الفهرس 
INTRODUCTIONOnly 14 pages are availabe for public view
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Abstract
The main idea behind correcting sight disorders using lasers is to modify its thickness and
geometry after applying the laser beam to specific layers of the cornea. The corneal shape relaxes
to a new equilibrium state to reflect this change in loading. The biomechanical response, along
with changes in mechanical properties of corneal tissue, partially determines final corneal
curvature. Alterations of these properties may contribute to medical complications, such as
overcorrections or undercorrections.
Intrastromal Photorefractive keratectomy (ISPRK) is a laser technique used to correct sight
disorders. The ideal refractive laser would evaporate tissue lenticels within the stroma without
mechanical incision. The evaporation of tissue results in small cavities that may coincide to form
a larger cavity. This large cavity will collapse to deform the curvature ofthe cornea and hence
correcting the sight disorder such as myopia. Efforts are being made to reach this ideal situation.
Excimer laser in situ keratomileusis (LASIK) has received considerable attention because it spares
the outer layers of the cornea and therefore reduces haze and regression, which are often observed
after a PRK procedure. A disadvantage of LASIK is that a flap must be surgically created to expose
the corneal stroma for Excimer laser ablation.
To ensure predictability and safety for refractive surgery, accurate biomechanical modeling
of-corneal structure is needed. Advanced models of corneas require spatial distrib tions of
material parameters, particularly the elastic (Young’s) modulus.
In this work, laser tissue effects were studied to understand how the intrastromal cavity is formed
inside the cornea, and also to know the side effects of the applied laser on the stroma. The
procedure itself was investigated to know how to select the appropriate laser parameters to make a
suitable cavity that is used to reshape the cornea.
Following that, we made a 3D finite element model of the cornea, with typical dimensions, material
properties, boundary conditions and loads. The model outcome was compared with another 2D model
used for the same purpose, so as to determine
its accuracy and reliability. The results were also discussed and compared with individual clinical
cases to further evaluate the accuracy of the model. Finally, a 3D finite element simulation was
made for the procedure for a virtual astigmatic case in order to visualize the effects on corneal
curvature and shape.
The results of this thesis show that this finite element simulation is relatively an accurate model
of the procedure taking into account the limitations outlined in this work.