SEP 2012

EyeWorld is the official news magazine of the American Society of Cataract & Refractive Surgery.

Issue link: https://digital.eyeworld.org/i/82503

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80 EWINTERNATIONAL September 2012 Scientific rationale for why femtosecond is the future of corneal refractive surgery by Dan Z. Reinstein, M.D. E W e are in the age of the femtosec- ond laser. Having first found utility in creating a flap for LASIK procedures, it became clear that the potential for other uses in eye surgery was significant. Most recently, cataract surgery femtosecond applications have been the focus of development for laser compa- nies. Now with the clinical trial of the VisuMax laser under way, a true refractive platform may be possible with only the use of a femtosecond laser. As with all new technology, there will no doubt be contin- ued improvement, and we can expect that the results will improve. Is intrastromal keratomileusis the future of corneal refrac- tive surgery? Dan Reinstein, M.D., makes a compelling case in this month's "Interna- tional outlook" column. One thing is cer- tain: Refractive surgeons around the world will be watching this carefully. Read for yourself and form your own opinion. John Vukich, M.D., international editor 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 incre- mental 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 proce- dure called femtosecond lenticule extraction (FLEx) in which a flap was created and lifted to allow the lenticule to be removed. Following the successful imple- mentation of FLEx, a new procedure called small incision lenticule extrac- tion (SMILE) was developed. This procedure involves passing a dissec- tor through a small 2-3 mm incision to separate the lenticular interfaces and allow the lenticule to be re- moved, 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 perform- ing 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 ele- ments 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 mini- mizes corneal distortion and a rapid, high precision femtosecond pulse placement to achieve sufficient 3D geometric cutting precision such that refractive lenticules can be cre- ated 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 suc- tion applied to the peripheral cornea (not the conjunctiva/sclera) allow- ing for a low suction force to immo- bilize the cornea, again reducing tissue distortion, (3) individual cali- bration of each contact glass based on confocal detection of radiation, (4) the combination of an optical beam path that is suspended on a fulcrum with force-feedback servo control of the height of the patient bed and headrest in order to main- tain a consistent contact glass force onto the cornea (again minimizing tissue distortion), (5) a high numeri- cal aperture of the beam designed to deliver a tight concentration of fem- tosecond energy with low per-pulse energy load, and (6) a high pulse repetition rate of 500 kHz in order to minimize treatment time. Advantages of ReLEx SMILE over LASIK Figure 1: Diagrams of the 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 μm cornea with a 100 μm ablation/lenticule and 120 μm flap/cap thickness. The LASIK RST of 280 μm consists only of posterior stroma. The SMILE RST has the same 280 μm of posterior stroma, but also has 70 μm of anterior stroma, for a total of 350 μm of stroma. However, since the anterior stroma is 50% stronger than posterior stroma, a further 35 μm can be added to make an effective total of 385 μm Figure 2: Diagrams demonstrating the difference between SMILE (top) and LASIK (bottom) in how the two procedures affect the anterior corneal nerve plexus These new femtosecond intrastromal lenticule procedures offer a number of potential advantages. 1. More accurate and repeat- 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, M.D. able tissue removal: Intrastromal lenticule procedures may have advantages over LASIK as all of the potential errors associated with excimer laser ablation are avoided, such as stromal hydration, laser flu- ence projection and reflection losses, and other environmental factors. In ReLEx, the tissue removal is defined only by the accuracy of the optome- chanics of the femtosecond laser, none of which is affected by envi- ronmental conditions. Therefore, it is likely that there will be less need for personalized nomograms to be used for different machines, loca- tions, or surgeons. In addition, the accuracy remains similar for low and high corrections as the only variable is the distance between the upper and lower lenticular cuts. 2. Increased biomechanical stability and reduced biomechani- cal variability: Potential benefits of SMILE are the relatively higher bio- mechanical strength of the remain- ing cornea as well as a reduction in the variability of the biomechanical effects produced by refractive tissue removal. Randleman et al3 published a study in 2008 in which they meas- ured the tensile strength of strips of stromal lamellae cut from different depths within the cornea. They found a strong negative correlation between stromal depth and tensile strength. The anterior 40% of the central corneal stroma was found to be the strongest region of the cornea, whereas the posterior 60% of the stroma was at least 50% weaker. We are accustomed to calculat- ing the residual stromal thickness in LASIK as the amount of stromal tissue left under the flap, so the first instinct is to apply this rule to SMILE. However, because there is no flap created in SMILE, the anterior stromal lamellae remain intact everywhere except for the small 2-3 mm incision. Therefore, the actual residual stromal thickness in SMILE is calculated as the stromal thickness below the posterior lenticule inter- face plus the stromal component of the overlying cap—between the an- terior lenticule interface and the ep- ithelium. Moreover, because anterior stroma is 50% stronger than poste- rior stroma, a further 50% of the untouched anterior stromal thick- ness can be added to get a residual stromal thickness value that can be compared to a LASIK residual stro- mal thickness. If this concept is applied to an example case (Figure 1), the signifi- cant improvement in biomechanics can be appreciated. Consider an eye with a spherical equivalent refrac- tion of –8.00 D and a central corneal pachymetry of 500 µm. We will as- sume that the epithelial thickness is 50 µm, that the LASIK excimer laser ablation depth and SMILE lenticule thickness are 100 µm, and that both procedures were performed under a

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