SMILE PROCEDURE

Refractive surgery is at an exciting stage of evolution. With newer techniques such as femtosecond lenticule extraction (FLEx) and small incision lenticule extraction (SMILE), patients wanting spectacle independence have a wider variety of options available. Here I will look at these latest developments, especially SMILE, which is developing as an even safer option than LASIK. TECHNIQUE:  All femtosecond laser-based refractive surgery developed following extensive work by Walter Sekundo and Rupal Shah, and was first developed as the FLEx procedure. While femtosecond LASIK involves creating a femtosecond laser flap followed by excimer ablation for refractive correction, FLEx uses the femtosecond laser itself to carve an intrastromal lenticule under the flap. This is done by creating two intersecting lamellar cuts – a posterior lenticule cut and an anterior flap/cap cut that correspond to the posterior and anterior surfaces of the intrastromal lenticule, respectively. The characteristics of this lenticule can be varied. Thickness varies according to the refractive error of the patient and the diameter of the lenticule determines the optic zone. Once the lenticule is created, the flap is lifted and the lenticule removed in order to achieve refractive correction. SMILE was the logical next step which made flapless treatment possible. Here, the flap side cut was shortened to allow only a small opening in the flap, thereby converting the flap into a cap. The lenticule is then dissected from under the cap and extracted through this small incision, the size of which can be varied and can be set to be as small as 2mm. Cap thickness can also be varied, however at present a thickness of 120 microns is most commonly preferred and used. ADVANTAGES OF SMILE: The presence of only a cap and no flap confers numerous advantages. The absence of any potentially displaceable flap makes the procedure safer in both the short-term as well as the long-term. SMILE is a minimally invasive procedure with incision size as small as 2mm. The absence of a flap and a very small incision together bring about a decrease in flap-related complications. This results in decreased risk of striae, flap dislodgement, epithelial ingrowth etc. The anterior corneal innervation is less disrupted as compared to a flap, resulting in faster recovery of dryness and corneal sensitivity as compared to LASIK. The retention of a nearly intact anterior stronger layer of the cornea allows better maintenance of biomechanical strength after SMILE as compared to LASIK. The entire procedure is completed with a single laser, thus decreasing surgical time and cost. These advantages, along with visual and refractive outcomes similar to LASIK, give SMILE a superior edge to LASIK. Disadvantages include an inability to treat hyperopia and absence of correction for cyclotorsion. Nomogram adjustments may however be made to achieve satisfactory correction of astigmatism. INTRAOPERATIVE COMPLICATIONS: Decentration may occur because of poor centration by the surgeon or with a patient unable to fix on the fixation light. Good centration needs to be confirmed before laser application and is generally not difficult in a cooperative patient. Suction may not build up in case of insufficient applanation, or if the eyelashes/drape or any other extraneous material is present within the applanating interface. These should be removed and the cornea reapplanated. Intraoperative suction loss is managed by repair cuts applied according to stage of suction loss. Excessive opaque bubble layer may indicate more difficult dissection. Uncut spots are rare and among other causes, may also be due to debris on the applanating glass which, if present, should be meticulously cleaned/replaced if necessary prior to laser application. Small uncut areas are generally not problematic, however large uncut zones can potentially cause difficult dissection and irregular astigmatism. During dissection, anterior and posterior lamellar planes should be carefully identified to avoid difficult separation and misdissection. Low powers have correspondingly thinner lenticules and these should be dissected with extra care to avoid lenticule tears  and retained lenticular fragments. To facilitate dissection in low powers, the lenticule thickness can be increased by increasing minimum lenticule thickness by adding a refractive neutral base. The cap may tear during dissection either due to faulty technique or sudden patient movement. If seen, the cap should be replaced carefully and allowed to adhere well without striae formation and a bandage contact lens applied. WHITE RING SIGN: The cap cut (anterior plane) should always be dissected prior to the lenticule cut (posterior plane). The reverse results in a lenticule adherent to the cap which is more difficult to separate and can result in cap tears. I have described this sign as a technique to differentiate anterior from posterior plane. The white ring seen intraoperatively is a light reflex from the lenticular side cut, and the instrument seen above this ring indicates anterior plane dissection. Conversely, when the shaft of the instrument lies below this ring, it indicates posterior plane dissection. POSTOPERATIVE COMPLICATIONS: Epithelial defects should be avoided. These and other predisposing factors such as bleeding from cut peri-limbal vasculature, sebaceous secretions and other foreign material in the interface, may lead to diffuse lamellar keratitis. Other postoperative complications are similar to LASIK and can include undercorrection, overcorrection, transient interface haze, striae, ectasia, infection etc. Some of these complications are generally expected to be less frequent than after LASIK because of the innate minimally invasive nature of the procedure. Enhancement after SMILE is possible either as a surface ablation, a thinner femto flap created on the SMILE cap, or by using the CIRCLE software that converts the existing cap into a flap. OTHER USES OF SMILE LENTICULE: The SMILE lenticule has been used to treat hyperopia in a recipient eye and to seal corneal defects. I have pioneered the PEARL (PrEsbyopic Allogenic Refractive Lenticule) corneal inlay which uses a 1mm SMILE lenticule for presbyopia correction. This has shown very promising results. CONCLUSION: SMILE is the latest form of refractive surgery which has come into its own because of good predictability, safety and efficacy, along with relative ease of surgery. Complications are rare and generally resolve favourably with no long-lasting effect on the patient's vision. Vision-threatening complications are rare. Dr Soosan Jacob is Director and Chief at Dr Agarwal’s Refractive and Cornea Foundation, Dr Agarwal’s Eye Hospital, Chennai, India, and can be reached at: dr_soosanj@hotmail.com