Comparison of Clinical Outcomes of Small-incision Lenticule Extraction (SMILE) Between Different Cap Thickness.

Summary

In the past two decades, the femtosecond laser (FSL) technology has been introduced in the corneal refractive surgery filed, and brought a remarkable innovation. It can make tissue dissection through photodisruption and plasma cavitation. Initially, the FSL was used predominantly to make a corneal flap when performing laser in situ keratomileusis (LASIK), which is followed by stromal ablation using excimer laser. A new surgical technique called femtosecond lenticule extraction (FLEx) has been developed that uses only FSL to dissect two interfaces to create refractive lenticule and then remove it, which is very similar with LASIK. Small incision lenticule extraction (SMILE) which is the advanced form of all-in-one FSL refractive technique does not make a corneal flap rather make small incision where the separated refractive lenticule is removed through, and the upper part of the corneal tissue is called cap. Since the clinical outcomes of SMILE were firstly published in 2011, SMILE has been widely used for correction of myopia or myopic astigmatism worldwide. SMILE provides excellent visual outcomes and has advantages including a lesser decrease in corneal sensitivity and absence of flap related complications compared to LASIK.

Because corneal ectasia after refractive surgery is the one of most terrifying complication, corneal biomechanics has been drawn interests to many researchers and clinicians. Theoretically, SMILE may preserve corneal biomechanics better than LASIK, because the anterior stroma which is stiffer than the posterior stroma remains intact in SMILE. However, there are some controversies, because previous studies investigating corneal biomechanics have been reported inconsistent outcomes, although SMILE has been reported equal to or better than LASIK. Weakening of corneal biomechanics and iatrogenic corneal ectasia have also been reported after SMILE. In addition, because the tensile strength of cornea gradually decreases as it goes backwards, creating deeper refractive lenticule may result in stronger cornea by preserving more of anterior lamellae of the cornea. But on the contrary, leaving sufficient residual stromal bed has been known to be important in preventing iatrogenic corneal ectasia, hence creating thin cap may be effective and desirable. Although many researches have been investigated the difference in biomechanical response between SMILE and LASIK, there are few studies evaluating the dependence of cap thickness on postoperative biomechanical strength after SMILE. El-Massry et al. reported that the thicker cap thickness showed higher postoperative corneal hysteresis (CH) and corneal resistance factor (CRF) with Ocular Response Analyzer (ORA; Reichert Ophthalmic Instruments, Depew, NY) which may not be optimal for a clear description of the viscosity and elasticity of the cornea,3 ; however, other studies have been presented that there is no significant difference of corneal biomechanics with cap thickness. There is no comparative human study using Corvis ST (Oculus, Wetzlar, Germany) despite presence of the study using Corvis ST in rabbit eyes. Furthermore, no prospective study with large number of subjects has been performed to date.

Conditions

• Myopia

Interventions

PROCEDURE

SMILE using 120μm cap thickness

The surgery was performed with standardized techniques with triple centration technique using the 500-KHz VisuMax system (Carl Zeiss Meditec AG, Jena, Germany). The superior cap depth was set as 120 or 140 µm, and the length of the side cut was set to 2 mm. Once the anterior (upper) and posterior (lower) planes of the lenticule were defined, the anterior and posterior interfaces were dissected using a micropetala with a blunt circular tip and extracted with midforceps. The integrity of the lenticule was assessed subsequently.

PROCEDURE

SMILE using 140μm cap thickness

The surgery was performed with standardized techniques with triple centration technique using the 500-KHz VisuMax system (Carl Zeiss Meditec AG, Jena, Germany). The superior cap depth was set as 120 or 140 µm, and the length of the side cut was set to 2 mm. Once the anterior (upper) and posterior (lower) planes of the lenticule were defined, the anterior and posterior interfaces were dissected using a micropetala with a blunt circular tip and extracted with midforceps. The integrity of the lenticule was assessed subsequently.

Sponsors & Collaborators

• Yonsei University

  lead OTHER

Study Design

Allocation

RANDOMIZED

Purpose

TREATMENT

Masking

NONE

Model

PARALLEL

Eligibility

Min Age

20 Years

Max Age

45 Years

Sex

ALL

Healthy Volunteers

No

Timeline & Regulatory

Start

2017-03-18

Primary Completion

2019-08-26

Completion

2019-08-26

Countries

• South Korea

Study Locations