Laser yields safe, predictable correction for astigmatism

January 1, 2008

An ongoing multinational study is evaluating LASIK for myopia and myopic astigmatism performed with a new-generation excimer laser (Schwind Amaris, Schwind eye-tech-solutions) using its aspheric, aberration-free ablation profile.

Key Points

Dr. Barraquer, a study investigator in private practice in Bogota, Colombia, reported that 364 patients have been enrolled in the study, and all were seen for follow-up at 3 months. Preoperatively, they had a mean spherical equivalent (SE) of –3.50 D with a range between –0.5 and –8.5 D. The patients had up to –9 D of sphere and up to –7 D of astigmatism.

At 3 months post LASIK, mean SE was –0.17 ± 0.24 D (range, –1 to +0.62 D). Uncorrected visual acuity was 20/16 or better in 55% of eyes, 20/20 or better in 95% of eyes, and 20/25 or better in 100%. Predictability was excellent, with refraction within 0.25 D of intended in 75% of eyes and ±0.50 D in 92%. All eyes were corrected within 1 D of attempted, and in an analysis of defocus, 89% of eyes had an outcome of 0 D.

Results of a wavefront analysis showed that, true to its name, the aspheric aberration-free treatment corrected the lower aberrations but left the preoperative higher-order aberrations (HOAs) essentially unchanged.

"The [new excimer laser] is demonstrating encouraging results in this preliminary analysis," Dr. Barraquer said. "Now we need long-term studies and to evaluate outcomes from treatments performed with the customized ablation profiles of this new laser platform."

The patients enrolled in the study had a mean age of 28 years. All flaps were created using a proprietary microkeratome (Carriazo-Pendular, Schwind eye-tech-solutions).

Dr. Barraquer said that refractive stability appeared promising, but with only 3 months of follow-up, it was too early to review that issue.

Correction of astigmatism was investigated with vector analysis. Dr. Barraquer noted the results were good overall, although there were a few outliers.

About the laser system

The new excimer laser operates with a repetition rate of 500 Hz and has automatic fluence adjustment with two fluence levels, a high fluence level that enables rapid treatment and a low fluence level used for fine correction to improve the resolution. The result is shortened treatment time without jeopardizing treatment precision and safety.

The device is a flying spot laser featuring a super-Gaussian beam profile with a very small beam size of 0.54 mm to deliver accurate ablations and a smooth surface. It also has an advanced eye tracker that works at 1,050 Hz and compensates for eye movements in five dimensions: horizontal and vertical displacement, horizontal and vertical rotation, and cyclotorsion.

"The tracking system is based on pupil position and the limbus, has a rapid response time of less than 3 milliseconds, and offers static and dynamic cyclotorsional control along with pupil centroid shift control," Dr. Barraquer said. "The [excimer laser] platform also has an option for integrated online, high-resolution pachymetry."