They say that corneal GP lens fitting is as much of an art as it is a science. However, don’t let that intimidate you. While contact lens fitting pearls tend to focus on the fundamentals, many practitioners (including myself) tend to struggle with what steps to take if at first you don’t succeed. Thus, my goal here is to discuss what to do when that corneal GP lens doesn’t look quite right.
What Kind of Cornea Are You Fitting?
The starting point is to know with what you are working. Do your patients have regular corneas and simply want crisp vision, or are they keratoconus or post-corneal transplant patients? For keratoconus patients, it is important to know the size, location, and severity of the cone.
Small or “nipple” cones tend to be less than 5mm in diameter, at or near the center of the visual axis, and very steep, and they tend to flatten rapidly. With many proprietary keratoconus fitting sets, eyecare practitioners (ECPs) can select lenses that match the steep keratometry (K) readings and have parameters that will appropriately match the cornea’s rapid flattening.
An oval cone will usually be located below the visual axis; thus ECPs may experience issues with decentration. With this type of cornea, a slightly larger diameter will aid in fitting, but also you may have to increase the optic zone, as blurred vision will result if a patient is viewing through both the central optic zone and the peripheral zone.
The steep part of the cornea is even more inferior with pellucid marginal degeneration, but ECPs should follow similar guidelines of a larger-diameter design and potentially a bigger optic zone (Sindt, 2015).
Another consideration for corneal GP lens selection for keratoconus patients is the severity of the condition. Mild keratoconus patients who have average K reading ≤ 45D can usually get by with a conventional GP design. For moderate cones of 45D to 50D, an overall diameter of 9.0mm will usually suffice. For advanced-stage cones of 50D to 55D, decrease the overall diameter to about 8.6mm to 8.8mm. For severe-stage cones of ≥ 55D, the diameter will usually be smaller (less than 8.0mm) (GP Lens Institute, 2019).
For post-corneal transplant corneas, the shape of the graft will impact the GP fitting relationship. I completely depend on corneal topography for initial lens selection. Is it a prolate graft that is steep centrally and flattens toward the periphery, an oblate graft that is flatter in the center and steeper peripherally, or a mixed graft that has areas that are very steep and some that are very flat? Finally, a steep-to-flat graft will have a steep area adjacent to a flat area (Chang and DeLoss, 2018; Szczotka and Lindsay, 2003). For oblate grafts, I like reverse geometry designs. Prolate shapes can sometimes work with conventional spherical GP designs or a post-graft lens that mirrors the graft’s shape. A mixed or flat-to-steep graft will usually have a mind of its own, so I usually start with a basic design and troubleshoot based on how it centers on the eye.
Where Does the Lens Travel?
Once a lens is chosen and an in-office diagnostic fitting is performed, examine how the lens does or does not move on the eye. Does it tend to settle superiorly, inferiorly, or laterally? Is it too steep or flat, or is it OK centrally?
Start with the central relationship (i.e., is there fluorescein or not?). If there is fluorescein, does it look OK, or is there too much? If there is none, then you will need to make a change. To evaluate the fit, you sometimes must manually center a lens that travels off of the center of the cornea. If it looks like it is too steep, flattening it by at least 0.1mm or 0.5D is the minimum amount of change needed to see a difference. The opposite is true if there is no fluorescein—steepening of 0.1mm or 0.5D is the minimum to see a change (Bennett et al, 2013).
If the lens tends to drop inferiorly, flatten the base curve by a minimum of 0.1mm. Or, consider going to a larger-diameter lens with a 0.2mm to 0.5mm change. Superior or lateral decentration warrants a steeper base curve. In rare cases, a smaller optic zone for bubbles can help. CLS
For references, please visit www.clspectrum.com/references and click on document #287.