A Review of GP Lens Edge Clearance
BY JOHN MARK JACKSON, OD, MS, FAAO
There are many factors that influence the on-eye performance of a corneal GP lens. One of the most important is the amount of edge clearance (i.e., the “gap” between the edge of the lens and the cornea), which gives us the bright band of fluorescein seen in the far periphery of the lens.
Though often confused, edge “lift” and edge “clearance” are not the same thing. “Lift” refers to a physical parameter of the lens (how the peripheral curves differ from the base curve). “Clearance” refers to the on-eye gap.
There are two main ways that edge lift/edge clearance influence how a lens performs.
Lens Position and Lid Interaction
In general, GP contact lenses work best with some degree of lid attachment. During blinks, the upper edge of the lens will stay under the upper lid, and the lens will not have an excessive downward drift between blinks. This helps to minimize the sensation of the lens.
The amount of edge clearance affects how well a lens will lid attach. In Figure 1, the lens has an axial edge lift (AEL) of 0.16mm. The peripheral band of fluorescein looks appropriate for a corneal GP lens (i.e., it has good edge clearance). Note that the lens “hangs” on the upper lid.
Figure 1. Good lid attachment with more edge clearance.
In contrast, Figure 2 shows a lens with AEL of only 0.07mm. Note that the peripheral fluorescein is much narrower here, and the lens drops easily to the bottom lid.
Figure 2. Poor lid attachment with less edge clearance.
All other parameters of these lenses are the same; base curve, optical zone diameter, overall diameter, lens material, and power are identical. With these held constant, it is easy to see the influence of edge clearance on lens positioning.
Corneal Chafing and Desiccation
The edge lift/clearance also can affect comfort and corneal health in another way. If the edge clearance is too small, the edge of the lens can rub against the cornea. This will result in corneal staining near the lens edge. However, if the clearance is too great, this can result in the classic 3 o’clock and 9 o’clock staining pattern, with staining at the horizontal limbal areas. This may occur due to evaporation of tears near the edge of the cornea due to surface tension at the lens gap.
If unchecked, 3 o’clock and 9 o’clock staining can lead to vascularized limbal keratitis (VLK). It is important to monitor GP patients for these issues and make changes to avoid long-term problems. Your GP lab can guide you on the appropriate change to make if you are not satisfied with the edge clearance.
By adjusting the amount of edge lift, the on-eye clearance can be increased or decreased. Consider taking a photo or video of the lens with a digital camera and sending it to the lab for consultation. The consultant can make a much better recommendation this way. CLS
Dr. Jackson is a 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 email@example.com.