Modifying the Refractive Power of the Eye

here are currently five methods of surgically modifying the corneal curvature presently in general usage. These are:

1. Keratomileusis
2. Radial Keratotomy
3. Epikeratophakia
4. Keratophakia
5. Keratoplasty

Of these five, the latter — keratoplasty (corneal transplantation), does not have as its primary goal (except in cases of keratoconus — a special kind of progressive myopic astigmatism) the modification of the corneal curvature and so will be discussed only incidentally.

As you have already seen in the section on refractive errors, we can correct the optics of myopia by either changing the curvature of the cornea or the length of the eye. Numerous surgical attempted have been made to do the latter. A procedure has been performed in which a band-like section is excised around the middle of the eye. The defect is then sutured together and the eye effectively shortened. However, the procedure is simply too radical to gain any support from other ophthalmologists. However, strapping of the myopic eye to prevent further elongation is infrequently done by a few specialists.

For less severe cases of myopia, a lensectomy can be done. In this operation the natural lens of the eye is removed permanently. This natural lens bends light rays entering the eye — by taking it out, it is hoped that the rays will be bent less and reach a focus at the retinal surface.

In myopic keratomileusis (MKM) surgery, a piece of the cornea is removed, frozen and reshaped to its proper degree. It is then sutured back on to the eye. In some instances this surgery can be done without freezing the tissue. A variant is used to treat some types of hyperopia.

In cases of extreme hyperopia due to absence of the natural lens (after cataract surgery in which no lens was implanted), or in cases of natural far-sightedness, an artificial lens can be implanted within the cornea of the eye. One such lens is made from material similar to that of an extended-wear contact lens and is called a KeratoGel^TM implant. Unfortunately, this method is no longer routinely available.

In children who have had cataract surgery and who cannot — for some reason — have a lens implant, it is possible to apply a lens to the outer surface of the cornea using a procedure called epikeratophakia. While useful for hyperopia, it is not recommended for myopia except in specialized types of myopic astigmatism known as keratoconus.

Depending upon which side of the fence you were standing, the results of the clinical excimer laser trials (PRK) were either encouraging or discouraging. If this reminds you of the tale of the Blind Men and the Elephant, you are not alone. Suffice that sufficient questions about the safety and efficacy remained and most of such uses are reserved for RK ‘touch-ups’ and irregular astigmatism. The excimer has been used mostly as a refractive tool to date, specifically to correct myopia and astigmatism. Attempts are also being made to correct hyperopia, and there was considerable hype that some day the laser would be used to perform radial keratotomy (RK) — but which it has yet to achieve in any way which surpasses current methods and probably never will.

Another method had been touted for hyperopia is — thermokeratoplasty (HTK). The late Professor Fyodorov and his group, as well as some investigators in the U.S., reported success in treating hyperopia with radially applied, deep peripheral corneal burns. Applied with a specially designed, computer controlled probe — these partial thickness burns were initially said to have corrected up to +5D of hyperopia. Subsequent reports were not as enthusiastic and results were mixed. The effect is variable and not well controlled and seems to decline over time with some cases reverting entirely. Scars can still be seen as long as 4 years later and recurrent erosions have been reported. Many of the previous practitioners have stopped performing this procedure. Attempts are currently being made using an erbium laser — laser thermokeratoplasty (LTK) but many of the objections raised against TK in general still apply to this procedure.

What's Changed

Some things have changed since the premier edition of this website first saw the light of day. For example, PRK — the darling of the laser crowd — has pretty much gone away except for ‘touching up’ post-RK cases and perhaps in myopia under –4 D. During this same period, thermokeratoplasty and hexagonal keratotomy (HK) have also gone the way of the Dodo and a good thing too. HK destabilized the cornea to a fare-the-well and changes in the corneal shape through denaturization of the stroma by heat continues to be both unpredictable and short lived into the bargain. Sunrises’ laser thermokeratoplasty technique was sent back by the FDA ‘for repair’ on the grounds that the results do not last (though it's back). Hyperopic lamellar keratotomy (HLK) also went down for the count as did automated lamellar keratectomy (ALK).

The king is dead

I have mixed emotions about LASIK is the new darling of the laser crowd and has caught on well around the world. It’s a good procedure but it requires thinking. Greater care needs be taken with lamellar surgery — the principles of which are laid out in the section on MKM. Unfortunately, too many practitioners treat this procedure as a ‘no-brainer’. Not good, especially since it can introduce corneal aberrations that reduce visual efficiency big time and then there’s this interesting phenomenon of increasing incidence of dry eye following LASIK.

Corneal topography

I had thought that, by now, everyone would have seen the error of their ways. But alas it was not to be. Practitioners still have not got the message that meridional slope values (provide by the sphere-based Placido’s Disk systems) do not provide enough information to describe the true shape of the cornea. All the mathematical gymnastics in the world will not provide the missing data and the fact remains that such devices are nowhere to be found in laboratories dealing with precision optics. Huxley said that this was the terrible tragedy of science: ‘When a beautiful theory is destroyed by an ugly fact.’ Is it any wonder that current topographical maps still have only entertainment value using as they do Placido’s disk and Slit-scan interpolation of corneal curvature? Fortunately, some laser manufacturers are beginning to catch on that wave-front and interferometric (initiated by yours truly) technology is the proper way to reshape the corneal surface. We can but hope.

Telescopic implants for macular degeneration

Alas, this saga is pretty much in suspended animation. The AMO lens did not work as well as first thought and the study was suspended. There is still hope, however, in a multi-objective Isræli implant being tested by Dr. Issac Lipshitz.

Long live the king

Radial keratotomy (RK) — which was to have been done to death by PRK — is still being performed in considerable and increasing numbers by the ‘unenlightened’ around the world for the simple reason that it works well for the degrees of myopia it was designed for. Though some lasers have been approved to treat corneal astigmatism nothing works better nor is more predictable than RK so far. More importantly, RK does not directly disturb the clear central cornea; lasers cut right across that area.