Verifying Soft Contact Lens Parameters
BY MARY JAMESON, BHS, COA, NCLC, CPOT
In a recent column, "One Size Doesn't Fit All" (Staffing Solutions, May 2003), Jane Beeman, COA, NCLC-AC, FCLSA, PRSA, mentioned the importance of verifying contact lens parameters. I whole-heartedly agree. The lenses ordered must match the practitioner's specifications to help ensure a proper contact lens fit. Verifying parameters also helps when you need to troubleshoot a contact lens that doesn't fit properly.
Soft contact lens verification is difficult, but not impossible. Verifiable parameters include back vertex power (BVP), overall diameter (OAD), and surface and edge inspection. You can also verify base curve radius (BCR) and center thickness (CT), but typically only specialty contact lens practices have the instrumentation that is used to verify these two parameters.
Measuring Back Vertex Power
Measure BVP with a lensometer. Remove the contact lens from the case and place the lens flat on a lint-free tissue. Fold the tissue over and blot the lens dry. Carefully remove the lens from the tissue and place it on the lens stop of the lensometer. You can read the soft lens power in the same way that you read the power of a spectacle lens.
Sometimes the mire image will be blurred. Just remove the lens from the lens stop, re-wet the lens and repeat the procedure. Make sure that the lens doesn't dehydrate too much or it will become easy to damage. This technique works for both spherical and toric soft lenses.
Measuring Overall Diameter
Use a handheld measuring magnifier to measure OAD. Remove the lens from the case and shake off the excess saline. Make sure the lens remains partly hydrated for this procedure.
Place the lens concave side down on the measuring magnifier. Align one side of the lens with the "zero" on the scale and read the point where the other side intersects the scale. You could also use a pupillary distance ruler to measure OAD.
Inspecting the Surface and Edge
You can perform contact lens surface and edge inspection with a measuring magnifier or a slit lamp biomicroscope. Again, blot the contact lens dry and place it on the measuring magnifier. Hold the contact lens up to a light source to inspect its surface. You can also turn the magnifier around and look through its flat surface. The magnification is higher from this side and can provide a closer look at any surface or edge abnormalities.
When you use a slit lamp, hold the contact lens behind the microscope with a pair of tweezers for magnified viewing. Most practitioners will perform "on eye" inspection of the lens as part of the examination.
Check the Lens First
Before you conclude that a contact lens is defective, make sure you have all of the information. You may find that the contact lens itself is correct and that you instead need to look elsewhere to discover the answer.
To obtain more information about lens verification, visit Indiana University School of Optometry's Web site at www.opt.indiana.edu and the University of Missouri St. Louis' Web site at www.umsl.edu.
Ms. Jameson is laboratory supervisor for the Department of Clinical Sciences at the Pennsylvania College of Optometry and past chair of the AOA Paraoptometric Section.