Low_resolution_Thesis_CDD_221009_public - Visual Optics and ...

INTRODUCTION
1.10.1. Tear studies
Already in the origins of wavefront theory of the human eye, Smirnov (Smirnov,
1961) made the important observation that the form of the tear layer might
significantly influence the ocular aberration (Charman, 2005).
The first surface of the eye is not the corneal epithelium of the eye but the surface
of the precorneal tear film. Changes in the tear volume and tear fluid dynamics can
induce changes in the HOAs even if the corneal shape is completely ideal (Maeda,
2009). During the 10-second period after opening the eye, 25% of the normal eyes
have a stable HOA pattern, 45% have small fluctuations, and 20% have a sawtooth
behaviour (Maeda, 2009). Subjects with short tear-film break-up time have a sawtooth
pattern with a marked upward curve that increase after blinking. From 5 to 9 s after
blinking, the total HOAs are significantly higher than that inmediately after blinking,
indicating that the optical quality might deteriorate in subjects with short tear-film
break-up time by supressing the blinking. When measuring HOAs, specially with
contact lenses (that can alter the tear dynamics), careful selection of the timing
(blinking vs measuring) is needed to avoid artifactual data.
1.10.2. Monofocal contact lenses
When a lens is placed on the eye, its HOAs will tend to add to, or substract from, those
of the eye, depending upon whether the signs of the wavefront errors are the same or
opposite. In the past, however, it has been difficult to predict how the lens and the eye
aberrations will combine. The aberrations of the lenses themselves have often been
unknown. Even if the design aberrations have been specified, the on-eye performance
may differ due to such effects as manufacturing errors, lens flexure, dehydration and
decentration, and the presence of the tear film.
With the generalization of the use of RGP contact lenses, it was generally
recognized that tear lens effects could result in the almost complete neutralisation of
many irregular aberrations and modest amounts of astigmatism associated with the
anterior corneal surface. However, in parallel with the early developments of this
thesis, only Hong and colleagues (Hong et al., 2001) had measured aberrations in
subjects wearing RGP contact lenses, finding that in 3 out of 4 subjects, RGP contact
lens provided lower aberrations than soft contact lenses and spectacle lenses.
However, a direct comparison of the optical changes produced by RGP contact lenses
on the anterior surface of the cornea and total optical system had not previously been
reported at the initial stages of this PhD project. Chapter 9 of this thesis will provide
new evidence on this field.
Concerning soft lenses for myopia, many studies have proven that they tend to
increase the HOAs (Roberts et al., 2006). The aberrations can be affected by the
manufacturing method (Jiang et al., 2006) and the power (Awwad et al., 2008). Dietze
and Cox (Dietze and Cox, 2004) showed that thin soft lenses with certain amount of
spherical aberration in air contributed the same level of spherical aberration on eye.
It is well known that the tear film on the anterior surface of the contact lens is
thinner and easier to break-up than the normal tear film on the anterior cornea,
affecting the aberrations variability. Besides, lens flexure (Collins et al., 2001),
movements, and decentrations will produce larger variability in the on-eye than offeye
measurements. Aberrometry provides a better understanding of the optical effects
of the contact lenses ex vivo or in situ.
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