Cataract & Refractive Surgery Today May, 2003
Spherical Aberration and Its Symptoms Theories on why it occurs and how new technology may address the problem.
BY MARIA REGINA CHALITA, MD, AND RONALD R. KRUEGER, MD, MSE
SYMPTOMS CORRELATED WITH SPHERICAL ABERRATIONS: Standard laser refractive surgery performed on patients with large scotopic pupil sizes is associated with nighttime vision problems such as halos.12 The increased amount of higher-order aberrations after standard LASIK is consistent with the relatively common patient comment, “I can read 20/20, but my vision is not as good as it was before.”13 We analyzed 105 eyes that underwent LASIK correction and correlated their symptoms with higher-order aberrations. Our analysis of optical symptoms and measured aberrations for a scotopic pupil size showed a statistically significant correlation between higher-order aberrations and glare (P=.041) as well as starburst (P=.004). When we broke down these aberrations into individual Zernike components, spherical aberration was the predominant cause, with a statistically significant correlation to glare (P=.010) and starburst (P=.014). Halos seemed to be associated with spherical aberration for the scotopic pupil size (P=.053). Table 1 shows the relationship of spherical aberration and coma with patients’ symptoms.
SPHERICAL ABERRATION PREVENTION AND CORRECTION: Surgeons must exercise care when treating eyes with larger scotopic pupils, especially if the procedure is expected to induce higher levels of spherical aberration (patients with large pupils will experience more symptoms with higher levels of spherical aberration). Customized laser ablations attempt to minimize these symptoms by more effectively avoiding laser-induced spherical aberrations. The ideal ablation profile for correcting refractive error without generating spherical aberration is to reshape the cornea with a lesser radius of curvature in the midperiphery rather than in the center. This difference in asphericity corrects the spherical aberration of the eye, because the flatter surface will cause less refraction of the peripheral rays.14