1.radial pattern of incisions in the cornea that cause the cornea to bulge; performed to correct myopia
1.(MeSH)A procedure to surgically correct REFRACTIVE ERRORS by cutting radial slits into the CORNEA to change its refractive properties.
definition of Wikipedia
Radial Keratotomy (n.) [MeSH]
radial keratotomy (n.)
Schematic diagram of RK, with incisions drawn in orange
Radial keratotomy (RK) is a refractive surgical procedure to correct myopia ("nearsightedness"), developed in 1974 by Svyatoslav Fyodorov, a Russian ophthalmologist. Though it has been largely supplanted by newer operations such as photorefractive keratectomy, LASIK, Epi-LASIK, and the phakic intraocular lens, RK remains popular for correction of astigmatism in certain people.
In 1974, Svyatoslav Fyodorov removed glass from the eye of a boy who had been in an accident. The boy, who required eyeglasses for correction of myopia caused by astigmatism, fell off his bicycle. His glasses shattered on impact, with glass particles lodging in both eyes. To save the boy's vision, Fyodorov performed an operation which consisted of making numerous radial incisions extending from the pupil to the periphery of the cornea in a pattern like the spokes of a wheel. After the glass was removed by this method and the cornea healed, Fyodorov found that the boy's visual acuity had improved significantly.
In RK, incisions are made with a diamond knife. Incisions that penetrate only the superficial corneal stroma are less effective than those reaching deep into the cornea, and consequently incisions are made quite deep. One study cites incisions made to a depth equivalent to the thinnest of four corneal-thickness measurements made near the center of the cornea. Other sources cite surgeries leaving 20 to 50 micrometres of corneal tissue unincised (roughly equivalent to 90% of corneal depth based on thickness norms).
The healing corneal wounds consist of newly abutting corneal stroma as well as fibroblastic cells and irregular fibrous connective tissue. Closer to the wound surface lies the epithelial plug, a bed of the cells that form the normal corneal epithelium, which have fallen into the wound. Often this plug is three to four times as deep as the normal corneal epithelium layer. As the cells migrate from the depth of the plug up to the surface, some die before reaching the surface, forming breaches in the otherwise healthy epithelial layer. This consequently leaves the cornea more susceptible to infection. This risk is estimated to be between 0.25% and 0.7% Healing of the RK incisions is very slow and unpredictable, often incomplete even years after surgery. Similarly, infection of these chronic wounds can also occur years after surgery, with 53% of ocular infections being late in onset. The pathogen most commonly involved in such infections is the highly virulent bacterium Pseudomonas aeruginosa.
Large epithelial plugs may cause more scattering of light, leading to the appearance of visual phenomena such as flares and starbursts—especially in situations like night driving, where the stark glare of car headlights abounds. These dark conditions cause the pupil to dilate, maximizing the amount of scattered light that enters the eye. In cases where large epithelial plugs lead to such aggravating symptoms, people may seek further surgical treatment to alleviate the symptoms.
The incisions of RK are used to relax the steep central cornea in people with myopia. Popularized by Svyatoslav Fyodorov of Russia, the original technique of incisions from periphery to center was called the Russian technique (Gulani AC, Fyodorov S: Future Directions in Vision course, June 1997) while the later advances of performing controlled incision from center to periphery was called the American Technique (Gulani AC, Neumann AC: Refractive Surgery Course, February 1996).
RK enjoyed popularity during the 1980s and was one of the most studied refractive surgical procedures. Its 10 year data was published as the PERK (Prospective Evaluation of Radial Keratotomy) study, which proved the onset of progressive hyperopia often found a decade after the original surgery is due to continued flattening of the central cornea.
A conceptually opposite technique of using hexagonal incisions in the periphery of the cornea is known has Hexagonal Keratotomy or HK (described by Antonio Mendez of Mexicali, Mexico), which was used to correct low degrees of Hyperopia. The idea of HK was to make six peripheral incisions to form a hexagon around the central cornea to steepen the hyperopic flat cornea and thereby focus the rays of light onto the retina. These incisions could further be of two types: connecting and non-connecting (Gulani AC: 10 Refractive Procedures for Hyperopia. ISOPT 2001).
RK may be performed with different types of incisions. They can have 4,8,16 or 32 incision surgeries and also all kinds of patterns and linearity based on their refractive errors, surgeon's style or training when it was initially done. Many of these people have had additional incisional surgeries like Astigmatic Keratotomy (AK) where incisions are placed at the steepest points of the cornea in people with astigmatism to relax and transform the cornea to a more spherical shape. Some people have had a combination of intraocular surgeries such as Pseudophakia or Phakic implants along with their keratotomies and many of them also underwent purse-string suture to control the over-correction (Dr. Green’s Lasso suture).
Due to the instability of the cornea along with age-related pathologies, it may be difficult to address visual acuity satisfactorily in people who have undergone RK surgery and later develop presbyopia (hyperopia caused by age-related changes in the crystalline lens). In these situations, the factors to be considered include:
Primary visual factors:
Secondary (Associated) Visual Factors:
The PERK study demonstrated that people who undergo RK continue to drift toward hyperopia ("farsightedness"). Many of these people have reached the age when presbyopia sets in. Some also develop cataracts. Their vision can still be restored with epilasik, photorefractive keratectomy, LASIK or phakic lens extraction, or cataract surgery. The corneal curvature has to re measured and modified by history, central keratometry or contact lens method.
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