Back to Basics with GP Lens Parameter Changes
BY JOHN MARK JACKSON, OD, MS, FAAO
Although we often talk about advanced GP lens topics in this column, some readers have asked for a review of basic parameter changes with standard GPs. Let’s look at some of the most common parameters that need to be adjusted to improve the fit of a corneal GP lens.
Keep this relationship in mind: If your alteration increases the sag depth, the lens will fit tighter; if it decreases the sag depth, the lens will fit looser.
The base curve radius (BCR) is the radius of curvature of the optical zone. Alter the BCR to get the best alignment with the cornea. For example, if the sodium fluorescein (NaFl) pattern appears too steep, you should flatten the BCR.
Change the BCR by at least 0.1mm (0.50D) to get a significant change in the lens fit. This amount will change the sag depth of the lens enough to change the physical fit of the lens.
Change the lens power by the same amount due to the tear film power changing with the new BCR. (Remember SAM-FAP: steeper-add-minus, flatter-add-plus.)
The overall diameter (OAD) can be adjusted to cover more of the cornea or to change the lens/lid interaction. The lid interaction is complicated; going larger could increase the lid interaction and make the lens ride higher, or it might make the lens ride a little lower because of the increased lens mass.
Be sure to change the OAD by at least 0.4mm. Changing the diameter will also change the lens sag and alter the alignment, so be sure to change the BCR along with the OAD. A larger contact lens has a deeper sag (Figure 1). Flatten the lens by 0.05mm (0.25D) for every 0.4mm increase in diameter, or steepen if you go smaller. Change the lens power to match the BCR change.
Figure 1. A larger-diameter lens (bottom) will fit tighter compared to a smaller lens (top) with the same BCR.
A thin GP contact lens will have less mass and could lid attach better compared to a thicker GP lens, but it may be more likely to flex and cause variable vision. Change the thickness by at least 0.02mm. Your lab consultant should be able to tell you if you have room to decrease the lens thickness for a particular material and lens design.
You can change the amount of edge clearance by altering the lens periphery. Excessive or minimal edge clearance can cause 3 o’clock and 9 o’clock staining. Increasing the edge clearance can improve tear exchange, and it can increase lid attachment. Many labs have moved to a “step” value system (i.e., “one step steep” or “one step flat”), so ask each lab for its system.
Corneal GP contact lenses remain an important part of clinical care, and understanding the basics of corneal GPs will also help you understand more complicated lens designs. CLS
Dr. Jackson is an associate professor at Southern College of Optometry, where he works in the Advanced Contact Lens Service, teaches courses in contact lenses, and performs clinical research. You can reach him at firstname.lastname@example.org.