The Eye

The Eye is a "living video camera". It has camera film (the retina) and a diaphragm like a camera shutter (the iris). [The pupil is an empty space formed by the inner edge of the iris. The light passes through this pupil (hole-in-the-iris) to the retina where images are seen.]

The eye is a camera with an imaging system consisting of two lenses. The cornea (the clear tissue in front of the iris and the pupil) is the main lens of the eye. It is an external lens and does two-thirds of the focusing. The cornea has a static power designed to maintain a constant power of focus constant throughout life. What we call the lens is immediately behind the pupil. It is the internal lens of the eye and does one-third of the focusing. It is designed to change the focus of the eye between distant and near objects as necessary.

Refractive Surgery

Refractive surgery is any surgery that changes the focus of the eye. Since the eye has two "lenses", its focus can be altered by doing surgery on either, the cornea or the lens. Surgery on the cornea could be LASIK or PRK which have superceded RK (Radial Keratotomy). Surgery involving the internal lens includes the Intraocular Collamer Lens (Visian ICL), cataract surgery, or Refractive Lens Exchange (cataract surgery done earlier due to a desire to realize the refractive surgery benefits of cataract surgery sooner). Cataract surgery effects its change in focus by removal of the natural lens (with a inherent power which requires the patient to wear glasses or contacts to see well) and replacing it with a new artificial lens (which will be of a different power and will therefore change the eye’s focus). Hence it is also considered to be refractive surgery.

The Cornea

The Cornea is approximately one-half millimeter (mm) thick. This is the same as 500 microns. Since the cornea does most of the focusing for the eye and is on the eye’s surface where it can easily reached for surgical alteration, it is a natural choice for refractive surgery. The epithelium or surface cells of the cornea are approximately 70 to 90 microns thick. The stroma, or body, of the cornea is the remaining 400 (+/-) microns thick. The endothelium or innermost layer of the cornea is only one cell-layer thick. However, it is very important because it pulls water out of the cornea constantly. This dehydration of the cornea is important in keeping it clear so that light can travel through it unimpeded to the lens and farther to the retina.

Excimer Laser Surgery

The EXCIMER laser, which is a specific type of "cool" laser, generates its power from light in the ultraviolet range. We do not see the laser because ultraviolet light cannot be visualized by the human eye. Because the laser does not heat the tissue, there is no tissue damage as the result of the laser light. The energy of the laser simply causes miniscule amounts of corneal tissue to dissociate at a microscopic level. As the treatment with the laser proceeds, microscopic layers of tissue, approximately 1/10th the width of a human hair are removed (ablated). The laser is programmed to ablate precisely the amount of tissue needed to achieve the desired visual result. The types of excimer laser surgery available today are PRK, LASIK, and their modifications. With excimer laser surgery, the attempt is to maintain 250 microns or approximately half of the corneal thickness after the treatment is completed. This limit implies a limitation on how much correction can be achieved by the excimer laser surgery and varies slightly with each patient’s individual corneal thickness measurement. Approximately twelve to fifteen microns of corneal tissue must be ablated for each diopter of corneal power correction.

PRK (Photorefractive Keratectomy)

PRK uses the laser’s energy to reshape the cornea to correct myopoia (near-sightedness), hyperopia (far-sightedness), or astigmatism. The epithelium or surface layer of the cornea differs in thickness among patients. Therefore it is necessary to remove the most superficial 90 to 110 microns of corneal tissue by any of several methods before performing the refractive surgery using the excimer laser itself. After the excimer laser has removed the prescribed amount of corneal tissue, the cornea behaves as if it has a large corneal abrasion. Application of a contact lens decreases discomfort during the two to four days it generally takes for the cornea to regrow its surface cells. After the surface cells have regrown and the contact lens is removed by the surgeon, the cornea sees much better and feels essentially normal. /p>

LASIK (Laser Assisted In Situ Keratomileusis)

With the development of precise surgical cutting instruments, the use of the excimer laser has been combined with a precise incision to produce the same precise laser-generated surgical result, but with less discomfort and with faster visual recovery than PRK. It has become the most commonly performed refractive surgery procedure used today by far. During LASIK the surgeon first creates a thin corneal flap (110 to 150 microns thick) using a device called a microkeratome, or alternatively and at added expense a 110 to 130 micron flap using an additional special femtosecond laser. The corneal flap is lifted up, and the laser beam is applied to the exposed interior surface of the cornea to reshape the tissue in the same way as is done in PRK. The flap is then replaced over the treated area. This corneal flap serves a natural bandage, which eliminates the discomfort associated with other types of refractive surgery, and expedites the healing process. Because of the extraordinary bonding properties of the corneal tissue, stitches are generally not needed to keep the flap in place postoperatively.