A note on definitions
In 2018 the Scleral Lens Education Society adopted an official terminology for scleral lenses, written by a committee of experienced fitters and published in 2020.[3] Fitters, manufacturers, and research papers still use some words differently, and size categories in particular vary between sources. The definitions below are written for patients and point out where usage differs.
A–C
Application bubble. An air bubble trapped under the lens at insertion. It sits in the fluid layer over the cornea, can blur vision, and can leave a dry patch. A fitter checks that the lens is on with no central corneal touch and no application bubbles.[1] The usual fix is to remove the lens, refill it, and reinsert it. See the guide to filling the lens and avoiding bubbles.
Clearance. The depth of the space between the back of the lens and the front of the cornea, usually measured in microns (thousandths of a millimeter). It’s checked at the microscope or with an imaging scan.[1] More isn’t better: a theoretical oxygen model recommended keeping central clearance at no more than 200 microns.[9] See also vault and settling.
Conjunctiva. The thin, clear membrane that lines the inside of the eyelids and covers the white of the eye. Where the eyelid lining meets the eye’s covering, it forms a continuous fold called the fornix, above, below, and to each side. That fold is why a lens can’t slip behind the eye.[6] A scleral lens actually rests on the conjunctiva over the sclera.[1]
Conjunctival prolapse. The conjunctiva bulging up into the space under the lens near the limbus. It’s one of several findings that are specific to scleral lens wear.[7]
Cornea. The clear, dome-shaped window at the front of the eye. In keratoconus and other conditions it becomes irregular, which is the most common reason people wear scleral lenses.
Corneo-scleral lens. A lens that rests partly on the cornea and partly on the sclera.[2] One size-based classification uses the name semi-scleral for the smallest lenses in the scleral family (see diameter classes).
D–F
Diameter classes. Several systems exist. One classification from the Scleral Lens Education Society, as reported in a 2018 review, uses these bands:[1]
| Name | Diameter[1] |
|---|---|
| Semi-scleral | 12.5–15 mm |
| Mini-scleral | 15–18 mm |
| Large scleral | 18–25 mm |
A 2014 review classifies lenses by where they rest instead: a lens resting partly on the cornea is corneo-scleral, and a lens resting entirely on the sclera, up to 25 mm across, is a scleral lens, subdivided into mini-scleral and large scleral.[2] Because eyes differ in size, the same diameter can behave differently on different eyes.
Dk. A measure of how easily oxygen passes through a lens material. Higher Dk means more oxygen gets through. Modern scleral lenses are made of high-Dk gas-permeable materials, which reduced many of the problems older scleral lenses caused.[7]
Fenestration. A tiny hole drilled through a lens. Fenestrations are one of the options a fitter may use when a cornea shows signs of swelling under the lens, along with changes to the lens shape, material, or wearing time.[1]
Fluid reservoir. See reservoir.
H–L
Haptic. Another word for the landing zone.
Landing zone. The outer part of the lens that rests on the conjunctiva over the white of the eye. Because the white of the eye is rarely perfectly round, landing zones come in different shapes. In an international survey of scleral lens fits, 64% were spherical, 26% toric, 7% quadrant-specific, and 3% custom.[5] See toric landing zone.
Large scleral lens. A scleral lens at the larger end of the size range. In one large Dutch practice, the median large scleral lens was 20 mm across, compared with 16 mm for mini-scleral designs.[4]
Limbus. The narrow ring where the clear cornea meets the white of the eye. A scleral lens is designed to vault the limbus as well as the cornea, so it shouldn’t rest on it.[1] Limbal bearing, where the lens does press on this area, is one of the fitting problems specific to scleral lenses.[7]
M–O
Midday fogging. Cloudy vision that develops during wear as particles build up in the reservoir. Studies have reported it in 26% to 46% of scleral lens wearers. Removing, cleaning, refilling, and reinserting the lens usually clears it.[8]
Mini-scleral lens. A scleral lens at the smaller end of the size range that still rests fully on the white of the eye. In the band system above, 15–18 mm.[1]
Optical zone. The clear center of the lens that you look through. Its front surface carries your prescription.
Over-refraction. A quick vision test done with the lens on, to fine-tune the prescription on the front of the lens.
P–R
Plunger. A small, soft suction tool used to insert or remove a scleral lens. Like the lens case, it needs to be kept clean.[12] See how to insert and how to remove a scleral lens.
Preservative-free saline. Sterile salt water without preservatives, used to fill the lens before insertion. Because the fluid sits against the cornea all day, it must be sterile, and single-use vials are preferred.[12]
PROSE. Short for Prosthetic Replacement of the Ocular Surface Ecosystem, a treatment from BostonSight in Needham, Massachusetts, that uses custom scleral devices.[11] It’s used for difficult ocular surface disease and for corneal ectasia.[10]
Reservoir. The saline-filled space between the back of the lens and the cornea, also called the post-lens fluid layer or tear reservoir. It masks corneal irregularity and keeps the cornea hydrated.[1]
Rigid gas-permeable (RGP or GP). The family of firm, oxygen-permeable plastics that scleral and smaller hard lenses are made from.
S–Z
Sagittal depth (sag). The height of the lens dome, measured from the plane of its edge to the center of its back surface. It’s the main way a fitter controls clearance: if a lens vaults too much, a lens with less sagittal depth is tried, and if it touches the cornea, one with more.[1] In one Dutch practice, median sagittal depth was 3.5 mm for mini-scleral and 4.3 mm for large scleral designs.[4]
Sclera. The tough white outer wall of the eye. The lens is named after it, though it actually rests on the conjunctiva covering it.
Scleral profile or impression. Measurements of the shape of the white of the eye, taken with a scanner or a mold, that some fitters use to design the landing zone. In one study, all lenses were designed from scleral topography or an impression of the eye’s surface.[13]
Settling. The lens sinking slightly into the conjunctiva during the first hours of wear. Most designs lose about 80 to 100 microns of central clearance over 1 to 8 hours, usually leveling off at around 2 hours.[1]
Toric landing zone. A landing zone shaped differently in different directions to match a white of the eye that isn’t evenly curved. In one large Dutch practice, 99.8% of landing zones were toric.[4]
Vault. The arch of the lens over the cornea and limbus, without touching them. Fitters often use “vault” and “clearance” interchangeably.
New to all of this? Start with what scleral lenses are and how they work.
Common questions
Is there an official set of scleral lens terms?
Yes. The Scleral Lens Education Society adopted a standard terminology in 2018, published in 2020. Not every fitter or manufacturer uses every term the same way, so if a word is unclear, ask your fitter what they mean by it.
What's the difference between vault and clearance?
They're used almost interchangeably. Vault describes the lens arching over the cornea; clearance is the measured depth of the space between the lens and the cornea.
Is a mini-scleral lens a weaker version of a scleral lens?
No. Mini-scleral describes size, not strength or quality. Mini-scleral and large scleral lenses both rest fully on the white of the eye. Which one suits you depends on your eye's shape and what the lens needs to do.
Is PROSE different from a scleral lens?
PROSE is a specific treatment from BostonSight that uses custom-made scleral devices. It works on the same principle as other scleral lenses: a fluid-filled lens that vaults the cornea and rests on the white of the eye.