Which device can be used to help determine if a GP lens is experiencing flexure?

• A. Lensometer
• B. Radiuscope
• C. Keratometer
• D. Lens clock

Answer: (C)

GP lenses can flex under eyelid pressure, which changes the posterior surface curvature and, in turn, how the cornea appears optically to the eye. A keratometer is useful here because it gauges the curvature of the surface that reflects light from the cornea. When a GP lens is on the eye, any flexure alters the effective curvature seen by the keratometer, causing a change in the measured radius that cannot be explained by the cornea alone. By comparing keratometry readings with the lens on versus the baseline corneal reading (or noting unexpected changes in the readings), you can infer that the lens is flexing. Other devices have their own roles—lensometers measure the back vertex power of a lens in a frame and aren’t as practical for assessing dynamic on-eye flexure, while radiuscopes or lens clocks quantify the lens’ curvature directly but are less informative about how the lens behaves on the eye.