fast visual recovery, relatively painless procedure, in addition to avoiding damage to the Bowman’s membrane, thus avoiding the possibility of corneal haze [13]; however, cutting the anterior strong corneal fibers was claimed to cause addition corneal weakness in addition to the weakness caused by the ablation process, thus increasing the possibility of postablation ectasia [2]. PRK on the contrary avoids cutting through the strong anterior corneal fibrils, so can preserve some corneal stability but is associated with pain for a few days, delayed refractive stability, prolonged topical steroid treatment with its possible complications like steroid-induced glaucoma and could be associated with postablation haze, which may decrease visual acuity and visual quality, and this had limited the use of PRK to low myopic and astigmatic errors [14]. However, the addition of topical mitomycin C application (which was claimed to decrease the incidence of post-PRK haze had encouraged many refractive surgeons to use PRK with mitomycin with high refractive errors, thus avoiding the Lasik flap creation, which was claimed to cause additional weakness of the corneal in addition to ablation-related weakness [7]. The ORA is one of the most commonly used ophthalmic devices that can evaluate biomechanical corneal properties in vivo [15]. This work, in which ORA parameters related to ectasia detection (mainly CH, CRF, and KS%) were studied before and after Lasik thin-flap (Moria 90 microkeratome) as well as PRK with mitomycin application for mild to high errors to have a better understanding of the biomechanical stability of the cornea after both effective refractive procedures.

Gaujoux and colleagues studied 37 eyes of 22 patients, in which LASIK (using 160 μm flap) was performed in 26 eyes and PRK in 11. Mean preoperative spherical equivalent was −4.2±1.4 D (range: −2 to −7.25 D). Before surgery, the mean CH value was 10.3±1.1 mmHg and the mean CRF was 10.2±1.0 mmHg. There was no difference between the Lasik and PRK groups for CH and CRF (P=0.43 and 0.24, respectively). Mean CH showed a statistically significant decrease following Lasik and PRK in all eyes (P<0.01). Postoperatively, CH decreased from 10.3 to 8.3 mmHg (19.6%) and CRF from 10.3 to 7.3 mmHg (29.8%) in the LASIK group, and CH decreased from 10.4 to 7.9 mmHg (24.1%) and CRF from 10.0 to 7.0 mmHg (30.8%) in the PRK group. Regarding reduction of biomechanical parameters, there was no statistically significant difference between post-LASIK eyes and post-PRK eyes (P=0.10 and 0.46 for CH and CRF, respectively). Decrease of CH and CRF was correlated with theoretic stromal ablation depth (r’s=0.55; P<0.001 for CH and r’s=0.75; P<0.001 for CRF) [16].

Gupta and colleagues performed a retrospective chart review carried out on 113 eyes that underwent LASIK and 121 eyes that underwent PRK for the treatment of myopia or myopia with astigmatism. Mean preoperative MRSE for LASIK and PRK eyes was −4.34 D (range, −0.68 to −8.35 D) and −6.64 D (range, −0.58 to −13.15 D), respectively. Mean CH showed a statistically significant decrease following LASIK and PRK in all eyes (P<0.01). Mean CH decreased from 10.77 mmHg (SD 1.50) to 8.92 mmHg (SD 1.21) for Lasik eyes and from 10.41 mmHg (SD 1.95) to 7.56 mmHg (SD 1.48) for PRK eyes. Mean CRF also showed a statistically significant decrease after surgery (P<0.01 for LASIK and PRK eyes). Mean CRF decreased from 10.5 mmHg (SD 1.82) to 7.3 mmHg (SD 1.48) for LASIK eyes and from 10.09 mmHg (SD 2.09) to 6.1 mmHg (SD 1.88) for PRK eyes. Comparison of mean change in CH showed that PRK induced a statistically significant greater change in CH compared with LASIK eyes (P<0.01). Comparison of change in CRF for LASIK and PRK failed to reach significance [17].

Reinstein and colleagues developed a mathematical model to estimate the relative differences in postoperative stromal tensile strength following PRK, LASIK, and small incision lenticule extraction. The postoperative total stromal tensile strength decreased for thinner corneal pachymetry for all treatment types and for LASIK [18]. They found that, postoperatively, the total stromal tensile strength was greatest after small incision lenticule extraction, followed by PRK, and then LASIK [18].

In this study, the CH decreased from a preoperative mean of 10.26±1.41 and 10.70±1.26 in group A (thin-flap Lasik group) and in group B (PRK with mitomycin group), respectively, to a postoperative mean of 7.65±1.35 in group A and 7.77±1.63 in group B, with a highly significant decrease (P<0.01) in both groups. Similarly, the CRF decreased from a preoperative mean of 10.37±1.32 and 10.71±1.58 in group A and group B, respectively, to a postoperative mean of 7.53±1.29 in group A and 7.70±1.55 in group B, with a highly significant decrease (P<0.01) in both groups. These results were in agreement with previous studies as done by Gaujoux and colleagues, Gupta and colleagues, and Reinstein and colleagues. The KS% increased from a preoperative mean of 6±7.27 and 7.8±10.44 in group A and in group B, respectively, to a postoperative mean of 38.25±19.87 in group A and 31.8±23.61 in group B, with a highly significant increase (P<0.01) in both groups, and this parameter was not studied in previously mentioned studies. The postablative decrease in CH had a mean value of 2.61±2.01 in thin-flap Lasik group and 0.8±1.44 in PRK with mitomycin group, with a nonsignificant difference (t test=0.57, P>0.05), and this finding was in agreement with the study by Gaujoux and colleagues but not with Gupta and colleagues, which was probably caused by a higher mean of refractive correction for PRK than for LASIK group in the study by Gupta and colleagues. The postablative decrease in CRF had a mean value of 2.8, in agreement with 5±1.63 in thin-flap Lasik group and 3.01±1.28 in PRK with mitomycin group with a nonsignificant difference (t test=0.34, P>0.05) and this finding was in agreement with Gaujoux and colleagues, as well as Gupta and colleagues studies but not with the study by Reinstein and colleagues, which was mathematically based only on the residual stromal layer for PRK or Lasik after obtaining results from in-vitro corneal stromal tensile strength changes rather than in-vivo studies.

The postablative increase in the KS% had a mean value of 32.25±21.49 in thin-flap Lasik group and 24.0±23.33 in PRK with mitomycin group, with a nonsignificant difference (t test=1.12, P>0.05), and this parameter was not studied in previously mentioned studies. A highly significant correlation (P<0.01) existed between total corrected spherocylindrical and the postablative decrease in the CH (r=0.66 in group A and r=0.66 also in group B) and the postablative decrease in the CRF (r=0.57 in group A and r=0.79 in group B), and this finding was in agreement with previously mentioned studies. A statistically significant correlation (P<0.05) existed between the total corrected spherocylindrical refraction and the postablative increase in the KS% for both group A (r=0.27) and for group B (r=0.21), and again this parameter was not studied in previously mentioned studies.

The age had no significant effect on the results, but the female sex showed a statistically significant effect on the postablative reduction of the CRF in particular (Pearson’s correlation r=0.25, <0.05).


  Conclusion Top


Both thin-flap Lasik and PRK with mitomycin for mild to high myopia and astimatism resulted in significant comparable reduction in the corneal biomechanical properties, the magnitude of which is highly dependent on the extent of correction with additional negative effect of the female sex on the CRF in particular.

Financial support and sponsorship

Nil.

Conflicts of interest

There are no conflicts of interest.

 
  References Top

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Kymionis GD, Kontadakis GA, Grentzelos MA, Panagopoulou SI, Stojanovic N, Kankariya VP et al. Thin-flap-laser in situ keratomileusis with femtosecond-laser technology. J Cataract Refract Surg 2013; 39:1366–1371.  Back to cited text no. 1
    
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Netto MV, Mohan RR, Sinha S, Sharma A, Gupta PC, Wilson SE. Effect of prophylactic and therapeutic mitomycin C on corneal apoptosis,cellular proliferation, haze, and long-term keratocyte density in rabbits. J Refract Surg 2006; 22:562–574.  Back to cited text no. 6
    
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Leccisotti A. Mitomycin C in photorefractive keratectomy: effect on epithelialization and predictability. Cornea 2008; 27:288–291.  Back to cited text no. 7
    
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Dupps WJ, Wilson SE. Biomechanics and wound healing in the cornea. Exp Eye Res 2006; 83:709–720.  Back to cited text no. 10
    
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Buratto L, Ferrari M. Indications, techniques, results, limits, and complications of laser in situ keratomileusis. Curr Opin Ophthalmol 1997; 8:59–66.  Back to cited text no. 11
    
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O’Brart DP, Corbett MC, Verma S, Heacock G, Oliver KM, Lohmann CP et al. Effects of ablation diameter, depth, and edge contour on the outcome of photorefractive keratectomy. J Cataract Refract Surg 1996; 12:50–60.  Back to cited text no. 12
    
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Schmack I, Auffarth GU, Epstein D, Holzer MP. Refractive surgery trends and practice style changes in Germany over a 3-year period. J Refract Surg 2010; 26:202–208.  Back to cited text no. 13
    
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O’Brart DP, Gartry DS, Lohmann CP, Muir MG, Marshall J. Excimer laser photorefractive keratectomy for myopia: comparison of 4.00- and 5.00-millimeter ablation zones. J Refract Corneal Surg 1994; 10:87–94.  Back to cited text no. 14
    
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Moreno-Montanes J, Maldonado MJ, Garcia N, Mendiluce L, Garcia-Gomez PJ, Segui-Gomez M. Reproducibility and clinical relevance of the ocular response analyzer in nonoperated eyes: corneal biomechanical and tonometric implications. Invest Ophthalmol Vis Sci 2008; 49:968–974.  Back to cited text no. 15
    
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Gaujoux T, Gavrilov JC, Sellam M, Kopito R, Chatel MA, Touzeau O et al. Changes in corneal biomechanics following corneal refractive surgery: LASIK versus PRK. Invest Ophthalmol Vis Sci 2008; 49:650.  Back to cited text no. 16
    
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Gupta N, Fry KL, Hersh PS. ORA measurementsafter PRK and LASIK. Invest Ophthalmol Visual Sci 2007; 48:5331.  Back to cited text no. 17
    
18.
Reinstein DZ, Arher TJ, Randelman BR. Mathematical model to compare the relative tensile strength of the cornea after PRK,LASIK, and small incision lenticule extraction. J Refract Surg 2013; 29:454–460.  Back to cited text no. 18
    


    Figures

  [Figure 1], [Figure 2], [Figure 3], [Figure 4], [Figure 5], [Figure 6]
 
 
    Tables

  [Table 1], [Table 2], [Table 3], [Table 4], [Table 5]



 

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  In this article
Abstract
Introduction
Patients and methods
Results
Discussion
Conclusion
References
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