WRITER Dr Paul Gifford

For most of their history, contact lenses and spectacles did one thing: they moved the focal point of a blurred world back onto the fovea. The lenses we now reach for in paediatric practice are doing something quite different. They are engineered to manipulate the retinal image deliberately, not only to give a child clear central vision, but to send the growing eye a signal to slow down. Across soft contact lenses, orthokeratology (OK) and the newest spectacle designs, the same idea recurs in different optical clothing. Understanding the technology, rather than the brand, is what lets us choose well at the chair.

ONE MECHANISM, OR TWO?

The dominant working hypothesis behind almost every myopia control lens is that the peripheral retinal image governs axial growth.1 A single-vision correction that is sharp on axis typically leaves the peripheral retina in relative hyperopic defocus,2 a stimulus that animal models link to elongation.3 Impose myopic defocus by bringing peripheral light to a focus in front of the retina and elongation slows.4 On this view, every modality discussed here is simply a different way of placing a competing, myopically defocused image in front of the eye while preserving usable central vision.

A second hypothesis that has gained ground is that these lenses work by degrading retinal image contrast rather than by imposing myopic defocus.5 Two observations drive it: lenslets of opposite sign can produce similar control,6 and lenses that simply scatter light also slow progression.7 Charman and Atchison’s recent modelling shows that multi-segment lenslets act as a low-pass spatial-frequency filter, attenuating fine detail so that any growth signal must reside in the low spatial frequencies.8

SOFT CONTACT LENS TECHNOLOGY

Two design philosophies dominate. The first is dual-focus, or concentric, optics: alternating zones of distance correction and constant relative plus power, so that a defocused image is presented simultaneously with the clear one.9 This is the principle behind the dual-focus design tested in the MiSight trials reporting roughly 59% slowing of progression over three years,10 with efficacy sustained across a six-year cohort,11 the longest soft-lens dataset reported to date.

The second philosophy shapes the optical profile continuously across the whole optic zone rather than adding discrete plus zones, manipulating the eye’s depth of focus and aberration structure to spread a myopically defocused image through a range of vergences. Two designs illustrate it. Extended depth of focus (EDOF) optics use higher-order aberration profiles to push the image shell forward across the periphery; randomised trials of White participants of European descent12 and Indian13 children have shown clinically meaningful slowing of axial elongation. Johnson & Johnson’s noncoaxial ring-focus design reaches the same goal by a different optical route and was found to slow axial elongation over six months relative to single-vision wear.14

The practical point for the chair is that these designs trade central image quality differently.

Dual-focus optics superimpose a constant defocused image that some children notice; EDOF and novel ring-focus designs spread the compromise across a range of vergences. Neither should be considered clinically ‘better’; instead, the right choice depends on the visual demands and tolerance of the individual child. The safety and compliance advantages of daily-disposable single-use wear are part of the technology and not a footnote to it.

ORTHOKERATOLOGY DESIGN

OK achieves the same optical goal through a wholly different route: it reshapes the anterior cornea overnight, flattening the centre and steepening the mid-periphery so that the cornea itself becomes the myopia-control optic, inducing peripheral plus power during the day.15 The efficacy evidence is robust and consistent. Meta-analyses of published research on OK for myopia control in children converge on around 50% slowing of axial elongation across different ‘conventional’ OK designs.16 A growing body of work now indicates that greater myopia control efficacy can be achieved by reducing the OK lens induced treatment zone diameter (TZD).17

Prescribing a smaller back optic zone diameter is the predominant approach that has been studied for reducing TZD.17 The proposed mechanism is that this increases the relative peripheral myopic defocus provided by the plus-powered ring surrounding the treatment zone.18 This effect may be further enhanced by bringing that ring closer to the central optical axis,19 where it is more likely to fall within the pupil and contribute to the retinal image, rather than being obscured.20

Studies analysing OK TZD optics are revealing that smaller TZD creates a steeper and more concentrated ring of relative peripheral plus,20-25 which intriguingly aligns with the same contrast logic raised earlier. Smaller TZD designs create a steeper corneal transition that increases spherical aberration, reduces modulation transfer, and increases light scatter.26 This specific optical degradation acts as a low-pass filter that selectively attenuates medium and high spatial frequencies, mirroring the mechanism described by Charman and Atchison for multi-segment spectacle lenslets.8

Despite evidence for smaller TZD providing greater myopia control, when we reliably reduced TZD without changing the refractive effect, relative peripheral refraction did not measurably change.27 That dissociation is hard to reconcile with a pure peripheral-defocus account and sits more comfortably with a contrast-degradation one. On this reading, a smaller treatment zone may enhance control as much by sharpening that contrast signal as by relocating defocus.

The practical message, though, is reassuring. Conventional OK already provides good myopia control, and reducing the treatment zone diameter is best seen as a possible refinement rather than a necessity. OK does not need to be customised in this way to be prescribed with confidence. A smaller treatment zone is an option worth considering for children who continue to progress, not a prerequisite for every fit.

SPECTACLE LENS TECHNOLOGY

The newest and fastest-moving category puts the same optics in front of the eye in a spectacle lens that carries none of the potential challenges of contact lens fitting and wear. Two distinct technologies have emerged, broadly mapping onto the two hypotheses above.

The first adds many small zones of plus power across the lens periphery, each casting a myopically defocused image while the clear spaces between them carry the distance correction. Most designs do this with discrete lenslets. The defocus incorporated multiple segments (DIMS) design, reported in a two-year randomised trial, slowed progression by around half,28 and the highly aspherical lenslet target (HALT) design achieved comparable results, with the added finding that greater lenslet asphericity improved efficacy.29

A more recent variant from ZEISS takes a different geometry: rather than discrete lenslets, its cylindrical annular refractive elements (CARE) impose the plus power as concentric rings. In a two-year randomised trial, CARE designs were shown to slow axial elongation by around 0.2 mm relative to single-vision wear.30

The increased efficacy finding from higher HALT asphericity is now driving a second generation of spectacle lens designs, with the two established lenslet designs taking different routes to the same end through increasing the volume of the myopic defocus signal.

The HALT route adds both power and asphericity: a maximised design slowed axial elongation by roughly 0.11 mm more over 12 months than the original in a contralateral crossover trial (about 0.043 versus 0.105 mm at six months),31 the clearest demonstration yet that strengthening the signal strengthens the effect.

The DIMS route instead keeps the lenslets spherical but raises their defocus power and reshapes the central zone. Its efficacy data remain confined to conference abstracts, but early visual-performance work is reassuring, with distance acuity preserved across both families, and only a modest near-vision reduction reported in one modified DIMS design.32

The open question is no longer whether more signal helps, but whether these stronger designs suit all myopes or chiefly younger children and faster progressors, and how availability and cost will place them within treatment algorithms. That those three quite different geometries – from discrete spherical lenslets to aspherical lenslets to concentric annuli – produce broadly comparable control, is itself a clue that the precise defocus pattern is not the whole story. Consistent with this, both honeycomb DIMS-type and highly aspherical lenslets have been reported to reduce contrast sensitivity at medium and high spatial frequencies while sparing the low frequencies, the same low-pass signature the contrast hypothesis predicts.33

The second technology takes the contrast route explicitly. Diffusion optics technology (DOT) scatters a controlled fraction of light using thousands of micro-diffusers, lowering retinal contrast on the hypothesis that high contrast itself drives elongation.5 The CYPRESS trial showed meaningful slowing sustained to four years.7 That a lens with no defocus-inducing optics at all can match the lenslet designs is perhaps the strongest single argument that contrast degradation belongs alongside defocus in any honest account of mechanism.

WHAT THIS MEANS IN CLINICAL PRACTICE, AND FUTURE DIRECTION

The optical similarities are the headline. Soft lenses, OK, and spectacles are not three unrelated treatments but three delivery systems for closely related optical signals, and increasingly it looks as though the shared currency may be the spatial-frequency content of the retinal image, not peripheral defocus alone. The design details that govern efficacy, namely add power and aberration profile in soft lenses, treatment-zone geometry in OK, and lenslet power profile or diffuser density in spectacles, are all variations on that single theme.

The future trajectory points toward customisation, tailoring the signal to the individual eye rather than selecting from fixed designs. For now, however, mechanism alone should not drive our clinical decisions. With well-designed options in each class performing broadly comparably, the choice between modalities is driven more by availability, lifestyle, compliance, and visual demand than by raw efficacy. Our understanding of how these lenses work, whether through defocus, contrast degradation, or both, is not yet firm enough to guide practice choices by lens design; these decisions should rest where the science is strongest and the studies most extensive; on the outcomes each of these treatments are proven to deliver.

Dr Paul Gifford is an eye care industry innovator drawing on experience that includes every facet of optometry clinical practice, transitioning to research and academia with a PhD in OK and contact lens optics, now consulting to industry and working on Myopia Profile, the world-leading educational platform that he co-founded with Dr Kate Gifford.

Dr Gifford is an Adjunct Senior Lecturer at the University of New South Wales and a visiting Associate Professor at University of Waterloo, Canada.

References available at mivision.com.au.