Fourier-based refractive treatment advantageous in myopic patients

Stanford, CA-Myopic patients who underwent CustomVue wavefront LASIK with the S4 laser (VISX, Advanced Medical Optics) with a Fourier-based program had better efficacy and predictability 1 year after the surgery than those with a Zernike-based system, according to Judy I. Ou, MD.

"The Fourier wavefront upgrade is currently available for CustomVue LASIK," said Dr. Ou, who is chief resident, department of ophthalmology, Stanford University, Stanford, CA. "The Fourier algorithm uses mathematics to derive an ablation pattern to improve the position of the wavefront ablation, which might be useful for eyes with a high degree of aberrations."

In explaining the differences between Fourier and Zernike analysis, she said that Fourier analysis uses about 240 Hartmann-Shack spots, which is equivalent to the 20th order Zernike polynomial. In contrast, the current Zernike analysis uses about 26 Hartmann-Shack spots and leads to a sixth-order Zernike polynomial.

Outcome measures

The outcome measures included the stability of the refraction, efficacy, predictability, safety, and higher-order wavefront aberration analysis.

The mean patient age was 41 years (range, 26 to 60 years). The preoperative mean spherical equivalent (SE) was similar in the two patient groups, –4.2 D in the Zernike group and –3.58 D in the Fourier group.

"One year after the procedure, the spherical equivalents were also similar between the two groups, with both about –0.25 D," Dr. Ou reported. "Both groups also had very stable refractions at 6 months and 1 year postoperatively."

At 1 year after surgery, 36% of eyes in the Zernike group and 46% in the Fourier group had 20/10 or better uncorrected visual acuity.

"This shows a slight trend toward better visual outcomes in the Fourier group," Dr. Ou commented.

Regarding predictability, 92% of eyes in the Fourier group and 79% of the eyes in the Zernike group were within 0.5 D of emmetropia.

Both groups had good safety profiles at 1 year. In the Zernike group no eyes lost a line of Snellen visual acuity and 63% of eyes gained one line of visual acuity. In the Fourier group, one eye lost one line of Snellen visual acuity and 43% of eyes gained one line of visual acuity.

The higher-order wavefront analyses were similar between the two groups in total root mean square, defocus, astigmatism, coma, trefoil, and spherical aberrations, Dr. Ou commented. No major complications developed in either group. Two eyes in the Zernike group were re-treated for residual myopia.

"We found that the Fourier group had slightly better efficacy and predictability than the Zernike group, while the safety profile was slightly better in the Zernike group. Overall, both treatment groups had similar results in the analysis of higher-order aberrations," she said.

However, numerous questions still remain, Dr. Ou said. For example, in the Fourier group with an ablation pattern that is equivalent to a 20th-order Zernike, can eyes be treated using the current technology? Does the Fourier algorithm detect noise, such as tear film, when it calculates a 20th-order Zernike polynomial?