Refractive surgery has received tremendous attention by the press, produced wide interest among patients, and has embraced a large percentage of the ophthalmic community. The main focus (no pun intended) is to correct myopia or nearsightedness, a condition that affects nearly one-fourth of the world's population.
In refractive surgery we change the optical properties so that patients no longer require spectacles to see at distance. Figure 1 is a schematic of a normal or emmetropic eye (no correction necessary for clear distance vision). In myopia the aberrant optical system results in the light rays being focused in front of the retina. Myopia can be corrected by flattening the cornea so that is has less converging power. Radial keratotomy and excimer laser are the two most popular approaches. In radial keratotomy a series of deep, radial incisions are made in the cornea and result in peripheral steeping and central flattening. The desired refractive change is typically controlled by adjusting the number or the length of the incisions. The objective is to obtain the desired effect without causing an overcorrection and thus pushing patients into an over corrected or farsighted state. Surgical decisions are based on statistical analysis of a large number of previous surgical procedures. RK can now be done in a relatively precise manner with rather early visual rehabilitation of the patients.
The excimer is currently an investigational device. The laser is currently under clinical study in the United States (FDA approval anticipated by many within the next 3-12 months), while it is estimated that over 200,000 cases have been performed throughout the world. This laser emits energy at 193 nanometers which causes photochemical breakdown of the corneal tissue and thus ablates or vaporizes tissue with minimal surrounding thermal damage. Delivery systems have been designed which permit the laser to shape the surface of the cornea much like one would lathe a correction on a contact lens. This allows the surgeon to flatten the central cornea and decrease the optical power of the cornea in a relatively precise manner.
At present we have an ongoing clinical study in radial keratotomy and are performing basic research studies with the Summit excimer laser. We anticipate that we will participate in an FDA sponsored trial of a new "second generation" excimer laser (the Schwind Keratom distributed by Coherent, Inc) in the fall of 1995. Our research interests focus on evaluating the safety and effectiveness of both modalities of surgery.
In our radial keratotomy study, we have three objectives:
1) We are studying the effects of RK on visual performance as measured by contrast sensitivity testing (the ability to distinguish sinusoidal wave patterns of increasing frequency). This addresses a different aspect of visual performance than tested with the high contrast Snellen acuity ("E") chart as typically used in the doctor's office.
2) We are measuring the surface topography of the cornea to determine whether problems of under correction can be ascribed to individual incisions so that any additional surgery to "enhance" the effect can focus on the individual under corrected incisions.
3) Finally, we are taking the topographical analysis data and performing sophisticated ray tracing analysis. We compute the actual position on the retina of individual light rays that are refracted by the eye. This allows us to predict the visual performance based on the corneal topography and centration of the procedure on the cornea.
Our overall outcomes with RK have been favorable with 100 percent of patients 20/40 or better without correction and no major complications. We have shown that centration of the procedure on the entrance pupil is important for optimizing visual performance in the mid-range frequency of contrast sensitivity testing. This is contrary to what some experienced surgeons have taught and should provide a scientific basis for developing optimum centering procedures.
We have also been very active in the basic investigation (non-clinical use) of the excimer laser. Although our Summit excimer has recently been approved for therapeutic keratectomy (removal of scars and surface irregularities) and FDA approval for refractive keratectomy has not yet occurred. Worldwide reports of clinical results with the first generation lasers are very similar to RK. Approximately 92 percent of eyes are 20/40 or better without correction. However, there is room for improvement. Approximately 3 percent of patients lose a significant amount of best corrected visual acuity and 8 percent remain significantly over or under corrected.
We have been involved with Coherent Lasers in the development of their new second generation laser, the Keratom. This laser can treat myopia as well as astigmatism. A number of advances, such as computer centration of the procedure, intraoperative eye tracking and enlarged ablation zones, should provide more precise surgery with more consistent results in a greater range of myopes. The early data from foreign countries with the Schwind Keratom Laser shows 98 percent of the patients are 20/40 or better, with less than 1 percent loss of two lines or more best corrected visual acuity.
A second major advance that is planned to be incorporated in the upcoming clinical trial of the Coherent Laser is LASIK (laser assisted in situ keratomieleasus). In this procedure, a thin flap of tissue is sectioned from the surface of the cornea and left attached with a thin adherent rim or hinge of tissue. The underlying stromal tissue is then treated very precisely with the laser and the superficial flap is then replaced back on top of the cornea. This results in a very predictable refractive correction with minimal scarring and pain with almost immediate visual rehabilitation. Patients with low to moderately high myopia can be corrected. Our personal experience with this technique is limited to patients we examined in Mexico. These patients did not experience pain and had excellent outcomes. Their surfaces were healed within 24 hours and it was difficult to even find the incision. The results from elsewhere in the world are outstanding. I believe that this may be the best refractive technique that the future holds and anxiously await the results of new FDA sponsored clinical trials.
Robert W. Snyder, M.D., Ph.D.
The University of Arizona
Head, Department of Ophthalmology