Greater defocus volume linked to slower axial growth in ortho-k

Paper title: Association between axial elongation and corneal topography in children undergoing orthokeratology with different back optic zone diameters

Authors:
 Tan Q (1), Kojima R (2), Cho P (3,4), Vincent SJ (5)

1. School of Optometry, The Hong Kong Polytechnic University, Hung Hom, Kowloon, Hong Kong SAR
2. College of Optometry, Pacific University, Oregon, USA
3. School of Optometry, The Hong Kong Polytechnic University, Hung Hom, Kowloon, Hong Kong SAR
4. Department of Optometry and Vision Sciences, West China School of Medicine, Sichuan University, Chengdu, China
5. Contact Lens and Visual Optics Laboratory, Optometry and Vision Science, Centre for Vision and Eye Research, Queensland University of Technology, Brisbane, Australia

Date: January 3, 2025

Reference: Tan Q, Kojima R, Cho P, Vincent SJ. Association between axial elongation and corneal topography in children undergoing orthokeratology with different back optic zone diameters. Eye Vis (Lond). 2025 Jan 3;12(1):3.

[Link to open acces paper]

Orthokeratology (ortho-k) is known to slow axial elongation in children, yet the underlying optical mechanism of this effect have not been confirmed. Previous studies have explored the relationship between corneal topography metrics, particularly corneal power shift (CPS) and axial growth, with mixed findings.1,2

This retrospective study examined whether a novel metric, volumetric myopic defocus dosage (MDD), derived from corneal topography over a 5-mm pupil zone, could predict axial elongation in children wearing ortho-k lenses with different back optic zone diameters (BOZD). The analysis included data from 80 Chinese children aged 6 to 11 years who had participated in the Atropine Combined with Orthokeratology (AOK) and the Variation of Orthokeratology Lens Treatment Zone (VOLTZ) trials, in which they wore ortho-k lenses with either 5mm or 6mm BOZD for 2 years. The association of other post-ortho-k topographical changes, such as primary spherical aberration and comatic aberration, and axial elongation was also explored.

Key points were as follows:

• Children fitted with 5-mm BOZD lenses had significantly less axial elongation (0.15 ± 0.21 mm) than those wearing 6-mm BOZD lenses (0.35 ± 0.21 mm) over 2 years.
• Volumetric MDD was consistently greater in the 5-mm BOZD group at 1 and 24 months of lens wear and was negatively associated with axial elongation over two years
• Other corneal topography metrics, including circumferential, flat, and steep MDD, and changes in corneal spherical aberration and coma, were not predictive of axial growth.
• No association was found between changes in primary spherical aberration or comatic aberrations and axial elongation over 2 years.

Children wearing 5-mm BOZD ortho-k lenses had greater volumetric myopic defocus dosage (MDD) and experienced less axial elongation over 2 years compared to those fitted with 6-mm BOZD lenses. The difference in axial growth was 0.20 mm over the study period, indicating that smaller BOZD designs may provide a more effective optical profile for myopia control. Volumetric MDD, calculated from corneal topography over a 5-mm pupil zone, was the only topography-based parameter associated with slower axial elongation.

These findings suggest that increasing the volume of myopic defocus across the pupil may enhance the efficacy of ortho-k treatment. Unlike the location or peak of the corneal power shift, volumetric MDD reflects the integrated optical effect over the full pupil area. While this metric is not currently available in clinical topography software, it could inform lens selection or customization in the future.

Changes in primary spherical aberration and comatic aberration were not associated with axial elongation, indicating they may have less influence for predicting treatment effect in ortho-k. 

While this study identified an association between volumetric MDD and axial elongation, there are limitations. While volumetric MDD was associated with slower axial growth, the clinical applicability of this metric is currently limited by the absence of automated tools in corneal topography software. Further research is needed to establish thresholds for ‘optimal’ MDD values and to determine how these may vary with pupil size, age, or baseline refractive error.

The study assessed only corneal metrics over a fixed 5-mm pupil diameter and did not account for individual variation in actual pupil size, retinal shape, or ocular aberrations, all of which may influence the effective defocus experienced at the retina. Moreover, while volumetric MDD was predictive of axial elongation, the causal role of relative myopic defocus remains to be confirmed.

Future studies combining corneal, ocular, and retinal imaging metrics may help clarify how these factors interact to influence myopia progression. Evaluation across different ortho-k lens designs is also warranted to confirm the broader applicability of volumetric MDD.

Purpose: To explore the associations between myopia defocus dosage (MDD), aberration coefficients (primary spherical aberration and coma), and axial elongation in children undergoing orthokeratology (ortho-k) with back optic zone diameters (BOZD) of 5 mm and 6 mm over 2 years.

Methods: Data from 80 participants from two ortho-k studies were analyzed: 22 and 58 children wore lenses with 5-mm and 6-mm BOZD, respectively. Four MDD metrics were calculated from corneal topography data over a 5-mm pupil for the 1-month and 24-month visits: the circumferential, flat, steep, and volumetric MDD. Corneal primary spherical aberration and comatic aberrations were also extracted from topography data over a 5-mm pupil. Linear mixed modelling was performed to explore the associations between the MDD, corneal aberrations, and axial elongation over 2 years, while controlling for confounding factors (e.g., baseline age and sex).

Results: Participants in the 5-mm BOZD group displayed less axial elongation than the 6-mm BOZD group over 2 years (0.15 ± 0.21 mm vs. 0.35 ± 0.21 mm, P < 0.001). A greater volumetric MDD was observed in the 5-mm BOZD group compared with the 6-mm BOZD group at the 1- and 24-month visits (both P < 0.001). No significant differences were observed between the two groups for the other MDD metrics or corneal aberration coefficients (all P > 0.05). Less axial elongation was associated with a greater volumetric MDD at the 1- and 24-month visits (both β = -0.01, P < 0.001 and P = 0.001), but not with any other MDD metrics or corneal aberrations (all P > 0.05).

Conclusions: The volumetric MDD over a 5-mm pupil after 1 month of ortho-k lens wear was associated with axial elongation after 24 months, and may be a useful predictor of future axial elongation in children undergoing ortho-k.

[Link to open acces paper]