What topography data do I need to fit orthokeratology lenses?

Orthokeratology (ortho-k) requires accurate corneal topography measures to identify suitable candidates, successfully fit lenses and monitor outcomes. Topography data is integral to initial lens fitting and to guide any modifications required, since lens-on-eye examination may not necessarily inform how the lens fits when worn overnight, in the closed eye condition. A full and accurate map of corneal shape is required, ensuring high-quality maps which are free from the impact of misalignment, poor tear film quality, eyelid occlusion or eyelash artifacts.¹ This article will discuss what data is required to fit ortho-k lenses.

The lens diameter for orthokeratology is determined by the HVID of the patient. Ortho-k lenses have a large diameter: typically around 10.6 to 11mm. An ideal ortho-k lens covers at minimum 95% of the cornea, which is usually around 0.8mm smaller than the HVID.² A lens which is too small for the HVID can lead to lateral decentration of the ortho-k lens and excessive movement.¹

Topography software typically includes a biometric ruler inbuilt which you can use to measure the HVID of the patient. It is important to go through the centre of the map and be as horizontal as possible - sometimes the topographer may cut off part of the cornea in the vertical plane. "White-to-white" measurements are the most helpful; this is when you take the measurement from the start of the sclera on one side, to the sclera on the other.²

Apical radius of curvature is a single measure taken at the corneal apex, denoting the point of maximum curvature of the corneal surface. While baseline apical curvature is not predictive of changes in refractive error in children,³ it can be a predictor of suitable candidature for ortho-k. Steeper apical corneal power (ACP) can indicate more capacity for corneal flattening, and hence higher treatment potential.^4,5

Ideal candidates for ortho-k tend to be those with myopia up to around 4.50D,¹ but the relationship between baseline apical curvature and treatment potential has not been explored. This means there is no defined 'ideal' baseline or final apical corneal power guidelines for candidate selection. As an example, a case series of 60 successful ortho-k fits for myopes with a mean of -2.19D starting refraction showed a mean initial ACP of 43.7D and final ACP of 41.5D.⁵

Once ortho-k treatment has commenced, comparing treated to baseline maps to evaluate change in ACP is a strong predictor of the measured refractive change.^4,5

Ortho-k lenses work on a reverse curve principle to flatten the central cornea while maintaining an alignment lens fit in the periphery for centration.¹ The sagittal (sag) height of the lens needs to match the sag height of the cornea - otherwise the periphery of the lens will not align with the periphery of the cornea, and the centre of the lens will not align with the corneal apex. Height can be obtained by a corneal topographer using the central corneal radii and eccentricity value, which describes the rate of corneal flattening towards the periphery. It is a single value measured over a specific chord (diameter).⁶

Sagittal height difference between flat and steep corneal meridians can necessitate fitting of a toric ortho-k lens. The chord and difference which denotes a toric design can vary between manufacturers, but a difference of around 30 microns over an 8mm chord is a generally agreed value.¹

Astigmatism is important in topography measurement, not only to determine the type of lens you may wish to use (spherical or toric), but also to determine if the patient is a good candidate for ortho-k.¹

In order for ortho-k lenses to maintain alignment, the lens must land 360 degrees around the peripheral cornea. If a patient has high astigmatism, limbal-to-limbal astigmatism or irregular astigmatism, the lens will have a tendency to decentre, and will most likely need a toric lens in order to combat this. While specific lens designs may vary in their recommendations for spherical versus toric ortho-k lenses, the generally agreed value is around 1.50D or greater of corneal toricity.¹

Identifying whether the refractive astigmatism measured in the prescription is due to the anterior cornea (modifiable with ortho-k) or internal eye structures (posterior cornea and lens, not modifiable) is also important in determining suitable candidates for ortho-k. In children with low to moderate myopia and healthy eyes, internal astigmatism typically compensates for anterior corneal astigmatism, so correcting the latter with ortho-k can expose the former as residual after treatment. This means that children (or adults) with a close match between refractive (total) and corneal astigmatism are likely to be better candidates for ortho-k.⁷

There have not been any studies on the relationship between internal (or ocular residual) astigmatism and outcomes in ortho-k.⁷

Corneal asphericity (Q) or eccentricity (e) is a measure of how rapidly the cornea flattens from central to peripheral locations. They are the same measure expressed differently, with Q = -e2. A higher absolute value of either metric indicates how different the overall corneal shape is from a sphere, which shows no rate of flattening from the apex to periphery, ie. a Q or e of zero. Children with myopia have a slightly less aspheric corneal shape than emmetropes or hyperopes, even with similar curvature at the corneal apex.³

Similarly as for ACP, a relationship has been found between higher baseline corneal eccentricity and higher potential refractive change possible in ortho-k. This is because ortho-k moves the corneal shape towards more sphericity (Q or e closer to zero). A case series of 60 successful fits found that a mean refractive change of -2.19D (range -1.00 to -5.00D) sat alongside a mean corneal eccentricity value of e = 0.475 (range 0.11 to 0.72).⁴

Another study found that initial corneal asphericity (Q) was not as strong a predictor of refractive change as apical corneal power.⁵

Ortho-k wear may result in visual outcomes of glare and haloes in lower light conditions, due to the significant change in higher order aberrations after treatment.¹  Larger pupils can make these visual effects more pronounced. It is a good idea to measure photopic and scotopic pupil sizes for this reason. For best visual acuity outcomes, pupil diameters of less than 6mm in dim illumination are recommended for candidate selection.¹

The relationship between pupil size and ortho-k lens design in myopia control is yet to be determined. A study in 2012 by Chen et al first raised the possibility that larger pupil diameters combined with ortho-k wear could result in a larger myopia control effect.⁸ By contrast, a study published in 2022 found that children with photopic pupil diameters of less than 4.8mm had slower axial elongation than those with larger pupils.⁹ This indicates that there's more to learn about the potential to optimize myopia control outcomes with ortho-k. Read more in our article Pupils and myopia management: what we know and need to learn.

The image below shows pupillometry measurement outputs from the Topcon MYAH instrument.

Different ortho-k lens designs typically have different fitting processes, depending on the manufacturer. The specific corneal topography data required can also vary, so reaching out to the manufacturer for a helping hand in lens design and ordering is the ideal place to start. Regardless of the topographer you are using or the lens design you choose, though, capturing good quality topography maps is vital. This enables the most accurate lens design and best possible outcomes for your patients. Taking several high quality maps is typically recommended for accurate ortho-k lens design, and some topographers have the ability to export directly to lens manufacturer websites supporting lens design.

• Case study: A low myope with long axial length

• Optimizing safety in orthokeratology

• The Topcon MYAH - Q&A with Mario Teufl

• Science Summary: Can we predict success with orthokeratology?

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