SHEAR PROPERTIES OF COLLAGEN CROSSLINKED PROCINE CORNEA

The cornea is the outer transparent part of the eye which provides
structural integrity and contributes vastly to refract light. The tissue acts
as a protective layer for internal contents of the eyeball and its structural
properties are important for proper vision. The optical properties of the
cornea include transmission and refraction of the light. The cornea is
subjected to intraocular pressure from the inside and could be subjected
to external forces caused by eye rubbing, for example. The mechanical
properties of the cornea are altered in several diseases such as
keratoconus, which is an eye disease changing the corneal
microstructure. Keratoconus affects about one in 2000 individuals and
causes the cornea thins and gradually becomes a cone-like bulge. Any
change in the shape of the cornea interferes with its optical properties
and causes the vision to become blurry and distorted. At initial stages of
the disease, eyeglasses or lenses can be used to correct for vision
abnormalities. However, keratoconus is a progressive disease making
the cornea to bulge more. Thus, the vision of individuals significantly
decreases if no medical intervention is done.
The corneal collagen cross-linking procedure has been shown to be
effective in halting or at least slowing the progression of keratoconus.
In this procedure, the corneas are soaked in riboflavin solution and
crosslinks are created in the corneal stroma by shining ultraviolet (UV)
light. It has been shown that corneal structural properties improve
because of this procedure, the progression of the disease stops, and
individuals may not need corneal transplants. The strengthening effects
of the collagen crosslinking treatment procedure have been commonly
investigated by performing uniaxial tensile experiments [1-2]. For
example, we have shown that corneal collagen cross-linking resulted in
a significant increase of tensile stress–strain response which is
dependent on the hydration of the samples [3]. The primary objective of
the present work was to conduct shear experiments in order to determine
how corneal collagen crosslinking procedure influences shear properties
of corneal samples in vitro. For this purpose, we used porcine corneas
and used the Dresden crosslinking protocol to create crosslinks in vitro.
We then performed oscillatory shear experiments to characterize the
dynamic modulus of treated and untreated samples.