Favorable comparison found with standard PRK and Zernike-based wavefront ablation.

By Charles Reilly, M.D.

The use of Fourier custom wavefront treatment can offer superior results when compared with standard PRK or Zernike surface ablation, according to results of a recent study my colleagues and I conducted. Our intention was to determine how newer wavefront Fourier and Zernike technologies compared with each other, as well as against our standard PRK results.
The recent trend in refractive surgery is to move back to the surface of the eye, and, as a result, PRK-style refractive surgery is becoming increasingly popular. When treating military patients in particular, we like to avoid creation and maintenance of a LASIK flap because of the high risk of eye trauma and other issues that may arise on the battlefield or at high altitude. With this in mind, we wanted to determine whether the newer Fourier technology would be a worthwhile addition to PRK for our elite military personnel who were undergoing the procedure.
Investigating the Approaches

In our study, we retrospectively examined 29 eyes that had undergone standard PRK, 25 treated with Zernike polynomial wavefront PRK and 40 that had undergone Fourier wavefront PRK. All the patients were treated on the VISX Star S4 with Eyetracker (Advanced Medical Optics [AMO], Santa Ana, Calif.) without iris registration. In addition, all of the procedures were performed at the same site by the same surgeon. Both the temperature and the humidity were controlled. Pupil sizes were equivalent and patients were all treated with the same 6-mm optical zone with an ablation zone of 8.0 mm.

Figure 1. At 6 months postop, 68% of Fourier patients achieved 20/12.5 UCVA or
better compared with 36% of Zernike-treated patients and 28% of standard PRK-treated patients.

Postoperative results at 6 months showed that approximately 68% of patients who had undergone Fourier PRK had attained 20/12.5 UCVA or better. We routinely expect vision of 20/15 � to have that number of patients with 20/12.5 acuity is stellar, in my opinion. By comparison, just 36% of patients who underwent Zernike treatment and 28% of those who were treated with standard PRK reached the 20/12.5 mark (Figure 1).

The BCVA outcomes mirrored these results. We found that 87% of patients who underwent PRK with the Fourier method had 20/12.5 BCVA, while just 60% of those who underwent Zernike PRK, and 48% of standard PRK patients reached this level � that is a substantial difference (Figure 2).
We also looked at contrast sensitivity, which I believe is important in terms of quality of vision. We found that there was no difference between those in the Zernike and those in the Fourier group in terms of cycles per degree after surgery. Patients attained excellent quality of vision with both of the custom approaches.

Figure 2. At 6 months postop, Fourier, Zernike and standard PRK-treated patients achieved 20/12.5 BCVA or better at 87%, 60% and 48%, respectively. The improved results of Fourier-treated patients are statistically significant.
 

The Fourier system,
which uses harmonic analysis, resembles analog technology
in that it describes the whole system rather than
splitting it into groups.

However, when we considered all of these results, we concluded that Fourier PRK was superior to the Zernike approach, as well as to standard PRK. The Fourier method showed a statistically significant improvement in BCVA in the 20/12.5 range. Meanwhile, Zernike PRK was statistically equivalent to the standard approach in this patient population.

Enhanced Detail
We believe that Fourier wavefront analysis ultimately offers greater detail and a truer representation of the patient�s actual wavefront and that this can be used for optimal correction (Figure 3). Fourier wavefront analysis is a way to describe complex shapes by using an infinite series of sines and cosines to mathematically describe the shape; this is known as Harmonic Analysis. This method differs from Zernicke Polynomials in which the wavefront is deconstructed into discrete polynomial terms to describe its overall shape. The difference between applying the Zernike polynomial and the Fourier harmonic analysis to the wavefront system is akin to the difference between analog and digital television technology. The digital approach takes chunks of information to form a picture. At low resolution it shows all the little pixels. In order to improve the picture, higher and higher resolution must be added.

The Fourier system, which uses harmonic analysis, resembles analog technology in that it describes the whole system rather than splitting it into groups. With this approach, a complex pattern is described using a series of sine waves to whatever resolution you desire. To describe a given shape, a series of sine waves can be added together. High resolution analysis of the wavefront error is paramount. It is able to capture more data points and has more fidelity as a result. I believe that this is one reason why we ended up with better outcomes with the technique.
By comparison, with the Zernike approach, complex wavefronts are broken down into discrete shapes using polynomials. There is also an inherent smoothing effect that takes place with the Zernike approach, resulting in some detail being lost.

Figure 3. The Fourier analysis provides a more detailed and accurate map of the patient�s wavefront vs. the Zernike.

Raising the Bar
Ultimately, our study shows that the Fourier technology not only works but leads to superior acuity than the standard and Zernike approaches. We are now closer than ever to achieving our goal of better than 20/20 uncorrected �super-vision� for patients following refractive surgery. As a result, we are currently in the midst of a paradigm shift. While in the past 20/20 acuity was our goal, I believe that this will no longer be the accepted norm for refractive surgery. New targets are being set. It is now a question of how many of your patients have 20/15 acuity � and even how many are at the 20/12 mark. The Air Force has always regarded 20/20 vision as substandard. Among our pilot population, 20/20 vision is two standard deviations below the mean.
In using this new approach, I think we must broaden our definition of who deserves a wavefront treatment. Instead of a criterion based upon a certain amount of higher-order aberration (HOA) present within a patient�s ocular system, I think we should focus on outcomes. We should consider which wavefront analysis method will give the patient the best visual result.
Overall, our results show that patients� acuity clearly benefited from undergoing Fourier wavefront over standard treatment, regardless of how much HOA the patient had. Ultimately, I think that when it comes to visual results, Fourier PRK will be very difficult to beat. OM

Charles Reilly, M.D., is chief of Cornea and Refractive surgery at Wilford Hall Medical Center, Lackland Air Force Base, San Antonio, Texas, and chief consultant to the Air Force Surgeon General for Refractive Surgery. He can be reached at (210) 292-2010 or via
e-mail at charles.reilly@lackland.af.mil.