Although scleral lenses are rapidly gaining ground, we clinicians can best serve our patients if we keep up our skills in fitting corneal GP lenses, too. A benefit of rigid lenses in general is the ability to fine-tune the fit for each patient. We can make numerous changes to the lens parameters, and it helps to periodically review how these changes will affect the lens performance.
The best base curve (BC) is one that provides the best overall alignment relationship with the cornea, minimizing any flatness (touch) or steepness (excessive pooling). Flattening a lens will lower the sagittal depth and loosen the fit; steepening the lens will increase the sagittal depth and tighten the fit.
Besides corneal bearing, an excessively flat lens will tend to drop in an arc along the limbus. A steep lens will tend to drop straight down. A slightly flat lens will tend to lid attach well but may ride too high. The bottom line: adjust the BC to get best alignment, but you can adjust a little flatter or steeper to help the lens position correctly, too. Make at least a 0.1mm (0.50D) change in the BC.
Once you have the best BC, the overall diameter (OAD) can be adjusted to achieve several goals. A larger diameter will usually lid attach a bit better due to the increased surface area interacting with the upper lid. A larger diameter can also provide more coverage of the pupil area to prevent lens flare in lower-light conditions. A smaller diameter can be used if the lower edge of the lens bumps against the bottom lid with blinking.
Change the OAD by at least 0.4mm. To maintain the same lens alignment, change the BC by 0.05mm (0.25D) for every 0.4mm change in OAD (flatter BC if making the OAD larger, steeper if smaller). Change the lens power by 0.25D as well.
Pay close attention to the amount of edge clearance, as this can affect the amount of tear exchange as well as the positioning of the lens. A flatter peripheral curve system produces more edge clearance and also increases the amount of lid attachment (Figure 1). A lens with steeper peripheral curves will tend to drop more (Figure 2). Check with your GP lab about how to adjust the edge clearance, as many designs have their own system for describing these changes.
A thicker lens will tend to drop more compared to a thinner lens. Though not usually the first choice, adjusting the thickness can fix a poorly positioning lens.
Be aware that making a lens thinner can result in on-eye lens flexure and fluctuating visual acuity. The general guideline for thickness change is at least 0.02mm. Your GP lab can guide you as to whether this will compromise the lens integrity.
I hope that these tips will help you when fine-tuning your next corneal GP lens fit! CLS