|Year : 1990 | Volume
| Issue : 3 | Page : 103-106
Radial keratotomy : Procedures
Jeffrey B Robin
Department of Ophthalmology, University of Illinois College of Medicine, Chicago, USA
Jeffrey B Robin
1855 West Taylor Streat: Chicago Illinois 60612
Source of Support: None, Conflict of Interest: None
The presently employed procedure of radial keratotomy is essentially unchanged from that developed by Fyodorov and introduced into the United States by Bores in 1978. The surgical procedure essentially consists of six basic steps: 1) application of appropriate anesthesia; 2) marking the visual axis; 3) marking the optical zone; 4) measuring the corneal thickness; 5) setting the depth of the blade; and 6) marking the corneal incisions. This review will carefully consider the pros and cons of many potential variations associated with each of these steps.
|How to cite this article:|
Robin JB. Radial keratotomy : Procedures. Indian J Ophthalmol 1990;38:103-6
Radial keratotomy, as presently practised, was developed by Yenaliev  and Fyodorov , in the early 1970s. In the mid-1970s, a large series of cases from Hyderabad, India was first reported by Siva Reddy.  Radial keratotomy was introduced into the United States by Bores in 1978. Essentially, except for refinements based upon collective surgical experience and the development of new technology, the surgical procedure itself has remained relatively unchanged. 
The goal of radial keratotomy is to reduce myopia by flattening the central cornea. This is accomplished by making radial incisions through the epithelium and stroma of the mid-peripheral and peripheral cornea; the central cornea is spared. The mid-periphery of the cornea then bows anteriorly and the curvature of the central cornea subsequently flattens.
The procedure of radial keratotomy is deceptively simple. There are six basic steps: 1) administration of appropriate anesthesia; 2) accurate marking of the visual axis; 3) marking the appropriate sized optical zone; 4) measuring the corneal thickness; 5) accurately setting the depth of the blade; and 6) making the corneal incisions. In this review, each step and its variations will be carefully considered.
| Anesthesia for radial keratotomy|| |
Radial keratotomy has been performed using general, retrobulbar and topical anesthesia. When first introduced, the majority of cases were performed using general or retrobulbar anesthesia. However, as surgical skill increased and technology improved, operative time was markedly reduced. General anesthesia no longer became justifiable in the vast majority of cases. There is still, however, a place for general anesthesia for those patients who are too anxious to tolerate other anesthetic options. It is important to remember, in those few cases that require general anesthesia, to mark the visual axis (v.i) prior to the induction of anesthesia. Retrobulbar anesthesia has been all but abandoned by radial keratotomy surgeons because of case reports of globe perforations associated with anesthetic administration. This complication is not surprising, considering that these are myopic eyes. Additionally, when used in other ophthalmic surgical procedures, retrobulbar anesthesia has been associated with other ocular, orbital and even central nervous system complications. The recently introduced technique of peribulbar anesthesia has been successfully used with radial keratotomy. This procedure, in which anesthetic is placed outside the muscle cone, appears to be safer than routine retrobulbar anesthesia.
Topical anesthesia, with or without concomitant intravenous sedation, is the method of choice for radial keratotomy. Tetracaine or proparacaine can be used, depending upon the patient's tolerance. The topical route is preferred for several reasons: 1) the eye must be freely mobile in order to mark the visual axis; 2) the procedure is very short (usually less than 15 minutes); and 3) the lack of serious complications. Although routine topical adminstration can readily achieve corneal anesthesia, it is essential to remember that radial keratotomy involves perilimbal fixation. The perilimbal area is usually quite sensitive and frequently is not adequately anesthetized with routine anesthetic drops. This situation can be alleviated by soaking a cotton tip applicator with the anesthetic and applying it directly onto the perilimbal area for two to three minutes.
Additional recommended preoperative procedures include the administration of a weak strength(1/2% or 1 %) pilocarpine solution and proper positioning of the patient and operating microscope. Pilocarpine is helpful because a constricted pupil makes for a more comfortable, less light-sensitive patient. Additionally, the visual axis is easier to visualize and mark against the backdrop of a constricted pupil. Care should be taken to ensure that the patient's head is positioned so that the iris is in a plane parallel to operating table; the microscope should be postitioned perpendicular to this plane.
| Marking the visual axis|| |
Accurate centering of the radial incisions is dependent upon precise identification of the visual axis. Inaccurate centering can result in glare, irregular astigmatism, over or undercorrection, and visual distortion.
Most radial keratotomy surgeons utilize the mirror quality of the anterior cornea-tear film to mark the visual axis. The light of the microscope bulb should be reduced as much as possible for patient comfort. The patient is then asked to look directly at the filament of the coaxial light bulb. Using the Zeiss OpMi 6 microscope, the surgeon closes his/her nondominant eye and views the reflection of the filament on the anterior cornea. By adjusting for parallax, the visual axis can be located by identifying the opposite end of the filament reflection and moving one-half the filament width inferiorly. For example, a right-eyed-dominant surgeon would find the visaul axis by identifying the left end of the filament reflection and moving one-half the filament width toward 6:00. Some newer operating microscopes have built-in optical centering systems that utilize either the coaxial light or a seperate fixation light. Additionally, if microscopic coaxial illumination is not available, the visual axis can be marked preoperatively using a monocular ophthalmoscope set with a +3.00 viewing lens on the dial. 
Once the visual axis is identified, it is marked with a blunt instrument. A Sinskey hook or a blunt 25-guage cannula can be used. A small depression should be created in the epithelium; care should be taken to avoid damage to the underlying basement membrane. The use of a sharp needle to mark the visual axis is probably too dangerous.
| Marking the optical zone|| |
Collective surgical experience has demonstrated that one of the essential parameters in predicting the effect of radial keratotomy is the size of the optical zone.  It is well-established that the smaller the optical zone, the more effective the procedure is in reducing myopia. The vast majority of cases will utilize optical zones between 3.0 and 4.0 mm. The exact size chosen depends upon surgeon's preference. There is a myriad of recommendations available that relate optical zone size to degree of preoperative myopia, age of the patient, and other factors. My personal recommendation is for the beginning radial keratotomy surgeon to begin by using the P.E.R.K. protocol  with minor adjustments for older patients (experience has shown that the surgical effect increases with age and that older patients are more susceptible for overcorrection, if given the same surgical procedure that would accurately correct a younger patient.) As the surgeon acquires more experience, his/her procedure should be adjusted based upon his/her own results. In any event, it is important for the radial keratotomy surgeon to have a complete array of optical zone markers at his/her disposal.
The appropriate-sized optical zone marker is selected and centered onto the previously-placed visual axis mark. Most optical zone markers consist of a round, blunt cylinder that has a centering device attached to the innner walls of the cylinder. This device, usually a set of fine wires placed 90° apart, serves to accurately center the optical zone marker around the visual axis mark. Once centered, the cylinder is firmly pressed onto the epithelium and the optical zone mark is created.
After the appropriate-sized optical zone is marked, it is helpful to outline the path for the subsequent corneal incisions. This can be done with any of a variety of incision markers that are commercially available. This incision markers can have either 4, 6, 8, 12 or 16 blades radiating off of a standard-sized optical zone marker. These "blades" are dull and, as with the optical zone marker, they should be pressed firmly onto the cornea. Anterior segment surgeons may also find these blades useful for positioning suture placement in lamellar or penetrating keratoplasty.
| Measuring corneal thickness|| |
Collective laboratory and clinical experience has determined that maximum effect of radial keratotomy is dependent upon making the incisions as close to Descemet's membrane as is possible. This requires accurate measurement of corneal thickness. For this step, most radial keratotomy surgeons utilize intraoperative ultrasonic pachymetry.
After the visual axis and optical zones are marked, the cornea is hydrated with balanced saline solution. Any of a variety of accurate, easy to use, ultrasonic pachymeters are commercially available.  The transducer tip should be sterilized according to manufacturer's recommendations (some of these tips cannot be heat-or gas-sterilized). It is helpful to have a sterile glove or cloth sheath around the distal end of the wire connectiong the transducer tip to the pachymeter unit.
Because the cornea thins as its center is approached, the thinnest area that will be cut will be at the optical zone mark(since the incisions will not be placed central to this mark.) Therefore, pachymetry measurements should betaken at the optical zone mark. Most surgeons take four readings, one in each quadrant, around the circumference of the optical zone mark. Each reading, for maximal accuracy, should be repeated three times. The major benefit of intraoperative pachymetry is that the surgeon will have the exact measurement of the corneal thickness as near to the time of incision as possible.
Although most surgeons use intraoperative ultrasonic pachymetry, several radial keratotomy practitioners recommend preoperative pachymetry, either ultrasonic or optical. They contend that the difference in accuracy is negligible, and that intraoperative pachymetry is a cumbersome step that unnecessarily prolongs the procedure. Some surgeons believe that a single central pachymetry reading is sufficient to plan their surgeries. Although optical pachymetry is more widely available to the beginning refractive surgeon, Salz and Coworkersi 1 have demonstrated that it is not as accurate as ultrasonic measurements.
| Setting the knife blade depth|| |
Because of the depth precision required for radial keratotomy, knives with adjustable blades must be used. Steel blades, because of stromal tissue disruption, have been all but abandoned for diamond and other gem blades.  All of these gem blades have adjustable blade depths and gross micrometers.  The accuracty of the micrometer setting should be verified using any of a veriety of blade gauges. The two basic types of gauges are the coin and ruler varieties. The style of gauge selected depends upon both personal preference and the recommendations of the blade manufacturer (certain gauges are made for certain blades). The micrometer verification should be performed under the microscope to ensure accuracy. Additionally, the surgeon should bear in mind that during the blade depth setting/verification procedure, the exposed cornea will be dehydrating (as much as 10% in 10 minutes) jeopardizing the accuracy of the intraoperative pachymetry readings. The cornea should be protected, either by rehydration or by removing the lid speculum, during this procedure.
The depth that the gem blade should be set is another topic of controversy among radial keratotomy surgeons. Each surgeon applies a different amount of pressure on the blade. Thus, the most practical method to attain optimal depth incisions is for the beginning surgeon to set the blade at 100% of the thinnest of the four optical zone pachymetry readings. Again, by keeping meticulous records of intraoperative technique and postoperative results, the surgeon can adjust the balde depth to improve results for future cases.
| Making the radial incisions|| |
Once the blade depth is set, the radial incisions are made. The first factor to consider is that of fixation. The globe must be properly fixated in order to ensure accurately placed incisions. There ae a variety of fixation devices available. The most commonly used are the two-point fixation forcepts and ring fixation devices. It is recommended to fixate the globe in more than one point, in order to minimize torsional rotation. My personal preference is to use the larger, limbus-to-limbus fixation forceps (Kremer), as opposed to the smaller two-point variety; torsional rotation is still a real.problem with these smaller fixation forceps. Several surgeons prefer the fixation ring, e.g. the Thornton ring, which fixes the entire circumference of the limbus. Use of these rings is associated, however, with increased patient discomfort and increased intraocular pressure. Suction rings have also been described for radial keratotomy; these have
the possible disadvantages of mechanical failure, vascular occlusion, conjunctival chemosis, and patient discomfort.
There are other points of some controversy regarding keratotomy, as deveoped by Fyodorov, involved incisions made from the periphery to the optical zone. However, this technique has been reversed by most surgeons, so that the incisions are made from the border of the optical zone to the limbus. The change was made because of the tendency to drag the peripheral-tocentral incisions into the optical zone, producing glare and unwanted refractive results. The next somewhat controversial point involves incision number. Radial keratotomy was introduced as a 16-incision procedure. Laboratory studies then demonstrated that the difference in myopia-reducing effect between 8 and 16 incisions was not stastically significant.  Therefore, the majority of radial keratotomies performed in the United States have used 8 incisions. Recently, however, Salz  introduced the concept of 4 incision radial keratotomy. Preliminary reports have demonstrated that this number of incisions can be effective for smaller degrees of myopia. As collected experience has shown that overcorrection is a major potential complication of radial keratotomy, an increasing number of surgeons are primarily using 4 incisions and, if necessary, erroring on the side of undercorrection; additional incisions can always be added at a later date. My personal preference is to use 4 incisions in all cases of myopia less than -4.00 D and in all patients (regardless of the degree of myopia) who are over 40 years old.
The incision is made by firmly plunging the blade in a perpendicular manner at the outlined border of the optical zone. It is helpful to let the blade remain stationary for a few seconds; additionally, it can be gently wiggled from side to side to help ensure smooth passage through the stroma. If no perforation has occurred, the incision is then extended peripherally (along the previously created marks) by a combination of knife motion and globe rotation. The knife motion should be slow and smooth; starts/stops are to be avoided. The goal is to keep the blade perpendicular to the underlying conrea throughout the knife's excursion. At what meridian to begin the surgery is of some controversy. Most surgeons begin in the horizontal meridians. I prefer the temporal horizontal meridian, as this is the thinnest and at most risk for perforation. I then move to complete the remainder of the 4 cardinal incisions. If 8 incisions are to be performed, the 4 cardinal incisions are then bisected. Following completion of the incisions, they should be spread and gently irrigated with balanced saline solution to remove blood, epithelial debris and particulate matter. Incision depth can also be grossly verified during this procedure.
Another incision-related controversial technique is primary redeepening. Some surgeons, including Fyodorov, recommend this to add more effect. The procedure involves deepening the incisions in the periphery only. This technique is not widely accepted because the effect of the procedure is minimal and the potential dangers of perforation, incision irregularity and surgical trauma are not warranted.
| References|| |
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Fyodorov SN, Durnev VV: Operation of dosaged dissection of corneal circular ligament is cases of myopia of a mild degree. Ann Ophthalmol 11:1885-1890, 1979.
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