Lower Femtosecond Laser Energy Levels Lead to Better Visual Recovery After Small-incision Lenticule Extraction for Myopia

Based on the review of literature, we regarded SMILE using energy levels of 115 nJ or higher as 'conventional' energy-SMILE (C-SMILE). Since 500kHz VisuMax femtosecond laser used in this study had a threshold photodisruption energy level of 100 nJ, lowest energy level of L-SMILE was set at 100 nJ. Because the spot distance must be changed by approximately 1 μm when laser energy changed by 50 nJ, we could set the maximum laser energy to 150 (100 + 50) nJ as highest one of C-SMILE while keeping the spot distance constant. Finally, we established two groups divided by different laser energies: L-SMILE group using lower energy levels of 100, 105, and 110 nJ, and C-SMILE group using 'conventional', not higher, energy levels of 115 to 150 nJ.

The SMILE procedures were conducted in the Eyereum Eye Clinic (Seoul, South Korea). The target postoperative refraction was emmetropia. The surgery, using standardized techniques, was performed by an experienced surgeon (D.S.Y.K.) using the VisuMax system. The surgical parameters used during SMILE were as follows: repetition rate of 500 kHz, pulse energy 100 to 150 nJ (100, 105, and 110 nJ, L-SMILE; 115 to 150 nJ, C-SMILE), spot distance 4.5 μm (regardless of FSL energy), cap thickness 120 μm, and side-cut width 2 mm in the 12-o'clock position with an angle of 90°. After the anterior (upper) and posterior (lower) delineated plane of the intrastromal lenticule were well defined, the anterior and posterior interface were dissected with a micro-spatula with a blunt circular tip and extracted with micro-forceps. The integrity of the lenticule was also checked subsequently.