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A. de Castro, P. Rosales, S. Marcos; Crystalline And Intraocular Lens Tilt And Decentration Measured With Scheimpflug Imaging: Experimental Validations . Invest. Ophthalmol. Vis. Sci. 2006;47(13):323.
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© ARVO (1962-2015); The Authors (2016-present)
The study presents a procedure to measure lens tilt α and decentration d from Scheimpflug anterior segment images. Comparisons with measurements from a previously reported Purkinje imaging system (PIS) were made on normal eyes, eyes with intraocular lenses (IOLs) and a realistic physical model eye.
Sections of the anterior segment of the eye were captured using a commercial Scheimpflug Imaging system (Pentacam) at 25 meridians. Images were processed using custom developed algorithms. The anterior and posterior corneal and lens surfaces were fit by spherical sections. The pupil center and the anterior corneal center of curvature were used to estimate the pupillary axis, used as a reference. The lens axis was defined by the center of curvature of its anterior and posterior surfaces. Lens α and d were obtained for each section and fit by a cosine function for 3–D estimates. For comparison α and d were also measured using a previously custom–built system, based on Purkinje reflections used for phakometry and lens positioning estimates. Measurements were performed on 3 young normal eyes (24 yrs), 4 eyes with IOLs (72 yrs), and a model eye. Measurements on human eyes were performed under pupil dilation. The model eye consisted on a PMMA cornea (diameter: 11.12 mm, anterior and posterior corneal radius: 7.80 and 6.48 mm respectively) attached to a water cell, in which intraocular lenses were mounted on x–y–z and rotational micrometers. Nominal α ranging from 0 to 3 deg, and d from 0 to 2 mm were set for testing.
In normal eyes α ranged from 1.83 to 2.00 deg (Scheimpflug) and 0.74 to 2.09 deg (PIS) with a mean discrepancy between techniques of 0.43 deg, and d ranged from 0.18 to 0.33 mm (Scheimpflug) and from 0.24 to 0.35 mm (PIS) with a mean discrepancy of 0.04 mm. For patients with IOLs α ranged from 1.6 to 4.5 deg (Scheimpflug) and 3.1 to 6 deg (PIS) with a discrepancy of 1.69 deg and d ranged from 0.98 to 1 mm (Scheimpflug) and 0.09 to 1.66 (PIS) mm with a discrepancy of 0.37 mm. For the physical model eye, the experimental data correlated well (r>0.9) with nominal values, with slopes of 1.53 (Scheimpflug ) and 1.77 (PIS) for α and 0.66 and 0.65 for d, with constant offsets between both techniques.
Measurements of tilt and decentration with both methods are repetitive and consistent to each other. Limitations of each technique depend on different factors, i.e. Purkinje imaging is limited for large anterior IOL radii and Scheimpflug by pupil size and detectability of the lens surfaces.
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