PUSHING SMILE LIMITS

Refractive and visual outcomes from small incision lenticule extraction (SMILE) rival those of femtosecond laser LASIK (fLASIK) for myopia up to -10.0D with less risk of complications, presenters told the XXXIII Congress of the ESCRS in Barcelona, Spain.

With clinical evidence mounting, several surgeons said they now prefer SMILE to any LASIK procedure. Introduced commercially in 2012, SMILE does not require cutting a flap or removing epithelium to reshape the stroma. Instead, a femtosecond laser cuts an intrastromal lenticule of about 6.5mm and an access tunnel of 2.0mm to 4.0mm. Via the tunnel, the lenticule is then completely separated from surrounding tissue with a spatula, and removed with forceps.

Detlev RH Breyer MD, Duesseldorf, Germany, credits this minimally invasive approach for better patient comfort after surgery and comparable visual and better safety results. “ReLEx SMILE with 130, 140 or 150-micron cap thickness replaced fLASIK completely for the last five years,” he said.

Petri Oksman MD, Helsinki, Finland, reported similar findings in a large retrospective study comparing 1,991 myopic eyes receiving SMILE with 1,895 receiving fLASIK. He said that “fLASIK and SMILE are equally accurate and stable”.

OUTCOMES AND SAFETY

Dr Oksman’s retrospective study involved patients with -0.75D to -10.0D myopia treated at two clinics from 2012 through to 2014. Three and six months after surgery, patients with initial myopia of -3.0D SEQ or less treated with fLASIK had about two ETDRS letters better mean uncorrected visual acuity (UCVA) than SMILE patients. At intermediate and high myopia fLASIK had a one to two letter advantage.

However, these differences are not clinically relevant, and are well within the repeatability range of visual acuity (VA) testing, which is ±3.5 to 9.0 letters, Dr Breyer pointed out. He believes other advantages of SMILE outweigh any slight VA advantage fLASIK may have.

“In patient counselling we discuss the speed and ease of the operation, reduced dry eye, no flap-related complications and corneal integrity,” he said.

In Dr Breyer’s retrospective study, fLASIK eyes had slightly better VA one day after surgery, but were similar to SMILE eyes with all cap thicknesses at one week, one month, three months, six months and one year after surgery (see Figure 1). The SMILE groups reported less dry eye, less pain and better comfort.

SMILE eyes also had a better safety profile (see Figure 2). Less than 12 per cent lost one line of corrected vision among 125 SMILE eyes with 130-micron caps and 90 eyes with 140-micron caps, Dr Breyer said. By contrast, 33 per cent of 40 fLASIK eyes lost one line or more. About 21 per cent of SMILE eyes with 150-micron caps lost one line, and two per cent two lines, though these results are less certain due to smaller numbers and shorter follow-up, he said.

Wavefront analysis found the SMILE eyes had lower ocular aberrations, including spherical and coma, over a 6.0mm optical zone, Dr Breyer said. Predictability and visual outcomes were similar for all cap thicknesses, suggesting that thicker caps do not affect outcomes. Looking at postoperative videokeratography, the ablation zone in ReLEx SMILE corneas is much more even than in fLASIK corneas, maybe explaining less corneal aberration induction and nearly no photopic phenomena drivinig at night time in ReLEx SMILE eyes.

Indeed, thicker caps may be resulting in less change in the total tensile strength of the cornea post-op. Speaking from the audience, Dan Z Reinstein MD, MA(Cantab), FRCSC, DABO, FRCOphth, FEBO, London Vision Clinic, UK, noted that the tensile strength of anterior stroma is about twice that of posterior stroma. Therefore, a thicker cap preserves more corneal strength since more of the lenticule is cut from the weaker posterior tissue. In effect, leaving 80 microns of anterior stroma is equivalent to leaving 160 microns of posterior stromal bed, making the cornea stronger with SMILE than an equivalent photorefractive keratectomy.

HIGHER MYOPIA

SMILE may also be preferable to LASIK for myopia exceeding -10.0D because it eliminates risk of heat build-up and environmental exposure seen with long excimer treatments, said Moones Abdalla MD, of the International Femto-Lasik Centre, Cairo, Egypt. As an off-lable, prospective non-comparative trial, 86 per cent of 365 eyes of -10.0D to -14.0D treated with SMILE were within ±0.5D of their refractive target at six months. Mean sphere was -1.28D and cylinder -0.83D, while corrected vision improved from a mean of 0.67 to 0.74 Snellen decimal.

No eye lost two lines, 1.0 per cent lost one line and 18 per cent gained one or more lines of distance vision, and overall quality of vision was better than expected, Dr Abdalla observed. “This marked flattening should be troublesome, but it is very acceptable with SMILE. You don’t cut a flap so the spherical aberrations are less,moreover the functional optical zone in SMILE is much wider than in LASIK, as tissue removal affects cornal biomechanics differently,” he said.

In a pilot study of SMILE for hyperopia as an investigator-initiated study, 24 patients with a mean preoperative sphere of +3.41D, cylinder 2.86D, achieved +0.29 sphere and -0.76 cylinder one week after surgery, Osama Ibrahim MD, PhD, Alexandria University, Egypt, reported. However, at six months this regressed to +0.74D sphere and -0.96D cylinder. This regression was improved by adding a transition zone and is expected to be even better with newer software and better algorithms.

“SMILE for hyperopic astigmatism is safe and effective but less predictable and less stable than for myopia or myopic astigmatism,” Dr Ibrahim said.

Higher hyperopia led to more regression, while increasing lenticule diameter and the transition zone lessened regression. Thickening the lenticule by a uniform, non-refractive 20-30 microns also made it easier to remove the thin central area without damage, he noted.

“Some less experienced surgeons can encounter problems with the SMILE surgical technique, and therefore may not achieve as good results,” Dr Breyer added.

Detlev RH Breyer: d.breyer@augenchirurgie.clinic

Petri Oksman: petri.oksman@medilaser.fi

Dan Z Reinstein: dzr@londonvisionclinic.com

Moones Abdalla: moones.abdalla@gmail.com

Osama Ibrahim: ibrosama@gmail.com