Low_resolution_Thesis_CDD_221009_public - Visual Optics and ...

ANTERIOR AND POSTERIOR CORNEAL ELEVATION MAPS AFTER REFRACTIVE SURGERY
7.2. INTRODUCTION
As described in detail in Section 1.7 of the Introduction, there are open clinical
questions in refractive surgery which may find an explanation in changes of the
posterior surface of the cornea. In this chapter, we will present the use of the
methodology developed in Chapter 6 to perform the first validated measurements (to
our knowledge) of the effect of LASIK on the back surface of the cornea. As the back
surface of the cornea is not directly affected by the procedure, we used these
measurements as an indicator of biological changes in the cornea resulting from
biomechanical processes and wound healing. The study was conducted longitudinally
to investigate potential changes over time.
The change in manifest refraction after LASIK differs from the measured change
in corneal power, estimated using the standard keratometric index (Tang et al., 2006).
The discrepancies may arise from the change in ratio of the anterior/posterior corneal
curvature with LASIK (and therefore the effective keratometric index) (Tang et al.,
2006, Jarade et al., 2006), change in the corneal effective index of refraction (due to
epithelial hyperplasia) (Spadea et al., 2000), or changes in the posterior corneal
curvature with LASIK. Moreover, theoretical calculations of post-operative corneal
shape after subtraction of standard ablation profiles differ dramatically from clinical
outcomes (Cano et al., 2003, Marcos et al., 2003, Gatinel et al., 2001, Jiménez et al.,
2003), as already explained in Section 1.7.2. To a large extent, this effect can be
explained by physical laws (Section 1.7.3) (Mrochen and Seiler, 2001, Jimenez et al.,
2002). The results of these predictions are very likely affected by the assumption of a
mechanically inert cornea (Munnerlyn et al., 1988), but when plastic corneas are
ablated (Chapters 3 and 5) (Dorronsoro et al., 2006a), we also find discrepancies with
clinical outcomes. Empirical corrections of the ablation algorithms (as those proposed
in Chapters 3 and 5) can compensate for laser efficiency losses (Dorronsoro et al.,
2006a), and perhaps for systematic deviations found in the average population, but are
unable to provide individual adjustments, since the cause of the inter-subject
variability in achieved correction is unknown (Dupps and Wilson, 2006). A greater
understanding and quantification of the biomechanical processes that take place in the
cornea after surgery would help to improve the predictability and stability of achieved
corrections.
Some previous experimental studies have reported significant changes in 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, these results
have been largely contested, because they may be due to artifacts caused by the optical
distortion produced by the anterior surface of the cornea (Ueda et al., 2005,
Donnenfeld, 2001).
In this study we use the Scheimpflug-based Pentacam topographer, validated in
Chapter 6, to study the changes in the back corneal surface produced by myopic
LASIK in patients, comparing them to physiological changes observed in control
subjects.
7.3. METHODS
7.3.1. Subjects
A total of 45 eyes (23 subjects) participated in the study. Fourteen subjects (27 eyes),
with ages ranging from 21 to 47 years (mean, std 32 ± 7 years), underwent myopic
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