Low_resolution_Thesis_CDD_221009_public - Visual Optics and ...

INTRODUCTION
there are still many unsolved issues (mainly related to fitting effects) that prevent the
satisfactory clinical implementation of aberration corrective contact lenses.
1.10.5. Models, simulations and predictions
Several studies performed computer simulations to understand the optical performance
of the contact lens (Atchison, 1995, Martin and Roorda, 2003, De Brabander et al.,
2003). Using computer modeling, they evaluated the interaction of the lens with a
model corneal surface and the optical contribution of the tear lens between the cornea
and contact lens. Validating those simulations is difficult, as they tend to simplify the
problem: Although some take into account contact lens decentration and corneal
irregularities, they do not consider the flexing and the influence of the internal optics.
Other studies (Collins et al., 2001) have measured the topography of the contact lens
on the eye to study flexure on eye, but the analysis is based on corneal elevation data
rather than on corneal wave aberrations.
1.11. OPEN QUESTIONS IN CONTACT LENS FITTING
Integrity of the contact lens design on-eye. It is well known that contact lenses
(specially soft, but also rigid) are affected by flexure and shape deformations, when
the lens is placed on the eye. An open question is, to which extent the design of the
lens (starting with radius and asphericity) will be preserved after fitting and therefore
the intended optical effect achieved. And in particular, with multifocal designs, how is
the effective power distribution of the fitted lens
Corneal biomechanics and corneal shape deformation: Long term or short term
corneal shape changes due to the extended wear of contact lenses can determine the
optical outcomes. To advance in this field, instrumental methods for the accurate
measurement of not only the anterior but also the posterior cornea are needed.
Hydration and tear film: It is clear that the tear film has an optical effect in the normal
eye. When fitting a contact lens, this tear film is modified, and the new tear film in the
front surface of the contact lens has different optical consequences. The actual
relevance of the optical effects induced by the tear film (introducing a refractive index
difference with the cornea, and also shape variations), and changes in the tear film
after fitting a contact lens need further exploration.
Optical interactions: What is the actual optical effect on the aberrations of the eye of
fitting contact lens How is the coupling between the aberrations of the contact lens
and the aberrations of the cornea What is the impact of the tear lens formed between
the contact lens and the cornea What is the optical relevance of this new source of
internal aberrations What is the impact of changing the balance with the aberrations
of the cristalline lens
Origin of inadaptation to multifocal contact lenses: Today, the fitting of multifocal
contact lenses very often fails to satify the patient, and a trial and error procedure is
often followed. New methods for the detailed study of successful and unsuccessful
adaptations are needed, in order to gain knowledge about the origin of the failures, to
anticipate or fix the problems, or to select good candidates for this kind of correction.
For example, some unsolved question are: What is the baseline of neural effects in the
failures to adapt simultaneous vision multifocal contact lenses And what is the
achieved multifocality, the actual depth of focus gain introduced by multifocal lenses
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