EyeWorld Asia-Pacific December 2012 Issue

September 2012 39 Dec EWAP CORNEA Scientific rationale for why femtosecond laser intrastromal keratomileusis is the future of corneal refractive surgery by Dan Z. Reinstein, MD Dan Reinstein, MD, makes a compelling case for femtosecond lasers E ver since femtosecond lasers were first introduced into refractive surgery, the ultimate goal has been to create an intrastromal lenticule that can be manually removed in one piece, thereby circumventing the need for incremental photoablation by an excimer laser. Following the introduction of the VisuMax femtosecond laser (Carl Zeiss Meditec, Jena, Germany) in 2007, the intrastromal lenticule method was introduced in a procedure called femtosecond lenticule extraction (FLEx) in which a flap was created and lifted to allow the lenticule to be removed. Following the successful implementation of FLEx, a new procedure called small incision lenticule extraction (SMILE) was developed. This procedure involves passing a dissector through a small 2-3-mm incision to separate the lenticular interfaces and allow the lenticule to be removed, thus eliminating the need to create a flap. The results of the first prospective trials of SMILE have been reported, 1,2 and there are now more than 50 surgeons routinely performing this procedure worldwide. A number of femtosecond lasers are commercially available, but the VisuMax is the only one being used for an intrastromal lenticular cut. There are six distinct design elements of the VisuMax that represent how the device was conceived from the ground up as a high precision intracorneal lenticular cutting tool. Central to the VisuMax mode of operation is the extremely light-touch coupling system that minimizes corneal distortion and a rapid, high precision femtosecond pulse placement to achieve sufficient 3D geometric cutting precision such that refractive lenticules can be created accurately within the body of the stroma. The main elements are (1) the curved coupling contact glass, which results in minimal corneal distortion, (2) coupling suction applied to the peripheral cornea (not the conjunctiva/ sclera) allowing for a low suction force to immobilize the cornea, again reducing tissue distortion, (3) individual calibration of each contact glass based on confocal detection of radiation, (4) the Figure 3. Mean corneal sensation for 39 eyes after SMILE compared with the corneal sensation after LASIK averaged over nine published studies. Source (all): Dan Z. Reinstein, MD Figure 2. Diagrams demonstrating the difference between SMILE (top) and LASIK (bottom) in how the two procedures affect the anterior corneal nerve plexus. Figure 1. Diagrams of intact stromal lamellae after LASIK (top) and SMILE (bottom) highlighting the anterior lamellae that remain intact after SMILE. The RST calculations are shown for a 500-micron cornea with a 100-micron ablation/lenticule and 120-micron flap/cap thickness. The LASIK RST of 280 microns consists only of posterior stroma. The SMILE RST has the same 280 microns of posterior stroma, but also has 70 microns of anterior stroma, for a total of 350 microns of stroma. However, since anterior stroma is 50% stronger than posterior stroma, a further 35 microns can be added to make an effective total of 385 microns. continued on page 40