Low_resolution_Thesis_CDD_221009_public - Visual Optics and ...

SOFT CONTACT LENS FITTING USING MODEL CORNEAS
8.1. ABSTRACT
Purpose: We developed an experimental model to study the optical
effects of fitting soft contact-lenses. Knowledge of the coupling
between the optics of the eye and of the contact-lens, and how the
lens flexes to conform the cornea is essential to understand fitting of
multifocal lenses, and why they do not work successfully in many
patients.
Methods: Artificial eyes were manufactured on PMMA cylinders.
The cornea was a polished spherical surface, and the retina was
located in the corneal focal plane. Total and anterior surface
aberrations were measured on naked artificial eyes and fitted with
positive and negative monofocal and multifocal soft contact lenses.
Laser Ray Tracing and videokeratoscopy were adapted to allow
measurements in the vertical direction. A special holder was used to
provide good centration and hydration of the lens, and uniform tear
surface.
Results: The shape of the contact lens on-eye was simulated based
on corneal topoghaphy measurements. We found differences
between monofocal and multifocal designs in the anterior surface
shape. The radii of the lenses were consistent with their nominal
power. Multifocal designs tend to produce aspherical surfaces, more
complex than simple conics. The analysis in terms of aberrations can
be simplified to spherical aberrations. Corneal and total spherical
aberration coincide in the artificial eye without lens, validating our
aberrometers. The presence of tear does not influence the average
result, although it increases the variability. Corneal spherical
aberration is reduced less than expected in positive multifocal
lenses. Negative multifocal lenses conform to the cornea, not
inducing multifocality at all their first surface. Monofocal lenses
reduce total spherical aberration. Total spherical aberration is
positive in the multifocal designs, which differs from the expectation
of the intended optical change.
Conclusions: A new experimental model for the study of soft
contact-lenses, based on aberrometry, has been developed which can
be used to evaluate the optical factors involved in contact lens fitting
without the variability associated to individual eyes. Physical causes
are more important than expected to account for the lack of
adaptation of patients to multifocal lenses.
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