I started performing Small Incision Lenticule Extraction in my clinic in 2015. I had been performing flap-based refractive surgery for just under a decade at that time, but the demand for SMILE came from the public. I was completely happy with my flap-based practice, and I was a little skeptical about the benefits of SMILE, but refractive surgery patients are well informed in Finland and interested in minimally invasive techniques. What was happening in our profession in 2015 was part of a wider trend in medicine, and I understood that the move to minimally invasive, endoscopic surgery represented a logical next step in refractive surgery as it did in every other field of surgery.
Now after careful clinical evaluation over the past 4 years, I am in the position to summarize my experience with SMILE and the VisuMax femtosecond laser technology (Carl Zeiss Meditec) used to perform the procedure (Figure 1).
Figure 1. The VisuMax femtosecond laser.
In my practice, roughly 60% of patients presenting for refractive surgery undergo SMILE, and the rest are treated with femtosecond LASIK (femto-LASIK). At the moment, the standard cap thickness for my SMILE procedures is between 100 and 135 μm, and the flap thickness for femto-LASIK is between 80 and 90 μm. Corneas thicker than 580 μm get a flap of more than 100 μm. For the first time in my clinical work, I am fully happy with the capacities of the technology available to us. In our clinic, we use the VisuMax femtosecond laser for SMILE and the MEL 90 excimer laser (Carl Zeiss Meditec) for femto-LASIK.
PATIENT SELECTION AND PERSONAL NOMOGRAMS
My interest over the past 4 years has been to identify what patients would benefit most from SMILE and what patients would be more suitable for an open-flap procedure.
For the most part, anterior corneal shape determines if a patient is suitable for SMILE. It is good to have symmetrically shaped corneas, with minimal difference between the astigmatic poles. It is also important that the corneal apex is located quite centrally. This makes positioning of the lenticule on the visual axis easier. Therefore, my current indications for SMILE are relatively young patients who have fairly symmetrical corneas; this represents the majority of clients coming to our clinic. On the other hand, patients with corneal astigmatism greater than 3.00 D and older patients are generally better candidates for a flap-based treatment like femto-LASIK.
If the cornea is very asymmetric, the first lens touch in the central cornea may dislocate the position of the lenticule. In these cases, femto-LASIK is preferable, although in my experience SMILE is more tolerant to fluctuations in positioning than excimer ablation. In both cases, it is important to track the position of the optical axis with topography maps because patients might not always properly fixate their sight on the alignment beam.
The SMILE nomogram can be refined over time by the surgeon together with a ZEISS regional clinical application specialist. Initially SMILE technology tends toward slight undercorrection, so it is important to have personal hands-on refraction data both pre- and postoperatively in order to be able to hit the sweet spot of optimal results.
optimizing the energy
Energy offset is an important topic. In early experience with SMILE, it is ok to use higher energy input to ensure smooth extraction of the lenticule. In reality, the VisuMax femtosecond laser rarely leaves adhesions between the lenticule and the cornea if the interface between the lens and the cornea is clear. Lowering the energy offset to the optimal level is best done with a ZEISS expert, but finer adjustments of energy offset can be done independently.
In my clinical experience, the best performance of the laser is obtained with a certain geometrical pattern in the cut. Although the appearance of black spots on the cornea is usually considered a complication of SMILE, a few visible at the cut site is nothing to worry about; it indicates that the energy offset is approaching the critical threshold level.
It is important to keep in mind that the less energy used in creating the lenticule, the quicker the visual rehabilitation will be. Most often, patients leave the clinic on the day of the procedure with binocular vision between 0.7 and 1.0, and the next day they have 1.0 to 1.2 monocular vision with clear corneas if the energy level during the procedure was right. The incidence of haze is a sign that energy levels were too high during the cut.
It is rare to see a prolonged dry eye episode in SMILE patients. Therefore, I always recommend SMILE in patients with signs of transient dry eye symptoms.
FUTURE TRENDS
In the future, I anticipate a continued drive toward even better optical outcomes, as has always been the case in refractive surgery. The next step is that we probably will start routinely concentrating on higher-order aberrations, particularly coma levels. I currently measure coma levels in every patient after surgery, and these results indicate that SMILE is better tolerant of coma compared to femto-LASIK. The most likely explanation is that, during lenticule formation, the eye does not move in relation to the laser beam. On the other hand, during an excimer laser ablation, the eye tracker fixes the ablation beam according to pupillary borders through the corneal tissue. When an eye moves slightly during the ablation, the location of the ablation on the cornea deviates from the planned treatment, as the surface of ablation is not on the same plane as the pupil. For that reason, I perform high myopia excimer ablations in two to three sequences, and it seems to help lower postoperative coma levels in highly myopic eyes.
I have also encountered postoperative coma with SMILE, usually if the interface is not carefully docked and does not take into account patient fixation and the location of the optical axis on topographical maps. The difference, however, is that the presence of coma is usually not a clinically significant phenomenon in SMILE. For that reason, I think the technique is more forgiving in regards to coma.
In order to achieve premium results, we must keep our lenses clear—metaphorically and literally. As professionals, we know the importance of defining the quality of our outcomes, but even patient satisfaction does not tell the whole story about quality. Making the effort to dig down into the details of postoperative residual refraction and use it to improve personal nomograms will lead to increased precision and benefit our future patients.
Achieving an excellent result always comes from a combination of surgical skill, thinking outside the box, and the performance of the laser technology. Very often, we tend to blame complications on the machine and explain to the patient that complications just happen. My view is that using ZEISS technology in recent years has heightened my clinical career. Now I know that when a complication happens, it likely originated from my thinking process rather than the limitations of the technology. This sense of trust and confidence in the ZEISS technology is the best gift that a surgeon could possibly ask for.
CONCLUSION
Laser vision correction technology is always advancing. There will continue to be a place for open-flap LASIK surgery, but I believe that SMILE will take over the market in time. A flapless procedure like SMILE has become the preferred choice for my clients in Finland, and to work in my country without this technology would be demanding. We await the availability of hyperopic SMILE, and this will be a welcomed addition to our refractive surgery practice. I believe that SMILE represents the future of laser vision correction elsewhere, as it delivers a minimally invasive approach with high-quality results in healthy eyes.
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