Femto-LASIK Sets High Bar for Myopic Correction

Despite the growing popularity of lenticule extraction procedures such as SMILE in recent years, femtosecond LASIK (FS-LASIK) is still considered the gold standard for myopia correction in normal corneas, according to Dr Miguel A Teus.

“We now have more than 20 years of experience with FS-LASIK, with its safety and efficacy well established by numerous studies in the scientific literature,” he said. “SMILE is also a very good procedure, with similar safety and refractive outcomes to FS-LASIK, and it may be particularly suitable for young patients with moderate myopia, those with risk of ocular trauma and who accept a slower visual recovery and a slightly lower uncorrected visual acuity.”

In a broad overview of the current options for laser vision correction, Dr Teus said surface ablation still has a role in the refractive surgeon’s arsenal.

“Surface ablation or PRK is still the technique of choice in thin corneas, asymmetric topography, epithelial dystrophy, and low myopia,” he said.

Using the femtosecond laser for flap creation further improved LASIK outcomes compared to those achieved with a mechanical microkeratome.

“The femtosecond laser obtains a planar, thinner, and more predictable flap than the meniscus-shaped flap obtained with a mechanical microkeratome,” Dr Teus explained. “The advantages are less biomechanical impact, lower incidence of intraoperative complications, and more flap adherence.”

The scientific literature also reflects this, he added, with several meta-analyses showing FS-LASIK provides fast visual recovery, similar efficacy and safety, better predictability, and lower induction of higher-order aberrations than MK-LASIK for myopia correction.

Lenticule extraction

For lenticule extraction, most published studies pertain to SMILE procedures using the VisuMax platform (Carl Zeiss Meditec).

“SMILE benefits include avoidance of flap-related complications, theoretically fewer dry eye symptoms, and improved biomechanical stability,” he said. “The downsides include a steeper learning curve, potential complications related to lenticule dissection and removal, increased risk of treatment decentration, delayed visual recovery, and limited retreatment options.”

Although dry eye has often been touted as a theoretical advantage of SMILE over FS-LASIK, Dr Teus said that the evidence is not so clear cut.

A 2022 meta-analysis showed SMILE had lower corneal nerve damage and better reinnervation within 3 months postoperatively.¹ Another study found FS-LASIK eyes had less corneal nerve fibre and branch densities, less nerve fibre length, and significantly more nerves with sprouting than SMILE eyes after 4 years postoperatively.² However, researchers still observed nerve regeneration activity after 2.7 years for both SMILE and LASIK, and the sub-basal nerve plexus had not returned to a normal level after 5.5 years with both techniques.

For corneal sensitivity, SMILE performed better than LASIK at 1 and 3 months but returned to baseline values in both groups at 12 months.³ Another study showed similar OSDI scores with FS-LASIK and SMILE during the follow-up period.⁴

“No eye had severe OSDI scores. The patient-reported dry eye symptoms improved after both FS-LASIK and SMILE procedures, so SMILE does not seem significantly better for dry eye prevention,” he said.

Biomechanical stability

SMILE preserves more anterior corneal stromal fibres in biomechanical stability—believed to have the greatest tensile strength in normal corneas.

“Some studies have shown a lower corneal biomechanical impact after SMILE, but others have found no significant differences between SMILE and FS-LASIK,” Dr Teus said.

“The biomechanical advantages of SMILE seem modest in normal corneas.”

He noted a reported incidence of ectasia of 0.15% after SMILE from a cohort of 7,024 eyes with 12 months follow-up, whereas FS-LASIK reported prevalence rates between 0.02% and 0.6%.⁵

“The main risk factor is abnormal preoperative topography,” he said. “There are the same exclusion criteria for LASIK and SMILE, so SMILE is not the solution if the eye is a risk for ectasia with LASIK.”

For visual rehabilitation, SMILE has slower visual recovery than FS-LASIK at 1 and 3 months but no significant differences at 6 or 12 months postoperatively.

“This is perhaps due to higher corneal optical density with SMILE in the early postoperative period due to higher surgical manipulation and trauma,” he said.

Although SMILE has demonstrated good efficacy and safety for low myopia, Dr Teus warned the surgery could be more challenging.

“The very thin lenticule required for low myopia correction may suffer from retained fragments during removal. We, therefore, need to enlarge the optical zone to 7 mm and set a thicker lenticule thickness than needed for the refractive correction, meaning more corneal tissue is removed for these low myopes compared to excimer laser surgeries.”

Dr Teus presented at the 2023 ESCRS Winter Meeting in Vilamoura, Portugal.

For citation notes, see page 55.

Miguel A Teus MD, PhD is Professor of Ophthalmology at the Department of Surgery, Medical, and Social Sciences, University of Alcalá, Madrid, Spain. miguel.teus@uah.es