How big they really are
Scleral lenses come in a range of sizes, chosen for each eye.
- In an international survey of fitters covering 419 eyes, the average scleral lens was 16.2 mm across. The smallest was 11.8 mm and the largest 23.0 mm.[1]
- In a study of 157 eyes with keratoconus, the average lens was 15.8 mm across.[3]
- For comparison, the cornea, the clear dome at the front of your eye, is about 11 to 12 mm wide.[2]
So a typical scleral lens is only a few millimeters wider than your cornea. It looks huge in your palm partly because it’s deep: it’s shaped like a small bowl, and when you fill it with saline it holds a visible pool of liquid.
Why the size is the point
A scleral lens is designed to do something a small lens can’t: arch over the entire cornea without touching it, and land on the white of the eye beyond it.
That matters for three reasons.
Comfort. The cornea is one of the most densely innervated and sensitive tissues in the body.[5] The white of the eye is much less sensitive. A lens big enough to rest there doesn’t press on the part of your eye that feels everything.
A fluid cushion. The space between the lens and the cornea is filled with saline. That keeps the cornea bathed all day, which is why sclerals are used for severe dry eye, and it fills in the bumps and irregularities of a misshapen cornea, which is why they can sharpen vision in conditions like keratoconus.
Stability. Resting on the white of the eye, a larger lens tends to stay put when you blink, rather than sliding around the way a small hard lens can.
A smaller lens would have to touch the cornea, and then it would no longer work this way.
Where it actually goes
The white of your eye extends well beyond what you see between your lids. A thin membrane called the conjunctiva covers it and continues around to line the inside of your eyelids.[4] When the lens is on, its outer edge rests on that membrane, with part of it tucked under your upper and lower lids.
That also means the lens has nowhere to go but where it belongs. There’s no opening behind the eye for it to slip into. See what if it gets stuck, or lost behind my eye?
Getting it past your lids
This is the part where the size really shows. Your normal eye opening is narrower than the lens, so you need to hold your lids wide while the lens goes in. Once it’s on, your lids close over its edge naturally.
What helps:
- Hold both lids at the lash line. Use your non-dominant hand to hold the upper and lower lids apart, pressing right at the lashes so they can’t slip.
- Face parallel to the floor. Look straight down into a flat mirror so the full lens can come straight up to your eye.
- Look into the center of the lens as it comes up, so it lands centered.
- Keep both eyes open. It’s easier to keep the target eye steady.
The full steps are in how to insert a scleral lens. If your lids keep slipping, see your lids keep slipping out of your fingers. If your eye opening is small or your eyes are deep-set, see small eye opening or deep-set eyes.
The first time it goes in
Most people are surprised that the lens fits at all. The saline touches your eye first, which can feel cool and strange. Then the lens settles, you blink, and it’s on. You may be aware of it at first, especially when you look far to one side. If it hurts, or you see a bubble, take it out, refill it, and try again. If it hurts every time, tell your fitter. See do scleral lenses hurt?
Common questions
How big is a scleral lens?
Sizes vary with the eye and the design. In an international survey of fitters, scleral lenses averaged 16.2 mm across, with a range from 11.8 mm to 23.0 mm. Your fitter chooses the size for your eye.
Why can't they just make it smaller?
The size is what lets the lens clear the whole cornea and rest on the white of the eye instead. Fitters do choose different diameters for different eyes, including smaller ones for small eye openings, but the lens still has to reach past the cornea to work as a scleral lens.
Will other people be able to see it?
The lens is clear. Its edge rests on the white of the eye, partly under your lids, and may be visible up close. Ask your fitter to let you look closely in a mirror at the fitting so you can judge for yourself.
What if my eyes are small or deep-set?
Insertion can take a different technique, and your fitter may choose a smaller lens. See the guide to inserting scleral lenses with small or deep-set eyes, and tell your fitter if you're struggling.