Excimer: Ablation on Human Hair
The unparalleled precision of the excimer laser makes it uniquely suited to the task of refractive corneal surgery. Each pulse of the laser removes 0.25 microns of tissue. Think of it as slicing 1/200 of a human hair, 1/28 of a red blood cell, or 1/39 millionth of an inch in 4 billionths of a second. This allows the surgeon to literally sculpt the cornea, gently and precisely, into a more desirable shape that allows rays of light to focus properly on the retina.
As explained previously, patients with nearsightedness have corneas that are too steep for the length of their eyes. The excimer laser is used to flatten the cornea so that the light rays that pass through it come to a point of focus on the retina, rather than in front of it.
Patients with hyperopia have corneas that are too flat for the length of their eye. The excimer laser is used to steepen the cornea so that light rays are focused on the retina, rather than behind it.
Photorefractive Keratectomy (PRK)
This is a procedure in which the front surface of the cornea is directly sculpted by the excimer laser. The surgeon prepares the eye by gently removing the surface layer known as the corneal epithelium. This layer regenerates itself within a few days. Computer-controlled pulses are directed at the exposed surface (corneal stroma) to reshape the cornea. Less than ten percent of the cornea is affected, with the deeper layers remaining untouched. The entire procedure takes approximately ten minutes per eye and is virtually pain free.
The primary advantage for the use of femtosecond lasers in cornealrefractive surgery is the improved safety over microkeratomes in creation of the lamellar flap.Additional advantages include increased precision,customization, and ease of surgery.
Although microkeratomesare generally safe with low complication rates,mostsight-threatening problems associated with LASIK occur as a result of flap complications.1The driving force behind the adaption of femtosecond lasers has been to minimize these complications.
Femtosecond lasers allow for customization of the flap for each patient. In addition to producing reliable flap thicknesses, the flapdiameter can be selected to one-tenth of a millimeter.
Femtosecond lasers are instruments that offer more surgical precision than current manual techniques.
Laser-Assisted In Situ Keratomileusis (LASIK)
The LASIK process also uses the excimer laser to reshape the cornea, but is done under a thin, protective, corneal flap. Refractive vision correction performed in the interior of the cornea (LASIK) offers numerous advantages over refractive vision correction performed on the cornea's surface (PRK). Rather than vaporizing the epithelial cells to expose the corneal stroma, a specialized instrument known as a microkeratome creates a flap of corneal tissue that is attached by a “hinge”. This flap is gently pulled back like a tiny, clear, hinged lid and the corneal stroma is exposed. The laser part of the LASIK procedure takes place in the exposed corneal bed (corneal stroma). The laser application itself usually takes about thirty to ninety seconds.
After the exposed corneal stroma is treated by the laser and minute amounts of cells are vaporized, the flap is replaced in its original position. Amazingly, the flap is held in position by the eye's natural suction facility, providing increased comfort and decreased recovery time for the patient. The entire procedure takes approximately ten to fifteen minutes per eye and , again, is virtually without discomfort.
LASIK (Laser-Assisted In Situ Keratomileusis)