Low_resolution_Thesis_CDD_221009_public - Visual Optics and ...

INTRODUCTION
and further-more, at least in a laboratory setting, to correct most of the aberrations of
the eye and provide the visual system with an almost perfect optics.
Custom phase plates (Navarro et al., 2000) have demonstrated to correct 80% of
the wave-aberrations in the human eye, and to improve contrast of confocal retinal
imaging (Burns et al., 2002). Phase plates have also been used to correct high order
aberrations in keratoconic patients (Sabesan and Yoon, 2009). However, phase plates
have to be optically coupled to the optics of the eye by other optical elements, what
limits their practical usefulness.
Other static corrections, i.e. in form of customized ablations (MacRae et al., 2000,
Mrochen et al., 2000) or custom contact lenses (Lopez-Gil et al., 2002) have more
potential interest for widespread use. They will be discussed in Sections 1.10.4 and
1.7.5. Wavefront-corrected intraocular lenses will be straightforward, once the
techniques for ablation in plastic curved surfaces will be well controlled, although
centration remains a critical issue in all these correction strategies.
The great advantage of adaptive optics is the possibility to produce dynamic wavefront
corrections. One can measure the optical aberrations of the eye and correct them on a
closed loop with an active optical element, typically a deformable mirror. This
technique is called adaptive optics (Fernandez et al., 2001, Hofer et al., 2001), and
allows providing the eye with unprecedent resolution and contrast (Marcos et al.,
2008) or to investigate the role of the ocular aberrations on the accommodative
response (Gambra et al., 2009) or in the visual function (Sawides et al., 2009). While
great advances have been done in the laboratory, the results obtained so far in the
correction of the ocular aberrations of the eye clinically are promising, but still distant
from being optimal. There are many open questions, and it is necessary to revisit the
procedures, and push the involved techniques and methodologies further.
In any case, wavefront correction (specially static corrections) will never provide
unlimited optical quality, as there there other limits for vision, apart from optical
aberrations and diffraction: Neural limits, scattering, temporal changes of aberrations
(Marcos, 2002) or chromatic aberrations.
1.6. LASER REFRACTIVE SURGERY
1.6.1. The origins of refractive surgery
Besides providing most of the power of the eye, the cornea has easy access. The idea
of changing its power to correct ametropias is straightforward. The early attempts of
incisional corneal refractive surgery were made more than 200 years ago (Sakimoto et
al., 2006, Donders, 1864) (see page 95). The techniques have been evolving since
then. Incisional refractive surgery (Fyodorov and Durnev, 1979) has been abandoned
now. The first laser refractive surgery applied to the corneal epithelium of a patient
eye to treat his myopia was reported by MacDonald et al. in 1989 (McDonald et al.,
1989). Previously, Seiler (Seiler and Wollensak, 1986) had applied the same
procedure (PRK) to a blind human eye. Munnerlyn et al. (Munnerlyn et al., 1988)
calculated the thickness of tissue necessary to correct a given quantity of myopia or
hyperopia. Pallikaris (Pallikaris et al., 1990) combined PRK with Keratomielousis
(removing a thin layer of cornea to change its power, that had been introduced by
Barraquer in the 1960’s (Barraquer, 1967), creating the Laser Assisted Keratomileusis
(LASIK), which is one the most popular surgical approach to correct myopia.
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