| Radial keratotomy | |
|---|---|
| Intervention | |
Schematic diagram of RK, with incisions drawn in orange. |
|
| ICD-9-CM | 11 |
| MeSH | D007646 |
Radial keratotomy (RK) is a refractive surgical procedure to correct myopia.
Contents |
The procedure was discovered by Svyatoslav Fyodorov who removed glass from the eye of one of his patients who had been in an accident. A boy, who wore eyeglasses, fell off his bicycle and his glasses shattered on impact, with glass particles lodging in his eyes. A procedure was performed consisting of making numerous radial incisions which extended 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, he found that the patient's eyesight had improved significantly.
In radial keratotomy (RK), incisions are made with a precision calibrated diamond knife. It has been found that incisions that penetrate only the superficial corneal stroma are less effective than those reaching deep into the cornea,[1] 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 centre of the cornea.[2] Other sources cite surgeries leaving 20 to 50 micrometres of corneal tissue unincised (roughly equivalent to 90% of corneal depth based on thickness norms).[1]
Accurate keratotomy is still popular to correct astigmatism. It is also done with a diamond knife but in these cases, cuts are made circumferentially, parallel to the edge of the cornea.
The healing corneal wounds consist of newly abutting corneal stroma as well as fibroblastic cells and irregular fibrous 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.[3][4][5] This risk is estimated to be between 0.25%[6] and 0.7%[7] Healing of the RK incisions is very slow and unpredictable, often incomplete even years after surgery.[8] Similarly, infection of these chronic wounds can also occur years after surgery,[9][10][11] with 53% of ocular infections being 'late' in onset.[12] The pathogen most commonly involved in such infections is the highly virulent bacterium Pseudomonas aeruginosa.[13]
Large epithelial plugs may cause more scattering of light, leading to symptoms of flare and 'starbursts'. This can happen 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, patients may seek further surgical treatment to alleviate the symptoms.[3]
Increasing altitude can cause partial blindness in radial keratotomy patients, as discovered by mountaineer Beck Weathers (who had had the surgery) during the 1996 Mount Everest disaster.
The incisions of RK are used to relax the steep central cornea in patients with myopia. Popularized by Dr. 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, Feb 1996).
Radial keratotomy 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 Dr. 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).
Patients with Radial Keratotomy (RK) may present with a variety 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 patients have had additional incisional surgeries like Astigmatic Keratotomy or AK where incisions are placed at the steepest points of the cornea in patients with astigmatism to relax and transform the cornea to a more spherical shape. Some patients 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).
Technological challenges of how to calculate the IOL power post RK surgery for an associated cataract as well as the aberrations induced along with stability questions exist. The presenting visual complaints are outlined in a new classification system below wherein the patient could be presenting with hyperopia as stated by the PERK study and that too at an age of Presbyopia (both require Plus lenses to correct them and therefore are a compounded problem) or with associated age-related deterioration from cataracts. Due to the instability of the cornea along with age-related pathologies, many physicians find it difficult to address these patients' visual acuity satisfactorily. In these situations, the factors to be considered include:
Primary Visual factors:
Secondary (Associated) Visual Factors:
As shown by the PERK study, patients who underwent radial keratotomy continue to drift toward hyperopia (far sightedness). Many of these patients have reached the age when presbyopia sets in. Some also develop cataracts. Their vision can still be restored with epilasik, PRK, 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.
|
||||||||||||||||||||||||||||||||||||||||||||||||