The treatment zone decentration and corneal refractive profile changes in children undergoing orthokeratology treatment

Abstract

Background: To confirm the association between treatment-zone (TZ) decentration and axial length growth (ALG) in children who underwent orthokeratology; and to explore the association between TZ decentration and relative corneal refractive power (RCRP) profile, which was known to be significantly associated with ALG retardation.

Methods: Four hundred myopic children of age 12 years participated in the study, with 200 wearing orthokeratology lenses and the other 200 wearing single-vision spectacle as the controls. Cycloplegic refraction was performed at baseline. Axial length was measured at baseline and 12 months after initial lens wear, and ALG was defined as the difference. In the ortho-k group, TZ decentration and the RCRP map were calculated from the topography map obtained at the 12-month visit. RCRP were summed within various chord radii from the cornea center, and the association to TZ decentration, spherical equivalent (SE), ALG were analyzed with linear regressions.

Results: Compared to the controls, children wearing orthokeratology lenses had significantly smaller ALG over 1 year (0.1 ± 0.15 mm vs. 0.32 ± 0.17 mm, p < 0.001). ALG was significantly and negatively associated with summed RCRP within the central cornea of 2 mm in radius. The mean TZ decentration was 0.62 ± 0.25 mm, and the mean direction was 214.26 ± 7.39 degrees. ALG was negatively associated with the TZ decentration magnitude (p < 0.01), but not the direction (p = 0.905). TZ decentration caused an asymmetrical distribution of the RCRP with the nasal side plus power shifting towards the corneal center. For chord radius ranging 1-2 mm, the association between TZ decentration and the summed RCRP were significant, and the proportion of variance accountable increased with chord radius. For chord radius beyond 1.5 mm, the association between baseline spherical equivalent (SE) and summed RCRP was significant. The portion of variance accountable by SE increased and peaked in 2.5 mm chord radius.

Conclusions: A larger TZ decentration was associated with a larger summed RCRP in the central cornea. It may be one of the possible reasons why TZ decentration is beneficial to retarding myopia progression.

Keywords: Corneal refractive power; Myopia control; Orthokeratology; Treatment zone decentration.

Conflict of interest statement

The authors declare that they have no competing interests.

© 2022. The Author(s).

Figures

Fig. 1

Methods to determine treatment zone decentration. A topographic map before and after orthokeratology treatment. B Method to determine TZ decentration during the 12-month follow-up visit. The red cross indicates the geometric center, the black circle represents the fitted treatment zone, and the black cross indicates the center of the treatment zone

Fig. 2

Methods to determine RCRP. A) Axial map at baseline, (B) axial map in 12 months after treatment, (C) The RCRP map

Fig. 3

Histogram showing axial length growth of children in the ortho-k (blue line) and spectacle groups (gray line). Data are expressed as the mean ± SD

Fig. 4

Histogram showing the distribution of RCRP, TZ decentration, and SE, the triangle representing the mean values. A) RCRP. B) TZ decentration. C) SE

Fig. 5

The association between ALG and RCRP, TZ decentration, and SE. A Correlation between ALG and RCRP. B Correlation between TZ decentration and RCRP. C Correlation between SE and RCRP

Fig. 6

RCRP map and RCRP profiles on four major meridians. A RCRP map of a subject with a small TZ decentration. B RCRP profiles along four meridians of this subject. Red line: horizontal. Dark line: vertical. Blue and green: oblique. C RCRP map of a subject with a large TZ decentration. D. RCRP profiles along four meridians of this subject

Fig. 7

The association between TZ decentration and summed RCRP at different chord radii. A Correlation between TZ decentration and summed RCRP at radius 1.5 mm. B Simple linear regression showed that the percentage of the variance of Summed RCRP could be explained by TZ decentration at different chord radii. Solid symbols indicate statistical significance

Fig. 8

The association between SE and summed RCRP at different chord radii. A Correlation between SE and summed RCRP at radius 3.5 mm. B Simple linear regression showed that the percentage of the variance of summed RCRP could be explained by SE (black block) at different chord radii