Comparison of Clinical Outcomes of Small-incision Lenticule Extraction (SMILE) Between Different Cap Thickness.
Condition: Myopia
MeSH terms: Surgical Wound
Study type: Interventional

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.
    


Intervention name(s): SMILE using 120μm cap thickness; SMILE using 140μm cap thickness
Intervention type(s): Procedure; Procedure
Intervention description(s): 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.; 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.
Allocation: Randomized
Intervention model: Parallel Assignment
Intervention model description: Enrollment period : 24 months after IRB approval Methods: The subjects are randomly divided into two groups. One group underwent SMILE surgery using 120 μm cap thickness, and the other group underwent SMILE surgery using 140 μm cap thickness. Before surgery, all patients underwent a detailed ophthalmological examination that included evaluation of logarithm of the minimum angle of resolution (logMAR) uncorrected-distance visual acuity (UDVA) and CDVA, manifest refraction, slit-lamp examination (Haag-Streit, Köniz, Switzerland), keratometry, and Scheimpflug-based corneal topography (Pentacam HR, Oculus). Dynamic corneal response (DCR) parameters were examined using Corvis ST. Corneal wavefront aberrations were measured using Keratron Scout (Optikon 2000, Rome, Italy). All examinations were repeated at 1, 3, and 6 months after surgery.
Masking: None (Open Label)
Primary purpose: Treatment
Keywords: Small incision lenticule extraction; Cap thickness