Customizing ortho-k: what does it mean and is it needed?

The concept of customized ortho-k can sometimes be misleading as by virtue of following a bespoke lens fit protocol all ortho-k lenses inherently involve some degree of customization. An ortho-k lens that is described as “customized” may simply mean that it is bespoke ordered, or specific changes to the contact lens parameters have been requested.

This article will discuss the three main changes that can be made to ortho-k lenses when fit for myopia control and how they might affect ortho-k efficacy for myopia control:

1. Increase to compression factor
2. Reduction to back optic zone diameter in an effort to reduce treatment zone diameter
3. Adjustment to ortho-k induced Multifocal optics.
    

Compression factor is an extension of the “Jessen factor”, whereby the difference between an ortho-k lens back optic zone radius and baseline corneal curvature measured in dioptres is matched to the desired change in refraction.

Early in the history of ortho-k it was realized that the cornea gradually rebounds after lens removal, thereby benefiting from an increase in the Jessen Factor calculations to create a compensatory over correction at lens removal. This compensation for the regression of the orthokeratology effect during the no lens-wear period (i.e. regression of corneal shape towards baseline) is designed to ensure the wearer obtains clear distance vision throughout the day. A Compression Factor of 0.75D was suggested by Mountford from his analysis of corneal change in response to ortho-k lens wear,¹ and became widely accepted as the standard.² 

Recently, Lau et al in 2022 reported greater myopia control effect from ortho-k lenses fit using a higher Compression Factor.³ The study involved children aged 6-12 years old with low to moderate myopia (-0.50D to -4.00D) and astigmatism (≤1.25 D), randomly assigned to a compression factor of 1.75D (High) or 0.75D (conventional), with change to axial length measured across 2-years.

Although it was found that the increased compression factor resulted in a slowing of myopia progression by 34% when compared to the control (i.e., conventional compression factor), it is also important to consider other factors. For example 63% of participants in the control group dropped out of the study due to concerns about myopia progression. A closer look also reveals that the mean axial elongation of participants wearing ortho-k were 0.53 ± 0.29mm in the control group and 0.35 ± 0.29 mm in the increased compression factor group. 

In a meta-analysis by Sun et al in 2015, axial eye growth over a 2-year period was 0.27mm in eyes wearing conventional ortho-k lens designs.⁴ This indicates that the increased compression factor lenses used in the current Lau et al study created what appears to be a slightly less myopia control effect than standard ortho-k lens designs previously investigated, and their 'conventional' (control) lenses had much less effect than typically found in other studies. 

The additional 1D of compression factor, or lens treatment factor, only gave a mean 0.30D extra myopia reduction, indicating individual variability. Furthermore, the conventional ortho-k lens design (control) group showed undercorrection from 12 to 24 months, with mean acuity of 0.12 logMAR (6/7.5-1 or 20/25-1). The study design indicates that single vision spectacles were prescribed as a ‘top-up’ for any children whose acuity dropped to less than 0.18 logMAR (6/9+1 or 20/30+1) or had residual refractive error exceeding 0.50D in either eye. Undercorrection is known to increase myopia progression,⁵ and no comment is made on compliance with over-spectacles. 

Another way that ortho-k lenses can be customized is by changing the back optic zone diameter, with decreasing the size of the back optic zone diameter gaining recent traction from research suggesting an improved myopia control effect over conventional ortho-k.^6-8 

The mechanisms are not entirely understood, however the intent is that a smaller back optic zone diameter (BOZD) will induce a smaller treatment zone diameter (TZD) on the eye, which in turn increases relative peripheral refraction^9-10 and higher spherical aberrations,^11-12 thereby enhancing myopia control. This theory developed from the observation that children with larger pupils compared to the ortho-k treatment zone had slower axial elongation and a hyperopic shift, while those with pupil diameters within the treatment zone had more axial elongation.¹³

The 2-year VOLTZ study investigated the effect of 6mm (conventional) and 5mm (reduced BOZD) ortho-k lenses on change to axial length in children aged 6 to 11 years old.⁸ Across one year axial eye length increased by 0.04 ± 0.15 mm the 5mm group by 0.17 ± 0.13 mm in the 6mm group. However, the slower growth in the 5mm optic group relative to the 6mm group was significant only across the first six month measurement interval. 

Although indicating a favorable outcome, these results need to be considered carefully in a clinical setting. At 1-year the VOLTZ study reported a comparable initial fitting success rate, with 100% for the 6mm group and 94% for the 5mm group, however four subjects of the 5mm group dropped out due to fitting issues even after attempts to modify the alignment zone to compensate were made.¹⁴ 

Changing the TZD may also have visual consequences. For example, a study involving young adults found that those wearing the 5mm lens measured lower contrast sensitivity.¹⁵ This may result in a deterioration in vision in low lighting level conditions and is hence important to be aware of.  

In a contralateral-eye study, conventional ortho-k lenses were compared to a novel multifocal ortho-k (MOK) lens design for 18 months with axial length monitored. The MOK lens was designed to create a “molded, center-distance, multifocal surface onto the anterior cornea, with a concentric treatment zone power of +2.50 D”.¹⁶

The MOK-treated eye progressed 0.17mm less than the conventional ortho-k lens-treated eye across the 18-month study period. Interestingly, no changes in peripheral refraction profiles were evident between the two lens designs: hence, relative peripheral refraction does not appear to be driving the enhanced myopia control effect. Additionally, visual acuity was reduced in MOK-treated eyes compared to conventional ortho-k.¹⁶

Ortho-k designs vary considerably, making it unreasonable to expect a simple measure like Compression Factor to remain consistent across all lens designs. Lau et al used the Menicon Bloom Night^TM lens, which is an empirical design that automatically compensates sag height to maintain the same apical lens clearance when altering the Compression Factor. The construct of the Menicon Bloom Night^TM is proprietary, making it highly unlikely that other ortho-k lens designs will provide the same kind of effect when Compression Factor is altered.^* 

The key takeaway here is that fitting ortho-k lenses to provide the full refractive treatment effect is likely to yield optimal myopia control results, consistent with previous findings of slowing axial length growth, rather than applying an universal increase to Compression Factor that could be easily interpreted from the Lau et al study. 

Reducing BOZD to achieve a smaller TZD achieved a better myopia control outcome; however, this comes at the cost of increased complexity to achieving a successful lens fit and greater potential for failure as shown by the higher study dropout rate for the smaller BOZD lens reported by Guo et al. Following a similar vein to changing Compression Factor, the effect that reducing TZD has on TZD should not be expected to be consistent across all lens designs. Whereas Guo et al reported a 0.94mm reduction in TZD from the 5mm compared to 6mm BOZD lens, Carracedo et al instead reported 0.3mm difference in TZD for the same 5mm to 6mm BOZD comparison using Paragon CRT lenses.¹⁵ 

In a clinical context, attempting to provide a greater myopia control effect by increasing Compression Factor or reducing BOZD is not as simple as may seem. Current research offers compelling evidence for beneficial effect; however, more investigations are required to validate these findings before the same approaches are adopted into clinical practice. The current weight of scientific validation for myopia control rests with providing optimal fit and vision outcomes using conventional ortho-k lens designs.

* Main studies for myopia control with Menicon lenses have been conducted with Menicon Z Night™. At the time they were performed, there was no availability of Menicon Bloom Night™ lenses in the markets where they occurred. Menicon Bloom Night™ design is based on Menicon Z Night™ design. The difference between both designs is the definition of the sagittal height.