We now know there are aberrations in the human optical system that can be measured by a variety of wavefront measurement devices. However, it is important to keep in mind that although we are enthusiastic about the possibilities of wavefront corrections, there are many unresolved issues. How can we correct a patient at one age and then handle the changes in the same optical system that will continue to evolve with time? Currently, it is still unclear whether some degree of optical aberration in the human visual system is beneficial for vision and therefore best left uncorrected. It is essential to remember the current limitations of our knowledge regarding the optical aberrations of the human eye as we evaluate this exciting innovation for LASIK.
INITIAL CHALLENGES
The tear film is the major refractive interface of the eye, and the refractive properties of the tear film can change with each blink, affecting the aberrations that are measured by the aberrometer. In addition, scars, corneal or lens opacities, and small pupils (<6 mm) can make obtaining an accurate aberration map very difficult. Therefore, standardization of the testing methods for wavefront analysis is crucial if customized wavefront LASIK is to be successful.
The mydriatic drops used to dilate the pupil for the aberration test can cause a toxic keratopathy which further alters the aberration map. This was a significant problem when we initially used the Zyoptix Custom Wavefront LASIK system (Bausch & Lomb, Claremont, CA), as we used multiple drops of 2.5% phenylephrine to achieve maximal pupillary dilation. Roberto Zaldivar, MD, of Mendoza, Argentina suggested that we could use just one drop of phenylephrine and then perform the aberration map 5 minutes later. This has allowed us to avoid the toxic keratopathy effect of the drops, and has greatly enhanced the quality and consistency of our aberration maps.
THE ZYOPTIX SYSTEM
When using the Zyoptix system, the first step is to obtain the wavefront map. The Zywave aberrometer uses the Hartmann-Shack method of analyzing the outgoing wavefront. The outgoing wavefront is captured by a lenslet array that focuses the points of light onto a cathode-coupling device. This device produces the grid pattern to be analyzed by the computer in creating the wavefront map (Figure 1). The wavefront map is then produced, and presented in a manner very similar to topography maps (Figure 2). A useful feature of this map for patients is the point spread function (PSF). This feature allows the surgeon to illustrate to the patient the visual distortion that they are currently experiencing from their current optical aberrations, and how the PSF would appear if custom wavefront LASIK were performed.
The next step is to go into the Zylink software, which combines all the data of both the Zywave as well as the Orbscan reading that is also obtained preoperatively. The Zylink software allows us to adjust the prescription as well as set the optical zones of the ablation program. The custom ablation is saved onto a disk, and is then used with the Bausch & Lomb 217Z laser. Although the current system is very functional, the ideal system would consist of a direct link between the diagnostic instruments and the laser.
ABLATION TIMES AND ZONE SIZES
We are generally accustomed to LASIK ablation times ranging from 30 to 40 seconds for a -6 D correction with a 6-mm optical zone. With the Zyoptix system, however, because it does four steps at a 2-mm spot size and then goes into a 1-mm spot size at another four to eight steps, this prescription and zone would take about a minute and a half. If we use an even larger optical zone, such as 7 mm, the ablation times increase substantially. Because there is such a huge blend zone with Zyoptix, it is probably best not to perform optical zones much higher than 6 mm in size, as the number of shots is too large, and the ablation time is too long. With ablation times longer than 2 minutes, the results have been less predictable in my experience. The size of the ablation zone equates to the number of shots. In my technique, I try to make sure that I am not doing much more than four and a half thousand shots, because that equals an approximate 2-minute treatment time.
The 217Z laser used to ablate much deeper than other lasers, but now with the new Zyoptix software, it is much more comparable to the other scanning lasers. This is beneficial because it removes less tissue. Due to the prolate ablation and wide zone ablation, patients have fewer postoperative problems with night glare.
Because of the prolonged ablation time, we initially noted a tendency for overcorrection with the Zyoptix system. After data analysis with Bausch & Lomb, we found that we required a nomogram adjustment in our spherical component of approximately a 15% reduction, which is very similar to the nomogram that I use for the Star S3 laser (VISX, Santa Clara, CA). After I instituted this nomogram adjustment, the results improved dramatically.
EVOLVING POSTOPERATIVE RESULTS
The results with Zyoptix may not always be evident on the first postoperative day. In a recent case, we performed Zyoptix on a high myope with 3 D of cylinder in both eyes. The preoperative best-corrected vision was 20/25 OD and 20/20 OS. On postoperative day 1, results of +1 -0.50 X 75 OD and +2 -1.0 X 90 OS were found. This may not seem to be the best result, despite the fact that the patient had quite a difficult initial prescription. However, after 1 month, there was a little less cylinder in the right eye, and the left eye seemed to be stabilizing, but it was still hyperopic. At 3 months, there was +0.50 sphere in both eyes, but the BCVA was now 20/15 OU. This evolution of the refractive result is very different from what we see with standard LASIK. The eye takes time to adjust to these fine-tuned treatments.
LASIK, ZYOPTIX, AND SUPER VISION
The super vision or hypervision potential of custom wavefront LASIK offers the potential to make the patient's vision better than it was using soft contacts and perhaps his or her glasses. This would mean that everyone would become a potential candidate for LASIK in the future, even if they have virtually no prescription. The bottom line is that custom wavefront LASIK now offers patients the best chance of achieving 20/20 vision or better. Of course, there are still no guarantees, and enhancements may always be necessary for some patients.
