The effect of varying prism and truncation on the performance of soft contact lenses

A group of 10 subjects were optimally fitted with soft senses by photoelectronic keratoscopic analysis of corneal form. Lenses with diameters of 12.8 and 13.5 mm were worn incorporating prism ballasting varying between 0 and 1.5 delta in 0.25 delta steps and truncation between 0 and 2.0 mm in 0.5-mm steps. The performance of each lens was evaluated according to eight commonly applied clinical criteria: visual stability, lens position, recentering ability, amount and direction of movement, subjective comfort, meridional location of the lens, and amount of blink-initiated rotation. No benefit in performance was obtained from incorporating larger amounts of prism and truncation in the lens design, the best combination being 0.75 delta with 0.5-mm truncation. As a single stabilizing feature, truncation appeared more effective in locating a lens in the correct meridian than prism ballasting. It was found that corneal-fitting soft lenses of differing diameters (between 12.8 and 13.5 mm) performed similarly with the same stabilizing features.