PRECISION AND RELIABILITY OF A TECHNIQUE FOR MEASURING THE BACK OPTIC ZONE RADIUS OF SOFT CONTACT LENSES

Nadia-Marie O.D.

Abstract

The back central optic radius (BCOR) of soft contact lenses cannot be measured with conventional methods used for rigid lenses because of their flexibility and hydration. Although a wide variety of techniques have been proposed, none has received wide clinical acceptance or come into common used. A study was conducted to determine the precision, reliability and limitations of a method using a Zeiss keratometer (model D-7082). Afocal lenses from each of the four FDA classified groups were used for 10 repeated measurements of the BCOR of the same lens during 4 separate sessions. During the procedure, the lens was placed concave side up in a contact lens case screwcap filled with unpreserved saline solution. Because the keratometer scale is calibrated for readings of convex surfaces in air; a correction factor has to be derived and incorporated in the calculation of the true or corrected BCOR. This was done by repeatedly measuring the BCOR of PMMA lenses with this technique and with a radiuscope and then comparing the results. A calibration graph of this comparison was plotted. The results indicate that the corrected BCOR or base curve of all lenses measured were flatter than the base curve specified by the manufacturer. The reliability of the technique, as expressed by the standard deviation, varied from 0.01 to 0.05 mm depending on the lens material. Repeated measurements for the same lens show statistically different mean values. For most lenses the reliability of the technique is sufficient to meet the American standards for soft contact lens tolerances. However, the clinical use of this method is limited by some practical factors.

Details

Year: 1992

Program Number: Poster 7

Author Affiliation: n/a

Co-Authors: Pierre O.D., Daniel L.Sc.O.

Co-Author Affiliation: n/a

Room: Great Hall