Precise determination of parameters is essential to the proper fit of a rigid gas permeable contact lens. It is also important that the lens be manufactured to specifications ordered. One hundred rigid gas permeable lenses from four labs were verified, and their parameters compared to ANSI standards. A considerable number of the lenses studied had one or more parameters which failed to meet these standards. The percentages of lenses which failed to meet ANSI standards for a specific parameter were as follows: optic zone -- 7%; back vertex power-- 9%; overall diameter-- 10%; center thickness -- 15%; base curve -- 25%; and, peripheral curve widths -- 55%. Therefore, it is to the optometrist's benefit to verify incoming lenses, and to be able to modify them when needed to help ensure a proper fit.
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