Scleral Lens Vault: Central Clearance, Settling, and OCT

What is scleral lens vault?

Vault is the distance from the posterior surface of the lens to the anterior surface of the cornea, filled with saline while the lens is worn.

Writing in Contact Lens Spectrum (September 2021), Ariel Cerenzie describes the arrangement precisely: a scleral lens vaults the cornea, rests on a fluid-filled reservoir above it, and lands on the sclera, with the fluid masking corneal irregularity and improving the optics. The same quantity travels under several names — tear reservoir height, corneal vault height, apical clearance, central corneal clearance — and it is quoted in micrometres. In “Landing in the Zone” (January/February 2024), Mile Brujic defines corneal clearance the same way: the distance from the posterior lens surface to the anterior cornea.

Vault is a result, not an ordered parameter. What the laboratory makes is a lens of a given sagittal height; what the eye returns is the clearance that height produces on that particular ocular surface. The geometry that connects the two — the closed form, the chord dependence, and why a sag figure is meaningless without a stated chord — is defined on Sagittal Depth of Contact Lenses, which owns that lemma on this site.

How is vault measured — OCT or slit lamp?

Both are used. The slit lamp gives an estimate against the known centre thickness of the lens; anterior segment OCT gives a direct measurement.

Cerenzie describes the optical-section method: narrow the slit-lamp beam to an optic section and compare the known lens thickness against the height of the fluid layer, reading the reservoir in multiples of a dimension you already know. It is quick, it needs no extra instrument, and it is an estimate. The more accurate assessment, she writes, is anterior segment OCT — which also shows something the slit lamp cannot, namely the extent of conjunctival compression underneath the landing zones, and, with OCT angiography, compression of the conjunctival microvasculature.

Fluorescein in the bowl of the lens is the third reading, and it has a floor. Brujic notes that fluorescein is only visible at a minimum thickness of about 15 µm, so an absence of visible fluorescein at the limbus is not proof of clearance at fine tolerances. He also notes the follow-up problem: a patient arriving with lenses already in has no fluorescein in the bowl, so limbal clearance after a day of wear cannot be judged that way at all.

Do you need an OCT to fit scleral lenses? No. Sclerals were fitted for decades on the optical section, and the sequence on Scleral Lens Fitting Process runs without one. What OCT changes is resolution and confidence — a measured reservoir instead of an estimated one, a visible limbal transition, and a direct view of how the landing zone is compressing tissue. Brujic gives one caution worth knowing before trusting an image: at the extreme edge of the lens the scan crosses from measuring through plastic to measuring bare tissue, and the resulting image shift makes the conjunctiva appear elevated — a well-aligned edge can look as though it is digging in.

How much central clearance is enough?

There is no single rule. Published practice runs from around 100 µm as a minimum to 300–500 µm on large-diameter designs, and the ceiling is set by oxygen, not by comfort.

Eef van der Worp’s A Guide to Scleral Lens Fitting states there are no rules for exact central corneal clearance, but that a minimum of about 100 µm is typically desired; that with true scleral lenses 200–300 µm is usually considered sufficient, going up to 500 µm if desired on end-stage large-diameter lenses; and that corneo-scleral lenses have been reported as low as 20–30 µm. He also gives the reference every optical-section estimate is made against: average corneal thickness in a normal eye is in the region of 530 µm.

That is the Visser scale for large-diameter scleral lenses as van der Worp reports it, and the convention that travels with it is the useful part: a grade one on any variable is usually considered acceptable, while a grade two means action is required. He also notes that fenestration shifts the whole window — non-fenestrated lenses typically run 200–600 µm, while the same design fenestrated can sit at 100–200 µm or less, because a fenestrated lens sinks onto the surface rather than floating on it.

The ceiling is physiological. Cerenzie summarises Michaud and colleagues’ oxygen-transport modelling, which concluded that the post-lens tear film should not exceed 200 µm, that lens thickness should not exceed 250 µm, and that the highest-Dk material available should be used. Clinical tests of that model are less alarming than the model: Esen and colleagues compared 100–200 µm, 200–300 µm and above-300 µm clearances in keratoconus over eight hours and found no clinically significant difference in corneal swelling, and Tan and colleagues found at most 1.65% corneal oedema across 200–400 µm on normal corneas, against roughly 3.6% physiological swelling after overnight eye closure. Both sets of authors still concluded in favour of the highest Dk, the thinnest lens and the smallest workable reservoir, because the data ran to eight hours and patients wear these lenses for years. Why thickness is the divisor in that calculation is on Oxygen Transmissibility and Dk/t of Contact Lenses.

The University of Iowa EyeRounds atlas puts good scleral clearance at 100–500 µm centrally and notes typical scleral lens thickness of 250–500 µm as the slit-lamp yardstick, with excessive sag reducing both oxygen transmission and best-corrected acuity. Too little clearance is the opposite failure: Cerenzie notes that if initial clearance is too low, settling can bring the lens into bearing on the cornea or limbus, with epithelial disruption and mechanical damage following.

What is limbal clearance, and why is it read separately?

Limbal clearance is the gap over the limbus, and it is read separately because a lens can vault the corneal apex perfectly and still bear where the stem cells are.

Brujic states the reason directly: the limbal region contains the stem cells of the cornea, and it is critical to respect their role in corneal health and physiology, so limbal clearance must be identified and confirmed present. Van der Worp reports roughly 100 µm as the optimal figure on the Visser scale, with less than that graded down and more than 200 µm graded as excessive — while noting that in smaller corneo-scleral designs limbal clearance is typically absent by design, because the limbal region is where those lenses rest.

On OCT the limbus has to be found before clearance over it can be judged, and Brujic gives the visual cues: the corneal epithelium is highly organised and looks different from conjunctival epithelium, so the transition between them marks the limbal region; the sclera appears whiter on the scan because it is non-transparent while the cornea appears darker; and beneath the limbal cornea there is a small wedge of more hyperreflective tissue that helps locate it. Most contemporary designs carry a reverse curve between the landing zone and the limbal clearance region, which is what produces the lift.

What is the landing zone, and how is it read?

The landing zone — also called the haptic zone — is the part of the lens intended to rest on the bulbar conjunctiva and underlying sclera outside the cornea.

Brujic gives that definition and notes its size varies with manufacturer and lens diameter. Van der Worp records the synonym and its origin: the word haptic comes from a Greek root meaning “to fasten” or “to attach.” It is worth being clear that “haptic” in this sense is a scleral lens zone; the same word is used for the arms of an intraocular lens, which is a different device entirely and not what this page is about.

An ideal landing zone, in Brujic’s description, disperses the lens evenly onto the conjunctival surface all the way round with no visible blanching anywhere. Where the lens presses unevenly, the vessels under the high-pressure region blanch. The working vocabulary is anatomical: the landing zone has a toe, the most distal part from the lens centre, and a heel, the most proximal.

• Toe-down — the landing zone is steeper than the sclera and the distal edge digs into conjunctival tissue; blanching shows at the toe. Lift the toe, or bring the heel closer.
• Toe-up — the landing zone is flatter than the sclera and the distal edge is elevated; blanching shows at the heel instead. Lift the heel, or bring the toe down.
• Impingement — additional pressure in one region, read as blanching of conjunctival vessels under it.
• Edge lift — hard to see head-on; shine the beam along the landing zone from the lens centre outward and read the shadow the elevated edge casts on the conjunctiva beyond it. Bigger shadow, more lift.

Whether the landing zone needs toricity is decided by rotation, not by corneal cylinder: rotate the lens manually and watch. If it returns to its original position, the sclera is likely toric; if it stays put, it likely is not. That most scleras are not spherical is now measured rather than assumed — Brujic cites DeNaeyer and colleagues on 140 eyes, of which only 5.7% were spherical, 28.6% toric-regular, 40.7% carrying asymmetric high or low points, and 26% with periodicity different from 180°. Van der Worp adds the scale of the consequence: about one degree of difference in scleral angle represents roughly 60 µm of sagittal height, so within the limbal area a typical difference of about 100 µm can occur, and in the scleral zone it can approach 400 µm.

Vertical and horizontal cross-sections also report centration. Brujic notes that even clearance superiorly and inferiorly means the lens is vertically well aligned; less clearance superiorly than inferiorly means the lens is sitting low, and the same logic read horizontally shows decentration toward whichever side carries the greater clearance.

What is settling, and how much of the vault does it take?

Settling is the landing zone sinking into the compressible conjunctiva, which flattens and tightens the fit and reduces the vault — most of it in the first two hours.

Cerenzie’s account is the fullest available: the landing curves carry the weight of the lens and compress the conjunctiva and Tenon’s capsule beneath, compounded by lid pressure, so the distance between cornea and lens back surface decreases. The process begins immediately after application and can continue for eight or more hours; a landing zone and vault that read as ideal can already read as less than ideal after 60 minutes. Kauffman and colleagues found 70% of total settling within two hours, and Nau and colleagues found tear clearance down by nearly half at two hours.

Two predictors are consistent across studies. Lenses fitted with a lower initial vault settle less than those fitted high — Esen and colleagues found initial vault and total settling inversely related over eight hours. And larger-diameter lenses settle less than smaller ones, because a wider landing zone spreads the same weight over more conjunctiva; Cerenzie relays a mentor’s image of a snowshoe against a high heel in snow. Larger lenses also land further from the limbus, where the conjunctiva may be less compressible.

Because a full day of settling cannot be waited out in the chair, allowances are built into the order. Esen and colleagues suggested adding roughly 100 µm to the desired post-settling clearance, or about 75 µm on larger-diameter lenses; Michaud and colleagues reported that for an 18 mm lens the total settling can be estimated by doubling the amount that settled in the first 30 minutes. Cerenzie’s own recommendation is to apply the fitting assessment after 20 to 30 minutes of settling, which reduces chair time and remakes without pretending to be the eight-hour figure. What settling does to power is on Power Adjustment of Scleral Lenses After Settling.

What goes wrong when the vault is too high?

Excess vault costs vision and oxygen before it costs comfort.

Cerenzie lists the consequences: higher vaults increase lower-order aberrations and reduce vision; debris accumulates in a deep reservoir, typically driven by excessive negative pressure beneath the lens; the lens is more likely to decentre inferiorly; and extended wear of an excessively vaulted lens can produce tight-lens syndrome with discomfort and hyperaemia. Prolonged wear of a thick reservoir under a tight periphery could, in theory, contribute to hypoxic complications, which is the concern that drives the Michaud modelling above — and matters most where the endothelium is already compromised. The Iowa atlas states the same trade in one sentence: excessive sag reduces oxygen transmission, treating the saline layer as part of the thickness term in Dk/t, and reduces best-corrected acuity.

None of that argues for a minimal vault either — the floor is corneal bearing after settling. The fit sits between two failures, and where it sits is read on the eye, not selected from this page.

Starting parameters are not a prescription

Every figure above is a reported range, not a target to order to.

Read the site-wide bound on Starting Contact Lens Parameters Are Not a Prescription. For the geometry that produces the vault, read Sagittal Depth of Contact Lenses. For the order these measurements are taken in, read Scleral Lens Fitting Process; for the parameter set as a whole, Scleral Lens Parameters.

Sources

Clinical claims on this page are attributed to the publications below.

• Contact Lens Spectrum — Alright Folks, Let’s Settle Down(Ariel Cerenzie, OD; September 2021) — vault as the fluid reservoir and its synonyms; the optic-section method against known lens thickness and AS-OCT as the more accurate assessment, including conjunctival compression and OCT-A microvasculature; settling as conjunctival and Tenon’s compression, immediate onset, eight or more hours, visible change at 60 minutes; Kauffman et al 70% within two hours; Nau et al clearance halved at two hours; inverse relation of initial vault and total settling (Esen et al); larger diameters settling less; adding ~100 µm (~75 µm for larger diameters); Michaud et al doubling the 30-minute figure on an 18 mm lens; Michaud et al oxygen modelling (post-lens film ≤200 µm, lens thickness ≤250 µm, highest Dk); Esen et al 100–200 / 200–300 / >300 µm groups over eight hours; Tan et al ≤1.65% oedema at 200–400 µm against ~3.6% overnight physiological swelling; aberration, debris, decentration, tight-lens syndrome and bearing-after-settling consequences.
• Contact Lens Spectrum — Landing in the Zone(Mile Brujic, OD, FAAO; January/February 2024) — corneal clearance defined posterior-lens to anterior-cornea; landing zone definition and size variation; even dispersion with no blanching; toe and heel, toe-down and toe-up, impingement, and the edge-lift shadow method; manual-rotation test for scleral toricity and diagnostic-lens meridian markings; DeNaeyer et al 140-eye scleral shape distribution; fluorescein visible only at ≥15 µm and unavailable at follow-up; OCT identification of the limbal region and the edge image-shift artefact; vertical and horizontal cross-sections as centration reads; reverse curve producing limbal lift; limbal stem cells as the reason limbal clearance is required.
• Eef van der Worp — A Guide to Scleral Lens Fitting(Pacific University College of Optometry, open access) — no exact rule for central clearance, minimum ~100 µm typically desired, 200–300 µm usually sufficient on true sclerals and up to 500 µm on large diameters, 20–30 µm reported on corneo-sclerals; average corneal thickness ~530 µm as the optical-section reference; the Visser grading scale for central and limbal clearance and the grade-one / grade-two convention; fenestrated 100–200 µm against non-fenestrated 200–600 µm; landing zone as haptic zone and the Greek derivation; limbal clearance typically absent in corneo-scleral designs; one degree of scleral angle ≈ 60 µm of sagittal height, ~100 µm in the limbal area and up to ~400 µm in the scleral zone.
• University of Iowa, EyeRounds Ophthalmic Atlas — Appropriate Scleral Lens Sagittal Depth(Tressa Larson, OD, FAAO) — good scleral clearance 100–500 µm centrally; typical scleral lens thickness 250–500 µm; excessive sag reducing oxygen transmission, with the saline layer treated as part of the thickness term in Dk/t, and reducing best-corrected acuity.