Comparison of corneal hysteresis and corneal resistance factor after small incision lenticule extraction and femtosecond laser-assisted LASIK: A prospective fellow eye study

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Abstract

Purpose

To compare corneal hysteresis (CH) and corneal resistance factor (CRF) between eyes treated with small incision lenticule extraction (SMILE) and femtosecond laser-assisted laser in situ keratomileusis (femto-LASIK).

Setting

Beyoğlu Eye Training and Research Hospital.

Design

Prospective comparative case series.

Methods

Sixty eyes from 30 patients with bilateral myopia or myopic astigmatism were studied. Inclusion criteria were spherical equivalent of subjective manifest refraction (SE) <10 diopters (D) and a difference ≤0.50 D between the SEs of both eyes. One eye of each patient was treated with SMILE, and the fellow eye underwent femto-LASIK. Randomization was performed using a sealed envelope system. The main outcome measures were CH and CRF measured preoperatively and postoperatively (1 and 6 months).

Results

Preoperative SE was similar in both groups (p = 0.852). CH and CRF values were reduced postoperatively in both groups compared to their corresponding preoperative values (p < 0.001). At the 6-month follow-up visit, the mean CH values in the SMILE and femto-LASIK groups were 8.95 ± 1.47 and 9.02 ± 1.27, respectively (p = 0.852), and the mean CRF values were 7.77 ± 1.37 and 8.07 ± 1.26, respectively (p = 0.380).

Conclusion

CH and CRF decreased after SMILE. There were no differences between SMILE and femto-LASIK treatments in postoperative CH or CRF values.

Introduction

Femtosecond lenticule extraction is a new method for refractive correction of myopia and myopic astigmatism. The procedure involves the creation of an intrastromal lenticule between two photodisruption planes that is mechanically removed for refractive correction. If the procedure involves creating and lifting a hinged flap above the lenticule, it is called femtosecond lenticule extraction (FLEX) [1]. If a flap is not created, and the lenticule is extracted from a 3- to 4-mm arcuate side cut close to the edge of the lenticule, the procedure is referred to as small incision lenticule extraction (SMILE) [2]. These procedures can only be carried out with the Visumax femtosecond laser platform (Carl Zeiss Meditec AG, Jena, Germany). Lenticule extraction is an efficient and safe surgical procedure with refractive results comparable to those achieved with laser-assisted in situ keratomileusis (LASIK) [3].

LASIK alters corneal biomechanical properties that are thought to play an important role in the development of post-LASIK ectasia [4]. SMILE may have biomechanical benefits over LASIK because it does not involve the creation of a flap and leaves the stroma over the lenticule untouched. However, there are no published studies regarding the biomechanical effects of SMILE.

The ocular response analyzer (ORA; Reichert Inc., Buffalo, NY, USA) is designed to obtain in vivo measurements of corneal biomechanical properties [5]. Corneal hysteresis (CH) and corneal resistance factor (CRF) are two metrics used in this device to describe the biomechanical properties of the cornea. Previous studies reported that CH and CRF significantly decrease after LASIK surgery [6], [7], [8], [5]. Here, we performed the first analysis of CH and CRF values after SMILE and compared them with the results of femtosecond LASIK (femto-LASIK) in fellow eyes.

Section snippets

Patients and methods

This prospective pilot study was approved by the ethics committee of Beyoğlu Training and Research Hospital. All patients provided informed consent, and the study complied with the Declaration of Helsinki.

Patients with bilateral myopia or myopic astigmatism were included in the study if the spherical equivalent of subjective manifest refraction (SE) was <10 diopters (D) and if the difference in SEs between both eyes was ≤0.50 D. Other inclusion criteria were mesopic (4 lux) pupil size ≤6.5 mm and

Results

Preoperative patient characteristics are shown in Table 1. Preoperative SE of manifest refraction and maximum thickness of the removed tissue were not statistically different between the SMILE and LASIK groups (p > 0.05).

There were no statistically significant differences in CH and CRF values between the SMILE and LASIK groups in any pre- or postoperative measurements (Table 2, Table 3).

In both groups, postoperative CH values at 1 and 6 months were significantly lower than the preoperative values

Discussion

The ORA device measures the biomechanical parameters CH and CRF; the manufacturer claims that these values characterize the viscoelastic properties of the cornea. Previous studies found that CH and CRF were significantly decreased after LASIK surgery, and greater attempted corrections correlated with greater reductions in CH and CRF [6], [7], [8], [5]. In addition, eyes with keratokonus have lower CH and CRF values compared to normal eyes [6]. These facts suggest that CH and CRF are lower in

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