Integration of topography and wavefront technology provides hope for the future in more complex cases

March 1, 2006

Chicago—Topographic- and wavefront-guided approaches to re-treatment of complex cases have advantages and disadvantages, but may be best viewed as complementary rather than competing techniques for managing the rare but suffering patients with poor quality of vision after excimer laser refractive surgery, said Terrence P. O'Brien, MD. He was speaking at the refractive surgery subspecialty day meeting sponsored by the International Society of Refractive Surgery of the American Academy of Ophthalmology.

"There has been debate over what is the optimal modality for correcting these destructive errors involving degradation of the optics over the entrance pupil. Used alone, wavefront- and topographic-based techniques have each proven to be invaluable," said Dr. O'Brien, professor of ophthalmology and director of the Refractive Surgery Service, Bascom Palmer Eye Institute, Miami.

"However, similar to institutions such as love and marriage, the two are inseparable and you can't have one without the other," he said. "Ongoing research integrating wavefront and topography data is expected to lead to an exciting future."

"Although topography takes into account corneal aberrations, there has been difficulty measuring and interpreting the true corneal contour, and that has implications for determining the optimal shape for re-treatment," he said. "Furthermore, an inability to treat residual refractive error and aberrant shape simultaneously may necessitate significant sacrifice of corneal tissue using a topographic-guided approach."

Nevertheless, the Custom Contoured Ablation Pattern (C-CAP) method, which used VISX software and laser technology together with data from the Humphrey Atlas topography system (Carl Zeiss Meditec), showed promise for being able to regularize the corneal surface and improve BSCVA in patients with irregular corneas after decentered ablations. Some patients also secondarily benefited with improved UCVA and overall quality of vision.

"In some cases, the results were spectacular, but the planning was complex and not often reproducible for widely variable clinical conditions," Dr. O'Brien said.

Thus the search for a better alternative continued, and the emergence of wavefront-guided customized surgery was met with considerable enthusiasm. Early efforts with therapeutic ablations, however, were somewhat disappointing due to the inability to capture even moderately aberrated corneas reliably in the wavefront evaluation. Furthermore, the Zernike reconstructions significantly smoothed the aberrations in complex shapes.

Therefore, the development of more robust Fourier algorithms has been a welcome advance because it provides higher fidelity of resolution and reconstruction.

"Repeatability analysis indicates the Fourier reconstruction is not capturing and treating noise but rather is a robust method," Dr. O'Brien said.

Combination approach

However, there are still some patients who should be advised to wait for future technology. At the University of Ottawa, Bruce Jackson, MD, and coworkers are researching ablation planning based on both topography and wavefront data. The aberrations are captured with the WaveScan aberrometer (AMO/VISX) and combined with data from the Humphrey Atlas topography system using tandem software that generates advanced ablation tables and exports them into the STAR S4 excimer laser (AMO/VISX).

"This software is needed because topography measures corneal aberrations, which involves many data points and a wide range, especially in a corneal surface that is highly aberrated after laser vision correction, while the wavefront measures whole eye aberrations but with a limited number of data points and is pupil-size dependent," Dr. O'Brien said.

Initially, the software is expected to be used to help plan topography-guided ablations in highly aberrated eyes and to automate the C-CAP process for the most complicated cases. It could also be used to simulate and compare outcomes for topography- and wavefront-based re-treatments and to predict corneal topography and wavefront profiles using various treatment parameters.