Lid interaction and toric soft lens axis location.

A group of 15 subjects were fitted with truncated toric soft lenses varying in cylinder power from 1 to 4 D, in 1 D steps, with axes 90 and 180 degrees. After 30 min of wear, the location of the truncation was measured from a slitlamp photograph and calculated from a spherocylindrical over-refraction. Blink-initiated rotation of the lenses was measured by slitlamp photography. Comparison of the location of the truncation of lenses with cylinder at axes 90 and 180 degrees showed no significant difference using matched pair analysis, nor was cylinder power found to effect truncation location. However, analysis of the incidence of acceptable location (truncation within 25 degrees of the horizontal) showed significantly more acceptable locations for those toric lenses with the cylinder at axis 90 degrees (against-the-rule). A significant product moment correlation between the amount of mislocation of the axis and blink-initiated rotation was found. A high correlation was found between axis location measured from photographs and that calculated from over-refraction. However, because of the large variance of the differences between the two values, over-refraction is not considered to be a clinically acceptable method of determining toric lens axis location. The significance of the results of this study are discussed with respect to toric lens design.