Eye Surgery 2000

By Dr. Sawatzky

For the eye to work, the image must be focused on the retina. The front of the eye (cornea) does approximately 75% (45 diopters) of the refraction and the lens inside the eye completes the refraction (15 diopters) and adjusts the focus depending upon the distance of the object from the eye. In many eyes, the cornea is too curved for the length of the eyeball (bends the light too much) so that the image is focused in front of the retina. This results in blurred vision when looking at distant objects. When the object is closer, more bending of the light is required and the object can be focused on the retina so vision is clear. Because these people can see clearly up close but not far away, they are said to be near-sighted. The proper name for this refractive error is myopia and it is by far the most common visual problem (25% of the western world is more than six diopters myopic).

If the cornea is not perfectly round but squished, it will have more curvature in one direction than another. This visual error is called astigmatism and is also very common. The last common visual problem is when the cornea is too flat for the length of the eyeball. In this case the image will be focused behind the retina and the person will be able to see clearly at a distance but not up close. These individuals are said to be far-sighted or hyperopic. For all of these refractive problems, the solution is to place a lens in front of the eye to correct the vision (glasses or contact lenses) or to surgically alter the eye.

There are now four readily available (in Canada) surgical procedures to alter the shape of the cornea to correct these visual abnormalities. The oldest, radial keratotomy (RK), is when radial cuts are made in the cornea to weaken and flatten it to correct for near sightedness. This procedure permanently weakens the eye and has many undesirable visual side effects. It should not be considered (see the May 97 column for a more complete discussion).

The second oldest procedure is surface photorefractive keratectomy (PRK). In this procedure the front of the cornea is carved with a laser to change its shape and thereby change the way in which it bends light. The third procedure is laser in situ keratomileusis (LASIK). In this procedure a flap is created on the surface of the cornea and the laser is used to carve the centre of the cornea to change its shape. The flap is then replaced over the operative site. The newest procedure is intracorneal rings. Very small cuts are made in the outer edges of the cornea and ring segments are inserted. These ring segments stretch the cornea and flatten it out, thereby correcting for near-sightedness or myopia.

Each of the last three procedures has advantages and limitations. This is one of the reasons why determining the best procedure for each individual's unique eyes requires considerable expertise. It is NOT simply a matter of entering the correction desired into the laser and letting the machine do the work. The following comments are for information only. I am not a corneal surgeon, nor do I work in this field. However, I am very interested in this area and follow it quite closely. If you are considering corneal surgery, you should consult with a specialist who does the surgery.

PRK's main advantage is that it is fast and easy. The laser carves the surface of the cornea and minimal skill is required by the person who operates the laser. The procedure can take as little as five minutes per eye and therefore a large number of people can be done each day with one laser. The primary disadvantages of this procedure are that the eye takes quite a long time to heal and the vision takes a long time to stabilize after the procedure (up to one month for each diopter of correction). The eye is quite painful for several days and eye drops must be used for several weeks. In addition, the eye often attempts to repair the damage and therefore, the end result is not as predictable. Sometimes the procedure must be repeated to undo the change in refraction caused by the eye's attempt to heal! Nevertheless, PRK is still an excellent procedure, especially for eyes that need only small amounts of myopic correction (1-3 diopters) and many individuals have had completely acceptable results with PRK. Over the last few years however, it has largely been replaced by LASIK.

The main difference between LASIK and PRK is in LASIK, a flap is created on the surface and the correction is applied to the centre of the cornea under the flap. The flap means that the surface of the cornea is not damaged and the basement membrane is left intact. Therefore, the only healing that has to take place is the cut. The eye is far less painful and the vision stabilizes much faster (days vs. months). In addition, because the correction was applied to the inside of the cornea, the eye is much less likely to try to heal the damage (less likely to change the refraction and require a second procedure). LASIK can be used to correct higher amounts of near-sightedness than PRK, up to six diopters of astigmatism and up to six diopters of far-sightedness. The disadvan tages of LASIK are that a special tool (microkeratome) is required to cut the flap (approximately $110,000 for the most recent model) and more skill is required of the operator. The procedure takes approximately 15 minutes per eye and therefore, fewer eyes can be corrected in a day with one laser. In addition, there are possible complications with the flap. Nevertheless, LASIK is much less painful than PRK, the results are turning out to be better, and it can be used for a much wider range of corrections.

The advantages of intra-corneal rings are that no part of the cornea is removed and the rings can be removed or replaced at any time in the future. The disadvantages are that there are a limited number of rings available (five in Canada, three in the US as of June 1999). The shape of the ring determines the amount of correction made to the cornea and therefore, the correction is NOT individualized. This is similar to going to the drugstore to get a pair of glasses. There are only five pairs and you choose the pair that gives you the best vision. The rings do not correct for astigmatism and can be dislodged by rubbing the cornea (only time will tell how significant a problem this is). However, for some eyes intra-corneal rings give excellent results and, no permanent changes are made to the cornea.

As can be seen from the above discussion, determining which procedure is best for your eyes is not easy. However, we have only touched the tip of the iceberg! There are many different models of laser being used to carve corneas and each has advantages and disadvantages. The older machines applied the correction to the centre five mm of the cornea. At night, the pupil would often open up wider than five mm and the visual distortion from the edge of the corrected area caused significant difficulties for the person. Newer machines correct an area of six to nine mm but even then, there is another level of complexity. Some machines that are advertised to correct an area of seven mm actually apply the correction over the entire seven mm area. Others flare the edges out to seven mm and the actual correction is applied to a smaller area. With these machines, if the pupil dilates to seven mm, the person will still have visual problems. In addition, people have pupils of different sizes. Finally, the larger the area that is cor rected, the more thickness of cornea that has to be removed for a given correction. In eyes that require a large correction (6+ diopters) and a wide area of correction, the amount of cornea that has to be removed in the centre might be so large that the cornea is weakened. The cornea is not the same thickness in every eye. Therefore, to properly determine the best correction for a specific eye, the pupils must be measured, the thickness of the cornea must be measured, and the optimal correction determined. For eyes that have large pupils and require significant correction, this process can be quite complicated and in some eyes only a partial correction can be safely performed.

Just when you thought life was getting complicated, there are also several different generations of laser machines being used. I had my eyes done with a third generation laser (new in 1996). The fourth generation laser became available in 1999 and, properly used, gives better results. The basic difference is that the third generation lasers were "wide beam" while the fourth generation lasers are "narrow beam or scanning". The fourth generation lasers carve a one to two mm spot on the cornea and cycle up to 50 times a second. The end result is that the surface of the cornea is smoother after being carved with a fourth generation laser and the cornea is less traumatized. The centre where I had my eyes done in 1996 has had a fourth generation laser since early 1999. When patients come back for their first checkup, 24 hours after having LASIK, they routinely see 20/20 to 20/25. That is equivalent to the best correction most of them had with glasses!

I hope I have given the impression that determining the best procedure to correct your vision can be a very complicated process. However, I do not mean to scare you out of having the procedure. I am strongly supportive of corrective corneal surgery and am extremely pleased with the results of my own surgery. If I were considering having the procedure done now, I would insist on the following: A fourth generation, scanning laser that the centre had used for long enough to determine its unique characteris tics (six months, each machine can be slightly different). I would insist that the person performing the procedure be a corneal surgeon (not required for PRK) with considerable experience. I would be extremely cautious of "cut-rate" or "discount" surgery (the centre where I had my eyes done still charges $2,400.00 per eye for LASIK). We are talking about your eyes, and PRK /LASIK permanently changes the cornea.

What about diving after this kind of surgery? No change from my column in 1997. I recommend waiting approximately six months after PRK and one month after LASIK or intra-corneal rings.

What about the future? This field is rapidly changing but the next procedure, expected to be available in Canada by the fall of 1999, is surgery for presbyopia (reading glasses). I do not have room to discuss this exciting new development here but I am following it closely (I am now 45 and expect to need "readers" in a couple of years). I will write about this in a year or so when the procedure is available and some experience has been obtained. In the meantime, stay safe and have fun diving.