On-eye power characteristics of soft contact lenses

Background. The power of a soft contact lens on the eye is a function of its off-eye power, the manner in which the lens flexes on the eye, lens hydration changes, and the corneal topography. Methods. In the present study, we used a high illumination keratometer, which allowed us to obtain front and back surface keratometric readings when the lenses were in position on the eye. The on-eye power of the lens could then be calculated from these readings, with the assumption that the center thickness and refractive index of the lens corresponded to those in vitro. Results. The estimates of the on- eye powers agreed closely with the results indicated by over-refraction. Moreover, comparison of in vitro with in vivo power estimates indicated that the positive lenses lost power on-eye, whereas the negative ones maintained their power. Conclusions. The present study confirms the results of earlier workers, who suggested that soft lenses drape to fit the cornea. Our findings appeared to be in agreement with the predictions of most of the models developed in the past.