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 Table of Contents  
ORIGINAL ARTICLE
Year : 2019  |  Volume : 20  |  Issue : 3  |  Page : 107-111

Corneal densitometry and topography after femtosecond laser-assisted laser in-situ keratomileusis compared with small-incision lenticule extraction


1 Damanhour ophthalmology Hospital, Egypt
2 Ophthalmology Department, Faculty of Medicine, Alexandria University, Egypt

Date of Submission15-Jan-2019
Date of Acceptance24-May-2019
Date of Web Publication26-Sep-2019

Correspondence Address:
BSC, MSC Enas Ahmed Hanafy Bakr
Al Buhaira Damanhour 34 Galal Qeratum street
Egypt
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Source of Support: None, Conflict of Interest: None


DOI: 10.4103/DJO.DJO_2_19

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  Abstract 


Purpose The aim of this study was to compare between femtosecond laser-assisted laser in-situ keratomileusis (FS-LASIK) and small-incision lenticule extraction (SMILE) regarding corneal densitometry (CD) and corneal topography.
Setting The study was conducted at Department of Ophthalmology, Faculty of Medicine, University of Alexandria, Alexandria, Egypt.
Patients and methods The study included 60 eyes of 30 patients divided into two equal groups: 30 eyes were treated using FS-LASIK and 30 eyes were treated using SMILE surgery for myopia and myopic astigmatism to evaluate CD and topography before and after 1 week and 2 months postoperatively using Pentacam HR.
Results There was an insignificant difference between the two groups in CD preoperatively (P=0.154), 1 week postoperatively (P=0.180), and 2 months postoperatively (P=0.138). In FS-LASIK group, there was a significant flattening in keratometric values (K1 and K2) of anterior corneal surface as the mean of K1 was 36.33±1.45 and 36.24±1.43 D and the mean of K2 was 37.09±1.55 and 37.00±1.52 D in comparison between 1-week postoperative and 2-month postoperative follow-up visits, respectively. There was an insignificant difference between the two groups in K1 and K2 of the posterior surface preoperatively, 1 week postoperatively, and 2 months postoperatively.
Conclusion There was an insignificant difference between the two groups in CD. In the anterior surface, FS-LASIK exhibited more flattening in K1 and K2 than SMILE. Regarding the posterior surface, there was an insignificant difference between FS-LASIK and SMILE.

Keywords: corneal densitometry, femtosecond laser-assisted laser in-situ keratomileusis, small-incision lenticule extraction, topography


How to cite this article:
Bakr EH, Ragab M, Elhennawy H, osman I. Corneal densitometry and topography after femtosecond laser-assisted laser in-situ keratomileusis compared with small-incision lenticule extraction. Delta J Ophthalmol 2019;20:107-11

How to cite this URL:
Bakr EH, Ragab M, Elhennawy H, osman I. Corneal densitometry and topography after femtosecond laser-assisted laser in-situ keratomileusis compared with small-incision lenticule extraction. Delta J Ophthalmol [serial online] 2019 [cited 2019 Oct 20];20:107-11. Available from: http://www.djo.eg.net/text.asp?2019/20/3/107/267941




  Introduction Top


Laser-assisted laser in-situ keratomileusis (LASIK) and small-incision lenticule extraction (SMILE) have proven to be safe and highly successful procedures with excellent outcomes and high patient satisfaction [1],[2].

Refractive surgery greatly affects the cornea. Laser is used to reshape the cornea to improve visual acuity. However, one of the common adverse effects is corneal haze. Corneal haze, the pathologic light scattered back to an observer during an examination, has been used as a mean of assessing the condition and optical quality of the cornea [3].

Haze can be measured by different methods. These methods can be divided into subjective and objective methods of assessment. Previously, the optical quality of the cornea was assessed and monitored by standard slit-lamp examination, that is, light scattering back to the observer, and the extent of severity was graded subjectively. This subjective evaluation tends to vary between observers [4]. Commencement of objective method was imperative to produce more reliable and reproducible results. Corneal densitometry (CD) provides information about corneal clarity and thus an indication of cornea health. Densitometry is expressed in gray scale unit from 0 to 100, where 0 means minimum backscattering of light (no clouding) and 100 means maximum backward scattering of light (opaque). Pentacam Scheimpflug camera is a noninvasive system that facilitates analysis of anterior segment and backscattered light simultaneously with automated detection of correct focus and alignment [5].

LASIK is intended to change the shape of the anterior surface of the cornea, thus changing its refractive power. As long as surgery affects the anterior corneal surface and weakens the cornea, a modification in the posterior corneal surface may be expected [6].

In this study, Pentacam HR was used to evaluate CD and topography 1 week and 2 months postoperatively after femtosecond (FS)-LASIK and SMILE.


  Patients and methods Top


The study was approved by the Local Ethical Committee of Alexandria University, and all patients signed a written informed consent before participation in the study. This is a prospective, nonrandomized, interventional comparative clinical study that included 60 eyes of 30 patients divided into two equal groups: 30 eyes were treated using FS-LASIK and 30 eyes were treated using SMILE surgery for myopia and myopic astigmatism.

All patients underwent, preoperatively, an ophthalmic evaluation that included uncorrected distance visual acuity, corrected distance visual acuity, manifest and cycloplegic refraction, slit-lamp examination of the cornea, dilated fundus examination, and finally corneal topography and CD analysis using Pentacam HR (Oculus, Wetzlar, Germany). Postoperative evaluation of CD and keratometry (K1 and K2) of anterior and posterior surfaces of the cornea were recorded after 1 week and 2 months postoperatively using Pentacam HR.

Inclusion criteria

The following were the inclusion criteria:
  1. Age more than 21 years and less than 40 years.
  2. Myopia from −6.0 to −10.0 D.
  3. Stable refraction for at least 6 months.
  4. Discontinuation of wearing soft contact lenses for at least 1 week.
  5. Discontinuation of wearing rigid contact lenses for at least 1 month.
  6. Regular corneal topography.


Exclusion criteria

The following were the exclusion criteria:
  1. Evidence of ocular diseases.
  2. Progressive unstable myopia.
  3. Corneal diseases like corneal dystrophy and keratoconus.
  4. History of trauma or surgery within the optical zone.
  5. Medications that affect wound healing like steroids and antimetabolites.
  6. Pregnancy and immunocompromised status.


Surgical technique

All surgeries were performed under topical anesthesia by the same surgeon (I.O.).

In FS-LASIK, after a flap was created with the VisuMax femtosecond laser system (Carl Zeiss Meditec AG, Jena, Germany), using pulse energy of 120–130 nJ, the refractive ablation was performed with the Allegretto excimer laser system (WaveLight Laser Technologie AG, Erlangen, Germany). The optical zone was 6.0–6.5 mm. The flap had a thickness of 100 µm.

SMILE is a fully flapless procedure during which only one type of laser is used. VisuMax femtosecond laser system with a repetition rate of 500 kHz was used to create an intrastromal refractive lenticule and a small incision. The surgeon then separates the lenticule from the surrounding tissue and extracts it through the small incision. In this study, the diameter of the optical zone was 5.5–6.5 mm. The thickness and diameter of the cap were 100 µm and 7.5 mm, respectively, with 2–3 mm side cut incision. The FS laser energy used during all SMILE procedures was 120–180 nJ.

Postoperative evaluation

All patients were followed up at 1 week and 2 months postoperatively. In each visit, complete ophthalmic examination was carried out. CD and K1 and K2 of the anterior and posterior corneal surfaces were measured at each visit using the same Pentacam HR.

Statistical analysis

Statistical analysis of the collected data was performed using IBM SPSS software package version 20.0. (IBM Corp., Armonk, New York, USA). A P value of less than 0.05 was considered statistically significant. Post-hoc test (Bonferroni) for comparison between different periods was used.


  Results Top


The study included 60 eyes of 30 patients divided into two groups: 30 eyes of 15 patients (six males and nine females), with mean age of 27.13±4.10 years, in the FS-LASIK group, and 30 eyes of 15 patients (five males and 10 females), with mean age of 27.20±5.23 years in the SMILE group. The mean spherical equivalent was −8.18±1.06 and −8.03±3.47 D in the FS-LASIK group and SMILE group, respectively.

There was an insignificant difference in CD between the two groups preoperatively, 1 week postoperatively, and 2 months postoperatively (P=0.138, [Table 1]).
Table 1 Comparison between the two studied groups according to corneal densitometry (Gray Scale Unit)

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In FS-LASIK group, there was a significant flattening in K1 and K2 of the anterior corneal surface in comparison between 1 week postoperatively and 2 months postoperatively. On the contrary, in the SMILE group, there was an insignificant flattening in K1 and K2 in comparison between 1 week postoperatively and 2 months postoperatively ([Table 2]).
Table 2 Comparison between preoperative and postoperative K1 and K2 of the anterior corneal surface in both groups

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There was an insignificant difference in K1 and K2 of the posterior corneal surface in comparison between the two groups preoperatively and postoperatively ([Table 3]).
Table 3 Comparison between the two studied groups according to K1 and K2 of the posterior corneal surface

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In the FS-LASIK group, the achieved optical zone was less than the intended optical zone as the mean of the intended optical zone was 6.22±0.25 mm and the mean of the achieved optical zone was 6.0±0.31 mm. There was a decrease in the mean by 0.21 mm. On the contrary, in the SMILE group, the achieved optical zone was larger than the intended optical zone as the mean of intended optical zone was 6.05±0.15 mm and the mean of the achieved optical zone was 7.33±0.27 mm (an increase in the mean by 1.28 mm, [Table 4]).
Table 4 Comparison between intended optical zone and achieved optical zone in the two groups (mm)

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  Discussion Top


The present study revealed an insignificant difference between the two groups in the CD preoperatively, 1 week postoperatively, and 2 months postoperatively. In both groups, there was an increase in CD 1 week postoperatively and significant reduction in CD 2 months postoperatively. Lazaridis et al. [7] studied the quantification of corneal clarity using the Scheimpflug CD and showed similar results before and 3 months after SMILE compared with FS-LASIK. No significant difference of corneal clarity was found 3 months postoperatively. There was a limitation of this study that it did not evaluate the short-term and transient effects over the first postoperative weeks, for example, 1 week and 1 month, whereas in the present study, we could evaluate the early outcome [7].

Such increase in CD after 1 week was explained by Yao et al. [8] who studied microdistortions in Bowman’s layer after SMILE using Fourier-domain optical coherence tomography and investigated the possible sources and potential visual effects. Microdistortions in Bowman’s layer were more in SMILE eyes than in FS-LASIK eyes. The amount decreased at 1 week and then remained stable [8]. However, there was an insignificant difference between the two groups in the current study, and this was explained by Dong et al. [9] who evaluated and compared early corneal wound healing and inflammatory responses after SMILE versus FS-LASIK. SMILE induces less keratocyte apoptosis, proliferation, and inflammation compared with FS-LASIK.

The present study revealed significant flattening in K1 and K2 of the anterior corneal surface 1 week postoperatively in both groups; this flattening changed the shape of the anterior surface of the cornea to achieve the refractive outcome. Two months postoperatively, FS-LASIK group showed further significant flattening in K1 and in K2. Gyldenkerne et al. [10] showed that corneal curvatures changed significantly in the anterior corneal surface but not in the posterior corneal surface, in both groups; after SMILE, the sagittal curvature was constant for the central 4-mm diameter, whereas in FS-LASIK, the curvature showed a gradual steepening with increasing diameter. These results were different from the current study. The effect of creating a corneal flap on corneal curvature is similar to that associated with large limbal relaxing incisions. Keratometric flattening after FS-LASIK, which regresses with wound healing, may be associated with excimer laser scanning and flap creation. Lim et al. [11] reported that healing and remodeling of the cornea occur mainly within the first 10 weeks following limbal relaxing incisions, and this augmented our results.

In the present study, there was an insignificant difference between FS-LASIK and SMILE in preoperative, first postoperative, and second postoperative K1 and K2 of the posterior corneal surface, but there was a significant difference in the SMILE group only in comparison between first and second postoperative values of K1. Ganesh et al. [12] reported that ReLEx SMILE causes significant changes in posterior corneal keratometric power and asphericity in moderate and high myopia. Gyldenkerne et al. [10] and Khairat et al. [13] found that corneal curvatures changed significantly in the anterior corneal surface but not in the posterior corneal surface, in both groups. These studies were in accordance with the current study results.

In the present study, in the FS-LASIK group, the achieved optical zone was less than the intended optical zone as the mean of the intended optical zone was 6.22±0.25 mm and the mean of the achieved optical zone was 6.0±0.31 mm reflecting a decrease in the mean by 0.21 mm (3.47%). However, in the SMILE group, the achieved optical zone was larger than the intended optical zone as the mean of the intended optical zone was 6.05±0.15 mm and the mean of the achieved optical zone was 7.33±0.27 mm with an increase in the mean by 1.28 mm (21.22%). Hou et al. [14] compared the effective optical zone after SMILE and FS-LASIK. The magnitude of decreases in effective optical zone was significantly smaller in the SMILE group than in the FS-LASIK group at all-time points after surgery .This study was similar to the present study results in the FS-LASIK group and different in the SMILE group, because in the current study, the achieved optical zone was larger than the intended optical zone [14].


  Conclusion Top


There was an insignificant difference between the two groups in CD. The FS-LASIK group exhibited more flattening in K1 and K2 of the anterior corneal surface than the SMILE group in comparison between the first and second postoperative visits. SMILE has better stability. Regarding the posterior corneal surface, there was an insignificant difference between FS-LASIK and SMILE. SMILE achieved larger optical zone than the intended, but FS-LASIK achieved smaller optical zone than the intended optical zone.

Financial support and sponsorship

Nil.

Conflicts of interest

There are no conflicts of interest.



 
  References Top

1.
Hjortdal JØ, Vestergaard AH, Ivarsen A, Ragunathan S, Asp S. Predictors for the outcome of small-incision lenticule extraction for myopia. J Refract Surg 2012; 28:865–871.  Back to cited text no. 1
    
2.
Kamiya K, Igarashi A, Ishii R, Sato N, Nishimoto H, Shimizu K. Early clinical outcomes, including efficacy and endothelial cell loss, of refractive lenticule extraction using a 500 kHz femtosecond laser to correct myopia. J Cataract Refract Surg 2012; 38:1996–2002.  Back to cited text no. 2
    
3.
Patel SV, Winter EJ, McLaren JW, Bourne WM. Objective measurement of backscattered light from the anterior and posterior cornea in vivo. Invest Ophthalmol Vis Sci 2007; 48:166–172.  Back to cited text no. 3
    
4.
Olsen T. Light scattering from the human cornea. Invest Ophthalmol Vis Sci 1982; 23:81–86.  Back to cited text no. 4
    
5.
Dhubhghaill SN, Rozema JJ, Jongenelen S, Ruiz Hidalgo I, Zakaria N, Tassignon MJ. Normative values for corneal densitometry analysis by Scheimpflug optical assessment. Invest Ophthalmol Vis Sci 2014; 55:162–168.  Back to cited text no. 5
    
6.
Dupps WJ Jr. Biomechanical modeling of corneal ectasia. J Refract Surg 2005; 21:186–190.  Back to cited text no. 6
    
7.
Lazaridis A, Droutsas K, Sekundo W, Petrak M, Schulze S. Corneal clarity and visual outcomes after small-incision lenticule extraction and comparison to femtosecond laser-assisted in situ keratomileusis. J Ophthalmol 2017; 2017:1–10.  Back to cited text no. 7
    
8.
Yao P, Zhao J, Li M, Shen Y, Dong Z, Zhou X. Microdistortions in Bowman’s layer following femtosecond laser small incision lenticule extraction observed by Fourier-Domain OCT. J Refract Surg 2013; 29:668–674.  Back to cited text no. 8
    
9.
Dong Z, Zhou X, Wu J. Small incision lenticule extraction (SMILE) and femtosecond laser LASIK: comparison of corneal wound healing and inflammation. Br J Ophthalmol 2014; 98:263–269.  Back to cited text no. 9
    
10.
Gyldenkerne A, Ivarsen A, Hjortdal JØ. Comparison of corneal shape changes and aberrations induced by FS-LASIK and SMILE for myopia. J Refract Surg 2015; 31:223–229.  Back to cited text no. 10
    
11.
Lim R, Borasio E, Ilari L. Long-term stability of keratometric astigmatism after limbal relaxing incisions. J Cataract Refract Surg 2014; 40:1676–1681.  Back to cited text no. 11
    
12.
Ganesh S, Patel U, Brar Sh. Posterior corneal curvature changes following Refractive Small Incision Lenticule Extraction. Clin ophthalmol 2015; 9:1359–1364.  Back to cited text no. 12
    
13.
Khairat YM, Mohamed YH, Moftah IA, Fouad NN. Evaluation of corneal changes after myopic LASIK using the Pentacam. Clin Ophthalmol 2013; 7:1771–1776.  Back to cited text no. 13
    
14.
Hou J, Wang Y, Lei Y, Zheng X. Comparison of effective optical zone after small-incision lenticule extraction and femtosecond laser-assisted laser in situ keratomileusis for myopia. J Cataract Refract Surg 2018; 44:1179–1185.  Back to cited text no. 14
    



 
 
    Tables

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



 

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