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 with conventional methods used for rigid lenses. Although a variety of techniques has been proposed, none has received wide clinical acceptance. A study was conducted to determine the precision and limitations of a method using a Zeiss keratometer. A total of 192 lenses from categories 1 (Optima 38) and 4 (Hydrocurve II 55%) of the FDA classification were used. These lenses were divided into six groups according to their water content and refractive power. The difference versus the mean of two different measurements (repeatability) has been plotted to determine the bias (mean of the differences relative to zero), the 95% limits of agreement and the repeatability coefficient of the technique. Variation between measurements was also assessed by calculating the reproducibility coefficient R for each soft lens. The precision expressed 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 +/- 0.058 to +/- 0.198 mm) meets the American Standards Tolerances for each group of soft lenses studied. The coefficient of reproducibility R for each soft lens studied seems to be related to lens flexibility and dioptric power. The technique used gives the same degree of precision as other keratometric methods described in previous studies. The clinical use of this method is discussed with regard to practical factors.
Ecole d'Optométrie, Université de Montréal, Québec, Canada.
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Quesnel
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Simonet
D003263:Contact Lenses, Hydrophilic / Q000592:standards*; D004867:Equipment Design; D008722:Methods; D015203:Reproducibility of Results