Low_resolution_Thesis_CDD_221009_public - Visual Optics and ...

HYBRID PORCINE/PLASTIC MODEL
6.2. INTRODUCTION
Previous chapters of this thesis were dedicated to the study of the physical changes in
the anterior surface of the cornea with refractive surgery. The aim of these studies was
achieving the necessary control over the changes in corneal shape to perform
successful wavefront guided surgery. The optical influence of the posterior cornea is
small compared to the anterior cornea. However, monitoring possible changes on the
posterior is important to assess the biomechanical integrity of the cornea, in particular
in response to surgery. The FDA recommends preserving at least 250 m of stromal
bed to prevent ectasia (bulging of the cornea). Ectasia is a rare complication in
LASIK, but the debate whether the posterior corneal surface is affected in LASIK is
still open.
This and the next chapter of this thesis will be devoted to the study of the
posterior corneal surface. This chapter will introduce a new experimental model to
validate a Scheimpflug-based instrument to characterize the posterior corneal shape.
Chapter 7 will present study where the validated instrument is applied to study the
effect of myopic laser ablations in humans on the posterior corneal surface.
There is a relatively large body of literature addressing on the posterior surface of
the cornea after LASIK (Wang et al., 1999, Seitz et al., 2001, Baek et al., 2001, Twa et
al., 2005, Grzybowski et al., 2005). However, measurements of the posterior corneal
shape after refractive surgery have been largely contested because they may be due to
artifacts caused by the optical distortion produced by the anterior corneal surface
(Ueda et al., 2005, Donnenfeld, 2001), which changes dramatically after the
procedure. Recently, a Scheimpflug imaging – based commercial corneal topographer
which nominally corrects for this distortion, Pentacam (Oculus GmbH, Germany) has
become available. The principles of Scheimpflug imaging were presented in the
Introduction (Section 2.1.3) as well as methods for correction of the optical and
geometrical distortion of this particular instrument (Rosales and Marcos, 2009). The
first studies on refractive surgery using this device have reported no significant
changes in the posterior surface of the cornea after LASIK and PRK (Ciolino and
Belin, 2006, Matsuda et al., 2008). Although some studies have reported a high
repeatability of the Pentacam instrument in normal (Lackner et al., 2005) and post-
Lasik eyes (Jain et al., 2007), a validation of the accuracy of this system to measure
the posterior cornea has never been presented.
The goal of this study is the validation of the reconstruction of the posterior
cornea provided by the Pentacam Scheimpflug imaging system. For that, we
developed a hybrid porcine/plastic eye model: with the scattering properties and
refractive index of corneal tissue and known posterior corneal geometry. This model
can be used in anterior segment research and validation/calibration of other biometry
and imaging instruments, as OCT.
6.3. METHODS
A validation of the accuracy of the measurements of the posterior corneal surface
geometry was performed in vitro using a hybrid porcine/plastic eye model. The use of
corneal tissue was motivated to achieve similar intracorneal scattering in the images –
which appears to be critical to achieve good edge-detection- and a similar index of
refraction –which is a parameter in the reconstruction algorithms.
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