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A study comparing the 30-kHz femtosecond laser (IntraLase, Advanced Medical Optics) and the mechanical microkeratome (Zyoptix XP, Bausch & Lomb) showed both created thinner-than-intended flaps, but with similar variance and independent of preoperative SE, keratometry, or pachymetry, reports one physician.
"Thinner flaps are associated with less risk of post-LASIK ectasia and faster visual recovery," said Dr. Utine, assistant professor, Yeditepe University Eye Hospital, Istanbul, Turkey. "However, we recommend that if the flap thickness is 90 μm or less in the first eye, surgeons should change the blade in the mechanical microkeratome or check the settings for the femtosecond laser before proceeding to making the flap in the fellow eye."
The study included 28 eyes that underwent flap creation with the femtosecond laser and 69 eyes that were treated with the microkeratome. Preoperative characteristics were similar in both groups with respect to mean spherical equivalent (SE) (–2.5 versus –2.4 D), mean keratometry (42.5 versus 43.7 D), and pachymetry (563 versus 551 μm).
Flap thickness was calculated by subtracting the pachymetric stromal bed thickness obtained after the flap was lifted from the preoperative ultrasonic pachymetry. The mean obtained flap thicknesses were 115 ± 15.7 μm in the microkeratome group and 110.4 ±14.1 μm in the femtosecond laser group. They were not significantly different between groups. There were no complications related to flap creation in any eyes and no difference between groups in best-corrected visual acuity (BCVA) outcomes, Dr. Utine reported.
Further analyses were conducted to identify possible correlations between deviation from intended thickness and preoperative SE, pachymetry, or keratometry. No correlations were found for any of those variables for either of the two devices.
"Plots of data for individual eyes show the scatter is high and illustrate why there were no significant correlations in any of these analyses," Dr. Utine said.
She noted that findings in this study are consistent with published literature for the femtosecond laser with respect to reported standard deviation of flap thickness and creation of thinner than intended flaps. They also indicate more reliable performance using the microkeratome compared with its predecessor (Hansatome, Bausch & Lomb).
"Kezirian et al. used the [predecessor microkeratome] to create flaps with an intended thickness of 180 μm and achieved a mean thickness that was 24 μm thinner than intended and with a standard deviation of 29 μm," Dr. Utine said. "In other studies, the standard deviation for mean flap thickness with the [predecessor device] has ranged between 17 and 33 μm. With such high standard deviations, there is a risk of creating flaps that are 50 μm or more thinner or thicker than intended."
When queried by panel members about how these results might affect selection between the femtosecond laser and the microkeratome in individual patients, Dr. Utine indicated the decision is guided by corneal characteristics.
"We prefer the laser in eyes with very steep or flat corneas and also believe it is better than the mechanical device in thinner corneas," she said.