Touch-up techniques following SMILE
Research shows that enhancement rates after LASIK range from approximately 7.8% to 20%, with regression of approximately 1 diopter after 10 years.
Research shows that enhancement rates after LASIK range from approximately 7.8% to 20%, with regression of approximately 1 diopter after 10 years.1-4 However, in small-incision lenticule extraction (SMILE), we found that after 5 years, regression is approximately half a diopter lower, with enhancement rates of between 2.1% to 2.9% after 1 to 2 years.5 Even with this low enhancement rate, there is still a need for surgeons to be aware of the options for enhancements after SMILE.
The circle procedure
There is just one official touch-up technique for SMILE, known as the circle procedure. A series of four circle patterns that are programmed into the femtosecond laser (VisuMax, Carl Zeiss Meditec) can be used to create a corneal flap after a previous SMILE procedure.
The surgeon will convert the original SMILE incision pocket into a LASIK-like flap that is easily lifted for stromal ablation of the residual refractive error with an excimer laser. With the circle procedure, the patient loses the flap-free approach, which comes along with all the possible complications of creating the flap.
In a prospective, single-centre case study series, researchers followed 27 patients 3 months after the circle procedure to assess its efficacy and safety. Efficacy was determined by the surgeon’s ability to lift the created corneal flap.
In 100% of cases, the flap lift was possible, as planned. No eyes lost two or more Snellen lines of corrected distance visual acuity and no procedure or flap-related complications or serious adverse events occurred. Researchers concluded that the circle technique is a suitable method for correction following SMILE.6
Secondary SMILE
As another option, surgeons can also perform an additional SMILE procedure, known as sub-cap-lenticule-extraction (Sub-Cap-LE), which is an off-label use. However, in a case report researchers described this modified SMILE in a 53-year-old woman who initially underwent bilateral SMILE for moderate myopia.
Just as with the initial procedure, a refractive lenticule and small incision are created inside the intact cornea in one step. The lenticule is then removed through this small incision, leaving the remainder of the superficial cornea intact.
However, during the second procedure, the surgeon used the previously created interface as the new superior plane of the new lenticule. Postoperatively, uncorrected visual acuity improved from 20/63 to 20/16. Certainly, more research is needed to develop a consistent surgical protocol.
The only difference between the first and second procedure is that the second procedure should be performed anteriorly or posteriorly to the initial one depending on how deep the SMILE pocket is. From a theoretical point of view, the procedure should work well, but surgeons may resist this approach because they do not know where all the laser spots are going and where they will end.
LASIK
To perform LASIK after SMILE, the surgeon should have an accurate measurement of the corneal thickness. During a routine LASIK procedure, there might be a flap thickness of 90 to 120 microns. During LASIK after SMILE, the flap should be approximately between 10 and 30 microns thinner than the cap, which is more difficult to handle.
Many laser devices do not provide a measurement that is accurate enough to cut a very thin flap, so the surgeon needs a powerful anterior-segment OCT or ultra-sound based machines (Artemis, Eigen) to ensure that after the excimer laser application, the origin interface is not disturbed.
PRK
Photorefractive keratectomy (PRK) is well known and easy to perform. From a surgical point of view, PRK after SMILE is the same.
The surgeon starts by removing the epithelium, applying the laser energy and placing a sponge with mitomycin C on the eye for 30 seconds to 2 minutes. The surgeon will rinse the eye and then put on the contact lens.
It is important to note that during PRK after SMILE, the surgeon should use mitomycin C because, otherwise, there can be complications with haze formation postoperatively. Also, the surgeon should keep the optical zone the same as in the SMILE procedure without making it larger or smaller.
My colleagues and I have also found that the aspheric profile from the excimer laser tends to overcorrect in low corrections, and as a result we do not use a fixed aspheric correction as a touch-up procedure. We have found that tissue-saving profiles and topographic guided profiles work well and we did not find any statistically significant differences among them. As with traditional PRK, note that the downsides of PRK are pain and slow visual rehabilitation.
Retrospective analysis
My colleagues and I conducted a retrospective multicenter study from four centres in Germany and Austria. We included 43 eyes out of 1,963 eyes. There was a retreatment rate of 2.2%, and three patients were excluded due to loss of follow-up.
In the study, 40 eyes were included with a follow-up of at least 3 months. Manifest refractive spherical equivalent (MRSE) before SMILE was 6.35 diopters and 0.86 before PRK. The target was, in every case, plano refraction.
For two-thirds of the cases, we used the advanced ablation algorithm (AAA) profile from the MEL 90 Excimer Laser platform (Carl Zeiss Meditec); in 20% of the cases, we used tissue saving ablation (TSA) profile; in 7% we used the topographic guided profile; and in 5% the aberration smart ablation (ASA) profile.
The mean refractive surgery equivalent showed a significant reduction after 1 week and remained stable for up to 3 months at 0.03 diopters.
In this group, before the PRK treatment, 27.5% were within 0.5 diopters, improving up to 80% after PRK. Seventy-five percent were within 1.0 diopter, improving to 92.5% and 25% of the patients were over 1.0 diopter refractive error, improving down to 7.5%, which was a significant improvement.
In addition, the uncorrected visual acuity showed a significant improvement at 6 weeks and remained stable up to 3 months at a level of 0.08 LogMAR. The corrected distance visual acuity after 6 weeks was equal to corrected distance visual acuity preoperatively, which is average. However, we had some outliners, so there was some loss of lines. Fifteen percent lost one line, and no patients lost more than one line (see Figure 1).
Interestingly, of the 15% of patients who lost one line, two-thirds achieved the best visual acuity of 20/20. They came from 20/18 to 20/20 and one-third was 20/25. No patient was below 20/25 after PRK.
The safety index was 1.06. We had one significant haze after PRK, showing resolution after 3 months of topical steroid medication. There were some unsatisfactory refractive outcomes in the form of two major undercorrections. One patient showed –1.75. This was the patient with the highest spherical equivalent pre-SMILE, and we did enhancement after 4 months.
Another patient was approximately -1.0 diopter, and the enhancement was done after 4 months as well, which was perhaps too close to the initial procedure.
There were two overcorrections, including 1.75 and 1.38 diopters, both with the ASA profile, which indicates that the ASA profile is not suitable for touchup-procedures. To compare the ablation profiles, AAA, TSA and topographic-guided are almost identical and no statistically significant difference between the profiles were found.
Surface ablation combined with a mitomycin C application seems to be safe and effective to treat residual refractive errors after SMILE. Due to low residual refractive errors, the ASA profile is not recommended due to the tendency of overcorrection. Of course, long-term results are necessary to compare it to different and other touch-up techniques as secondary SMILE and LASIK.
Conclusion
With many options for enhancement techniques after SMILE, the ultimate decision is based on the patient’s needs. If the patient wants a flapless procedure, I recommend PRK. If the patient wants a painless, procedure, all other options are viable, keeping in mind that these options are all more challenging to perform than PRK.
In the end, there is no golden rule and it is a personal approach that depends on the patient’s preference and the surgeons’ abilities.
References
1. Alio JL, et al. Ten-year follow-up of laser in situ keratomileusis for myopia of up to -10 diopters. Am J Ophthalmol. Jan 2008;145(1):46-54.
2. Perez-Santonja JJ, et al. Retreatment after laser in situ keratomileusis. Ophthalmology. Jan 1999;106(1):21-28.
3. Hersh PS, Fry KL, Bishop DS. Incidence and associations of retreatment after LASIK. Ophthalmology. Apr 2003;110(4):748-754.
4. Lyle WA, Jin GJ. Retreatment after initial laser in situ keratomileusis. Journal of cataract and refractive surgery. May 2000;26(5):650-659.
5. Blum M, et al. Five-year results of Small Incision Lenticule Extraction (ReLEx SMILE). The British journal of ophthalmology. Jan 8 2016.
6. Chansue E, et al. Safety and efficacy of VisuMax® circle patterns for flap creation and enhancement following small incision lenticule extraction. Eye and Vision. 2015;2:21. doi:10.1186/s40662-015-0031-5.
Dr Martin Dirisamer, MD, PhD, FEBO
Dr Dirisamer is the head of the department for refractive surgery at the University Eye Clinic, LMU Munich, Germany. Dr Dirisamer reports no relevant financial disclosures.
