GP Contact Lens Design: Zones, Curves, and Edge Lift

What is a gas permeable lens design made of?

A GP design is a central powered curve surrounded by progressively flatter curves, cut to an overall diameter.

The central curve is the back optic zone radius (BOZR) — the value a prescription prints as the base curve, in millimeters of radius, and the value the laboratory lathes at the apex of the back surface. The zone it occupies is the back optic zone diameter (BOZD). Outside it sit one or more flatter peripheral curves, and the clearance they open at the lens edge is edge lift. The sum of all of them is the overall diameter (OAD).

According to Ento Key, Rigid Corneal Lens Design and Fitting (Contact Lens Practice), the BOZD is usually fixed for a given design at a given total diameter and is generally 1 mm to 1.5 mm smaller than the total diameter, and it should be large enough to cover the pupil in most conditions including low illumination. The two numbers move together: reducing BOZD without reducing BOZR leaves a shallower sagittal depth and therefore a flatter fit, and that chapter's rule of thumb is that an increase in BOZD of 0.5 mm requires an increase in BOZR of about 0.05 mm to keep a clinically equivalent fit.

What do GP lens designs look like on an order form?

GP lens designs are named by how many back-surface curves they carry: bicurve, tricurve, tetracurve, multicurve, or aspheric.

According to Ento Key, Rigid Contact Lenses: Basics (from Stein, Slatt, Stein and Freeman, Fitting Guide for Rigid and Soft Contact Lenses), most rigid corneal lenses are bicurve or tricurve. A bicurve lens has one base curve and one flatter secondary curve; a multicurve lens has a base curve and three or more peripheral curves. The junction between two adjacent radii is the blend, cut with a tool whose radius falls between the two, and a blend may be light — the zones are still identifiable — or heavy, so the zones run into each other.

The design chapter above adds the working ranges: a peripheral zone is generally 1 mm to 2 mm wide and composed of one to four peripheral curves, tricurve designs are probably the most commonly used form, bicurve designs appear on small lenses below about 8.5 mm, and tetracurve or other multicurve designs are used on larger lenses or where a smoother transition is wanted. Aspheric back surfaces take the other route: the periphery flattens continuously, so the design does not need a progressively flattening sequence of spherical curves at all.

Overall diameter is the field most likely to be misread as a soft-lens number. Ento Key's basics chapter gives most corneal rigid lenses a chord diameter between 8 mm and 10 mm, against 12 mm to 15 mm for a soft lens; the design chapter quotes typical corneal lens diameters of 8.30 mm to 9.80 mm. A GP diameter and a soft diameter are not the same measurement of the same thing, and Diameter of Contact Lenses keeps the soft ordered value.

Which pages cover each GP parameter?

Four pages sit under this one, and each takes a single parameter or the relationship it produces.

• Fluorescein Patterns and Fit Troubleshooting of GP Lenses — how the tear layer under the lens is read, and which parameter to change when the pattern is flat, steep, or decentred.
• Tear Lens and Lacrimal Lens Power of GP Contact Lenses — the fluid lens the BOZR creates against the cornea, and why changing the base curve changes the power you order.
• Bitoric GP Contact Lenses: Design and Power Effect — what happens to the back surface when corneal cylinder is too high for a spherical curve to align.
• Peripheral Curves and Edge Lift of GP Contact Lenses — the flatter radii outside the optic zone, their widths, and the edge clearance they produce.

Three live pages carry the rest of the path. RGP Contact Lens Parameters and Starting Power is the method: keratometry to a starting base curve, fluorescein, bench checks, and over-refraction, in sequence. RGP Contact Lens Power Calculator is the arithmetic. Sagittal Depth of Contact Lenses is what the whole back surface adds up to over a stated chord.

When is a GP lens indicated instead of a soft lens?

A GP lens is considered when the eye needs an optical surface the cornea cannot supply itself — high or irregular astigmatism, high ametropia, or a cornea a soft lens simply drapes over.

The reason is in the material rather than the design. A soft lens takes the shape of the cornea underneath it, so a corneal irregularity passes straight through to the front surface. A rigid lens keeps its own shape, and the tear film fills the difference. Ento Key's basics chapter puts it plainly: the fluid interface between a spherical rigid lens and the cornea fills out irregularities in the anterior corneal contour, in effect converting an astigmatic cornea into a spherical one.

That indication is a clinical decision, not a parameter, and it is made with a practitioner. The boundary and the candidacy discussion sit on RGP Contact Lens Parameters and Starting Power; the on-eye consequences of getting a fit wrong are on Contact Lens Fit Assessment: Movement, Centration, and Lag.

Which measurements are the inputs to a GP design?

Three measurements set the starting design: keratometry or topography, corneal astigmatism, and the lid and aperture geometry seen at the slit lamp.

Keratometry gives the flat and steep K readings the base curve is chosen against, in diopters or in millimeters of radius. Those two units are one conversion — D = 337.5 ÷ r, with r in millimeters — and the RGP Contact Lens Power Calculator runs it before the tear lens. Ento Key's basics chapter notes the keratometer reads only the central optic cap, roughly 5 mm to 7 mm across, while the periphery of the lens actually rests on the flatter intermediate zone — which is why a design chosen on K alone still has to be read with fluorescein.

Corneal astigmatism is the difference between the two K readings, and it decides whether the back surface can stay spherical. The design chapter's rule of thumb is that 1.00 D of corneal astigmatism corresponds to about a 0.2 mm difference in K readings. Where that difference goes past the spherical limit, the back surface becomes toric — that threshold is on Bitoric GP Contact Lenses.

Lid position and palpebral aperture set diameter and edge lift. According to the design chapter, when the upper lid sits relatively high the lens tends to decentre low, and a larger diameter is often required so the upper lid grips and holds it — a lid attachment fit. A relatively low upper lid with a narrow aperture tends to produce upward decentration instead. Those are observations, not calculations, and they are made on the patient in front of you.

Empirical or diagnostic fitting: what does the laboratory need?

Empirical fitting is designing the lens without putting a diagnostic lens on the eye — the refraction and keratometry go to the laboratory, and the first lens the patient wears is their own.

According to Review of Cornea and Contact Lenses, Empirical Fitting of GP Lenses (Ed Bennett, OD, February 2024), that method is now more the rule than the exception, on the back of manufacturer nomograms, online calculators, corneal topography with lens design software, and better lathing. The article reports a survey of prescribing habits in which five modalities were predominantly fitted empirically: multifocals 86%, torics 83%, spherical designs 79%, corneal reshaping 71%, and hybrids 67%.

The following are what an empirical order is built from, per that article:

• A careful manifest refraction, vertexed to the corneal plane where the power calls for it
• Keratometry, or simulated K values from topography
• Corneal topography, ideally with lens design software that renders a simulated fluorescein pattern
• Lid position and pupil size
• A laboratory consultant, who manages that laboratory's designs daily and can be sent slit lamp photographs or video

Diagnostic fitting is the other method: a trial set of known base curves and diameters, read on eye with fluorescein and finished with an over-refraction. Ento Key's design chapter describes a standard set as a single diameter and back vertex power with base curves in 0.1 mm steps, and prefers sets carrying two diameters. Bennett's article names the trade-offs honestly — diagnostic lenses are standard designs rather than custom ones, and every one of them has to be cleaned, disinfected, and logged under ISO 19979 — while noting that segmented translating multifocal designs and small-diameter GPs on irregular corneas are still predominantly fitted diagnostically.

Two cautions belong with that. First, a nomogram is named here, never cloned: the laboratory guides and calculators that design these lenses are the property of the people who validated them, and this site links to them rather than reproducing them. Second, no empirical design is a first-fit promise. The published figures are success rates, not certainties — Ento Key's design chapter cites 91% of eyes fitted successfully by empirical ordering in one study and topographer-assisted fitting success of 77% to 93% — and when the first lens does not align, the correct move is a diagnostic set, not a third empirical re-order.

Starting GP parameters still require on-eye fit

Every value on this page is a starting parameter, because a rigid lens is judged by the tear layer under it and that layer cannot be calculated from a spectacle prescription.

Rigid gas permeable contact lens fitting resolves at the slit lamp: the base curve is proved against the fluorescein pattern, the periphery is proved against edge clearance and tear exchange, and the power is proved against an over-refraction after the lens settles. Read Starting Contact Lens Parameters Are Not a Prescription for the bound that applies across this site.

Sources

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

• Ento Key, Rigid Contact Lenses: Basics(Stein, Slatt, Stein and Freeman, Fitting Guide for Rigid and Soft Contact Lenses) — bicurve, tricurve and multicurve designs; the blend and light versus heavy blending; optic zone as diameter minus peripheral curve widths; chord diameter 8–10 mm rigid against 12–15 mm soft; the fluid interface converting an astigmatic cornea to a sphere; the keratometer reading the central optic cap only.
• Ento Key, Rigid Corneal Lens Design and Fitting(Contact Lens Practice) — BOZD 1–1.5 mm smaller than total diameter; 0.5 mm BOZD change against 0.05 mm BOZR; peripheral zone 1–2 mm wide with one to four curves; typical corneal lens diameters 8.30–9.80 mm; 1.00 D of corneal astigmatism as about 0.2 mm of K difference; lid attachment and decentration; diagnostic sets in 0.1 mm base curve steps; 91% empirical and 77–93% topographer-assisted success figures.
• Review of Cornea and Contact Lenses, Empirical Fitting of GP Lenses(Ed Bennett, OD, February 2024) — empirical fitting defined as designing without diagnostic lenses; the empirical prescribing survey (multifocals 86%, torics 83%, spherical 79%, corneal reshaping 71%, hybrids 67%); topography and lens design software; the laboratory consultant; ISO 19979 disinfection of diagnostic lenses; segmented translating designs and irregular corneas still fitted diagnostically.