Radial Keratotomy

Note - the author of this article is my Dad, who (at the time of writing) performs the RK procedure in Tehran, Iran.

1. Introduction
2. Structure of the Eye
3. Different Types of Refractive Error
4. Correction through Corrective Lenses
5. Correction through Surgery
6. Conditions under which RK can be Performed
7. Alternatives

Introduction

There is little doubt that by the end of this century, corneal surgery for the correction of myopia and astigmatism will be one of the most frequent performed surgeries of the eye.

However, before we discuss the surgical procedure, we should review the structure of the eye and understand how it manages to focus and form images, such that the explanation of the surgical procedure to repair the refractive error will be clear.

The Structure of the Eye and How It Forms Images

The eye is one of the most complicated organs in the human body. Even with modern technologies, the most sophisticated cameras and computers are unable to replicate its function. The human eye is able to detect the movement of objects and distinguish between different colors, as well as ascertaining depth and being able to adapt to different types of lights in varying intensities.

Our eyes are carrying out this difficult task every minute we are awake. To understand their functioning and the image formation therein, we are going to look at the eye from an optical perspective. For the sake of simplicity, it is convenient to relate it to the workings of a camera.

Any camera has two important parts: one or more lenses, and a photographic paper or film. Light is focused by the lenses onto the film, thus forming a clear picture. In the eye, the cornea and crystalline lens take the place of the camera's lens, and the neurofibre layer - known as the retina - takes the place of the film. As the light passes through the cornea and then the crystalline lens, it is focused on to the retina, from where the image is passed on to the brain through the optic nerve. In this way, we are able to perceive objects (see figure 1).

The eye is a spherical object. The first part of the refractive system, the cornea, is in the front of the eye. It is completely transparent and slightly oblong. Most of the refractive power of the eye is attributed to the cornea. Behind the cornea is the iris, which is responsible for the color of the eye. In the center of the iris is a hole that is called the pupil. The purpose of the pupil is to regulate the amount of light that enters the eye. The crystalline lens falls behind the iris and is suspended by fine fibers called the zonula. Being the second part of the refractive system in the eye, the crystalline lens is able to change its convexity (called accommodation) and thus change its refractive power. It is in this manner that we bring objects of different distances into focus.

Having described how a healthy eye is supposed to work, we shall go on to describe what the "refractive error" is, and how it can cause problems in vision.

The Different Types of Refractive Error

1. Myopia - also known as short sightedness or near sightedness. In this disease, the picture (which is supposed to be imposed on the retina) is instead imposed in front of the retina. The effect is that far objects appear visually blurred. The cause of this illness is that either the distance between the cornea and the retina has become too long, or the curvature of the cornea has increased.

2. Hypermetropia - also known as far sightedness. In this disease, the picture (which again is supposed to be imposed on the retina), is instead imposed behind the retina, with the effect of a blurred picture. The cause of this disease is that either the distance between the cornea and the retina has become too short, or the curvature of the cornea has decreased. However, depending on the amount of defect in refraction and the age of the patient, the crystalline lens may be able to compensate by changing its curvature (called accommodation) such that the picture is brought forward and is perceived clearly. However, if the amount of hypermetropia is excessive, or if the crystalline lens has lost some of its ability to focus due to age, then the crystalline lens will not be able to complete this task. The result being that neither far objects are focussed clearly, nor are near objects.

3. Astigmatism - if we view the eye as a spherical object, the light which is focussed by this sphere comes together at one point, and the picture formed on that point. In astigmatism the eye is more elliptical than spherical (similar to the back side of a spoon), and the light which should come to focus at one point, focuses in two points.

(see figure 2)

To find out if a patient has myopia, we have to place him at a distance of 6 meters from an eye chart, and ask him to read the chart from top to bottom. Normal vision is 6 by 6 which means the patient can read the lowest line on the chart. The visual acuity represented by the lines from bottom to top on the chart are 6/6, 6/9, 6/12, 6/18, 6/24, 6,36, and 6/60 respectively. This mean that a healthy eye is able to distinguish the top line of the chart from 60 meters, whereas one which is struck with severe myopia can only distinguish the top line from 6 meters and no further. The amount of refractive error is measured in diopteres. There is generally no relation between how many lines of the chart a patient can read and the number of diopteres of correction he requires.

The Correction of Myopia through Corrective Lenses

To correct myopia, we generally place glasses of different negative refraction powers in front of the eye. This causes the total refractive index of the eye to decrease and as a result, light focuses correctly on the retina. Contact lenses can be used to the same effect, instead of glasses.

The Correction of Myopia and Astigmatism through Surgery

An alternative to corrective lenses for correcting myopia and astigmatism is surgery. This surgery is called Radial Keratotomy. It simply reduces the refractive power of the cornea by decreasing its curvature, such that the focused picture falls on the retina. The effect of this surgery is generally permanent and without any relapse. The exception is in the case where the distance between the cornea and the retina (called the anteroposterior distance) increases. Such an increase means that the myopia is still in progression.

Radial Keratotomy, like many medical discoveries, was an accident. In 1970 Professor Fyodorov came across a case where a patient's cornea had been scratched by glass in a car accident. While removing the glass and treating the eye, he noticed that his patient's eyes, which previously had a high degree of myopia, were much corrected. This led him to believe that if by making cuts on the cornea one could reduce its curvature, it may be possible to cure myopia. With this observation, he tried his method on various patients, and in 1977, after 7 years of study and excellent results in Russia, American ophthalmologists seized this finding and made the procedure safer and more reliable through the design and use of more precise instruments.

In this operation it is possible to reduce the curvature and thus the refractive power of the cornea by making cuts on the cornea in a pattern resembling bicycle spokes. As such, light focuses on the retina instead of in front of the retina, giving the patient clear vision. The number of cuts required on the cornea can vary from 4 to 16 (and possibly sometimes more) and is dependent on the degree of near-sightedness of the patient (i.e. the severity of the myopia).

It is important to point out that no cuts are made in the center of the cornea where the pupil is located, and as such they will not interfere with the patients vision. Furthermore, upon healing the cuts on the cornea cannot be seen with the naked eye and thus cause no cosmetic problems. Prior to surgery, the thickness of the cornea is measured by ultrasound. The cuts are then made under a microscope with a diamond knife to a precision of 1/100th of a millimeter. Their deepness is gauged such that the cuts do not penetrate more than 90% of the cornea. The length of the operation is not more than 20 minutes and is done under a local anesthetic. As mentioned, the operation is carried out under a microscope, and the patient does not need to be admitted into a hospital. The eye that has been operated upon must remain covered for a period of several hours. It is opened the next day in the doctor's office, and the patient will notice that their vision is already considerably improved. However variations in vision may continue to occur for no longer than two to three months.

Sometimes all of the myopia (or near-sightedness) is not corrected in one session, in which case a second, similar operation may be required. Normally the operation is carried out first on one eye and then the second eye one week later. However, some ophthalmologists, in specific situations and with the approval of the patient, prefer to operate on both eyes in one session or reduce the waiting period in between operations. There are certain advantages and disadvantages to performing the operation on both eyes in one session and if the decision is made to proceed in this manner, they are explained to the patient.

Conditions under which Radial Keratotomy can be Performed

All cases of myopia and astigmatism are not necessarily good candidates for Radial Keratotomy. As such, the condition of the eye must first be carefully examined by the ophthalmologist and only accepted for surgery if the suitable conditions exist. The following points will be considered when examining a patient for Radial Keratotomy.

1. Age - Since myopia is generally progressing until at least the age of 18, the patient must normally be at least 18 years old (older if the myopia is still progressing at that age). There is no upper age limit for surgery.

2. Sex - The results of Radial Keratotomy is almost universally equal in both sexes. The exception being that better results are obtained in young males than young females.

3. The eye should be whole and in good shape, and not afflicted with any other diseases or illnesses. It is especially important that the cornea not be previously afflicted with herpes, or have any recurrent abrasions.

4. The patient has to realize that their vision may not necessarily be corrected 100%. Patients who expect their vision to be improved 100%, or who need perfect vision as a requirement of their profession are not good candidates for this operation.

5. Patients who previously were the users of hard or soft contact lenses, and for whatever reason are no longer able to wear contact lenses are excellent candidates for this operation.

6. We divide our patients into three groups in terms of myopia and astigmatism - low, medium and high:
   • Low - the first group has myopia of less than 2 diopteres. On these patients, the operation must be done with extreme care since it would be very easy to overcorrect their myopia (near-sightedness), causing them to be farsighted. Although the same possibility exists in the second and third groups of patients, the risk is greatest in the first.
   • Medium - the second group has myopia of between 2 to 8 diopteres. These patients are the best candidates for Radial Keratotomy.
   • High - the third group has myopia of greater than 8 diopteres. In these patients, there is a possibility that their post operation vision will not be perfect. However, it is possible to significantly reduce their myopia, for example, from 12-14 diopteres down to 3-4 diopteres.

7. Ocular pressure - This refers to the pressure of the fluids inside the eye. Patients whose ocular pressure is higher or lower than average (i.e. outside of the range considered to be average) are not good candidates for this operation and conversely, patients whose ocular pressure is average, up to the upper allowances of average, are good candidates for this operation.

Again it is important to note that immediately after the operation, the patient's vision will not be stable and may vary from day to day. Specifically, there will be a noticeable difference in vision between the morning and evening. After a short period (generally two to three months) the patient's vision will become stable. Clearly it is the goal of both the doctor and the patient to ensure perfect vision (i.e. 6/6) after the operation. However due to the large number of factors which influence the operation, many of which have been discussed above, vision may not necessarily ultimately be 6/6.

With the precise instruments that today's technology has created, this operation is largely free of complications. However, one should remember that any operation carries the possibility of complications, and that nobody can guarantee a perfect complication free operation.

Alternatives

The patient should note that other than Radial Keratotomy, alternative methods are available for correcting the eye's refractive error. Among the available treatments, some include corneal scratching, in order to reduce the cornea's curvature, the removal of some layers from the cornea through the use of a laser, or a combination of methods.

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