|Year : 2020 | Volume
| Issue : 3 | Page : 146-152
Corneal biomechanical properties after laser-assisted in situ keratomileusis and small-incision lenticule extraction in myopic eyes
Basma ElShourbagy1, Alaa A El Dorghamy2, Ahmed M Ghoneim2, Hazem A Elbedewy2
1 Department of Ophthalmology, Tanta Health Insurance Hospital, Tanta, Egypt
2 Department of Ophthalmology, Faculty of Medicine, Tanta University, Tanta, Egypt
|Date of Submission||20-Mar-2020|
|Date of Decision||21-Apr-2020|
|Date of Acceptance||07-May-2020|
|Date of Web Publication||23-Sep-2020|
MD, FRCS Hazem A Elbedewy
Department of Ophthalmology, Faculty of Medicine, Elgeish Street, Tanta 31511
Source of Support: None, Conflict of Interest: None
Purpose The aim of this study was to compare the differences in corneal biomechanical changes after laser-assisted in situ keratomileusis (LASIK) and femtosecond laser small-incision lenticule extraction (SMILE) using the corneal visualization Scheimpflug technology (Corvis ST).
Patients and methods This is a prospective study that included two groups of myopic patients. Group 1 included 20 eyes that underwent LASIK surgery, and group 2 included 20 eyes that underwent SMILE surgery. The corneal biomechanical properties were assessed preoperatively and three months postoperatively using the Corvis ST in both groups. The main evaluated parameters included the first and second applanation (A1 and A2) time, absolute velocity and length, highest concavity (HC) time and radius, peak (P) distance, HC deflection amplitude, deformation amplitude, and Corvis biomechanical index.
Results There was no significant difference between the two groups regarding all preoperative biomechanical parameters measured by Corvis ST. At the third postoperative month, there was no significant difference between the two groups regarding A1 length or absolute velocity, whereas A1 time, A2 time, A2 length, absolute A2 velocity, and HC time were significantly lower among patients in the LASIK group than patients in the SMILE group. On the contrary, HC radius, P distance, HC deflection amplitude, deformation amplitude, and Corvis biomechanical index were significantly higher among patients in the LASIK group than patients in the SMILE group.
Conclusion SMILE and LASIK procedures substantially alter the corneal biomechanical properties measured by the Corvis ST. The LASIK procedure seemed to result in less tensile strength and more compliant cornea when compared with the SMILE procedure in myopic corrections. This denotes greater reduction in biomechanical stability in the LASIK procedure than in SMILE.
Keywords: corneal biomechanics, Corvis biomechanical index, laser-assisted in situ keratomileusis, myopia, small-incision lenticule extraction
|How to cite this article:|
ElShourbagy B, El Dorghamy AA, Ghoneim AM, Elbedewy HA. Corneal biomechanical properties after laser-assisted in situ keratomileusis and small-incision lenticule extraction in myopic eyes. Delta J Ophthalmol 2020;21:146-52
|How to cite this URL:|
ElShourbagy B, El Dorghamy AA, Ghoneim AM, Elbedewy HA. Corneal biomechanical properties after laser-assisted in situ keratomileusis and small-incision lenticule extraction in myopic eyes. Delta J Ophthalmol [serial online] 2020 [cited 2020 Oct 20];21:146-52. Available from: http://www.djo.eg.net/text.asp?2020/21/3/146/295878
| Introduction|| |
Laser-assisted in situ keratomileusis (LASIK) is the current laser refractive procedure of preference to deal with myopia. Nowadays, refractive lenticule extraction has been introduced as a single-laser refractive system without using an excimer laser. Small-incision lenticule extraction (SMILE) is a variation of refractive lenticule extraction that needs no retractable flap . Laser refractive surgical treatment is broadly accepted as a secure and efficient treatment of myopia. However, the removal of corneal tissue unavoidably causes biomechanical weakening of the cornea .
The fear of developing postoperative ectasia drew the eyes of investigators toward the significance of corneal biomechanics. The cornea acts as a viscoelastic material having factors of each viscosity and elasticity .
It is realized that corneal refractive surgery influences corneal biomechanical properties. There are many reviews showing that techniques including flaps specifically negatively affect corneal biomechanical properties . Corneal biomechanics are a focal point of studies nowadays, not only owing to their influence on the intraocular pressure (IOP) evaluation but also for their influence on the corneal structural integrity after refractive surgical procedures .
Before 2005, the main technique for evaluating the corneal biomechanics was just in cadaveric eyes. The Ocular Response Analyzer was created as a noncontact tonometer discrediting the effect of corneal biomechanics and corneal thickness on IOP estimation. The corneal visualization Scheimpflug technology (Corvis ST) was presented in 2010 as a brand new device for measuring IOP and corneal biomechanics .
The Corvis ST makes use of a steady air puff of a noncontact tonometer which forces the cornea through several stages that are detailed on the Scheimpflug images and are recorded on a video. There are nine phases of the dynamic corneal deformation; the initial phase, preceding the deformation, then the ongoing phase in which the cornea deforms inward while keeping its convex shape, the next phase is the moment of the first applanation (A1) or flattening of the cornea, after which there is the ongoing phase of concavity, where the cornea deforms and takes on a concave shape. The next phase is called the phase of oscillation, containing the moment of maximum corneal bending, and then the outgoing concave phase, when the cornea with its concave shape starts to return to its normal convex shape. After that, the cornea reaches the phase of the second applanation (A2) followed by the outgoing convex phase, until the cornea reaches its normal convex shape, in which occurs the last phase, the so-called phase after corneal deflection .
The effects of LASIK, which requires flap, and SMILE, which is a flapless technique, on corneal biomechanics have not been fully investigated yet . The aim of this study was to compare the corneal biomechanical changes after LASIK and SMILE procedures using the Corvis ST parameters.
| Patients and methods|| |
This is a prospective study that was carried out at El-NOKHBA Eye and Laser Center from January 2018 to May 2019. The study was approved by the Ethics Committee of the Faculty of Medicine, Tanta University, and was done in accordance with the Declaration of Helsinki and its later amendment. A written informed consent to participate and publish data was taken from every participant or his/her legal guardian if he/she was less than 21 years old before the study.
The study was conducted on 40 eyes divided into two groups. The first group included 20 eyes which had undergone LASIK procedure, and the second group included 20 eyes which had undergone SMILE procedure. The Corvis ST (OCULUS Corvis; Corneal Visualization Scheimpflug Technology, Wetzlar, Germany) measured the corneal biomechanical parameters preoperatively and three months postoperatively. The Corvis ST parameters included the following ,:
- The first and second corneal applanation (A1 and A2): expressed as time [time from the measurement beginning to the first or second applanation moment (ms)], length [length of flattened cornea at the first or second applanation (mm)], and velocity [velocity of corneal apex during the first or second applanation (mm/ms)].
- Highest corneal concavity (HC): expressed in time and radius (radius of curvature of the corneal concavity at point of HC).
- Peak distance (P distance): distance between corneal peaks at point of HC.
- Maximum deformation amplitude (DA): maximum inward movement of corneal apex at point of HC.
- Highest concavity deflection amplitude (HCDefl Amp): corneal deflection amplitude at point of HC.
- Corvis biomechanical index (CBI): calculated by the machine from the previous parameters.
The patients in both groups were matched for age, sex, and preoperative Corvis ST parameters. The patients were allocated randomly in either of the two groups by simple equal randomization using randomly generated numbers by a computer system.
The study included patients 18 years of age or older with a spherical equivalent (SE) of −3.00 to −8.00 diopters (D) and refractive cylinder less than −4.00 D, with stable refraction for at least 1 year before surgery. Soft contact lens wearers must have removed contact lenses at least 1 week before the baseline measurement.
Patients with central corneal thickness (CCT) less than 500 μm, residual corneal bed thickness less than 300 μm, progressive or unstable myopia and/or astigmatism, and clinical or corneal topographic evidence of keratoconus were excluded. Patients with residual, recurrent, or active ocular disease such as uveitis, media opacities, history of herpetic keratitis, severe dry eye, severe allergic eye disease, glaucoma, visually significant cataract, and retinal disease, previous corneal surgery or trauma within the corneal flap zone, or corneal vascularization within 1 mm of the corneal flap zone were also excluded. In addition, patients taking systemic medications or having diseases likely to affect wound healing (such as corticosteroids, antimetabolites, diabetes, history of connective tissue disease) were also excluded from the study.
All patients were subjected to routine preoperative ophthalmologic examination, including uncorrected distance visual acuity, manifest and cycloplegic refraction, corrected distance visual acuity, slit-lamp examination, and fundus examination. Preoperative IOP was measured by Goldman applanation tonometer (Haag-Streit International, Koeniz, Switzerland). Corneal topography and CCT were measured with the Pentacam (Allegro Oculyzer, WaveLight, GmbH, Erlangen, Germany). Corneal biomechanics were measured by the Corvis ST.
All LASIK procedures were done using the M-2 microkeratome (Moria, Antony, France) to create the flap, which was adjusted to 110 µm. The laser was applied using the Wavelight Allegretto excimer laser-400 (Alcon, Fort Worth, Texas, USA), and the surgical steps were performed according to Steinert et al. .
All SMILE procedures were done using Platform Visumax 500 (Carl Ziess Meditec, Jena, Germany) with constant cap thickness of 100 µm. All SMILE surgical steps were performed according to Reinstein et al. .
During the follow-up visits, all patients were assessed for uncorrected/best-corrected distance visual acuities, manifest refraction, and slit-lamp examination. Corneal topography and CCT were assessed by Pentacam, whereas corneal biomechanics were assessed by the Corvis ST.
Data were analyzed by Statistical Package for the Social Sciences, version 23 (SPSS Inc. Released 2015. IBM SPSS statistics for Windows, version 23.0; IBM Corp., Armonk, New York, USA). Student t test was used for comparison of quantitative variables between two groups of normally distributed data, whereas Mann–Whitney’s test was used for comparison of quantitative variables between two groups of not normally distributed data. χ2 test was used to study association between qualitative variables. Whenever any of the expected cells were less than five, Fisher’s exact test was used. Two-sided P value less than 0.05 was considered statically significant.
| Results|| |
The mean age of the LASIK group was 28.90±6.15 years and of the SMILE group was 26.80±3.20 years. Males represented 60% of the LASIK group and 80% of the SMILE group. As the two groups were matched, there was no statistically significant difference between them regarding age or sex (P=0.184 and 0.168, respectively).
There was no statistically significant difference between the LASIK and the SMILE groups regarding preoperative sphere, cylinder, SE, CCT (μm), and IOP (mmHg) ([Table 1]). In addition, there was no statistically significant difference between the two groups regarding Corvis ST parameters [A1 time (ms), A1 length (mm), A1 velocity (mm/ms), A2 time (ms), A2 length (mm), A2 velocity (mm/ms), HC time (ms), HC radius (mm), P distance (mm), HCDefl Amp (mm), and DA (mm) during HC or CBI].
Both groups have shown very close postoperative results regarding sphere, cylinder, SE, CCT, and IOP with no statistically significant difference between them ([Table 2]).
At the third postoperative month, there was no statistically significant difference between the two groups regarding A1 length or absolute A1 velocity. However, A1 time, A2 time, absolute A2 velocity, HC time, and A2 length were significantly lower among patients in the LASIK group than patients in the SMILE group. On the contrary, HC radius, P distance, HCDefl Amp, DA, and CBI were significantly higher among patients in the LASIK group than patients in the SMILE group ([Table 3]).
|Table 3 Comparison between the two groups regarding preoperative and third postoperative month Corvis parameters|
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A1 time decreased postoperatively (at the third postoperative month) in both groups but only significantly in the LASIK group. A1 length was reduced but not to a significant level in both groups. A1 velocity was reduced in both groups but not to a significant level. A2 time, HC time, and A2 length were reduced significantly in both groups, but the reduction was more in the LASIK group. HC radius was reduced significantly in both groups but the reduction was more in the SMILE group. The P distance, HCDefl Amp, DA, and CBI increased significantly in both groups, but the increment was more in the LASIK group. The absolute A2 velocity was increased significantly in both groups but the increment was more in SMILE group ([Table 3] and [Figure 1]).
|Figure 1 A1: Preoperative Pentacam of a patient in LASIK group, A2: preoperative Corvis ST, and A3: Corvis ST at the third postoperative month of the same patient. B1: preoperative Pentacam of a patient in SMILE group, B2: preoperative Corvis ST, and B3: Corvis ST at the third postoperative month of the same patient. LASIK, laser-assisted in situ keratomileusis; SMILE, small-incision lenticule extraction.|
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| Discussion|| |
The application of laser to the corneal tissue produces inevitable changes to the corneal biomechanics. In this study, there was no statistically significant preoperative difference between the LASIK and SMILE groups regarding the potentially confounding factors like age, sex, preoperative sphere, cylinder, SE, CCT, or IOP.
The results of this work showed that the DA during HC had a significant increase from preoperative to postoperative values in both groups. The increment was more in the LASIK group (about two folds) than in the SMILE group, suggesting a more severe inward deformation during the air pulse after LASIK, possibly owing to a more compliant cornea, denoting much lower biomechanical changes. This was in agreement to the study done by Ahmed et al.  but was different from the study by Osman et al.  who found a fivefold increase of DA in the LASIK group.
The HCDefl Amp showed a significant increase from the preoperative to the postoperative values in both groups. This increase was much more in the LASIK group (about two folds) than in the SMILE group, denoting much more mobility of the cornea in this group.
The mean postoperative A1 time and A2 time were longer in the SMILE group than in the LASIK group, denoting slower movements of the cornea in respect to the air puff (i.e. stiffer cornea), which reflects a less compliant cornea after the flap-free procedure. Similar results were found in previous studies by Osman et al.  and Damgaard et al. .
The HC time was significantly shorter after LASIK compared with SMILE procedure. This can be attributed to the corneal flap creation, where more collagen fibers are cut in the LASIK procedure, which may lead to a more compliant cornea that reaches HC faster than in SMILE. The same observation was stated by Osman et al. , Damgaard et al. , and Pedersen et al. .
The A1 length showed nonsignificant postoperative decrease in both groups, whereas A2 length deceased significantly in both groups, with a significant decrease in the LASIK group than in the SMILE group. Osman et al.  and Ahmed et al.  did not find significant differences between preoperative and postoperative A1 and A2 lengths in LASIK or SMILE groups.
The absolute A1 velocity did not show any significant change preoperative or postoperatively or even between the two groups. The absolute A2 velocity was significantly increased in both groups, with more significant increase in the SMILE group than in the LASIK group. The P distances showed also a significant postoperative increase in both groups, which was higher in the LASIK group. The postoperative HC radius decreased significantly in both groups, with more reduction in the SMILE group. Both findings denote a more compliant cornea after LASIK procedure. The same was reported by Osman et al. .
The different Corvis parameters are used by the device to automatically calculate the CBI. The CBI was significantly much higher in the LASIK group compared with the SMILE group, which again means a more compliant cornea after the LASIK procedure.
In the LASIK group, the mean postoperative values of deformation and deflection amplitudes were nearly two times higher than in the SMILE group, suggesting a more severe inward deformation during the air pulse. In addition, A1 time, HC time, and A2 time were significantly shorter after LASIK procedure compared with the SMILE procedure, which means that the cornea reached first applanation, HC, and second applanation faster in the LASIK group. This means more stiffed corneas in the SMILE group which resist deformation during air pulse.
Some authors explained the difference of biomechanical properties after LASIK and SMILE. Most of the tensile strength of the cornea is located in the anterior part which is characterized by more interwoven collagen fibers than the rest of the cornea . SMILE induces less damage to the collagen lamellae than LASIK; hence, we can expect less changes in the biomechanical indices after SMILE than after LASIK. The LASIK flap created by the microkeratome extends deeply in the peripheral anterior corneal layers, which are stronger than the center, thus breaking through the more biomechanically important collagen bundles . Another important point is the difference between the flap to cap diameters in both procedures. In LASIK surgery, the flap should be bigger than the transition zone (> 8.5 mm), but in SMILE, the cap is usually less than 8 mm which saves cutting through the stronger peripheral anterior corneal collagen bundles .Sonigo et al.  and Schmack et al.  suggested that the healing pattern can also affect the biomechanical properties of the cornea in both procedures. It tends to be more inflammatory after SMILE surgery, resulting in stronger fibrotic scarring.
| Conclusion|| |
According to the Corvis ST parameters, it was noticed that both SMILE and LASIK procedures substantially altered the corneal biomechanical properties. LASIK procedure seemed to result in less tensile strength and more compliant cornea when compared with, SMILE procedure in myopic corrections denoting greater reduction in biomechanical stability in the LASIK procedure.
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Conflicts of interest
There are no conflicts of interest.
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[Table 1], [Table 2], [Table 3]