How scleral lenses work

The shape: a dome on the white of the eye

Think of a scleral lens as a shallow dome with a brim. The brim rests on the white of the eye, and the dome arches over the cornea.

Its parts, from the center out:

• The optical zone. The clear center you look through. Its front surface carries your prescription.
• The vault. The arch over the cornea and the limbus, the ring where the clear cornea meets the white of the eye. A scleral lens is designed to clear both completely.^[1]
• The landing zone. The outer brim, also called the haptic, that rests on the eye. It sits on the conjunctiva, the thin clear membrane that covers the white of the eye, not on the sclera itself.^[1][2]

The landing zone has to match the shape of your eye, which is rarely a perfect sphere. In an international survey of scleral lens fits, 64% of landing zones were spherical, 26% toric (shaped differently in different directions), 7% quadrant-specific, and 3% fully custom.^[3]

The fluid layer: why vision gets sharper

Before insertion, you fill the bowl of the lens with sterile saline. Once the lens is on, that saline is trapped between the back of the lens and the front of the cornea. It’s called the reservoir or post-lens fluid layer.

An irregular cornea scatters light because its surface is uneven. Glasses can’t fix that, because they can only correct regular blur. Under a scleral lens, light passes through the smooth front of the lens and then through the fluid, which fills in the hollows of the cornea’s surface. The result is that the irregular surface is largely masked, much as a corneal rigid lens masks irregular astigmatism, and higher-order aberrations (the distortions behind ghosting and halos) are reduced.^[1]

Because the lens rests on the much less sensitive white of the eye instead of the cornea, it also avoids rubbing one of the most densely innervated tissues in the body.^[12]

The fluid layer: why it protects the surface

For people with severe dry eye and other ocular surface disease, the reservoir works as a liquid bandage. It keeps the cornea continuously hydrated and protects it from the shearing force of the eyelids with each blink.^[1] That is why scleral lenses are used for conditions such as Sjögren’s syndrome, graft-versus-host disease, and exposure problems, not just for vision.

Settling: the lens sinks a little

A scleral lens doesn’t stay exactly where it lands. Over the first hours of wear it settles slightly into the conjunctiva. Studies summarized in a 2018 review found most designs lose about 80 to 100 microns of central clearance over 1 to 8 hours of wear, usually leveling off at around 2 hours.^[1] Fitters plan for this: a lens that looks just right at insertion may end up touching the cornea later, so clearance is often checked after the lens has been on for a while.

Very little fresh tear fluid gets in

A soft or small rigid lens moves with each blink and pumps tears underneath. A scleral lens moves very little. Some tear movement does happen, but tear exchange under a scleral lens is minimal compared with other lens types.^[4]

This has two consequences:

• Whatever is in the reservoir stays there. That’s why the filling solution must be sterile and preservative-free.
• Debris can build up. Cells shed by the cornea and other particles collect in the fluid over the day and can cloud vision. This is known as midday fogging, and its causes are still being studied.^[4]

The landing zone can also create other problems unique to scleral lenses, such as the conjunctiva bulging up into the space under the lens near the limbus (conjunctival prolapse) or the lens edge bearing on the limbus.^[5]

Oxygen: how the cornea breathes under the lens

The cornea has no blood vessels, which is part of why it’s clear, so it takes in oxygen from the tear film and air in front of it. Under a scleral lens, oxygen has to pass through two layers: the lens itself and the fluid reservoir. Modern scleral lenses are made of highly oxygen-permeable materials for this reason.^[5]

Research on oxygen and corneal swelling points the same way:

• A theoretical model concluded that most scleral lenses fitted with the techniques of the time would cause some oxygen-related corneal swelling. Its authors recommended the most oxygen-permeable material available, a thin lens, and clearance of no more than 200 microns.^[6]
• A study of 8 healthy eyes measured oxygen at the corneal surface under 18 mm lenses. A lens fitted with about 400 microns of clearance reduced the oxygen available to the cornea by 30% compared with one fitted at about 200 microns.^[7]
• A study of 10 people with healthy corneas found that corneal swelling rose with a thicker fluid layer (from 0.69% with a thin reservoir to 2.11% with a thick one) but leveled off at around 600 microns.^[8]
• A randomized crossover study of 15 young adults found lenses made from the lowest-oxygen material tested caused more corneal thickening after 8 hours than higher-oxygen materials. The authors advised using the higher-oxygen materials for daily wear.^[9]
• A combined modeling and clinical study concluded that, with modern materials and typical fits, open-eye wear causes less than 2% swelling in healthy corneas. That’s below the roughly 4% the authors describe as the cornea’s normal, physiological level of swelling. Closed-eye wear, by contrast, appeared clinically unsafe.^[10]

These are mostly short studies in healthy eyes. Corneas that are already compromised can respond differently. In 9 eyes that had had a full-thickness corneal transplant, a few hours of scleral lens wear produced about three times as much swelling as in healthy eyes, with more variation from eye to eye.^[11] That’s why people with transplants and other fragile corneas are monitored closely.

For a plain-language list of the terms used here, see the scleral lens glossary. For what wearing the lenses involves day to day, see the wearing guides.