Purchase this article with an account.
L. A. Cavuoto, X.-R. Huang, R. W. Knighton; Corneal Birefringence Mapped by Scanning Laser Polarimetry. Invest. Ophthalmol. Vis. Sci. 2007;48(13):3532.
Download citation file:
© ARVO (1962-2015); The Authors (2016-present)
To increase our understanding of the polarization properties of the cornea and their affect on retinal retardance measurements. Current corneal measurement schemes treat the cornea as having uniaxial birefringence with the slow axis tangential to the cornea surface and oriented nasally downward. A more detailed model (van Blokland, JOSA A, 1987) treats the cornea as a curved biaxial crystal with its fastest axis oriented perpendicular to the corneal surface and its slowest axis coincident with the tangential axis of the uniaxial model. Rays that enter at angles not perpendicular to the corneal surface experience birefringence from both tangential and perpendicular corneal axes. Corneal compensation performed for retinal measurement only accounts for the tangential component. The perpendicular component could lead to errors in measured retinal retardance values.
Measurement of retinal retardance of 15 naturally dilated eyes were acquired with scanning laser polarimetry (GDx-VCCTM) without corneal compensation. The first image was acquired with the pupil of the eye in the standard position centered on the acquisition screen. For subsequent measurements, the pupil was moved to different position on an overlaid pattern to obtain images through different corneal locations. In each image, the bow tie pattern formed at the macula revealed the axis and retardance experienced by the beam coincident with the visual axis and thus provided a measurement of birefringence at one corneal location. The corneal points were mapped and the data were compared to a numerical simulation of the birefringence map predicted by the curved biaxial crystal model.
Corneal birefringence varied greatly across a 5x5 mm corneal area and among different eyes. The locations of zero birefringence, where the incident beam was aligned with the optic axes of the cornea, were found for each eye. A good fit to the measured data was obtained with the numerical simulation of van Blokland’s model.
These results confirm by another method the curved biaxial crystal model of corneal birefringence. Although the uniform linear retarder model used for corneal compensation in GDx approximates the central cornea, the biaxial model enhances the cornea birefringence description away from the center and can possibly be used to predict uncompensated cornea contributions to retinal retardance measurements.
This PDF is available to Subscribers Only