PRK

What is PRK?

Photorefractive Keratoplasty Surgery (PRK) is a type of refractive surgery that incorporates the technology of the Excimer Laser to reshape the anterior surface of the cornea. It has been performed clinically since 1991 world-wide, with tens of millions of patients having undergone this procedure with excellent results. The surgery is designed to eliminate or reduce your dependency on glasses or contact lenses. PRK can correct nearsightedness (myopia), farsightedness (hyperopia), and astigmatism. To understand how PRK offers certain advantages among a variety of refractive procedures, it is important to understand the structure of the cornea. The human cornea is comprised of five layers as shown in the histology section below: Epithelium (Ep), Bowan’s Membrane (Bo), Stroma (St), Descemet’s Membrane (De), and Endothelium (En).

More than 90% of the cornea is made of Stroma, a series of parallel collagen lamellae arranged in perfect molecular registration to provide optical clarity. In addition, the cornea has an intrinsic curvature which works in conjunction with the crystalline lens behind the iris to focus light on the retina. If light rays can focus on the retina without the aid on an optical device, the eye is considered Emmetropic for distance. However, if light naturally focuses in front of the retina, the eye is Myopic. If light focuses behind the retina, the eye is Hyperopic. And if two planes of light 90 degrees apart do not focus on the retina, then Astigmatism is present. Photorefractive Keratectomy Surgery achieves its refractive objectives by re-shaping the corneal stroma. To treat myopia, the cornea is flattened, and for hyperopia, the cornea is steepened. Since astigmatism arises from a cornea that has two different curvatures, it may be necessary to steepen the flattest axis while flattening the steepest curvature. This is referred to as a cross-cylinder ablation. Nidek, the developer of the excimer laser used by Dr. Rubman, developed this technology and has been assigned the patent (U.S. 6,315,771 B1). A combination of ablation strategies may be utilized to achieve the best possible results.

The Treatment

To alter the refractive power of the cornea, the excimer laser must ablate (remove tissue from) the corneal stroma. To access the stroma, the first layer of the cornea, the epithelium, must be removed. The epithelium is approximately 1/10th the thickness of the entire cornea and will regrow in several days following the surgery. The corneal stroma does not regenerate and changes made to it by the excimer laser are permanent. Epithelial removal is accomplished with the excimer laser and a specialized device, the Amoils Rotary Brush. The epithelium is approximately 50m thick and a plano (non-refractive) excimer ablation is used to remove 90% of this tissue. Next, the Amoils Brush is applied to the cornea to gently remove any residual epithelium. The refractive ablation is then performed with the excimer laser. The Nidek EC-5000 uses slit scanning technology which achieves extremely smooth tissue removal across the corneal surface. The illustration below shows how the ablation wave passes across the cornea, changes orientation, and again sweeps over the cornea.

After the ablation, a bandage contact lens is placed on the cornea. The lens is worn continuously for five days allowing the epithelial surface to regenerate. Antibiotic and non-steroidal anti-inflammatory drops are administered while the lens is worn. On the fifth day, the contact lens is removed in the office and the cornea assessed as to whether complete re-epithelialization has occurred. Following re-epithelialization, a topical steroidal drop is used and then discontinued after several weeks.

Pre-operative Evaluation

To determine if you are a candidate for PRK Surgery you will require a series of measurements that will be performed by Dr. Rubman in the office. These tests will determine the amount of refractive error that needs to be corrected, the corneal thickness, the degree and uniformity of your corneal curvature as well as the general health of your eye. The gold standard used to acquire this information is the Pentacam, a device that takes Scheimpflug slit photographs of the eye to create important diagnostic corneal maps of thickness and elevation, as well as generating other important corneal data. A particular pre-operative concern is whether the patient has an early manifestation of corneal pathology known as keratoconus. The earliest appearance of this condition is referred to as forme fruste keratoconus and it is very important to detect this in patients who may be considering refractive surgery. The refractive map shown below (left) illustrates a more advanced case of keratoconus and the unsuitability for any refractive laser procedure. However, even subtle corneal pathology can be revealed with the Pentacam. As shown below (right), this -3.00 myope has a cornea with posterior keratoconic changes and should not undergo any refractive surgery.