The use of the EXCIMER LASER in corneal refractive surgery has greatly increased the safety of the procedure as the refractive correction is achieved by the removal of a little as 10 to 20% of the total corneal thickness.
The excimer laser is a “cool” laser, and does not generate heat in the cornea. The laser energy destroys intermolecular bonds in the corneal tissue resulting in tissue removal by a process termed “photoablative decomposition”. The exquisite precision with which tissue can be removed and the absence of thermal damage render this laser particularly suitable for surgery on the delicate cornea.
Removal of corneal tissue alters the corneal contour. By specifically removing tissue from different portions of the cornea, the surgeon is able to correct the different refractive errors. Removal of central corneal tissue in a circular fashion results in central flattening and corrects myopia.
Surface Photorefractive Keratectomy (SURFACE PRK), has been in use all over the world since 1987 and has proven to be safe and effective, particularly in the treatment of myopia and astigmatism. However, when very high refractive errors are treated by SURFACE PRK, the predictability of the procedure is lower with increased occurrence of undesirable side effects. Most of these adverse results appear to be due to excessive corneal healing. Since removal of corneal tissue from the surface is a powerful stimulus for corneal healing, the treatment of higher errors (requiring increased removal of corneal tissue) produces poorer results.
ALLEGRETTO WAVE – EXCIMER LASER
A recent advance in the treatment of such high refractive errors is a procedure termed Laser In-Situ Keratomileusis (LASIK). In this surgery, a flap of superficial corneal tissue is cut using a motorized blade. The flap is lifted exposing the underlying corneal stroma. The excimer laser is then used to perform the appropriate tissue removal in the corneal bed. At the conclusion of laser delivery, the corneal flap is repositioned in its original position. The flap adheres to the underlying corneal stroma without the need of sutures.
How Does the Laser Work?
Many patients ask, “How can a laser correct my vision?” The laser removes tissue from the center of the cornea (in the case of myopia) to flatten its curvature and correct nearsightedness. In the case of farsightedness, tissue is removed from the periphery of the cornea to steepen its curvature. The laser essentially reshapes the cornea's front surface. To do this, the corneal stroma (tissue beneath the corneal epithelium) must be exposed. This can be accomplished by directly removing the corneal epithelium with a laser (PRK) or by creating a corneal flap with a microkeratome (LASIK).
The Human Cornea. With PRK, treatment is performed on the surface after the epithelium has been removed. With LASIK, the treatment is performed in the stroma, and the anterior architecture is preserved
Many types of lasers are used in laser eye surgery. Argon lasers heat tissue and have been used for years to treat disorders such as diabetic retinopathy and glaucoma. YAG lasers break tissue bonds by creating a shock wave and are used following cataract surgery and to treat certain types of glaucoma. The excimer laser is a gentle, cold-beam laser uniquely suited to the task of refractive corneal surgery.
The goal is to reshape the cornea so that rays of light that enter the eye are focused clearly onto the retina. The excimer laser produces a cool, ultraviolet beam of light (193 nanometers in length) that literally vaporizes tissue as it breaks carbon-to-carbon bonds without harming adjacent tissue. Tissue is removed in a precise fashion on a microscopic level, leaving adjacent tissue unharmed. This vaporizing process is called photoablation.
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 in the previous chapter, 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.
As explained in Chapter Two, 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.
Laser 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 In Situ Keratomileusis)
Cornea is made flatter in Myopia
Cornea is made steeper in Hyperopia
The amount of tissue removed in each of these procedures is determined by the patient's degree of refractive error. Before the laser is employed to vaporize the tissue, the degree of refractive error is translated into numbers that are entered into the laser's computer. The quantity and pattern of tissue removal unique to each patient are then calculated. Both PRK and LASIK are refractive procedures that utilize the precision of the excimer laser to reshape the cornea by vaporizing