the contact lens exam
Verifying GP Contact Lenses
BY JULIE DEKINDER, OD, & VINITA ALLEE HENRY, OD

Verification of GP contact lenses remains an important tool for practitioners who fit these lenses. Verification can prove beneficial before dispensing GP lenses; upon follow up to confirm lens damage, warpage or reversed lenses; in instances in which diagnostic lenses are mismarked; and when determining the current lens parameters for a GP contact lens wearer.

Steps for Verification

Following are five important parameters to check when verifying GP lenses:

1. Base Curve Radius (BCR) is one of the most important parameters to verify, primarily because it affects the lens-to-cornea fitting relationship. You can measure BCR with a radiuscope or a radiusgauge.

When using a radiuscope, make sure the lens is clean and dry, so it will provide clear images. Fill the lens mount with saline. You'll find the real image on the lower end of the scale and the aerial image at approximately 6mm to 9mm on the scale. Both images appear as spokes or stars. After locating and centering the aerial image, lower the instrument to the real image. When the real image is in clear focus, zero the radiuscope and then return to the aerial image. At the point where the aerial image is in clear focus, read the BCR from the scale. Here's a troubleshooting tip: If the lens is warped or has toric curves, then you'll see a partial aerial image, and the image that's 90 degrees away will appear at a separate reading. The first image is the steeper curve and the second image is the flatter curve.

2. Power Practitioners most commonly determine the power of a GP lens as back vertex power using a lensometer. You can verify GP lens power similar to the way you verify spectacle power. Place the concave side of the lens on the lens stop and move the dial until you observe a clear image. Then read the power off the lensometer drum.

3. Lens Thickness Lens thickness is an easily verified parameter. A dial gauge will determine the thickness of the lens at the center or the edge. It's a good parameter to keep in mind when troubleshooting lens problems such as flexure, oxygen permeability, lens position and patient comfort.

4. Lens Edge Inspecting the lens edge is important for patient comfort. From the patient's perspective, a rough edge could result in symptoms of foreign body sensation and irritation.

You can most quickly and effectively inspect the lens edge by using the palm method, which consists of placing the lens, concave side down, on the palm of your hand. Gently slide the lens across your palm to determine the smoothness of the lens.

Other methods of edge inspection include using a projection magnifier, slit lamp biomicroscope or measuring magnifier. In addition, Young Enterprises offers a Contact Lens Edge Profile Analyzer, which uses radiuscope optics to view the edge profile.

5. Overall Diameter Practitioners most commonly verify overall diameter with a measuring magnifier or reticule. A PD ruler, V-channel gauge or dial gauge are other instruments that you can use to determine lens diameter. In addition, you can use the measuring magnifier to determine the optic zone diameter, blend the peripheral curve widths and, in a limited manner, to inspect the lens surface.

Get Verifying

On a final note, GP lens verification is simple and quick. In the time it took you to read this column, you could have verified the parameters of a GP lens.

Dr. DeKinder is currently completing a Cornea and Contact Lens Residency at the University of Missouri-St. Louis, College of Optometry.

Dr. Henry is the chief of the Contact Lens Service and is the Director of Residencies at the University of Missouri-St. Louis, College of Optometry.