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Flap thickness can be predetermined to virtually any depth up to 400 ? 10 ?m from the laser interface.
"The interest in the IntraLase femtosecond laser has grown out of a few factors. One is the patient's perception of the safety of cutting tissue with laser versus a blade. In addition, another factor is concern about safety, predictability, and the reproducibility of flaps using a traditional blade microkeratome. Finally, there is increased surgeon comfort when using a laser versus a blade," said Dr. Yoo, assistant professor of ophthalmology, Bascom Palmer Eye Institute, Miami.
"In our practice, we found improved visual acuity and quality of vision after LASIK with the femtosecond laser, perhaps in part because there may be less optical aberrations induced by the planar flap created with the IntraLase femtosecond laser compared with the meniscus flap created with the blade microkeratome," she said.
"With flaps created using the IntraLase femtosecond laser, there is less epithelial ingrowth, probably because the side cut is at a steeper angle. Since July 2003, we have had no buttonhole flaps, and partial or incomplete flaps almost never occur," she explained.
The precision of the microkeratome can vary greatly, and in the literature the flap thickness has been reported to vary more than ±100 µm from the predicted thickness. The meniscus flap created by a blade microkeratome tends to be thicker in the periphery and thinner in the center, which induces increased optical aberrations.
The hinge angle also differs between the two instruments in that a blade microkeratome creates a D-shaped stromal bed and the hinge angle tends to be larger and requires a larger flap diameter to accommodate the ablation, according to Dr. Yoo.
Few complications "The flap created with the IntraLase femtosecond laser offers greater dimensional and positional control and development of few complications. A study performed by Dr. Brian Will found that safety with the femtosecond laser was high. In 1,827 consecutive eyes, the instrument lost suction in eight cases. No other complications occurred. This seems to be the experience across practices as well as in our own," she explained.
Dr. Yoo explained that a number of dimensions can be programmed with the femtosecond laser. Flap thickness can be predetermined to virtually any depth up to 400 ± 10 µm from the laser interface. The device offers minimal biological variability, so that the flap thickness is not as dependent on the preoperative corneal pachymetry as with a blade microkeratome. The flap diameter, which also is not affected by biological variability, can be programmed depending on the white-to-white measurement. The laser begins at the bottom or side based on the desired location of the hinge. A "pocket" is created for excess gas, which avoids an opaque bubble layer.
The laser separates the lamella in a raster pattern. The vertical side cut places the laser pulses on top of each other in a circular pattern, which avoids the pre-programmed hinge location. The side cut angle provides a well-defined edge at almost any angle. Finally, the hinge angle and hinge position can be set as desired. The flap position can also be moved according to the position of the pupil in relation to the geometric center of the cornea, she explained.