Low_resolution_Thesis_CDD_221009_public - Visual Optics and ...

INTRODUCTION
alcohol is used to loosen the epithelium. In Epi-LASIK, a purely mechanical means of
epithelial dissection (using Epikeratome) allows the creation of the epithelial sheet.
1.6.3. Safety and efficacy of laser refractive surgery
An average of 700,000 patients in the U.S. have LASIK anually, with procedures
growing worldwide from 3.6 million to 4.2 million (MarketScope, 2008). To date,
more than 28.3 million LASIK procedures have been performed worldwide.
Collectively, 7,830 patients (representing 16,502 eyes) participated in clinical trials
from 1993 – 2005. In April 2008 the Food and Drug Administration reaffirmed the
Safety and Efficacy of the LASIK procedure (Ellin, 2008). Although the number of
patient complains has risen in the last few years, the post-operative visual outcomes
have improved in the most recent studies. In FDA studies recruiting patients that had
undergone surgery before 2000, 1.4% of patients lost 2 lines or more of BSCVA
versus 0.6% in studies after 2000. Before 2000, 1.68% of patients with a preoperative
BSCVA 20/20 or higher had a postoperative BSCVA 20/25 or higher, compared with
0.16% after 2000. The surveys determining patient satisfaction with LASIK have
found most patients satisfied, with satisfaction range being 92–98 percent. A metaanalysis
dated March 2008 performed by the American Society of Cataract and
Refractive Surgery over 3,000 peer-reviewed articles published over the past 10 years
in clinical journals from around the world, including 19 studies comprising 2,200
patients that looked directly at satisfaction, revealed a 95.4 percent patient satisfaction
rate among LASIK patients worldwide (Ciccone, 2008, Solomon et al., 2009).
1.6.4. Laser ablation
All models of excimer laser corneal refractive surgery (experimental, analytical or
numerical) rely on the validity of the assumption of energy deposition occurring in a
well-defined layer which is then removed by ablation. For absorbing media, which is
the case for cornea and most polymers at the processing wavelength (193nm), the
intensity I of the electromagnetic wave propagating into the material (z-direction)
decreases exponentially following the Beer-Lambert expression, according to
with
I
z
( z)
I0
e , (1.5)
k
4 . (1.6)
This physical model of ablation is called the blow-off model. The inverse of the
absorption coefficient is the optical penetration depth, i.e. the depth in which the
major fraction of the energy is deposited (1/e of the total amount absorbed). k is the
extinction coefficient. As a consequence of Eq. (1.5), the etch rate d (etch depth/pulse)
is expected to relate linearly to the natural logarithm of the laser fluence F according
to
1 F
d ln
, (1.7)
F th
where F th is the ablation threshold.
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