Bifocal Contact Lenses

The performance of bifocal contact lenses was studied in vivo and in vitro. A pilot study was undertaken to determine the lenses to be used in the long-term study. In the long-term study four types of contact lenses were fitted to each of 10 patients to assess the optical performance. Included in the study were single vision soft contact lenses for monovision, the Alges bifocal, the Nissel PS45 lens and the Diffrax lens.

A series of tests was carried out on thelenses after one and two months established wear, and also with their spectacle correction. These included assessment of visual acuity under different testing conditions, contrast sensitivity and stereopsis. A simultaneous practitioner grading of the physiological response to the lenses and patient subjective assessment of the lenses was also carried out. The physiological response to all lenses was acceptable.

Although the visual performance with thelenses at distance and near was satisfactory, monovision and spectacles gave better monocular results in most cases. The binocular performance with the five modes of correction showed no statistically significant differences apart from stereopsis which was significantly worse with monovision. This suggests that bifocal contact lenses are a viable option for the presbyopic contact lens wearer. In vitro studies were also carried out. These included interferometry and transverse power measurements on several of the available lenses. The results suggest that some improvements could be made in the design of certain lenses.