Careful inspection of GP lenses is useful for several reasons. It is important to verify lenses when they arrive from the laboratory to ensure that they have the correct parameters and that they are free of defects, such as edge or surface issues. Inspection is also helpful for checking the condition of a patient’s old lenses and for collecting lens data if the patient was fit elsewhere.

What to Evaluate and How

As a refresher, here are the five main things to evaluate when inspecting GP lenses.

1. Base Curve Radius The base curve is usually measured with a radiuscope. To get a good reading, make sure that the lens is clean. Also make sure that the back of the lens is dry when mounted on the stage. If you need a radiuscope, used instruments are frequently available on online auction sites.
2. Overall Diameter (OAD) and Optic Zone Diameter (OZD) The OAD and OZD are measured with a handheld loupe with a scale. To visualize the optic zone, slowly move the loupe back and forth while aiming at the edge of a light source (Figure 1). Move the lens so that one end of the optic zone is at zero, then read the scale at the other end. In Figure 1, the OZD is 8.6mm.
3. Power Make sure that your lensometer has the smaller lens “stop” made for contact lenses. Using the larger one for spectacles will result in an inaccurate reading. The smaller stop places the lens at the correct position, and it is easier to balance the lens on it. Be sure to put the back of the lens against the stop so that you are measuring the back vertex power of the lens.
4. Center Thickness Often overlooked, this parameter can influence lens flexure and on-eye positioning. To me, it is most important to check future lenses against old lenses for consistency. If you need one, contact lens thickness gauges can also be found at online auctions.
5. Lens Surface and Edge Use the slit lamp to inspect the lens surface for scratches and deposits as well as to see chips or other edge defects. The quality of the edge polish has a significant impact on lens comfort. A well-polished lens edge will have a thin, bright reflection when you view the side closest to you (Figure 2).

It’s Best to Be Certain

Take the time to inspect and verify all GP lenses in your practice. As the saying goes, you can’t tell the players without a scorecard, and you can’t know that the lenses are correct and in good shape unless you check them. CLS