Eccentricity is a value that describes how much the peripheral curve radius flattens with respect to the apical curve radius. Much of the fitting of RGPs takes place in the periphery; this is why it is important to take into account eccentricity when fitting lenses. 100 subjects had eccentricities measured of their right eye at 7, 8, and 9 mm chords and the data analyzed. The average eccentricities are 0.572, 0.603, and 0.635 for the 7, 8, and 9 mm chords respectively. The average values show greater eccentricities as you move into the periphery of the cornea, demonstration that eccentricity can change a great deal with small changes in chord length. We have shown that there is a large, well distributed range of values at each chord. Very few of those sampled would have eccentricities that would not change the peripheral curve radius by ±1/8th D from the expected peripheral curve radius at a given chord. Furthermore, small changes in eccentricity can change the peripheral curve radius a great deal. Because of this large distribution of values, and because a small change in eccentricity can have great impacts on peripheral steepness, it is important to take into account eccentricity, and the position where the lens fits on the cornea when fitting RGP lenses.
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