PRECISION AND RELIABILITY STUDY OF A MODIFIED KERATOMETRIC TECHNIQUE FOR MEASURING THE RADIUS OF CURVATURE OF SOFT CONTACT-LENSES

The back optic zone radius of soft contact lenses cannot be measured w ith conventional methods used for rigid lenses. Although a variety of techniques has been proposed, none has received wide clinical acceptan ce. A study was conducted to determine the precision and limitations o f a method using a Zeiss keratometer. A total of 192 lenses from categ ories 1 (Optima 38) and 4 (Hydrocurve II 55%) of the FDA classificatio n were used. These lenses were divided into six groups according to th eir water content and refractive power. The difference versus the mean of two different measurements (repeatability) has been plotted to det ermine die bias (mean of the differences relative to zero), the 95% li mits of agreement and the repeatability coefficient of the technique. Variation between measurements was also assessed by calculating the re producibility coefficient R for each soft lens. The precision expresse d by the SD of a series of ten measurements on one lens in each group, was computed for comparison with previous studies. The repeatability of the technique expressed in term of 95% limits of agreement (range /- 0.05 8 to +/- 0. 198 mm) meets the American Standards Tolerances fo r each group of soft lenses studied. The coefficient of reproducibilit y R for each soft lens studied seems to be related to lens flexibility and dioptric power. The technique used gives the same degree of preci sion as other keratometric methods described in previous studies. The clinical use of this method is discussed with regard to practical fact ors.