Advantages of aspheric aberration-free method

Patients in need of visual corrections whose vision is not affected by existing high-order aberrations (HOAs) may have an alternative to wavefront treatment using an aberration-free method.

Results of a three month myopia study¹ with 140 eyes as well as a hyperopia study² with 33 eyes has been conducted by Dr de Ortueta, MD, a refractive surgeon in the Recklinghausen eye clinic in Germany. "I employ the aberration-free treatment in approximately 85 percent of my refractive laser treatments. This method provides fast, stable and reproducible results," he claims.

In the study none of the eyes lost more than one line of Best Spectacle-Corrected Visual Acuity (BSCVA) postoperatively, whereas 6% of the myopic and the hyperopic eyes postoperatively gained two or more lines of BSCVA. In both groups, the corneal wavefront showed an extremely low induction of spherical aberrations; as compared to the study results published for other laser platforms the lowest induction of aberrations after LASIK treatment was observed.

The Aberration-free method is based upon the hypothesis that it is not always advantageous to remove preoperatively existing HOAs. Studies with untreated eyes^3,4 have shown that patients with above-average visual acuity can have high-order aberrations. It was also observed that patients with the fewest HOAs were not always those who achieved the best visual acuities.³ Because the brain, with help of neural compensation, adapts to aberrations,⁵ the patient has less need of adapting to a new visual situation. In such a way, there are at least five criteria (native aberrations, neural compensation, chromatic blur, depth of focus, wide field vision) favouring the target of leaving minor amounts of not clinically relevant aberrations.

A further disadvantage of wavefront-guided treatment is that it aims to remove high-order aberrations, but may generate new high-order aberrations - up to twice as many.⁶ Because the brain requires a longer adaptation phase for good sight due to postoperative induced aberrations,⁵ a greater number of patients possibly do not profit from an ocular wavefront-guided treatment.⁶ Furthermore, HOAs change with increased age. Although wavefront-guided treatment may achieve an optimum result according to the present visual performance, it is only for a limited time.

In conventional spherical and aspherical ablation methods, HOAs are induced during the laser treatment. The aspheric aberration-free profiles of the Schwind Amaris software used are reported to compensate HOAs that may result from the flap cut, change in the corneal shape through the laser ablation or from loss of efficiency by ablation at the corneal periphery. For the compensation calculations, Schwind developed a complex mathematical model including simulated ray tracing through the cornea described by its anterior surface and its pachymetry. The aspheric ablation profile is based on comprehensive analysis of the extent to which certain high-order aberrations provide optimized asphericity and therefore good visual results.

The clinical six month results of the SCHWIND AMARIS for myopia treatments show that the lowest induction of aberrations is observed in comparison with published study results for other laser platforms. The total root mean square of HOAs (for 6 mm analysis diameter) increased an average of only 0.07 µm postoperatively. This also resulted in a significantly higher postoperative contrast sensitivity of the patients.⁷

References1. D. de Ortueta, S.Arba Mosquera, H. Baatz. J. Refract Surg; 2009 (in press).
2. D. de Ortueta, S.Arba Mosquera, H. Baatz. J Refract Surg. 2009 Feb;25(2):175-84.
3. Pablo Artal et. al., What aberration pattern (if any) produces the best vision?, 6th International Wavefront Congress, Athens, Greece; February 2005.
4. Y. Levy et al., Am J Ophthalmol,; 2005; 139: 225-228.
5. P. Artal et al.,. Journal of Vision; 2004; 4:281-287.
6. P.S. Binder, et al., J Cataract Refract Surg. 2007;33:1158-76.
7. M.C. Arbelaez et al., J Refract Surg; 2009 (in press).