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Paul P. van Saarloos; Comparison of the iTrace vs. Nidek Optical Path Difference (OPD) on a Model Eye Central Anomaly. Invest. Ophthalmol. Vis. Sci. 2012;53(14):3593.
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© ARVO (1962-2015); The Authors (2016-present)
To determine if a central anomaly on a model eye is detected, and if it affects the wavefront measurements of the model eye, when using the Nidek OPD and the iTrace from Tracey Technologies.
The center of the outer lens of a Heine Model Eye, set to emmetropia, was altered by the application of a very small spot of epoxy which produced a "central island" anomaly. The Model Eye was centered and scanned first using the iTrace Aberrometer and then recentered and scanned with the Nidek OPD II.
The iTrace displayed a clear added power to the eye of 1 micron wavefront change centrally in a circular pattern of approximately 2mm diameter. The peripheral error was relatively uniform. The OPD maps significant peripheral aberrations of up to 10 microns, but no added power centrally. The glue measured 1.6 by 1.9mm when measured with a micrometer.
The iTrace maps showed the expected patterns from the created anomaly, with central added power, and the peripheral region left unaberrated, but the OPD maps did not. The OPD missed the central added power and reported aberrations in the periphery that were not there. The measurement method of the OPD uses a peripheral rotating slit with a camera looking through a small central area that is not measured. The scanning slits appear to have missed the created anomaly and the camera looking through the central aberrations the anomaly induced, created false readings of aberrations in the periphery. This has significant clinical implications for patients with central keratoconus, using aberration measurements to correct eyes with central aberrations eyes with a laser, and presbyopia corneal inlays.
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