Newer refractive technologies, particularly laser refractive technologies, have sparked debate. However, it is necessary to consider the research and observations of the clinical outcomes. Patient satisfaction and safety are what it is really about.
Laser Refractive Surgery (LRS) has been around for a few decades and over time there have been marked enhancements in this area. We’ve seen LRS develop from PRK (Photorefractive keratectomy) through LASIK (Laser-Assisted in Situ Keratomileusis), a combination of these and LASEK (Laser Epithelial Keratomileusis). Now we have SMILE (Small Incision Lenticule Extraction).
A review of recent research papers was undertaken to look at aspects of laser refractive surgery in order the better understand the current evidence. Anterior segment specialists, Dr. Gerard Sutton, Dr. Michael Lawless and Dr. Dean Corbett provide their comments on the research based on their clinical experience.
Summarising and in part paraphrasing a recent paper, Clinical outcomes of small-incision lenticule extraction and femtosecond laser–assisted wavefront-guided laser in situ keratomileusis by Pi~nero1 et al, we note the following;
It is hard to prove yet but SMILE does seem more accurate the higher the level of myopia
“Both small-incision lenticule extraction SMILE and wavefront-guided femtosecond laser–assisted LASIK provide good visual outcomes and effective correction of the refractive error… slightly better UDVA (unaided distance visual acuity) outcome seems to be present after wavefront-guided femtosecond LASIK.
“Although there was a trend toward a higher level of coma aberration in patients having SMILE… (possibly) related to the presence of mild levels of treatment de-centration, (there was) no consistent evidence of the superiority of one technique over the other in this regard.
“Various studies have confirmed the decrease in corneal sensitivity in the early period after SMILE, but this decrease has been found to be less than the decrease after femtosecond LASIK… primarily due to a smaller decrease in sub-basal nerve density in the early postoperative period in eyes having SMILE. At six months no differences (were found). More studies are needed to confirm potential differences at the inflammatory level between the techniques in the early postoperative period.
“TBUT decreases after both SMILE and femtosecond LASIK, with apparently less reduction after SMILE in the early postoperative period. A similar trend has been reported for the outcomes of the Schirmer test. This may be related to the generation of more dry eye symptomatology after femtosecond LASIK than after SMILE, as suggested by studies evaluating this issue by means of validated questionnaires. More research is needed on this issue to extract consistent conclusions.
“There is increased backscattered light intensity at the intrastromal level after SMILE compared with after femtosecond LASIK, which disappears six months after surgery. This may be related to a more limited visual recovery in the early postoperative period. The significantly higher number of microdistortions of Bowman’s layer after SMILE in the early postoperative period may be another reason for this potential limitation in the visual recovery. Future studies should confirm whether there is a direct relationship between the levels of laser energy used and the level of intrastromal backscattering in the initial postoperative period of SMILE.
“Corneal biomechanical changes occur after both SMILE and femtosecond LASIK, with no scientific evidence supporting the superiority of one technique over the other in this area. There may be a potential benefit of SMILE extraction over femtosecond LASIK in eyes with high myopia, but this has to be confirmed in future studies
“Scientific evidence supporting the stability of results exists for WFG (wavefront-guided LASIK) in the long term (four years) but not for SMILE (12 months only).
“There is limited evidence of the outcomes of SMILE retreatments, and the results of surface ablation retreatments in SMILE eyes are poor”.
Additional Evidence
In another paper, Bilateral Ectasia After Femtosecond Laser-Assisted Small Incision Lenticule Extraction (SMILE)2 by Mattila et al, they mention corneal ectasia “has been reported with an incidence of one in 2,500 after LASIK and one in 3,300 after photorefractive keratectomy (PRK)” while stating “Small incision lenticule extraction (SMILE) leaves the anterior cornea intact and thus is thought to carry less risk for ectasia. As far as we know, just three case reports of ectasia after SMILE have been published so far with some concern about whether SMILE was indeed the definitive cause of ectasia. One of the reports showed a case of ectasia after SMILE in a normal eye, whereas there were preoperative signs of forme fruste keratoconus in the other two cases.”
In their discussion the authors state; “This case illustrates the development of bilateral ectasia after SMILE, although the ectasia in the left eye was subclinical. The right eye presented a preoperative early keratoconus, whereas the left eye was topographically unremarkable. As far as we know, SMILE was chosen because of the presumably lower risk for postoperative ectasia. There were no known published incidences of ectasia after SMILE at the time of the operations. It can be questioned whether the patient’s topographic changes would have progressed to clinical keratoconus with time. However, the postoperative results were less than optimal in the right eye, leading to PRK enhancement and the development of progressive keratoconus in both eyes shortly after the operations. Our case underlines the importance of thorough preoperative assessment for possible keratoconus suspect changes with corneal topography to avoid postoperative ectasia. Furthermore, the preoperative assessment should follow the same principles in all laser refractive procedures and no refractive surgery should be performed in keratoconus (KC) or forme fruste keratoconus (FFKC) regardless of the surgical technique, whether it be LASIK, PRK or SMILE.”
Another study, Comparison of biomechanical effects of small incision lenticule extraction and laser-assisted subepithelial keratomileusis3 by Chen et al, looked at things from a different perspective. They analysed corneal hysteresis (CH) and the corneal resistance factor (CRF) parameters at one and three months postoperatively. They found “significant differences in the preoperative manifest refraction spherical equivalent (MRSE), central corneal thickness (CCT), the planned ablation depth (AD) and CRF values between the two groups. The CH and CRF values at one and three months postoperatively were significantly lower than the preoperative values in both groups. The postoperative CH values were significantly lower in the LASEK group than in the SMILE group at both follow-up visits. No significant differences were observed in postoperative CRF between groups at one month and three months. No significant correlation was found between the planned AD and the changes in CH or CRF at any follow-up visit in the SMILE group. The preoperative magnitude of CH and CRF may be predictors of postoperative changes in CH and CRF in both groups. They concluded that CH and CRF decreased after SMILE and LASEK. However, the changes in the CH values were less after SMILE than after LASEK.”
In a final review of a paper, Four-year observation of predictability and stability of small incision lenticule extraction4 by Han et al they investigated long-term refractive outcomes, wavefront aberrations and quality of life after small incision lenticule extraction (SMILE) for moderate to high myopia. “Measurements included uncorrected distance visual acuity (UDVA), corrected distance visual acuity (CDVA), manifest refraction, wavefront aberrations, and quality of life. At four years postoperatively, UDVA was better than or equal to 20/20 in 92 per cent of eyes. The efficacy index was 1.07 ± 0.16.
“89 per cent of eyes were within ± 0.5 D of the intended refractive target. No eye lost any Snellen lines. The safety index was 1.16 ± 0.14. No significant changes of SE (spherical equivalent) occurred among postoperative follow-ups at months one, three, six, and years one, two and four.
“Higher-order aberrations, coma, spherical aberration and higher-order astigmatism increased postoperatively, and no significant changes of aberrations were detected among the one-month, six-month or four-year follow-ups postoperatively.
“Compared to the spectacles group, the surgery group showed a significantly higher total score on quality of life (45.71 ± 2.61 vs 39.96 ± 3.56).”
They conclude that SMILE provides “a predictable and stable correction of moderate to high myopia as documented by long-term follow-up”.
Clinical Observations and Experience
Corneal specialist ophthalmologist Dr. Dean Corbett for his view of SMILE and he commented;
“Neither PRK or LASIK have shown beneficial results over wavefront optimised (WFO) excimer treatment. This is almost certainly because a good deal of higher order aberrations reside in the tear film; eliminating this is impossible due to the inability to track the eye satisfactorily in three planes. Both LASIK and PRK are further affected by healing – PRK by ET and LASIK by the creation of a flap. There is little doubt that flap creation will destabilise and weaken the eye, and has the potential to induce a lot of errors in the wavefront. The issues of the Stiles-Crawford effect and induction of spherical aberration with excimer treatments is well known, hence the development of WFO treatments. This is because the Gaussian beam is meeting the ablated surface more and more tangentially as the beam ‘peripheralises’ on the cornea. This has led to algorithms to estimate additional ‘shots’ to be placed in the periphery so as to minimise the induced spherical aberration (SA).
“With SMILE- these problems all go away; there is no excimer, no induced SA, just a shape that is planned. No flap wrinkles/dislocation/induced higher order aberrations etc. Again, avoided with SMILE.
“SMILE will quite possibly do to LASIK what LASIK did to PRK: I am 90 per cent SMILE and would not care if I never did another LASIK.”
The tear film is arguably the greatest source of aberrations in the eye. By its nature this occurs with each and every blink, around 28,000 times a day, causing variability! We cannot compensate for this with aberration profiles in wavefront guided or optimised laser refractive surgery, nor with IOLs, ICLs or contact lenses. This is one of the reasons why practitioners have noted improved visual outcomes when using the latest generation of highly wettable contact lenses with improved surfaces.
Essentially LRS continues to evolve and the results are better, more consistent and safer than ever before, as they should be, with evolution over time and as technology, screening and instrumentation improves.
There are many controversies in eye care and ophthalmology in particular; as we have seen with the differing opinions regarding femtosecond Laser-aided cataract surgery.
It would seem that PRK, LASIK, LASEK and SMILE are all sound techniques and surgeons will recommend the best and safest procedure that provides the best outcomes for their patients.
Dr. Gerard Sutton comments as follows: “I think this is a reasonable review of the state of play of SMILE at this point in time. I recently gave a Plenary Lecture on this topic at RANZCO in Melbourne and my position on this technique is clear. In order to adopt a new procedure there needs to be a perceived and ideally a proven clinical advantage. Many new procedures don’t stand the test of time but I don’t think SMILE will be one to disappear.
“There are five reasons to consider SMILE as a surgical option for the treatment of myopia in place of LASIK or PRK. If it provided: 1. More predictable visual outcomes, 2. Better visual quality 3. Improved safety 4. Stronger biomechanics or 5. Less dry eye.
“I think the evidence so far is that the visual outcomes are similar or slightly better with SMILE for higher levels of myopia and that the visual quality studies are mixed. “There is no evidence of better biomechanics but I think the evidence of better dry eye outcomes and less reduction of corneal sensitivity are clear.
“It was the paper by Ganesh5 at el that convinced me that SMILE was worth adding to my surgical quiver. This was not quoted in the above review but is worth a look. SMILE makes up approximately 60 per cent of my corneal refractive procedures, so LASIK and PRK are still an important part of the choices I offer to my patients. Some of the mixed outcomes in the literature are due to the variability in technical aspects of the system. There is no doubt that our results have improved dramatically since we standardised the temperature, humidity and energy settings of the laser. We will publish this data in 2017 but in our laser setting, this data will show a clear role for SMILE in the treatment of myopia.“There are a number of improvements that will be made that will make the surgery easier for the novice surgeon and I expect competition from other companies to drive these changes.
“I disagree with Dr. Corbett that LASIK is a redundant procedure but I do think that as a refractive surgeon it is important to have SMILE in your surgical armamentarium,” concluded Dr. Sutton.
Dr. Michael Lawless, like Drs. Corbett and Sutton, shared similar sentiments and said, “SMILE promises so much it’s difficult to remain rational. No flap to dislodge, less variability in healing, less damage to corneal nerves, less weakening effect on the cornea.
“I avoided SMILE for as long as I could because it seemed impossible that it could provide as accurate a correction as LASIK.
“Our group started in August 2014. A little over two years of experience has now led me to perform SMILE in 60 per cent of cases, LASIK in 30 per cent and PRK in 10 per cent. So I need all technologies. It is hard to prove yet but SMILE does seem more accurate the higher the level of myopia, so for anybody above -3.00D of myopia I would normally suggest SMILE rather than LASIK. I have the same exclusion criteria for SMILE as I do for LASIK. The cornea needs to have the appropriate anatomy to do either procedure and it is not reasonable to perform SMILE on patients where you think they might be at risk of ectasia with LASIK because they would probably still be at risk of ectasia with SMILE. It probably is less of a risk but this is not known yet and it is premature to offer those patients SMILE. Where the corneas are a little thin or where I am mildly concerned, I will perform surface laser or more likely with large errors, use a phakic implantable lens.
“So this is a technology in transition. SMILE is limited by its inability to centre perfectly in every patient and it is very surgeon dependent in this regard and the ability to fix the toric axis accurately is not ideal. These things will improve as competition increases amongst the manufacturers.
“SMILE definitely has a place, and in my view, a majority place these days but it will not eliminate PRK and it will not eliminate LASIK. I think they coexist and I am grateful that I have the ability to offer what I think is the right operation in the correct circumstances,” concluded Dr. Lawless.
Some evidence and clinical views that seem to show that SMILE is fast becoming a procedure that is gaining acceptance. It is already a preferred laser refractive surgery option for a number of surgeons. It is expected that in time more surgeons will adopt SMILE as a refractive surgery option.
Professor Gerard Sutton is an internationally recognised ophthalmic surgeon with Vision Eye Institute and the Sydney Medical School Foundation Chair of Cornea and Refractive Surgery. He specialises in laser eye surgery, laser cataract surgery and corneal transplantations. He has a special interest in the treatment of keratoconus.
Clinical Associate Professor Michael Lawless was one of the first ophthalmic surgeons in the world and the first surgeon in the Southern Hemisphere to perform femtosecond laser for cataract surgery. His areas of specialisation are laser vision correction, cataract surgery, lens surgery, refractive lens exchange and corneal transplants.
Dr. Dean Corbett is a specialist in laser vision correction, implantable contact lenses and cataract surgery. He also specialises in presbyopia correcting surgery and is a recognised glaucoma surgeon. Dr. Corbett is a consultant ophthalmologist at Greenlane Clinical Centre, Auckland, is a Clinical Lecturer, and practices from Auckland Eye.
The information contained in this article is general in nature and has been provided in good faith, without taking into account your personal circumstances. While all reasonable care has been taken to ensure that the information is accurate and opinions fair and reasonable, no warranties in this regard are provided.
