Changes in hydrogel contact lens power due to flexure

When a hydrogel lens is placed on the eye, its power may change because of flexing of the lens. The correcting power of the lens when on the eye can be calculated by assuming that the lens flexes to the shape of the underlying cornea. Actual measurements of the correcting power of Hydrocurve lens on the eye were not in agreement with the calculated values. Assuming that the front and back surfaces of the lens flex by the same amount, the presence of a fluid lens between the hydrogel lens and the cornea would account for this discrepancy. For minus lenses of moderate and high power, it was found that the change in power was in the direction of more plus; for low minus and all plus lenses, the change was in the direction of more minus. Under the conditions of the present investigation, correcting power measured with the lens on the eye was in agreement with the calculated value for regular-thickness Hydrocurve lenses of -2.00 D and thin lenses of -0.75 D. This is interpreted to mean that lenses of these powers conform to the shape of the cornea so that the fluid lens is thin and plano. The clinical implications are discussed.