We all strive to make our GP lens fits as comfortable as possible. One of the most critical factors for corneal GP success is the edge clearance (EC), which is essentially the gap between the back of the lens and the cornea under the peripheral curves (PCs). We indirectly measure the EC when we evaluate the amount of fluorescein dye under the peripheral area of the lens. Ideally, we want an even band of dye around the lens perimeter that is roughly 0.5mm to 0.75mm wide—not too wide, not too narrow, and showing symmetry in the width of the band.

I am very picky about EC, because I know that an incorrect EC will have a negative impact on lens positioning, comfort, and corneal staining. Excessive EC will cause too much lid interaction and will decrease comfort or will cause the lens to position too far under the upper lid. Insufficient EC can cause the lens to position inferiorly and will inhibit tear exchange. Both excessive and insufficient EC can cause 3 o’clock and 9 o’clock corneal staining, which can lead to vascularized limbal keratitis.

One potential issue that can occur is when there is asymmetry in the amount of EC in different meridians. Ideally, the peripheral band of fluorescein should be the same width around the entire contact lens. When it’s not, there are a few options to try.

Case for Toric PCs

The lens in Figure 1 shows less EC on the temporal and nasal sides of the lens and slightly excessive EC at the top and bottom of the lens. This can occur in normal astigmatic corneas that are on the borderline for needing a toric base curve; it is also sometimes observed in keratoconus patients (such as in Figure 1). When you have a patient such as this, order the lens with toric PCs. In this example, that would result in more EC horizontally and less EC vertically to produce a more even EC all the way around.

Case for Asymmetric PCs

The lens in Figure 2 has an asymmetric EC profile. This eye also has keratoconus, but the problem that I want to illustrate with this fit is the large amount of edge standoff at the bottom of the lens. In this case, the lab can steepen the PC in the bottom quadrant to even out the EC. Fixing this will improve comfort and will decrease the chance of the lens ejecting from the eye.

Get Help When You Need It

If you aren’t sure whether you need either of these options, send a photo or video of the lens fit to your lab consultant, who can help you make the correct choice. A photo can also help consultants determine how much alteration to the PCs is needed.

Conclusion

An ideal corneal GP lens fit is in the details. Correcting uneven EC can make an already good fit great. Take that extra step to give your GP wearers their best chance for successful, comfortable wear. CLS