Validity of Contacheck in Measuring the Base Curve of Hard Contact Lens

The standard method to measure the base curve radius of hard contact lenses (RGP) is to employ a tool namely Optical Spherometer which is also called Radioscope. As Radioscope is an expensive and not easily accessible tool compared to keratometer, using the keratometer clamps seems to be more reasonable. In order to remove these problems, in this research a small clamp was designed and built which is a combination of the two above-mentioned clamps and the name Contacheck was selected for it. The purpose of this research is to find an answer to this question that whether or not the sizes of radius of the base curve of hard contact lenses resulting from Contacheck are reliable and valid compared to the standard device of Radioscop.

In this study 43 hard contact lenses in different sizes were randomly selected and their curve which had been specified based on the manufacturing company were measured two times by an optometrist; once by a Radioscope device and then by the Contacheck. Another optometrist had gone through the same measurement procedure. The correlation of the values resulting from two devices (Inter-rater reliability coefficient), the equality of the values obtained from the observations by two independent evaluators were also tested. Bland –Altman method was employed to determine the approximate agreement of 95% in all mentioned stages.

The mean and Standard deviation of curvature of hard contact lenses were 6.75±0.75 &6.76±0.73 by radioscope and contachek respectively. Intraclass correlation coeffitiont (ICC) was 0.998 and the confidence interval for the mean differences between the average values ​​of both devices by the Bland-Altman plot was (-0.11, 0.12).

The contachek is a valid and reliable instrument for measuring the Base curve radius of hard contact lenses.