Low_resolution_Thesis_CDD_221009_public - Visual Optics and ...

REFRACTIVE SURGERY PMMA MODEL
3.2. INTRODUCTION
Refractive surgery has become a popular alternative for the correction of refractive
errors. However, as described in Section 1.7.1 of the Introduction of this thesis, one of
the major issues of myopic refractive surgery is the increase of corneal asphericity
with the procedure (Marcos et al., 2001a), which causes an important increase of
spherical aberration (Moreno-Barriuso et al., 2001b) with significant visual
consequences (Applegate et al., 2000). Specific details on the magnitudes of these
changes and an extensive literature review can be found in the Introduction (Section
1.7.1). Some theoretical studies (Mrochen and Seiler, 2001) pointed at laser efficiency
effects as the origin of the problem (see Section 1.7.3 of the introduction). The
effective laser energy delivered onto the cornea changes as the laser beam moves from
the center to the periphery. As a result the effective ablation pattern applied on the
cornea varies with respect to the theoretical ablation pattern and the actual postoperative
corneal shape differs from the expected one. Understanding the physical
factors affecting the effective transfer of the energy to the cornea is essential.
The aim of the study presented in this chapter was to investigate the causes for the
change in corneal asphericity (and spherical aberration) produced by refractive
surgery. For this task we developed an experimental model of ablation on PMMA,
which we compared with computer predictions and experimental data in patients
obtained in previous studies in our laboratory.
In this study we used plastic corneal models to assess the laser ablation profiles
programmed in standard LASIK surgery and estimate the geometrically-related laser
efficiency effects.
Plastic models have been used in refractive surgery research and calibration for a
long time, especially for the assessment of roughness and calibration of fluence
(Gottsch et al., 1996, Odonnell et al., 1996, Roszkowska et al., 2003). However, it has
not been until recently that plastic models have been used to study in detail the shape
changes induced by refractive surgery on flat and spherical surfaces (Marcos et al.,
2005b). Plastic models are also interesting for quality assessment and market approval
of new laser platforms (2007, Drum, 2003, Drum, 2005), to check experimentally the
calibration state of individual laser units, and the diagnosis of appropiate alignment.
They will also help to validate theoretical models of refractive surgery (Arba-
Mosquera and de Ortueta, 2008, Kwon et al., 2008).
In this study the model corneas were made out of PMMA, a widespread material
for ophthalmic applications. The next chapters will present further elaborations of the
model using a novel material, Filofocon A, which proved even better ablation
properties. Spherical PMMA surfaces were made with similar radii of curvature than
that of human corneas. Profilometric measurements of PMMA flat surfaces after
ablation will test the actual ablation profile programmed into the laser system, and
whether this is close to the theoretical Munnerlyn pattern. A comparison between the
ablated spherical PMMA surfaces and identical ablations on flat surfaces will show
the effects of laser efficiency changes across a curved PMMA surface. Once the
difference in ablation rates between PMMA and corneal tissue are considered, as well
as the differences in ablation threshold in PMMA and cornea, it is possible to estimate
the effect of laser efficiency on post-operative corneal shapes. A direct comparison of
real and model post-operative corneal shapes will give insight into the possible
contribution of biomechanical and wound healing effects to the increase of corneal
asphericity in myopic refractive surgery. The conclusions will be relevant both in the
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