Orthokeratology Customisation: A Necessity for Myopia Control?

With the increase in myopia management options, Dr Paul Gifford and Jagrut Lallu consider the issue of customising OK lenses.

While efficacy for slowing progression of myopia is widely accepted, the mechanism behind why OK induces a myopia control effect remains unclear. OK lenses temporarily mould anterior corneal topography to create a central treatment zone of flattening to correct myopia, surrounded by an annular zone of para-central corneal steepening.

The change to corneal profile from OK has been shown to create both a positive shift in peripheral refraction and a positive shift in spherical aberration.^3,4 Both of these optical effects have been linked to slowing progression of myopia in children.^5,6

ENHANCING OK’S EFFICACY FOR MYOPIA CONTROL

OK lenses are essentially rigid gas permeable lenses, modified to create a back surface profile that fits flat on the central cornea and steepens in the mid periphery to realign with the peripheral cornea, ensuring a stable lens fit. Practitioners can calculate the back surface curves required to create this profile themselves or use commercial designs that remove this complexity by following a defined lens fit process.

The capacity to easily modify OK lens design has led to practitioners and scientists modifying different aspects of lens back surface curvature to increase myopia control efficacy. The perceived association between the OK-induced corneal steepened peripheral zone and change to peripheral hyperopia has concentrated most investigation on increasing the positive shift in refractive power across the peripheral steepened zone. The assumption being that this will create a greater positive shift in peripheral refraction, and consequently, a greater myopia control effect.

INCREASING COMPRESSION FACTOR

Lau et al. recently published research reporting that “participants wearing OK lenses of increased compression factor further slowed axial elongation by 34%, when compared with the conventional compression factor”.⁷ This is a considerable improvement – should we all be using higher compression factors when fitting OK to children for myopia control?

The compression factor is an extension of the Jessen factor, named after George Jessen who was the first to publish on the concept of OK in 1962.⁸ The simple premise is that the back optic zone radius (BOZR) of the lens is calculated to be flatter than the cornea’s refractive power by the amount of correction required. The compression factor defines the extra amount of BOZR flattening that is applied to compensate for loss of refractive effect from OK across the day, and is typically set at 0.75D.⁹

Lau et al. reported 0.53±0.29 mm axial elongation over two years with a conventional compression factor (0.75D) and 0.35±0.29 mm with an increased compression factor (1.75D).⁷ These results clearly indicate an enhanced myopia control effect with the increased compression factor, however it needs to be recognised that this ‘enhanced’ effect is no better than the 0.27 mm change in axial elongation reported for a meta-analysis of previously published research on conventional OK studies.¹⁰ Using this benchmark, their results could be interpreted as achieving a ‘standard’ myopia control effect with an increased compression factor and a ‘less than standard’ effect with a conventional compression factor.

A fundamental difference between research and practice implementation is that OK fitting in a research study must follow a defined study protocol, which in Lau et al. resulted in the increased compression factor cohort being undercorrected by -0.18±0.74D at two years. Undercorrection has been reported to accelerate myopia progression,¹¹ which may account for the loss of myopia control effect in this cohort. Ongoing assessment in clinical practice allows lens fit to be altered to reinstate full refractive correction. In doing so, this breaks the rules of the study and thereby invalidates any expectation to achieve the same outcomes.

Earlier work by Lau et al. explains their experimental pathway. In 2018 they reported that children experiencing slower eye growth had higher measured spherical aberration.⁵ In 2020 they reported that OK lenses fit with 1.75D compared to a 1.00D compression factor created higher spherical aberration.¹² Combining these outcomes naturally leads to hypothesising that increasing the compression factor will increase myopia control effect, hence the study.

In clinical practice, the compression factor value is chosen at the start point in the lens fit process. At subsequent follow-up visits, any residual refractive error is likely to be corrected by altering the BOZR, thereby changing the effective compression factor. This effectively reduces compression factoring to a blunt tool for modifying OK’s myopia control effect in clinical practice.

REDUCING TREATMENT ZONE DIAMETER

The treatment zone induced by OK is the area of central corneal flattening and is typically seen to reduce in size with increasing myopic correction, which in turn has been shown to increase spherical aberration. By manipulating the lens design parameters, it is also possible to reduce treatment zone size independent of refractive correction, thereby breaking the link that encumbers compression factor for enhancing OK myopia control efficacy in clinical practice.

Altering treatment zone size has been shown to be dependent on OK lens design. Kang et al. altered back optic zone diameter (BOZD) and the tangent angle in the BE (Capricornia) lens design, to find it made no difference to measured peripheral refraction.13 In the Paragon CRT design (CooperVision), reducing BOZD from 5 mm to 6 mm was found to reduce treatment zone diameter (TZD) by 0.3 mm.¹⁴ More recently, a 0.92 mm difference in TZD was reported between a 5.5 mm and 6 mm BOZD using a custom OK lens design.¹⁵

There is one paper to date that has reported the effect of reducing OK TZD for myopia control efficacy. It reported 0.22 mm less axial elongation from 5 mm vs 6 mm BOZD in the BE Free (KATT Design Group, Precision Technology Services) lens design.16 Unlike the earlier reported compression factor study, the test group in this BOZD study did show enhanced myopia control efficacy compared to the Sun et al. metaanalysis of conventional OK lens designs.¹⁰

We need to be mindful that this is the only study published to date, and further validation is required to confirm repeatability of effect. Statistically significant effects were observed only across the first six months of wear. Thereafter, axial elongation progressed at the same rate with both 5 mm and 6 mm BOZD lens design. Further research is needed to understand the true mechanism of myopia control effect, and why the enhanced effect didn’t perpetuate beyond six months.

Reducing TZD raises the question on whether vision is compromised from a smaller area providing refractive correction. The limited publications to date indicate that contrast sensitivity, visual acuity, and subjective vision are not adversely affected.¹⁴ However, optical modelling revealed that corneal power correction reduces with increasing pupil diameter, raising the suggestion that reduced TZD OK may become undercorrected when pupil size increases in dimmer lighting.

Guo et al. also reported that the first lens fit success rate reduced from 100% with the 6 mm BOZD design to 5 mm BOZD design.¹⁶ In their first year results, they reported that “four subjects dropped out from the study because the 5 mm BOZD lenses were unable to achieve a smooth well-centred TZ and/or good visual performance even after modifications to the lens parameters”. These four subjects were successfully fit with the 6 mm BOZD, indicating potentially greater complexity of the lens fit process with smaller BOZD OK lens designs.

TREATMENT ZONE DECENTRATION

Decentration of the OK treatment zone is defined as 0.5 mm to 1.5 mm difference between the centre of the treatment zone and the pupillary centre, and in clinical practice is typically reported toward the inferior temporal quadrant. Conversely, the centre of the entrance pupil of the eye was found to be located superior temporally to the corneal light reflex and its position was unaffected by OK lens wear.¹⁷

Treatment zone decentration has been shown to be associated with increases to spherical aberration, coma, and relative peripheral refraction.¹⁸ The same study reported a positive correlation between amount of decentration and change to relative peripheral refraction, leading the authors to speculate that OK decentration might contribute to efficacy of myopia control.¹⁸

Spherical aberration manipulation can be achieved by modifying the back optic zone diameter as described above and, in addition to this, modifying the eccentricity of the back optic zone diameter. The multifocal orthokeratology (MOK) lens design uses this principle by creating a 3.6 mm zone of clear central vision with a surrounding 1.2 mm annulus for myopic treatment, with an aim to achieve +2.50D of myopic retinal defocus. Design alterations like this can be achieved by altering the width of the treatment zone and applying eccentricity to the optic centre.¹⁹

WHAT OK LENS SHOULD YOU FIT FOR MYOPIA CONTROL?

Before considering modifying OK lens design, it needs to be remembered that traditional OK has been demonstrated to provide, on average, around 50% myopia control effect compared to single vision controls across numerous different lens designs. This is comparable to published efficacy for myopia control spectacles and soft contact lenses. By fitting an OK lens that you are comfortable prescribing, following the manufacturer’s recommendations, you will likely be meeting the requirements to replicate this effect.

If you prefer to try increasing myopia control efficacy in OK, we have made a strong case that increasing compression factor is a blunt tool, inducing clinical research results that cannot be replicated in clinical practice. Different OK lens designs will undoubtedly manage the compression factor in different ways, thereby adding greater unpredictability. Targeting reduced TZD has been reported to provide greater myopia control efficacy and is more controllable.

Reducing BOZD to reduce TZD can be replicated in clinical practice, however the myopia control effect is likely to be dependent on lens design. Guo et al. reduced BOZD from 6 mm to 5 mm, resulting in 0.92 mm reduction to TZD. Gifford et al. achieved the same 0.92 mm reduction in TZD by reducing BOZD from 6 mm to 5.5 mm, and Carracedo et al. reported a much smaller 0.3 mm reduction in TZD when reducing BOZD from 6 mm to 5 mm. It is the change to TZD that is active during the day, and these studies are reporting a non-uniform relationship between BOZD and TZD between different lens designs.

Also, keep in mind that the enhanced myopia control efficacy from reduced TZD OK was only apparent across the first six months of wear. When considered alongside the potential increased complexity of lens fit, this raises the question whether the potential for greater myopia control efficacy across the first six months of wear is worth the extra chair time that it may take to achieve a successful lens fit outcome.

The current take home message for managing OK decentration is that as long as we achieve good visual acuity unaided, it may be prudent not to ‘fix’ some mild decentration with good vision and a healthy cornea. We would encourage all practitioners to at least note the location of the pupil when fitting orthokeratology to myopic patients.^20-22

Our recommendation overall, is that those new to OK lens fitting should first be comfortable fitting and prescribing a conventional OK lens design for myopia control. The research evidence reports these to slow myopia progression in children by equivalent amounts to the best soft contact lens and spectacle options for myopia control, and that customisation for myopia control is not essential.

For experienced OK lens fitters who are comfortable with altering OK lens design parameters, there is now compelling evidence that reducing TZD enhances myopia control efficacy in OK, however there is only one published paper to date.

What remains unclear is whether fitting first with a conventional OK lens, and then subsequently altering lens design to reduce TZD, will create the same effect as published by Guo et al.¹⁶ This latter approach would potentially simplify the lens fit process by allowing successful lens fit to be established using a traditional OK lens design. Once a good fit is established, lens design modification could then be attempted, having already established that the patient is comfortable with OK and has a fall back if successful lens fit cannot be achieved with a smaller BOZD design.

Dr Paul Gifford is a research scientist and industry innovator based in Brisbane, Australia. He is the cofounder of Myopia Profile and My Kids Vision.

Dr Gifford is also an Adjunct Senior Lecturer at University of New South Wales, visiting Associate Professor at the University of Waterloo, Ontario, Canada, and a consultant to the contact lens industry. He has won several prestigious research awards, published numerous scientific and educational papers, and lectured internationally.

Jagrut Lallu MSc is a practising optometrist at Rose Optometry in Hamilton, New Zealand with a special interest in myopia management and keratoconus fitting. A clinical senior lecturer at Deakin University and a Teaching Fellow at the University of Auckland, Mr Lallu is a founding member of the Australia and New Zealand Child Myopia Working Group. He is the Past President of the Cornea and Contact Lens Society of New Zealand and served 10 years on the Orthokeratology Society for Oceania as the New Zealand Liaison.

References

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