Low_resolution_Thesis_CDD_221009_public - Visual Optics and ...

CHAPTER 6
no preferential orientation. These retrieved values of astigmatism of the posterior
corneal surfaces conformed to spherical plastic surfaces are similar to those found in
the calibrations of spherical model eyes (anterior surfaces) in the previous section.
There was no correlation between the astigmatism of the anterior and posterior corneal
surface (p=0.57 for all eyes).
6.4.3.3. Asphericities
Finally, there was no correlation between the anterior and posterior corneal
asphericities (p=0.28 for all the eyes), as shown in Fig. 6.5, where anterior
asphericities are plotted vs posterior asphericities (ideally zero). However, the
posterior corneal asphericity differed from zero, and was quite variable (-0.33 ± 0.44)
for the back surface).
Fig. 6.5. Measurements of Asphericities. Each point is the average across repeated
measurements in the same conditions.
6.5. DISCUSSION
In this chapter we have presented a validation of the Pentacam Scheimpflug system to
measure the posterior corneal surface geometry. The experiments using a hybrid
porcine/plastic eye model indicate that the posterior corneal radius can be obtained
with high repeatability and that the reconstructed posterior surface is not affected by
changes or asymmetries in the anterior cornea. Posterior corneal radius seems to be
slightly underestimated and asphericity overestimated in comparison to the nominal
values of the posterior cornea in the model eye used for calibration. While studies on
the repeatability of this system had been published before (particularly of the anterior
radius of curvature and pachymetry (Lackner et al., 2005, Jain et al., 2007)) this is, to
our knowledge, the first time that the accuracy (both for anterior and posterior surface)
is studied.
6.6. CONCLUSIONS
We have developed an artificial eye model that shows corneal properties (scattering,
refractive index) comparable to those of living eyes and can be used for calibration of
anterior segment imaging systems. The accuracy of the posterior corneal geometry
data provided by the Pentacam Imaging System was verified using our model. The
independence of the retrieved data from the anterior corneal topography indicates
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