Scleral Lens Vault: Why the Fit Is Measured in Microns
A lens that never touches the cornea
Every other contact lens you have met rests on the cornea or is shaped by it. A scleral lens does neither. It bridges over the cornea entirely and bears on the sclera, the white of the eye, leaving a fluid-filled gap between the back of the lens and the front of the cornea. That gap is filled with unpreserved saline at insertion, and it stays there for as long as the lens is worn.
The gap is called the vault, or clearance, and it is the single most important number in a scleral fit. It is measured in microns, which is the first hint that this is a different order of precision from spectacle work. A frame fit is judged in millimetres and a soft lens in tenths of a millimetre; a scleral lens is judged in hundredths.
Why the tolerance is so tight
Central corneal thickness in an adult is roughly 550 microns. That single figure explains the whole fitting philosophy: the entire clearance you are trying to achieve is a fraction of the tissue it is protecting. A change of 50 microns is under a tenth of corneal thickness, and yet in this geometry it is the difference between a lens that clears and a lens that touches.
Too little vault and the lens lands on the corneal apex. Even without an obvious abrasion, the cornea is being denied the fluid layer that is doing the work, and the fit becomes unstable. Too much vault and three things happen at once: the fluid layer becomes thick enough to distort vision and to trap metabolic waste, the lens mass increases, and the lens tends to settle further during the day, so a fit that looked generous in the chair may be excessive by evening.
That settling is why timing matters. Scleral lenses are assessed after a defined settling period, not at the moment of insertion, because the numbers change as the fluid squeezes out from under the landing zone.
How clearance is actually measured
Vault is read from an optical section: the slit lamp is set to a narrow beam, and the gap between the back surface of the lens and the front of the cornea is estimated against the beam's thickness. Corneal topography and anterior segment OCT can quantify the same gap more repeatably, which is why they are increasingly used for documentation. Either way the number is a few hundred microns, and either way it is read by someone looking through a microscope rather than by the wearer looking in a mirror.
The fluid is an optical element
Here is the part that catches people out. The saline reservoir is not a neutral spacer; it is a lens. Light passes from the back surface of the scleral lens into fluid, then from fluid into the cornea, and each of those boundaries has its own curvature and its own refractive index step.
The power contributed by the fluid layer depends on the difference between the back curve of the lens and the front curve of the cornea, and on the index difference at each surface. Saline sits around 1.334 against a cornea near 1.376, so the fluid-to-cornea boundary is a real optical surface, not a formality. The practical consequence: the same lens specification placed on two eyes with different corneal curvature does not deliver the same correction, and a lens ordered to the same power on both eyes may need different power on each.
This is also why the prescription on a scleral lens is not simply the spectacle prescription converted. The laboratory designs the lens so that lens plus fluid plus cornea together produce the intended result, which is a different calculation from anything on a spectacle order form.
The landing zone is the other half of the fit
Vault describes the space over the cornea. The landing zone describes where the weight goes, and the two are independent. A lens can have ideal clearance and still be wrong because the haptic lands too tightly on the conjunctiva, compressing vessels and leaving the eye red after a few hours. Landing too loosely is its own failure: the lens moves, bubbles get under it, and the fluid layer does not stay put.
You cannot assess either from the outside with a mirror. Both require a slit lamp and, usually, fluorescein and a lens fit assessment. What a wearer can report is the pattern over time: whether the eye is white or red at the end of a day, whether vision clouds after a few hours, whether the lens feels different at hour eight than at hour one. Those reports are what tell a fitter which direction to move.
Three misconceptions
- "More clearance is safer." Excess vault reduces oxygen reaching the cornea through a thicker fluid layer, invites mid-day fogging, and adds mass. Clearance is a target with a range, not a maximum to exceed.
- "The power on the box is the power I get." The fluid lens contributes its own power, which is why the ordered power is not a simple conversion of the spectacle prescription.
- "If it feels fine, the fit is fine." Comfort is compatible with a compressed landing zone and with excessive clearance. Both cause problems that appear weeks later rather than immediately.
When to go back rather than adapt
Scleral lenses are not a lens you break in. A correct fit is comfortable from the first week, and the fluid layer either stays clear or it does not. Redness that persists after removal, a white ring on the sclera, fogging that returns within two hours of insertion, halos around lights, or a lens that needs to be reinserted because it will not settle are all fit findings rather than adaptation problems.
The useful thing to bring to that appointment is a timeline rather than a description: how long after insertion the symptom starts, whether it changes with the amount of time worn, and whether the other eye behaves the same way. A fitter can measure vault and landing in minutes, but only the wearer knows the pattern across a day. Those two pieces of information together are what turn a vague complaint into a specification change.