ADVANCED SURFACE ABLATION (ASA)

Photorefractive keratectomy (PRK) using the VISX laser was the first method of laser vision correction approved by the FDA. Since the approval of PRK, there have been many improvements in the laser and the techniques used for surface laser vision correction. Advanced Surface Ablation uses laser treatment on the surface of the cornea after removing the surface cells (called the epithelium) which then regrow. For the painless procedure of advanced surface laser ablation, a drop of anesthetic is placed upon the eye followed by removal of the surface epithelium (cells) using the laser, alcohol, or scraping gently (or a combination of techniques). The “cold” laser called an Excimer laser then adjusts the focus of the eye to correct vision by changing the front curve on the cornea to be flatter for nearsightedness and steeper for farsightedness. To correct astigmatism, the laser changes the curves on the cornea to make light focus simultaneously on the retina of the back of the eye. This is bloodless with no cutting. The laser pulses correct curvature with an accuracy of one-millionth of an inch. Best uncorrected vision may improve more slowly with advanced surface ablation and there may be more discomfort for the first 48-72 hours than with LASIK. However, most of our patients have no discomfort because diluted topical anesthetic drops are used until the epithelial cells have healed under a bandage soft contact lens which is placed on the cornea at the end of the laser treatment. Usually, the contact lens is removed in the office and the anesthetic drops stopped about 72 hours after treatment.

It is important to realize that haze or clouding of the cornea may occur with any kind of laser treatment and the risk of haze increases for higher degrees of correction. This risk may be higher for advanced surface laser ablation. An antibiotic called mitomycin may be applied topically for a few seconds for patients requiring higher amounts of correction to reduce this risk.

• There are a number of advantages of advanced surface laser ablation over LASIK which uses a cut with a blade or laser to create a flap.
•  Because of structural weakness, the central cornea may “bulge” causing an irregular surface following laser treatment which is called “ECTASIA”.  Generally the risk of “ECTASIA” is thought to be higher following LASIK than after ASA (Advanced Surface Ablation).
  An average cornea is about 540 microns thick. (About 0.02 inches or about 1/50 of an inch). In surface ablation, the thickness of the cornea is reduced only by the amount of tissue ablated (removed) from the surface of the cornea by the excimer laser in order to correct vison. However, in a LASIK treatment a flap is first created with the femtolaser or with the mechanical microkeratome before tissue is removed under the flap by the excimer laser. The flap is then repositioned and replaced over the area where tissue was ablated (removed). The replaced flap does not add to the strength of the cornea as it did before the flap was cut. (EXAMPLE: A cornea is 540 microns thick and 100 microns must be ablated (removed) for vision correction. After ASA (Advanced Surface Ablation) removes 100 microns for correction there remains 440 microns of residual cornea tissue. If the same cornea has LASIK a flap may be created of 110 microns (which is a common  flap thickness when a flap is created with the femtolaser). The excimer laser then removes the 100 microns  of cornea under the flap for the vision correction. In this example, 540 microns minus 210 microns (110 micron flap plus 100 microns of tissue removed) leaves only 330 microns of residual tissue of the central cornea. This increases the risk of central cornea weakness and eventual development of “ECTASIA” compared to the Advanced Surface Ablation vision correction which left a thickness of 440 microns (33% more tissue).
• In general, most eye surgeons prefer to leave the cornea with a thickness of at least 275 microns after laser correction. Even surgeons who prefer LASIK will often suggest surface ablation for laser vision correction of patients who have thin corneas. In order to have a stronger eye, surface ablation is also often recommended to patients with a higher occupational or recreational risk for blows or injury to the eye such as firemen, boxers, etc.  Surface ablation is also often recommended for patients with high degrees of nearsightedness requiring more tissue removal for vision correction so that additional weakness is not created with a LASIK flap.
• Using Advanced Surface Ablation will allow an extra safety margin of 110-180 microns of cornea thickness which is lost with creation of a flap with LASIK.  Dr. Nepple feels this safety margin to reduce the risk of early or late onset ECTASIA should be offered to ALL patients.
• Advanced surface laser ablation (ASA) may results in less dry eye problems after the treatment, immediate or long term, and eliminates post-flap problems that can occur even years later.

DESCRIPTION OF ADVANCED SURFACE ABLATION

• After placing a small speculum which holds the eye open in a painless manner, a drop of anesthetic is placed on the eye.
• A small ring is placed on the surface of the cornea and the ring is filled with a few drops of an alcohol solution. The alcohol solution is removed after waiting a few seconds. The ring is removed and the layer of surface cells are then removed (or this surface cell layer is folded back creating a flap layer of epithelial cells (this is NOT the middle layer (called the “stroma” ) of the cornea which is cut with a LASIK procedure) to expose just the surface of the stroma (middle layer of the cornea). Alternatively, the surface “epithelial” cells may just be scraped off.
• When an epithelial flap is created it is left attached at one side and the epithelial flap is folded back over the attachment out of the way. Next, the cornea is reshaped using the excimer laser to treat the surface of the stroma of the cornea now that the epithelial cells are off the surface. If an epithelial flap was created, it is unfolded and repositioned over the treated cornea. A bandage contact lens is placed to help the flap stay in place and for comfort. The repositioned epithelial cells are replaced by new cells growing on the surface of the eye over the next two to five days. If the epithelial cells were simply removed, a bandage contact lens is placed and new epithelial cells grow in under the contact lens. After the epithelium is healed, the contact lens is removed. This is usually 3 – 5 days after the laser treatment.

Thus, in advanced surface laser ablation, only the surface of the stromal layer of the cornea is ablated and reshaped by the laser. Note that the deepest layers of the cornea stroma are left untouched and intact because no cut is made with either the microkeratome or the femtolaser. The laser ablation starts at the surface of the cornea stroma layer thus there is more stroma which can be treated without leaving the cornea excessively thin and weakened with a risk of “ectasia”. This may allow laser vision correction treatment for patients who otherwise could not be treated because either they have thin corneas and/or have highly myopic eyes requiring more tissue removal for correction of their vision.

Summary of the Advantages of Advanced Surface Ablation (ASA)

• Less thinning , leaving a stronger cornea for all patients. This is particularly important for patients with higher degrees of myopia (thus requiring more tissue removal by the laser for vision correction) or patients involved in contact sports, etc.
• No stromal flap complications such as lost, displaced, or buttonholed stromal corneal flaps. Should these problems happen after Advanced Surface Ablation (ASA), the epithelial flap simply regenerates and grows back into place, unlike LASIK.
• No DLK, inflammation or infection that occurs in the interface with LASIK.. Even if infection occurs in Advanced Surface Ablation (ASA), it is much more amenable to treatment directly with antibiotic drops since the infection is not covered by a stromal flap.
• Less dry eye. The deeper cornea stromal nerves are not cut by the femtolaser or microkeratome.
• There is no stromal flap with striae as in LASIK which may cause irregular astigmatism with loss of vision.
• In ASA there is no stromal cut created by the femtolaser or the microkerotome. Thus no epithelial ingrowth into the interface cut is possible. In fact, we want the epithelium to grow on the surface after ASA and it usually does so very rapidly.
• Possibly decreased risk of retina detachment.

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