Exploring laser vision correction for myopia with/without astigmatism

Reviewed by Dr Mounir Khalifa

Take-home:  A prospective masked study enrolling eyes with low to moderate myopia with or without astigmatism found significantly better efficacy, safety, and predictability with wavefront-guided LASIK compared with small incision lenticule extraction (SMILE). 

Results of a prospective, masked study show significant differences favoring wavefront-guided (WFG) LASIK compared with small-incision lenticule extraction (SMILE) for the treatment of low to moderate myopia with or without astigmatism.

The study, which was conducted by Dr Mounir A Khalifa and colleagues in Egypt, included 110 eyes with spherical equivalent (SE) up to -6 D who were followed for six months after their refractive procedure. All patients but one underwent bilateral surgery.

WFG LASIK was performed in 51 eyes of 51 patients with the Star S4 IR excimer laser (Abbott) using a high-resolution wavefront aberrometer (iDesign system, Abbott), mechanical microkeratome (M2, Moria), 6.0 mm optical zone, and 8.0 mm treatment zone. Fifty-nine eyes of 59 patients underwent SMILE using the VisuMax femtosecond laser (Carl Zeiss Meditec) to create a 6.5 mm lenticule.

Visual and refractive outcomes were good with both procedures. WFG LASIK, however, was associated with faster visual recovery compared with SMILE as well as significantly better efficacy, safety, predictability, and preservation of visual quality with less induction of higher-order aberrations (HOAs) and less contrast sensitivity loss.

The recovery of UDVA in relation to preoperative CDVA (efficacy index with time) was slower in the SMILE group than in the WFG LASIK-treated eyes, and the difference between groups was more prominent in eyes with low myopia versus those with moderate myopia because of a virtually flat efficacy index curve in the WFG LASIK group (see Figure on Page 24).

“Previously published studies comparing SMILE and LASIK primarily included eyes treated with conventional LASIK and found visual and refractive outcomes were similar for the two procedures whereas results for visual quality and aberrometry were more variable,” said Dr Khalifa, professor of ophthalmology, Tanta University, and chairman, Horus Vision Correction Center, Alexandria, Egypt.

“SMILE offers some advantages compared with LASIK because there is no flap, and we found it to be an efficacious and safe procedure for correction of myopia with or without astigmatism,” he said.

“However, WFG LASIK remains the state-of-the-art laser vision correction technique,” Dr Khalifa added. “It offers more predictable outcomes and better aberrometric control and delivers rapid visual recovery and better UCVA outcomes, which are important factors influencing overall satisfaction of refractive surgery patients.”

Dr Khalifa performed all of the LASIK procedures at the Horus Vision Correction Center. The SMILE procedures were done by Dr Ahmed Ghoneim, professor of ophthalmology, Tanta University, Tanta, Egypt.

Diving deeper

Diving deeper

The two refractive surgery groups were well-balanced with respect to mean age, sex, mean manifest spherical equivalent (MSE), and distribution of eyes with low (≤-3.0 D) and moderate (>-3.0 to -6.0 D) myopia.

Preoperatively, mean UDVA was significantly better in the WFG LASIK group whereas the SMILE eyes had a significantly better mean CDVA.

Based on data collected at the 6-month postoperative visit, mean UDVA and CDVA were significantly better in the WFG LASIK group compared with SMILE, and the proportion of eyes achieving logMAR UDVA 0.00 or better was also significantly greater in the WFG LASIK group compared with SMILE (90.2% versus 78.0%). The mean efficacy index was 1.12 for WFG LASIK and 0.92 for SMILE, and the difference was also statistically significant.

Corresponding with the functional differences, the refractive outcomes analyses showed significantly lower values for mean manifest cylinder and SE in eyes that had WFG LASIK compared with the SMILE group.

In addition, there were statistically significant differences favoring WFG LASIK versus SMILE in analyses of the proportion of eyes with postoperative SE ±0.50 D of target (98.0% versus 81.5%) and residual cylinder ≤0.50 D (100% versus 84.7%).

Vector analyses of astigmatic change also showed statistically significant differences favoring WFG LASIK for TIA, SIA, and DV.

“Our finding that SMILE may undercorrect astigmatism up to 4.0 D is consistent with results reported by Zhang et al. showing better astigmatic outcomes after WFG LASIK compared with SMILE,” Dr Khalifa said.

“The lack of iris registration and cyclorotation compensation when performing SMILE but not the potential off-axis application of the treatment may explain the better astigmatic correction achieved with WFG LASIK,” he added. “Treatment decentration with SMILE, however, may be the reason why induced more coma than WFG LASIK.”

Measuring level of HOA

Measuring level of HOA

HOAs in both groups were measured with the same high-resolution aberrometer and analysed for changes from baseline since there were significant differences between groups preoperatively.

Compared with WFG LASIK, SMILE induced a significantly higher level of total HOAs and primary coma.

“The greater induction of primary coma after SMILE as well as an increase in ocular scattering after that procedure may explain our finding of greater loss of CDVA after SMILE compared with WFG LASIK,” Dr Khalifa said.

Safety analyses showed WFG LASIK was associated with a significantly higher safety index compared with SMILE (1.20 vs 0.98) and no loss of CDVA. In the SMILE group, 6.8% of eyes had a one line or greater decrease from baseline CDVA.

Measurements of contrast sensitivity at 6 months showed decreases from baseline after both refractive procedures, but the loss was significantly greater after SMILE compared with WFG LASIK at the spatial frequencies of 6, 12, and 18 cycles/degree.

“Changes to reduce bridges and the size of the cavitation bubble created with the femtosecond laser, including increasing its frequency, shortening the focal length, reducing the pulse energy, and reducing spot distance, should significantly improve the predictability of SMILE,” Dr Khalifa added. “In addition, implementation of axial and torsional automatic registration should provide benefits for reducing coma induction and improving astigmatic correction.”

Dr Mounir Khalifa, MD, PhD

E: Mounir.khalifa100@gmail.com

Dr Khalifa received an unrestricted educational grant from AMO Germany GmbH. Dr Ghoneim has no relevant financial interest to report.