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REFRACTIVE CORNEAL SURGERY

The inconvenience of spectacles to many wearers and the complications associated with contact lenses have resulted in a search for surgical solutions to the problem of refractive error.

Radial Keratotomy

In the late 1940s, Sato of Japan created anterior and posterior corneal incisions to alter the curvature of the cornea. Results were poor, and endothelial decompensation with corneal edema occurred frequently. In 1972, Fyodorov of the USSR began to use anterior corneal cuts only. Currently, the operation consists of radial incisions involving 90% of the corneal thickness and extending from a clear optical zone (usually the central 3 mm or more of the cornea) toward but not reaching the limbus. The amount of correction achieved is modified by the size of the optical zone and the number and depth of the incisions. Various formulas and computer programs are used to determine the value of these parameters in each case.

There is general agreement that radial keratotomy does reduce the degree of myopia and is most effective for myopia in the lower range (-2 to -4 diopters). There is a significant degree of unpredictability in the final result, with under- or overcorrection or even progressive hyperopia. Glare and fluctuations of vision during the day are commonly reported side effects. Delayed healing of corneal incisions, with corneal infections occurring up to 2 years after the procedure, have been reported. Endophthalmitis, traumatic cataract, and endothelial cell loss are rare but have been reported.

Keratomileusis

In 1961, Barraquer of Colombia reported on the technique of myopic keratomileusis for the correction of high degrees of myopia. The procedure has been performed in other countries but by relatively few surgeons. A deep lamellar corneal autograft is cut; the tissue is frozen and then reshaped with a cryolathe to obtain a flatter curvature after thawing; and the autograft is then sutured back into position. Expensive cryolathe and microkeratome equipment is required. The procedure has also been used for hyperopia. Automated lamellar keratoplasty (ALK) is a new form of this procedure.

Complications of keratomileusis include improper depth of the lamellar bed, delayed epithelialization over the resutured tissue, interface epithelial growth and opacity, and irregular astigmatism.

Procedures to Correct Astigmatism

Various patterns of keratotomy have been described to correct corneal astigmatism. Irregular astigmatism continues to be a serious problem following most corneal operations, including radial keratotomy and penetrating keratoplasty, and after cataract surgery. Troutman and others have described relaxing incisions, compression sutures, and wedge resections for postkeratoplasty astigmatism, utilizing a surgical keratometer. Various techniques for cataract incision, such as scleral tunnel incisions and clear corneal incisions as well as altering incision location, have been reported as useful in preventing postoperative astigmatism after cataract surgery.

Alloplastic Corneal Implants

Disks of many different materials have been inserted into corneal stromal pockets, initially to control corneal edema but more recently to correct refractive errors. In most cases, the corneal tissue anterior to the implant undergoes necrosis. Hydrogel and polysulfone lenses have been more successful than other types of lenses tried so far. Use of alloplastic corneal implants would remove the need to rely on autologous or homologous material in refractive surgery. A plastic ring meant to be implanted intrastromally is under investigation and shows promise as a refractive surgery procedure.

Clear Lens Removal & Phakic Lens Implants

A few surgeons around the world have advocated the removal of clear lenses in high degrees of myopia, suggesting that the risk of doing so is minimal owing to the safety of extracapsular lens extraction. The procedure is controversial because of a significant risk of retinal detachment in high myopes. Phakic lens implants are under investigation for treatment of high refractive errors.

Lasers

A further approach to refractive corneal surgery involves the use of lasers (see Chapter 24). The excimer laser has received the most publicity, but other machines such as the solid-state neodymium:YAG laser and "minilasers" have been shown to be effective also. Laser photorefractive keratectomy (PRK) produces precisely controlled flattening of the anterior cornea to reduce myopia. The procedure also is done for astigmatism and hyperopia. Anterior stromal haze, irregular astigmatism, and regression have been observed after PRK. In the United States, theFood and Drug Administration (FDA) has given approval for PRK to two laser companies; the procedure has been done in many other countries for years.

Laser in situ keratomileusis (LASIK) is a procedure that utilizes a motorized microkeratome to cut a shallow lamellar corneal disk which is folded back to allow excimer laser photoablation to the stromal bed. The flap is then folded back into position. The patient is more comfortable immediately after surgery than with PRK, and best vision is restored earlier. Irregular astigmatism and interface problems are among the complications of this procedure.

Controversy exists over the choice between surface ablation (PRK) and LASIK. In general, PRK is used for lower (-6.00 D or less) and LASIK for higher levels of myopia. Improved methods of surface ablation or other techniques may eliminate the need for LASIK in the future. Generally, LASIK is not approved by the FDA.

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