Does orthokeratology treatment zone influence myopia control?

Paper title: Variation of Orthokeratology Lens Treatment Zone (VOLTZ) Study: A 2-year randomised clinical trial

Authors: Guo, Biyue (1); Cheung, Sin Wan (1); Kojima, Randy (2); Cho, Pauline (1)

1. Centre for Myopia Research, School of Optometry, The Hong Kong Polytechnic University, Kowloon, Hong Kong, SAR, China
2. College of Optometry, Pacific University, Forest Grove, Oregon, USA

Date: Jan 2023

Reference: Guo B, Cheung SW, Kojima R, Cho P. Variation of Orthokeratology Lens Treatment Zone (VOLTZ) Study: A 2-year randomised clinical trial. Ophthalmic Physiol Opt. 2023 Aug 6

[Link to open access paper]

Orthokeratology (Ortho-K) lenses can be effective in reducing axial elongation (AE) and therefore myopia progression. The reverse geometry fitting relationship with the cornea provides a steeper paracentral zone and a flatter central zone. This flatter zone is commonly referred to as the treatment zone (TZ) and is thought to be integral to regulating axial growth.^1,2

Using lens designs with smaller back optic zone diameters (BOZD) give smaller TZ size areas. This is expected to increase the extent of peripheral retina that receives myopia defocus from the steeper corneal periphery, and hence improve the treatment effect, although research has shown that reducing TZ diameter does not alter relative peripheral refraction.¹

The Variation of Orthokeratology Lens Treatment Zone (VOLTZ) Study compared the axial elongation for children wearing Ortho-K lenses with either a 5mm or 6mm BOZD over 2yrs wear.

The VOLTZ clinical trial randomly assigned 45 Chinese children aged between 6 and 11yrs with myopia between -0.75 and -4.00D and astigmatism less than 2.50D, to be fit with either 5mm BOZD (n = 22) or 6mm BOZD (n = 23) Ortho-K lenses.

Baseline examinations included cycloplegic auto-refraction, corneal topography, sub-foveal choroidal thickness (ChT) and axial length. The examinations were repeated at 6-monthly intervals.

The on-eye TZ parameters were determined by using subtractive tangential topography images to compare before and after lens wear and statistical modelling was used to assess the association of factors such as TZ size and choroidal thickness changes with AE.

There were no differences found between the groups for baseline parameters. At the 24-mth visit, the 5mm TZ group showed significant reductions in AE and TZ size:

• Axial elongation was reduced by 0.20mm, with most change taking place in the first 6mths.
• The horizontal and vertical TZ diameters were 2.69 ± 0.28 vs. 3.84 ± 0.39 mm and 2.65 ± 0.22 vs. 3.42 ± 0.34 mm for the 5mm and 6mm groups, respectively.
• An historical control group (single vision spectacles) was used to compare AE values. The 5mm and 6mm BOZD groups showed 76% and 44% less elongation, respectively.

No significant between-group differences were found for ChT changes at 6, 12 and 24-mth intervals. However, a transient increase in choroidal thickness was noted at the 6-mth and 12-mth intervals, compared to baseline data. An association for changes in ChT and AE was found, but the effect size was close to zero.

Clinically significant changes in spherical equivalent refractive error (SER) were seen for the 5mm group at 6, 12 and 18-mth visits, but not at 24-mths.

Ortho-K lenses with 5mm back optic zone diameters created a smaller treatment zone, consequently giving 0.20mm less axial elongation over 2 years than the 6mm BOZD design.

• These findings agree with Pauné et al, who also investigated reduced AE with smaller BOZDs.³
• The saving on axial length growth with the smaller BOZD was similar to that found using 0.5% atropine. Chia et al found a 67% myopia control effect with 0.5% atropine compared with 0.01%,⁴ although a significant rebound effect has been found with this concentration of atropine.⁵

However, eye care practitioners may find the majority of the saving for AE occurs in the first 6 months of wearing a smaller BOZD ortho-k lens design, presumably due to stabilisation of the TZ area or perhaps an adaptation effect. 

1. Sub-foveal choroidal thickness changes were not impacted by the TZ size and were assumed to have little effect on axial elongation. However, the ChT changes were predominantly in the first 6mths of wear, along with changes in the TZ diameter, SER and axial elongation. Repeating this RCT study design would reveal if this short-term effect is repeated, and if any adaptation effect is potentially at play.
2. The ability of ortho-k lenses to slow AE may be related to increased defocus and aberrations created by a smaller BOZD. Studies have shown relationship between aberrations and myopia progression^6,7 but not specifically in the case of 5mm versus 6mm BOZD lens design.
3. A limitation of this study was no single vision spectacle control group. Single vision lenses have been shown to have no myopia slowing effect for young children and as such, there is an ethical dilemma to using young myopia children as a spectacle control group. Instead, this study used an historical control group from a previous study for comparison.⁸

Title: Variation of Orthokeratology Lens Treatment Zone (VOLTZ) Study: A 2-year randomised clinical trial

Authors: Guo, Biyue; Cheung, Sin Wan; Kojima, Randy; Cho, Pauline

Purpose: To compare axial elongation (AE) and treatment zone (TZ) characteristics in children wearing 6 mm or 5 mm back optic zone diameter (BOZD) orthokeratology (ortho-k) lenses over 2 years

Methods: Forty-five (6 to <11 years of age) myopic (−4.00 to −0.75 D) children of Chinese ethnicity were randomly assigned to use the two different lens designs (23 and 22 wore the 6 and 5 mm lenses, respectively). Data collection was performed at baseline and every 6-months after commencing lens wear.

Results: After 24 months, subjects wearing lenses with a 5 mm BOZD achieved smaller TZ diameter (horizontal: 2.69 ± 0.28 vs. 3.84 ± 0.39 mm; vertical: 2.65 ± 0.22 vs. 3.42 ± 0.34 mm, p < 0.001) and less AE (0.15 ± 0.21 vs. 0.35 ± 0.23, p = 0.005) compared to those using the 6 mm design, with no difference in choroidal thickness (ChT) changes (p = 0.93). A significant increase in ChT, using pooled data analysis, was noted at the 6-month (11.8 ± 19.77 μm, p < 0.001) and 12-month (12.0 ± 23.7 μm, p = 0.004) visits, compared to baseline, indicating a transient change in ChT. Significant associations were noted, using linear mixed models, between AE and the TZ diameters (p < 0.003) after adjusting for baseline data. A very weak association was found between ChT changes and AE, with the effect size close to zero.

Conclusions: Smaller BOZD ortho-k lenses resulted in a smaller TZ diameter, which was associated with less AE after 2 years of treatment. The changes in ChT played a very weak role, suggesting that other factors may contribute more to the reduced AE in subjects wearing lenses having a smaller BOZD.

[Link to open access paper]