Low_resolution_Thesis_CDD_221009_public - Visual Optics and ...

INTRODUCTION
corneal surface may be expected, since the surgery weakens the cornea (see Fig. 1.21).
As corneal biomechanical properties are largely unknown, it is not possible to predict
theoretically the extent of this potential deformation (Dupps and Wilson, 2006).
Fig. 1.21. From (Dupps and Wilson, 2006). Major biomechanical loading forces in
the cornea and a model of biomechanical central flattening associated with disruption
of central lamellar segments. According to Roberts (Roberts, 2000, Roberts, 2002), a
reduction in lamellar tension in the peripheral stroma reduces resistance to swelling
and an acute expansion of peripheral stromal volume results. Interlamellar cohesive
forces (Smolek, 1993) and collagen interweaving, whose distribution is greater in the
anterior and peripheral stroma and is indicated by grey shading, provide a means of
transmitting centripetal forces to underlying lamellae. Because the central portions of
these lamellae constitute the immediate postoperative surface, flattening of the optical
surface occurs, resulting in hyperopic shift.
1.7.5. Current trends in refractive surgery
Refractive surgery has been improved over the past years, thanks to the refinement of
the involved technologies and the improvement in the ablation pattern design.
Presumably, the improvements of the ablation patterns with the estimated correction
factors obtained theoretically are now included in the proprietary ablation algorithms
of most laser platforms, which are now most likely compensated for efficiency effects.
The final goal of refractive surgery is the possibility for correction of higher optical
aberrations of the eye, and not only conventional refractive errors. Most laser
manufacturers claim that current ablation algorithms are now wavefront-optimized or
customized to the patients’ optical aberrations (aspheric, wavefront-guided or
topography-guided) (Kohnen, 2006, Kohnen, 2008), with the aim of avoiding the
increase of spherical aberration that was a major issue with standard ablation profiles
(Moreno-Barriuso et al., 2001b, Marcos et al., 2001a). Customized wavefront guided
surgery is assisted by wavefront sensors and topographers (to measure optical
aberrations) and precise lasers provided with precise control of the energy delivery at
each point, thanks to flying spot technology, pupil tracking, and fast repetition rates.
Femtosecond laser technology allows for flap creation that induces less change in
high-order aberrations and corneal biomechanics (Kim and Chuck, 2008).
Recent studies (Arba-Mosquera and de Ortueta, 2008, Kwon et al., 2008) describe
sophisticated numerical models that take into account most of the knowledge gathered
in the last years on the physical effects affecting refractive surgery (in particular some
of the results of this thesis), with less approximations, and considering the temporal
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