Are scleral lenses analogous to corneal GP lenses when it comes to the fitting process? The answer: It’s complicated.

To start, when it comes to the materials from which the lenses are made, both corneal GP and scleral contact lenses share a common bond. Additionally, although vastly different in size, diameter is diameter. Where the process gets complicated for many is the fitting relationship of the contact lens in relation to the cornea. And beyond diameter and material, this is where the two roads diverge.

With respect to corneal GP lenses, practitioners are taught to take keratometry readings into account when selecting the initial base curve. From there, practitioners also know that when it comes to evaluation of the fit, they need to look at the central sodium fluorescein pattern and adjust the base curve of the lens to improve how the lens fits on the cornea. Furthermore, our old friends “SAM and FAP” (i.e., steep add minus, flat add plus) come into play when making the adjustment, and they directly relate to refractive error. Eyecare providers know that the tear film plays a role in the resultant power of the lens and that they must calculate the outcomes. They also know that if you increase or decrease the diameter of the lens, this will need to be factored in, as it impacts the fitting relationship to the cornea as well as the tear film power. This is so easy, right?

Vault Considerations

When fitting scleral lenses, one of the main fitting points is the scleral vault. This is the space between the posterior surface of the lens and the anterior cornea. It is generally acceptable to have about 200 or 250 microns, or about half the width of the cornea when using an optic section on your slit lamp, of fluid reservoir or “tear film” in this area. Increasing or decreasing this amount, however, will not change the lens power or your resultant over-refraction for the majority of fits.

What is confusing is that many manufacturers do not have a uniform term for the part of the lens that adjusts this space, and nomenclature can be a problem. Some companies refer to it as the “sag,” which is the lens sagittal height; some use the term corneal “vault”; and others use “base curve” to define this area of the lens, to name a few.

Of note: In many cases when fitting scleral lenses, the base curve is independent of the vault. This means that you can raise or lower the lens vault without impacting the tear lens power; however, the base curve changes of most lenses can still impact the refractive outcome. Therefore, a steeper base curve will require more minus power in the lens and vice versa for a flatter base curve.

Don’t Forget Toricity

Another area that can be confusing when switching from fitting corneal GP lenses to fitting scleral lenses is toricity. Contrary to how the term is used with soft lenses, for sclerals it does not refer to any portion of the refractive error. Rather, it refers to the fitting relationship on the sclera. As such, it may be easier to think of this area either as peripheral alignments or landing zones. Practitioners can adjust this area to be flatter or steeper in different quadrants to provide a better fitting relationship and also to improve comfort for patients.

Bringing things back together, both corneal GPs and sclerals do share front-surface toricity in common. This directly refers to remaining sphero-cylindrical over-refraction that can be incorporated onto the front surface of the lens to improve visual acuity.

In Summary

While the end goal—to achieve the best possible vision and comfort—is the same, and sclerals and corneal GPs share some things in common, the fitting process for the two lenses tends to be different. CLS