Corneal biomechanical properties after SMILE and Femto-LASIK procedures (comparative study)

 INTRODUCTION

The cornea is a dome-shaped, clear, avascular tissue that clearly focuses light rays on the retina, forming a protective and impermeable layer against infectious pathogens and mechanical damage. The cornea must be smooth and transparent and must have a constant arch surface for proper refraction. In contrast, strong and elastic components are needed to support intraocular pressure and maintain regenerative biological protection.¹

Biomechanics is the study of mechanical laws determined by the structural elements of an organism or object. It helps to characterize function and understand the factors that affect pathophysiology. Corneal biomechanics study has been an interesting research matter and a focus of attention for many years. ²

This has led to the development of several methods of measuring ex vivo and in vivo corneal biomechanics. Current technology includes an imaging system combined with a non-contact air-puff tonometer, as in the Corvis ST (Oculus Optikgerate GmbH, Germany) and Ocular Response Analyzer (ORA, Reichert Ophthalmic Instruments, Buffalo, NY). Further techniques include Brillouin spectroscopy and optical coherence elastography. ³

The results of ORA may not directly explain the corneal mechanical behavior. Optical coherence tomography (OCT) or Scheimpflug imaging techniques can now directly measure corneal deformation. ⁴

Corvis ST and ORA are non-contact devices, both based on the use of an air-puff system. However, the parameters determined by both devices cannot be compared to each other, and there are some differences. The maximum pressure of the air puff is variable in ORA (depending on P1) and constant in Corvis ST. Also Corvis ST gives much more information regarding corneal deformation than ORA. Lastly the main parameters of ORA are based on applanation pressures P1 and P2, but Corvis ST depends on the dynamic corneal response (DCR). ³

The Corvis ST device takes 140 Scheimpflug images in a time of only 30 milliseconds (4,330 images per second). It focuses on the central corneal eight mm in a single horizontal meridian. It uses a 455 nm wavelength, UV-free blue LED. This device produces an air jet with a pressure up to 25 kilopascals. The cornea undergoes a well-characterized flattening cycle (applanation) until it is completely deformed, followed by a second flattening state to achieve a complete reformation. Immediately after the measurement, a quality score is available to assess the reliability of the measurement. The device is CE marked on all measurements, but can only be used in the United States to measure IOPs and CCTs. ⁵

The common refractive surgeries nowadays include photorefractive keratectomy, laser-assisted in situ keratomileusis and SMILE. ⁶

The femtosecond laser produces corneal flaps that are safer and more predictable in thickness than those produced by microkeratomes. In addition, flap adhesions are stronger and less susceptible to traum. Femtolasik also provides higher contrast sensitivity, less dry eye, and lower epithelial ingrowth rates. In addition, femtoseconds reduce the incidence of short flaps, buttonhole perforations, epithelial abrasions and blade marks. Although it is more likely to develop corneal haze, photosensitivity and rainbow glare after FS-LASIK. ⁷

SMILE was approved by the U.S. Food and Drug Administration in 2016 for the correction of nearsightedness with or without astigmatism as a "flapless" refraction correction laser technology using a single femtosecond laser system. ⁸

Utmost of the anterior stromal corneal lamellae are kept untouched in SMILE, it has been hypothesized that the biomechanical influence in SMILE is lower than in other laser vision correction procedures, implying that it may be secure for modulation of high refractive defects with a lower risk of ectasia. ⁹

In this study, we evaluated and compared the biomechanical behavior of the cornea after SMILE and FS-LASIK procedures to check the theoretical advantage of SMILE over FS-LASIK.

PATIENTS AND METHODS

This was a prospective comparative observational study. The study was carried out on forty eyes underwent SMILE procedures and other forty eyes underwent Femto-LASIK procedure. The study was carried out in the International Femto-LASIK Center (New Cairo).

All patients provided written informed consent after the nature of the procedure was explained in addition to the benefits and possible risks explained.

Both groups were subjected to detailed clinical assessment including assessment of unaided and best aided visual acuity, manifest and after cycloplegic drops refraction, Slit-lamp biomicroscopy to assess anterior segment, ocular fundus examination, intraocular pressure measurement: obtained by the Corvis ST system (biomechanically corrected IOP), corneal topography with Pentacam system(Oculus gmbh, Wetzlar, Germany) and measuring corneal biomechanical parameters with the Corvis ST preoperatively and postoperatively (at day one, three months and one year after the procedure). All participants were informed about the indications & the hazards of the procedures and signed a written informed consent.

We included candidates who were 18 years old or older, any sex is accepted, patients who had myopia or compound myopic astigmatism excluding those with hypermetropia or mixed astigmatism. All patients had stable refraction in the previous 2 years and had normal corneal morphology with normal corneal tomography. Contact lenses wear was prevented at least 2 weeks before surgery.

We excluded those with any corneal disease or had a previous ocular operation, those who had Keratoconus or suspected keratoconus, patients with previous ocular trauma, patients developed post-operative complications, females had pregnancy or lactation at time of follow-up and patients who had systemic disorders as diabetes.

Surgical techniques:

Topical anesthesia:

Multiple drops of Benoxinate hydrochloride 0.4% were used.

FS-LASIK procedure:

a flap was generated using a VisuMax femtosecond laser (Carl Zeiss Meditec®, Germany), the flap thickness was 110 μm and its diameter ranged from 7.9 to 9.0 mm. After flap creation and elevation, photoablation correction was applied in a diameter of 5.8 to 6.9 optical zone with a MEL-80 Excimer laser (Carl Zeiss Meditec®, Germany); then the flap was repositioned.

SMILE Procedures: The VisuMax femtosecond laser system (Carl Zeiss Meditec AG, Germany) was employed to design the corneal stromal lenticule, the laser pulse frequency was 500 kHz and pulse energy ranged from 130 to 160 nJ. The cap thickness designed to be between 110 and 120 μm, the diameter of the lenticule varied from 6.0 to 7.0 mm, and the corneal cap diameter designed to be 1.0 mm larger than the lenticule. A blunt spatula was used to first the front surface of the lenticule and then it’s back surface. After that the surgeon manually brought out the lenticule through a tiny opening (Figure 1).

Fig. 1: extraction of the lenticule through small incision

Postoperative medications:

We used topical 0.5% gatifloxacin q.i.d for one week, 1% prednisolone acetate t.i.d for one week and one tears substitute containing carboxy methyl cellulose sodium 0.5% for three months.

Assessment of corneal biomechanical parameters:

We used Corvis ST machine (software version Recalc 1.2b1036 RC) for measurement of corneal biomechanical parameters, IOP, CCT, and Scheimpflug video capture of the corneal deformation (Figure 2). The accepted videos only showed a smooth progression with no disturbances from eyelash or eyelid obstruction. If the video indicated an error, or the biomechanical parameters revealed a blank value, then up to 3 more attempts were performed in an effort to get a valid reading.

Fig. 2: video capture of corneal deformation parameters.

RESULTS

The collected data was tabulated, and statistically analyzed using SPSS program (Statistical Package for Social Sciences) software version 26.0, Microsoft Excel 2016 and MedCalC program software version 19.1. Descriptive statistics were performed for numerical parametric data as mean ± SD (standard deviation) and minimum & maximum of the range and for numerical non parametric data as median and 1st & 3rd inter-quartile range.

At the first follow up, assessment of Ocular Biomechanics revealed that the two studied groups were also comparable in respect to A1 length, A2 length, A1 velocity and A2 velocity showing no significant difference (p= 0.554, 0.221, 296 & 0.123 respectively). Also, there was no statistically significant difference between both groups regarding change in mean deformation, peak distance and radius (p= 0.079, 0.861 & 0.210 respectively) (table 1).

Table 1: Comparison between the studied groups regarding Ocular Biomechanics at first follow up.

At the second follow up, the two studied groups were also comparable in respect to A1 length, A2 length, A1 velocity and A2 velocity showing no significant difference (p= 0.430, 0.882, 1.0 & 0.491 respectively). Also, there was no statistically significant difference between the two groups regarding change in mean deformation, peak distance and radius (p= 0.831, 0.880 & 0.195 respectively) (table 2).

Table 2: Comparison between the studied groups regarding Ocular biomechanics at second follow up.

At the third follow up, the two studied groups were also comparable in respect to A1 length, A2 length, A1 velocity and A2 velocity showing no significant difference (p= 0.223, 0.895, 0.529 & 0.781 respectively). Also, there was no statistically significant difference between the two groups regarding change in mean deformation, peak distance and radius (p= 0.895, 0.263 & 0.455 respectively) (table 3).

Table 3: Comparison between the studied groups regarding Ocular biomechanics at third follow up.

Variation from preoperative values of the DCR and all time of follow- up of the SMILE group showed a statistically significant difference (reduction) as regard radius of curvature (p<0.001) and a statistically significant difference (increase) (p<0.001) as regard A1 velocity, deformation amplitude and peak distance (table 4).

Table 4: Difference between parameters during follow up in SMILE group

Variation from preoperative values of the DCR and all time of follow- up of the FS-LASIK group showed a statistically significant difference (reduction) as regard A1 length (p<0.001) and non-statistically significant difference (reduction) as regard radius of curvature and a statistically significant difference (increase) (p<0.001) as regard deformation amplitude (table 5).

Table 5:  Difference between parameters during follow up in Femto- LASIK group