Low_resolution_Thesis_CDD_221009_public - Visual Optics and ...

HYBRID PORCINE/PLASTIC MODEL
6.1. ABSTRACT
Purpose: Quantitative analysis of posterior corneal Scheimpflug
images requires proper correction of geometrical and optical
distortion from the anterior surfaces. We developed a hybrid
porcine/plastic eye model (with the scattering properties and
refractive index of corneal tissue and known posterior corneal
geometry) for validation of the posterior corneal reconstruction of
the Pentacam Scheimpflug imaging system.
Methods: The hybrid porcine/plastic eye models consisted on
excised porcine corneas mounted on a 12-mm diameter plastic
piston finished on spherical surfaces with known radii of curvature
(7.47, 7.93 and 8.75 mm, as validated by non-contact profilometry).
Special care was taken to preserve the endothelium integrity. The
porcine’s posterior cornea conformed to the underlying plastic
surface. Scheimpflug images were obtained within 4 hours postmortem.
The posterior corneal surface was evaluated using the
fitting routines of the Pentacam software. We used 12 porcine
corneas. The retrieved posterior corneal geometry was compared to
the plastic surfaces’ nominal data. Correlations between anterior and
posterior corneal data were performed to assess that the optical
distortion correction was successfully applied.
Results: Measurements on the hybrid model eye of known posterior
corneal geometry show that the retrieved parameters (radius,
astigmatism, asphericity) are more precise for the anterior surface of
the cornea than for the posterior surface. The measured posterior
corneal radius of curvature is minimally affected by the geometry of
the anterior surface, although consistently underestimated
(0.26±0.12 mm on average). There was larger variability and
underestimation of the posterior surface asphericity (Qpost=-
0.33±0.44) than of the same spherical surfaces tested anteriorly
(Qant=-0.02±0.22). The anterior porcine corneas varied in anterior
radii (from 8.0 to 9.6) and astigmatism (1.9 ±1.0 D). However, there
was no significant correlation between anterior and posterior corneal
radii of curvature, asphericity or astigmatism (p>0.26).
Conclusions: This model has allowed testing the accuracy of the
posterior corneal geometry estimates from Pentacam. The
independence of the retrieved data from the anterior corneal
topography suggests proper correction of optical distortion. This
model shows more comparable properties to those of living eyes and
could be used for calibration of other anterior segment imaging
systems. The Pentacam Scheimpflug system can be used reliably to
assess changes in the posterior corneal radius of curvature after
LASIK.
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