The correction of the LCA in the eye has been long been debated and proposals of LCA-correcting IOLs have been made, mostly in the form of diffractive elements.
22,23 While the monochromatic MTF of the eye clearly exceeds the polychromatic MTF,
13,14 and there is evidence that, in the absence of both chromatic and monochromatic aberrations, visual performance exceeds that with noncorrected chromatic aberrations,
15,18 correction of LCA alone has not yielded remarkable vision correction.
33 Reasons for this relatively low benefit of correcting LCA include the presence of TCA,
13,14,17,18 and the fact that monochromatic aberrations and LCA interact favorably in eyes with physiological amounts of aberrations. McLellan et al.
14 and Ravikumar et al.
18 reported that, in fact, the presence of monochromatic aberrations attenuated the degrading effect of the chromatic aberration, particularly for shorter wavelengths, in contrast with a diffraction-limited eye where chromatic defocus produced large differences in the MTF across wavelengths. As the IOLs become more sophisticated in design (ultimately aiming at correcting the HOA of the individual eye),
1,4,6,22,23 the correction of LCA may become more relevant. Both IOLs of the study had aspheric surfaces, and aimed by design at correcting (or at least reducing) the positive spherical aberration of the average cornea,
3,6 similarly to the corneal/internal balance of spherical aberration in the young eye.
34,35 The measured HOA in the pseudophakic patients of the study (average RMS_HOA = 0.21 ± 0.08 μm [Tecnis] and 0.17 ± 0.04 μm [AcrySof], green light, 4-mm pupil) were of the order of magnitude of those found in a young population (average RMS_HOA = 0.70 ± 0.11 μm, green light, 6.51-mm pupil).
16 Our simulations of the PSFs reveal the image quality degradation produced by the HOAs in the tested pseudophakic patients, which changed in shape and magnitude across eyes. According to the mentioned prior literature, physiological amounts of LCA are not greatly detrimental to retinal image quality, as a result of the positive interaction with the existing natural HOA. In fact, in most eyes, the interactions of HOA and astigmatism with chromatic defocus attenuated the impact of the chromatic difference of focus on the PSF, very much like the effect of pure defocus, having a lesser relative impact on image degradation in the presence of HOA and astigmatism than in a diffraction-limited eye. As seen in
Figures 3 and
4, the relative impact of the chromatic difference of focus in eyes with HOAs (with or without astigmatism), is much lower than that expected in a diffraction limited eye (
Figs. 4a,
4d), with no significant differences in the degradation of the PSF with LCA between groups. Other functions that have been suggested to be helped by the presence of LCA include emmetropization and accommodation.
36,37