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C-58844

SmILE – the new paradigm in laser refractive surgery

This article outlines the technique, applications and relative merits of small incision lenticule extraction (SmILE)

Introduction 

Laser refractive surgery did not get off to a good start at the beginning. Although the technology initially was comparatively crude, it also suffered from the fact that retailers rather than clinicians promoted it. This led to a situation where many believed the risks involved in laser vision correction were significant and best avoided. However, recent developments in technology have advanced so much that it is now possible to alter the shape of the cornea with innovative approaches. Just like contemporary cataract surgery, the process can be carried out through a 2mm incision on the cornea without the need to create a flap as in the case of laser in situ keratomileusis (LASIK). So, small incision lenticule extraction (SmILE) may become the laser correction of choice.

The breakthrough is centred on the technology of the femtosecond laser. A femtosecond is 1 quadrillionth of a second. Such short pulses can be used to generate a plasma with minute accuracy within the stroma of the cornea. In this way a lenticule can be fashioned within the stroma and removed by the surgeon through a small incision. The small incision itself is also created by the femtosecond laser (see Figure 1: The main planes of cut. The contour of the deeper cut determines the refractive effect).

It is best to imagine the cornea in a myopic eye as being too strong a lens, which refracts light in front of the retina. So, if the surgeon can remove a lenslet from the cornea its refractive power can be adjusted so that it focuses the light exactly on the retina. In the laser control system there is an algorithm that calculates the exact lenticule parameters that must be removed to achieve emmetropia in that eye. Under topical anaesthesia the laser glass ‘kisses’ the cornea with very low-level suction; this holds the eye steady with exactly the correct corneal curvature for the procedure. Then, the femtosecond laser makes the refractive cut, the characteristics and dimensions of which are determined by the refractive change required for that eye. All of this is achieved intrastromally without any incision. Next, the laser creates what is termed the ‘cap cut;’ this is anterior to the refractive cut and is usually about 130μm below the surface of the epithelium and again it is carried out without an incision and all within the stroma (see Figure 2: The appearance of the eye immediately after the femtosecond laser cut)By this stage, the laser has cut the anterior and posterior surfaces of the lenticule so the only task the surgeon has is to extract it. To do this, the laser cuts a small side cut about 2mm long at the edge. Any remaining adhesions are broken with a miniature spatula and the freed lenticule is withdrawn through the 2mm opening (see Figure 3: Removal of the lenticule. Once removed, the lenticule is examined to ensure it is complete and then it can potentially be used as donor material for a patient with keratoconus). There is no flap and no excimer ablation. Recovery is very rapid, and most patients achieve 6/6 unaided vision by the next day but some further improvement takes place over the following few weeks.

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Dry eye symptoms

Dry eye does occur following SmILE but it is much less and of shorter duration than with LASIK.1–3 Patients do use topical lubricants but most can stop them after a few weeks. This is thought to be due to the fact that fewer of the corneal nerves are cut with SmILE than with LASIK. In creating a flap during LASIK the superficial corneal nerves are severed almost through 360° and thereby it takes much longer for them to regrow. Additionally, the absence of a flap in SmILE means that the cornea remains stronger as nearly all of the stromal fibres remain uncut. It remains to be seen if the risk of ectasia is less with SmILE compared with LASIK but theoretically it should be.

Who is suitable? 

SmILE can easily treat refractive errors of ~+3.00 to -10.00D and up to 3.00D of astigmatism. Patients must have sufficient corneal tissue such that the residual stromal thickness is adequate to prevent ectasia later. Keratoconus will be exacerbated by a SmILE procedure but ironically this group of patients may inadvertently benefit from this technology in an unforeseen way (this will be discussed in more detail later). 

How is it different from LASIK? 

In LASIK, the cornea is cut almost 360° and lifted off the stromal bed; this means that almost all of the stromal fibres are cut as well as the nerves. The corneal flap is then hinged by the surgeon to expose the stromal bed beneath. The patient is then placed under the excimer laser and the laser ablation is delivered to the cornea. At the end of the excimer treatment the corneal flap is replaced over the treated stromal bed.

In SmILE, the femtosecond laser cuts the excess lenticule within the stroma of the cornea without having to lift a flap. The surgeon then delivers the lenticule through a small 2mm incision and the treatment is complete. No excimer laser is needed. Fewer stromal fibres are cut and also fewer corneal nerves are severed. As there is no flap the risk of traumatic flap dislocation later is eliminated. If the patient were to need a retreatment at any time later it is possible to do a standard LASIK procedure if there is sufficient residual corneal stroma. During the procedure, no tracking is required as the eye is locked on to the laser for the refractive cut, whereas in LASIK the refractive cut is performed after the flap is lifted and the eye is free to wander. For LASIK, therefore, an eye tracker is required but it is not required for SmILE.

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Risks

It is probably best to divide the risks of laser refractive surgery into two categories: the risks of the surgery itself damaging the eye; and the risks that the intended refractive outcome is not achieved. Also, it is important to understand that risk is relative and that there are no risk-free options. All decisions in life are choices between sets of risks.

Many contact lens wearers are not aware that there is a small risk of losing some vision with soft contact lens wear.4–6 The most dramatic is acanthamoeba keratitis, which although rare, is devastating. Most contact lens wearers who suffer this complication do not recover useful vision. Less devastating but more common among contact lens wearers is bacterial keratitis, where if it affects the visual axis will often lead to permanent reduction in vision.

It is also important to compare similar risks and not confuse the risk profile of the early lasers with the technology that is available today. Recent data suggests that refractive surgery with a modern laser is less risky than soft contact lens wear even with daily disposables.4–6

Risks of surgery itself

Complications occurring during or immediately after the surgery include infection and damage to the corneal tissue leading to scarring. Later complications include progressive weakening of the cornea leading to bulging and distortion of the contour causing the vision to deteriorate (ectasia). Severe complications in this category might lead to the need for a corneal transplant; these risks are extremely rare. Most, but not all complications can be successfully treated and the vision restored to the predicted level.

Risks of not achieving intended focus

In excess of 95% of patients achieve the intended focus with one laser procedure.7,8 Almost all of the remaining 5% achieve intended focus with a second enhancing laser procedure. A very small percentage of patients do not achieve the intended focus, but for almost all of those patients the final refractive outcome is better than it was prior to any treatment. Occasionally, patients find that even though they have achieved intended focus they realise they would like to have something slightly different. Such patients can have a further procedure to alter that focus.

Regression

The term regression refers to the possibility that the eye gradually becomes out of focus again and requires glasses. Regression was a very real problem in hyperopic treatments with early lasers. Even patients who do not wear glasses will undergo fluctuations in their focus. The glasses or contacts the patient had five years ago would be out of date now and will be different from the glasses or contacts they might need in five years’ time. However, it is probable that their vision will remain within the ‘spectacle-free zone’ for a very long time and perhaps forever. In the early days of laser correction regression was not uncommon. The latest lasers seem to have a more stable outcome although by definition very long-term data is not available yet. Nevertheless, it appears that with the latest technology, enhancements can be carried out many years later if required.

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The spectacle-free zone

Vision is like any biometric measurement: there is a range of normal rather than one single measurement. Google says that the average height of men is 1.7m. That does not mean that anyone who is taller or shorter is abnormal; rather, there is a range within which most of us fall. The range of spectacle-free vision is described in (Figure 4: The spectacle-free zone – most people whose refraction falls within the blue area would regard themselves as independent of glasses). The numbers indicate the lenses required to achieve ‘perfect’ focus. A person whose focus is at point X in the figure has a spectacle prescription of -0.50D of myopia and 0.25D of astigmatism. Wearing this prescription would theoretically give the person better vision but in practice it is so weak that very few people would bother wearing such a pair of glasses. Such people regard themselves as spectacle-free. Many people whose spectacle prescription falls in the blue zone would regard themselves as spectacle-free for most of the time and can see the world adequately without glasses. They can perform the majority of visual tasks perfectly with no glasses or contacts. You will notice that there no sharp edge to the zone but rather there is a gradual transition into the zone were glasses are necessary to see clearly. The purpose of laser refractive surgery is to move the patient’s vision from the zone where glasses are necessary into this zone of spectacle-free vision. We do not need to get the patient to the very centre of the zone to be successful. This is where the brain comes in. Think of good vision as 20% optics (focus) and 80% data processing. If the visual cortex gets an image from within the zone it will process it so that it will appear to come from the very centre. That is perfect vision. The software the brain uses to analyse vision is designed to work with eyes that are focused in the spectacle-free zone. Evolution demanded this. That is why we say that glasses, and to a lesser extent contact lenses, do not fully solve the problems of ametropia. Laser refractive surgery gives the vision nature intended us to have. 

Figure 4

How can SmILE help patients with keratoconus? 

Research is well under way where the excised lenticule from a SmILE patient is transplanted into the cornea of a patient with keratoconus. In the recipient keratoconic patient the femtosecond laser is used to fashion a single plane of dissection within the stoma, which is then opened by the surgeon. Into this pocket the excised donor lenticule is inserted thereby adding tissue to the thinned cornea. The early results are looking very promising and because the stroma is the least antigenic part of the cornea, rejection is very much less a problem.9 It might also be eventually possible to use the boosted cornea to carry out refractive procedures to improve the vision and thereby the quality of life of the keratoconus sufferer. 

Conclusion 

Femtosecond laser technology will give people with ametropia a real life changing treatment. As a cataract surgeon the author often asks patients who were ametropes all their lives and are now emmetropic what different path their lives would have taken if they’d had corrective surgery when they were young. The answers are often very moving: many people feel they would have been able to live an easier life without the burden of glasses; others feel that they would have pursued different careers. In fact, it’s only when they are free of the burden that they are able to realise how being ametropic was so restrictive. To turn it around the other way, if I had a patient who was emmetropic all their lives and following my cataract surgery they were a moderate hyperope or myope, they would be very clear that I had forced a handicap on them, and rightly so. 

About the author

John Bolger FRCS, DO, FEBOS-CR is a consultant ophthalmologist who completed his training at the Royal Free Hospital and Moorfields Eye Hospital. Mr Bolger has worked as an independent eye surgeon since 1994 and is the founder and co-owner of My-iClinic. 

References

  1. Kobashi H, Kamiya K, Shimizu K (2017) Dry Eye After Small Incision Lenticule Extraction and Femtosecond Laser–Assisted LASIK: Meta-Analysis. Cornea 36(1):85–91
  2. Ganesh S, Rishika G (2014) Comparison of Visual and Refractive Outcomes Following Femtosecond Laser- Assisted LASIK With SMILE in Patients With Myopia or Myopic Astigmatism. J Refract Surg 30(9):590-596
  3. Denoyer A, Landman E, Trinh L, et al (2015) Dry eye disease after refractive surgery: comparative outcomes of small incision lenticule extraction versus LASIK. Ophthalmology 122(4):669-76
  4. Schein OD and Poggio EC (2008) Ulcerative keratitis in contact lens wearers. Cornea 27(1):22-7
  5. Stapleton F, Naduvilath T, Keay L, et al (2017) Risk factors and causative organisms in microbial keratitis in daily disposable contact lens wear. PLoS ONE 12(8): e0181343
  6. Masters J, Kocak M, Waite A (2017) Risk for microbial keratitis: Comparative meta-analysis of contact lens wearers and post-laser in situ keratomileusis patients. J Cataract Refract Surg 43(1):67-73
  7. Vestergaard A, Ivarsen AR, Asp S, et al (2012) Small-incision lenticule extraction for moderate to high myopia: predictability, safety, and patient satisfaction. J Cataract Refract Surg 38:2003–2010
  8. Vestergaard A, Ivarsen A, Asp S, et al (2013) Femtosecond (FS) laser vision correction procedure for moderate to high myopia: a prospective study of ReLEx® flex, and comparison with a retrospective study of FS-laser in situ keratomileusis. Acta Ophthalmol 91:355–362
  9. Zhao J, Shen Y, Tian M, et al (2017) Corneal Lenticule Allotransplantation After Femtosecond Laser Small Incision Lenticule Extraction in Rabbits. Cornea 36(2):222–228.