One of the most interesting recent developments in laser refractive surgery is the emergence of the new ultra-short pulse picosecond and femtosecond lasers.1,2 Femtosecond lasers produce more predictable flap thicknesses, cause fewer increases in higher-order aberrations and backscatter light, have better UCVA outcomes, and generate less epithelial damage compared with mechanical microkeratomes.3-7
Recently, we compared the efficacy, safety, predictability, stability. Using a femtosecond laser versus standard LASIK with a mechanical microkeratome) for myopia8,9 we also studied the changes in corneal higher-order aberrations and contrast sensitivity after LASIK. Efficacy indexes for both procedures were good, although the femtosecond group showed slightly better results than mechanical LASIK (1.07 vs 1.00). In both procedures, satisfactory visual outcomes in relation to the safety index were observed, and most eyes maintained or gained multiple lines of BCVA. Patients who underwent LASIK with the femtosecond laser gained more lines of BCVA than those who underwent mechanical LASIK.
Predictability was also good, and 100% of eyes had a spherical equivalent (SE) within 1.00 D. The femtosecond group showed a slightly higher percentage of eyes within the 0.50 D range in SE compared with the mechanical group (98% vs 92%). In terms of stability, both groups experienced a small regression between months 1 and 6 of follow-up, although it was larger for standard LASIK patients. The stability index at 6 months was excellent in the femtosecond group (0.08 D change of SE).
OPTICAL QUALITY OF THE CORNEA
Our results indicate that standard LASIK surgery was associated with higher values of corneal higher-order aberrations after surgery than LASIK with the femtosecond laser for both small (3.5 mm) and large (6 mm) pupil diameters. Spherical aberration increased significantly for both procedures, although the increase was higher for standard LASIK than with the femtosecond laser (P<.01). Geometric differences in the stromal bed that are created by either the femtosecond laser or mechanical microkeratome may play a role in the differences between the surgeries.
A postoperative increase of coma-like aberrations occurred as a result of both surgeries. In the microkeratome group, this increase may be related to the difference in the hinge angle between both flaps. Here, the flap depends on corneal diameter and corneal curvature, and the variation in the hinge angle between patients is beyond the surgeon's control. In contrast, the hinge angle with the femtosecond laser is always constant. In both surgeries, the increase of coma-like aberrations after surgery is expected, due to the effect of the flap hinge on aberrations.
CONTRAST SENSITIVITY
There were no statistically significant differences in contrast sensitivity values for 1.5, 3, and 6 spatial frequencies (CPD) at any luminance level between the femtosecond and microkeratome groups. We did, however, report significant differences at 12 CPD under mesopic conditions and at 18 CPD under all luminance levels. Femtosecond laser for LASIK showed better contrast sensitivity compared with standard LASIK under mesopic conditions at 12 CPD and photopic and mesopic conditions at 18 CPD.
Why does LASIK with the femtosecond laser show better contrast sensitivity than standard LASIK at these spatial frequencies and luminance levels? We believe that it is because increased higher-order aberrations show poor contrast sensitivity at the high spatial frequency pattern followed by standard LASIK. This finding corroborates the relationship between higher-order aberrations and contrast sensitivity at high spatial frequencies. If the femtosecond laser group showed lower values of higher-order aberrations after surgery than the mechanical group, better results in contrast sensitivity would be achieved.
Geometric differences in the stromal bed, as previously suggested, may play a role in higher-order aberration differences. Under mesopic levels, these differences may be attributed to the increase of higher-order aberrations at large pupil diameters. One should also consider that differences in the flap dimension or postoperative pharmaceutical treatment may also play a significant role.
In my opinion, LASIK with the femtosecond laser is a safe and effective procedure for treatment of myopia and should be considered as a surgeon's first choice. Visual acuity and contrast sensitivity improvements after use of the femtosecond laser for LASIK are related to the differences in postoperative optical quality of the cornea found with femtosecond and microkeratome flap creation.
Montés-Micó, PhD, is Associate Professor in the Optics Department, Faculty of Physics, University of Valencia, in Spain. Dr. Montés-Micó states that he has no financial interest in the products or companies mentioned. He may be reached at robert.montes@uv.es.
