Diets & Weight Loss


Incision into the cornea

Question: Whats the difference between Photorefractive keratectomy vs Radial Keratotomy?

Answer: Refractive keratoplasty is a generic term, which includes all surgical procedures on the cornea to improve vision by changing the shape, and thus the refractive index, of the corneal surface. Refractive keratoplasties can be broadly subdivided into keratotomies, i.e., corneal incisions; keratectomies, i.e., removal of corneal epithelium; and keratomileusis, i.e., reshaping a stromal layer of the cornea. Additionally ocular implants are now commercially available for the treatment of mild myopia. The following is a description of the most common type of keratoplasties performed as a treatment of myopia, hyperopia, and astigmatism. Radial keratotomy (RK) Using a high-powered microscope, the physician places microincisions (usually eight or fewer) on the surface of the cornea in a pattern much like the spokes of a wheel. The incisions are very precise in terms of depth, length, and arrangement. The microincisions allow the central cornea to flatten, thus reducing the convexity of the cornea, which produces an improvement in vision. Photorefractive keratectomy (PRK) uses a computerized laser for corneal reshaping. The excimer laser produces a beam of ultraviolet light in pulses that last only a few billionths of a second. Each pulse removes a microscopic amount of tissue by evaporating it, producing very little heat and usually leaving underlying tissue almost untouched. Overall, the surgery takes approximately 10-20 minutes; however, the use of the laser beam lasts only 15-40 seconds. In patients with myopia, corneal tissue is removed in its center in order to flatten it, while in hyperopia, corneal tissue is removed at its periphery, in order to create corneal steepening. Astigmatism describes a corneal contour which is not perfectly symmetrical, similar to the shape of a back of a spoon. The amount of tissue removal thus varies along different corneal meridians. Automated lamellar keratoplasty (ALK) This is also referred to as keratomileusis in situ and is used for treating hyperopia. In this procedure a "cap" of corneal tissue is removed and a refractive cut is made with a microkeratome in the stromal bed of the cornea. When the "cap" is positioned back into its original location on top of the eye, microscopic scar tissue is formed, causing the "cap" to bulge out, thus correcting the overly flattened cornea that is associated with hyperopia. The cornea and "cap" adhere to each other, eliminating the need for sutures. Normally, one eye is treated at a time, with about 3 to 4 weeks allowed between each eye surgery. To ease any discomfort, the eye is anesthetized with special drops, and the patient is given a mild sedative to remain relaxed and aware throughout the procedure. Epikeratophakia or Epikeratoplasty (lamellar keratoplasty) These procedures involve suturing a prelathed donor cornea onto the surface of the recipient's cornea. A microkeratome is used to remove a layer of the patient's cornea and the donor cornea is sutured into the keratectomy bed. This surgery has been proposed as a means of correcting adult and pediatric aphakia, keratoconus (a conical protrusion of the cornea, caused by thinning of the stroma, and resulting in major changes in the refractive power of the eye), and myopia. As a treatment of myopia, this technique has been largely abandoned because of its lack of predictability and poor optical results. Laser in-situ keratomileusis (LASIK) In this technique the epithelial layer of the cornea is pulled back, creating a flap and the stromal bed of the cornea is reshaped with the laser. Finally, the protective layer is repositioned without sutures and is secure after five minutes of air-drying. The LASIK procedure appears to be gaining in popularity. Removal of tissue from the stromal bed is more precise in comparison to PRK or ALK. Additionally, compared to PRK, LASIK is associated with fewer healing complications and is less painful since the epithelial surface of the cornea remains intact. The location and amount of tissue removed is similar to that described for PRK. Ocular implants. In April 1999 the FDA approved ocular implants composed of tiny transparent crescents that can be implanted in the periphery of the cornea, resulting in a flattening of the cornea. The ocular implants, called KeraVision Intacs, are designed to treat mild myopia and astigmatism. The implants, although considered permanent, can be removed if complications develop. Clean Lens Extraction (CLE) Phacoemulsification of the lens with intraocular lens implantation (also called clear lens (CLE) extraction) has been proposed as an alternative to corneal refractive surgery in patients with moderate to severe hyperopia and high myopia. The available clinical evidence consists of case series with small numbers of patients. The outcomes reported indicate a postoperative uncorrected visual acuity ranging from 20/25 to 20/50 in patients with severe hyperopia and 20/40 in eyes with high myopia. There


Keratotomy Related Products and News