Why This Laser Now?

Currently there are several types of laser platforms available for refractive surgery. Which type of laser platform to use depends upon a variety of factors.

Wavefront-guided (conventional) Module

The conventional refractive platform involved a flattening of the cornea, which generated a transition zone that was a major cause of spherical aberration. There is an area of the cornea that the laser does not touch and is thus left untreated, and there is an area on the cornea where the laser is able to deliver full treatment. The disparity, or transition zone, in between these areas is the source of problems with visual aberration, specifically spherical aberration. Originally, spherical aberrations were improved with aspheric designs, but there were still issues with the lasers in terms of optical zones, due to the fact that many of these calculations were all made in plastic, and did not take into consideration what a curved cornea would do. So, in the periphery, the beam would hit the cornea in an oval pattern instead of a circular pattern. The ovalization caused loss of peripheral energy.

Wavefront-optimized (standard) module

Wavefront optimization (Figure 1) was meant to compensate for this loss. It does so by increasing the energy delivered in the periphery, creating optical zones that are quite large, resulting in reduced induction of spherical aberration. Wavefront-optimized treatments were developed by Michael Mrochen and Theo Seiler in an attempt to reach several specific goals. Unlike previously available technology, wavefront-optimization was designed to create a pure refractive treatment that would leave higher-order aberrations unchanged, or the amount of induced aberrations reduced. Specifically, the design was to reduce the amount of induced spherical aberration originally observed with the conventional platforms.

When utilized in patients with up to 10 D of myopia and up to 3.5 D of cylinder, significant improvements were achieved through treatment with the WaveLight laser (Table 1).¹ A correlation coefficient (r²) equal to 1 reveals a perfect correlation, and this study demonstrated an r² value of 0.994 for attempted and achieved sphere, as well as an r² value of 0.959 for absolute attempted and achieved cylinder or astigmatism. These results indicate that the WaveLight laser, with an optimized nomogram, provides great visual outcomes.

Table 1. WaveLight Laser With Up To 10 D Of Myopia and 3.5 D Of Cylinder Source: Karl G. Stonecipher, MD

Clinical trials examining the efficacy of wavefront optimization demonstrated that there was very little change in spherical aberration. Moreover, an essential goal of laser treatment is preventing the need for further treatment. A patient that requires an enhancement following surgery may consider that endeavor a failure, and therefore it is vital to achieve their goals during the initial procedure. With the wavefront-optimized laser platform, the enhancement rate has been determined to be 0.88% in up to 10 D with 3.5 D of cylinder. This improves the enhancement rate of 3% in the original FDA study.

Patient Selection: Wavefront-optimized vs. Wavefront-guided Modules

Care must be taken to determine whether a patient is an appropriate candidate for wavefront-guided treatment or wavefront-optimized treatment. This laser platform produces outstanding results with wavefront-guided treatments otherwise known as custom treatments, but patient selection is critical. In order to meet such objectives, surgeons can ask their patients a series of questions, and their answers can be used to help determine which strategy should be employed. Once all such details are collected, along with measurements from tools such as with the wavefront analyzer, the ideal options can be identified.

Figure 1. Standard Module (Wavefront-Optimized) Wavefront-optimization was designed to leave higher orderaberrations unchanged without inducing spherical aberrations.It does do by creating large optical zones, in contrast to conventional treatment, which did not take into consideration what a curved cornea would do. Source: Karl G. Stonecipher, MD

For example, if a patient with aberrations on the cornea wants to be treated, detailed questions must be asked to determine whether they have significant visual aberrations. Questions such as, “Do you have glare?” “Do you have halos?” “Do you have night vision problems?” and “What do you see in your glasses or contact lenses?” can help meet such objectives. If the patient reports something unusual in their glasses or contacts, a measurement with the wavefront analyzer can be obtained in order to determine whether the aberration is high enough to treat. If there is symmetrical computer topography, sometimes an analyzer is not needed, but is used anyway. Physicians are more likely to use wavefront-guided treatments if their higher-order aberrations are > 0.3 µ.¹ It must be taken into consideration that most eyes have low preoperative aberrations. Moreover, eyes with very low aberrations have similar results with wavefront-guided LASIK and wavefront-optimized LASIK. Lastly, the effectiveness of wavefront-guided LASIK depends on preoperative higher-order aberration levels and spherocylinder treatment amount.

References

1. Stonecipher KG, Kezirian GM. Wavefront-optimized versus wavefront-guided LASIK for myopic astigmatism with the ALLEGRETTO WAVE: three-month results of a prospective FDA trial. Journal of Refractive Surgery. 2008 Apr;24(4):S424-30.

Case Presentation Case 1: One important advantage to the wavefront-optimized platform is that it can be utilized in patients with a variety of problems. Consider a female post-premium IOL patient. The lens was expensive and the patient expects success. After surgery she has 20/40 uncorrected vision, which is not what patients perceive as a positive outcome. Her best corrected vision is 20/20, but her desired outcome, and what she expected for the high cost, was to be out of glasses. She has a residual refractive error that is hyperopic; what can be done? Stephen Lane, MD: It is vital to ensure that our patients’ best corrected vision is indeed 20/20 and there is not an extenuating circumstance that would prevent the achievement of this goal. For example, if the above patient’s best corrected vision was 20/30, possibilities such as ocular surface disease, cystoid macular edema (CME), and other issues that present an obstacle to achieving a best corrected vision of 20/20 must be considered. But for this particular patient, the issue is that after premium IOL implantation, she has residual refractive error. What should be done to correct it? Daniel S. Durrie, MD: Due to the fact that this patient corrects to 20/20+, and it is three months after surgery, I would perform a surface ablation. I use the Allegretto platform, and I have found that the wavefront-optimized platform works very well on these types of patients. It is interesting that while physicians view surface ablation as a bigger procedure, most patients do not. In fact, we have corrected refractive errors much smaller than this with surface ablation if the patient is extremely unhappy that the target was not achieved. This is especially true with lenses intended to provide both near and distance vision, where it is very important to correct the astigmatism, and the laser is an excellent option. You can also perform LASIK in this case, but I prefer surface ablation in my post-IOL patients. At three months after surgery would you be comfortable performing LASIK on top of a cataract incision? Durrie: I have done it, but that is one of the reasons that I prefer surface ablation. I do not have to worry about the risks of LASIK on the cataract incision. Maybe this case had limbal-relaxing incisions (LRIs) at the time of surgery, which you would not want to disrupt by putting a suction ring on the eye. Kerry D. Solomon, MD: Three months after surgery, I would not proceed right to treatment. Patients aged 60 years and older are at risk for ocular surface disease, lid margin disease, aqueous insufficiency, and a variety of other ocular disorders. So first, I would examine the ocular surface and retina very carefully. I would look for residual astigmatism. I would get a good refraction and good topography, bring the patient back in a few weeks or a month, and if there is stable refraction, I would proceed with treatment options. Karl G. Stonecipher, MD: I agree, and there are several points to consider: The first point to discuss is timing; when is the optimal time to operate on someone? Many patients do not take into account that our clear corneal incisions are LRIs, and in fact, are cutting corneal nerves much like LASIK does. Thus I think that dry eye is a significant issue, and it is very important to consider. Another issue with many of these multifocal IOLs is that it is difficult to get a good wavefront image. Thus if you are trying to perform a conventional Visx vs. wavefront-optimized treatment, there is not a comparison. If I cannot acquire a good custom measurement, then I will not be able to use what I feel is the best option for the patient. Again, you want to make sure that their best corrected vision is 20/20. We have noticed that, at about 0.74 D, patients start to complain. They notice glare, halos, and other problems. However, that should not be the cut-off, because there are many symptomatic patients with lesser amounts of refractive error. Most importantly, pay attention to the patient. Many want to be treated early postoperatively; however, you do not want to move too fast. You want to ensure that the refractive treatment is stable, and you want to confirm that the patient does not have dry eye.