Ortho-K Lens Design and Post-Wear Topography

What is an ortho-k lens design?

An ortho-k lens is a rigid gas permeable lens in reverse geometry: a back optic zone flatter than flat K, then a steeper reverse curve, then an alignment zone, then peripheral curves.

On a conventional GP lens each curve is flatter than the one inside it. Reverse geometry breaks that sequence once, deliberately, so the lens can sit flat centrally while still landing on the mid-periphery. The zone names, widths and typical dimensions — optic zone around 6 mm, a reverse curve roughly 3.00 D steeper than the back optic zone radius over 0.50 to 1.00 mm, an alignment zone about 0.70 to 1.00 mm wide, and the flattest peripheral curves about 0.50 mm wide — are set out on RGP Contact Lens Parameters and Starting Power, because an ortho-k lens is a GP lens and is described in GP units.

This page picks up where that description ends: what controls the lens's sagittal height, and what the cornea looks like the next morning.

What controls the sagittal height of a reverse-geometry lens?

Not the base curve. According to John Mountford and Don Noack, Corneal Topography and Orthokeratology: Post-fit Assessment (Contact Lens Spectrum, June 2002), lens sagittal height in a reverse-geometry design is controlled predominantly by the radius, or depth, of the reverse curve and the radius or angle of the peripheral alignment zone.

That is the structural difference from every other GP lens on this site. On a spherical or aspheric GP design, sagittal height is set by the base curve radius and the posterior optic zone diameter — the pairing described on Sagittal Depth of Contact Lenses. On a reverse-geometry design the base curve is chosen to produce a target amount of central flattening, so it is not available to also set the vault. The reverse curve takes that job instead. When a laboratory orders this parameter as a depth in microns rather than as a radius, it is usually called the return zone depth: it is the sagittal contribution of the reverse curve, and it is the number changed when the lens sits too high or too low.

Mountford and Noack publish the tear profile a five-curve reverse geometry lens is calculated to produce, which is what those two parameters are aiming at:

• 5.0 µm of apical clearance
• 10 µm of clearance in the area of alignment curve 1
• Alignment in the area of alignment curve 2
• 80 to 100 µm of clearance in the area of the peripheral curve

Those values are for that lens geometry, and the article states they vary if a tangent or hyperbolic alignment system is used instead of radial curves. The tolerance is the striking part: 5 µm of apical clearance is a target no keratometer can verify, which is why the fitting is calculated from topography and confirmed from topography.

What does a post-wear topography map show?

It shows where the lens actually sat during closed-eye wear, which is not reliably where it sits when you watch it at the slit lamp.

Mountford and Noack make that the reason the map exists: the position of the lens in the open-eye environment is not necessarily representative of where it positions in the closed eye, so corneal mapping is the only reliable means of knowing where the lens was during sleep. They describe three map presentations — axial, tangential (also called “true”), and refractive power — viewable in radius (mm) or power (D), and state that each has its use and none is inherently superior.

The subtractive, or difference, map is the one that carries the verification. It subtracts the baseline map from the post-wear map, so it shows what was done to the original cornea rather than what the cornea currently looks like. Mountford and Noack call it essential and vastly superior to viewing a pre- and post-wear map side by side.

Each presentation answers a different question on the same fit:

• Axial — the power at the apex is an independent indicator of the best-sphere refractive change.
• Tangential — shows the centration of the effect, and on a difference map, the degree of any decentration.
• Refractive power — shows the actual extent of the treatment zone.

What does a centred bullseye look like?

Three features, and Mountford and Noack are explicit that this is the only post-wear pattern that yields valid refractive data.

• A well-centred area of corneal flattening — the treatment zone, or TxZ
• A circle of mid-peripheral corneal steepening
• Little or no peripheral corneal change

The treatment zone is not at its final size on day one. The central treatment area varies in diameter depending on the amount of change induced, and Mountford and Noack state it reaches its full diameter after seven to 10 nights of wear. A treatment zone read at the first morning visit is therefore an early reading, not a final one.

One procedural instruction follows from all of this and is easy to get backwards: map before you refract. Mountford and Noack take topography first specifically because a refraction resulting from a decentred zone or a central island provides no relevant clinical data — the topography makes the diagnosis and the refractive number is redundant. On a true bullseye, the refractive change and the subtractive map's apical power change should agree closely, and that agreement is itself part of the verification.

How do you read a decentred map?

By which way the ring of steepening has moved, and each direction points at a different parameter.

The tangential difference map quantifies it. In the worked example Mountford and Noack publish, the lens had decentred by approximately 1.0 mm, and the axial map showed the red ring impinging on the inferior pupil zone. They note that the over-refraction in that case would still show some myopic reduction but with an increase in with-the-rule astigmatism, and flare and halos at night — a reminder that a partially working result is still a failed fit, and that the visual symptom and the map are describing the same thing.

Diameter is doing real work here. Ortho-k lenses are larger than conventional corneal GPs for exactly this reason: Myopia Profile's topography-data article states that an ideal ortho-k lens covers at minimum 95% of the cornea, usually around 0.8 mm smaller than the horizontal visible iris diameter, and that a lens too small for the HVID can lead to lateral decentration and excessive movement. HVID as a measurement, and its relationship to overall diameter, is on Diameter of Contact Lenses.

What is a central island, and what does it mean?

A central island is a small 1.0 mm to 3.0 mm central zone of corneal steepening sitting inside the flattened treatment area — the opposite error to a decentration.

Mountford and Noack attribute it to overestimating corneal sagittal height or underestimating eccentricity, which produces a lens with excessive central apical clearance — the lens bridges across the central cornea instead of touching it. On the map the centration can look perfect: a small steep island surrounded by a moat of corneal flattening, then the red ring.

They distinguish two kinds, and the distinction changes what you do:

• An island that is slightly flatter than the original cornea — about 0.50 D — but still steeper than the surrounding area will usually disappear during the first week of wear, and may simply be tissue resistant to change.
• The second kind does not resolve with time, because the lens fit is too tight. Best-corrected acuity is typically about two lines down, and the refractive endpoint is not clear cut. Correct it by decreasing the assumed corneal sagittal height by 8.0 µm and recalculating, or increasing the eccentricity value by 0.05, and by flattening the reverse curve and the alignment curve or curves.

Note the symmetry with the smiley face row above: the same two numbers, 8.0 µm of sagittal height and 0.05 of eccentricity, move in opposite directions for the two opposite errors. That is the practical reason both are worth remembering.

What is corneal eccentricity, and what does it decide?

Eccentricity (e) describes how rapidly the cornea flattens from centre to periphery. With the central radii, it is what a topographer uses to compute sagittal height over a chord.

Myopia Profile states the relationship between the two conventions plainly: corneal asphericity Q and eccentricity e are the same measure expressed differently, with Q = −e², and a higher absolute value of either means a shape further from a sphere. A perfect sphere has Q and e of zero, because it shows no flattening from apex to periphery. Ortho-k moves corneal shape toward sphericity — Q or e closer to zero — which is a geometric description of what the treatment zone is, not a claim about what it achieves clinically.

Eccentricity is also where the largest ordering errors originate, because a small error in e is not a constant error in elevation. Simard, Gagnon and Michaud, The Most Frequent Orthokeratology Problems to Troubleshoot (Review of Cornea & Contact Lenses, February 2025) publish the arithmetic on a 10.6 mm custom lens: at a K of 40.00 D, using an eccentricity of 0.1 instead of 0.2 produces an elevation error of only 12 µm, but the same 0.1 difference at e 0.8 versus 0.9 produces 31 µm, and at a K of 46.00 D the same 0.8-versus-0.9 difference produces 50 µm. Their rule of thumb is that it is easier to make an elevation mistake on a steep cornea with high eccentricity — and that a 10 µm difference can affect lens centration.

One further finding from the same article belongs on any page about verifying this design against what was ordered: the reshaped corneal profile never equals the lens shape. Simard and colleagues note that adding −1.00 D of correction may require changing the radius by about −1.50 D, because the curvature change does not translate fully into the moulding effect. That is precisely why the post-wear map is taken at every visit rather than trusting the ordered curve.

Why is astigmatism a design limit here rather than a target?

Because in this design corneal astigmatism is handled by a toric landing zone, not by ordering a cylinder power. The lens has to land 360° on the mid-periphery, and an elevation difference between the principal meridians is what stops it.

Two published thresholds mark the boundary. Myopia Profile gives a generally agreed value of around 1.50 D or more of corneal toricity as the point where a toric ortho-k design is indicated, and a sagittal height difference between flat and steep meridians of around 30 µm over an 8 mm chord as the sag-based equivalent, while noting that the chord and difference denoting a toric design vary between manufacturers. Simard and colleagues express the same boundary in elevation terms: on a regular cornea, a 25 µm elevation difference between principal meridians corresponds to about −1.25 D of astigmatism at an 8 mm chord, and they treat that as the threshold for going to a toric landing zone. They also identify switching between a spherical and a toric peripheral landing zone as the first problem-solving step, since most decentration traces back to a misjudged corneal elevation difference.

This page covers that as a design limit and goes no further. Soft toric rotation, the LARS compensation and starting toric power belong on the Toric Contact Lens Calculator, and they are a different problem — a soft toric is stabilised and compensated optically, whereas a toric landing zone is a mechanical alignment change with no cylinder power in it.

Where do the Jessen factor and the compression factor sit?

Upstream of this page. They set the back optic zone radius before any of the verification described here happens.

The Jessen factor is the amount a back optic zone radius is flattened relative to flat K; the compression factor is the additional flattening applied on top of it. Both are worked through, with the published arithmetic and the disagreement in the literature about the correct figure, on RGP Contact Lens Parameters and Starting Power. That guide is the owner of that calculation and it is not restated here.

The division is worth stating because it is the useful one clinically. The Jessen and compression factors decide how flat the centre of the lens is. The reverse curve depth and alignment zone decide where the lens sits. The post-wear map tells you whether the second group worked, which is why a map that is not a bullseye is a fitting problem rather than a power problem — and why re-calculating the base curve in response to a smiley face is the wrong move.

What this page does not cover

Everything that happens to a patient over time, which is a large fraction of what is written about this modality.

This site does not publish ortho-k candidacy criteria, starting ages, wearing schedules, corneal-response monitoring, review intervals, axial length targets, or myopia-management outcomes. Those are clinical care decided by a practitioner who has examined the eye and can follow it, and they are stated as out of scope here for the same reason they are stated as out of scope on the RGP guide: they are not ordered lens parameters, and a site that returns parameters should not imply it can return them.

What is here is the geometry and the map. That is enough to read a post-wear topography against the lens that produced it, and it is not enough to decide whether that lens should have been fitted.

Read Starting Contact Lens Parameters Are Not a Prescription for the bound that applies across this site, Myopia Control Contact Lenses: Optics and Ordered Parameters for the category, and Base Curve of Contact Lenses for the radius this design starts from.

How we calculate·Medical disclaimer

Sources

Clinical claims on this page are attributed to the publications below. Where a source also reports treatment outcomes or candidacy guidance, those parts are deliberately not reproduced.

• John Mountford, Dip App Sc, FAAO, FCLSA, and Don Noack, Dip Opt (WA), Corneal Topography and Orthokeratology: Post-fit Assessment (Contact Lens Spectrum, June 2002) — axial, tangential and refractive power maps and the subtractive map; the closed-eye position is not the open-eye position; the centred bullseye as flattening plus a circle of mid-peripheral steepening plus little peripheral change, and the only pattern giving valid refractive data; full treatment zone diameter after seven to 10 nights; reverse-geometry sagittal height controlled by reverse curve depth and alignment zone radius or angle; the 5 µm / 10 µm / alignment / 80–100 µm tear profile; smiley face, frowny face and lateral decentration with their corrections; approximately 1.0 mm of decentration in the published example; the 1.0–3.0 mm central island and its two types; map before refracting.
• Myopia Profile, What topography data do I need to fit orthokeratology lenses?— an ideal ortho-k lens covers at minimum 95% of the cornea, usually around 0.8 mm smaller than HVID, and a lens too small for the HVID can cause lateral decentration and excessive movement; sagittal height from central radii and eccentricity over a specific chord; Q = −e² and what a value further from zero means; ortho-k moves corneal shape toward sphericity; a sagittal height difference of around 30 µm over an 8 mm chord, and around 1.50 D of corneal toricity, as generally agreed toric-design thresholds that vary between manufacturers.
• Patrick Simard, OD, Rémy Gagnon, OD, and Langis Michaud, OD, The Most Frequent Orthokeratology Problems to Troubleshoot (Review of Cornea & Contact Lenses, February 2025) — a 25 µm elevation difference between principal meridians equals about −1.25 D of astigmatism at an 8 mm chord, taken as the threshold for a toric landing zone; the eccentricity-to-elevation table on a 10.6 mm lens (12 µm at K 40.00 D for e 0.1 versus 0.2; 31 µm for e 0.8 versus 0.9; 50 µm at K 46.00 D for e 0.8 versus 0.9); a 10 µm difference can affect centration; adding −1.00 D of correction may require about −1.50 D of radius change because the lens shape never equals the reshaped corneal profile.