Is an extra compression factor needed in orthokeratology for myopia control?

Paper title: Orthokeratology lenses with increased compression factor (OKIC): A 2-year longitudinal clinical trial for myopia control

Authors: Jason K Lau (1), Kin Wan (1), Pauline Cho (1)

1. Centre for Myopia Research, School of Optometry, The Hong Kong Polytechnic University, Hong Kong SAR, China.

Date: February 2023

Reference: Lau JK, Wan K, Cho P. Orthokeratology lenses with increased compression factor (OKIC): A 2-year longitudinal clinical trial for myopia control. Cont Lens Anterior Eye. 2023 Feb;46(1):101745.

Link to paper abstract

This two-year prospective study in 6 to 12-year-old children, found that higher ortho-k compression factor (1.75D) provided a better myopia control effect than the same lens design fitted with conventional (0.75D) compression factor.

The authors state that the increased compression factor lenses slowed myopia control by 34% when compared to the conventional compression factor lenses. While this sounds like a compelling reason to use a higher compression factor when fitting children with ortho-k lenses for myopia control, reading into the paper reveals that axial eye length increased by 0.35±0.29 mm of 2 years for the eyes fitted with high compression factor lenses.

By comparison, in their 2015 meta-analysis on efficacy of ortho-k for myopia control, Sun et al reported 0.27mm axial eye growth over the same 2-year period from conventional ortho-k lens designs. This indicates that the high compression factor lenses used in the current Lau et al study created slightly less myopia control effect than standard ortho-k lens designs previously investigated, and their 'standard' (control) lenses had much less effect than typically found in other studies. The chart below illustrates these comparative outcomes, with the range of findings from the Sun et al meta-analysis shown on the right of the chart in yellow.

A potential reason for the difference between the conventional and high compression factor cohorts identified by the authors is that the conventional compression factor lenses resulted in under correction by the end of the 2-year study period. Under correction has been previously reported to accelerate myopia progression, which in the case of the conventional compression factor cohort, perhaps offering a more plausible reason for the reported difference in myopia control effect.

The above scrutiny of the paper reveals that authors' claim of greater myopia control effect from increasing compression factor in ortho-k needs to be considered with caution, and does not provide any evidence for increasing compression factor in ortho-k when fit to children for myopia control.

Ortho-k was first described by Jessen in 1962 using the simple concept that fitting a rigid lens that has the back surface radius equivalent to the keratometry + refractive error, will cause the cornea to bend to match the shape of the lens and consequently overcome the refractive error. As ortho-k continued to develop, the refractive error component became known as the Jessen Factor.

We have since seen over 60 years of ortho-k development, and it's safe to say that modern ortho-k lenses have come a long way over that time, benefiting from high precision corneal topography, high oxygen permeable materials, improvements to lens design, and greater accuracy in lens manufacture. Despite all these changes, Jessen Factor has been retained as a term to calculate the back optic zone radius in ortho-k and doesn't take into consideration that all modern lens designs are fit using different principals to the original lenses described by Jessen.

It turns out that Jessen Factor is quite unreliable, which, while not shown in a peer-reviewed study, is nicely shown in data published in the book Orthokeratology published by John Mountford et al in 2005. For 131 subjects fit with ortho-k lenses targeted to -4.00D Jessen Factor, less than 30 achieved the targeted correction with the rest either being under or over corrected.

Compression Factor describes the extra amount of refractive change that is added to the Jessen Factor to overcome the loss of refractive effect that occurs during the day when the ortho-k lenses are not worn and is generally accepted to be 0.75D.(Mountford, 1998) Compression Factor uses the same principals as Jessen Factor, so it's not surprising to find that it is also not a reliable measure, as revealed in a study by Wan et al (2019) who targeted 1.00D difference in compression factor between two study cohorts, only to achieve around 0.50D difference achieved refractive effect.

The take home message from this critique is that the Jessen Factor and Compression Factor are not reliable absolute measures and are likely to vary considerably across different ortho-k lens designs. Consequently, applying a blanket set increase in compression factor, as may be interpreted from this current Lau et al study, should be approached with caution. The authors actually reported less myopia control effect from this approach when compared to previously published research.

Instead, the paper reveals that under correcting the control group as an alternative and perhaps more plausible reason for the difference in the reported effect on axial eye growth, which can easily be overcome by fitting ortho-k lenses to ensure that the required refractive effect is achieved. Taking this approach with conventional ortho-k lens designs rather than applying a blanket increase to compression factor is backed by a greater volume of published research for providing a better myopia control effect. Essentially - fit an ortho-k lens design to provide the full refractive treatment effect, and the myopia control result should fit within previous evidence showing a robust efficacy to slow axial length growth.

Title: Orthokeratology lenses with increased compression factor (OKIC): A 2-year longitudinal clinical trial for myopia control

Authors: Jason K Lau, Kin Wan, Pauline Cho

Purpose: To investigate the effectiveness of orthokeratology (ortho-k) lenses and corneal changes with increased compression factor for myopia control over a 2-year period.

Methods: Young participants (age: 6-<12 years), with low myopia (0.50-4.00 D) and low astigmatism (≤1.25 D), were recruited and allowed to choose to wear either single-vision spectacles or ortho-k lenses (randomly assigned to compression factor of either 0.75 or 1.75 D). Axial length and cycloplegic refraction were measured at six monthly intervals for two years by a masked examiner. The myopia control effectiveness was determined by axial elongation.

Results: A significant number of control (63 %) dropped out, mainly due to concern about myopia progression (58 %). A total of 75 participants (mean age: 9.3 ± 1.0 years; control: n = 11, ortho-k [0.75 D]: n = 29, ortho-k [1.75 D]: n = 35) completed the study. Considering ortho-k groups only, the mean axial elongation of participants wearing ortho-k lenses of conventional compression factor (0.75 D) and increased compression factor (1.75 D) were 0.53 ± 0.29 and 0.35 ± 0.29 mm, respectively, over the 2-year study period. The between-group differences in corneal health were not significant at all visits.

Conclusion: Participants wearing ortho-k lenses of increased compression factor further slowed axial elongation by 34%, when compared with the conventional compression factor without compromising corneal health. Further investigations are warranted to confirm the potential mechanism of an increased compression factor for improved myopia control effectiveness.

[Link to paper abstract]