From glass to gas permeable plastics
The first documented scleral lens, from 1887, was a glass shell.[1] The big change came with gas permeable plastics. Fluorosilicone acrylates combine the oxygen permeability of silicone with the wettability and deposit resistance of fluorinated compounds, and hexafocon A and hexafocon B are among the most widely used.[1] The same review lists silicone acrylate, fluorosilicone acrylate, and PMMA as the rigid materials scleral lenses are made from.[1]
Research lenses give a sense of what’s in use today: one recent study used a material called roflufocon A with a Dk of 100,[3] and another used hexafocon B with a Dk of 141.[4] Your fitter chooses the material along with the lens design.
What Dk means
Dk is a measure of how easily oxygen passes through a material. Higher numbers mean more oxygen. But what reaches your cornea also depends on thickness, which is why fitters also talk about Dk/t, the material’s permeability divided by its thickness.
Scleral lenses face two extra barriers:
- Thickness. Scleral lenses are thicker than regular contact lenses. One review describes typical thickness as 250 to 500 microns or more, which reduces the oxygen that gets through.[1]
- The fluid layer. The saline under the lens has its own low oxygen permeability, about 80 by the same measure, so a deep fluid layer adds to the barrier.[1]
Theoretical studies suggest corneal swelling may be avoided with a lens at least 250 microns thick, a fluid layer no more than 200 microns deep, and a material Dk of at least 125, though many lenses in use are thicker or have deeper fluid layers.[2]
What this means for your cornea
In healthy eyes, the effects are small. Highly oxygen-permeable scleral lenses typically cause around 2% swelling in the center of the cornea during open-eye wear.[3] A 2026 meta-analysis of 22 studies (830 eyes) found the cornea thickened slightly while the lens was on, by about 8 microns on average, with no significant difference after the lens came out.[5] In a small keratoconus study, lenses between 200 and 400 microns thick didn’t cause clinically significant swelling after 6 hours of wear.[6]
Eyes with a weakened endothelium, the inner cell layer that pumps fluid out of the cornea, are different. In 12 eyes with corneal transplants wearing lenses with a Dk of 100 for 8 hours, the cornea swelled by about 7% on average, more in older grafts.[7] That’s why fitters choose materials and designs carefully after a transplant, and may add fenestrations or reduce vault.
Material isn’t everything
Materials also differ in how well they wet and resist deposits, which affects comfort and clarity. Many lenses get a surface coating for that reason. But in a study of 48 habitual wearers, lens material, coatings, diameter, and solutions together explained only a modest share of who had midday fogging, and no single factor stood out on its own.[8]
If you have a corneal transplant, Fuchs’ dystrophy, or another condition that affects the endothelium, ask your fitter which material and thickness they’ve chosen and how they’ll monitor for swelling.
Common questions
Can I sleep in my scleral lenses if the material lets oxygen through?
Not unless your eye doctor specifically tells you to. The studies showing small changes looked at daytime, open-eye wear. Overnight wear is used only in specific medical situations, under close supervision. Follow your fitter's wearing schedule.
Are scleral lenses made of glass?
Not anymore. The earliest scleral lenses, in the late 1800s, were glass shells. Modern lenses are made of rigid gas permeable plastics that let oxygen through.
Does a higher-Dk material always mean a healthier lens?
Not by itself. Thickness and the depth of fluid behind the lens also limit oxygen. A high-Dk lens that's very thick or sits over a deep fluid layer may deliver less oxygen than you'd expect from its rating. Your fitter balances all three.