Femtosecond laser flap thickness highly predictable

A correlation was found between room temperature and corneal edema due to temperature-related changes in laser spot delivery.

ByMichela Cimberle

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MONTE CARLO – Flap creation with the IntraLase FS femtosecond laser has high predictability and a low complication rate, according to a study on a large number of eyes. Interesting correlations were also found in the study between room temperature and procedure outcomes.

The study, presented by Bojan Pajic, MD, at the European Society of Cataract and Refractive Surgeons Winter Refractive Surgery Meeting, was carried out in 405 consecutive eyes treated with LASIK for either myopia (383 eyes, mean –4 D) or hyperopia (22 eyes, mean +2.75 D).

“The aim of our study was to assess the safety and predictability of the IntraLase femtosecond laser in relation to both flap outcomes and visual results,” said Dr. Pajic of Olten, Switzerland, where the study was carried out.

LASIK was performed in all eyes using a Technolas (Bausch & Lomb) excimer laser.

High predictability

In all 405 eyes, target flap thickness was 120 µm. After the procedure, central flap thickness was measured by confocal corneal laser scanning microscopy.

“Results demonstrated that femtosecond laser flap cutting is a highly reproducible technique with a high rate of predictability. The mean flap thickness was 120.3 µm with a standard deviation of 3.6 µm. Maximum flap thickness was 126 µm, and minimum flap thickness was 114 µm,” Dr. Pajic said. “Unlike what happens with traditional microkeratomes, we can definitely rely on this instrument to cut LASIK flaps of exactly the thickness we want.”

No significant endothelial cell loss was found at 6 months postop.

Visual results were good in both myopic and hyperopic eyes. Predictability was high in both groups of patients. Approximately 98% of patients were within ±0.5 D of attempted correction, and results were stable over time, Dr. Pajic said.

Best corrected visual acuity was unchanged in 27% of the patients, improved by one line in 55% and improved by two lines in 18%. No patient lost lines of visual acuity.

“A few patients required a second LASIK procedure for enhancement, but this was not related to the femtosecond laser use,” Dr. Pajic said.

Complications were few. Striae were seen in two eyes, and three eyes were reported to have diffuse lamellar keratitis.

Confocal microscopy. Corneal edema is more pronounced in the cornea on the right, exposed at a room temperature of 22° C.

Images: Pajic B

Hotspot of femtosecond laser at room temperature of 17° C.

Hotspot of femtosecond laser at room temperature of 24° C. The spot is less homogenous.

Room temperature, edema

“An interesting correlation was found between interface edema and room temperature,” Dr. Pajic said.

Confocal microscopy observation of the cornea after surgery showed that eyes that were exposed to a warmer environment tended to develop proportionally more edema than eyes operated on at a slightly lower temperature.

“Just a matter of a couple of degrees made the difference,” Dr. Pajic noted.

The external temperature, he explained, has an impact on the delivery of femtosecond laser energy to the tissues.

“If we compare the hotspot of the laser at 17° C and at 24° C [room temperature], we can see that with the raise in temperature the spot loses homogeneity. Temperature is a main issue if we want good results with femtosecond lasers,” he said.

He suggested that the ideal room temperature should be between 17° and 22°C.

Treatment time is also important, Dr. Pajic said.

“We found that the best results are obtained when treatment time is under 82 seconds from the moment suction is started to the moment it is released,” he said.

For more information:

Bojan Pajic, MD, can be reached at Klinik Pallas Augenzentrum, Louis Giroud Strasse 20, CH-4600 Olten, Switzerland; +41-62-2866200; fax: +41-62-2866220; e-mail: bpajic@datacomm.ch.

Michela Cimberle is an OSN Correspondent based in Treviso, Italy, who covers all aspects of ophthalmology. She focuses geographically on Europe.