This the sclera is higher than the modulus of

This study was performed by three
researchers named B K Pierscionek, M Asejczyk-Widlicka, and R A Schachar. This study
was done to figure out the effects of IOP when the cornea and sclera were
deformed.  The authors wanted to
specifically see how the corneal and scleral radii reacted to IOP. They
deformed the cornea and sclera by injecting through the optic nerve to the
vitreous. Then they measured the IOP and ocular rigidity after the deformation.

The final conclusions of this study were that the elastic moduli of both the
cornea and sclera are independent of IOP. Some differences in results between
the cornea and sclera are that the modulus of elasticity of the sclera is
higher than the modulus of elasticity in the cornea and the curvature of the
sclera changes when IOP increases while the curvature of the cornea does not
change with increasing IOP. Lastly, it was seen that the porcine scleral
rigidity seemed to be similar to the human scleral rigidity.

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The authors were able to obtain 16
porcine eyes to use in their experiments. Since they are eyeballs they need to
stay fresh so the authors kept the eyes on ice as they transported them and
then during the preparation and experiment the authors kept the eyes moist by
putting saline solution on them. Also, the extraocular muscles and extraneous
fat were removed from the eyeballs.  In
order to keep the eyes in one position the authors placed the eyeballs on a
base that was made of perspex tubing with gradation along the edge. The radii
for the corneal and scleral profiles were measured by the authors taking pictures
of the eyeball and then uploading the pictures onto a computer. In order to
find the center of curvature of each eyeball the authors had to use an
optimization procedure. By using a corneal applanation tonometer, that had an
accuracy of plus or minus 2 mm the authors were able to figure out the baseline

In order to be accurate the authors
measured the IOP four different times after each of the 5 injections of 100 ul
increments were injected into the vitreous. The authors were also able to
measure the ocular rigidity from this as well. By using equations applicable to
thin-walled pressure vessels the authors were able to measure the elastic
properties of the cornea and sclera. The assumption to allow us to apply these
equations to the eyeball was that the authors had to treat the eyeball as a
thin-walled pressure vessel. One equation that we did not go over in class that
was used is the equation for circumferential stress, which is P times R divided
by 2 times t. The variables are as follows: 
P is the IOP, R is the radius of the scleral shell, and t is the
thickness of the scleral shell. The authors also used an equation for
volumetric strain within a thin- walled sphere which is 3 divided by E, which
is the elastic modulus, all multiplied by stress minus Poisson’s ratio (for the
sclera) which is then multiplied by stress. They then use the volume of the eye
and the equation for volumetric strain to derive an equation for elastic
modulus, which ends up being 3 times 1 minus Poisson’s ratio all multiplied by
stress over volumetric strain. 


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