The word ‘laser’ stands for “light amplification by stimulated emission of radiation”. It is a special kind of light that is of a single frequency (wavelength) and highly synchronised (coherent). This allows lasers to be precisely focused and attain tremendous energy levels.
The mention of lasers being a form of radiation leads some to question whether they can cause cancer. This is not so because lasers are not related to either X-rays or radioactive materials. In fact, all kinds of light are considered as forms of radiation.
The most commonly used laser in LASIK surgery is the excimer laser. This laser is produced by a combination of argon and fluorine gases. The light of the excimer laser is a far ultraviolet light and is invisible to the eye. The excimer laser is unique because of its ability to remove corneal tissue without scarring or burning the cornea. Excimer lasers are extremely precise, and are able to etch and shape the cornea into any form or shape.
The excimer laser literally vapourises the cornea and “ejects” corneal molecules without charring the cornea. This reduces inflammation and promotes superficial healing. At the same time, it prevents deep central “healing” of the cornea, and this ensures that the cornea remains in the form or shape intended.
Improvements in technology have made excimer lasers safer and more precise. The old broad-beamed lasers (first and second generation) are less accurate and expose patients to more risks. Currently, the best available lasers are fast, scanning lasers with extremely small beam sizes (less than 1 mm in diameter). These lasers gently spread a purified beam of laser energy in very small pulses over the cornea, creating a smooth ablation surface. The newer-generation lasers available now are able to perform various types of treatment such as standard, wavefront-guided, abberration free, topography-guided and corneal wavefront guided LASIK (PerfectShape® LASIK).
A femtosecond laser is an infra-red laser that is used to create the LASIK flap. Presently, AMO Corp. (California, USA) is the most established company producing a femtosecond laser to create the LASIK flap.
Instead of cutting a flap in the cornea the way a microkeratome does, the laser creates tiny bubbles within the cornea to split the layers of corneal tissue. It joins these bubbles into a contiguous layer to create the flap, making the size and contour of the flap more even and predictable. This is particularly beneficial for very flat or very curved corneas. The latest generation of femtosecond laser, creates LASIK flaps in 10 to 12 seconds, which is similar to a mechanical microkeratome.
The risk of a buttonhole or a partial flap is considerably less likely with femtosecond lasers. The flap created by these lasers also induce less visual aberrations and distortions compared to the microkeratome-cut flap. Femtosecond lasers are also more precise in creating the diameter and thickness of the flap when compared to the mechanical microkeratomes. However, if the femtosecond laser is set at too high an energy level to create a flap, it will induce more inflammation and the possibility of light sensitivity. However, this light sensitivity can be treated with topical eyedrops and is a temporary phenomenon.